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A Level Biology Paper 1 > Mass Transport In Plants > Flashcards

Mass Transport In Plants Flashcards

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1
Q

Explain how water enters the xylem

A

Root hair cells actively transport ions from soil across their membranes
Into their cytoplasm
Via carrier proteins
Lowering water potential so it becomes more negative in cytoplasm than in soil
Water moves into root hair cells via osmosis down a water potential gradient
Water passes from cell, across endodermis and into xylem vessels

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2
Q

How is water transported to leaves

A

Transpiration from leaves
Creates cohesion tension
Due to hydrogen bonding between water molecules
Adhesion of water molecules binding to xylem creates a continuous column of water
Which is pulled up xylem
Due to negative pressure

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3
Q

Factors that affect transpiration

A

Light intensity
Temperature
Air movement
Humidity

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4
Q

What causes root pressure

A 
Active transport by endodermis
Of ions/salts
Into xylem
Lowers water potential in xylem
So water enters by osmosis
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5
Q

Root adaptations

A

Hair like extentions: Increase surface area so increase uptake of water and increase the area for channel and carrier proteins

Thin cell wall: Shorter diffusion pathway

Mitochondria: Lots for ATP synthesis to provide the energy needed for active transport of ions/salts

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6
Q

Osmosis

A

Movement of water from a region of higher water potential to an area of lower water potential across a partially permeable membrane
Via aquaporins

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7
Q

Why do plants need nitrates

A

DNA
RNA
To make the nitrogenous bases in nucleic acid

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8
Q

Why do plants need magnesium ions

A

To make chlorophyll

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9
Q

Why do plants need phosphate ions

A

To make nucleotides of DNA and RNA
To make ATP
To make phospholipids in the phospholipid bilayer

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10
Q

Function of xylem

A

Transports water from roots up the stem to leaves

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11
Q

Function of phloem

A

Transports sugar and organic substances
From leaves where they are formed in photosynthesis
To where they are needed (shoots, roots, flowers, fruits)

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12
Q

Xylem adaptations

A

Dead cells form hollow tubes with no cytoplasm/organelles
~Allows easier water flow with no impediments from organelles

No end walls because end walls break down and form continuous tubes
~Water can form a continuous column

Cell walls are strengthened with lignin
~Xylem waterproof and rigid so provides support and allows them to withstand tension/pressure

Xylem pits that are little holes allowing water to move laterally between xylem vessels
~If one vessel gets blocked water can still get around it so flow of water isn’t slowed or stopped

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13
Q

What are xylem pits

A

Little holes in xylem vessels
Allowing water to move laterally between xylem vessels
So water can get around blocked vessels
Flow of water isn’t slowed or stopped

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14
Q

What is transpiration

A

Evapouration of water from a plant

Driving movement of water up a xylem vessel

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15
Q

Explain transpiration and how this leads to a transpiration stream

A

Stomata open
Water diffuses from airspaces (higher water potential) inside the leaf to (lower water potential) outside of leaf
Loss of water from air space causes movement down a water potential gradient
From mesophyll to air spaces
Lowers water potential of mesophyll cells
Water moves by osmosis from adjacent mesophyll cells
Creating a water potential gradient across the leaf to the xylem vessels
Water enters leaf via osmosis down a water potential gradient
Causing it to be pulled up under tension through xylem from roots
Water forms a continuous column in the narrow xylem vessels
Water molecules form weak hydrogen bonds between each other so stick together (cohesion)
Water attracted to hydrophilic walls of xylem forming forces of adhesion
Pulling force is great
Column of water is under tension
Movement of water through plant from roots to leaves is known as a transpiration stream

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16
Q

What is a transpiration stream

A

Movement of water through plant from roots to leaves

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17
Q

Explain the cohesion tension theory

A

Water evaporates from leaves
Lowering water potential in cells
Water is drawn out of xylem
Creating tension/negative pressure
Due to cohesive forces between water molecules due to hydrogen bonding
Adhesion between hydrophilic walls of xylem and water molecules
Water is pulled up as a continuous column

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18
Q

How is a high pressure produced in the leaves

A

Sugars enter the phloem
So water potential becomes lower/more negative
Water enters leaves by osmosis down a water potential gradient
Increased volume of water in the leaves
Causes an increased pressure in the leaves

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19
Q

Explain the negative pressure in the xylem

A

Water evaporates from leaves
Lowering water potential in cells
Water is drawn out of xylem
Creating cohesion tension/negative pressure
Due to cohesive forces between water molecules due to hydrogen bonding
Adhesion between hydrophilic walls of xylem and water molecules
Water is pulled up as a continuous column

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20
Q

How does light affect transpiration

A

Doesn’t affect transpiration directly
Stomata open in the light and close in the dark
Rate of transpiration is higher in the light

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21
Q

How can the factor of temperature be managed to stop transpiration

A

Use a tank of water to absorb heat from the lamp in experiment
Since heat/temperature is a factor affecting the rate of transpiration

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22
Q

How does temperature affect transpiration

A

As temperature increases so does the kinetic energy of water molecules
Move more rapidly
Temperature increasing causes an increase in the rate of diffusion
So rate of transpiration increases

23
Q

How does humidity affect transpiration

A

Air spaces in the leaf are saturated with water vapour
Air outside contains much less water vapour
Increased humidity means greater difference in humidity between air spaces and air means greater rate of transpiration
Due to a larger water potential gradient
Water leaves leaf down a water potential gradient

24
Q

How does air movement/wind speed affect transpiration

A

Air movement over leaf moves water vapour away from the stomatal pores
Increases water potential gradient between inside and outside of the leaf
Greater rate of air movement means faster the movement of water vapour
So the greater the rate of transpiration

25
Q

Explain the potometer experiment

A

Leafy shoot cut cut diagonally under the water
potometer filled completely with water so no air bubbles
Rubber tube used to fit leafy shoot to potometer under water
Potometer removed from under water
All joints sealed with waterproof jelly
Air bubble introduced into capillary tube
By slightly tilting a beaker full of water at the end of the tube
As transpiration occurs water moves through the capillary tube, air bubble moving with it
Distance moved over a period of time recorded and a mean is calculated from repeats
Volume lost can be calculated over a period of time

26
Q

Precautions for potometer experiment

A 
Seal joints
Cut shoot underwater
Cut shoot diagonally
Dry leaves
No air bubbles present other than one introduced
Note the start point of air bubble
27
Q

Why are joints sealed in potometer experiment

A

Sealed with a waterproof jelly
Stops water getting out and air getting in
Making potometer airtight

28
Q

Why is the shoot cut underwater in the potometer experiment

A

Stops air bubbles being taken into xylem and breaking the cohesion tension

29
Q

Why is the shoot cut diagonally in the potometer experiment

A

Stops the end of xylem closing

30
Q

Why do you dry the leaves in the potometer experiment

A

Water on leaves lowers the water potential gradient and blocks the stomata opening

31
Q

Limitations of photometer experiment

A

Only a proxy measure of transpiration
Absence of roots
Volume of water taken up isn’t equal to the volume of water lost in transpiration
Because water used for other processes like photosynthesis, hydrolysis, support and turgidity

32
Q

Mass flow hypothesis

A

In source (leaf) sugars are actively transported into phloem
By companion cells
Lowering water potential of sieve tubes
So water enters via osmosis
Increase in volume of water causes an increase in pressure
Causing a mass movement towards sink (roots) due to hydrostatic pressure gradient
Sugars converted in roots for respiration or storage

33
Q

Explain what happens at a source

A

Active transport is used to load solutes (e.g. sucrose from photosynthesis)
From companion cells into the sieve tubes of the phloem at the source
Lowering the water potential inside the sieve tubes
Water enters the sieve tubes by osmosis from the xylem and companion cells
Creating a high pressure inside the sieve tubes at a source end of phloem
Resulting in a hydrostatic pressure gradient from a source to sink
Which pushes solutes along sieve tubes towards sink

34
Q

Explain what ringing experiments are

A

A ring of bark (containing phloem but not xylem) is removed from a woody stem
Causing a bulge to form above ring
Fluid from bulge has a higher sugar concentration than the fluid below the ring
Because sugars can’t move past since the bark removed contains phloem
Evidence of a downward flow of sugars

35
Q

Evidence for mass flow from ringing experiment

A

Accumulation of sucrose/amino acids in bulge above cut phloem but a lower sugar concentration below

Sucrose below cut carries on moving to sink until it runs out

36
Q

Evidence against mass flow hypothesis from ringing experiments

A

Sieve plates would create a barrier to mass flow

Sugar travels to many different sinks not just the one with highest water potential as the model suggests

37
Q

Explain what happens at a sink

A

Solutes are removed from the phloem to be used in respiration for energy for active transport or to be stored as starch
Increasing the water potential inside sieve tubes
So water leaves the tubes by osmosis
Into the xylem
Lowering pressure inside sieve tubes
And creating a hydrostatic pressure gradient from source to sink
Gradient pushes solutes along sieve tubes towards sink

38
Q

What are radioactive tracer experiments

A

Supplying leaves with radioactive carbon 14 (carbon dioxide)
Which gets converted to Glucose in photosynthesis
Then sucrose or other organic substances
Allowing radioactive carbon 14 to be tracked as it is transported throughout the plant

39
Q

What do radioactive tracers show

A

Movement of radioactive carbon 14 through the plant
When converted into glucose in photosynthesis
Then sucrose
Shows translocation

40
Q

What could potentially hinder mass transport of sugars

A

Sieve plates

41
Q

How are sieve cells adapted for mass transport of sugars

A

Living cells that have no nucleus and few organelles so no obstructions to mass flow
Each sieve tube element has a companion cell to carry out living functions for sieve cell
Connected to eachother through sieve plates
Pores allow sucrose/amino acids to move through

42
Q

How are companion cells adapted for mass flow of sugars

A

Each companion cell carries out living functions for one sieve cell
Contain many mitochondria for ATP synthesis through aerobic respiration for the active transport of solutes
Plasmodesmata connecting companion cells and sieve tubes allow movement of substances like ATP, proteins, solutes

43
Q

How can heat treatment affect a plant

A

Damages/denatured enzymes involved in ATP synthesis to produce energy needed for active transport of solutes
So damages phloem
Some water is transported in the phloem but most in xylem
Flow of water not really affected since it is a passive process
Osmosis occurs down a water potential gradient which doesn’t require ATP

44
Q

How is pressure generated in the phloem

A 
Active transport used to load solutes from companion cells
Into sieve tubes of phloem
At the source
Lowering the water potential in sieve tubes
Water enters via osmosis
Down a water potential gradient
From xylem and companion cell
Increased volume of water in phloem
Increases the hydrostatic pressure
At the source
45
Q

How does the mass movement of water relate to ion dispersion

A

Mass movement of water ensures mineral ions are transported around plant

46
Q

Advantage of evaporation other than in xylem and water transport

A

Has a cooling effect

Reduces chance of denaturation of enzymes

47
Q

Graph for rate of transpiration vs temperature

A

Directly proportional

Passes through the origin
Straight line

Increase in kinetic energy increases speed water molecules diffuse at

48
Q

Graph of rate of transpiration against light intensity

A

Positive y intercept
Proportional increase for a bit
Begins to plateau

Increase in light intensity causes increase in stomata opening (increase photosynthesis)

Plateaus when all stomata open
Stomata limiting factor

49
Q

Explain the graph of rate of transpiration against humidity

A

Positive y intercept
Straight line with negative gradient

Increasing humidity decreases water potential gradient

50
Q

Structure of phloem

A

Phloem tissue formed from cells arranged in tubes
Sieve tubes
Sieve plates
Companion cells

51
Q

How can aphids be used to show mass flow

A 
To investigate pressure in phloem
They pierce phloem
Then their bodies removed
Mouth left behind
Allowing sap to flow out
Flows out quicker nearer leaves than further down stem
Evidence of a pressure gradient
52
Q

How do metabolic inhibitors show mass flow

A 
Stop ATP production
Put into phloem 
Translocation stops
Therefore active transport involved
Can be tracked using a technique called autoradiography
53
Q

Sinks

A

Where sugars and organic substances are needed

Roots
Shoots
Flowers
Fruits

A Level Biology Paper 1 (16 decks)

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