Gas exchange in plants

State that the main function of a leaf is photosynthesis and explain how its external features (shape, thinness) are adapted for this process.
Identify the key layers in a cross-section of a leaf (waxy cuticle, epidermis, palisade, spongy, lower epidermis) and describe the main function of each.
Describe the structure and function of stomata and guard cells in regulating gas exchange and water loss.

Leaves are the powerhouses of a plant.
Their main job is photosynthesis – the process of using sunlight, water, and carbon dioxide to make the plant's own food (glucose).
Leaves come in many shapes and sizes, but most share key features designed for this vital task.

Leaves are brilliantly adapted for making food:
- Green Colour: Most leaves are green because they contain chlorophyll, a pigment inside structures called chloroplasts. Chlorophyll absorbs the sunlight energy needed for photosynthesis.
- Thin: Being thin allows carbon dioxide gas to travel (diffuse) quickly from the air to the inner cells where photosynthesis happens.
- Large Surface Area: A wide, flat shape helps the leaf capture as much sunlight as possible.
- Veins: These contain tiny tubes: xylem brings water up from the roots, and phloem takes the sugar made during photosynthesis away to other parts of the plant.

Stomata (singular: stoma): These are tiny openings, usually found mostly on the underside of the leaf.
Function: They control the movement (diffusion) of gases into and out of the leaf.
- Carbon Dioxide: Enters the leaf through the stomata to be used in photosynthesis.
- Oxygen: Leaves the leaf through the stomata as a waste product of photosynthesis.
- Water Vapour: Also leaves the leaf through the stomata (this process is called transpiration).
Guard Cells: Each stoma is surrounded by a pair of special guard cells. These cells control whether the stoma is open or closed. Stomata are usually open during the day (when photosynthesis happens) and closed at night (to save water).
Why are most chloroplasts near the top? The palisade cells are packed near the top surface of the leaf to absorb the maximum amount of sunlight needed for photosynthesis.

Photosynthesis: The process used by plants to convert light energy into food (glucose) using sunlight, water, and carbon dioxide.
Chlorophyll: The green pigment found in chloroplasts that absorbs sunlight for photosynthesis.
Chloroplasts: Organelles within plant cells (especially in leaves) where photosynthesis takes place.
Xylem: Tubes in plant veins that transport water from the roots to the leaves.
Phloem: Tubes in plant veins that transport sugars (glucose) made during photosynthesis to other parts of the plant.
Epidermis: A single layer of tightly packed, transparent cells found on the upper and lower surface of a leaf.
Palisade Layer: The layer of cells in a leaf, packed with chloroplasts, where most photosynthesis occurs.
Spongy Layer: A layer of cells in a leaf with many air spaces, allowing for gas movement.
Stomata: Tiny pores, usually on the underside of a leaf, that control gas exchange.
Guard Cells: Pairs of specialised cells that surround and control the opening and closing of stomata.

Investigate how environmental factors such as light intensity, carbon dioxide concentration, and temperature can affect the rate at which photosynthesis occurs in a leaf.
Research different types of plant leaves and describe how their structures (internal and external) are specially adapted for survival in different environments (e.g., hot and dry, aquatic, low light).