BiologyHow Plants Make Food From Light
A sunlit greenhouse bursting with leafy green plants, where Medi crouches beside a broad-leaved plant, magnifying glass in hand, watching tiny bubbles rise from a submerged leaf in a beaker of water.
- Explain how plants use light energy, water, and carbon dioxide to produce glucose and oxygen.
- Identify the role of chlorophyll and chloroplasts in capturing light energy.
- Describe where each reactant enters the plant and where each product exits.
- Predict what happens to a plant's photosynthesis rate when one of its inputs is removed.
Key terms
- Photosynthesis
- The process plants use to turn light energy, water, and carbon dioxide into glucose and oxygen.
- Chloroplast
- The plant-cell organelle where photosynthesis takes place.
- Chlorophyll
- The green pigment in chloroplasts that absorbs light energy and gives leaves their color.
- Glucose
- The sugar a plant builds during photosynthesis to use for energy and growth.
- Stomata
- Tiny pores on a leaf through which carbon dioxide enters and oxygen exits.
Inputs and Outputs of the Reaction
Photosynthesis takes simple ingredients and rebuilds them into food. The inputs are carbon dioxide drawn through the stomata, water pulled up from the roots, and light energy captured by chlorophyll inside the chloroplasts. The light energy powers a rearrangement of the atoms so that the plant assembles glucose, a sugar it uses for energy and growth, and releases oxygen as a byproduct. The balanced equation, 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂, captures this exactly: three things go in, and two come out, with light driving the change.
Where a Plant's Mass Really Comes From
It feels obvious that a plant grows by eating the soil, but that is not what happens. Van Helmont's classic willow experiment showed the soil barely lost any mass even as the tree gained huge weight, because almost all of a plant's body is carbon that was once carbon dioxide in the air. The plant captures that CO₂ and locks the carbon into glucose and then into wood and leaves. This is why clearing forests releases carbon dioxide: burning or rotting wood returns its stored carbon back to the atmosphere.
Worked examples
Predict what happens to a plant kept in total darkness for several days.
- List the inputs photosynthesis needs: carbon dioxide, water, and light energy.
- Remove the missing input: in darkness there is no light energy for chlorophyll to capture.
- Apply the rule: without light, the chloroplasts cannot rearrange atoms to build glucose.
- Conclude: the rate of photosynthesis drops to zero, though the plant can still respire.
Answer: Photosynthesis stops because light, a required input, is missing, so the plant makes no new glucose while in the dark.
Activity
Sort the matter and energy below — does each one go IN to the plant (input) or come OUT of the plant (output)?
Practice
Sort carbon dioxide, water, light, glucose, and oxygen into photosynthesis inputs and outputs.
Explain where most of a giant oak tree's mass actually comes from and why.
Common mistakes to avoid
- Plants get most of their mass from soilMost plant mass comes from carbon dioxide absorbed from the air, not from minerals in the soil.
- Plants only do photosynthesis and never respirePlants also carry out cellular respiration to release the energy stored in the glucose they make.
Check your understanding
Which organelle in plant cells is the main site of photosynthesis?
A plant is placed in a sealed, dark container for several days. Which change is MOST likely to occur?
A student says, 'Plants get most of their mass from the minerals and nutrients in soil.' What is wrong with this claim?
Recap
Photosynthesis uses light energy, carbon dioxide, and water inside chloroplasts to build glucose and release oxygen, and most of a plant's mass comes from carbon dioxide in the air rather than from the soil.
Reflect
How does it change the way you see a tree to know it is built mostly from air and sunlight?