Lesson 2: LeavesUnit 5: Plants

What are the functions of leaves?

Site of photosynthesis Leaf cells absorb energy from sunlight through organelles called chloroplasts.

When solar energy is absorbed, the plant cell uses it to convert carbon dioxide and

water into glucose and oxygen

Functions of Leaves

Gas exchange Gas exchange between the interior of

the plant and its environment Epidermis contains pores through which

gases can pass in and out. In photosynthesis, the plant uses CO2

and releases O2 and H2O vapours through these pores.

Functions of Leaves

Storage The sugars (glucose) that are produced

during photosynthesis are stored in the leaves as well as other parts of the plants

Function of Leaves

Protection from predators Ex: cacti leaves are reduced to sharp

spines; some leaves have surface hairs or toxins that prevent herbivores from eating it

Structure of Leaves - External

Structure of Leaves - Internal

Leaf Structure

The epidermal cells are tightly packed in a single layer and covered by a waxy coating called a cuticle.

Leaf Structure

The cuticle prevents water loss provides a physical barrier against

bacteria, moulds and insects.

Epidermis

The Epidermis

Epidermal cells do not contain chloroplasts, so they do not perform photosynthesis.

Epidermal cells are transparent, so light can pass through them to the cell within the leaf.

Mesophyll

Chloroplasts are found mainly in the cells of the mesophyll, which means ‘middle leaf’. This ground tissue is specialized for photosynthesis. There are two parts to the mesophyll:

 

Mesophyll

palisade mesophyll (region directly under the upper epidermis). The tightly packed, elongated arrangement maximizes the amount of light the plant can collect for photosynthesis

  spongy mesophyll (region directly beneath

the palisade). The loosely packed layer with lots of air spaces allow for gas exchange between the mesophyll cells and the atmosphere through stomata.

 

Guard Cells & Stomata

A stoma (plural: stomata) is an opening in the epidermis of a leaf, through which gases pass in and out.

Two kidney-shaped cells, called guard cells, control the opening and closing of a stoma.

In terrestrial plants, most of the stomata are in the lower epidermal layer, below the spongy mesophyll.

When stomata are open, carbon dioxide can enter the leaf and oxygen can escape. This helps the plant photosynthesize.

Ideally, plants would open their stomata whenever it was sunny. However, when stomata are open, water vapour can also escape.

How is this problematic?

A very thin leaf could dry out and die very quickly on a sunny or windy day. Preventing too much water loss is therefore a major concern for many plants.

If there is a good supply of water within the leaf, the guard cells expand and bend apart -- opening the stomata.

If there is a shortage of water, the guard cells become soft and collapse -- closing the stomata.

Guard cells also have a mechanism that responds to light levels. This lets them close the stomata at night when carbon dioxide is not needed because there is not light for photosynthesis.

Vascular Tissue in Leaves In leaves, the vascular tissue (xylem and

phloem) are arranged into veins that run through the spongy mesophyll.

Xylem carries water, mineral and nutrients upwards from the roots to the top of the plant.

Phloem carries food(sugars) from one part of a plant to another. It can move upwards or downwards.

 

Photosynthesis & Cellular Respiration

Photosynthesis Light is used with Carbon dioxide from the air and

water from the root system to produce glucose and oxygen. This occurs during the day.

6CO2 + 6H2O + Light Energy = C6H12O6 + 6O2

 Cellular Respiration The glucose is broken down (with oxygen from the

air) to create ATP with carbon dioxide and water as by products.

C6H12O6+6O2 ----------> 6CO2+6H2O+36ATP 

Photosynthesis

https://www.youtube.com/watch?v=eo5XndJaz-Y