HESI A2 · Biology · 6 min read
Cellular Respiration and Photosynthesis
- Trace the pathways of aerobic cellular respiration, identifying the key inputs, outputs, and cellular locations of glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.
- Compare the mechanisms and yields of aerobic respiration with lactic acid and alcohol fermentation under anaerobic conditions.
- Describe the structural components of chloroplasts and explain how the light-dependent reactions and the Calvin cycle convert solar energy into chemical energy.
Glycolysis and the Transition into the Citric Acid Cycle
Cellular respiration is a series of metabolic pathways that break down organic molecules, such as glucose, to produce ATP. When oxygen is present, this process is summarized by the overall equation: glucose + 6 oxygen → 6 carbon dioxide + 6 water, with energy released for ATP production. To manage this energy conversion efficiently, the cell divides aerobic cellular respiration into three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. You can think of glucose as an unopened fuel canister; cellular respiration carefully releases its stored energy in increments rather than igniting it all at once.
The first stage, glycolysis, takes place in the cytoplasm of the cell. Glycolysis is an anaerobic process because it does not directly require oxygen. During its initial energy-investment phase, the cell consumes two molecules of ATP to prime the pathway. Glycolysis then breaks down one molecule of glucose into two molecules of pyruvate. In doing so, it produces a net gain of two ATP molecules via substrate-level phosphorylation and reduces two NAD+ molecules to two NADH molecules.
In eukaryotic cells, the transition to the next stage requires that pyruvate enters the mitochondrial matrix to undergo pyruvate oxidation. During pyruvate oxidation, each pyruvate is converted into a two-carbon acetyl group attached to coenzyme A, forming acetyl CoA. This transition step releases one molecule of carbon dioxide per pyruvate and produces one molecule of NADH per pyruvate.
Next, acetyl CoA enters the citric acid cycle, which is also known as the Krebs cycle and occurs within the mitochondrial matrix. At the start of the citric acid cycle, acetyl CoA combines with oxaloacetate to form citrate. Because each glucose molecule splits into two pyruvates during glycolysis, two turns of the citric acid cycle are required to process the products of one glucose molecule. Per turn, the cycle yields one ATP (or GTP), three NADH, and one FADH2, while generating two molecules of carbon dioxide per acetyl CoA. Substrate-level phosphorylation occurs in both glycolysis and the citric acid cycle to generate these early ATP molecules.
Common mistakes
- Thinking that fermentation directly synthesizes ATP, when its actual biochemical function is only to regenerate NAD+ from NADH so that glycolysis can continue.
- Confusing the cellular locations within mitochondria and chloroplasts, such as placing the Calvin cycle in the thylakoid membrane instead of the stroma, or placing the citric acid cycle in the intermembrane space instead of the mitochondrial matrix.
- Believing that the oxygen released during photosynthesis originates from carbon dioxide, rather than from the photolysis of water during the light-dependent reactions.
- Assuming that human skeletal muscle cells produce carbon dioxide during lactic acid fermentation, when lactic acid fermentation actually reduces pyruvate directly to lactate without releasing carbon dioxide.
Key points
- Aerobic cellular respiration converts glucose and oxygen into carbon dioxide, water, and ATP across three main stages: glycolysis, the citric acid cycle, and oxidative phosphorylation.
- Glycolysis occurs in the cytoplasm without oxygen, consuming two ATP to yield two pyruvate, two NADH, and a net gain of two ATP via substrate-level phosphorylation.
- In the mitochondrial matrix, pyruvate oxidation converts pyruvate to acetyl CoA, which enters the citric acid cycle to generate carbon dioxide, ATP (or GTP), NADH, and FADH2.
- Oxidative phosphorylation along the inner mitochondrial membrane pumps protons to establish an electrochemical gradient; chemiosmosis via ATP synthase then produces the vast majority of cellular ATP, with oxygen acting as the final electron acceptor to form water.
- When oxygen is absent, fermentation regenerates NAD+ from NADH so glycolysis can continue producing two net ATP per glucose; lactic acid fermentation yields lactate without carbon dioxide, while alcohol fermentation yields ethanol and carbon dioxide.
- Photosynthesis takes place in chloroplasts, where light-dependent reactions across thylakoid membranes split water by photolysis, releasing oxygen and generating ATP and NADPH.
- In the chloroplast stroma, the Calvin cycle uses RuBisCO for carbon fixation, consuming ATP and NADPH to produce glyceraldehyde-3-phosphate (G3P), which is used to synthesize glucose.
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