Review Test
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The lipid bilayer is composed primarily of what two biological molecules?
(A) Sugars and fats
(B) Carbohydrates and proteins
(C) Proteins and fats
(D) Sugars and proteins
Which of the following eukaryotic intracellular components are not organelles?
(A) Golgi apparatus
(B) Cytoskeleton
(C) ER
(D) lysosomes
What is the typical width of the lipid bilayer?
(A) 3 nanometers
(B) 5 nanometers
(C) 7 nanometers
(D) 9 nanometers
Which of the following is the main component of the cytoplasm?
(A) Cytosol
(B) Cytosine
(C) Ectoderm
(D) Chlorophyll
Which of the following phrases best matches the definition of the term hydrophobic?
(A) "water-hating"
(B) "water-loving"
(C) "water-impartial"
(D) "water-storing"
The cytosol composes up to what percent of a cell's volume?
(A) 10%
(B) 30%
(C) 50%
(D) 90%
The term hydrophilic means which of the following?
(A) Repelled by water
(B) Insoluble in water
(C) Non-water containing
(D) Attracted to water
The function of the cytoskeleton is most similar to the function of what other cellular structure?
(A) Peroxisome
(B) Lipid bilayer
(C) Cytoplasm
(D) Mitochondria
A molecule that has both polar and nonpolar regions is called ___________.
(A) Hydrophobic
(B) Hydrophilic
(C) Amphipathic
(D) Hydrated
The cytoskeleton is primarily responsible for __________.
(A) Cell shape
(B) Cellular energy
(C) Cellular respiration
(D) Cell density
The most abundant class of lipids found in the lipid bilayer are the ___________.
(A) Phospholipids
(B) Glycolipids
(C) Sphingolipids
(D) Liposomes
Which of the following are not a cytoskeletal protein filament?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
What is the name of the process by which plant cells convert light energy into biological energy?
(A) Photosynthesis
(B) Photorespiration
(C) Light-conversion
(D) Oxidative phosphorylation
Which of the following is not a property of the lipid bilayer?
(A) Fluidity
(B) Impermeability
(C) Polarity
(D) Permeability
What is the name of the structure around which microtubules grow?
(A) Centers
(B) Matrices
(C) Growth cones
(D) Centrosomes
Membrane proteins that cannot be easily removed from the cell membrane are called __________.
(A) Peripheral proteins
(B) Integral proteins
(C) Rigid proteins
(D) Locked proteins
What organelle is responsible for carrying out photosynthesis?
(A) Mitochondria
(B) Nucleus
(C) Vacuole
(D) Chloroplasts
Which of the cytoskeletal protein filaments has the largest diameter?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
Membrane proteins that span the entire lipid bilayer are called __________.
(A) Membrane-spanners
(B) Integral proteins
(C) Peripheral proteins
(D) Transmembrane proteins
Which cytoskeletal filament is responsible for forming the nuclear lamina?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
What configuration does a protein adopt when it crosses the lipid bilayer?
(A) Alpha-helical
(B) Beta-helical
(C) Gamma-helical
(D) Delta-helical
Molecules done being processed in the ER are often transported to which structure?
(A) Peroxisomes
(B) Mitochondria
(C) Golgi apparatus
(D) Vacuoles
The face through which molecules enter the golgi apparatus is called the __________ face.
(A) Cis
(B) Trans
(C) Medial
(D) Dorsal
Which cytoskeletal protein filaments line the cell membrane?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
Which of the following is not a common destination of vesicles secreted from the golgi apparatus?
(A) Lysosome
(B) Cell membrane
(C) ER
(D) Mitochondria
Which of the following best describes the properties of a lipid-bound protein?
(A) Having its tail-end attached to the lipid bilayer
(B) Having its head-end attached to the lipid bilayer
(C) Being entirely located in the lipid bilayer
(D) Being loosely associated with the lipid bilayer
Which of the following eukaryotic organelles is primarily responsible for celluar digestion?
(A) Nucleus
(B) Lysosome
(C) Mitochondria
(D) Golgi apparatus
Which of the following is the largest organelle in eukaryotic cells?
(A) ER
(B) Golgi apparatus
(C) Lysosome
(D) Nucleus
Which of the following is not a specialized plant cellular structure?
(A) Lysosome
(B) Cell wall
(C) Chloroplast
(D) Vacuole
What is the name of the organelle that helps mediate endocytosis and exocytosis?
(A) Vacuole
(B) Peroxisome
(C) Lysosome
(D) Endosome
What is the name of the carbohydrate coat found on the outside of the lipid bilayer of higher- order cells cells?
(A) Glycosylation
(B) Glycocalyx
(C) Glycogen
(D) Glutamine
What structure in plants replaces the function that the mitochondria performs in animal cells?
(A) Vacuole
(B) Cytoskeleton
(C) Chloroplast
(D) Nucleus
In eukaryotes, the cell nucleus composes what fraction of the total cell volume?
(A) 10%
(B) 15%
(C) 20%
(D) 25%
What is the name of the process by which molecules naturally flow from an area of higher concentration to one of lower concentration?
(A) Respiration
(B) Transfusion
(C) Dialysis
(D) Diffusion
Which of the following organelles do not contain a double-membrane?
(A) Nucleus
(B) Lysosome
(C) Mitochondria
(D) Chloroplasts
Membrane transport that occurs without the input of extra energy can be classified as what type of transport?
(A) Passive
(B) Active
(C) Catalytic
(D) Inhibitory
The cell nucleus is not found in direct contact with which of the following cellular structures?
(A) Cytosol
(B) Cytoskeleton
(C) ER
(D) Mitochondria
Membrane transport that requires the input of additional enery is called _________.
(A) Passive
(B) Active
(C) Catalytic
(D) Inhibitory
What is the name of the inner-most space in a mitochondrial cell?
(A) Matrix
(B) Intermembrane space
(C) Cytosol
(D) Chlorophyll
Membrane proteins that mediate active transport are generally __________.
(A) Lipid-bound proteins
(B) Ionophores
(C) Channel proteins
(D) Carrier proteins
What is the main function of the mitochondria?
(A) Molecular digestion
(B) DNA storage
(C) Space-filler
(D) Enery-producer
The mitochondrial matrix is analogous to which structure in plant chloroplasts?
(A) Stoma
(B) Intermembrane space
(C) Cytosol
(D) Vacuole
Membrane proteins that mediate passive transport are generally _________.
(A) Lipid-bound proteins
(B) Ionophores
(C) Channel proteins
(D) Carrier proteins
Which eukaryotic organelle contains oxidizing enzymes?
(A) Lysosome
(B) Peroxisome
(C) Golgi apparatus
(D) ER
What is a typical thickness for a cell wall?
(A) 50 nanometers
(B) 50 micrometers
(C) 10 nanometers
(D) 10 micrometers
__________ are a class of membrane proteins that increase a cell's permeability to certain ions.
(A) Lipid-bound
(B) Ionophores
(C) Channel proteins
(D) Carrier proteins
__________ endoplasmic reticulum has ribosomes attached to its outer membrane.
(A) Smooth
(B) Spiked
(C) Rough
(D) Cytoskeletal
Vacuoles can occupy up to what percent of plant cells?
(A) 90%
(B) 75%
(C) 50%
(D) 25%
Membrane transport is important for which of the following biological processes?
(A) Protein synthesis
(B) Cell communication
(C) Maintenance of cellular pH
(D) All of the above
The ER plays a major role in the processing of which of the following biological molecules?
(A) Carbohydrates
(B) Proteins
(C) Adenosine triphosphate
(D) DNA
Which of the following is not a normal function of plant cell vacuoles?
(A) Cellular digestion
(B) Space-filling
(C) Energy production
(D) Storage
Terms
Active transport - The transport of molecules across a membrane and against their natural flow; mediated by carrier proteins and requiring outside energy.
Carrier protein - A protein responsible for mediating the active transport of molecules from one side of the lipid bilayer to the other. Transport is carried out by a conformational change that occurs within the protein that forms an opening for specific molecules to pass through.
Channel protein - A protein responsible for mediating the passive transport of molecules from one side of the lipid bilayer to the other. Transport is carried out by its membrane-spanning hydrophilic structure which, when open, allows molecules to pass through.
Diffusion - The transport process in which molecules naturally travel from an area of higher concentration to an area of lower concentration.
Glycocalyx - A layer of carbohydrates that coats the exterior of higher-ordered cells. Functions in protecting the cell from damage.
Hydrophilic - A polar molecule that selectively associates itself with water through hydrogen bonds.
Hydrophobic - A nonpolar molecule that does not readily associate with water through hydrogen bonds.
Integral protein - A membrane protein that cannot be easily removed from the lipid bilayer.
Ionophore - A class of membrane transport proteins. Small, hydrophobic molecules that increase membrane permeability to certain ions.
Lipid bilayer - A thin double layer of phospholipid molecules. Provides the structure of a cell membrane. Structure is a result of hydrophobic and hydrophilic forces.
Lipid-bound protein - Membrane proteins that are located entirely within the lipid bilayer, having no part touching either the inside or outside of the cell.
Multi-pass protein - Transmembrane proteins that cross the lipid bilayer more than one time.
Osmosis - The process by which water naturally travels from an area of high concentration to one of lower concentration.
Passive transport - Transport mediated by channel proteins. The movement of molecules across a membrane according to the natural flow.
Peripheral protein - A membrane protein that can be easily removed from the lipid bilayer.
Single-pass protein - A transmembrane protein that only crosses the lipid bilayer one time.
Transmembrane protein - A membrane protein that spans the lipid bilayer having portions in contact with both the inside and outside of the cell. Area within the lipid bilayer forms an alpha-helix.
The Lipid Bilayer
Lipid Bilayer Structure
The lipid bilayer is a universal component of all cell membranes. Its role is critical because its structural components provide the barrier that marks the boundaries of a cell. The structure is called a "lipid bilayer" because it is composed of two layers of fat cells organized in two sheets. The lipid bilayer is typically about five nanometers thick and surrounds all cells providing the cell membrane structure.Lipids and Phospholipids
Figure %: Basic Lipid Structure
The hydrophilic region is attracted to aqueous water conditions while the hydrophobic region is repelled from such conditions. Since a lipid molecule contains regions that are both polar and nonpolar, they are called amphipathic molecules.
The most abundant class of lipid molecule found in cell membranes is the phospholipid. The phospholipid molecule's polar head group contains a phosphate group. It also sports two nonpolar fatty acid chain groups as its tail. 
Figure %: Phospholipid Structure
The fatty acid tail is composed of a string of carbons and hydrogens. It has a kink in one of the chains because of its double-bond structure. The Bilayer
The phospholipids organize themselves in a bilayer to hide their hydrophobic tail regions and expose the hydrophilic regions to water. This organization is spontaneous, meaning it is a natural process and does not require energy. This structure forms the layer that is the wall between the inside and outside of the cell.
Figure %: Lipid Bilayer
Properties of the Lipid Bilayer
As we have already mentioned, the most important property of the lipid bilayer is that it is a highly impermeable structure. Impermeable simply means that it does not allow molecules to freely pass across it. Only water and gases can easily pass through the bilayer. This property means that large molecules and small polar molecules cannot cross the bilayer, and thus the cell membrane, without the assistance of other structures.Another important property of the lipid bilayer is its fluidity. The lipid bilayer contains lipid molecules, and, as we will discuss later, it also contains proteins. The bilayer's fluidity allows these structures mobility within the lipid bilayer. This fluidity is biologically important, influencing membrane transport. Fluidity is dependent on both the specific structure of the fatty acid chains and temperature (fluidity increases at lower temperatures).
Structurally, the lipid bilayer is asymmetrical: the lipid and protein composition in each of the two layers is different.
Cell Membranes
Problems
Problem : Identify the lipid bilayer in the following diagram of a cell.
Cell
Solution for Problem 1 >>
Solution
The lipid bilayer is the outer-most layer surrounding the cell.
Close
Problem : What is the main function of the lipid bilayer?
Solution for Problem 2 >>
The lipid bilayer acts as a barrier between the inside and outside of the cell. It is highly impermeable and does not allow most molecules to freely pass through it into or out of the cell.
Close
Problem : Why is the structure called a lipid bilayer?
Solution for Problem 3 >>
Problem : Fill in the blanks.
A phospholipid molecule contains two distinct regions. The __________ region is attracted to water and the ___________ region is repelled from water. As a result of its both polar and nonpolar regions, it is classified as a(n) __________ molecule.
Solution for Problem 4 >>
Hydrophilic. Hydrophobic. Amphipathic.
Close
Problem : Are the two layers of the lipid bilayer identical in composition?
Solution for Problem 5 >>
No. As a result of the bilayer's fluidity, structures such as lipids and proteins can freely move around within the lipid bilayer.
Membrane Proteins
In addition to the lipid bilayer, the cell membrane also contains a number of proteins. We have already mentioned the presence of certain proteins in the cell membrane. In this section we will discuss the different classes of proteins found there. While the lipid bilayer provides the structure for the cell membrane, membrane proteins allow for many of the interactions that occur between cells. As we discussed in the previous section, membrane proteins are free to move within the lipid bilayer as a result of its fluidity. Although this is true for most proteins, they can also be confined to certain areas of the bilayer with enzymes. Membrane proteins perform various functions, and this diversity is reflected in the significantly different types of proteins associated with the lipid bilayer.Classifications of Membrane Proteins
Integral Proteins
Integral proteins are embedded within the lipid bilayer. They cannot easily be removed from the cell membrane without the use of harsh detergents that destroy the lipid bilayer. Integral proteins float rather freely within the bilayer, much like oceans in the sea. In addition, integral proteins are usually transmembrane proteins, extending through the lipid bilayer so that one end contacts the interior of the cell and the other touches the exterior. The stretch of the integral protein within the hydrophobic interior of the bilayer is also hydrophobic, made up of non-polar amino acids. Like the lipid bilayer, the exposed ends of the integral protein are hydrophilic.
Figure %: Membrane Proteins
When a protein crosses the lipid bilayer it adopts an alpha-helical configuration. Transmembrane proteins can either cross the lipid bilayer one or multiple times. The former are referred to as single-pass proteins and the later as multi-pass proteins. As a result of their structure, transmembrane proteins are the only class of proteins that can perform functions both inside and outside of the cell. Peripheral Proteins
Peripheral proteins are attached to the exterior of the lipid bilayer. They are easily separable from the lipid bilayer, able to be removed without harming the bilayer in any way. Peripheral proteins are less mobile within the lipid bilayer.Lipid-Bound Proteins
Lipid-bound proteins are located entirely within the boundaries of the lipid bilayer.The Cell Surface
The protein and lipid cell membrane is covered with a layer of carbohydrate chains on its outer surface. This layer is called a cell coat or glycocalyx. The exact composition and distribution of these chains is very diverse. The chains are thought to provide the cell with protection against damage. Glycocalyx are only found on the surface of the cells of higher organism's.
Figure %: A detailed view of a Cell Membrane (phospholipid bilayer and associated proteins)
Problems
Problem : Will you find the same set of membrane proteins in each cell membrane?
No. Membrane proteins perform a number of functions within cells, as a result, different proteins are necessary in different regions of cells depending on the function of the cell and the interactions it may take part in.
Problem : What are the names of the two main classes of membrane proteins and how could you tell one from the other?
The two main classes of membrane proteins are integral versus peripheral proteins. Since peripheral proteins are easily dissociated from the lipid bilayer, one could treat a cell with a mild detergent that does not disrupt the cell membrane and then see if the specific protein remains associated with the lipid bilayer or is removed.
Problem : What is the name of the configuration that membrane proteins adopt in regions that span the lipid bilayer?
This configuration is called an alpha-helix. It is the same structure that DNA adopts naturally.
Problem : Which class of proteins, integral or peripheral, are freer to move around within the lipid bilayer?
Integral proteins can be thought of as icebergs that float in a lipid bilayer sea. They are relatively mobile in the cell membrane.
Problem : The cell surface is covered with an additional set of molecules. What name is given to these structures and what is their function in the cell?
The cell surface is covered with a cell coat or glycocalyx which consists of carbohydrate chains. They help protect the cell from damage.
Structures Responsible for Membrane Transport
We have discussed how the lipid bilayer acts as an efficient barrier by only allowing a very small number of non-polar molecules to freely enter or exit a cell. While for the most part this selectivity is a valuable function and allows the cell to maintain its integrity, cells do need to move certain large, polar molecules such as amino acids, sugars, and nucleotides across their membranes. As a result, cell membranes require specific structures that allow for the transport of certain molecules.Membrane Transport
There are a number of different ways that molecules can pass from one side of a cell membrane to the other. Some such means, like diffusion and osmosis, are natural processes that require no expenditure of energy from the cell and are called passive transport. Other methods of transport do require cellular energy and are called active transport. In addition to these two forms of transport, there exist other forms of transport such as endocytosis and exocytosis, which will be discuss later and do not require the same set of membrane proteins for their function.
Passive Transport
Diffusion is the natural phenomenon in which nonpolar molecules naturally flow from an area of higher concentration to an area of lower concentration. Osmosis is a similar process, but refers specifically to water molecules. Both of these classes of molecules we have already discussed as capable of crossing the lipid bilayer. As seen in , neither diffusion nor osmosis require the expenditure of energy.Active Transport
Active transport occurs when a cell actively pumps a molecule across its membrane, against the natural direction dictated by diffusion, osmosis, or polarity. As seen in , such transport requires energy. 
Figure %: Active and Passive Transport Proteins
Transport Proteins
Both of passive and active transport are mediated with the help of transmembrane proteins that act as transporters. shows the two main classes of transport proteins: carrier proteins and channel proteins. For the most part, carrier proteins mediate active transport while channel proteins mediate passive transport. Carrier proteins create an opening in the lipid bilayer by undergoing a conformational change upon the binding of the molecule. Channel proteins form hydrophilic pores across the lipid bilayer. When open, these pores allow specific molecules to pass through. There is one other class of transport proteins called ionophores. These are small, hydrophobic proteins that increase bilayer permeability for specific ions.Transport proteins are critical to cell life and cell interactions. They allow for the proper distribution of ions and molecules in multicellular organisms. Additionally, they can help to maintain proper intra- and extra-cellular pH levels, facilitate communication between cells, and are involved numerous other essential functions including protein sythesis.
Cell Membranes
Problems
Problem : Why is it necessary for cell membranes to have proteins that help transport molecules?
The cell membrane is composed of a lipid bilayer that is highly impermeable to most molecules. As a result, outside structures are required to help transport essential large, polar molecules across the cell membrane.
Problem : What is the name of the natural process by which molecules flow from an area of higher concentration to one of lower concentration?
Diffusion.
Problem : What is the difference between the behavior of carrier and channel proteins?
Problem : What is the function of an ionophore?
Ionophores function to increase a membrane's permeability to a specific ion thereby facilitating its movement across that cell's membrane.
Problem : Name two specific functions of membrane transport.
Membrane transport helps maintain the proper distribution of ions across a cell membrane; helps maintain proper cellular pH, and helps mediate communication between cells in multi-cellular organisms.
Cell Differences
Terms
Cell wall - The thick and rigid layer that covers the plasma membrane in plant cells. Composed of fat and sugar molecules in a matrix.
Chlorophyll - A pigment located within a chloroplast that absorbs light in plant cells, helping to convert light energy into biological energy in the process of photosynthesis.
Chloroplast - A double membrane-bound organelle found in plant cells that contains chlorophyll and is responsible for mediating photosynthesis.
Photosynthesis - A process in which plants convert sunlight into energy sources that can be used inside the cell to sustain life.
Vacuole - Membrane-bound, fluid-filled organelle found only in plant cells. Can compose up to 90% of a cell's volume and performs diverse functions in plant cells, including digestion of intracellular molecules.
Plant Cells
Figure %: Generalized Plant Cell
Structurally, plant and animal cells are very similar because they are both eukaryotic cells. They both contain membrane-bound organelles such as the nucleus, mitochondria, endoplasmic reticulum, golgi apparatus, lysosomes, and peroxisomes. Both also contain similar membranes, cytosol, and cytoskeletal elements. The functions of these organelles are extremely similar between the two classes of cells (peroxisomes perform additional complex functions in plant cells having to do with cellular respiration). However, the few differences that exist between plant and animals are very significant and reflect a difference in the functions of each cell. Plant cells can be larger than animal cells. The normal range for an animal cell varies from 10 to 30 micrometers while that for a plant cell stretches from 10 to 100 micrometers. Beyond size, the main structural differences between plant and animal cells lie in a few additional structures found in animal cells. These structures include: chloroplasts, the cell wall, and vacuoles.
Figure %: Plant Cell v. Animal Cell
Chloroplasts
In animal cells, the mitochondria produces the majority of the cells energy from food. It does not have the same function in plant cells. Plant cells use sunlight as their energy source; the sunlight must be converted into energy inside the cell in a process called photosynthesis. Chloroplasts are the structures that perform this function. They are rather large, double membrane-bound structures (about 5 micrometers across) that contain the substance chlorophyll, which absorbs sunlight. Additional membranes within the chloroplast contain the structures that actually carry out photosynthesis.Chloroplasts carry out energy conversion through a complex set of reactions similar to those performed by mitochondria in animals. The double membrane structure of chloroplasts is also reminiscent of mitochondria. The inner membrane encloses an area called the stoma, which is analogous to the matrix in mitochondria and houses DNA, RNA, ribosomes, and different enzymes. Chloroplasts, however, contain a third membrane and are generally larger than mitochondria.
The Cell Wall
Another structural difference between in plant cells is the presence of a rigid cell wall surrounding the cell membrane. This wall can range from 0.1 to 10 micrometers thick and is composed of fats and sugars. The tough wall gives added stability and protection to the plant cell.Vacuoles
Vacuoles are large, liquid-filled organelles found only in plant cells. Vacuoles can occupy up to 90% of a cell's volume and have a single membrane. Their main function is as a space-filler in the cell, but they can also fill digestive functions similar to lysosomes (which are also present in plant cells). Vacuoles contain a number of enzymes that perform diverse functions, and their interiors can be used as storage for nutrients or, as mentioned, provide a place to degrade unwanted substances.Cell Differences
Problems
Problem : Which of the following cellular components are not shared between plant and animal cells?
- Mitochondria
- Nucleus
- Chloroplasts
- Vacuoles
- Cell membrane
- Endoplasmic Reticulum
- Golgi apparatus
- Lysosomes
- Peroxisomes
- Cell wall
In the list all structures are shared between plant and animal cells except cholorplasts, vacuoles, and cell wall.
Problem : Why are chloroplasts found in plant cells and not in animal cells?
Plant cells use sunlight for energy. In order to effectively use the light, it must be converted within the cell. Chloroplasts are structures that perform this function. Animal cells do not use this special mechanism for energy and therefore do not need the additional structure.
Problem : What is the name of the process by which plant cells convert sunlight into energy a cell can use for biological functions?
Photosynthesis
Problem : Which is generally larger, an animal or plant cell?
Plant cells are generally larger than animal cells. They can be as large as 100 micrometers in diameter.
Problem : Vacuoles are most similar to what other organelle found in both plant and animal cells? Why?
Vacuoles are most similar to lysosomes because both have digestive functions in cells. Vacuoles and lysosomes both contain enzymes that can break down unwanted molecules in a cell.
Cell Differences
Prokaryotic Cells
Unlike eukaryotic cells, prokaryote cells lack membrane-bound organelles. However, whereas prokaryote cells are less structurally complex than eukaryotes, they are more chemically complex, since all of the prokaryote cell's biomolecules are floating around together. These biomolecules must interact only with other appropriate molecules to perform biological function.
Prokaryotic cells contain a single compartment enclosed within the cell membrane. In this space reside DNA, RNA, ribosomes and other molecules. Prokaryotes lack a defined nucleus (which is where DNA and RNA are stored in eukaryotic cells), mitochondria, ER, golgi apparatus, and so on. In addition to the lack of organelles, prokaryotic cells also lack a cytoskeleton. Recall that in addition to its role as structural support for the interior of the cell, the cytoskeleton is also involved in intracellular organelle transport. Since there are no organelles to be transported in prokaryotic cells, such a function is unnecessary.
Like the eukaryote cell, the prokaryote cell is filled with cytosol. The prokaryote cytosol is filled with enzymes, which carry out respiratory processes reserved in eukaryotes for the mitochondria. Prokaryote and eukaryote ribosomes also differ slightly, reflect minor differences in prokaryotic versus eukaryotic processing of DNA.
Cell Differences
Problems
Problem : Fill in the blank. Prokaryotic cells differ structurally from eukaryotic cells because they lack membrane-bound __________.
Organelles.
Problem : Fill in the blank. Eukaryotic cells are more complex __________, but prokaryotic cells are more complex ____________.
Structurally. Chemically.
Problem : What structures of a cell that we have discussed are conserved between prokaryotic and eukaryotic cells?
The cell membrane in both prokaryotic and eukaryotic cells have the same structure. Additionally, prokaryotic cells also contain cytosol in its cytoplasm.
Problem : Are multi-cellular organisms more likely to be prokaryotic or eukaryotic?
Eukaryotic.
Problem : What is a reason for the lack of a cytoskeleton in prokaryotic cells?
The cytoskeleton functions in eukaryotic cells in intracellular organelle transport. Since prokaryotic cells do not have organelles, this function is unnecessary.
Terms
Actin - A very abundant protein in eukaryotic cells that is the main component of actin filaments.
Actin Filaments - Approximately 5-9 nanometers in diameter. Provide structural support to the plasma membrane. As a cytoskeletal protein provides for movement of organelles within cells.
Centromere - A round structure that holds together sister chromatids.
Centrosome - A region of the cell near the nucleus from which microtubules sprout. Centrosomes are not found in all cells. Centrosomes are comprised of two centrioles.
Chromosome - A structure composed of DNA and proteins containing all the genetic material of a cell. Found in the cell nucleus.
Cytoplasm - A fluid found in the main compartment of eukaryotic cells. Includes everything outside the cell nucleus but the organelles and the cytoskeleton. The main component is cytosol.
Cytoskeleton - A system of protein filaments found throughout the cytoplasm of eukaryotic cells that help provide for cell structure. Composed of actin, intermediate filaments, and microtubules.
Cytosol - The main component of the cytoplasm that fills the main compartment of eukaryotic cells.
Endoplasmic reticulum - A membrane-bound organelle found in eukaryotic cells. Makes direct contact with the cell nucleus and, since it is dotted with ribosomes, is the site of lipid and protein synthesis. Comes in two forms, smooth and rough.
Endosome - A membrane-bound organelle found in eukaryotic cells. Responsible for delivering molecules to the lysosome for digestion.
Eukaryote - An organism composed of one or more cells with defined intracellular components including a nucleus and cytosol. Includes all organisms except bacteria and viruses.
Golgi apparatus - A membrane-bound organelle found near the cell nucleus in eukaryotic cells. Responsible for sorting and packaging proteins for secretion to various destinations in the cell.
Intermediate filament - One of three protein components of the cytoskeleton. A fibrous protein filament approximately 10 nanometers in diameter. Forms the nuclear lamina that helps protect the cell nucleus.
Intermembrane space - The space between the outer and inner membrane in a mitochondria.
Lysosome - A membrane-bound organelle found in eukaryotic cells. Contain acids and enzymes that degrade unwanted molecules.
Matrix - The space inside the inner membrane of mitochondria.
Microtubule - One of three protein components of the cytoskeleton. Long, cylindrical structures approximately 25 nanometers in diameter. Extend from the centrosome to all parts of the cell, forming tracks on which organelles can travel within the cell. Microtubules can be either kinetocore microtubules or non-kinetocore microtubules. Kinetocore microtubules bind to sister chromatids during mitosis; non-kinetocore microtubules do not.
Mitochondria - An organelle within the cell. Much of cell respiration is carried out within its bounds.
Nucleus - A large, double membrane-bound organelle found in eukaryotic cells. Contains DNA and RNA.
Organelle - A membrane-bound sub-cellular structure found in eukaryotic cells. The Cell nucleus, mitochondria, ER, and golgi apparatus are all examples.
Peroxisome - A small, membrane-bound organelle found in eukaryotic cells. Contains oxidizing enzymes that oxidize organic molecules and process hydrogen peroxide in the cell.
Prokaryote - An organism composed of usually one, but occasionally more, cells that lack defined sub-cellular compartments. All essential material is enclosed within the cell membrane. Includes all bacteria and close relatives.
Ribosome - A molecule composed of ribosomal RNA* {biology/molecularbiology/translation}* and proteins, and located on the endoplasmic reticulum**. Responsible for mediating protein synthesis.
Rough endoplasmic reticulum - Endoplasmic reticulum that is coated with ribosomes and involved in protein synthesis.
Smooth endoplasmic reticulum - Naked endoplasmic reticulum that lacks ribosomes and is more involved in lipid synthesis.
Intracellular Components
The Cytoskeleton and Cytosol
In this section we will discuss the intracellular components that are not organelles. The cytoskeleton and cytosol are structural elements that help provide the cell with its structure. The cytoskeleton is composed of protein filaments and is found throughout the inside of a eukaryotic cell. The cytosol is the main component of the cytoplasm, the fluid that fills the inside of the cell. The cytoplasm is everything in the cell except for the cytoskeleton and membrane-bound organelles. Both structures, the cytoskeleton and cytosol, are "filler" structures that do not contain essential biological molecules but perform structural functions within a cell.The Cytosol
Figure %: Location of the cytosol within a cell.
The Cytoskeleton
The cytoskeleton is similar to the lipid bilayer in that it helps provide the interior structure of the cell the way the lipid bilayer provides the structure of the cell membrane. The cytoskeleton also allows the cell to adapt. Often, a cell will reorganize its intracellular components, leading to a change in its shape. The cytoskeleton is responsible for mediating these changes. By providing "tracks" with its protein filaments, the cytoskeleton allows organelles to move around within the cell. In addition to facilitating intracellular organelle movement, by moving itself the cytoskeleton can move the entire cells in multi-cellular organisms. In this way, the cytoskeleton is involved in intercellular communication.The cytoskeleton is composed of three different types of protein filaments: actin, microtubules, and intermediate filaments.
Actin
Actin is the main component of actin filaments, which are double-stranded, thin, and flexible structures. They have a diameter of about 5 to 9 nanometers. Actin is the most abundant protein in most eukaryotic cells. Most actin molecules work together to give support and structure to the plasma membrane and are therefore found near the cell membrane.Microtubules
Microtubules are long, cylindrical structures composed of the protein tubulin and organized around a centrosome, an organelle usually found in the center of the cell near the cell nucleus. Unlike actin molecules, microtubules work separately to provide tracks on which organelles can travel from the center of the cell outward.Intermediate filaments
Intermediate filaments are the final class of proteins that compose the cytoskeleton. These structures are rope-like and fibrous, with a diameter of approximately 10 nanometers. They are not found in all animal cells, but in those in which they are present they form a network surrounding the nucleus often called the nuclear lamina. Other types of intermediate filaments extend through the cytosol. The filaments help to resist stress and increase cellular stability.
Figure %: Organization of actin, microtubules, and intermediate filaments within a cell.
These three types of protein are distinct in their structure and specific function, but all work together to help provide intra-cellular structure. Because they are so diverse, it is very difficult to study the specific functions of the cytoskeletal components. Intracellular Components
Problem : What are the main functions of the cytosol and cytoskeleton?
The cytosol is the site of protein synthesis and the cytoskeleton helps provide intracellular structure and helps with organelle movement in cells.
Problem : Label the locations of the cytoskeleton and cytosol in the following diagram.
Figure %: Cell
Figure %: Solution
The cytosol is a liquid that fills the interior of a cell. The cytoskeleton is composed of protein filaments found in the cytosol.
Problem : In what way is the function of the cytoskeleton similar to the function of the lipid bilayer?
The lipid bilayer provides the structure of the cell membrane. Similarly, the cytoskeleton helps provide the structure of the interior of the cell.
Problem : The cytosol makes up most of what intracellular component?
The cytoplasm.
Problem : Name the three proteins that compose the cytoskeleton. Which of these is responsible for forming the nuclear lamina?
The three types of protein filaments are actin molecules, microtubules, and intermediate filaments. Intermediate filaments form the nuclear lamina.
Intracellular Components
Eukaryotic Organelles: The Cell Nucleus, Mitochondria, and Peroxisomes
We will now begin our discussion of intracellular organelles. As we have mentioned, only eukaryotic cells have intracellular sub-divisions, so our discussion will exclude prokaryotic cells. We will also focus on animal cells, since plant cells have a number of further specialized structures. In this section we will discuss the importance of the cell nucleus, mitochondria, peroxisomes, endoplasmic reticulum, golgi apparatus, and lysosome.The Cell Nucleus
The cell nucleus is one of the largest organelles found in cells and also plays an important biological role. It composes about 10% of the total volume of the cell and is found near the center of eukaryotic cells. Its importance lies in its function as a storage site for DNA, our genetic material. The cell nucleus is composed of two membranes that form a porous nuclear envelope, which allows only select molecules in and out of the cell.The DNA that is found in the cell nucleus is packaged into structures called chromosomes. Chromosomes contain DNA and proteins and carry all the genetic information of an organism. The nucleus gains support from intermediate filaments that both form the surrounding nuclear lamina and makes direct contact with the endoplasmic reticulum. The nucleus is also the site of DNA and RNA synthesis.
Figure %: Location of the cell nucleus, mitochondria, and peroxisomes in a cell.
Mitochondria
The mitochondria, with its specialized double-membrane structure, generate adenosine triphosphate (ATP), a molecule that provides organisms with energy.
Figure %: Mitochondrial structure
The outer and inner membranes of the mitochondria form two sub-compartments: the internal matrix space and the intermembrane space. Those few proteins found withn the mitochondria are located within the inner membrane. Mitochondria synthesize ATP with energy supplied by the electron transport chain and a process called oxidative phosphorylation.
Peroxisomes
Peroxisomes are single-membrane structures found in all eukaryotic cells. They are small, membrane-bound structures that use molecular oxygen to oxidize organic molecules. The structure is one of the major oxygen utilizing organelles, the other being the mitochondria. Peroxisomes contain oxidative enzymes and other enzymes that help produce and degrade hydrogen peroxide.Because of their varying enzymatic compositions, peroxisomes are diverse structures. Their main function is to help breakdown fatty acids. They perform specific functions in plant cells, which we will discuss later.
The Endoplasmic Reticulum
The endoplasmic reticulum, or ER, is a very important cellular structure because of its function in protein synthesis and lipid synthesis. For example, the ER is the site of production of all transmembrane proteins. Since nearly all proteins that are secreted from a cell pass through it, the ER is also important in cellular trafficking. In addition to these major roles, the ER plays a role in a number of other biological processes. There are two different types of ER: smooth ER and rough ER (RER).The rough ER has its name because it is coated with ribosomes, the structures most directly responsible for carrying out protein synthesis. Smooth ER lacks these ribosomes and is more abundant in cells that are specific for lipid synthesis and metabolism.
Figure %: The Endoplasmic Reticulum
In addtion to protein and lipid synthesis, the ER also conducts post-synthesis modifications. One such modification involves the addition of carbohydrate chains to the proteins, though the function of this addition is unknown. Another major modification is called protein folding, whose name is rather self- explanatory. Another role of the ER is to capture calcium for the cell from the cytosol. Finally, the ER can secrete proteins into the cell that are usually destined for the golgi apparatus. 
Figure %: The location of the Endoplasmi Reticulum, golgi apparatus, and lysosome in a eukaryotic cell.
The Golgi Apparatus
The golgi apparatus is usually located near the cell nucleus. It is composed of a series of layers called golgi stacks. Proteins from the ER always enter and exit the golgi apparatus from the same location. The cis face of the golgi is where proteins enter. A protein will make its way through the golgi stacks to the other end called the trans face where it is secreted to other parts of the cell.
Figure %: Structure of the Golgi Apparatus
In the golgi apparatus, more carbohydrate chains are added to the protein while other chains are removed. The golgi stacks also sort proteins for secretion. After sorting, the membrane of the golgi buds off, forming secretory vesicles that transport proteins to their specific destination in the cell. A protein's destination is often signaled with a specific amino acid sequence at its end. A protein secretion most often travels back to the ER, to the plasma membrane where it can become a transmembrane protein, or to the next structure we will discuss, the lysosomes. Lysosomes
Lysosomes are sites of molecular degradation found in all eukaryotic cells. They are small, single-membrane packages of acidic enzymes that digest molecules and are found throughout eukaryotic cells. As such, Lysosomes are a sort of cellular "garbage can," getting rid of cellular debris. Proteins that are not correctly folded or have significant mutations can be secreted to the lysosomes and be degraded instead of taking up space in the cell. Detritus proteins and other molecules can find their way to the lysosome in a variey of ways.Molecules from outside a cell can be taken in through a process called endocytosis. In this process, the cell membrane invaginates, forming a vesicle containing the transported molecule that will eventually reach a lysosome. The reverse of endocytosis is exocytosis. In this process, molecules within a cell are secreted into an endosome, a membrane-bound structure that delivers the molecule to the lysosome. After reaching the lysosomes, the molecules are secreted from a cell in membrane vesicles. Proteins secreted by the golgi apparatus into the plasma membrane can also be taken back to the lysosome by endosomes.
Intracellular Components
Problems
Problem : Identify the cell nucleus, mitochondria, and peroxisomes in the following diagram.
Figure %: Eukaryotic cell
Figure %: Solution
Problem : What is the name of the structures into which DNA is packaged inside the cell nucleus?
DNA is packaged into chromosomes with proteins in the cell nucleus.
Problem : Does the nucleus allow molecules to pass across its double membrane?
Yes. The nuclear membrane contains pores that allow only certain molecules to pass across its membrane either in or out of the cell.
Problem : Why does the mitochondria have two distinct sub-compartments?
Mitochondrial membranes have two layers—an inner and outer membrane. As a result, there are two distinct spaces in the organelle. One between the inner and outer membranes, called the intermembrane space, and one within the inner membrane, called the matrix space.
Problem : What is the chemical process that occurs in the peroxisomes?
Oxidation. The peroxisomes contain enzymes that oxidize intracellular molecules like fatty acids.
Problem : Identify the endoplasmic reticulum, golgi apparatus, and lysosome in the following diagram.
Figure %: Eukaryotic cell
Figure %: Solution
Problem : Why is it called "rough" endoplasmic reticulum?
It is called rough endoplasmic reticulum because it is coated with small ribosomal particles that for a bumpy coat on the cytosolic side of the ER membrane. It is contrasted with smooth ER which lacks these ribosomes.
Problem : How are proteins segregated in the golgi apparatus for secretion?
Most proteins contain a signal sequence at its end that tell the golgi to where in the cell it should be secreted.
Problem : Into which face of the golgi apparatus do proteins from the ER enter? Which face do they exit from?
Proteins enter the golgi apparatus through the cis face and exit through the trans face.
Problem : What is the function of the lysosome?
The lysosome functions to remove intracellular debris by digesting it with the acidic enzymes found inside it.
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Review of Cell Structure
Review Test
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Scroll through the page to review your answers. The correct answer is highlighted in green. Your incorrect answers (if any) are highlighted in red. If you'd like to take the test over again, click the reset button at the end of the test.
The lipid bilayer is composed primarily of what two biological molecules?
(A) Sugars and fats
(B) Carbohydrates and proteins
(C) Proteins and fats
(D) Sugars and proteins
Which of the following eukaryotic intracellular components are not organelles?
(A) Golgi apparatus
(B) Cytoskeleton
(C) ER
(D) lysosomes
What is the typical width of the lipid bilayer?
(A) 3 nanometers
(B) 5 nanometers
(C) 7 nanometers
(D) 9 nanometers
Which of the following is the main component of the cytoplasm?
(A) Cytosol
(B) Cytosine
(C) Ectoderm
(D) Chlorophyll
Which of the following phrases best matches the definition of the term hydrophobic?
(A) "water-hating"
(B) "water-loving"
(C) "water-impartial"
(D) "water-storing"
The cytosol composes up to what percent of a cell's volume?
(A) 10%
(B) 30%
(C) 50%
(D) 90%
The term hydrophilic means which of the following?
(A) Repelled by water
(B) Insoluble in water
(C) Non-water containing
(D) Attracted to water
The function of the cytoskeleton is most similar to the function of what other cellular structure?
(A) Peroxisome
(B) Lipid bilayer
(C) Cytoplasm
(D) Mitochondria
A molecule that has both polar and nonpolar regions is called ___________.
(A) Hydrophobic
(B) Hydrophilic
(C) Amphipathic
(D) Hydrated
The cytoskeleton is primarily responsible for __________.
(A) Cell shape
(B) Cellular energy
(C) Cellular respiration
(D) Cell density
The most abundant class of lipids found in the lipid bilayer are the ___________.
(A) Phospholipids
(B) Glycolipids
(C) Sphingolipids
(D) Liposomes
Which of the following are not a cytoskeletal protein filament?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
What is the name of the process by which plant cells convert light energy into biological energy?
(A) Photosynthesis
(B) Photorespiration
(C) Light-conversion
(D) Oxidative phosphorylation
Which of the following is not a property of the lipid bilayer?
(A) Fluidity
(B) Impermeability
(C) Polarity
(D) Permeability
What is the name of the structure around which microtubules grow?
(A) Centers
(B) Matrices
(C) Growth cones
(D) Centrosomes
Membrane proteins that cannot be easily removed from the cell membrane are called __________.
(A) Peripheral proteins
(B) Integral proteins
(C) Rigid proteins
(D) Locked proteins
What organelle is responsible for carrying out photosynthesis?
(A) Mitochondria
(B) Nucleus
(C) Vacuole
(D) Chloroplasts
Which of the cytoskeletal protein filaments has the largest diameter?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
Membrane proteins that span the entire lipid bilayer are called __________.
(A) Membrane-spanners
(B) Integral proteins
(C) Peripheral proteins
(D) Transmembrane proteins
Which cytoskeletal filament is responsible for forming the nuclear lamina?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
What configuration does a protein adopt when it crosses the lipid bilayer?
(A) Alpha-helical
(B) Beta-helical
(C) Gamma-helical
(D) Delta-helical
Molecules done being processed in the ER are often transported to which structure?
(A) Peroxisomes
(B) Mitochondria
(C) Golgi apparatus
(D) Vacuoles
The face through which molecules enter the golgi apparatus is called the __________ face.
(A) Cis
(B) Trans
(C) Medial
(D) Dorsal
Which cytoskeletal protein filaments line the cell membrane?
(A) Actin
(B) Intermediate filaments
(C) Primary filaments
(D) Microtubules
Which of the following is not a common destination of vesicles secreted from the golgi apparatus?
(A) Lysosome
(B) Cell membrane
(C) ER
(D) Mitochondria
Which of the following best describes the properties of a lipid-bound protein?
(A) Having its tail-end attached to the lipid bilayer
(B) Having its head-end attached to the lipid bilayer
(C) Being entirely located in the lipid bilayer
(D) Being loosely associated with the lipid bilayer
Which of the following eukaryotic organelles is primarily responsible for celluar digestion?
(A) Nucleus
(B) Lysosome
(C) Mitochondria
(D) Golgi apparatus
Which of the following is the largest organelle in eukaryotic cells?
(A) ER
(B) Golgi apparatus
(C) Lysosome
(D) Nucleus
Which of the following is not a specialized plant cellular structure?
(A) Lysosome
(B) Cell wall
(C) Chloroplast
(D) Vacuole
What is the name of the organelle that helps mediate endocytosis and exocytosis?
(A) Vacuole
(B) Peroxisome
(C) Lysosome
(D) Endosome
What is the name of the carbohydrate coat found on the outside of the lipid bilayer of higher- order cells cells?
(A) Glycosylation
(B) Glycocalyx
(C) Glycogen
(D) Glutamine
What structure in plants replaces the function that the mitochondria performs in animal cells?
(A) Vacuole
(B) Cytoskeleton
(C) Chloroplast
(D) Nucleus
In eukaryotes, the cell nucleus composes what fraction of the total cell volume?
(A) 10%
(B) 15%
(C) 20%
(D) 25%
What is the name of the process by which molecules naturally flow from an area of higher concentration to one of lower concentration?
(A) Respiration
(B) Transfusion
(C) Dialysis
(D) Diffusion
Which of the following organelles do not contain a double-membrane?
(A) Nucleus
(B) Lysosome
(C) Mitochondria
(D) Chloroplasts
Membrane transport that occurs without the input of extra energy can be classified as what type of transport?
(A) Passive
(B) Active
(C) Catalytic
(D) Inhibitory
The cell nucleus is not found in direct contact with which of the following cellular structures?
(A) Cytosol
(B) Cytoskeleton
(C) ER
(D) Mitochondria
Membrane transport that requires the input of additional enery is called _________.
(A) Passive
(B) Active
(C) Catalytic
(D) Inhibitory
What is the name of the inner-most space in a mitochondrial cell?
(A) Matrix
(B) Intermembrane space
(C) Cytosol
(D) Chlorophyll
Membrane proteins that mediate active transport are generally __________.
(A) Lipid-bound proteins
(B) Ionophores
(C) Channel proteins
(D) Carrier proteins
What is the main function of the mitochondria?
(A) Molecular digestion
(B) DNA storage
(C) Space-filler
(D) Enery-producer
The mitochondrial matrix is analogous to which structure in plant chloroplasts?
(A) Stoma
(B) Intermembrane space
(C) Cytosol
(D) Vacuole
Membrane proteins that mediate passive transport are generally _________.
(A) Lipid-bound proteins
(B) Ionophores
(C) Channel proteins
(D) Carrier proteins
Which eukaryotic organelle contains oxidizing enzymes?
(A) Lysosome
(B) Peroxisome
(C) Golgi apparatus
(D) ER
What is a typical thickness for a cell wall?
(A) 50 nanometers
(B) 50 micrometers
(C) 10 nanometers
(D) 10 micrometers
__________ are a class of membrane proteins that increase a cell's permeability to certain ions.
(A) Lipid-bound
(B) Ionophores
(C) Channel proteins
(D) Carrier proteins
__________ endoplasmic reticulum has ribosomes attached to its outer membrane.
(A) Smooth
(B) Spiked
(C) Rough
(D) Cytoskeletal
Vacuoles can occupy up to what percent of plant cells?
(A) 90%
(B) 75%
(C) 50%
(D) 25%
Membrane transport is important for which of the following biological processes?
(A) Protein synthesis
(B) Cell communication
(C) Maintenance of cellular pH
(D) All of the above
The ER plays a major role in the processing of which of the following biological molecules?
(A) Carbohydrates
(B) Proteins
(C) Adenosine triphosphate
(D) DNA
Which of the following is not a normal function of plant cell vacuoles?
(A) Cellular digestion
(B) Space-filling
(C) Energy production
(D) Storage
Cell Respiration: Introduction
Terms
Acetyl Coenzyme A - A small molecule that carries acetyl functional groups in cells. Composed of an acetyl group attached to a coenzyme A molecule. The starting product of the citric acid cycle.
Adenosine Triphosphate (ATP) - The molecule from which cells derive energy. Comprised of an adenosine molecule bonded to three phosphates, each phosphate bond contains energy, especially the third bond. By breaking that one bond and reducing ATP to adenosine diphosphate (ADP), the cell can get the energy to carry out its various processes.
Aerobic respiration - A metabolic process involving oxygen in the breakdown of glucose.
Anabolic - Term describing enzyme-catalyzed reactions in a cell that involves the synthesis of complex molecules out of simpler subunits and which uses energy.
Anaerobic respiration - A metabolic process that does not involve oxygen in the breakdown of glucose.
Carbohydrate - A molecular compound containing carbon, hydrogen, and oxygen. Subunits are sugars.
Catabolic - Term describing enzyme-catalyzed reactions in a cell that involve the degradation of molecules into more simple subunits with the release of energy.
Chemotroph - An organism that derives its energy from the ingestion of food molecules.
Citric acid cycle - Also known as the Krebs Cycle; a metabolic pathway found in aerobic organisms that oxidizes acetyl coA groups to carbon dioxide and water.
Coenzyme - A molecule that participates in an enzyme-catalyzed reaction and functions to transfer atoms or electrons between itself and various molecules.
Elimination reaction - A reaction that involves the ejection of a specific group from a molecule, often resulting in the formation of a carbon-carbon double bond.
Glycolysis - A metabolic pathway occurring in the cell *cytosol that during a series of reactions converts glucose to pyruvate and synthesizes ATP**.
Isomerization - A reaction that does not change the atomic make-up of a molecule, but rather changes its geometric conformation, yielding a slightly different molecule.
Metabolism - All the reactions occurring in an organism that participate in the acquisition or conversion of energy for use in the organism.
Nicotinamide adenine dinucleotide - A coenzyme that participates in oxidation and reduction reactions. An important electron carrier in oxidative phosphorylation.
Oxidation - A reaction that involves the overall loss of electrons from a specific molecule or atom. Can occur with the addition of an oxygen or by the removal of a hydrogen.
Oxidative phosphorylation - A process occurring in the mitochondria that results in the formation of ATP from the flow of electrons to oxygen.
Photosynthesis - A process in which plants convert sunlight into energy sources that can be used inside the cell to sustain life.
Phototroph - Organisms that obtain energy from sun light through photosynthesis.
Protein - An essential molecule found in all cells. Composed of amino acid subunits.
Reduction - A reaction that results in the overall gain of electrons to a specific molecule or atom. Can occur with the addition of a hydrogen atom or by the removal of an oxygen atom.
Respiration - A process that occurs in cells in which cells breakdown food molecules to yield ATP. Can be either aerobic or anaerobic.
Cellular Energy Sources
The goal of cellular respiration and metabolism in animals and plants is, ultimately, the conversion of one type of energy source to another. Presumably, the original energy source comes in a form that cannot be immediately used to support cellular activities. For humans, our external energy sources are the foods we eat. Once we ingest and digest the food, our cells metabolic processes convert the energy contained within the food into a form of energy that can function in our cells. These constant conversions are what allow us to perform our day-to-day activities.
Since energy is the ultimate goal of metabolism, it will be helpful to understand what these various external and internal energy sources really are. As we have mentioned, food is the external energy source for humans. Different foods are composed primarily of one of the following three macromolecules: carbohydrates (breads and pastas), lipids (fats and oils), or proteins (meats and beans). During digestion of food, when the food is first broken down internally, these large molecules are broken into subunits. Depending on their type, subunits can be metabolized in different ways and then used as internal energy sources.
The distinct means of metabolizing specific subunits all have the same goal, the production of the primary cellular energy source: adenosine triphosphate.
Figure %: Chemical structure of ATP
As you can see in the figure above, ATP contains three phosphate groups. These groups are primarily responsible for ATP's role as an energy source. During metabolic reactions, these phosphate groups can be transferred from ATP to yield either adenosine diphosphate (ADP) or adenosine monophosphate (AMP).
ATP -> ADP + P + energy, or
ATP -> AMP + 2P + energy
ATP -> AMP + 2P + energy
The release of one or more phosphate groups is energetically favorable: the reaction produces energy. ATP can also undergo a reaction with water to yield ADP or AMP to release energy. The cell can use the energy produced from the breakdown of ATP for whatever purpose is necessary. Often, the energetically favorable breakdown of ATP is often coupled with another, energetically unfavorable reaction that is designed to drive the first reaction forward through the synthesis of additional ATP.
ATP synthesis is almost exactly opposite to the process by which ATP is broken down to produce energy: phosphate groups are brought in contact with either ADP or AMP. While this process is not as favorable, it is able to occur with the energy derived from metabolizing foods. In addition to ATP, there are a number of other reactive molecules that are involved in the production of cellular energy. These are called coenzymes and their role is to help transfer other chemical groups like hydrogens. Coenzymes work in conjunction with metabolic enzymes to drive metabolic reactions. Among these are nicotinamide adenine dinucleotide (NADH) and acetyl coenzyme A. We will discuss the specific roles of both these molecules more in following sections.
Metabolism
Basics of Metabolism
Metabolism is a process of energy acquisition and conversion. It is necessary because organisms are constantly undergoing cellular changes--they are not in a state of equilibrium. Metabolism is an attempt to regulate cellular conditions by making internal changes to maintain a steady cellular state. As a general rule, nature's tendency is towards conditions of disorder. This means that disorderly conditions are energetically favorable--they release energy. Highly ordered and organized conditions are not energetically favorable and require energy to occur. As a result, the thousands of reactions that constantly occur inside us to maintain cellular organization need energy. The body produces this needed energy by breaking down ATP, and then using this energy to promote energetically unfavorable, but biologically necessary reactions.Among chemotrophs, there are two major categories of metabolic pathways. The distinction between the two is that one involves degradation reactions while the other involves synthesis reactions. Catabolic pathways involve the breakdown of ingested food molecules. Anabolic pathways involve the synthesis of essential biomolecules. Along each of these pathways, a number of enzymes work in combination to help drive the reactions. The catabolic pathways are involved in breaking down carbohydrates and proteins into their polysaccharide, or sugar, and amino acid subunits. These reactions release energy needed by the cell (this is why food, the source of carbohydrates and proteins, is essential for survival). Anabolic pathways take the simple products of catabolic degradation--ATP, for example--and use energy from their degradation to synthesize complex biomolecules.
As we have mentioned, the breakdown of ATP is an energetically favorable reaction. This is true because it involves splitting one larger, more organized molecule into two smaller ones. The energy that is released in this process can be used to drive other, less favorable reactions forward. In this way, ATP acts as a major energy source for cells.
As one can imagine, there are many different anabolic and catabolic reactions going on at any second in our bodies. As a result, metabolic pathways must be highly regulated as to ensure that the proper enzymes for synthesis and degradation are active at the appropriate times. Some of this regulation is made possible by different metabolic processes occurring in distinct parts of the cell.
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