Photosynthesis

Essential Question

How do producers capture energy from sunlight?

Almost every food chain starts with a plant, an alga, or a bacterium that captures sunlight. In this lesson, you’ll see how photosynthesis turns light energy into chemical energy stored in sugar. You’ll find where in the cell it happens, how the raw materials get there, and why leaves are green.

Photosynthesis Stores Light Energy in Sugar

• Inputs: CO₂, H₂O, and light energy
• Outputs: glucose (C₆H₁₂O₆) and O₂
• Light energy becomes chemical energy in glucose
• Done by plants, algae, and some bacteria

Photosynthesis is the process that captures light energy and stores it as chemical energy in the bonds of glucose: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Carbon dioxide and water are rearranged into a sugar and oxygen gas. The sugar holds much more chemical energy than the carbon dioxide and water it was built from, and the extra energy came from light. Plants and algae carry out photosynthesis, and so do some bacteria. These producers make the food that the rest of a food web depends on.

Inside the Chloroplast

• Chloroplasts: organelles where photosynthesis happens
• Double membrane surrounds the chloroplast
• Thylakoids: flat sacs stacked into grana
• Stroma: the fluid around the thylakoids

In plants and algae, photosynthesis takes place in chloroplasts. A chloroplast has a double membrane. Inside is a fluid called the stroma and a system of flat, disk-shaped sacs called thylakoids. Thylakoids are stacked into piles called grana (one pile is a granum). The green pigment chlorophyll is built into the thylakoid membranes, where it absorbs light. Photosynthetic bacteria have no chloroplasts, so they carry out photosynthesis in their own membranes and cytoplasm.

From the Leaf to the Chloroplast

• CO₂ enters through stomata
• Air spaces let CO₂ reach the cells
• Water travels up from the roots in xylem
• O₂ leaves through the stomata

A leaf is built to bring the raw materials to its chloroplasts. Carbon dioxide enters through tiny pores called stomata (one pore is a stoma), then spreads through air spaces to the cells. Water is pulled up from the roots through the xylem, the water tubes you met in Unit 1. Light shines through the thin, flat leaf to reach the chloroplasts. The oxygen made by photosynthesis leaves through the same stomata. Both CO₂ and O₂ are carried in and out of the leaf by diffusion.

Why Leaves Look Green

• Chlorophyll absorbs blue and red light best
• Chlorophyll reflects green light
• Absorbed light powers photosynthesis
• Reflected light reaches your eyes

Sunlight looks white, but it is a mixture of colors. The graph shows how much light of each wavelength chlorophyll absorbs (sample values). Chlorophyll absorbs blue light (about 450 nm) and red light (about 675 nm) strongly, but it absorbs green light (about 550 nm) weakly. The green light that is not absorbed is reflected back to your eyes, so leaves look green. Only the light that is absorbed can supply energy for photosynthesis.

What Plants Do With the Glucose

• Cellular respiration: release energy as ATP
• Starch: store energy and carbon
• Cellulose: build cell walls
• Amino acids and lipids: build other molecules

Glucose is the starting point for most of what a plant needs. The plant breaks down some glucose in cellular respiration to make ATP, the energy molecule cells use. It links glucose into starch to store energy and carbon, and into cellulose to build cell walls. It also uses the carbon, hydrogen, and oxygen from sugar, together with other elements such as nitrogen, to make amino acids, lipids, and other large molecules (Unit 1). Animals that eat the plant get all of these molecules, and the energy stored in them.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Balancing the Inputs and Outputs

A student writes the equation for photosynthesis, 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. If a leaf takes in 18 molecules of CO₂, how many molecules of water does it need, and how many molecules of glucose and oxygen does it make? Check that the atoms balance.

Show the solution

1. Scale the equation: 18 CO₂ is 3 times 6 CO₂, so multiply every coefficient by 3.
2. Inputs: 3 × 6 = 18 H₂O
3. Outputs: 3 × 1 = 3 C₆H₁₂O₆ and 3 × 6 = 18 O₂
4. Check carbon: 18 C in 18 CO₂; 3 × 6 = 18 C in 3 glucose. Check oxygen: 18 × 2 + 18 × 1 = 54 O in; 3 × 6 + 18 × 2 = 54 O out.
5. Answer: 18 H₂O in; 3 glucose and 18 O₂ out. All atoms balance.

Example 2: Which Colors Does Chlorophyll Use?

Use the absorption graph. (a) Which colors does chlorophyll absorb most, and which does it absorb least? (b) How many times as much blue light (450 nm) as green light (550 nm) does it absorb? (c) A plant grows under a lamp that gives off only green light. Predict its rate of photosynthesis and explain.

Show the solution

1. (a) Read the graph: absorption is highest at 450 nm (blue, 0.95) and 675 nm (red, 0.90), and lowest at 550 nm (green, 0.10).
2. (b) Compare: 0.95 ÷ 0.10 = 9.5, so chlorophyll absorbs about 9.5 times as much blue light as green light.
3. (c) Predict: the rate will be low.
4. Reasoning: chlorophyll absorbs only about 10% of green light, so little light energy is captured, and the plant stores little energy as glucose.
5. Answer: Most: blue and red. Least: green. Blue is absorbed 9.5 times as much as green. Under green light, photosynthesis would be slow.

Example 3: Label the Chloroplast

Use the model. Name each labeled part of the chloroplast, and tell which part holds the chlorophyll.

Show the solution

1. A: the double membrane that surrounds the chloroplast.
2. B: the stroma, the fluid that fills the space around the thylakoids.
3. C: a thylakoid, one flat, disk-shaped sac.
4. D: a granum, a stack of thylakoids.
5. Chlorophyll: it is built into the membranes of the thylakoids (C and D), where it absorbs light.
6. Answer: A: double membrane. B: stroma. C: thylakoid. D: granum. Chlorophyll is in the thylakoid membranes.

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Practice

Label a chloroplast, scale the photosynthesis equation, and use an absorption graph to explain why leaves are green. Try it on your own first, then check your work with the answer key.

• Photosynthesis Practice Worksheet (PDF)
• Photosynthesis Practice Answer Key (PDF)

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Lesson Slides

• 8.1 Photosynthesis Slides (PDF)

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Big Ideas

• Photosynthesis turns light energy into chemical energy in glucose
• 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂
• It happens in chloroplasts, in plants and algae
• Chlorophyll absorbs blue and red light, and reflects green
• Plants use glucose for energy, storage, and building materials

Back to the essential question: producers capture light energy with chlorophyll in their chloroplasts and store it as chemical energy in glucose, using carbon dioxide from the air and water from the soil. Oxygen is released as a by-product. The plant then uses the glucose to make ATP, to store energy, and to build its body. Next, you’ll look inside the process and see the two stages of photosynthesis.

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