Photosynthesis

STEP Grade 9

Simple Diagram of a Leaf

Cross Section of a Leaf

The Chloroplast

The Chloroplast

Stroma:  The matrix surrounding the grana in the inner membrane of chloroplasts. The area between membranes (thylakoids, grana) inside the chloroplast.

Grana: A series of stacked thylakoid disks containing chlorophyll; found in the inner membrane of chloroplasts.

Chlorophyll

Reactions of Photosynthesis

6CO2 + 6H2O C6H6O6 + 6O2

The process is a lot more complex Actually takes place in two separate stages:

The Light reactions – occurs in grana The Dark reactions – occurs in stroma

The Light Reactions

The purpose of the light reactions is to change light energy into chemical energy in the form of 2 chemicals ATP and NADPH. Requires light Light energy used to make ATP. The light splits water into hydrogen ions (H+) and

oxygen atoms. The oxygen is released to the air. The H+ ions are used to make NADPH ATP carries energy to the dark reactions. NADPH carries energy and H+ to the dark

reactions.

The Dark Reactions

The purpose of the dark reactions is to change CO2 into sugar by adding energy & the H+ ions from ATP & NADPH. Can occur in light or dark. 6 CO2 & lots of ATP & NADPH are needed to

make one sugar molecule. The sugar is then used for plant growth or stored

for use in fruits and leaves of plants as food for animals

Light & Dark Reactions

Limiting Factors

Factors affecting the rate of photosynthesis.

As the rate of photosynthesis increases so will the plants growth rate.

Three factors limit photosynthesis from going any faster: Light level, carbon dioxide level, and temperature.

Without enough light a plant cannot photosynthesize very fast, even if there is plenty of water and carbon dioxide. Increasing the light intensity will make photosynthesis faster.

Limiting Factors

Sometimes photosynthesis is limited by the level of carbon dioxide. Even if there is plenty of light a plant cannot photosynthesize 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 photosynthesize if it gets

Limiting Factors

Questions

1. Which graph best shows the effect of increasing light intensity on the rate of photosynthesis?

2. Which graph best shows the effect of increasing carbon dioxide concentration on the rate of photosynthesis? A

3. Which graph best shows the effect of increasing temperature if light and carbon dioxide are not limiting (i.e. the levels of light and carbon dioxide are high). A