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Biology Unit 3: Cellular energetics

Cells capture and release energy through enzyme-driven pathways. Photosynthesis stores energy in sugar, and respiration releases it as ATP.

Covers: Enzymes and activation energy, ATP, photosynthesis, cellular respiration, fermentation

Study guide

Cells capture and use energy through chemical reactions controlled by enzymes. Enzymes are proteins (catalysts) that speed reactions by lowering activation energy. A substrate binds the enzyme's active site, which has a specific shape. Temperature and pH affect enzyme activity, and extreme conditions denature the enzyme by changing its shape. Competitive inhibitors block the active site, and noncompetitive inhibitors bind elsewhere and change the enzyme's shape.

ATP is the cell's energy currency. Breaking its last phosphate bond releases energy that powers cellular work, and ATP is regenerated from ADP and phosphate.

Photosynthesis happens in chloroplasts: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂. The light-dependent reactions in the thylakoid membranes use light energy captured by chlorophyll to split water (releasing O₂) and make ATP and NADPH. The Calvin cycle in the stroma uses that ATP and NADPH to fix CO₂ into sugar.

Cellular respiration releases energy from glucose: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. Glycolysis in the cytoplasm splits glucose into two pyruvates, making a small amount of ATP and NADH. In mitochondria, the Krebs (citric acid) cycle releases CO₂ and makes more electron carriers. The electron transport chain uses those electrons to pump protons across the inner membrane, and ATP synthase uses the proton gradient to make most of the ATP (oxidative phosphorylation). Oxygen is the final electron acceptor, forming water.

Without oxygen, cells use fermentation, which regenerates NAD⁺ so glycolysis can keep making a little ATP. Animals produce lactic acid, and yeast produce ethanol and CO₂.

In feedback inhibition, the final product of a metabolic pathway binds to an earlier enzyme, usually at an allosteric site away from the active site, and shuts it down. This keeps the cell from making more product than it needs and from wasting resources. Aerobic respiration makes roughly 30–32 ATP per glucose, while glycolysis followed by fermentation makes only 2. If the electron transport chain is blocked (cyanide stops the final step that passes electrons to O₂), proton pumping and ATP synthesis stop and the cell has to rely on fermentation. If ATP synthase itself is blocked, protons can't flow back, so the gradient builds up while ATP production falls.

The rate of photosynthesis rises with light intensity and CO₂ concentration until another factor becomes limiting, and like any enzyme-driven process it has an optimal temperature. It can be measured by the oxygen released or the CO₂ taken in. Chlorophyll absorbs red and blue light best and reflects green. Plotted against temperature or pH, an enzyme's activity peaks at its optimum and drops on either side. Above the optimum the enzyme denatures, and at low temperature its molecules just move more slowly. Many enzymes also need cofactors (such as metal ions) or coenzymes (such as vitamins) to work.

Key ideas

• Enzymes lower activation energy; temperature, pH and inhibitors affect them.
• Photosynthesis: light reactions make ATP and NADPH; the Calvin cycle uses them to fix CO₂ into sugar.
• Cellular respiration: glycolysis → Krebs cycle → electron transport chain, which makes most ATP.
• Oxygen is the final electron acceptor in aerobic respiration.
• Without oxygen, fermentation regenerates NAD⁺ so glycolysis can continue.

Formulas and rules

Common mistakes

• Thinking enzymes are used up. They're reused.
• Saying plants don't do cellular respiration. They do.

Words to know

Activation energy

The energy needed to start a reaction; enzymes lower it.

Final electron acceptor

Oxygen, in aerobic respiration.

Light reactions

The first stage of photosynthesis; make ATP, NADPH and O₂.

Glycolysis

Splitting glucose into pyruvate in the cytoplasm, making a little ATP.

Fermentation

Anaerobic process that regenerates NAD⁺ so glycolysis can keep making ATP.

Photosynthesis equation

6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Cellular respiration equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Enzyme

A protein catalyst that speeds reactions by lowering activation energy.

Active site

The region of an enzyme where the substrate binds.

Denaturation

Loss of a protein's shape (and function) from heat or pH.

Competitive inhibitor

Blocks the active site.

Noncompetitive inhibitor

Binds elsewhere and changes the enzyme's shape.

ATP

The cell's energy currency; energy is released when a phosphate is removed.

Calvin cycle

Uses ATP and NADPH to fix CO₂ into sugar in the stroma.

Krebs (citric acid) cycle

Releases CO₂ and makes electron carriers in the mitochondria.

Electron transport chain

Pumps protons across the inner membrane using electrons from NADH and FADH₂.

ATP synthase

Uses the proton gradient to make most of the cell's ATP.

Chlorophyll

The pigment that absorbs light for photosynthesis.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.