Transport in plants

• Transport systems in plants are needed to meet the plants needs, as exchange via diffusion would be too slow. They also give the plant structure.
• The xylem tissue transports mineral ions + water via. the transpiration stream which only travels upwards.
• The xylem tissues are made up of singular elongated dead cells that form tubes called xylem vessels, as well as tracheids. They are both perforated by pits to allow water to leave the vessel.
• The xylem vessels contain spirals of lignin which allow adhesion between the vessel walls and the water. Cohesion allows the water to travel against gravity.
• The phloem transports sucrose up and down the plant via. translocation.
• The phloem consists of sieve tubes and surrounded by companion cells which assist in the loading of sugars.
• The companion cells surrounding the phloem are smaller and thinner and contain many organelles, mainly mitochondria, to provide energy for translocation. They also provide nutrients to the sieve cells.
• Factors affecting transpiration include the light intensity, as the stomata open up more to allow more CO2 in, so more water is lost. Temperature also affects the rate, as the higher the temp the faster the leaves evaporate water so increase the water potential gradient, so more water is lost. Humidity also affects the rate as the higher the humidity, the less steep the water potential gradient so less water is lost. The stronger the wind, the more water molecules being blown away from the plant, increasing the wate potential gradient therefore more water is lost.
• There are two pathways which the water enters the plant via the root hair cells: symplast pathway and apoplast pathway.
• The symplast pathway is when water travels through the cytoplasm via. osmosis and travels along the plasmodesmata to pass into each sequential cell.
• The apoplast pathway is when the water travels through the dead cell wall by a pressure gradient. At the endodermis, the casparian strip forces the water to take the symplast pathway.
• Water moves through the leaves by moving through the spongy mesophyll by osmosis, where it then evaporates as it leaves. The vapour diffuses out of the leaf.
• Translocation is the movement of assimilates (sucrose + amino acids). Sucrose moves from source to sink.
• Mass flow is the movement of fluids down a pressure/ temperature gradient.
• Active loading occurs during translocation when the hydrogen ions leave the companion cells by active transport into the surrounding tissue, then diffuse back via. cotransporter proteins, due to the created concentration gradient, bringing with them sucrose, increasing the concentration. They then move into the sieve tubes via. plasmodesmata as the concentration gradient is lower in here.
• Translocation in the phloem then occurs as the sucrose concentration increases the water potential decreases. Water proceeds to move from the xylem by osmosis. This increases the hydrostatic pressure in this area which creates a gradient pushing assimilated in either direction.
• Once at the sink, the sucrose diffuses into sink cells or moves via. active transport. It is either used for respiration or stored as starch. This reduces the concentration of sucrose, so more sucrose moves into this are to maintain the gradient. Water also moves into this area, then leaves to the surrounding tissues, due to the increased water potential.
• Xerophytes live in dry conditions and are adapted to reduce water loss.
• Hydrophytes live in wet conditions and are also adapted to prevent water loss, but not as much as xerophytes.
• Some adaptions of the xerophytes include deep roots to reach underground water sources and a large area of shallow roots to reach any rainwater. They also have fewer leaves or rolled leaves to reduce transpiration. Rolled leaves trap moist air which increases humidity to slow diffusion.
• Xerophytes also have thick waxy cuticles which are impermeable to water, preventing evaporation. They also have stomata which open only at night and close in the day. Water can also be stored in the succulent tissues allowing lots of storage.
• Hydrophytes adaptions include large leaves to allow photosynthesis, thin cuticle to allow water loss, absent xylem tissue as the whole plant tends to be submerged therefore all cells can absorb water. They also have no strengthening tissues as water provides support. They also have deep roots for anchorage to keep the plant in place. They have no root hair cells as the plant doesn't need to absorb excess water. Their stomata are also situated on the top layer so water can be lost.