Exchange

veins

• Bring blood back to the heart • Blood flows in one direction • Valves stop backflow • Pressure low • Strong but thin walls • Large lumen • Thin outer wall

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capillaries

• Link between arteries and veins • Diffusion occurs • One cell thick wall • Short diffusion pathway • Small lumen • Steady blood flow

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Formation of tissue fluid

1. Blood in arteriolar end of the capillary as high hydrostatic pressure meaning it has a high water potential 2. Lots of ions are dissolved in the water 3. Molecules are forced out of the capillary because of the pressure (ultrafiltration) 4. blood

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Mass transport in plants

Vascular bundle – xylem and phloem are found here Xylem- hollow tube, dead cells, transport water Phloem – transport minerals

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Young root

• Central core of xylem often in an X shape • Phloem found in-between x arms • Provides strength for roots

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Stem

• Discrete in non woody plants • Xylem found towards inside • Layer of meristem cells

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xylem functions

• Long cells • Thick wall impregnated by lignin • Hollow tube • Lignin strengthens and waterproofs walls • Lignin pattern allows flexibility • Bordered pit allows for water to pass in and out of xylem vessels

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Phloem functions

Sieve tubes • Lined up end to end • Form a tube ]sugars dissolve in water forming sap • Cross walls are called sieve plates • Thin wall

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Phloem functions

Companion cells • Lie between sieve tubes • Large nucleus dense cytoplasm • Mitochondria • Carry out metabolic processes • Gaps allow for flow of minerals between cells.

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Moving between cells

When cells touch diffusion can occur. Water will move from a high WP to Low WP

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Apoplast pathway

• Water moves through spaces between cells

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Symplast pathway

• Passes through cytoplasm then passes through plasmodesmata of next cell

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Vacuolar pathway

• Water can pass through vacuole as well as cytoplasm

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Movement into roots

• Plant roots surrounded by soil • Root hairs use ATP to transport minerals from the soil to the root • Reduced WP • Water taken up across the plasma membrane by osmosis

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Movement into xylem

• Endodermis cells are a layer surrounding the xylem and block the apoplast pathway • Can move minerals from the cortex in the xylem • Creates a water potential • Water moves along symplast pathway from root hair cells across the cortex to the xylem

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Movement up the stem

Root pressure • Action of moving nitrate ions into xylem drives water in by osmosis • Pulls water up xylem

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Transpiration pull

• Water lost though leaves • Water must be replaced • Water molecule joined by hydrogen bonds • Water molecules move and pull upwards to high pressure • Tension causes • Cohesion tension theory

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Mechanism of translocation

Co- Transport • Sucrose diffuses by facilitated diffusion from photosynthesising tissues to companion cells • Hydrogen ions are actively transported from companion cells into the spaces in cell walls • Sucrose and hydrogen ions are co transported

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Mass flow

• Sieve tubes have a low WP • Sink cells sucrose is used up or converted to starch • Cells have low sucrose so sucrose is actively transported which lowers the WP • Water moves in these cells so hydrostatic pressure is lowered also • Water entering t

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Transfer into sink

• Sucrose actively transported by companion cells out of the sieve tube into the sink cells

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Tracer and ringer experiment

• Cylinder of bark removed from tree • Tracer used so photosynthesis products can be observed • Phloem content can be measured

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Mass flow of hypothesis

• Supports the idea sugar is made in photosynthesis and travels around the plant via the phloem • Sugar accumulates in source cells and diffuse into the xylem • Areas of low sugar require sugar which are called sink cells • Respiring cells eg flower

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