How Plants Make Food: Modeling Photosynthesis
Students use a model and simple data to explain how plants use light energy, carbon dioxide, and water to produce sugars and release oxygen.
Illustrations are auto-generated and may be placeholders. They can be refreshed to match the narration.
What Do Plants Need?
Plants need light, carbon dioxide, and water to carry out photosynthesis. Leaves capture light energy, usually from the Sun. Carbon dioxide enters a leaf through tiny openings called stomata. Roots absorb water from the soil, and tubes in the stem carry it to the leaves. Soil minerals support growth, but soil is not the plant’s food. Environmental characteristics affect how easily plants obtain these inputs. For example, a cactus in Arizona receives intense sunlight but has limited water, so it stores water and reduces water loss. A fern in a shaded, rainy forest has plentiful water but less direct light. A map showing sunlight and rainfall patterns can help explain why different plants grow in different locations. Wherever a plant lives, it must obtain all three photosynthesis inputs.
Inputs and Products of Photosynthesis
Photosynthesis changes carbon dioxide and water into glucose, a type of sugar, and oxygen. Light supplies the energy for this change. A balanced model is 6 carbon dioxide molecules plus 6 water molecules plus light energy produce 1 glucose molecule and 6 oxygen molecules. It can be written as 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. The numbers show that atoms are rearranged rather than created or destroyed. Glucose contains carbon, hydrogen, and oxygen atoms that came from the input molecules. The plant uses glucose for energy and to build materials such as cellulose. Oxygen leaves through the stomata. For example, an aquatic plant placed in bright light may release visible oxygen bubbles into the surrounding water.
Modeling Matter and Energy
A useful model separates the movement of matter from the movement of energy. Matter includes carbon dioxide, water, glucose, and oxygen. During photosynthesis, their atoms are rearranged, but the total number of each kind of atom stays the same. Matter can cycle through an ecosystem. For example, a plant stores carbon in glucose, a rabbit eats the plant, and both the rabbit and plant release carbon dioxide through cellular respiration. Energy follows a different path. Light energy enters the plant and becomes chemical energy stored in glucose. When an organism uses glucose, some energy supports life processes and some moves to the environment as heat. Unlike matter, energy does not cycle back to the Sun. Models should therefore use looping arrows for matter and one-way arrows for energy.
Analyzing Light and Sugar Data
Data can show how light intensity is related to sugar production. In a simplified investigation, identical plants receive 20, 40, 60, or 80 light units while water, carbon dioxide, and temperature stay constant. The plants produce 1, 2, 3, or 4 milligrams of sugar per hour. When light doubles from 20 to 40 units, sugar production doubles from 1 to 2 milligrams per hour. The ratio is constant: 1 milligram of sugar for every 20 light units. A graph of these data forms a straight line through the origin, showing a proportional relationship within the tested range. The model predicts 5 milligrams per hour at 100 light units. Real plants may stop following this pattern at very high light levels if water, temperature, or carbon dioxide becomes limiting.
Explaining Photosynthesis with Evidence
A scientific explanation includes a claim, evidence, and reasoning. One claim is that light energy allows plants to make sugar from carbon dioxide and water while releasing oxygen. Evidence can come from the data: as light increased from 20 to 80 units, sugar production increased from 1 to 4 milligrams per hour. Another investigation might show that a water plant produces more oxygen bubbles in light than in darkness. The reasoning connects these observations to the photosynthesis model. Light provides energy for rearranging atoms from carbon dioxide and water into glucose and oxygen. The glucose stores chemical energy and matter that can move to another organism when the plant is eaten. For example, a caterpillar obtains both carbon-containing matter and chemical energy by eating a leaf. This evidence supports photosynthesis as a process that cycles matter and moves energy into living systems.
