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Z A level Biology > Transport in plants 2 > Flashcards

Transport in plants 2 Flashcards

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1
Q

What does transpiration do?

A

Transpiration reduces the water (hydrostatic) pressure at the top of a xylem vessel compared with the pressure at the base, so causing the water to flow up the vessels

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2
Q

How can plants also increase the pressure difference between the top and bottom?

A

By raising the water pressure at the base of the vessels

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3
Q

How is this pressure raised?

A
  1. The active secretion of solutes, e.g. mineral ions, into the water in the xylem vessels in the root
  2. Cells surrounding the xylem vessels use energy to pump solutes across their membranes and into the xylem by a active transport
  3. The presence of the solutes lowers the water potential of the solution in the xylem, thus drawing in water from the surrounding root cells
  4. This influx of water increases the water pressure at the base of the xylem vessel
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4
Q

Why is root pressure not very significant?

A
  • Although root pressure may help in moving water up xylem vessels, it is not essential and is probably not significant in causing water to move up xylem in most plants
  • Water can continue to move up through the xylem even if the plant is dead
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5
Q

What is water transport in plants largely?

A
  • A passive process driven by transpiration from the leaves

- The water simply moves down a continuous water potential gradient from the soil to the air

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6
Q

How are xylem vessel arranged in a root?

A

The xylem vessels are in the centre of the root unlike the arrangement in stems, where they are arranged in a ring and are nearer the centre

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7
Q

What happens to water taken up by roots hairs?

A
  • It crosses the cortex of the root and enters the xylem in the centre of the root
  • It does this because the water potential inside the xylem vessels is lower than the water potential in the root hairs
  • Therefore, the water moves down this water potential gradient across the root
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8
Q

What route does the water take?

A
  • The water takes two routes through the cortex

- Individual molecules can switch from one route to the other at any time

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9
Q

Describe the apoplastic pathway

A
  1. The cells of the cortex, like all plant cells, are surrounded by cell walls made of several layers of cellulose fibres, criss-crossing one another
  2. Water can soak into these walls, rather as it would soak into blotting paper and can seep across the root from cell wall to cell wall without every entering the cytoplasm of the cortical cells
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10
Q

Describe the symplastic pathway

A
  1. The water to move into the cytoplasm or vacuole of a cortical cell by osmosis, and then into adjacent cells through the interconnecting plasmodesmata
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11
Q

Which pathway is more important?

A
  • The relative importance of these two pathways varies from plant to plants and in different conditions
  • Normally it is probable that the symplastic pathway is more important, but when transpiration rates are especially high more water travels by the apoplectic pathway
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12
Q

What happens once the water reaches the endodermis?

A
  1. The apoplastic pathway is abruptly blocked
  2. The cells in the endodermis have a thick, waterproof waxy band of suberin in their cell walls
  3. This band called the Casparian strip, forms an impenetrable barrier to water in the walls of the endodermis cells
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13
Q

How can water cross the endodermis?

A
  1. Through the cytoplasm of the endodermal cells
  2. As the endodermal cells get older, the suberin deposits become more extensive, except in certain cells called passage cells, through which water continue to pass freely
  3. It is thought that this arrangement gives a plant control over what mineral ions pass into its xylem vessels, as everything has to cross cell surface membranes and it may also help with the generation of root pressure
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14
Q

What happens once across the endodermis?

A
  1. The water continues to move down the water potential gradient across the pericycle and towards the xylem vessels
  2. Water moves into the xylem vessels through the pits in their walls
  3. It then moves up the vessels towards the leaves as a previously described
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15
Q

Summarise the apoplastic pathway

A
  1. Water enters the cell wall
  2. Water moves through the cell wall
  3. Water may move from cell wall to cell wall through the intercellular spaces
  4. Water may move directly from cell wall to cell wall
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16
Q

Summarise the symplastic pathway

A
  1. Water enters the cytoplasm by osmosis through the partially permeable cell surface membrane
  2. Water moves into the sap in the vacuole through the tonoplast by osmosis
  3. Water may move from cell to cell through the plasmodesmata
  4. Water may move from cell to cell through adjacent cell surface membranes and cell walls
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17
Q

Describe a young root and root hairs

A
  1. The tip of the root is covered by a tough protective root cap and is not permeable to water
  2. Just behind the top some of the cell in the epidermis are drawn out into long thin extensions called root hairs
  3. These reach into spaces between the soil particles, from where they absorb water
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18
Q

How does water move into the root from the soil?

A
  1. Water moves into the root hairs by osmosis down. water potential gradient
  2. Although soil water contains some inorganic oils in solution, it is relatively dilute solution and so has a relatively high water potential
  3. However the cytoplasm and cell sap inside the root hairs have considerable quantities of inorganic ions and organic substances such as proteins and sugars dissolved in their, and so have relatively low water potential
  4. Therefore water diffuses down this water potential gradient though the partially permeable cell surface membrane and into the cytoplasm and vacuole of the root hair cell
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19
Q

What is the benefit of root hairs?

A
  • The large number of very fine root hairs provides a large surface area in contact with the soil surrounding the root, thus increasing the rate at which water can be absorbed
  • Root hairs are also important for the absorption of mineral ions such as nitrate and magnesium
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20
Q

What is a disadvantage of root hairs?

A

-These root hairs are very delicate and often only function a few days before being replaced by new ones as the root grows

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21
Q

What are mycorrhizas?

A
  1. Many plants especially trees, have fungi located in or on their roots, forming associations called mycorrhizas, which serve a similar function to root hairs
  2. The mycorrhizas act like a mass of fine roots which absorb water and nutrients, especially phosphate, from the soil and transport them into the plant
  3. Some trees, if growing on poor souls are unable to survive without these fungi
  4. In return the fungi recipe organic nutrients from the plant (relationship in which two organisms of different species both benefit is mutualism)
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22
Q

What are needed by plants?

A
  • Apart from the carbohydrates made in photosynthesis , plants need a supply of mineral ions to complete their nutrition
  • E.G. nitrate, phosphate, sulfate, potassium, magnesium and calcium
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23
Q

How are mineral ions absorbed?

A
  1. Mineral ions in solution are absorbed along with water by the roots, particularly by the root hairs
  2. Their route through the plant is the same as that for water, crossing the root by apoplastic and symplastic pathways before moving the the mass flow of the xylem sap up the xylem to the rest of the plant
  3. From the xylem they enter the apoplastic and symplastic pathways again
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24
Q

How else do mineral ions move?

A
  • As well as moving by mass flow through the apoplastic pathway and xylem, mineral ions can also move by diffusion and active transport
  • E.G they can diffuse into the apoplastic pathway of the root from the soil and once in the apoplastic pathway can diffuse in any direction according to the concentration gradients
  • They can also enter cells by missions, facilitated diffusion and active transport
25
Q

Describe facilitated diffusion and active transport of mineral ions

A
  1. Facilitated diffusion dan active transport allow cells to control what ions enter or leave cells
  2. One important control point is the root endodermis, were the Casparian strip forces ions to pass through living cells before they can enter the xylem
26
Q

What is translocation?

A
  • Translocation can be applied to transport in both xylem and phloem and it means moving from one place to another
  • It tends to be used more commonly to describe the transport of soluble organic substances within a plant
  • These are substances which the plant itself has made, e.g. sugars which are made by photosynthesis in leaves and these substances are sometimes called assimilates
27
Q

How are assimilates transported?

A

In sieve elements

28
Q

What are sieve tubes?

A
  1. Phloem contains unique tube like structures called sieve tubes
  2. They are made of living cells
  3. A sieve tube is made up of many elongated sieve elements (also know as sieve tube elements), joined end to end vertically to form a continuous tube
29
Q

Describe a sieve element

A
  • Each sieve element is a living cell
  • Has a cellulose cell wall, a cell surface membrane and cytoplasm contain ER and mitochondria
  • However the amount of cytoplasm is very small and only forms a grin layer linen the inside of the wall of the cell
  • There is no nucleus and no ribosomes
30
Q

What is another feature of sieve elements?

A
  • End walls
  • Where the end walls of two live elements meet, a sieve plate is formed
  • This is made up of the walls of both elements, perforated by large pores
  • These pores are easily visible with a good light microscope
  • In living phloem, the pores are open, presenting little barrier to the free flow of liquids through them
31
Q

What does each sieve elements have?

A

At least one companion cell lying close beside it

32
Q

Describe the structure of a companion cell

A
  • Cellulose cell wall
  • Cell surface membrane
  • Cytoplasm
  • Small vacuole
  • Nucleus
  • But number of mitochondria and ribosomes is rather larger than normal and how cells are metabolically very active
33
Q

How are companion cells related to sieve elements?

A
  • Companion cells are very closely associated with their neighbouring sieve elements
  • They are regarded as a single functional unit
  • Numerous plasmodesmata pass through their cell walls, making direct contact between the cytoplasm of the companion cell and that of the sieve element
34
Q

What is the liquid inside phloem sieve tubes called?

A

Phloem sap

35
Q

Describe the contents of the sieve tubes

A
  • The contents of the sieve tubes are under very high. pressure
  • When a sieve tube is cut, the release of pressure inside the tube causes a surge of its contents towards the cut
  • When the contents comes up against a sieve plate they may block it
  • This helps to prevent escape of the contents of the sieve tube
  • Then within minutes get sieve plate is properly sealed with a carbohydrate called calls, a process sometimes called ‘clotting’
  • However castor oil plants are usual in that their phloem sap does continue to flow from a cut for some time, making relatively easy to collect
36
Q

What may be used to sample sap from other plants?

A
  1. Aphids, such as a greenfly feed using using tubular mouthparts called stylets
  2. They insert these through the surface f the plant’s stem or leaves into the phloem
  3. Phloem sap flows through the stylet into the aphid
  4. If the stylet is cut near the aphids head, the sap continues to flow
    - It seems that the small diameter of the stylet does not allow sap to flow out rapidly enough to trigger the plants phloem ‘clotting’ mechanism
37
Q

How does phloem sap move?

A

Phloem sap, like the contents of xylem vessels moves by mass flow

38
Q

Describe mass flow

A
  • Mass flow moves organic solutes about 1mh-1 on average about 10,000 times faster than diffusion would
  • Whereas in xylem vessels, the difference pressure that causes mass flow is produced by water potential gradient between soil and air, requiring no energy input from the plant
  • To create the pressure differences needed for mass flow in phloem, the plant has to use energy
39
Q

What is phloem transport?

A

An active process and a pass transport in xylem

40
Q

How is the pressure difference produced in the phloem?

A

By active loading of sucrose into the sieve elements at the place from which sucrose is to be transported

41
Q

What is any area of a plant in which sucrose is loaded into the phloem called?

A
  • A source

- This is usually a photosynthesising leaf or a storage organ

42
Q

What is any area where sucrose is taken out of the phloem called?

A
  • A sink

- E.G the roots

43
Q

What does loading a high concentration of sucrose in to a sieve element do?

A
  1. Greatly decreases the water potential in the sap inside it
  2. Therefore water enters the sieve element, moving down a water potential gradient by osmosis
  3. This causes a correspondingly high build up of pressure (equivalent to about six times atmospheric pressure) and the pressure is referred to as hydrostatic pressure, organ pressure or pressure potential
  4. A pressure difference is therefore created between the source and the sink
44
Q

What does the pressure difference in the phloem cause?

A
  1. A mass flow of water and dissolved solutes through the sieve tubes, from the high pressure area to the low pressure area
  2. At the sink, sucrose may be removed and used, causing the water to follow by osmosis, and thus maintaining the pressure gradient
45
Q

Where are sinks?

A
  • Sinks can anywhere in the plants, both above and below the photosynthesising leaves
  • Thus saps flows both upwards and downwards in the phloem (in contrast to xylem in which flow is always upwards)
  • Within any vascular bundle, phloem sap may be flowing upwards in some sieve tubes and downwards in other, but it can only flow one way in any particular sieve tube at any one time
46
Q

Describe the sucrose in the phloem

A
  1. In leaf mesophyll cells photosynthesis in chloroplasts produce triose sugars, sine if which are converted into sucrose
  2. The sucrose in solution then moves from the mesophyll cells across the leaf to the phloem tissue
  3. It may moveably the symplastic pathway, moving from cell to cell via plasmodesmata
  4. Alternatively, it may move by the apoplastic pathway, travelling along cell walls
    - Which route is more important depends on the species
47
Q

how do companion cells and sieve elements work together?

A
  1. Sucrose is loaded into a companion cell or directly into a sieve element by active transport
  2. Hydrogen ions (H+) are pumped out of the companion cell into its cell wall, using ATP as an energy source
  3. This creates a large excess of hydrogen ions in the apoplastic pathway outside the companion cell
  4. The hydrogen ions can move back into the cell down their concentration gradient, throughs. protein which acts as a carrier for both hydrogen ions and sucrose at the same time
48
Q

How are the sucrose molecules carried through?

A
  1. The sucrose molecules are carried through this co-transporter molecule into the companion cell, against the concentration gradient for sucrose
  2. The sucrose molecules can then move from the companion cell into the sieve tube, through the plasmodesmata which connect them (the symplastic pathway)
49
Q

Where does unloading sucrose from the phloem occur?

A
  1. Unloading occurs into any tissue which requires sucrose
  2. It is probablele that sucrose moves out of the phloem into these tissues using both symplastic and apoplastic routes as with loading
50
Q

What is needed for the unloading of sucrose form the phloem?

A
  1. Phloem unloading required energy, and similar methods to those used for loading are probably used
  2. Once in the tissue, the sucrose is converted into something else by enzymes, so decreasing its concentration and maintaining a concentration gradient
  3. One such enzyme is invertase, which hydrolyses sucrose to glucose and fructose
51
Q

What is similar about sieve tubes and xylem vessels?

A

-Liquid moves by by mass flow down a pressure gradient, through tubes formed by cells stacked end to end

52
Q

What is different about sieve tubes and xylem vessels regarding water transport?

A
  • Unlike water transport through xylem, which occurs through dead xylem vessels, translocation through phloem sieve tubes involves active loading of sucrose at sources and unloading at sick, thus requiring living cells
  • Sucrose would leak out of xylem vessels because they have no living membrane and membranes can control entry or loss of solutes
53
Q

What is different about sieve tubes and xylem vessels regarding lignin?

A
  1. Xylem vessel have lignified cell walls, whereas phloem sieve tubes do not
  2. This is an advantage because it means the cells can be dead and therefore entirely empty
  3. Water can therefore flow unimpeded, and the dead xylem vessels with their strong walls also support the plant
54
Q

What is different about sieve tubes and xylem vessels regarding reduced resistance?

A
  1. Sieve tubes reduce resistance to flow by having only a thin layer of cytoplasm and no nuclei, but whereas the end walls of xylem elements disappear completely those of the phloem sieve elements form sieve plates
  2. It is calculated that without the resistance of the sieve plates along the pathway, the steep positive pressure gradient inside the side tubes would quickly be lost, with the different pressures at source and sink quickly equilibrating
  3. Xylem on the other hand has to withstand doing negative pressure (tension) inside its tubes and buckling is prevented by its lignified walls
55
Q

Why are sieve plates beneficial?

A
  1. Sieve plates also allow the phloem to seal itself up rapidly with callose if damaged e.g. by a grazing herbivore rather as a blood vessel in an animal is sealed by clotting
  2. Phloem sap has a high turgor pressure because of its high solute content, and would leak out rapidly if the holes in the save lates were not quickly sealed
  3. Phloem sap contains valuable substances such as sucrose, which the plant cannot afford to lose in large quantity
    - The ‘clotting’ of phloem sap may also help to prevent the entry of micro-organisms which might feed not e nutritious sap or cause disease
56
Q

Describe and explain how sucrose is transported from the phloem in the leaves of sugar beet to storage tissues in the root

A
  1. Diffusion into phloem sieve tube element from companion cell through plasmodesmata
  2. Presence of sucrose (in phloem sieve tube element) lowers water potential
  3. Water enters (sieve tubes) by osmosis /down the the water potential gradient (into sieve tubes)
  4. Increase in hydrostatic pressure (in sieve tube elements)
  5. Low hydrostatic pressure in root/ sink by removal of sucrose
  6. Movement of phloem sap/ sucrose in solution, down hydrostatic pressure/ from high to low hydrostatic pressure
  7. Mass flow
57
Q

Explain how transpiration is responsible for the movement of water across the root

A
  1. Water potential gradient between leaves and roots
  2. Diffusion out (via stomata) of water vapour
  3. Evaporation of water from mesophyll cell surfaces/walls/membranes
  4. Transpiration from leaves creates transpiration pull
  5. Tension is set up/ present in xylem vessels
  6. Cohesion between water molecules in xylem
  7. Hydrogen bonding between water molecules
  8. Adhesion of water molecules to cellulose in wall of xylem vessels
  9. Movement of water from apoplasit through endodermis by osmosis
    10 e.g. idea that water column extension from centre of root to root hairs (and to soil water)
58
Q

How are sugars transported in phloem sieve tubes from source to sink?

A
  1. Mass flow
  2. Sucrose decrease the water potential
  3. Water enters sieve tubes by osmosis
  4. Water enters down water potential gradient
  5. Increased volume therefore higher hydrostatic pressure
  6. Unloading / removal of sucrose at sink
  7. Lowers hydrostatic pressure at sink
  8. Movement is down pressure gradient from high to low hydrostatic pressure
59
Q

What are structural differences between guard cell and lower epidermal cells?

A
  1. Guard cell have chloroplasts
  2. Varying thickness of cell wall
  3. No plasmodesmata

Z A level Biology (15 decks)

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