Mass Transport in Plants A1 Flashcards

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1
Q

What labels does the cross section of a root have?
(6)

A
  1. phloem
  2. xylem
  3. epidermis
  4. root hair
  5. endodermis
  6. cortex
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2
Q

Describe the features of a root and how they help its function.
(2)

A
  1. extensions to increase surface area to increase uptake of water
  2. thin cell wall for a shorter diffusion pathway
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3
Q

Explain how roots absorb minerals, ions and water.
(3)

A
  1. actively transport ions from soil across their membrane from soil across their membrane into cytoplasm, via carrier proteins
  2. this lowers the water potential of the cytoplasm, to below the water potential in the soil
  3. water enters the root hair cells via osmosis down a water potential gradient (high to low) through a partially permeable membrane
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4
Q

How does water get into xylem from root hair cell?

A

Water passes from root hair cells, across the cortex cells to the xylem.

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5
Q

Describe the symplastic pathway.
(5)

A
  1. water passes through the cytoplasm of the cell
  2. across the cell membranes
  3. through plasmodesma (hole in cell wall) from one cell to another
  4. via osmosis down a water potential gradient
  5. it is relatively slow as there is resistance to the movement of water (cell membrane and solutes)
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6
Q

Describe the apoplastic pathway.
(4)

A
  1. water passes through the adjoining cell walls from one cell to another
  2. movement is relatively fast (less resistance to water molecules as larger gaps in between cellulose molecules)
  3. this is via diffusion
  4. the water then reaches the endodermis
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7
Q

What do the cells of the endodermis do to water and ions?
(6)

A
  1. the endodermis has a waterproof strip (casparian strip) which prevents water movement through walls of the endodermis
  2. water is forced to pass through cytoplasm of cell of the endodermis
  3. the cells actively transport ions to xylem
  4. this lowers water potential in xylem
  5. so water passes into xylem from endodermal cells down a water potential gradient
  6. this enables movement of water from root hairs to centre of root, down a water potential gradient.
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8
Q

Describe the structure and function of the xylem.
(4)

A
  1. dead cells which form hollow tubes - this allows water potential to not be affected, so water stays within the xylem vessels and doesn’t leave
  2. end walls break down so xylem forms a continuous tube - so water can form a continuous column
  3. cell walls are strengthened with lignin - this makes the xylem waterproof
  4. there are pits in the xylem - these are little holes which allow water to move between xylem vessels.
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9
Q

Explain how water enters xylem from the endodermis in the root and is then transported to the leaves.
(8)

A

(in the roots)
1. casparian strip blocks apoplastic pathway/only allows symplastic pathway
2. active transport by endodermis
3. of ions/salts into xylem
4. lowers water potential in xylem/water enters xylem by osmosis/down a water potential gradient
(xylem to leaf)
5. evaporation / transpiration (from leaves)
6. creates cohesion/tension (H-bonding) between water molecules (negative pressure)
7. adhesion/water molecules mind to xylem
8. creates continuous column of water

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10
Q

Root pressure moves water through the xylem.
Describe what causes root pressure.
(4)

A
  1. active transport by endodermis
  2. ions/salts into xylem
  3. lowers water potential (in xylem)
  4. water enters by osmosis
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11
Q

Name a factor that can affect transpiration.

A

light / temperature / air movement / humidity

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12
Q

Describe the process of transpiration.
(2)

A
  1. open stomata (so CO2 can enter leaf) causes water to diffuse from air spaces in leaf to out leaf from high-low water potential (transpiration)
  2. this causes a continuous concentration gradient through cells in the plant causing a continuous column in xylem vessels
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13
Q

What is the cohesion tension theory of water movement up a plant?
(6)

A
  1. water moves out stomata from a high to low concentration gradient
  2. water forms continuous column in narrow xylem vessel
  3. water moles have forces of attraction so they stick together - they have cohesion
  4. they are also attracted to the walls of the xylem (adhesion)
  5. pulling force is great and column is under tension
  6. this is called transpiration stream
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14
Q

Root pressure moves water through the xylem.
Describe how a high pressure is produced in the leaves.
(3)

A
  1. water potential becomes lower/becomes more negative (as sugar enters phloem)
  2. water enters phloem by osmosis
  3. increased volume (of water) caused increased pressure
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15
Q

What type of pressure aids water movement from root to top of tree?

A

Hydrostatic pressure is higher in roots than top of tree so aids water movement upwards

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16
Q

Name and explain the factors affecting the rate of transpiration.
(4)

A
  1. light - doesn’t directly affect it, stomata open in light and closed in dark (rate is higher in light)
  2. temperature - increases kinetic energy, water diffusion increases
  3. humidity - greater difference in humidity, the greater rate of diffusion
  4. air movement - moves water away from stomatal pores increases potential gradient and diffusion rate
17
Q

Explain how to measure the rate of transpiration in plants using a potometer.
(8)

A
  1. leafy shoot cut under water
  2. potometer completely filled with water so no air bubbles
  3. leafy shoot fitted to potometer under water using rubber tube
  4. potometer removed from under water and all joints sealed with waterproof jelly
  5. air bubble introduced to capillary
  6. as transpiration occurs water moves through tube and bubble moves with it
  7. distance recorded over a period of time, mean is calculated from n. of repeats
  8. volume of water lost over a period of time calculated
18
Q

Give two precautions the students should have taken when setting up the potometer to obtain reliable measurements of water uptake by the plant shoot.
(8)

A
  1. seal joints / ensure airtight /watertight
  2. cut shoot under water
  3. cut shoot at a slant
  4. dry off leaves
  5. insert into apparatus under water
  6. ensure no air bubbles are present
  7. shut tap
  8. note where bubble is at start / move bubble to start position
19
Q

Name some xerophyte adaptations plants have to reduce water loss.
(6)

A
  1. reduced number of stomata
  2. stomata trapped in pits (maintain water vapour in pits) - less concentration gradient
  3. hair to trap water
  4. rolled leaves
  5. leaves reduced to spines
  6. thick waxy cuticles
20
Q

What is translocation?
(5)

A
  1. the movement of solutes (assimilates) to where they are needed in a plant
  2. its an energy requiring process that happens in the phloem
  3. moves solutes from sources (where assimilates are produced, so high concentrations) to sinks (where assimilates are used up, so low concentrations)
  4. enzymes maintain a concentration solutes at sink to other storage substances
  5. this makes sure there is always a lower concentration at the sink than source
21
Q

Describe the mass flow hypothesis for the mechanism of translocation in plants.
(6)

A

(transporting substances from source to sink in plant)
1. in source/leaf sugars actively transported into phloem
2. by companion cells
3. lowers water potential of sieve cell/tube and water enters by osmosis
4. increases in pressure causes mass movement (toward sink/root) - active transport down concentration gradient
5. sugars used/converted in root for respiration for storage
6. this causes a lower pressure in sieve tube, creating a pressure gradient from source to sink, continuing the pattern

22
Q

Give supporting evidence of mass flow.
(5)

A
  1. if a ring of bark (which includes a phloem not xylem) is removed from a woody stem, a bulge forms above the ring
  2. the fluid in bulge has a high concentration of sugar that fluid below ring because sugars can’t move past area where bark removed (provided there is a downflow of sugars)
  3. when an aphid pierces a leaf further down the stem, sap flows out quicker, evidence that there is a pressure gradient
  4. radioactive tracers eg. carbon (14C) can be put in organic substances in plant
  5. if metabolic inhibitor (which stops ATP production) put in phloem, translocation stops - providing active transport is involved
23
Q

Give evidence again the mass flow theory.
(2)

A
  1. sugar travels to many sinks, not just the one with the highest water potential
  2. sieve plates would create a barrier to mass flow. A lot of pressure needed for a solutes to get through at reasonable rate
24
Q

Give the evidence from radioactive tracers for mass flow theory.
(5)

A
  1. done by supplying part of a plant (leaf) with an organic substance that has a radioactive label eg CO2 (gets in sugars during photosynthesis etc)
  2. then tracking its movement by autoradiography to reveal where tracker has spread
  3. plant is killed (frozen using liquid nitrogen) then placed onto photographic film
  4. where the film turns black, radioactive substances are present
  5. this demonstrates translocation of substances from source to sink over time (during photosynthesis)
25
Q

Describe correlation and causation relationships.

A
  1. data from an experiment may show a continuous pattern with results eg. width of bark compared to mg of carbohydrate transported to bottom of tree
  2. however this does not mean it is caused by the width of bark
  3. other factors may affect the results eg. more bark taken away means less phloem, reducing the amount of carbohydrates that can be transported etc.
26
Q

Sodium chloride and potassium chloride had different effects for the opening and closing of stomata, explain why.

A
  1. K+ (potassium) ions lower water potential inside / Na+ (sodium) ion lowers water potential outside
  2. different guard cell wall thickness causing opening stomata.