The leaf's structure:
Important featues of a leaf
How leaves are adapted for efficient photosynthesis:
1) They are broad, so there is a large surface area exposed to light
2) They are thin, so carbon dioxide and water vapour have a short distance to travel to reach the photosynthesising cells
3) There are air spaces in the spongy mesophyll layer which allows gases like carbon dioxide and oxygen to move easily between cells. Also means they have a large surface area to volume ratio for gas exchange
4) They contain lots of chlorophyll (a pigment that absorbs light energy for photosynthesis). Chlorophyll is found in the chloroplasts, most of which are in the palisade mesophyll layer at the top of the leaf where they can get most light
Important featues of a leaf (cont)
5) The upper epidermis is transparent so that light can pass through it to the palisade mesophyll layer
6) The lower surface is full of little holes called stomata. They're there to let gases like carbon dioxide and oxygen in and out. They also allow water to escape (known as transpiration)
7) Leaves have a network of veins. These deliver water and other nutrients to every part of the leaf and take away food produced by the leaf. They also help to support the leaf structure
Leaf palisade cells
How leaf palisade cells are designed for photosynthesis:
1) They're packed with chloroplasts, containing chlorophyll
2) Their tall shape means a lot of surface area is exposed down the side for absorbing carbon dioxide from the air in the leaf
3) The tall shape also means there's a good chance of light hitting a chloroplast before it reaches the bottom of the cell
Diffusion in leaves
Plants exchange gases through diffusion
Definition: Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration
When plants photosynthesise they use up carbon dioxide from the atmosphere and produce oxygen.
When plants respire they use up oxygen and produce carbon dioxide.
This means there is a lot of movement of gases going into and out of plants, and this movement happens by diffusion.
Diffusion of gases in leaves is vital for photosyn
When the plant is photosynthesising it uses up lots of carbon dioxide, meaning there is hardly any in the leaf. This makes more carbon dioxide move into the leaf, through diffusion.
At the same time, lots of oxygen is being produced as a waste product from photosynthesis. Some of the oxygen is used in respiration, and the rest diffuses out the stomata.
At night time, no photosynthesis takes place because there is no sunlight. This means that lots of carbon dioxide is being made and lots of oxygen is being used up, through respiration. This means at night time, carbon dioxide diffuses out of the leaf, and oxygen diffuses into the leaf.
Water vapour also escapes from the leaf by diffusion. The diffusion of water out of a leaf is known as transpiration.
Osmosis is a type of diffusion.
Definition: Osmosis is the movement of water molecules across a partiallypermeable membrane from a region of higher water concentration to a region of lower water concentration.
A partially permeable membrane is just a membrane with very small holes in it. It is so small only tiny molecules like water can pass through. Large molecules like sucrose cannot pass through.
Water molecules pass through the partially permeable membrane in both directions, because water molecules move about randomly all the time.
However, if there are more water molecules on one side than the other side of the partially permeable membrane, more water molecules may pass through in one direction than the other.
Here is a diagram where osmosis will be happening:
Because osmosis will be happening, it will cause more water particals to move to the right in this diagram. This means the stonger concentrated solution will become more dilute. The water acts like it's trying to 'even up' the concentration on either side of the membrane.
Turgor pressure in plant cells
There are four types turgor pressure in plant cells:
- Turgid, Normal, Flaccid and Plasmolysed
This diagram shows the different types:
Turgor pressure in plant cells (cont)
- When cells absorb water they become plump and swollen, the contents push against the cell wall and the pressure helps to support the plant tissue.
- If there's no water in the soil, a plant starts to wilt. This is because the cells lose their turgor pressure and become flaccid.
- If a plant is really short of water the cytoplasm inside the cells start to shrink and the membrane pulls away from the cell wall. The cell is now plasmolysed. The plant cell doesn't lose its shape completely though, as it has an inelastic cell wall.
- Plants aren't too bothered by changes in the amount of water because the inelastic cell wall keeps everything in place.
- Animal cells don't have a cell wall.
- This means if an animal cell takes in too much water, it bursts - this is known as lysis.
- If an animal cell loses too much water it gets shrivelled up - known as crenation.
- Animal cells have to keep a constant amount of water in their cells, whilst plants are a bit more tolerant of periods of drought.
How root hairs take in water by osmosis:
- Each branch of a root is covered in millions of microscopic hairs, giving the plant a large surface area to volume ratio, for absorbing maximum water from the soil.
- There is usually a higher concentration of water in the soil than there is inside the plant, so the water is drawn into the root hair cell by osmosis.
Transpiration is the loss of water from the plant.
1) Transpiration is caused by evaporation and diffusion of water from inside the leaf.
2) This creates a slight water shortage in the leaf, so more water is drawn up from the rest of the plant through the xylem vessels to replace it.
3) This in turn means more water is drawn up from the roots, and so there is a constant transpiration stream of water through the plant.
Transpiration is a side-effect of the way leaves are adapted for photosynthesis. They have to have stomata in them so that gases can be exchanged easily. Because there is more water inside the plant than in the air outside, the water escapes from the leaves through the stomata.
Benefits of transpiration
1) There is a constant stream of water from the ground which helps to keep the plant cool
2) It provides the plant with a constant supply of water for photosynthesis
3) The water creates turgor pressure in the plant cells, which help support the plant and stop it wilting
4) Minerals needed by the plant can be brought in from the soil along with the water