Unit 5 Cell Energy Test Review- Answers and Study Guide
Unit 5 Cell Energy Test: What You Actually Need to Know
Your Unit 5 Cell Energy test covers photosynthesis and cellular respiration. That's it. Two big topics, but the questions are predictable if you know the patterns. This guide cuts through the textbook fluff and gives you exactly what will be on the test.
What Unit 5 Actually Tests
Most teachers structure Unit 5 around three core concepts:
- ATP as cellular currency
- Photosynthesis (converting light to sugar)
- Cellular respiration (converting sugar to ATP)
If you understand how energy flows in and out of cells, you're already halfway there. The test usually mixes diagram identification, equation balancing, and "explain what happens when" questions.
ATP: The Energy Carrier
ATP is adenosine triphosphate. You need to know:
- Structure: Adenine + ribose + three phosphate groups
- Why it matters: The third phosphate bond is unstable. Break it, release energy. That's the whole point.
- ADP: adenosine diphosphate. Two phosphates. You can add one back with energy from food or sunlight.
Your teacher might ask you to explain ATP's role in one sentence. Here's that sentence: ATP stores a small amount of energy that cells can use immediately for any reaction that needs it.
Photosynthesis: The Big Picture
Photosynthesis happens in chloroplasts. It uses sunlight, water, and carbon dioxide to make glucose and oxygen.
The Equation
6CO₂ + 6H₂O + Light Energy → C₆H₁₂O₆ + 6O₂
Know this equation cold. You'll need to balance it, identify reactants/products, or explain what happens if you remove one component.
Two Stages: Light-Dependent vs. Light-Independent
Light-dependent reactions happen in the thylakoid membrane. They:
- Capture light energy with chlorophyll
- Split water molecules (photolysis)
- Produce ATP and NADPH
- Release oxygen as a byproduct
Light-independent reactions (Calvin cycle) happen in the stroma. They:
- Use ATP and NADPH from the light reactions
- Fix CO₂ into organic molecules
- Produce glucose (C₆H₁₂O₆)
The light-independent part doesn't literally need darkness. It just doesn't use light directly. That's why scientists stopped calling it "dark reactions."
Where Things Go Wrong
If light decreases, light-dependent reactions slow down. Less ATP and NADPH form. The Calvin cycle grinds to a halt. Students often get tricked by questions like "what happens if you remove the chlorophyll?" The answer: photosynthesis stops because light energy can't be absorbed.
Cellular Respiration: The Payoff
Cellular respiration breaks glucose down and captures the energy in ATP. It happens in the mitochondria.
The Equation
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
This is the reverse of photosynthesis. Remember that. It's a test favorite.
Three Stages
1. Glycolysis — happens in the cytoplasm
- Breaks glucose (6 carbons) into two pyruvate molecules (3 carbons each)
- Net gain: 2 ATP, 2 NADH
- Doesn't need oxygen (anaerobic)
2. Krebs Cycle (Citric Acid Cycle) — happens in the mitochondrial matrix
- Processes pyruvate from glycolysis
- Net gain: 2 ATP, 6 NADH, 2 FADH₂ per glucose
- Releases CO₂ as waste
3. Electron Transport Chain — happens in the inner mitochondrial membrane
- Most ATP production happens here (about 28-34 ATP)
- Oxygen is the final electron acceptor
- Forms water as a byproduct
Aerobic vs. Anaerobic
With oxygen: complete respiration. Without oxygen: fermentation.
Fermentation has two types. Lactic acid fermentation happens in muscles during intense exercise. Alcoholic fermentation happens in yeast. Both regenerate NAD⁺ so glycolysis can keep running.
Your test might ask: why does muscle soreness happen after exercise? Lactic acid buildup from fermentation. The answer is always that.
How Photosynthesis and Respiration Connect
Here's the table that will save you:
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Location | Chloroplasts | Mitochondria |
| Energy Source | Sunlight | Glucose |
| Energy Output | Chemical energy (glucose) | ATP |
| Reactants | CO₂ + H₂O + Light | Glucose + O₂ |
| Products | Glucose + O₂ | CO₂ + H₂O + ATP |
| Type | Anabolic (builds up) | Catabolic (breaks down) |
They are essentially opposite reactions. The products of one are the reactants of the other. Plants do both. Animals only do respiration.
How to Actually Prepare
Step 1: Memorize the two main equations. Write them out five times each. 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂ and C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. These will show up guaranteed.
Step 2: Draw the cell diagrams. Chloroplast and mitochondria. Label the compartments: thylakoid, stroma, matrix, cristae. Know where each stage happens.
Step 3: Track the carbon atoms. Glucose has 6 carbons. Pyruvate has 3. CO₂ has 1. If a question asks what happens to the carbons in glycolysis, you should be able to say "they end up in two pyruvate molecules."
Step 4: Count ATP. Total aerobic respiration yields about 36-38 ATP per glucose. Know the approximate breakdown: 2 from glycolysis, 2 from Krebs, 32-34 from ETC.
Step 5: Practice explaining. Your teacher will ask "explain what happens to ATP production if oxygen is removed." The answer: glycolysis keeps going briefly, but without oxygen the ETC stops, NADH can't be recycled, and fermentation kicks in. ATP production drops dramatically.
Common Test Questions with Answers
Q: What is the function of chlorophyll?
A: Absorb light energy to drive photosynthesis. It captures photons, especially in the red and blue wavelengths.
Q: Why is water necessary for photosynthesis?
A: Water is split during the light reactions in a process called photolysis. This replaces electrons lost by chlorophyll and releases oxygen.
Q: What happens to a plant if you keep it in complete darkness?
A: The light-dependent reactions stop. No ATP or NADPH forms. The Calvin cycle can't function without them. Glucose production halts. The plant eventually dies unless it has stored glucose.
Q: Where is the most ATP produced in cellular respiration?
A: The electron transport chain. It produces roughly 90% of the total ATP from glucose.
Q: Why does anaerobic respiration produce far less ATP than aerobic?
A: Without oxygen, the ETC doesn't work. Fermentation only recycles NAD⁺ and allows glycolysis to continue, but glycolysis only yields 2 ATP. Most of the glucose's energy remains untapped.
What to Skip
Don't waste time memorizing every intermediate in the Calvin cycle or Krebs cycle. You don't need to know the names of all 10 molecules in the Calvin cycle. Your teacher probably won't ask that. Focus on the big picture: what goes in, what comes out, where it happens.
Also skip the detailed chemistry of chlorophyll's ring structure or the exact mechanism of ATP synthase rotation. That's for later. Unit 5 is about the overview.
The Bottom Line
Photosynthesis and cellular respiration are two sides of the same coin. Know the equations. Know where they occur. Know that one builds glucose while the other breaks it down. If you can explain the relationship between them, you've passed the test.