how water enters a plant
- Water has to get from the soil, through the root and into the xylem
- the system vessels that transports water throughout the plant.
- the bit of the root that absorbs water is covered in root hairs. these increase the roots surface area, speeding up water intake.
- once its absorbed, the water has to get through the cortex, including the endodermis, before it can reach the xylem.
how water travels into the xylem
water can travel through the roots into the xylem by two different paths:
- the symplast pathway - goes through the living parts of cells - the cytoplasm. the cytoplasm of neighbouring cells connect through plasmodesmata (small gaps in the cell walls).
- the apoplast pathway - goes through the non-living parts of the root - the cell walls. the walls are very absorbent and water can simply diffuse through them, as well as passing through the space between them.
- when water in the apoplast pathway gets to the endodermis cells through, its path is blocked by a waxy strip in the cell walls, called the caspaarian 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.
both pathwayss are used, but the main one is the apoplast pathway because it provides the least resistance.
Symplast and Apoplast
how water moves up a plant
water can move up a plant in two ways:
- cohesion and tension help water move up plants, from roots to leaves, against the force of gravity.
- water evaporates from the leaves at the top of the xylem
- this creates tension (suction) which pulls more water into the leaf
- water molecules are cohesive (they stick together) so when some are pulled into the elaf others follow. this means the whole column of water in the xylem from the leaves down to the roots moves upwards.
- water enters the stem through roots
- root pressure also helps move the water upwards. when water is transported into the xylem in the roots, it creates a pressure and shoves water already in the xylem further upwards. this pressure is weal and couldn't move water to the top of bigger plant by itself. but it helps, especially in young, small plats where the leaves are still developing
transpiration is the evaporation of water from a plants surface, especially the leaves.
- water evaporates from the moist cell walls and accumulates in the space between cells in the leaf.
- when the is stomata open, it moves out of the leaf down the concentration gradient (theres more water inside the leaf than in the air outside)
transpiration is really a side effect of photosynthesis - the plant needs to open its stomata to let in CO2 so that it can produce glucose, but this also lets water out.
factors affecting transpiration rate
- light - the lighter it is the faster the transpiration rate. this is because the stomata open when it gets light. when its dark the stomata are usually closed, so there's little transpiration.
- temperature - the higher the temperature the faster the transpiration rate. warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. this increases the concentration gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.
- humidity - the lower the humidity, the faster the transpiration rate. if the air around the plant is dry, the concentration gradient between the leaf and the air is increased, which increases transpiration.
- wind - the windier it is, the faster the transpiration rate. lots of air movment blows away water molecules from around the stomata. this increases the concentration gradient, which increases the rate of transpiration.