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A-Level Biology - OCR A > 3.9 - Transport in Plants > Flashcards

3.9 - Transport in Plants Flashcards

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1
Q

Why do plants need transport systems?

A
  • SA:V Ratio is often small
  • To distribute materials
  • Large distances needed to transport (diffusion is not enough)
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2
Q

What are dicotyledonous plants?

A

A plant with 2 food stores in the seed. They have transport systems arranged as vascular bundles.

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3
Q

Outline the structure of the root

A
  • Vascular tissue situated centrally
  • Roots are subjected to pulling forces
  • Vascular tissue position gives strength
  • Pericycle and endodermis surround vascular tissue
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4
Q

What is the pericycle?

A

Point of lateral growth in roots/branches.

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5
Q

What is the function of the endodermis?

A

Regulates movement of water and mineral ions into vascular tissue.

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6
Q

Outline the structure of the stem

A
  • Vascular bundle arranged towards outside of root
  • VBs and collenchyma and sclerenchyma provide support.
  • Ring of support is discontinuous allowing flexibility
  • Cambium separates phloem and xylem.
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7
Q

What are parenchyma, collenchyma and sclerenchyma?

A

Cells with thickened cell wall used for support.

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8
Q

Outline the structure of the leaf

A
  • Network of tiny bundles throughout the leaf
  • Tiny bundles fuse into side veins which run parallel to each other
  • Side veins merge into the main vein
  • Main vein runs along the centre of the leaf, getting wider towards the leaf stalk.
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9
Q

Outline the structure of xylem

A
  • Parenchyma and sclerenchyma cells in xylem walls
  • Xylem fibres
  • Vessels
  • Tracheids
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10
Q

How is xylem’s structure related to its function?

A
  • Cells are long and thin, arranged end-to-end
  • Cell contents die when they are mature
  • Cell walls are thickened with lignin
  • Annular, reticular and spiral thickening
  • Pits throughout cells
  • Narrow lumen.
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11
Q

Name the three pathways for movement of water in plants

A
  • Apoplast (dead cell walls)
  • Symplast (cytoplasm)
  • Vacuolar (vacuole)
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12
Q

How does water move via the apoplast pathway?

A

Water moves through cell walls and intracellular spaces which offer low resistance to the flow of water.

As water molecules move through the network of cellulose fibres in the cell walls, water molecules are pulled through as a result of cohesion.`

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13
Q

How does water move via the symplast pathway?

A

water moves through the cytoplasm of the root cells which are connected by plasmodesmata.

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14
Q

What is the casparian strip?

A

A band of waxy, waterproof material called suberin found in the walls of the endodermis.

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15
Q

What is the function of the casparian strip?

A

Casparian strip forces water travelling via the apoplast pathway into the cytoplasm of the cells (symplast pathway).

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16
Q

What is the function of endodermal cells?

A

Endodermal cells actively transport mineral ions into the xylem vessels lowering their water potential, so that water enters via osmosis.

This generates root pressure which pushes water a short way up the xylem.

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17
Q

Outline the evidence for root pressure

A
  • Pressure in xylem increases with temp. increase and decreases with temp. decrease. -> suggests that an active process is involved.
  • Metabolic inhibitors cause root pressure to cease -> “”
  • Decreasing O₂ and glucose levels causes a decrease in root pressure -> Active transport requires ATP, made in respiration ∴ decreasing O₂ slows rate of respiration.
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18
Q

Outline cohesion-tension theory

A
  • Cohesion hold water molecules in a long chain
  • Loss of water vapour from top of plant lowers pressure at top of xylem pulling more water up.
  • This creates tension in the column of water
19
Q

What evidence is there supporting cohesion-tension theory?

A

Tree trunk diameter decreases as rate of transpiration increases

  • Evaporation from the leaves draws water from xylem via osmosis water is pulled up - creating a tension.
  • Tension pulls xylem vessel walls in creating a negative pressure.
20
Q

What is capillary action?

A

Adhesion attracts water molecules to sides of xylem vessel - pulls water up the sides.

21
Q

Define transpiration

A

Loss of water vapour from aerial parts of the plant via the stomata.

22
Q

How do stomata open and close?

A

Guard cells have irregular thickness in cell walls.
When guard cells are flaccid, stomata are closed.
When guard cells are turgid, stomata are open.

23
Q

Outline the process of transpiration

A
  1. Water diffuses from xylem to mesophyll cells via osmosis
  2. Evaporation of water from the surface of mesophyll cells into intracellular air spaces
  3. diffusion of water vapour from intracellular air spaces out through the stomata.
24
Q

List the factors affecting transpiration

A
  • Number of leaves
  • Number, size, positioning of stomata
  • Presence of a cuticle
  • Temperature
  • Relative humidity
  • Air movement or wind
  • Light intensity
  • Water availability
25
Q

What is a xerophyte?

A

Plants which have adaptations for living and reproducing in dry environments which enable them to conserve water or to acquire as much as possible.

26
Q

List the adaptations of xerophytes

A
  • Thick waxy cuticle
  • Sunken stomata
  • Reduced stomatal density
  • Reduced leaves
  • Hairy leaves
  • Curled leaves
  • Water storage tissues
  • Loss of leaves
  • Root adaptations (long tap roots/shallow network)
  • Folded stems (like in cacti)
  • Avoidance (become dormant/die completely leaving seeds behind to germinate upon rainfall)
27
Q

What is a hydrophyte?

A

Plants which live in water (submerged or on the surface or at the edges of bodies of water) - need special adaptations to cope with growing in water or permanently saturated in oil.

28
Q

List the adaptations of a hydrophyte

A
  • Very thin/no waxy cuticle
  • Many always-open stomata on upper surfaces
  • Reduced plant structure
  • Wide, flat leaves
  • Small roots
  • Large SA of stems and roots underwater
  • Air sacs
  • Aerenchyma - speacialised parenchyma tissue formed in the leaves, stems and roots of hydrophytes. It makes the leaves and stems more buoyant.
29
Q

Define translocation

A

The movement of sucrose and other substances up and down the plant (from source to sink).

30
Q

What are assimilates?

A

Organic compounds made when Carbon from CO₂ is incorporated into new substances as a result of photosynthesis.

31
Q

Define sink

A
  • Part of the plant where sucrose is removed from phloem
  • Any part of the plant which is growing
  • Where sugar is used for respiration or stored as starch.
32
Q

Define source

A
  • Any part of the plant where sucrose is released into phloem
  • Where sugar is produced by photosynthesis or by breakdown of storage products.
33
Q

What is the phloem?

A

Plant tissue used to transport dissolved sugars and other assimilates.

34
Q

How are the sieve tube elements related to their function?

A
  • Elongated, little cytoplasm, no nucleus to save space
  • Joined end-to-end forming a column
  • End walls have many pores to allow sap to flow (sieve plates).
35
Q

How are companion cells related to their function?

A
  • Small with nucleus and other elements
  • Many mitochondria to provide ATP for active processes e/g/ sucrose loading
  • Cytoplasm linked to sieve tubes by plasmodesmata
36
Q

Define mass flow

A

The bulk movement of substances from one area to another due to differences in pressure.

37
Q

What is phloem loading?

A

The transfer of sucrose into sieve elements from photosynthesising tissue.

38
Q

Outline the process of phloem loading

A
  1. Active transport of H⁺ ions out of companion cell (requires ATP)
  2. Creates H⁺ ion concentration gradient
  3. Facilitated diffusion of H⁺ back into companion cell
  4. Sucrose (assimilates) move back into the companion cell with H⁺ through cotransport proteins
  5. Sucrose diffuses through plasmodesmata from companion cell into sieve tube (symplast route).
39
Q

Outline the process of pressure flow theory

A
  1. Loading sugar into phloem from source increases solute conc. and decreases Ψ inside sieve tube cells. This causes them to take up water from surrounding tissues
  2. Water reabsorption creates a hydrostatic pressure which forces the sap to move along the tube.
  3. Gradient of pressure in the sieve-tube reinforces by the active unloading of sugar and consequent loss of water by osmosis at sieve.
  4. Xylem reabsorbs water from sink to source.
40
Q

Outline the evidence supporting translocation of organic molecules occurring in the phloem

A
  • Pressure in the sieve tubes (sap released when cut)
  • Concentration of sucrose is higher in leaves (source) than in roots (sink)
  • Downward flow in phloem occurs in daylight, ceases at night or when leaves are shaded
  • Increases in sucrose levels in leaf are followed by similar increases in phloem a little later
  • Metabolic poisons / lack of Oxygen inhibit translocation
  • Companion cells contain many mitochondria and readily produce ATP.
41
Q

Outline the evidence questioning pressure flow theory

A
  • Function of sieve pates is unclear - they seem to hinder mass flow
  • Not all solutes move at the same speed - they should do so if movement is by pressure flow
  • Sucrose is delivered at more or less the same ratio to all regions rather than going more quickly to the ones with the lowest sucrose levels.
42
Q

What is the puncture experiment?

A

When phloem is punctured with a hollow tube, sap oozes out showing there is high pressure (compression) inside the phloem.

If the xylem is punctured then the air is sucked in, showing that there is low pressure (tension) inside the xylem.

43
Q

What evidence is there supporting translocation being an active process?

A
  • Metabolic poisons including the formation of ATP stop translocation
  • Sugars move up to 10,000x faster by mass flow than they could by diffusion
  • pH of companion cells is higher than that of surrounding cells (due to H⁺ ion pump).

A-Level Biology - OCR A (16 decks)

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