Form 4 Biology Exam Papers 2023-2024 | Somaliland

Internal Structure of a Leaf:

When the leaf is cut in cross-section and seen under a microscope, the below structures are seen:

Figure: Internal Structure of a Leaf

1. Cuticle:

Cuticle is a transparent waxy layer covered on the upper surface of the leaf

Cuticle is made up of wax which is secreted by the epidermal cells

2. Epidermis:

Epidermis is a single layer of cells on the upper and lower surface of the leaf.

The epidermal cells of the leaf don’t contain chloroplast.

Leaf epidermis keeps the leaf’s shape.

Epidermis of the leaf is divided into, upper epidermis and lower epidermis

(A) Upper epidermis:

Upper epidermis is found on the upper surface of the leaf and can be seen when the leaf is cut in cross-section and observed under microscope.

Cells of the upper epidermis are transparent

No stomata are present

(B) Lower epidermis:

Lower epidermis is found on the lower surface of the leaf and can be seen when the leaf is cut in cross-section and observed under microscope.

Stomata are present

3. Mesophyll tissue:

Mesophyll is the tissue between upper and lower epidermis of the leaf

Mesophyll tissue consists of:

(A) Palisade cells (palisade mesophyll cells) ad

(B) Spongy cells (spongy mesophyll cells)

(A) Palisade Mesophyll:

Palisade cells are elongated cells and box-like shape

Palisade cells are found below the upper epidermis

Palisade cells are closely packed together to absorb maximum light.

Palisade cells contain maximum amount of the leaf’s chloroplasts

Palisade cells are the main region for photosynthesis in the leaf

Palisade cells receive carbon dioxide by the diffusion from air spaces in the spongy mesophyll.

Chloroplasts in the palisade cells are able to move within the cytoplasm.

During dim light, chloroplasts in the palisade cells move to the upper parts of the cell allowing them maximum absorption of light.

In bright light, chloroplasts in the palisade cells move to the lower parts of the cell for protection from the bleaching effects of intense light.

(B) Spongy Mesophyll:

Spongy cells are irregularly shaped cells with large air spaces between them

The air spaces between spongy cells are known as intercellular air spaces.

Air spaces between spongy cells allow gaseous exchange in the leaf.

Spongy cells allow of gaseous exchange in the leaf.

Spongy cells contain viewer chloroplasts

(4) Vascular bundles:

This is the leaf vein, made up of both xylem and phloem.

Xylem vessels bring water and minerals to the leaf cells.

Phloem vessels transport sugars and amino acids away from the leaf cells through the a process called translocation.

They also provide support for the leaf

(5) Stomata and Guard Cells:

Stomata are tiny pores found at the lower epidermis or underside of the leaf.

Stomata are always open during day time and close in the night time.

Each stoma is surrounded by a pair of guard cells.

Guard cells are pair of cells that surround each stoma.

Guard cells control whether the stoma is open or closed.

 Stomata opening during day light

1. Potassium ions (K⁺) move into the guard cells
2. Solute concentration in the guard cells increase
3. Water moves into the vacuoles of the guard cells (Endosmosis of water)
4. The guard cells expand and become turgid
5. The stomata open

 

Figure: Mechanisms of Stoma Opening

Stomata closing during night/dark time

1. Potassium ions (K⁺) move out of the guard cells
2. Solute concentration in the guard cells decrease
3. Water moves out of the vacuoles of the guard cells (Exosmosis of water)
4. The guard cells shrink in size and become flaccid
5. The stomata close

Figure: Mechanism of Stoma Closing

Figure: Internal Structure of a Leaf