Biology - Leaf Structures and Functions PDF

# Chapter 12: Living Together - Plants, Animals and Ecosystems ## 12.1 What Are the Leaf Structures and Their Functions? ### Learning Outcomes - Identify the cellular and tissue structure of a dicotyledonous leaf, as seen in transverse section using the light microscope and describe the significan...

# Chapter 12: Living Together - Plants, Animals and Ecosystems ## 12.1 What Are the Leaf Structures and Their Functions? ### Learning Outcomes - Identify the cellular and tissue structure of a dicotyledonous leaf, as seen in transverse section using the light microscope and describe the significance of these features in terms of their functions, such as the distribution of chloroplasts for photosynthesis, stomata and mesophyll cells for gaseous exchange, and vascular bundles for transport. - Describe how carbon dioxide reaches mesophyll cells in a leaf. The leaf is sometimes referred to as one of the principal 'factories' of the world because it is where photosynthesis occurs. It is also the organ where other food substances such as fats and proteins are formed. These substances eventually become the food of all other living organisms, either directly or indirectly. As a 'factory', the leaf must be well equipped with the machinery for manufacturing food. How is the leaf adapted to its function of photosynthesis? ### External Features of a Leaf A typical green leaf consists of the leaf blade and the leaf stalk. * **Network of veins:** Veins carry water and mineral salts to the cells in the leaf blade. Veins also carry manufactured food from these cells to other parts of the plant. * **Leaf blade:** The leaf blade has a large flat surface compared to its volume. This enables it to obtain the maximum amount of light for photosynthesis. A large, thin leaf blade also means that carbon dioxide can rapidly reach the inner cells of the leaf. * **Leaf arrangement:** Leaves are always organised around the stem in a regular pattern. In general, leaves grow either in pairs (opposite one another on the stem) or singly in an alternate arrangement. This ensures that the leaves are not blocking one another from light and that each leaf receives sufficient light. * **Leaf Stalk:** The leaf stalk holds the leaf blade away from the stem so that the leaf blade can obtain sufficient light and air. In some leaves, for example, grasses and maize, the leaf stalk is absent. Such leaves have long leaf blades. ### Internal Structure of the Leaf A leaf, cut transversely | Structure | Function | | :------------------------------------------- | :------------------------------------------------------------------------------------------------------------------- | | **Upper Epidermis** | The leaf blade has an upper epidermis made up of a single layer of closely packed cells. The upper epidermis is covered on the outside by a waxy and transparent cuticle. | | **Mesophyll** | The mesophyll lies just between the upper and lower epidermis. <br/> It is the main site of photosynthesis. The mesophyll consists of two types of tissue: palisade mesophyll and spongy mesophyll. | | **Palisade mesophyll:** | - consists of one or two layers of closely packed, long and cylindrical cells. <br/> - contains numerous chloroplasts <br/> - specialised for photosynthesis | | **Spongy mesophyll:** | - cells with an irregular shape. <br/> - has numerous large intercellular air spaces among the loosely packed cells. <br/> - carries out photosynthesis but contains fewer chloroplasts than the palisade mesophyll. <br/> - cells covered with a thin film of moisture. <br/> - contains transport tissue - xylem and phloem which are grouped together to form a vascular bundle | | **Lower Epidermis** | Beneath the mesophyll is the lower epidermis. Like the upper epidermis, the lower epidermis consists of a single layer of closely packed cells. | | **Cuticle** | The cells are covered by an outer layer of cuticle, which reduces water loss through epidermal cells. | | **Stomatal pore (stoma)** | The lower epidermis contains many minute openings called stomata (singular: stoma). | | **Guard cell** | Guard cells are found in the lower epidermis. A pair surrounds each stoma and helps to regulate the rate of transpiration by opening and closing the stoma. Guard cells contain chlorophyll which are not present in other epidermal cells. | ### How Is the Leaf Adapted for Photosynthesis? | Adaptation | Function | |----------------------------------------------------------|-------------------------------------------------------------------------------------------------------------------| | Waxy cuticle on upper and lower epidermis | It reduces water loss through evaporation from the leaf. It is transparent for light to enter the leaf. | | Stomata present in the epidermal layers | Stomata open in the presence of light, allowing carbon dioxide to diffuse in and oxygen to diffuse out of the leaf. | | Chloroplasts containing chlorophyll in all mesophyll cells | Chlorophyll absorbs energy from light and transfers it to chemical stores of energy in glucose molecules. | | More chloroplasts in upper palisade tissue | More light can be absorbed near the upper leaf surface. | | Interconnecting system of air spaces in the spongy mesophyll | The air spaces allow rapid diffusion of carbon dioxide and oxygen into and out of mesophyll cells. | | Veins containing xylem and phloem situated close to mesophyll cells | Xylem transports water and mineral salts to mesophyll cells. Phloem transports sucrose away from the leaf. | ### How Do Guard Cells Control the Size of Stomata? Guard cells help to regulate the passage of gases between the leaf and the environment. For example, the guard cells can reduce the amount of water vapour escaping from the leaf. Stomata can close to reduce water loss even when a plant is in sunlight, such as on extremely hot days. In such situations, the excess loss of water causes the guard cells to become flaccid. Thus, the stomatal pore closes. As the guard cells control the opening and closing of the stomata, they regulate the passage of gases such as carbon dioxide and oxygen into and out of the leaf. **Note:** Stomata in the leaf are generally kept open during the day which allows carbon dioxide and oxygen to diffuse into and out of the leaf. [ Figure 12.5 Stomata on a leaf viewed under 750x magnification ] Guard cells manufacture glucose by photosynthesis. The increase in glucose concentration lowers the water potential of the cell sap in the cell. Water enters the guard cells by osmosis. They become turgid and more curved. This is because the cell wall around the stomatal pore (stoma) is thicker than the other parts of the cell. Hence, the cell curves around the stoma and the stoma opens. When the leaf loses too much water in strong sunlight, the stomata close. Excess evaporation of water causes the guard cells to become flaccid, thus the stoma closes. This prevents excessive loss of water by the leaf. [ Figure 12.6 Opening and closing of a stoma ]