Photosynthesis and Cellular Respiration Study Guide

Key terms

Photosynthesis

Process that uses light energy to build glucose from carbon dioxide and water, releasing oxygen. It happens in plants, algae and some bacteria.

Chlorophyll

Green pigment that absorbs light energy, mostly red and blue light, for photosynthesis.

Chloroplast

Organelle where photosynthesis takes place in plant cells and algae.

Light-dependent reactions

First stage of photosynthesis, in the thylakoid membranes. Light splits water, releasing oxygen, and the energy is stored in ATP and NADPH.

Calvin cycle

Second stage of photosynthesis, in the stroma. It uses ATP and NADPH to build glucose from carbon dioxide.

Stomata

Pores in a leaf that let carbon dioxide in and let oxygen and water vapor out.

Cellular respiration

Process that breaks down glucose, using oxygen, to release energy stored as ATP. Carbon dioxide and water are the waste products.

ATP

Molecule that carries usable chemical energy for cell activities such as building molecules, moving and transporting substances.

Glycolysis

First stage of respiration. It splits glucose in the cytoplasm into two pyruvate molecules, needs no oxygen and makes a small net gain of ATP.

Krebs cycle

Stage of aerobic respiration in the mitochondrial matrix that finishes breaking down glucose-derived molecules, releasing carbon dioxide and loading electron carriers.

Electron transport chain

Stage in the inner mitochondrial membrane where most ATP is made. Oxygen is the final electron acceptor, forming water.

Fermentation

Way to make a little ATP without oxygen by recycling the carriers glycolysis needs. It produces lactic acid in muscle cells and alcohol and carbon dioxide in yeast.

Two equations that mirror each other

• •Photosynthesis: 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
• •Cellular respiration: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP and heat)

Matter cycles and energy flows. The atoms are reused, but the energy comes in as light and leaves as heat.

Where each stage happens

• •Light-dependent reactions: thylakoid membranes inside the chloroplast.
• •Calvin cycle: the stroma, the fluid around the thylakoids.
• •Glycolysis: the cytoplasm.
• •Krebs cycle: the mitochondrial matrix.
• •Electron transport chain: the inner mitochondrial membrane.

Following the energy and the atoms

1. 1Chlorophyll absorbs light energy.
2. 2Water is split and oxygen is released; ATP and NADPH capture the energy.
3. 3The Calvin cycle uses that ATP and NADPH to turn carbon dioxide into glucose.
4. 4The glucose is stored or used. Respiration breaks it down to make ATP.
5. 5Carbon dioxide and water are released and can be used again.

With and without oxygen

Aerobic respiration makes far more ATP than anything that happens without oxygen: roughly 30 to 32 ATP per glucose, depending on the textbook, compared with a net of 2 from glycolysis alone. When oxygen is missing, cells fall back on fermentation, which keeps glycolysis running but makes no additional ATP.

Common mistakes

• Mistake: Plants do photosynthesis and animals do respiration.

  Plants do both. Chloroplasts make glucose and mitochondria break it down. Animals do respiration only.

• Mistake: Photosynthesis creates energy.

  Energy is never created. Photosynthesis converts light energy into chemical energy stored in glucose.

• Mistake: The oxygen released in photosynthesis comes from carbon dioxide.

  It comes from water, which is split in the light-dependent reactions.

• Mistake: Cellular respiration is the same as breathing.

  Breathing moves gases in and out of the body. Cellular respiration is the chemical process inside cells that uses the oxygen.

• Mistake: All of respiration happens in the mitochondria.

  Glycolysis happens in the cytoplasm. The later stages happen in the mitochondria of eukaryotic cells.

Common questions