MYP Biology 3.5 Photosynthesis Notes

What Is Photosynthesis?

1. Photosynthesis is the process plants use to make their own food.Plants absorb light energy and convert it into chemical energy stored in glucose.
2. This takes place inside chloroplasts, which contain the pigment chlorophyll.
3. Most chloroplasts are found in the palisade mesophyll of the leaf, where cells are packed tightly to absorb as much light as possible.
4. Plants rely on photosynthesis for growth, repair, and energy release during respiration.

How Is the Leaf Adapted for Photosynthesis?

1. A large surface area absorbs more light.
2. A thin structure allows gases to diffuse quickly.
3. Many chloroplast-rich palisade cells maximise light absorption.
4. Stomata on the underside of the leaf allow carbon dioxide to enter and oxygen to leave.

What Are the Inputs and Outputs of Photosynthesis?

1. Photosynthesis requires carbon dioxide and water, and produces glucose and oxygen.
2. These substances enter and leave the leaf through specialised structures.
3. Inputs
   1. Carbon dioxide: enters through stomata
   2. Water: absorbed by roots and transported in the xylem
4. Outputs
   1. Glucose: stored, used, or transported through the phloem
   2. Oxygen: released through stomata

How Does Photosynthesis Work?

1. Light energy is absorbed by chlorophyll in the chloroplasts.
2. Water, brought to the leaf by the xylem, is split inside the leaf.
3. Carbon dioxide, entering through stomata, provides the carbon needed to build glucose.
4. Glucose is formed, storing chemical energy in its bonds for later use.
5. Oxygen is released through the stomata into the air.

What Are the Limiting Factors of Photosynthesis?

1. The rate of photosynthesis is controlled by whichever essential factor is in shortest supply.
2. The three main limiting factors are light intensity, carbon dioxide concentration, and temperature.

Light Intensity

1. At low light levels, photosynthesis increases as light intensity increases.
2. Once light is plentiful, the rate plateaus because another factor becomes limiting.

Carbon Dioxide Concentration

1. More carbon dioxide increases the rate until CO₂ is no longer limiting.
2. Beyond that point, increasing CO₂ has no effect because light or temperature becomes limiting instead.

Temperature

1. Photosynthesis is enzyme-controlled.
2. As temperature rises, enzyme activity increases until the optimum temperature is reached.
3. Above this, enzymes begin to denature, causing the rate to fall sharply.

Tip

• Because photosynthesis relies on enzymes, temperature affects the shape of the rate graph more dramatically than light or CO₂.
• These limiting factors therefore closely resemble the factors that affect enzyme activity

Why Is Photosynthesis Essential for Life?

1. It provides glucose, the main energy source for respiration.
2. Plants use glucose for growth, repair, and making other essential substances.
3. Photosynthesis releases oxygen, needed by most organisms for respiration.
4. All animals depend on plants directly or indirectly for food and energy.