Lesson 4 of 9 / Nutrition and transport in flowering plants
Photosynthesis makes food
Where do the carbon and energy in a plant's food come from?
In this lesson: State the photosynthesis word equation, energy conversion, carbohydrate uses and why most life depends on it.
About 6 min
The key ideaChlorophyll 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
Photosynthesis uses light energy absorbed by chlorophyll in chloroplasts. Carbon dioxide and water are used to make carbohydrates, represented by glucose in the overall equation. Light energy is converted into chemical energy stored in organic molecules; energy is not created.
The word equation is carbon dioxide + water -> glucose + oxygen, in the presence of light and chlorophyll. Carbon dioxide and water are used to form products. Light supplies energy and chlorophyll absorbs it; they are conditions shown with the equation, not extra food molecules taken up by the roots.
Glucose can be used in respiration, converted to insoluble starch for storage, used to make cellulose for cell walls, or converted to sucrose for transport. Carbohydrates can also contribute to making fats and, with nitrogen from mineral ions such as nitrates, amino acids and proteins.
Most food chains depend directly or indirectly on photosynthetic producers for organic food and its chemical energy. Photosynthesis also releases oxygen used in aerobic respiration. Most, rather than all, life depends on it: some ecosystems begin with microorganisms using chemical energy instead of light.
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.
Is this true? "Light is converted into matter and roots absorb the plant's ready-made food."
Carbon dioxide and water supply matter for photosynthesis. Light supplies energy. Roots mainly absorb water and mineral ions; the plant makes organic food.