Mass Transport In Plants

Question 1

Explain how water enters the xylem

Answer 1

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

Question 2

How is water transported to leaves

Answer 2

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

Question 3

Factors that affect transpiration

Answer 3

Light intensity
Temperature
Air movement
Humidity

Question 4

What causes root pressure

Answer 4

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

Question 5

Root adaptations

Answer 5

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

Question 6

Osmosis

Answer 6

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

Question 7

Why do plants need nitrates

Answer 7

DNA
RNA
To make the nitrogenous bases in nucleic acid

Question 8

Why do plants need magnesium ions

Answer 8

To make chlorophyll

Question 9

Why do plants need phosphate ions

Answer 9

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

Question 10

Function of xylem

Answer 10

Transports water from roots up the stem to leaves

Question 11

Function of phloem

Answer 11

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

Question 12

Xylem adaptations

Answer 12

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

Question 13

What are xylem pits

Answer 13

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

Question 14

What is transpiration

Answer 14

Evapouration of water from a plant

Driving movement of water up a xylem vessel

Question 15

Explain transpiration and how this leads to a transpiration stream

Answer 15

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

Question 16

What is a transpiration stream

Answer 16

Movement of water through plant from roots to leaves

Question 17

Explain the cohesion tension theory

Answer 17

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

Question 18

How is a high pressure produced in the leaves

Answer 18

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

Question 19

Explain the negative pressure in the xylem

Answer 19

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

Question 20

How does light affect transpiration

Answer 20

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

Question 21

How can the factor of temperature be managed to stop transpiration

Answer 21

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

Question 22

How does temperature affect transpiration

Answer 22

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

Question 23

How does humidity affect transpiration

Answer 23

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

Question 24

How does air movement/wind speed affect transpiration

Answer 24

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

Question 25

Explain the potometer experiment

Answer 25

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

Question 26

Precautions for potometer experiment

Answer 26

``` 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 ```

Question 27

Why are joints sealed in potometer experiment

Answer 27

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

Question 28

Why is the shoot cut underwater in the potometer experiment

Answer 28

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

Question 29

Why is the shoot cut diagonally in the potometer experiment

Answer 29

Stops the end of xylem closing

Question 30

Why do you dry the leaves in the potometer experiment

Answer 30

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

Question 31

Limitations of photometer experiment

Answer 31

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

Question 32

Mass flow hypothesis

Answer 32

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

Question 33

Explain what happens at a source

Answer 33

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

Question 34

Explain what ringing experiments are

Answer 34

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

Question 35

Evidence for mass flow from ringing experiment

Answer 35

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

Question 36

Evidence against mass flow hypothesis from ringing experiments

Answer 36

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

Question 37

Explain what happens at a sink

Answer 37

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

Question 38

What are radioactive tracer experiments

Answer 38

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

Question 39

What do radioactive tracers show

Answer 39

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

Question 40

What could potentially hinder mass transport of sugars

Answer 40

Sieve plates

Question 41

How are sieve cells adapted for mass transport of sugars

Answer 41

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

Question 42

How are companion cells adapted for mass flow of sugars

Answer 42

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

Question 43

How can heat treatment affect a plant

Answer 43

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

Question 44

How is pressure generated in the phloem

Answer 44

``` 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 ```

Question 45

How does the mass movement of water relate to ion dispersion

Answer 45

Mass movement of water ensures mineral ions are transported around plant

Question 46

Advantage of evaporation other than in xylem and water transport

Answer 46

Has a cooling effect | Reduces chance of denaturation of enzymes

Question 47

Graph for rate of transpiration vs temperature

Answer 47

Directly proportional Passes through the origin Straight line Increase in kinetic energy increases speed water molecules diffuse at

Question 48

Graph of rate of transpiration against light intensity

Answer 48

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

Question 49

Explain the graph of rate of transpiration against humidity

Answer 49

Positive y intercept Straight line with negative gradient Increasing humidity decreases water potential gradient

Question 50

Structure of phloem

Answer 50

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

Question 51

How can aphids be used to show mass flow

Answer 51

``` 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 ```

Question 52

How do metabolic inhibitors show mass flow

Answer 52

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

Question 53

Sinks

Answer 53

Where sugars and organic substances are needed Roots Shoots Flowers Fruits