3.3 transport in plants


Vascular bundle

Vascular bundle= made of xylem and phloem tissue together 

Xylem - transport water and minerals. Thick, strong cell wall with lignin impregnated in the walls making the vessel dead. Lignin makes the cell waterproof strengthens the walls and prevents it from collapsing. This is called lignification. Lignin forms patterns preventing the vessel from being too rigid and allows some flexibility of the stem. In some places ligninfication does not happen leaving gaps called bordered pits which allow movement of water through vessels. Flow of water is only upwards 

Phloem - transports assimilates e.g sucrose or amino acids. The thin cell wall is made out of cellulose and living cells. The tissue consists of sieve tubes - made of sieve tube elements and companion cells. Companion cdells carry out metabolic processes needed to load assimilates into the sieve tube elements. They are linked by the fine strands called plasmodesmata. The flow of assimilates go up and down the plant. 

In the stem a cambium seperates the vascular bundle. The cambium is responsible for secondary growth of stems and roots. It can be refered as a meristem because it can differentiate into specialised cells e.g phloem and xylem. 

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Water transport

Osmosis and water potential= the more negative the water potential the water will move to the most negative number e.g water potential of 0 outside and -500 inside the water will move into the cell


Symplast pathway: movement of water through the living spaces of the cell=cytoplasm 

Vacuolar pathway: movement of water through the vacuole and the cytoplasm=slowest movement

Apoplast pathway: movement of water through cell wall=fastest movement

Before water moves through the root hair cell into the xylem vessel it has to go through the Casparian *****. The ***** is an impermeable layer of suberin. As a result all water in the apoplast pathway is forced into the symplast pathway. This means water has to travel through the cell memnrane so substances can be controlled 

Water moves from cortex into the medulla and xylem by osmosis. Water enters medulla, it can't pass back into cortex as apoplast pathway is blocked 

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Transpiration=is the lost of water from the stomatas on the underside of the leaf. This happens when water levels change in the guard cells around each stoma.

Stomata is closed - flaccid, water moves out of the vacuoles by osmosis. Stomata is open - turgid, water moves into the vacuoles by osmosis

How water enter and leaves the leaf:

Water enters the leaf through the xylem and moves by osmosis into the cells of the spongy mesophyll using apoplast pathway. Water evaporates from the cell wall of the spongy mesophyll. This creates water vapour in the air gaps between the cells. Water vapour moves by diffusion out of the leaf through the open. This relies on a difference n the concentration of water vapour molecules in the leaf compared with outside the leaf. This is known as the water vapour potential gradient. 

Processes that help move water up the stem: Root pressure=action of the endodermis moving minerals into the medulla and xylem by active transport. The pressure build up in the medulla forcing the water into and up xyelm. Transpiration pull=loss of water from leaves and water molecules having cohesive forces create a continuous column. Capillary action=adhesion forces to side of xyelm pulls the water up the side of the vessel. 

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Factors affecting transpiration

Potometer - the apparatus used to estimate the rate of transpiration. To make the results valid: do repeats and find the mean, set up under water to prevent air bubbles from forming, the the stem at an angel to provide a large SA, use leaf shoot so there is more stomatas. Dry leaves to maintain steep water vapour potential gradient. 

Light intensity - stomatas are open to allow gaseous exchange for photosynthesis. More light=increase transpiration rate

Temperature - higher temp=increase traspiration. Increase the ROE the water vapour potential in leaf rises. Increase ROD throught stomata cause water molecules have more KE

Humidity - higher humidity=decreased rate of water loss. Cause there will be a smaller water vapour potential gradient between air spaces inside and outside of the leaf

Air movement - air will carry water vapour away that has just diffused out of the leaf. This will maintain a high water vapour potentail gradient

Water avaliability - if there is little water in the soil, then the plant can't replace the water that is lost. So the stomata closes, the leaf wilts = transpiration decreases. 

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Translocation is the movement of assimlates around the plant from the source to the sink

Source - part of the plant that loads assimilates into the sieve tubes

Sink- part of the plant that removes assimlates from the sieve tubes e.g growing roots and stem

Active loading- sucrose is loaded into sieve tubes by an active process using enegry and ATP in the companion cells.

1) Energy is used to actively transport H+ ions out the companion cell creating a concentration gradient.  2) H+ ions diffuse back into the companion cell uses facillitated proteins. Only allow movement if H+ ions have a sucrose molecule.  3) As the concentration of sucrose in the companion cell increases it diffuses through plamodesmata into the sieve tube. 

Mass flow in the phloem

Water entering the sieve tube at the source increases the hydrostatic pressure. Water leaving the sieve tube at the sink reduces the hydrostatic pressure creating a pressure gradient. Since the sap in one tube is all moving in the same direction, this is mass flow. 

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Xerophytes and Hydrophytes

Xerophytes- water loss from transpiration is greater than water taken up. 

Adaptations - Thick waxy cuticle=impermeable. Stomata in pits (suken)=traps water vapour. Needles=smaller SA. Stomata on underside of the leaf=less exposure to sunlight. Stomata close during the day reduce diffusion of water vapour. Rolled leaves=traps water vapour. Hairs and spikes=traps water vapour so there is no gradient. Loss of leaves in the winter=no stomata present for water loss. 

Hydrophytes l either partially or completely submerged in water so no oxyen uptake

Adaptations - many large air spaces=keeping leaf afloat. Stomata only on the upper epidermis=so they are exposed to the air. Leaf stem has many air spaces=helps buoyancy but also allows oxygen to diffuse quickly to the roots for aerobic respiration. Have hydathodes= found at the tips and they can release water droplets which many evaporate from leaf surface. 

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