EOY IGCSE BIOLOGY – SECTION 1: PLANTS

Photosynthesis is defined as the biological process by which green plants manufacture their own food.
During this process, plants utilize energy captured from sunlight to facilitate the conversion of specific raw materials into chemical energy.
The primary reactants involved are carbon dioxide, which is obtained from the atmospheric air, and water, which is absorbed from the soil.
These components are converted into glucose ( $C_6H_{12}O_6$ ), a specific type of sugar that the plant uses for energy production and growth.
As a secondary result of this process, oxygen ( $O_2$ ) is generated as a waste product and is subsequently released into the atmosphere.

The process can be represented by a word equation:
- $\text{Carbon dioxide} + \text{Water} \rightarrow \text{Glucose} + \text{Oxygen}$
The balanced chemical equation for the process is:
- $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$

For photosynthesis to take place, a plant requires four essential components:
- Light: Provides the energy needed for the chemical reaction.
- Chlorophyll: The green pigment that absorbs light energy.
- Carbon dioxide ( $CO_2$ ): A raw material sourced from the air.
- Water ( $H_2O$ ): A raw material sourced from the soil.
If any one of these factors is absent or in short supply, the rate of photosynthesis will decrease or may stop entirely.

A limiting factor is defined as the specific factor that is in the shortest supply, thereby restricting or limiting the overall rate of photosynthesis.
Light Intensity:
- Acts as a limiting factor because higher light intensity provides more energy to drive the photosynthetic reaction.
Carbon Dioxide Concentration:
- Acts as a limiting factor because $CO_2$ is a fundamental raw material required to synthesize glucose.
Temperature:
- The process of photosynthesis is controlled by biological catalysts known as enzymes.
- As the temperature increases, these enzymes work at a faster rate until they reach their optimum temperature.
- If the temperature rises above this optimum level, the enzymes undergo denaturation (they lose their shape and function), causing the rate of photosynthesis to fall sharply.

Broad Shape: Leaves are typically broad to provide a large surface area for the maximum absorption of sunlight.
Thin Profile: The thin nature of leaves ensures a short distance for gases to diffuse quickly into and out of the cells.
Transparent Upper Surface: This allows light to reach the internal photosynthetic cells effectively.
Palisade Layer Organization: Contains a high density of chloroplasts to maximize the absorption of available light.
Spongy Mesophyll Air Spaces: Internal gaps increase the efficiency of gas exchange within the leaf tissue.

Mechanism of Absorption:
- Plants absorb water from the soil through specialized structures called root hair cells.
- Root hair cells are characterized by their long and thin shape, which provides a significantly large surface area for absorption.
The Process of Osmosis:
- Water enters the root hair cells via osmosis.
- Osmosis is defined as the movement of water molecules from a region of high water potential to a region of lower water potential across a partially permeable membrane.
Pathway of Water:
- Once water is absorbed into the root hair cells, it moves through the root cortex and finally enters the xylem vessels for transport.

Xylem is a specialized transport tissue responsible for carrying water and dissolved mineral ions from the roots up to the leaves.
Structural Characteristics:
- Xylem vessels are composed of dead cells arranged end to end to form continuous tubes.
- The cell walls of the xylem are thick and reinforced with a substance called lignin, which provides structural support and prevents the vessels from collapsing under pressure.
Direction of Transport:
- Transport within the xylem is strictly unidirectional, moving in one direction only: $\text{Roots} \rightarrow \text{Stem} \rightarrow \text{Leaves}$ .

Transpiration: Defined as the loss of water vapor from the leaves, primarily occurring through the stomata.
Mechanism:
- Water evaporates from the moist surfaces of the mesophyll cells into the air spaces.
- The resulting water vapor then diffuses out of the leaf through the stomata.
Transpiration Stream:
- As water molecules leave the leaf, they pull more water molecules upward from the xylem to replace them.
- This continuous, upward movement of water from the roots to the leaves is referred to as the transpiration stream.

Temperature: High temperatures increase the rate of transpiration because water molecules evaporate more quickly at higher thermal energies.
Wind: Increased air movement removes water vapor from the immediate vicinity of the leaf surface, maintaining a steep concentration gradient and thus increasing transpiration.
Humidity: High humidity levels decrease the rate of transpiration because there is a smaller difference in moisture concentration between the inside of the leaf and the outside air.
Light Intensity: Bright light increases transpiration because it triggers the stomata to open wide to allow carbon dioxide to enter for photosynthesis, which simultaneously allows more water vapor to escape.

Phloem: A transport tissue responsible for the movement of synthesized food substances, specifically sucrose and amino acids, throughout the plant.
Structural and Functional Differences from Xylem:
- Unlike xylem, phloem is composed of living cells.
- Transport in the phloem is bidirectional, meaning it can occur in both directions.
Translocation:
- Translocation is defined as the movement of sucrose and amino acids through the phloem.
- These substances move from the "source" (the leaves, where they are produced) to the "sinks" (areas where they are needed for growth or storage, such as the roots, flowers, fruits, and growing shoots).