OCR AS Biology F211 Transport in plants.

Megan Vann
Note by Megan Vann, updated more than 1 year ago


A Level AS OCR Biology Note on OCR AS Biology F211 Transport in plants., created by Megan Vann on 12/05/2014.
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Transport in plants- F211.( Large) Plants need transport systems because they need to take substances from their environment and return waste to their environment. Every cell of a multicellular organisms needs a regular supply  of water and nutrients. Cells close to the edge of the organism( epithelium ) can gain this by simple diffusion but there are many cells inside the plant further from the supply that cannot gain the oxygen and nutrients they need by simple diffusion. This is because larger plants have a small SA:V ratio.What substances need to be moved?Water- from soil, to roots then to the rest of the plant. Minerals-from soil, to roots and then to the rest of the plant. Sugars- from leaves to rest of plant.Why is water important? Keeping cells turgid therefore supporting the plant. Required in photosynthesis . absorption of substances dissolved in the soil. cooling effect on the surface of leaves in hot climates (xerophytes) Vascular tissues:Phloem and Xylem: The Phloem transports sucrose from the leaves up and down the plant. The Xylem transports water and minerals up the plant. 


Stem: 8 Vascular bundles. Xylem on inside and Phloem on outside. Cambium in-between the two (meristems to make new xylem and phloem cells) 

Root: 5 phloem vessels. Endodermis controls the water and minerals that enter the xylem. Xylem on inside and phloem on the outside.

Structure of Xylem:used to transport water and minerals from the roots up to the leaves and other parts of the plant. Xylem tissue consists of tubes to carry to water and dissolved minerals , fibres to help support the plant and living parenchyma cells.Xylem vessels.Long cells with thick walls that have been impregnated with lignin. The lignin waterproofs the vessles but as a result the cells die and their end walls and contents decay. This leaves a long column of dead cells with no contents- a tube with no end walls- a xylem vessel. Lignin- Strengthens the vessel walls and prevents the vessel from collapsing. This keeps the vessel open at all times. ( no content In cells mean less resistance to flow)The lignin thickening forms patterns in the cell wall. This prevents the vessel from being to rigid and allows flexibility of the stem or branch. In some places this lignification is not complete. It leaves pores in the walls or the vessel which are called pits or bordered pits. These allow water to leave one vessel and pass into another adjacent vessel or pass into the living part of the plant.Adaptions of xylem to its function:1)made of dead cells aligned end-to-end to form continuous column.2)the tubes are narrow so the water column does not break easily and capillary action can be effective.3)pits in the lignified walls allow water to move sideways between vessels.4)lignin deposits in the wall in spiral, ring shaped or rectangular patterns allows the xylem to stretch as the plant grows and enables the stem or branch to bendThe flow of water is not impeded because: There are no end walls. There are no cell contents. There is no nucleus or cytoplasm. lignin thickening prevents the walls from collapsing.

Leaf: the vascular bundles form midrib an veins of the leaf. Xylem is located above the phloem on the surface nearer the lamina of the leaf.

The structure of Phloem:Phloem tissue consists of two types of cell: the sieve tube elements and the companion cells.

Sieve tubes:  sieve tube elements not true cells as they contain very little cytoplasm and no nucleus. lined up end to end to form a tube, in which transports sucrose. The sucrose is dissolved in water to form sap. The sieve tubes have very thin walls and are usually 5 or 6 sided.This tube contains cross walls at intervals. These cross walls are called sieve plated and are perforated by many pores to allow sap to flow.Companion cells: In between the sieve tubes. Have a large nucleus and dense cytoplasm. Have lots of mitochondria to produce ATP for active processes. Companion cells carry out the metabolic processes needed by the sieve tube elements. This includes using ATM as a source of energy to load sucrose into the sieve tubes. The cytoplasm of the companion cell and the sieve tube elements are liked through the plasmodesmata. These are gaps in the cell walls allowing communication an flow of substances between the cells.

Plant cells and water: Pure water has a water potential of zero. cells have a negative water potential because they contain dissolved salts and sugars. water molecules move from less negative regions to more negative regions. when a plant cell is full of water it is described as being turgid. when a plant cell looses all its water is it plasmolysis- the plasma membrane will lose contact with the wall.

What route can water take between cells? The apoplast pathway The symplast pathway The vacuolar pathway

The apoplast pathway: Water moves through the many water filled spaces between the cellulose molecules in the cell wall. The water does not pass through any plasma membrane.The symplast pathway: Water enters cell cytoplasm through the plasma membrane. It can then pass through the plasmodesmata from one cell to the next. Water can move through the continuous cytoplasm from cell to cell.The Vacuolar pathway: Similar to the symplast pathway but the water is not confined to the cytoplasm of the cells. It is able to pass through the vacuoles as well.

Water uptake from the soil: plant roots are surrounded by soil particles. The epidermis contain root hair cells that increase the surface area of the root. These cells absorb minerals from the soil by active transport using ATP for energy. These minerals reduce the water potential of the cell cytoplasm . Water is taken up across the plasma membrane by osmosis as the molecules move down the water potential gradient.Movement across the root: the movement of water across the root is driven by an active process that occurs at the endodermis. The endodermis consists of special cells that have a waterproof strip in some of their walls. This strip is called the casparian strip. The casparian strip blocks the apoplast pathway, forcing water into the symplast pathway. The endodermis cells move minerals by active transport from the cortext into the into the xylem. This reduces the water potential in the xylem. As a result, water moves from the cortext to the xylem by osmosis. This reduces the water potential in the cells just outside the endodermis. This combined with water entering the root hair cells, creates a water potential gradient across the whole cortex. Therefore water is moved along the symplast pathway from the root hair cells, across the cortex and into the xylem.Root hair cell(conc gradient is created)------>across the cortex------>through the endodermis----->into the xylem.

How does water move up the stem? There are three processes that help water move up the stem: Root pressure, transpiration pull and capillary action.Root pressure: The action of the endodermis moving minerals into the xylem by active transport drives water into the xylem by osmosis. This forces water into the xylem and pushes the water up the xylem but only a few meters.Transpiration pull: The loss of water by evaporation from the leaves must be replaced by water coming up the xylem. Water molecules are attracted to each other by forces of cohesion. These cohesion forces are strong enough to hold the molecules together in a long chain or column . As molecules are lost at the top of the column, the whole column is pulled up as a chain. This is the transpiration stream. The pull from above can create tension in the column of water, this is why xylem vessels must be strengthened with lignin. If the water column is broken in one xylem vessel, the water column can still be maintained through another vessel via the pits.Capillary action: The same forces that hold the water molecules together also attract the water molecules to the sides of the xylem vessels( the lignin). This is called adhesion. Because the xylem vessels are very narrow, theses forces of attraction can pull the water up the sides of the vessel.

Transpiration: evaporation of water from the leaves of a plant. osmosis of water from xylem to mesophyll cells. evaporation from mesophyll cells into inter cellular space. diffusion of water vapour through the stomata. The loss of water vapour through the stomata is due to the water potential gradient between the calls in the leaf and the air outside the leaf. The diffusion of water is down a water potential gradient towards a more negative water potential. The stomata close when the guard cells that control the stomata loose too much water and become flaccid.

The potometer: Measures the rate of transpiration/ water loss. Must make sure to select healthy plant/shoot. Must assemble all components under water to avoid air bubbles. Must cut the shoot at an angle under water to make sure no bubbles block the xylem. Make sure there in an air-tight seal between the shoot and the rubber tubing. Make sure the leaves aren't wet to make sure the stomata aren't blocked .

Factors that effect the rate of transpiration: The number of leaves- a plant with more leaves has a large surface area over which water vapour can be lost. Number, size and position of stomata- if the leaves have many large stomata, then water vapour is lost more quickly. If the stomata are on the lower surface water vapour is lost slower. Presence of cuticle- a waxy cuticle reduces evaporation from the leaf surface. Light- in light, the stomata open to allow gaseous exchange for photosynthesis. Temperature- will increase the water loss in three ways: Increasing the rate of evaporation so water potential in leaves increases. It also increases the rate of diffusion through the stomata. Also decreases the relative water vapour potential in the air, allowing more rapid diffusion of water out the leaf. Relative humidity- higher relative humidity in the air will decrease the rate of water loss, vapour blocking the pores. Air movement- carry away water vapour that has just diffused out the leaf increasing the rate of transpiration. Water availability- if there is little water in the soil then the plant cannot replace the water that is lost so stomata close to reduce the loss of water.

Xerophytes:Unavoidable losses: The loss of water by transpiration is unavoidable . This is because plants exchange gasses with the atmosphere via the stomata. During the day plants take up a lot of carbon dioxide to be used in photosynthesis. Oxygen, a bi-product of photosynthesis, needs to be removed so the stomata must be open during the day. While the stomata are open, this is an easy route for water vapour to escape through.Leaves of xerophytes: How they reduce water loss by transpiration. Leaf rolled up and the inner surface is covered in hairs-The rolled leaf and hairs both serve to trap moist air so reducing transpiration. In addition, a smaller surface area of leaf is exposed to the drying effects of the wind. Thick waxy cuticle- this reduces water evaporation from the surface. Trapped air in centre with high vapour potential - water will not move out because this would be going against the concentration gradient. Stomata in pits to trap air with moisture close to the stomata- reduces the gradient in the water vapour potential between inside and outside the leaf.

Translocation: is the transport of assimilates throughout the plant, in the phloem tissue. A source releases sucrose into the phloem. A sink removes sucrose from the phloem.How does sucrose enter the phloem? : Sucrose is loaded into the phloem by an active process. The companion cells use ATP to transport hydrogen ions out of their cytoplasm and into surrounding tissues. This sets up a diffusion gradient and the hydrogen ions diffuse back into the companion cells . This diffusion occurs through special cotransporter proteins. These proteins enable the hydrogen ions to bring sucrose molecules into the companion cells. As the concentration of sucrose builds up inside the companion cells, they diffuse into the sieve tube elements through the numerous plasmodesmata.Movement of sucrose along the phloem: Sucrose is actively loaded into the sieve tube element and reduces the water potential. Water follows by osmosis and increases the hydrostatic pressure in the sieve tube element . Water moves down the sieve tube from higher hydrostatic pressure at the source to lower hydrostatic pressure at the sink. Sucrose is removed from the sieve tube element by surrounding cells, increasing the water potential of the sieve tube element Water moves out the sieve tube and reduces the hydrostatic pressure. This process produces a flow of water along the phloem. This flow carries sucrose and other assimilates along the phloem. This is called mass flow.Evidence for the mechanism of translocation: radioactively labelled CO2 appears in the phloem. ringing a tree to remove the phloem results in sugars collecting above the ring. an aphid feeding on a plant stem can be used to show that the mouthparts are taking food from the phloem the companion cells have many mitochondria. translocation can be stopped by using a metabolic poison that inhibits the formation of ATP. the rate of flow of sugars in the phloem is so high that energy must be needed to drive the flow. the pH of the companion cells is higher than that of surrounding cells( because of CO2). the concentration of sucrose is higher in the source than in the sink. Evidence against the mechanism of translocation: not all solutes in the phloem sap move at the same rate. sucrose is moved to all parts of the plant at the same rate, rather than going more quickly to areas with a low concentration. the role of sieve plates is unclear.



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