Cellular Energy: Metabolism, ATP, Enzymes
I. All cells must have a supply of energy to maintain their organization.
A. Work must be done to maintain the organization of a cell.
B. A cell must use energy to form macromolecules from subunit molecules and to arrange them into the cell’s structural components.
II. Energy is the capacity to do work, to bring about a change.
A. There are several forms of energy.
1. mechanical energy
2. light energy
3. electrical energy
4. heat energy
5. Chemical bond energy is stored in the bonds that hold atoms together.
6. One kind of energy can be transformed into another kind of energy.
B. The first energy law says that energy can be neither created nor destroyed, but it can change from one form to another.
1. It often seems as if energy is being used up, as in gasoline burning or food being metabolized.
2. Organisms and machines are not isolated but exist as part of a system with their surroundings.
3. The energy needed to run an organism or a machine comes from the surroundings, and when energy is used to do work, some of it is lost as heat that is accepted by the surroundings.
4. Energy that is put into a system is converted into both work and heat, resulting in the same amount of energy that leaves the system.
C. The second energy law says that when energy is changed from one form to another, some useful energy is always lost as heat, so that energy cannot be recycled.
1. While the total amount of energy is unchanged, the energy lost as heat is no longer useful in doing work.
2. This implies that only processes that decrease the amount of useful energy occur naturally or spontaneously.
3. The amount of disorder or entropy is always increasing in the universe.
4. To maintain organization of a cell or other systems, there must be a continual input of energy into the system from its surroundings.
5. Living cells convert energy to usable forms and heat and store energy in the form of chemical compounds.
D. Life does not violate the laws of energy.
1. A living thing represents a temporary storage area for useful energy.
2. Energy flows through a community of organisms.
a) As energy transformations occur, useful energy is lost to the environment as heat until all the energy is converted to heat.
b) Energy cannot be recycled, so there must be an ultimate source of energy that can continually supply all living things with energy, which is the sun.
c) Photosynthesis captures less than 2% of the solar energy that reaches the earth from the sun, but this energy makes the organic food that supports all types of living organisms on earth.
III. Cellular metabolism is the term for the array of chemical reactions that occurs within cells.
A. Nutrient molecules are broken down through oxidation, a degradative process that releases energy.
1. The chemical reactions occur in a sequence, with each product acting as the reactant or substrate of the next reaction.
2. Each reaction takes place through the action of its own enzyme.
3. The intermediate molecules in a reaction pathway can also be used as the starting points for other pathways, linking degradative and synthetic reactions.
B. Enzymes are organic catalysts, globular proteins that speed up chemical reactions.
1. Each enzyme is very specific in its action and works in only one particular reaction or group of related reactions.
2. Enzymes are usually named by adding the ending -ase to the name of the substrate molecule or the action performed by the enzyme.
3. No reaction can occur in a cell unless its own enzyme is present and active.
4. The energy of activation is the energy that must be supplied to cause molecules to react with one another.
a) The energy of activation required when an enzyme is present is considerably less than that required when the enzyme is not available.
b) Energy of activation is the difference between the nonreactant energy level and the reactive energy level, written as Ea.
c) Enzymes lower the energy of activation of a reaction by binding with the substrate(s) in such a way that a reaction can occur.
5. The enzyme-substrate complex forms when an enzyme’s active site forms a connection with the substrate(s).
a) This interaction is rather like a key fitting into a lock.
b) In this interaction the active site undergoes a slight change in its shape, allowing it to accommodate the substrate(s) more perfectly.
(1) This is called the induced fit model of complex formation.
(2) The change in the active site facilitates the reaction that occurs next.
(3) After the reaction occurs, the product(s) is/are released.
(4) The active site returns to its original shape.
c) Only a small amount of enzyme is needed because the molecules can be used repeatedly.
6. Some enzymes actually participate in the reaction they catalyze, as in the digestion of proteins by trypsin.
7. Sometimes particular reactants can produce more than one kind of product, as determined by the presence or absence of an enzyme that catalyzes one of the reactions.
C. Several conditions affect enzymatic reactions.
1. An enzyme’s activity generally increases with greater substrate concentration.
a) There are more collisions between substrate molecules and the enzyme.
b) If the concentration of substrate is so great that the enzyme’s active site is almost continuously filled with substrate, the maximum velocity of the enzyme has been reached.
2. Higher temperature generally results in an increase in enzyme activity, as molecules move more rapidly.
a) If the temperature rises above a certain point, enzyme activity levels out and then declines rapidly.
b) Enzymes are proteins and are denatured at high temperatures as the shape changes so that the active site cannot bind the substrate efficiently.
3. A change in pH can affect enzyme activity.
a) Each enzyme has an optimal pH that helps maintain its normal configuration.
b) A change in pH can alter the ionization of the R groups in the amino acids of the protein, disrupting the normal interactions and structure of the enzyme, so that it is inefficient in binding substrate.
D. Regulation of enzyme activity by the cell involves enzyme concentration and activity.
1. Cells regulate enzyme concentration through control of protein synthesis and can also regulate the actions of enzyme molecules already present in the cell.
2. Inhibition can be competitive or noncompetitive, reversible or irreversible.
a) In competitive inhibition, another molecule competes with the substrate molecule to occupy the active site of the enzyme.
b) In noncompetitive inhibition, the inhibitor binds to the allosteric site of an enzyme, a site different from the active site, so that the 3-dimensional structure of the enzyme is changed to an inactive form.
c) Inhibition in cells is usually reversible.
d) Irreversible inhibition is usually the result of a poison, such as penicillin’s inhibition of an enzyme needed by bacteria to build the cell wall.
3. Feedback inhibition is the method by which most cells regulate nearly every enzyme.
a) Negative feedback, as used by a cell, is shown by the action of a thermostat in regulating temperature; as an enzyme’s product accumulates, it turns off the action of the enzyme so that less product is constructed.
b) This keeps the concentration of the product within a certain range.
c) Most enzymatic pathways are also regulated by feedback inhibition so that the end product of the pathway binds to the first enzyme in the pathway and shuts down the entire sequence of reactions.
E. Coenzymes are organic cofactors required by some enzymes for enzymatic function to occur.
1. Some enzymes require nonprotein cofactors that may include ions such as magnesium, potassium, or calcium.
2. Some enzymes require coenzymes as carriers for chemical groups or electrons.
3. Coenzymes usually participate directly in the reaction.
4. Vitamins are relatively small organic molecules required in trace amounts in the diet, usually acting as a coenzyme or the precursor to a coenzyme.
5. NAD+ (nicotinamide adenine dinucleotide) is a coenzyme that carries electrons.
a) It works with dehydrogenases to remove a pair of hydrogen atoms from the substrate.
b) This is a degradative (oxidation) reaction that releases energy, transferring part of it to NAD+ in the form of high-energy electrons.
6. FAD (flavin adenine dinucleotide) is a coenzyme that also carries high-energy electrons, accepting 2 hydrogen ions to become FADH2.
7. Both NAD+ and FAD are involved in cellular respiration.
a) At various steps in this metabolic pathway, these coenzymes accept electrons from substrates.
b) NAD+ and FAD transfer electrons to an electron transport system of membrane-bounded carriers that pass electrons from one carrier to another.
c) As electrons are transferred, oxidation occurs and energy is released that is used to produce ATP molecules.
8. NADP + is similar to NAD+ but contains a phosphate group.
a) This coenzyme is used in the electron transport system of chloroplasts to produce ATP.
b) During photosynthesis, carbon dioxide is reduced to a carbohydrate using electrons supplied by NADPH and energy supplied by ATP.
F. Metabolism is possible in plants because ATP carries energy and NADPH carries electrons between degradative and synthetic pathways.
1. Organisms are able to store energy briefly in their cells.
2. Only photosynthesizers can make organic food by utilizing solar energy.
3. ATP and NADPH carry solar energy to make the macromolecules produced by plants that serve as the source of organic food for all organisms.
G. Organic food molecules are used as an energy source or as a source of unit molecules to construct macromolecules.
IV. ATP (adenosine triphosphate) is the common energy currency of cells, broken apart when a cell needs energy.
A. A large supply of ATP is produced and used continuously.
1. About 8kg (17 lb) of ATP is produced every hour in an average male.
2. The total amount present in the body at any one time is only about 50g (1.8 oz).
3. The cell’s entire amount of ATP is recycled about once a minute from ADP (adenosine diphosphate) and phosphate groups.
B. ATP is a nucleotide composed of adenine and the sugar ribose (together called adenosine) and 3 phosphate groups.
1. ATP is a “high-energy” compound that releases a large amount of energy when 1 or 2 of the phosphates are removed from the molecule.
2. About 7.3 Kcal per mole are released from ATP in the cell when the high-energy bond is broken.
3. ATP is advantageous as an energy carrier for several reasons.
a) The energy released can be captured in a step-by-step manner that prevents energy waste.
b) It provides a common energy currency that can be used in many different kinds of reactions.
c) When the phosphate bond is broken, enough energy is released to run certain cell reactions, with little energy wasted.
4. Most reactions in the cell, such as ATP breakdown and synthesis of other structures, are coupled reactions, usually using the same enzyme to allow both reactions to occur at the same time and in the same place.
C. There are at least 3 functions of ATP.
1. It supplies energy needed for the chemical work of synthesizing macromolecules.
2. It supplies energy for transport work of pumping materials across the plasma membrane.
3. It supplies energy needed for the mechanical work of muscle contraction, etc.
4. All prokaryotes and eukaryotes use ATP, illustrating the chemical unity of all living things.
D. The formation of ATP can take place in 2 ways.
1. Substrate-level phosphorylation occurs in the cytosol.
a) A high-energy substrate transfers a phosphate group to ADP to form ATP.
b) This is not the primary way of making ATP.
2. Most ATP is made in the mitochondria through the process of chemiosmotic phosphorylation, using the electron transport system.
1. Carriers of the electron transport systems are located in membranes of these organelles.
2. According to the chemiosmotic theory, hydrogen ions tend to collect on one side of the membrane as they are pumped there by certain electron carriers, establishing an electrochemical gradient.
3. ATP synthetase complexes cross the membrane and provide channels through which the hydrogen ions move down their concentration gradient.
4. This flow of hydrogen ions into the mitochondrial matrix provides the energy for the ATP synthetase to produce ATP from ADP and phosphate.
5. This combination of chemical and osmotic events is called chemiosmotic phosphorylation, since it produces ATP.