PHOTOSYNTHESIS

Photosynthesis is the chemical change which happens in the leaves of green plants. It is the first step towards making food not just for plants, but ultimately for every animal on the planet as well.

What is Photosynthesis?

Photosynthesis needs:

• chlorophyll (in chloroplasts of leaves)

• carbon dioxide (from the air)

• water (from the soil) (carried by the xylem)

• sunlight energy

Photosynthesis produces:

• glucose (which can be stored as starch) (transported around plant by phloem)

• Oxygen (by-product)

Formula for Photosynthesis:

As you can see there are two kinds of mesophyll cells - palisade mesophyll and spongy mesophyll. The mesophyll cells contain tiny bodies called chloroplasts which contain a green chemical called chlorophyll. Chlorophyll enables the light energy from sunlight to be converted into chemical energy for the photosynthesis reaction.

Where does Photosynthesis take place?

ChloroplastInternal Structure of a leaf

Electron Micrograph of a chloroplast in the Mesophyll Cells of the Leaf

Leaf tissue

Light Independent Phase

•Hydrogen + Energy combine with carbon dioxide to form carbohydrates in the form of glucose

•Takes place in the stroma of the chloroplast

Light Dependent Phase

•Light energy absorbed by chlorophyll.

•Water (H2O) is split, oxygen is released and the hydrogen joins into a hydrogen carrier molecule.

•Energy is released

Takes place in the Grana (membranes) of the chloroplast

Photosynthesis involves 2 steps: Both steps take place in the Chloroplasts of the mesophyll cells

6CO2 + 6H20 = C6H1206 + 602

The Photosynthesis Equation

It must be balancedi.e. the same number of atoms on each side of the equation

(1 molecule of glucose)

Factors limiting photosynthesis

The speed at which photosynthesis takes place depends on :

• Light

• Temperature

• Carbon Dioxide Level

Factors limiting Photosynthesis• Without enough light a plant cannot photosynthesise very fast, even

if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster.

• Sometimes photosynthesis is limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesise if it has run out of carbon dioxide.

• Temperature can be a limiting factor too. If it gets too cold the rate of photosynthesis will slow right down; equally, plants cease to be able to photosynthesise if it gets too hot.

If you plot the rate of photosynthesis against the levels of these three limiting factors you get graphs like the ones below.

Maximising Growth

• Understanding the factors that limit photosynthesis enables greenhouse farmers to maximise the conditions for plant growth. They often use paraffin lamps inside the greenhouse because burning paraffin produces carbon dioxide as well as heat, and so makes photosynthesis proceed faster. They may also use artificial light to enable photosynthesis to continue beyond daylight hours.

Day or Night?