Leaf Structure and Function, Slides of Biology

Download Leaf Structure and Function and more Slides Biology in PDF only on Docsity!

Form 5 KSSM Biology

Chapter 2

Leaf Structure

and Function

2.1 Structure of a Leaf

• Leaves are the main photosynthetic organs of a plant.
• They are adapted to carry out photosynthesis efficiently.
• The structure of a leaf can be divided into external and internal structures. External and internal structures of a leaf

How does a thin and flat lamina allow the leaf to carry out photosynthesis effectively? A flattened lamina provides a shorter diffusion distance for gases and allows sunlight to penetrate the mesophyll cells easily.

Internal structure of a leaf lamina The upper epidermis is usually thicker than the lower epidermis.

• The vascular bundles are present in the **veins of the leaves.

1. Xylem transports water and mineral** salts absorbed by the roots to the **leaves,
2. Phloem transports products of** photosynthesis away from the leaf. The air spaces within an eudicot leaf are relatively large due to the presence of loosely- packed spongy mesophyll cells. There are more stomata on the lower epidermis than on the upper epidermis. The mesophyll of eudicotyledon leaves consists of two regions, the palisade mesophyll and spongy mesophyll.

2.2 Main Organ for Gaseous Exchange

The necessity of gaseous exchange in plants

• Plants require oxygen from the external environment for cellular respiration, and carbon dioxide for photosynthesis. Both these gases also have to be removed from plant cells.
• Gaseous exchange between plant cells and the surrounding air occurs by diffusion mainly through stomata (singular: stoma) and lenticels.
• A stoma consists of a pore surrounded by a pair of guard cells, which control the opening and closing of a stoma by changing shape.

The mechanism of the opening and closing of the stoma depends upon the turgor pressure in the guard cells. The turgor pressure is determined by the

• uptake of potassium ions
• changes in sucrose concentration in the guard cells

What is observed if a whole

leaf is immersed in hot

water?

Explain your answer.

• Gas bubbles appear on the

leaf surface because the

higher temperature causes

the air in the air spaces of

the leaf to expand and

diffuse through the

stomata.

Mechanism of stomatal opening

• Higher solute potential causes the turgor pressure in the guard cells to increase, and the water potential to decrease. Therefore, water from the surrounding epidermal cells diffuses into the guard cells via osmosis.
• During the day, guard cells carry out photosynthesis to synthesise sugar at a fast rate. Energy is generated for the active transport of potassium ions (𝐾 ! ) into the guard cells from the neighbouring epidermal cells.
• At the same time, sucrose is transported from the surrounding mesophyll cells into the guard cells.

• In the absence of light or at night, photosynthesis does not occur.
• Sucrose concentration in the guard cells decreases.
• Potassium ions diffuse out of the guard cells into the neighbouring epidermal cells. - As a result, the guard cells shrink due to the loss in turgidity. - The thicker inner cell walls come together to close the stoma
• This causes the solute potential in the guard cells to decrease and the water potential to increase.
• The low turgor pressure in the neighbouring epidermal cells causes water to diffuse out of the guard cells to the surrounding epidermal cells by osmosis.

Mechanism of stomatal closing

Effects of water deficiency in plants on stomatal opening and

closing

• When the stomata are open, water is transpired, and carbon dioxide enters the leaf through the stomatal pore. It is essential for the stomata to stay open for plants to obtain sufficient carbon dioxide for photosynthesis.
• On a hot and dry day, water deficiency in plants occurs due to excessive water loss through transpiration.
• Most plants close their stomatal pores to reduce water loss. This response conserves water but at the same time, it also reduces carbon dioxide levels in the leaf.
• In order for photosynthesis to take place in plants and, at the same time, prevent the excessive loss of water, stomata open in response to high light intensity and low levels of carbon dioxide in the air spaces of the leaf.
• In general, stomata open during the day and close at night

Compare and contrast gaseous exchange in the leaf of a flowering plant

and in a human.

Flowering Plant Human The organ involved is the leaf. The organs involved are the lungs. Respiratory gases enter and leave the leaf through the process of diffusion. Respiratory gases enter and leave the lungs through the processes of inhalation and exhalation. High densities of palisade mesophyll and air spaces provide a large surface area for gaseous exchange A large number of alveoli provides a large surface area for gaseous exchange The moist surfaces of palisade mesophyll and spongy mesophyll allow oxygen to diffuse into the cells Alveoli are covered by a thin film of moisture to allow oxygen to diffuse through the wall into the blood capillaries

2.3 Main Organ for Transpiration

The necessity of transpiration in plants

• In plants, water is lost through a process called transpiration.
• Transpiration involves the loss of water vapour through evaporation from leaves.
• Water that is lost to the surroundings is replaced by the water absorbed from soil by plant roots. (a) Only 1% of the water is used by plant cells for photosynthesis and to maintain cell turgidity. (b) The remaining 99% evaporates from the leaves and is lost to the atmosphere through transpiration.