Grade 8 · Ecosystems and Energy Flow · Energy Flow and Matter Cycling
Photosynthesis and Food Production
How plants turn light, water and carbon dioxide into food.
Learning objectives
- Write the word equation for photosynthesis.
- Explain the role of chlorophyll and light.
- Use food chains to trace energy from the Sun to a consumer.
NGSS alignment
MS-LS1-6
Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy into and out of organisms.
HS-LS1-5
Use a model to illustrate how photosynthesis transforms light energy into stored chemical energy.
MS-LS2-3
Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion B — Inquiring and Designing
- Criterion C — Processing and Evaluating
ENGAGE
Where does the mass of a tree come from?
A tree can gain hundreds of kilograms of mass while the soil around it barely loses any.
Think about it
- If not from the soil, where does the extra mass come from?
- What would you need to measure to test your idea?
EXPLAIN
Photosynthesis
Photosynthesis transfers light energy into chemical energy stored in glucose.
Word equation
carbon dioxide + water, using light energy and chlorophyll, gives glucose + oxygen
- Chlorophyll in chloroplasts absorbs light
- Carbon dioxide diffuses in through stomata
- Water is drawn up from the roots
- Glucose is used for respiration, growth or stored as starch
Energy flow in ecosystems
Producers capture light energy. Consumers get their energy by eating producers or other consumers, so almost every food chain begins with photosynthesis.
| Level | Name | Example |
|---|---|---|
| 1 | Producer | Grass |
| 2 | Primary consumer | Grasshopper |
| 3 | Secondary consumer | Lizard |
| 4 | Tertiary consumer | Hawk |
Common misconception
Plants respire all the time, day and night. During daylight photosynthesis is simply faster than respiration.
INVESTIGATION
Light intensity and photosynthesis
research question
How does light intensity affect the rate of photosynthesis in pondweed?
hypothesis
Bubble rate will increase as light intensity increases until another factor becomes limiting.
variables
Independent: distance from lamp (10, 20, 30, 40, 50 cm). Dependent: bubbles produced per minute. Controlled: temperature, carbon dioxide concentration, same pondweed sample.
method
Place pondweed in sodium hydrogencarbonate solution. Count bubbles for one minute at each distance, allowing two minutes to acclimatise. Repeat three times.
safety
Keep the lamp away from water. Use a heat shield to keep temperature constant.
analysis
Plot mean bubble rate against distance and describe where the graph levels off.
Watch
Photosynthesis vs Cellular Respiration Comparison
Amoeba Sisters · 8 min
Shows how the two processes are mirror images of each other.
Before you watch: Do plants respire? Predict before watching.
- Write both word equations side by side.
- Why is the carbon cycle described as a loop?
Interactive simulation · PhET
Energy Forms and Changes
Select the light source and follow the energy symbols as light energy is absorbed and transferred.
While you explore
- Where does the energy in glucose originally come from?
- What energy transfer happens inside a chloroplast?
Key vocabulary
- Producer
- A living thing that makes its own food.
- Photosynthesis
- How plants use light to make their own food.
Practice questions
0/2 correct
Level 1 · Criterion A
Which two substances are the raw materials of photosynthesis?
Level 2 · Criterion A
In a food chain, grass is described as the:
MYP criterion tasks
Level 3 · Criterion B
Design an investigation to test whether carbon dioxide concentration limits the rate of photosynthesis in pondweed.
Reflect & track
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