Exchange And Transport Systems UNIT 2 Flashcards
Root hairs cells info
Each root hair is a long, thin extension of a root epidermal cell. They are efficient surfaces for the exchange of water and minerals due to a large surface area and thin surface layer. The cells arise from epidermal cells and grow into spaces around soil particles. In damp conditions they are surrounded by a soil solution which contains mineral ions, solution has high water potential. Root hairs have amino acids, sugars and mineral ions dissolved in them, has a lower water potential. Water moves by osmosis from soil into root hair cells down water potential gradient
How does water enter the plant
Through its root hair cells
Explain the movement of water through a plant
The soil around the roots has less negative water potential compared with the leaves which have a more negative water potential (because water is constantly evaporating from them). This creates a water potential gradient that keeps water moving through the plant in the right direction, from roots to leaves
Info on symplast route/ pathway
Goes through living parts of cells-cytoplasm. The cytoplasm of neighbouring cells connect through plasmodesmata (small channels in cell walls)
Info on apoplast route/ pathway
Goes through non-living parts of the root- cell walls. The walls are very absorbent and water can simply diffuse through them, aswell as passing through the spaces between them. When the water gets to the endodermis its path is blocked by a waxy strip of cell walls called the Casparian strip. Now the water has to take the symplast pathway. This is useful because it means the water has to go through a cell membrane. Cell membranes are able to control whether or not substances in the water get through. Once past this barrier, the water moves into the xylem.
2 ways in which water can move up a plant
Cohesion tension
Root pressure
Info on cohesion tension
Water evaporates from the leaves at the top of the xylem
This creates tension which pulls more water into the leaf
Water molecules are cohesive so when some are pulled into the leaf others follow. The water molecules bind to the xylem. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
Water enters the stem through the roots
Info on root pressure
caused by the active transport of ions and salts by the endodermis which then move into the xylem. This lowers the water potential in the xylem so water moves in via osmosis. This creates a pressure and pushes water already in the xylem further upwards. This pressure is weak and couldn’t move the water to the top of bigger plants by itself. But it helps in young, small plants where the leaves are still developing.
Transpiration
The evaporation of water from a plants surface
How does transpiration happen
Water evaporates from the moist cell walls and accumulates in the spaces between cells in the leaf
When the stomata open, it moves out of the leaf and down the water potential gradient
4 main factors that affect transpiration rate
Light
Temp
Humidity
Wind
How does light affect transpiration rate
The lighter it is the faster the rate. This is because the stomata open when it gets light. When it’s dark the stomata are usually closed
How does temp affect transpiration rate
The higher the temp the faster the rate. Warmer water molecules have more kinetic energy so move faster and evaporate from the cells of the leaf faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
How does humidity affect transpiration rate
The lower the humidity, the faster the rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration
How does wind affect transpiration rate
The windier it is, the faster the rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases transpiration
