Transport in plants (module 3, section 3)


1. Xylem and Phloem

Why do plants need transport systems?

Plants need substances like water, minerals and sugars to live as well as needing to get rid of waste substances. plants are multicellular so have a small SA:V ratio. they are also relatively big so have a high metabolic rate. exchanging substances by direct diffusion would be too slow to meet their metabolic needs. plant transport systems are needed to move substances to and from individual cells quickly.

Location of Xylem and Phloem tissues:

Xylem tissue: Transports water and mineral ions in solution from roots to eaves. Also for support. Phloem tissue: mainly transports sugars (also in solution) up and down plant.

Roots: Xylem and Phloem are in centre to provide support as it pushes through soil.

Stem: Xylem and Phloem are near outside to provide a sort of 'scaffolding' that reduces bending.

Leaves: Xylem and Phloem make up a network of veins which support the thin leaves.

Tip: Plants also need carbon dioxide, but this enters at the leaves(where it is needed)

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1. Xylem and Phloem

Adaptations of Xylem Vessels:

Very long tube like structures formed from cells (vessel elements) joined end to end. No end walls = an uninterupted tube that allows water to pass up through the middle easily. Cells are dead so no cytoplasm. Cell walls thickened with lingin which stops the walls collapsing in. Lignin deposited in different ways, e.g. spirals or distinct rings. Allows flexibility and prevents stem breaking. Amount of Lignin increases as plant gets older. Water and Mineral ions move in and out of small vessels where ther's no Lignin (through small pits). This is how other cells are supplied with water.

Adaptations of Phloem Tissue:

Tube like structures. Transports solutes (dissolved substances). Purely for transport, not structure. Contains Phloem fibres, Phloem Parenchyma, Sieve tube elements and companion cells.

Sieve Tube Elements: Living cells, form tube for transporting sugars through, joined end to end to form sieve tubes. 'sieve' parts = end walls with holes in to let solutes pass through. Usually for living cells, STE have no nucleus, very thin layer of cytoplasm and few organelles. Cytoplasm of adjacent cells connected through holes in sieve plates.

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1. Xylem and Phloem

Companion Cells: Lack of nucleus and other organelles in sieve tube elements means they can't survive on their own. For every Sieve tube element, there is a companion cell. Companion cells carry out the living functions for both themselves and their sieve cells. E.g. They provide the energy for the active transport of solutes.

Tip: Phloem tissue also transports small amounts of amino acids, certain ions and plant hormones - but mainly sugars.

Examination of stained plant tissue:          Practical

Plant Dissection: 1. Use a scalpel to cut a cross-section (transverse or longitudinal) of stem. Cut as thinly as possible. 2. Use tweezers to gently place cut sections in water to stop them from drying out until use. 3. Add drop of water to microscope slide, add plant section and carefully add 1 or 2 drops of a stain, e.g. Toluidine Blue O (TBO), and leave for about 1 minute. 4. Carefully apply cover slip so you have created a wet mount. 5. View specimen under a light microscope and draw a labelled diagram of what you observe.

Cells will look different (under microscope) depending on stain used. E.g. Staining with TBO will make Lignin in walls of Xylem cells blue-graan. The Phloem cells and rest of tissue will generally appear varying shades of pink and purple. 

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2. Water Transport

How does water enter a plant?

Water enters plant through the root hair cells and then passes through the root cortex, including the endodermis, to reach Xylem. Water is drawn into roots via Osmosis. Water always moves from areas of higher wate potential to areas of lower water potential (down a concentration gradient). The soil around roots generally has high WP and leaves has lower WP. This creates a WP gradient that keeps water moving through the plant in the right direction, from roots (high) to leaves (low).

Water Transport through the root:

Water travels through the roots into Xylem by two different pathways:

The Symplast Pathway: Goes through the living parts of cells - the cytoplasm. The cytoplasm of neighbouring cells connects through plasmodesmata (small channels in cell walls). Water moves through the Symplast Pathway via Osmosis.

Tip: Osmosis is the diffusion of water molecules across a partially permeable membrane from area of high WP to area of lower WP.     (WP = Water Potential)

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2. Water Transport

The Apoplast Pathway: Through non-living pats of cells - Cell walls. The walls are very absorbent and water can simply diffuse through, as well as passing through the spaces between them. The water can carry solutes and move from areas of high Hydrostatic pressure to areas of low Hydrostatic pressure. This is an example of Mass Flow.                                                             When water in apoplast pathway gets to endodermis cells in root, its path is blocked by a waxy ***** in the cell walls called the Casparian *****. Now the water has to take the Symplast pathway. This is useful because it means the water has to go through a plasma (cell-surface) membrane. Cell membranes are partially permeable and able to control whether or not substances in water get through. Once past the barrier, water moves into Xylem. 

Tip: Both pathways are used, but main one is Apoplast pathway because it provides least resistance.

Water transport through the leaves:

At leaves, Water leaves Xylem and moves into cells mainly by apoplast pathway. Water evaporates from cell walls into spaces between cells in leaf. When Stomata open, water evaporates. Evaporation of water from plant's surface is called Transpiration.

(starred out word is: s t r i p)

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2. Water Transport

Tip: Water passes from Xylem into leaf cells via Osmosis.

Tip: 'stomata' is plural. 'stoma' is singular. Stomata = tiny pores in surface of leaf.

Tip: 'mesophyll cells' - they're just a type of leaf cell, don't be put off.

Water movement up a plant:

Movement of water from roots to leaves = Transpiration Stream. Mechanisms:

Cohesion and Tension: Help water move up plant, against gravity. 1. Water evaporates from leaves at 'top' of Xylem. 2. Creates a tension (suction) which pulls up more water. 3. Water molecules stick together (cohesion). 4. Water enters through root cortex.

Adhesion: As well as being attracted to each other, water molecules are attracted to the walls of the Xylem vessels. This helps water to rise up through Xylem vessels.

Tip: Air bubbles can form in Xylem which block column of water, prventing water from reaching cells. w/o enough water, cells = flaccid and wilt. 

Tip: Cohesion and Tension allow mass flow of water over long distances up stem.

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3. Transpiration

Why does Transpiration Happen?

Happens as a result of Gas Exchange. A plant needs to open its stomata to let in carbon dioxide so that it can produce glucose (by photosynthesis). This also lets water out as there is a higher concentration of water in leaf rather than air outside, so water moves down concentration gradient when stomata opens. therefore, Transpiration is a side effect of Gas exchange needed for photosynthesis.

Factors affecting Transpiration rate:

1. Light Intensity: Lighter it is, faster rate of transpiration = stomata open when light, the lighter the wider they open. when dark, they're closed so little transpiration.

2. Temperature: Higher temperature, faster rate.Warmer molecules have more energy so evaporate from cells inside leaf faster. Increases WP between inside and outside leaf, making water diffuse out leaf faster.

3. Humidity: Lower humidity, faster rate. If air around eaf is dry, WP gradient between leaf and air increased, increases rate of transpiration.

4. Wind: Windier, faster rate. Air movement moves water molecules from stomata. Increases WP gradient, increases rate of Transpiration.

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3. Transpiration

Estimating Transpiration rate - potometers:          Practical:

Measures water uptake by a plant, but its assumed that water uptake is directly related to water loss by leaves. You can use it to estimate how different factors affect transpiration rate.

Using a potometer: (make sure you've carried out risk assessment before you begin) 

1. Cut a shoot underwater to prevent air from entering Xylem. Cut it at a slant to increase surface area available for water uptake. 2. Assemble potometer in water and insert the shoot under water so no air can enter. 3. Remove apparatus from water but keep end of the capillary tube submerged in beaker of water. 4. Check the apparatus is watertight and airtight. 5. Dry the leaves, allow time for the shoot to acclimatise and then shut the tap. 6. Remove the end of cappillary tube from beaker of water until one air bubble has formed, then put end of tube back into the water. 7. Record starting position of the air bubble. 8. Start a stopwatch and record distance moved by the bubble per unit time, e.g. per hour. The rate of air bubble movement is an estimate of the transpiration rate. 9. Remember, only change one variable (e.g. temperature) at a time. All other conditions must be kept constant (e.g. light intensity, humidity).

Tip: The air bubble is sometimes called the air-water meniscus.Tip: You need to carry out repeats to improve precision of results and to help identify and anomalies.

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3. Transpiration

Tip: Transpiration rate is not same as water uptake - some water is used for photosynthesis and to support the plant, and some water is produced during respiration.

Adaptations in Xerophytic Plants:

Xerophytes are plants like cacti and marram grass. they're adapted to live in dry climates. Adaptations prvrent them loosing too much ater by transpiration.

E.g: Cacti - 1. Have thick, waxy layer on epidermis - reduces water loss by evaporation because layer is waterproof (water cannot move through it). 2.They have spines instead of leaves - this reduces the surface area for water loss.3.Cacti also close their stomata at hottest times of day when transpration rates are highest.                                                                                        Marram grass - 1.Has stomata sunk in pits, so they're sheltered from wind. This traps moist air in pits and helps slow transpiration down by lowering WP gradient. 2.Layer of 'hairs' on epidermis - traps moist air round stomata, reduces WP gradient between leaf and air, slowing transpiration. 3.Rolls leaves in hot/windy conditions - traps moist air, slowing down transpiration. Also reduces exposed surface area for losing water and protects stomata from wind. 4. Thick, waxy layer on epidermis to reduce water loss by evaporation (like Cacti). (Water cannot move through it - waterproof)

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3. Transpiration

Adaptations in Hydrophytic Plants:

Hydrophytes are plants like water lillies, which live in aquatic habitats. Grow in water so don't need adaptations to reduce water loss, but they do need adaptations to help them cope with a low oxygen level.

1. Air spaces in the tissues help the plants float and can act as a store of oxygen for use in respiration. E.g. water lillies have large air spaces in their leaves. This allows them to float on surface of water, increasing amount of light they receive. Air spaces in roots and stems allow oxygen to move from the floating leaves down to parts of the plant that are underwater.

2. Stomata are usually only present on upper surface of floating leaves. This helps maximise Gas Exchange.

3. Hydrophytes often have flexible leaves and stems - these plants are supported by the water around them, so they don't need rigid stems for support. Flexibility helps to prevent damage by water currents.

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4. Translocation

What is Translocation?

The movement of dissolved substances (e.g. sugars like sucrose, and amino acids) to where they're needed in a plant. Dissolved substances are sometimes called assimilates. Translocation is an energy-requiring process that happens in the phloem. 

Translocation moves substances from 'sources' to 'sinks'. The source of a substance is where it is made (it's at a high concentration there). The sink is the area where it is used up (it's at a lower concentration there). E.g. source for sucrose is usually the leaves (where it's made following photosynthesis), and the sinks are other parts of plant like storage organs and meristems (areas of growth) in roots, stems and leaves. 

Some parts of a plant can be both sink and source. E.g. Sucrose can be stored in roots. During growing season, sucrose is transported from roots to leaves to provide leaves with energy for growth. (In this case, roots are source and leaves are a sink.)

Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink (e.g. by breaking them down or making them into something else). This makes sure there's always a lower concebntration gradient at the sink than......

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4. Translocation

..... at the source. E.g. In potatoes, sucrose is converted to starch in the sink areas, so there's always a lower concentration of sucrose at the sink than inside the phloem. This makes sure a constant supply of new sucrose reaches the sink from the phloem.                                                  In other sinks, enzymes such as invertase break down sucrose into glucose (and fructose) for use by the plant - making sure there's a lower concentration of sucrose at the sink.  

Tip: Assimilates are substances that become incorporated into the plant tissue.

Tip: Sugars transported as sucrose because sucrose is both soluble and metabolically inactive - so it doesn't get used up during transport.

Tip: The phloem transports solutes up and down a plant from sources to sinks.                           

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4. Translocation

Mass Flow hypothesis:  (it's a theory)

1. Source -  Active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (e.g. the leaves). This lowers the WP inside sieve tubes, so water enters the tubes by osmosis from Xylem and Companion cells. Creating a high pressure inside sieve tubes at source end of Phloem.

2. Sink -  At sink end, solutes are removed from Phloem to be used up. This usually happens by diffusion (a passive process) because the solutes are at a higher concentration in the Phloem than in the surrounding tissue at the sink. The removal of solutes increases the WP inside sieve tubes, so water also leaves tubes by osmosis, lowering the pressure inside sieve tubes.

3. Flow - The result is a pressure gradient from the source end to the sink end. This gradient pushes solutes along the sieve tubes toeards the sink. When they reach the sink the solutes will be uesd up (e.g. in respiration) or stored (e.g. as starch) The higher the concentration of sucrose at source, the higher rate of translocation.

Tip: Once they've left the Phloem cells, the solutes are transported to cells in the sink via the symplast or apoplast pathways.

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4. Translocation

Active Loading:

Active loading is used at the source to move substances into Companion cells from surrounding tissues, and from Companion cells into sieve tubes, against concentration gradient. The concentration of sucrose is usually higher in companion cells than the surrounding tissue cells, and higher in the sieve tube cells than companion cells.

Sucrose is moved to where it needs to go using active transport and co-transport proteins. Co-transport proteins are a type of a carrier protein that bind two molecules at a time. The concentration gradient of one of the molecules is used to move the other molecule against its own concentartion gradient. In active loading, H+ ions are used to move sucrose against its concentration gradient.

1.  In companion cell, ATP is used to actively transport hydrogen ions (H+) out of cell and into surrounding tissue cells. this sets up concentration gradient - there are more H+ ions in surrounding tissue than in companion cell.

2. An Hion binds to a co-transport protein in the companion cell membrane and re-enters the cell (down concentartion gradient). A sucrose molecule binds to co-transport protein at the.....

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4. Translocation

.... same time. The movement of the H+ ion is used to move sucrose molecule into cell, against its concentration gradient.

3. Sucrose molecules are then transported out of companion cells and into sieve tubes by same process.

ATP is one of the products of respiration. The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions. 

Tip: Active transport uses energy to move substances against their concentration gradient.

Tip: Carrier proteins are found in cell membranes - they're used to transport substances across the membrane.

Tip: Companion cells contain many mitochondria, which means they can make lots of ATP for active loading.

Tip: Aphids are small insects that feed on the sap carried in phloem of a plant.

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Here are potential equations you made need to know:


(This is for the area of a sphere)

V=4/3 πr^3

(This is for the volume of a sphere)

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Plant Transport

Side notes:

1: Check the OCR specification to make sure you have covered everything.

2: Pratice questions and test papers!

3: Find pictures to corespond to each of the processes to help your understanding. Drawing them yourself may help you to remember them more!

4: I could not add pictures. I recommend finding pictures or drawing them youself and sticking/stapling them to the flashcard.

Thank you! 

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