National 5 Biology Course Notes

Unit 1 : Cell Biology

Part 7 : Photosynthesis

Word equation for photosynthesis

In photosynthesis, a plant uses light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose. Oxygen is also produced as a by-product and is released from the plant.Photosynthesis can be summarised by this word equation:

Light

Absorbed

by chlorophyll

Carbon Dioxide + Water Glucose + Oxygen

However photosynthesis is not a single chemical reaction, it is a series of reactions, each controlled by its own enzyme.

Two stages of photosynthesis

The photosynthesis reactions can be divided into two main stages:

1. Light reactions – This stage uses light energy trapped by chlorophyll

2. Carbon fixation – In this stage, which does not require light, carbon dioxide is converted to glucose using substances produced in the light reactions

Light reactions

These reactions use light energy from the sun which is trapped by chlorophyll in the chloroplasts.There are two light reactions:

Photolysis

Light energy absorbed by chlorophyll is used to split water into oxygen and hydrogen atoms

Water Hydrogen + Oxygen

ADP + Pi ATP [Pi = phosphate]

Production of ATP

Light energy absorbed by chlorophyll is used to change ADP + phosphate to ATP

This reaction changes light energy into chemical energy (stored in ATP)

Oxygen produced by photolysis is released from the plant as a by-product.The other products of the light reactions – hydrogen atoms and ATP are used in the second stage of photosynthesis, carbon fixation

Carbon fixation

This is the second stage of photosynthesis. It involves a series of enzyme controlled reactions joining

together carbon dioxide and hydrogen to form glucose.

This stage requires ATP and hydrogen, passed on from the light reactions

Carbon dioxide Glucose

ATP ADP + Pi

The hydrogen needed to change carbon dioxide to glucose comes from splitting water (photolysis).

The energy needed to make glucose from CO2 comes from changing ATP (from the light reactions) back to ADP and Pi

How the plant uses glucose

The glucose produced during photosynthesis can be used in various ways by the plant cell

1. It can be broken down during respiration to release energy in respiration

2. It can be converted to starch and stored.

3. It can be converted to cellulose and used to create cell walls

Limiting Factors

A limiting factor is something that slows the rate of a process when it is in short supply.

Limiting factors for photosynthesis are

1. Light Intensity

2. Temperature

3. Carbon dioxide concentration

Ra

te o

f p

ho

tosy

nth

esi

s

Light intensityX

When the graph is like this the variable on the X axis is not the limiting factor

Limiting factor graphsThe graph shows the effect of light intensity on the rate of photosynthesis

When the graph is like this the variable on the X axis is the limiting factor

Up to point X, increasing light intensity will increase the rate of photosynthesis, so light intensity is the limiting factor

After X, increasing light intensity does not increase the rate of photosynthesis, so light intensity is no longer the limiting

factor (the limiting factor is now either temperature or carbon dioxide concentration)