Cellular Respiration and Enzyme Activity: Key Concepts
Cellular Respiration and Enzyme Activity
Cellular respiration, the breakdown of glucose in the presence of oxygen, is an example of a pathway of catabolism.
The synthesis of protein from amino acids is an example of anabolism.
A Cell Does Three Main Kinds of Work:
- Chemical
- Transport
- Mechanical
Energy coupling is the use of an exergonic process to drive an endergonic one.
ATP is composed of ribose (a sugar), adenine (a nitrogenous base), and three phosphate groups.
The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis.
Energy is released from ATP when the terminal phosphate bond is broken.
This release of energy comes from the chemical change to a state of lower free energy, not from the phosphate bonds themselves.
An enzyme is a catalytic protein.
Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction.
The reactant that an enzyme acts on is called the enzyme’s substrate.
The enzyme binds to its substrate, forming an enzyme-substrate complex.
The active site is the region on the enzyme where the substrate binds.
Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction.
The Active Site Can Lower an EA Barrier by:
- Orienting substrates correctly
- Straining substrate bonds
- Providing a favorable microenvironment
- Covalently bonding to the substrate
Cofactors are nonprotein enzyme helpers.
Cofactors may be inorganic (such as a metal in ionic form) or organic.
An organic cofactor is called a coenzyme.
Coenzymes include vitamins.
Competitive inhibitors bind to the active site of an enzyme, competing with the substrate.
Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective.
Allosteric regulation may either inhibit or stimulate an enzyme’s activity.
Allosteric regulation occurs when a regulatory molecule binds to a protein at one site and affects the protein’s function at another site.
Cooperativity is a form of allosteric regulation that can amplify enzyme activity.
In feedback inhibition, the end product of a metabolic pathway shuts down the pathway.
Metabolic Pathways
Fermentation is a partial degradation of sugars that occurs without O2.
Aerobic respiration consumes organic molecules and O2 and yields ATP.
Anaerobic respiration is similar to aerobic respiration but consumes compounds other than O2.
Cellular respiration includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration.
C6H12O6 + 6 O2 ® 6 CO2 + 6 H2O + Energy (ATP + heat)
Stages of Cellular Respiration
- Glycolysis (breaks down glucose into two molecules of pyruvate)
- The citric acid cycle (completes the breakdown of glucose)
- Oxidative phosphorylation (accounts for most of the ATP synthesis)
Glycolysis
- Occurs in the cytosol
- Does not require oxygen
- 1 glucose produces 2 Acetyl CoA
Citric Acid Cycle
- Pyruvate moved to mitochondria (by active transport)
- Acetyl CoA enters cycle
- Joins 4 carbon molecules
- Redox reactions occur and release CO2
- 4 carbon molecule regenerated
- Occurs twice
- 1 ATP, 1 FADH2, 3 NADH
Electron Transport
Inner membrane (cristae) of the mitochondrion.
Types of Fermentation
Two common types are alcohol fermentation and lactic acid fermentation.
Yeast and many bacteria are facultative anaerobes, meaning that they can survive using either fermentation or cellular respiration.
Obligate anaerobes carry out fermentation or anaerobic respiration and cannot survive in the presence of O2.
Photosynthesis
A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes.
The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center.
The Calvin Cycle Has Three Phases:
- Carbon fixation (catalyzed by rubisco)
- Reduction
- Regeneration of the CO2 acceptor (RuBP)