Lesson 4 of 12 / Nutrition and transport in flowering plants
Photosynthesis makes food
Where do the carbon and energy in a plant's food come from?
I can state the photosynthesis word equation, energy conversion, carbohydrate uses and why most life depends on it.
I can explain light capture, chemical energy and carbohydrate uses.Syllabus
Syllabus K224 / K225, Topic 8(d). Explain light capture, chemical energy and carbohydrate uses.
I can explain dependence of most life on photosynthesis.Syllabus
Syllabus K224 / K225, Topic 8(e). Explain dependence of most life on photosynthesis.
I can state the photosynthesis word equation.Syllabus
Syllabus K224 / K225, Topic 8(f). State the photosynthesis word equation.
About 6 min
Make a guess. You are not marked.
A small seed grows into a tree weighing several tonnes. Where did most of that mass come from?
Show the answer
Carbon dioxide from the air, plus water.
Photosynthesis builds sugars from carbon dioxide and water. The carbon in wood comes mainly from the air.
The key idea
Chlorophyll absorbs light energy, which is converted to chemical energy as carbon dioxide and water form carbohydrates and oxygen.
light and chlorophyll
-> glucose + oxygen
Glucose can be converted to insoluble starch for storage. Starch is not the main sugar transported in phloem.
Arrows distinguish inputs and output. Light provides energy; CO2 and water provide atoms. This is an overall process diagram, not the detailed biochemical pathway.
Explanation
In photosynthesis, chlorophyll in the chloroplasts absorbs light energy. The plant uses carbon dioxide and water to make glucose. Light energy is changed into chemical energy, which is stored in the glucose.
The word equation is: carbon dioxide + water -> glucose + oxygen. Light and chlorophyll are needed, so they are written next to the arrow. They are not taken in through the roots.
The plant uses glucose in respiration. It also turns glucose into starch to store it, into cellulose to build cell walls, and into sucrose to carry it round the plant. With nitrogen from nitrate ions, it can also make amino acids and proteins.
Almost all food chains start with plants and other photosynthesising producers. Photosynthesis also releases the oxygen that living things use in aerobic respiration.
Step by step
- 1
Identify inputs and conditions
Carbon dioxide and water are reactants; light and chlorophyll enable the process.
- 2
Account for matter and energy
Atoms are rearranged into products while light energy becomes chemical energy.
- 3
Follow the food
Link carbohydrate to plant uses and to consumers in food chains.
Worked example
Why a plant needs more than mineral ions
A student says most of a plant's new dry mass comes from minerals absorbed by roots. Correct the explanation.
One way to explain it
Much of the carbon in new organic material comes from carbon dioxide absorbed from air and fixed into carbohydrates during photosynthesis. Mineral ions supply essential elements such as nitrogen, but are not the main source of the plant's carbohydrate carbon.
Why this answer works
- Dry mass excludes the water content.
- Trace carbon from CO2 into organic molecules.
- Keep mineral-ion importance without calling minerals the plant's food.
Watch out for this
A student says: "Light is converted into matter and roots absorb the plant's ready-made food." What is wrong with this?
Show the answer
Carbon dioxide and water supply matter for photosynthesis. Light supplies energy. Roots mainly absorb water and mineral ions; the plant makes organic food.
Check your understanding
Which statement correctly follows photosynthesis into a food chain?
Show the answer
Plants store absorbed light energy as chemical energy in organic food, which consumers can obtain.
Photosynthetic producers supply organic matter and chemical energy to most food chains.
Quick recall
Where do the carbon and energy in a plant's food come from?