- Nutrition is the way in which organisms obtain and use food.
Water is a very important substance obtained and transported throughout plants.
- Transpiration stream is the movement of water through a plant.
Transpiration stream is maintained by:
- Root pressure
- Osmosis is the movement of water from high water concentration to low water concentration across a semi-permeable membrane.
- Root pressure is the force exerted by water within the xylem tissue of the roots.
- Transpiration is the loss of water from the aerial parts of a plant.
Water and mineral uptake:
- Water enters the root hairs by osmosis, moving from high water concentration to low water concentration.
- Minerals dissolve easily in water and move into the root by diffusion – either by passive transport or by active transport (requires ATP).
- Water moves across the ground tissue and into xylem tissue.
- Water is then transported up the plant.
Cohesion-tension model of water transport
- John Joly and Henry Dixon were two Irish scientists who first proposed the cohesion-tension model of water transport.
- Water moving into the xylem tissue of the root causes a pressure build up – this is called root pressure.
- Root pressure contributes to the upward movement of water molecules.
- Water molecules have hydrogen bonds between them maintaining them in the liquid form – this is cohesion of the water molecules.
- Water molecules also tend to stick easily to the sides of the xylem vessels – this is called adhesion of the water molecules.
- Transpiration of the water molecules occurs mainly from the leaves – this pulls the column of water molecules upwards through the xylem, creating a tension in the water molecules.
Control of transpiration
Transpiration is controlled by:
- Waxy cuticle – prevents direct water loss from the surface of leaves.
- Stomata – controls the rate of transpiration by opening and closing.
- Lenticels – allows a small amount of transpiration but also allows oxygen in (for respiration) and carbon dioxide out (excretion).
Carbon dioxide uptake
Carbon dioxide is mostly taken in from the atmosphere through stomata (see picture below), but can be taken from the mitochondria (that carry out respiration, producing CO2) within the leaf cells (respiration).
Stomatal opening and closing
- Stomata open during the day and close during the night, in general
- High levels of water can cause the stomata to open – the plant tries to get rid of excess water
- Low water levels causes stomata to close – plant tries to conserve water
- Windy conditions can cause stomata to close – plant tries to conserve water
- Guard cells are the structures that control the opening – when they become turgid they curve away from each other – opening the stoma; when they lose water (become flaccid) they remain together and the stoma closes.
- Carbon dioxide levels are the main controlling factor in whether stomata open or close – high CO2 levels cause the stomata to close and low levels of CO2 cause them to open.
Transport of the products of photosynthesis
- Oxygen – it is a by-product of photosynthesis and is either released into the atmosphere or is used by the leaf cells in respiration
- Glucose – the main product of photosynthesis, can be used immediately in respiration, stored in the cell as starch or converted to sucrose and transported to another area of the plant.
Modified plant food storage organs
Plants store food in mainly in the form of starch and can store it in different areas, depending on the species.
Root storage organ:
Tap roots, such as the carrot and parsnip are root storage organs where a lot of starch is stored.
Stem storage organ:
Potatoes and asparagus are examples of plants that can store starch in their stems.
Leaf storage organ:
Onions and garlic store food in modified leaves – in the form of a tightly packed ‘bulb’.
Petiole storage organ:
Celery and rhubarb are examples of plants that store food in their petioles.