Photosynthesis and Cellular Respiration
Grade 7 · Science · Study Guide
Discover how plants capture sunlight to make food and how all living cells release energy from that food.
Plants are the ultimate solar panels. Through a process called photosynthesis, they use sunlight, water, and carbon dioxide to build glucose — a sugar that stores chemical energy. This happens inside chloroplasts, the green organelles in plant cells. The overall equation is: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. That oxygen released as a byproduct is what you breathe. Without photosynthesis, there would be essentially no oxygen in Earth's atmosphere.
But making glucose is only half the story. That stored energy has to be released for the plant — or any organism — to actually use it. This release happens through cellular respiration, which takes place in the mitochondria of nearly every living cell. Animals can't photosynthesize, so they eat plants (or other animals that ate plants) to get glucose. Cellular respiration then breaks that glucose down: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP energy. ATP (adenosine triphosphate) is the actual fuel cells run on.
Notice that photosynthesis and cellular respiration are essentially opposites of each other. Photosynthesis uses CO₂ and produces O₂; respiration uses O₂ and produces CO₂. Together they cycle carbon and oxygen through ecosystems — a partnership that has kept Earth's atmosphere stable for billions of years.
Even plants perform cellular respiration 24 hours a day. They photosynthesize only when light is available, but they are always burning glucose for energy. In bright conditions, photosynthesis produces far more glucose than the plant can respire, so it stores the excess as starch.
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Location | Chloroplasts | Mitochondria |
| Reactants | CO₂ + H₂O + light | Glucose + O₂ |
| Products | Glucose + O₂ | CO₂ + H₂O + ATP |
| Who does it | Plants, algae, some bacteria | Almost all living organisms |
| When it occurs | During daylight | 24 hours a day |
Key terms
- Photosynthesis
- The process by which plants use sunlight, water, and CO₂ to produce glucose and oxygen.
- Cellular respiration
- The process by which cells break down glucose using oxygen to release ATP energy, CO₂, and water.
- Chloroplast
- Organelle in plant cells where photosynthesis takes place; contains chlorophyll.
- Chlorophyll
- The green pigment in chloroplasts that absorbs light energy for photosynthesis.
- ATP
- Adenosine triphosphate; the molecule cells use as direct energy currency.
- Glucose
- A simple sugar (C₆H₁₂O₆) produced by photosynthesis and used as fuel in cellular respiration.
Flashcards
- Photosynthesis equation
- 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
- Cellular respiration equation
- C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
- Chloroplast
- Plant cell organelle where photosynthesis occurs.
- Mitochondria
- Organelle where cellular respiration occurs in eukaryotic cells.
- ATP
- The direct energy currency used by cells to power all activities.
- Starch
- How plants store excess glucose for later use.
Practice questions
Where does photosynthesis take place inside a plant cell?
- Mitochondria
- Nucleus
- Chloroplast
- Ribosome
Show answer
Answer: Chloroplast
Chloroplasts contain chlorophyll and the enzymes needed to convert light energy into chemical energy (glucose).
Plants perform cellular respiration but animals do not.
Show answer
Answer: False
ALL living organisms — including plants — perform cellular respiration continuously to produce ATP from glucose.
The green pigment in plant cells that absorbs light energy is called ___.
Show answer
Answer: chlorophyll
Chlorophyll is the pigment that gives plants their green color and captures the light energy that drives photosynthesis.
What is the main product of cellular respiration that cells use for energy?
- Glucose
- Oxygen
- ATP
- Carbon dioxide
Show answer
Answer: ATP
ATP (adenosine triphosphate) is the universal energy molecule that cells use to power all biochemical reactions.
Photosynthesis takes in carbon dioxide and releases oxygen.
Show answer
Answer: True
Photosynthesis consumes CO₂ from the atmosphere and releases O₂ as a byproduct — this is the source of atmospheric oxygen.
Fun facts
- A single large tree can absorb about 48 pounds of CO₂ per year through photosynthesis.
- Your brain alone uses about 120 grams of glucose every single day just to keep you thinking.
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