GED Science: Energy for Life: Photosynthesis and Respiration Concepts
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Overview
This chapter explains how living things get the energy they need. Plants capture energy from sunlight, and almost all living things release energy from food through cellular respiration.
In photosynthesis, plants and algae use light energy to turn carbon dioxide and water into glucose and oxygen. It takes place in chloroplasts, which contain the green pigment chlorophyll. In cellular respiration, cells use oxygen to break down glucose, releasing energy and giving off carbon dioxide and water. It takes place in the mitochondria.
The two processes are linked: each uses what the other produces. When oxygen is short, some cells, including human muscle cells during hard exercise, use fermentation, which releases a small amount of energy without oxygen and makes lactic acid in muscle. GED questions on this topic often describe a simple experiment, such as counting oxygen bubbles from a water plant under a lamp.
Key concepts
Photosynthesis
Carbon dioxide + water + light energy → glucose + oxygen. Happens in chloroplasts.
Memory tipPlants make food and give off oxygen.
Cellular respiration
Glucose + oxygen → carbon dioxide + water + energy. Happens in mitochondria.
Memory tipRespiration is not the same as breathing; it happens inside cells.
Fermentation
Releasing energy from glucose without oxygen. In human muscle it makes lactic acid; yeast makes alcohol and carbon dioxide.
Memory tipNo oxygen, less energy.
Chloroplast and mitochondrion
Chloroplasts capture light for photosynthesis. Mitochondria release energy from food.
Memory tipChloroplast = capture light; mitochondrion = release energy.
Balanced equation for respiration
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. One glucose makes 6 carbon dioxide molecules.
Memory tipMultiply by the number of glucose molecules used.
Often tested
- The products of photosynthesis are the reactants of respiration, and the reverse.
- In a lamp-distance experiment, distance (light) is the independent variable and bubbles counted is the dependent variable.
- Describe the trend in data without claiming an exact pattern the numbers do not show.
Easy to confuse: Photosynthesis and cellular respiration
| Feature | Photosynthesis | Cellular respiration |
|---|---|---|
| Uses | Carbon dioxide, water, light | Glucose, oxygen |
| Makes | Glucose, oxygen | Carbon dioxide, water, energy |
| Where | Chloroplasts (plants, algae) | Mitochondria (in plant and animal cells) |
| Energy | Stored in glucose | Released from glucose |
3 sample questions
Verified practice questions from this unit.
Question 1. Plants use carbon dioxide, water, and light energy in photosynthesis. What does photosynthesis make?
- ① Salt and heat
- ② Protein and nitrogen
- ③ Glucose (sugar) and oxygen
- ④ Carbon dioxide and water
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Answer: ③ Glucose (sugar) and oxygen
Key point: Photosynthesis: CO₂ + water + light → glucose + oxygen
In photosynthesis, plants capture light energy and use it to turn carbon dioxide and water into glucose, a sugar that stores energy. Oxygen is released as a by-product.
Wrong choices
- Salt and heat: Photosynthesis does not make salt.
- Protein and nitrogen: Photosynthesis does not make protein or nitrogen.
- Carbon dioxide and water: Carbon dioxide and water are what photosynthesis uses up. They are also what cellular respiration gives off.
Question 2. In which part of a plant cell does photosynthesis take place?
- ① Mitochondrion
- ② Cell wall
- ③ Chloroplast
- ④ Nucleus
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Answer: ③ Chloroplast
Key point: Chloroplasts hold chlorophyll and carry out photosynthesis
Chloroplasts contain chlorophyll, the green pigment that captures light energy. That is where photosynthesis happens.
Wrong choices
- Mitochondrion: The mitochondrion is where cellular respiration releases energy from food.
- Cell wall: The cell wall gives the plant cell support and shape.
- Nucleus: The nucleus holds the cell's DNA.
Question 3. Cellular respiration releases the energy stored in glucose. Which gas do most cells, including human cells, need for this process?
- ① Nitrogen
- ② Oxygen
- ③ Carbon dioxide
- ④ Helium
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Answer: ② Oxygen
Key point: Respiration: glucose + oxygen → CO₂ + water + energy
In aerobic cellular respiration, cells combine glucose with oxygen to release usable energy. Carbon dioxide and water are given off as wastes.
Wrong choices
- Nitrogen: Nitrogen makes up most of the air, but our cells do not use it in respiration.
- Carbon dioxide: Carbon dioxide is a waste product of respiration. It is breathed out, not used.
- Helium: Helium is not used by living cells.
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