Where does photosynthesis take place, and how does it work?

Photosynthesis takes place in the chloroplasts of plant and algal cells. In a plant, most chloroplasts are in the leaves, packed into the mesophyll cells in the middle layers of the leaf. Inside each chloroplast, the first stage (the light-dependent reactions) happens on the thylakoid membranes and the second stage (the Calvin cycle) happens in the stroma, the fluid around them. The overall equation is carbon dioxide + water → glucose + oxygen, powered by light.

Where does photosynthesis take place?

The answer depends on how far you zoom in. Each level sits inside the one above it.

So the shortest answer to "where does photosynthesis occur?" is in the chloroplasts, and the most precise answer is on the thylakoid membranes and in the stroma of the chloroplasts.

Inside the leaf

A leaf is built like a sandwich. The upper and lower epidermis form the outside, covered by a waxy cuticle that cuts water loss. In between is the mesophyll. The upper layer, the palisade mesophyll, has tall, tightly packed cells. Below it, the spongy mesophyll has loosely arranged cells with air spaces between them, so carbon dioxide can spread to every cell. Both layers contain many chloroplasts.

Carbon dioxide enters, and oxygen leaves, through tiny pores called stomata, usually on the underside of the leaf. Two guard cells surround each stoma and open or close it. Water arrives from the roots in the xylem, and the sugar made in the leaf is carried away in the phloem. Both run through the veins of the leaf.

Not every plant cell photosynthesises (photosynthesizes in US spelling). Root cells, which grow in the dark, usually have no chloroplasts, and most cells of the leaf epidermis have none either. The guard cells are the exception in the epidermis: they do contain chloroplasts.

Inside the chloroplast

A chloroplast has a double membrane around it. Inside are stacks of flat, disc-shaped sacs called thylakoids; one stack is a granum (plural grana). The fluid around the stacks is the stroma. Our guide to chloroplast structure labels every part.

Photosynthesis without chloroplasts

Cyanobacteria are bacteria that photosynthesise, but like all prokaryotes they have no chloroplasts. Instead, their cell membrane folds inwards to make membranes that are also called thylakoids, and the Calvin cycle runs in the cytoplasm. Chloroplasts are thought to have evolved from ancient free-living cyanobacteria, which is why the two look so alike inside.

What is the equation for photosynthesis?

The word equation is:

The balanced symbol equation is:

The big numbers balance the atoms: one glucose has six carbon atoms, so six carbon dioxide molecules go in, and the six water molecules supply the hydrogen. Light is written above the arrow, not as a reactant, because it is energy, not a substance. Photosynthesis is an endothermic reaction: it takes in energy from light, and that energy ends up stored in the glucose.

What are the reactants of photosynthesis?

The reactants of photosynthesis are carbon dioxide and water.

• Carbon dioxide comes from the air. It diffuses into the leaf through the stomata, then through the air spaces into the mesophyll cells.
• Water comes from the soil. It is absorbed by the root hair cells and carried up to the leaf in the xylem.
• Light energy is needed too, and is absorbed by chlorophyll, but it is not counted as a reactant.

What are the products of photosynthesis?

The products of photosynthesis are glucose and oxygen. Some of the oxygen is used in the plant's own respiration and the rest diffuses out through the stomata. The oxygen comes from the water molecules, which are split during the first stage.

The plant uses the glucose in several ways:

• for respiration, to release energy for the cell;
• turned into starch, which is insoluble, to store;
• turned into fats and oils to store, especially in seeds;
• turned into cellulose, which strengthens cell walls;
• turned into amino acids to make proteins, using nitrate ions absorbed from the soil.

Now try building the word equation yourself. Drag each molecule to the side it belongs on, then press Mark to check it in the course.

How does photosynthesis work?

Photosynthesis happens in two stages, one after the other. The first captures light energy. The second uses it to build sugar from carbon dioxide.

Stage 1: the light-dependent reactions

These happen on the thylakoid membranes. Chlorophyll, held in groups called photosystems, absorbs light, mostly blue and red. Green light is reflected, which is why leaves look green. The absorbed energy is used to:

• split water molecules, which releases oxygen as a by-product;
• make ATP, the cell's energy carrier;
• make NADPH, which carries the hydrogen and electrons needed to build sugar.

Stage 2: the Calvin cycle (light-independent reactions)

This happens in the stroma. The enzyme RuBisCO joins carbon dioxide to a five-carbon molecule called RuBP. This is carbon fixation. The products are then turned into a three-carbon sugar, G3P, using the ATP and NADPH from stage 1, and most of the G3P is used to rebuild RuBP so the cycle can turn again.

One carbon dioxide molecule is fixed per turn. It takes three turns to make one G3P, and six turns, fixing six carbon dioxide molecules, to make enough G3P for one glucose. Across those six turns the cycle uses 18 ATP and 12 NADPH. Light is not an input to the Calvin cycle itself. Even so, it runs on the ATP and NADPH from stage 1, so it cannot keep going for long in the dark.

Why is photosynthesis important?

• It feeds almost every ecosystem. Photosynthesis is how the energy in sunlight gets into living things. The sugar plants make is the start of nearly every food chain, and OpenStax estimates that photosynthesis powers 99 percent of Earth's ecosystems.
• It makes the oxygen we breathe. The oxygen released by plants, algae and cyanobacteria is what animals and plants use in aerobic respiration.
• It takes carbon dioxide out of the air. In the carbon cycle, photosynthesis is how carbon moves from the atmosphere into living things.
• It builds the plant itself. The cellulose in wood, the starch in potatoes and the oil in seeds are all made from glucose from photosynthesis.

How are photosynthesis and cellular respiration related?

They are opposite processes. Photosynthesis turns carbon dioxide and water into glucose and oxygen, storing energy. Aerobic respiration uses glucose and oxygen to make carbon dioxide and water, releasing that energy as ATP. The products of one are the reactants of the other.

Plants do both. A common mistake is to think plants photosynthesise instead of respiring. They respire all the time, in every living cell. In bright light, photosynthesis runs faster than respiration, so a leaf takes in carbon dioxide and gives out oxygen overall. After dark, respiration carries on alone, so the leaf gives out carbon dioxide. Our guide to cellular respiration covers the other half of the cycle.

Rate of photosynthesis and limiting factors

The rate of photosynthesis depends on four main things: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. Whichever one is in shortest supply holds the rate back. That one is called the limiting factor.

Here x is the light intensity and y is the rate of photosynthesis, both in arbitrary units. On the steep part of the curve, more light gives a faster rate, so light is the limiting factor. On the flat part, more light makes no difference: carbon dioxide concentration or temperature is now holding the rate back. Raising that factor would lift the whole plateau higher.

• Light intensity: more light supplies more energy for stage 1, up to a point.
• Carbon dioxide concentration: more carbon dioxide speeds up fixation in the Calvin cycle, up to a point.
• Temperature: photosynthesis is controlled by enzymes, so warming speeds it up at first. Too hot, and the enzymes are denatured and the rate falls sharply.
• Chlorophyll: a leaf short of chlorophyll, for example because the soil lacks the magnesium ions needed to make it, cannot absorb as much light.

Greenhouse growers use limiting factors to decide whether paying for extra heating, lighting or carbon dioxide will grow enough extra crop to be worth it.

The inverse square law for light

In experiments, light intensity is changed by moving a lamp nearer or further away. Light intensity follows an inverse square law: double the distance from the lamp and the light spreads over four times the area.

Halve the distance and the light intensity goes up four times. Double it and the intensity falls to a quarter. GCSE Higher students in England are expected to use this.

Photosynthesis worked examples

Example 1: checking the equation balances

Count each type of atom on both sides of6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2.

Every atom balances, so the equation is correct. Atoms are rearranged, never made or destroyed.

Example 2: the rate from a gas volume

An aquatic plant under a lamp releases 6.0 cm³ of oxygen in 4 minutes. Find the rate of photosynthesis.

At that rate it would release 1.5 × 60 = 90 cm³ in an hour. Counting bubbles instead of measuring volume is quicker but less accurate, because bubbles vary in size.

Example 3: moving the lamp

A lamp is moved from 20 cm to 10 cm away from a plant. By what factor does the light intensity change?

The light intensity is 4 times as great. The rate of photosynthesis will rise, but by less than 4 times if another factor starts to limit it.

Example 4: mass of glucose from carbon dioxide

A plant fixes 88 g of carbon dioxide. What mass of glucose can it make? UseM_r(\text{CO}_2) = 44andM_r(\text{C}_6\text{H}_{12}\text{O}_6) = 180.

Moles of carbon dioxide: 88 ÷ 44 = 2 mol. The equation says 6 mol of carbon dioxide make 1 mol of glucose, so 2 mol make 2 ÷ 6 = ⅓ mol.

The plant can make 60 g of glucose, and releases 2 × 32 = 64 g of oxygen at the same time. Check: 88 g of carbon dioxide + 36 g of water = 124 g, and 60 g + 64 g = 124 g, so mass is conserved.

Common mistakes

• Saying photosynthesis happens in the mitochondria. That is respiration. Photosynthesis happens in the chloroplasts.
• Thinking plants only respire at night. They respire all the time; at night it is just the only process happening.
• Thinking the oxygen comes from carbon dioxide. It comes from the water molecules split in stage 1.
• Writing light as a reactant. Light is energy, so it goes above the arrow.
• Saying chlorophyll absorbs green light. It reflects green light, which is why it looks green. It absorbs mostly blue and red.
• Calling the Calvin cycle "the dark reaction" and saying it happens at night. It happens in the light, alongside stage 1, because it needs stage 1's ATP and NADPH.

Practise biology in a short timed paper

High School Biology and AP Biology each give you a short timed paper every day, with 8 questions worth 25 to 35 marks across the course, so photosynthesis turns up regularly but not in every paper. The Cell Biology course walks through photosynthesis and respiration with interactive models.

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