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Biology 1.2.3

AS OCR biology - transport in plants

Why do multicellular plants need transport systems? To provide cells with molecules for growth and respiration, and remove waste products. They have small surface-area-to-volume ratios and not all cells are in contact with the external medium.
What is the plant transport tissue? Vascular tissue, found as vascular bundles containing phloem and xylem
Describe distribution of vascular tissues in the stem of dicotyledonous, herbaceous plants Vascular bundles on the outer edge of the stem, xylem on the inner part of each bundle and phloem on the outer part, separated by a layer of cambium (meristem cells that are undifferentiated and can differentiate to form more xylem or phloem)
Describe distribution of vascular tissues in the leaf of dicotyledonous, herbaceous plants Vascular bundles form the midrib and the veins (which get smaller as they branch away from the midrib), xylem on top of phloem, separated by a layer of cambium cells
Describe distribution of vascular tissues in the roots of dicotyledonous, herbaceous plants Vascular bundle in the centre, xylem in a cross shape, phloem between each spike, surrounded by pericycle meristem cells, bounded by endodermis
What is the role of xylem tissue? To transport water and dissolved minerals up the plant for photosynthesis, and to support the plant
What is evidence for the role of xylem tissue? If celery is placed in water with blue die, the coloured water passes up the xylem not the phloem
What are the components of xylem tissue? A mixed tissue made up of xylem vessels, parenchyma cells and fibres
What are xylem vessels + structure? Continuous tubes of dead cells, end walls break down into transverse plate which breaks down. no cytoplasm or organelles. Lignin deposited in cell walls - strength + waterproof, annular, spiral, reticulate. Pits are gaps allow water to move laterally
How are xylem vessels adapted to function? Narrow to increase adhesion of water and allow capillary action. No organelles or cytoplasm to increase size of lumen and reduce resistance. Lignin deposits prevent water leaking out, and prevent vessel collapsing due to pressure from transpiration
What are fibres + parenchyma cells? Fibres - elongated, dead, lignified cells that provide mechanical strength and support. Parenchyma cells - loosely-packed living cells that provide turgidity and support, store starch and minerals, air spaces allow gas exchange.
What is the function of phloem? Phloem is involved in the translocation of assimilates - the movement of the products of photosynthesis (sucrose dissolved in water to form sap) up and down the plant.
How is sucrose transported? Sucrose is transported by mass flow from sources where it is in high concentration to sinks where it is in low concentration
What happens at a source? Sucrose is actively loaded into the companion cells at a source (e.g. a photosynthesising leaf) by H+ coupled cotransporter proteins and it diffuses into the sieve tube element
What happens at a sink? Sucrose is unloaded at a sink by facilitated diffusion - sinks can be growth centres e.g. meristem cells in root and shoot tips, or storage centres e.g. seeds
What is mass flow? Mass flow is when the plant uses energy to create pressure differences that allow assimilates to flow from a high pressure area to a low pressure area, down a hydrostatic pressure gradient.
What are the pressure differences in a plant? Source - high hydrostatic pressure - sucrose enters sieve tube element which lowers water potential, water moves in by osmosis. Sink - low hydrostatic pressure - sucrose diffuses out of sieve tube elements, high water potential, water diffuses out
What is some evidence for mass flow? Rate of transport is too high for simple diffusion. Presence of H+ coupled co transporter proteins. Radioactively-labelled carbon in CO2 appears in phloem. Ringing a tree causes sucrose to collect above the ring. Mouthparts of aphids enter phloem...
Continued Companion cells have many mitochondria. pH of companion cells is higher than surrounding cells. Concentration of sucrose is higher at source than sink.
What is some evidence against mass flow? Not all solutes in phloem sap move at same rate. Sucrose is moved to all parts of the plant at the same rate, rather than going to areas with low concentration more quickly. Role of sieve plates is unclear
What are the components of phloem tissue? Sieve tube elements, companion cells, fibres, parenchyma cells
What is the structure of sieve tube elements? Elongated living cells line up to form a long tube- cytoplasm and a few organelles (SER, plastids) along sides to minimise resistance, no nucleus. End cell walls have perforations called sieve plates, callose with pores in, longitudinal flow of sucrose
What is the function / structure of companion cells? One is found next to each sieve plate cell. Metabolically active - contain a nucleus, mitochondria for respiration, RER. Plasmodesmata allow communication and transport of materials between companion cells and sieve tube elements.
What is water potential, pressure potential and solute potential? Water potential - measure of the tendency for water molecules to move from one place to another. Pressure potential - pressure exerted on cell walls by water in a plant cell. Solute potential - pressure exerted on cell walls by solute in a plant cell
What is osmosis? The movement of water molecules from an area of high water potential to an area of low water potential, down a water potential gradient, across a partially permeable membrane.
What happens when a plant cell is placed in a hypotonic solution? Water potential in cell is lower than water potential in solution, so water moves into the cell by osmosis, cell becomes turgid (doesn't burst due to strength of cell wall)
What happens when a plant cell is placed in a hypertonic solution? Water potential in the cell is higher than in the solution, so water moves out of the cell by osmosis, cell becomes plasmolysed - cell membrane pulls away from cell wall
How does water move from the soil to the root hair cell? Root hair cells in epidermis absorb mineral ions from soil water by active transport using ATP energy, carried out by H+ coupled cotransporter proteins. Low water potential in root hair cells, water diffuses into cells by osmosis, across plasma membrnae
How does water move from the root hair cell to the xylem vessel? Water moves across the cortex, endodermis and pericycle and into the xylem by osmosis because the water potential inside the xylem vessel is lower than the water potential in the root hair cells. Apoplast, symplast, vacuolar pathways. Casparian strip.
What is the apoplast pathway? Water travels through the water-filled gaps in cellulose in cell walls, any dissolved minerals are carried as water doesn't pass through the plasma membrane?
What is the symplast pathway? Water travels through the cytoplasm, plasma membrane and plasmodesmata in cell walls (gaps in cell walls containing a thin strand of cytoplasm)
What is the vacuolar pathway? Water travels through the cytoplasm, vacuoles, plasma membranes and plasmodesmata in cell walls
What is the casparian strip + location + function? The cells in the outer layer of the endodermis have a waxy strip of suberin called the casparian strip, which blocks the apoplast pathway and forces water to pass through plasma membrane, transporter ions, plant controls which ions enter xylem
How does water move up the stem? Root pressure (minerals in xylem vessel lower water potential so water enters + forced up stem). Transpiration pull (low water potential in leaf - cohesion-tension theory, transpiration stream). Capillary action - water adheres to side of narrow xylem
What is transpiration? The loss of water from the aerial parts of the plant e.g. the leaves - water evaporated from the mesophyll cells and diffuses out of the stomata
What is the transpiration stream? The movement of water from the roots to the leaves and out of the plant through stomata, caused by a water potential gradient up the plant.
What are stomata? Pores between 2 guard cells that allow gases to diffuse in and out for gas exchange, respiration and photosynthesis.
Describe the stomata mechanism Proton pumps use energy from ATP to actively pump H+ ions out of guard cells. High conc of H+ ions outside. Co-transporter proteins have an affinity for K+ and H+ ions so they are both transported by facilitated diffusion into guard cells...
Continued Low water potential in guard cells, water enters by osmosis, turgid cells so thick, inelastic inner wall of guard cells produces a bend that opens the stomata.
What happens if not enough water is present? The guard cells are flaccid so stomata close.
What factors affect the rate of transpiration? Number of leaves-more leaves increases SA over which water vapour can be lost. Number/size/position of stomata-lots of large stomata on surface increases water loss.Thickness of hydrophobic waxy cuticle. Light intensity-high makes stomata open wider for..
Continued Photosynthesis. Temp- high increases rate of evaporation from mesophyll cells, faster diffusion, steeper conc gradient. Relative humidity-high reduces water potential gradient. Wind-removes water vapour so steep conc gradient. Water availability
How do you measure the rate of transpiration? Set up a potometer- leafy shoot in capillary tube in a water-filled beaker, top sealed with wax+rubber bung. Dry + measure leaves. Set up apparatus underwater to make it air/water tight. With syringe add water bubble - measure time taken to cover distance
What are xerophytes? Plants that are well adapted to reduce water loss so it can survive in conditions that are arid or promote excessive rates of transpiration
What are some adaptations of xerophytes? Small leaves reduce SA+water loss. Densely-packed spongy mesophyll reduces SA less water vapour in leaf air spaces. Thick waxy cuticle. Water potential gradient less steep by epidermal hairs, sunken stomata in pits, rolled leaves. High salt conc. Roots
What is an example of a xerophyte + adaptations? Marram grass lives on sand dunes - water drains away quickly, saline sand, windy. Adaptations: rolled leaves, sunken stomata in pits, epidermal hairs, moist air trapped in centre.
Created by: 11043



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