AP Biology 3.5 Cellular Respiration Study Notes - New Syllabus Effective 2025
AP Biology 3.5 Cellular Respiration Study Notes- New syllabus
AP Biology 3.5 Cellular Respiration Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the processes and structural features of mitochondria that allow organisms to use energy stored in biological macromolecules.
Key Concepts:
- Cellular Respiration
3.5.A.1 – Cellular Respiration
Cellular respiration is a set of metabolic reactions occurring inside the cells to convert biochemical energy obtained from the food into a chemical compound called adenosine triphosphate (ATP). Metabolism refers to a set of chemical reactions carried out for maintaining the living state of the cells in an organism. These can be divided into two categories:
Catabolism – the process of breaking molecules to obtain energy.
Anabolism – the process of synthesizing all compounds required by the cells.
Therefore, respiration is a catabolic process, which breaks large molecules into smaller ones, releasing energy to fuel cellular activities.
3.5.A.2 – Aerobic Respiration
It is the process of cellular respiration that takes place in the presence of oxygen gas to produce energy from food. This type of respiration is common in most of the plants and animals, birds, humans, and other mammals. In this process, water and carbon dioxide are produced as end products.
Glucose (C6H12O6) + Oxygen 6(O2) → Carbon-dioxide 6(CO2) + Water 6 (H2O) + Energy (ATP)
According to the above-given chemical equation, energy is released by splitting the glucose molecules with the help of oxygen gas. At the end of the chemical reaction, energy, water molecules, and carbon dioxide gas are released as the by-products or end products of the reactions.
The 2900 kJ of energy is released during the process of breaking the glucose molecule and in turn, this energy is used to produce ATP – Adenosine Triphosphate molecules which are used by the system for various purposes.
Aerobic respiration process takes place in all multicellular organisms including animals, plants and other living organisms.
During the respiration process in plants, the oxygen gas enters the plant cells through the stomata, which is found in the epidermis of leaves and stem of a plant. With the help of the photosynthesis process, all green plants synthesize their food and thus releases energy.
3.5.A.3 – Electron Transport Chain
The ETC transfers electrons in a series of oxidation-reduction reactions that establish an electrochemical gradient across membranes.
i. In cellular respiration, electrons delivered by and FADH are passed to a series of electron acceptors as they move toward the terminal electron acceptor, oxygen. Aerobic prokaryotes use oxygen as a terminal electron acceptor, while anaerobic prokaryotes use other molecules.
ii. The transfer of electrons, through the ETC, is accompanied by the formation of a proton gradient across the inner mitochondrial membrane, with the membrane(s) separating a region of high proton concentration outside the membrane from a region of low proton concentration inside the membrane. The folding of the inner membrane increases the surface area, which allows for more ATP to be synthesized. In prokaryotes, the passage of electrons is accompanied by the movement of protons across the plasma membrane.
iii. The flow of protons back through membrane-bound ATP synthase by chemiosmosis drives the formation of ATP from ADP and inorganic phosphate. This is known as oxidative phosphorylation in aerobic cellular respiration.
iv. In aerobic cellular respiration, decoupling oxidative phosphorylation from electron transport generates heat. This heat can be used by endothermic organisms to regulate body temperature.
3.5.B.1 – Glycolysis
Glycolysis is the primary step of cellular respiration, which occurs in all organisms. Glycolysis is followed by the Krebs cycle during aerobic respiration. In the absence of oxygen, the cells make small amounts of ATP as glycolysis is followed by fermentation.
This metabolic pathway was discovered by three German biochemists- Gustav Embden, Otto Meyerhof, and Jakub Karol Parnas in the early 19th century and is known as the EMP pathway (Embden–Meyerhof–Parnas).
There are three main steps in glycolysis: substrate level phosphorylation, conversion of glucose-phosphate to fructose-phosphate, and the formation of two molecules of phosphate. In addition, there are several intermediate steps that occur between these three main steps such as the conversion of fructose-bisphosphate to glyceraldehyde-phosphate and the conversion of dihydroxyacetone phosphate to glycerol-phosphate.
3.5.B.2 – Pyruvate
In order for pyruvate, the product of glycolysis, to enter the next pathway, it must undergo several changes. The conversion is a three-step process.
Step 1. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium. The result of this step is a two-carbon hydroxyethyl group bound to the enzyme (pyruvate dehydrogenase). This is the first of the six carbons from the original glucose molecule to be removed. This step proceeds twice (remember: there are two pyruvate molecules produced at the end of glycolysis) for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps.
Step 2. The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked up by NAD+, forming NADH. The high-energy electrons from NADH will be used later to generate ATP.
Step 3. The enzyme-bound acetyl group is transferred to CoA, producing a molecule of acetyl CoA.
3.5.B.3 – Krebs Cycle
The Krebs cycle or TCA cycle (tricarboxylic acid cycle) or Citric acid cycle is a series of enzyme catalysed reactions occurring in the mitochondrial matrix, where acetyl-CoA is oxidised to form carbon dioxide and coenzymes are reduced, which generate ATP in the electron transport chain.
Krebs cycle was named after Hans Krebs, who postulated the detailed cycle. He was awarded the Nobel prize in 1953 for his contribution.
It is a series of eight-step processes, where the acetyl group of acetyl-CoA is oxidised to form two molecules of CO2 and in the process, one ATP is produced. Reduced high energy compounds, NADH and FADH2 are also produced.
Two molecules of acetyl-CoA are produced from each glucose molecule so two turns of the Krebs cycle are required which yields four CO2, six NADH, two FADH2 and two ATPs.
Steps involved in this process: –
3.5.B.4 – Transfer of electron by NADH and FADH2
Although the citric acid cycle is considered to be part of aerobic metabolism, it does not itself use the oxygen. Only in the final catabolic reactions that take place on the inner mitochondrial membrane is molecular oxygen (O2) directly consumed. Nearly all the energy available from burning carbohydrates, fats, and other foodstuffs in the earlier stages of their oxidation is initially saved in the form of high-energy electrons removed from substrates by NAD+ and FAD. These electrons, carried by NADH and FADH2, are then combined with O2 by means of the respiratory chain embedded in the inner mitochondrial membrane. The large amount of energy released is harnessed by the inner membrane to drive the conversion of ADP + Pi to ATP. For this reason, the term oxidative phosphorylation is used to describe this last series of reactions.
Although the mechanism by which energy is harvested by the respiratory chain differs from that in other catabolic reactions, the principle is the same. The energetically favorable reaction H2 + ½O2 → H2O is made to occur in many small steps, so that most of the energy released can be stored instead of being lost to the environment as heat. The hydrogen atoms are first separated into protons and electrons. The electrons pass through a series of electron carriers in the inner mitochondrial membrane. At several steps along the way, protons and electrons are transiently recombined. But only when the electrons reach the end of the electron-transport chain are the protons returned permanently, when they are used to neutralize the negative charges created by the final addition of the electrons to the oxygen molecule.
3.5.B.5 – Oxidative Phosphorylation
The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation. The key steps of this process, shown in simplified form in the diagram above, include:
- Delivery of electrons by NADH and FADH2. Reduced electron carriers (NADH and FADH2) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. In the process, they turn back into NAD+ and FAD, which can be reused in other steps of cellular respiration.
- Electron transfer and proton pumping. As electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump H+ ions, moving them out of the matrix and into the intermembrane space. This pumping establishes an electrochemical gradient.
- Splitting of oxygen to form water. At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up H+ to form water.
- Gradient-driven synthesis of ATP. As H+ ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP.
The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron transport chain components are found in the plasma membrane.
As the electrons travel through the chain, they go from a higher to a lower energy level, moving from less electron-hungry to more electron-hungry molecules. Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient.
3.5.B.6 – Fermentation
Fermentation occurs in yeast cells and bacteria and also in the muscles of animals. It is an anaerobic pathway in which glucose is broken down.
The respiration that happens at the minute level in our body, viz., in the cell is called the cellular respiration. It occurs in the presence or absence of oxygen. Any type of cellular respiration begins with glycolysis where a 3-C molecule, pyruvic acid is formed as the end product. Different cells handle this pyruvate in two major ways, fermentation is one of them.
Types of Fermentation
There are three different types of fermentation:
Lactic Acid Fermentation – In this, starch or sugar is converted into lactic acid by yeast strains and bacteria. During exercise, energy expenditure is faster than the oxygen supplied to the muscle cells. This results in the formation of lactic acid and painful muscles.
Alcohol Fermentation – Pyruvate, the end product of glycolysis is broken down into alcohol and carbon dioxide. Wine and beer are produced by alcoholic fermentation.
Acetic Acid Fermentation – Starch and sugar present in grains and fruits ferment into vinegar and condiments. E.g. apple cider vinegar.