The Leaf

The Leaf - Functions

A leaf’s most important job is to provide a large surface area where photosynthesis can take place. If photosynthesis produces more glucose than the plant needs, the excess is converted into starch and stored in the leaf.

The Upper and Middle Leaf

The epidermis makes up the upper surface of the leaf. Its main function is protection, so it doesn’t perform photosynthesis. The cells of the epidermis secrete a waxy cuticle that helps reduce the amount of water that evaporates from the surface of the leaf. The mesophyll tissue (meso = middle) is between the upper and lower surface of the leaf. It consists of palisade cells (arranged in lines; packed with chloroplasts and mitochondria; perform photosynthesis) and spongy parenchyma cells (loosely packed to form a sponge-like network with open spaces containing gases needed or produced by photosynthesis: water vapour, oxygen, and carbon dioxide).

The Leaf - Functions

A leaf’s most important job is to provide a large surface area where photosynthesis can take place. If photosynthesis produces more glucose than the plant needs, the excess is converted into starch and stored in the leaf.

The Upper and Middle Leaf

The epidermis makes up the upper surface of the leaf. Its main function is protection, so it doesn’t perform photosynthesis. The cells of the epidermis secrete a waxy cuticle that helps reduce the amount of water that evaporates from the surface of the leaf. The mesophyll tissue (meso = middle) is between the upper and lower surface of the leaf. It consists of palisade cells (arranged in lines; packed with chloroplasts and mitochondria; perform photosynthesis) and spongy parenchyma cells (loosely packed to form a sponge-like network with open spaces containing gases needed or produced by photosynthesis: water vapour, oxygen, and carbon dioxide).

The Lower Leaf Surface

The lower surface of the leaf is made of an epidermis that is critical for the exchange of gases between the leaf and the outside environment. To allow gases to move in and out, guard cells are scattered across the lower surface of the leaf. They change their shape to open or close the stomata (singular stoma), thus allowing gases (carbon dioxide enters and oxygen and water vapour exit) to move in and out of open spaces in the spongy parenchyma cells. Stomata and guard cells play a significant role in transpiration.

Labe a diagram of the leaf.

The Leaf - X & P

The centre of the leaf contains xylem and phloem tissue arranged into vascular bundles, which form veins that dissect the interior of the leaf at regular intervals. At their tips, the vessels meet the open spaces in the parenchyma tissue. There, the xylem delivers water (in the form of water vapour) and minerals from the roots to the photosynthesizing cells. The cells in xylem die at maturity, so they do not use the plant’s energy stores. They leave behind their thick cell walls, forming long fibrous hollow pipes through which water can flow. The dead xylem cells are fortified with lignin, a hard substance that makes them strong, which helps them keep the plant upright. The phloem picks up sugars that have been produced in the leaf and delivers them to cells in the rest of the plant. Like xylem tissue, phloem tissue is made of vertically stacked tubes. Their cell walls are porous, which allows materials to be exchanged between the phloem and the neighbouring cells. Cells in phloem tissue are alive and actively (use energy) pump glucose from where it is made to where it is needed (ex. in the buds for growth, in the roots for storage). xylem - passive transport. phloem - active transport.

Chloroplast

A chloroplast is an organelle is specialized cells in the leaves of plants. They are filled with grana, which are stacks of little sacs called thylakoids, which contain light-trapping chlorophyll molecules. Chlorophyll is a pigment that gives plants their green colour and allows thylakoids to trap energy from the Sun. This light energy fuels photosynthesis, a chemical reaction where carbon dioxide from the air combines with water from the soil to produce glucose and oxygen. Carbon Dioxide + Water —> Glucose + Oxygen Chloroplasts can change their shape and location in a cell to increase the amount of light they capture.

Transpiration

Transpiration is the evaporation of water from a plant’s leaves. Xylem tissue ends when it reaches the leaves. Here, liquid water turns into water vapour in the spaces between the spongy parenchyma cells on the middle of the leaf. Some of this water will be used during photosynthesis, but most of it will evaporate when the stomata open to take in carbon dioxide and release oxygen. Transpiration makes room for more water from the xylem to move into the leaves, pulling the water column up. Transpiration is a much more important factor in water movement through the plant than root pressure, it pulls water up from the roots to the top of the plant. The transpiration of water from the leaves creates tension that pulls on the water column in the xylem. It is made possible by the cohesive and adhesive properties of water.

Cohesion

Cohesion is the ability of water molecules to cling to each other, which holds the water column in the xylem together.

Adhesion

Adhesion is the tendency of water molecules to stick, or adhere, to certain surfaces, such as the wall of a xylem vessel. The clinging of the water to the xylem walls helps to prevent the water from flowing back down to the roots due to the force of gravity.

Transpiration Steps

1. Water from the soil enters the xylem in the roots; tension in the water column extends from the roots to the leaves

2. The water column is held together by cohesion; adhesion keeps it in place

3. The Sun causes the water to evaporate

4. Transpiration (evaporation) of water from the leaves creates tension that pulls on the water column in the xylem