Photosynthesis process and factors affecting it

Photosynthesis is the process by which green plants make their own food. It is the reaction that turns the energy in sunlight into food energy for almost every food chain on Earth.

• It happens in the chloroplasts of plant cells, which contain the green pigment chlorophyll.
• Chlorophyll absorbs light energy (mainly from the Sun).
• That light energy is converted into chemical energy, which is stored in the bonds of the glucose (a carbohydrate) that is made.

So photosynthesis is really an energy conversion: light energy → chemical energy. Because plants make their own food this way, they are described as autotrophic.

Why it matters. The glucose made can be:

• used in respiration to release energy, or
• converted to starch for storage, or to cellulose for cell walls, or to other molecules.

Exam tip. When a question asks what photosynthesis is, the marking point is the energy conversion — say "light energy is converted (by chlorophyll) into chemical energy stored in glucose/carbohydrate". Don't just list the ingredients.

You must be able to recall both equations precisely.

Word equation

carbon dioxide + water → (light / chlorophyll) → glucose + oxygen

The light and chlorophyll are written above the arrow because they are needed for the reaction but are not "used up" as raw materials.

Balanced symbol equation

$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

Check the balancing — the same atoms appear on both sides:

• Carbon: 6 on the left (in 6CO₂) and 6 on the right (in C₆H₁₂O₆). ✓
• Hydrogen: 12 on the left (in 6H₂O) and 12 on the right (in C₆H₁₂O₆). ✓
• Oxygen: 12 + 6 = 18 on the left (6CO₂ has 12 O, 6H₂O has 6 O) and 6 + 12 = 18 on the right (C₆H₁₂O₆ has 6 O, 6O₂ has 12 O). ✓

	Raw materials (in)	Products (out)
Words	carbon dioxide + water	glucose + oxygen
Symbols	6CO₂ + 6H₂O	C₆H₁₂O₆ + 6O₂

Exam tip. Glucose is C₆H₁₂O₆ — count the atoms before you write it. Losing a subscript (e.g. writing C₆H₂O₆) loses the mark. The big numbers in front (the 6s) must be there for the equation to be balanced.

The rate of photosynthesis is how quickly a plant makes glucose (often measured by how fast oxygen is given off). Three things affect this rate:

1. Carbon dioxide concentration — CO₂ is a raw material. More CO₂ → faster rate, until something else becomes limiting.
2. Light intensity — light provides the energy. Brighter light → faster rate, until something else becomes limiting.
3. Temperature — photosynthesis is controlled by enzymes. Warmer (up to an optimum) → faster rate. Too hot and the enzymes denature, so the rate falls.

A limiting factor is the factor that is in shortest supply at a given moment — it is the one "holding back" the rate. Increasing it speeds up photosynthesis; increasing the others does nothing until they become limiting in turn.

Analogy. Think of a factory production line: the slowest worker sets the pace. Speeding up the others won't help until you speed up the slow one. That slow worker is the limiting factor.

How each factor behaves on a rate graph:

• CO₂ and light intensity each give a line that rises, then levels off (plateaus) when a different factor becomes limiting.
• Temperature gives a line that rises to an optimum and then falls as enzymes denature — this is the key difference, because the other two simply plateau.

Exam tip. When a graph plateaus, the marking point is: "another factor has become limiting" (e.g. "the rate is now limited by light intensity / temperature"). Don't just say "it stays the same".

Exam questions almost always test limiting factors through a graph. Learn to read the two parts of the curve.

How to describe this graph in the exam:

• The rising part (low light): the rate increases as light intensity increases, because light is the limiting factor — more light energy lets photosynthesis go faster.
• The plateau (high light): the rate stays constant, because light is no longer limiting — now a different factor (CO₂ concentration or temperature) is limiting, so adding more light has no effect.

The same S-then-flat shape appears for a CO₂ graph. For a temperature graph the line instead rises to a peak (the optimum) and then drops as the enzymes denature.

Exam tip. To raise the plateau higher you must increase the other factor. A favourite question is "how could you increase the maximum rate?" — answer: increase the factor that is now limiting (e.g. raise the CO₂ concentration or the temperature).

Temperature is the factor students get wrong most often, so it deserves its own look.

Photosynthesis is a series of reactions controlled by enzymes, and enzymes are affected by temperature:

• Low to optimum temperature: as it warms up, particles have more kinetic energy, there are more successful collisions, so the rate increases.
• Above the optimum: the high temperature changes the shape of the enzymes' active sites — the enzymes denature — so the rate falls sharply and eventually stops.

This gives a temperature–rate curve with a clear peak at the optimum, unlike the simple plateau seen for light and CO₂.

Watch the wording. Enzymes are denatured, not "killed" — enzymes are molecules, not living things. Use the word denature.

Exam tip. If a temperature graph falls after a peak, the marking point is "the enzymes have denatured / the enzymes' active sites have changed shape", which slows or stops photosynthesis.