Module 3 Section 3: Plant Transport

Question 1

Why do plants need transport systems

Answer 1

They are multicellular:
Increasing transport distances
Small surface area : volume ratio
Relatively high metabolic rate

Direct diffusion would be too slow to meet metabolic demands

Question 2

What do transport systems do in plants

Answer 2

Need transport systems to move substances to and from individual cells quickly

Question 3

Two types of plants

Answer 3

Herbaceous dicots (non woody stem)
Woody dicots (woody stem)

Question 4

What are vascular system of a plant

Answer 4

Transport vessels that run through the root, stem, leaves arranged in vascular bundles
Herbaceous dicots have vascular systems made up of two transport vessels: phloem and xylem

Question 5

Why does size of the plant cause the need for a transport system

Answer 5

The plant is made up of lots of cells and each one requires water, glucose and mineral ions
The roots take up water and mineral ions while the leaves produce glucose by photosynthesis
These molecules need to be transported to the other ends of the plant and this large distance means that simple diffusion cannot be used as it wouldn’t be fast enough to meet the metabolic demands

Question 6

Why does the surface area to volume ratio of a plant cause the need for a transport system

Answer 6

They have less surface area available for substances to diffuse through, so the rate of diffusion may not be fast enough to meet its cells requirements
Large plants cannot rely on diffusion alone to supply their cells with substances such as food and oxygen and to remove waste products.

Question 7

How are plants adapted to increase SA:V

Answer 7

Branching body shape
Leaves are flat and thin
Roots have root hairs

Question 8

Why does having a high metabolic rate cause the need for a transport system in plants

Answer 8

Have more cells so there is a high demand for oxygen and nutrients and more waste is produced

Question 9

What are the different plant transport systems

Answer 9

Transpiration System
The movement of water molecules and dissolved minerals ions
Xylem vessels
Passive process

Translocation system
The movement of sugars (Sucrose) & amino acids
Phloem vessel – sieve & companion cells
Active process

Question 10

How are xylem and phloem arranged

Answer 10

Xylem and Phloem are arranged in vascular bundles in the roots, stems and leaves
There is a layer of cambium between these vessels which contain meristem cells

Question 11

How is xylem and phloem arranged in the root

Answer 11

Xylem in the centre (cross shape) with phloem in four separate structure to provide a drill-like structure and support for the root as it pushes into the soil

Question 12

How is xylem and phloem arranged in the stems

Answer 12

Xylem and phloem are near the outside to provide a scaffolding that reduces bending
Xylem on the inside phloem on the outside

Question 13

How is xylem and phloem arranged in the leaf

Answer 13

Xylem and phloem make up a network of veins which supports the thin leaves

Question 14

Structure of xylem vessels

Answer 14

Lignified cell walls with spiralised lignin
No end plates (mature)
No protoplasm (no nucleus or cytoplasm)
Pits in wall (non-lignified)
Vessels have small diameter

Question 15

Function of lignified cell walls in xylem vessels

Answer 15

Adds strength to withstand hydrostatic pressure so vessels don’t collapse, impermeable to water

Question 16

Function of having no end plates in xylem vessels

Answer 16

Allows the mass flow of water and dissolved solutes to be cohesive (between water molecules) and adhesive (between water and the walls)
These forces would be disrupted with end plates

Question 17

Function of having no protoplasm in the xylem vessel

Answer 17

Doesn’t impede the mass flow of water and dissolved solutes (transpiration stream)

Question 18

Function of having pits in wall for xylem vessels

Answer 18

Lateral movement of water
Allows flow of water even if air bubbles form in vessels

Question 19

Function of xylem vessels having a small diameter

Answer 19

Helps prevent the water column from breaking and assists with capillary action

Question 20

Structures of phloem tissue

Answer 20

Made up of sieve tubes and companion cells

Question 21

Structure of sieve tubes

Answer 21

Living cells forming a tube for transportation
Joined end to end to form sieve tubes
Sieve section has holes in to allow solutes to pass through
Sieve tube elements have no nucleus, very thin cytoplasm and a few organelles
Cytoplasm of adjacent cells is connected through holes in sieve plates

Question 22

Function of companion cells

Answer 22

Cells accompany sieve tube elements and carry out living functions for both of them
e.g. they provide energy for active transport of solutes

Question 23

What is the need for water in plants

Answer 23

Mineral ions and sugars are transported in aqueous solution
Water is a raw materials of photosynthesis
Cooling effect (by transpiration)
Turgor pressure - hydrostatic skeleton

Question 24

Adaptations of root hair cells

Answer 24

Very thin cellulose walls to provide a short pathway
Microscopic in size
Large SA : V ratio
Concentration of solutes in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell

Question 25

What are the water movement pathways

Answer 25

Symplastic pathway (through cytoplasm)
Vacuolar pathway (through vacuoles)
Apoplast pathway (through cell wall)

Question 26

Process of water through the apoplast pathway

Answer 26

Water enters and moves through the cell wall
Moves by diffusion (not crossing a partially permeable membrane
Water may move from cell wall to cell wall, across the intercellular spaces or it may move directly from cell wall to cell wall
This is faster than the symplastic pathway

Question 27

Process of water through the symplastic pathway

Answer 27

Water enters the cytoplasm across the partially permeable plasma membrane
Water may move from cell to cell through the plasmodesmata
Water may move from cell to cell through adjacent plasma membranes and cell walls

Question 28

Process of water through the vacuolar pathway

Answer 28

Water enters the cytoplasm across the partially permeable plasma membrane
Water can move into the sap in the vacuole, through the Tonoplast
Water may move from cell to cell through the plasmodesmata
Water may move from cell to cell through adjacent plasma membranes and cell walls
Similar to symplast pathway
Slowest route

Question 29

What is the casparian strip and where is it found

Answer 29

Found in the endodermis
The caspian strip is an impermeable layer of suburin - a waxy material
It forces all the water in the apoplast pathway into the symplastic pathways

Question 30

What is the endodermis

Answer 30

This is a continuous cylinder of endodermal cells which surrounds the central vascular tissue (xylem and phloem)

Question 31

Process when water reaches the casparian strip

Answer 31

When water reaches the endodermis of the root, it’s path is blocked.

The endodermis has a waterproof, impenetrable layer called the Casparian strip in its walls.
This is because of the waxy layer of suberin in the walls of endodermal cells.

To cross the endodermis, water in the apoplast pathway moves into the symplast pathway

In this way the selectively permeable plasma membrane of the cells can control what enters the xylem tissue.

This is important as the cell surface membrane can remove any toxic solutes from the soil, and only allow necessary water molecules and mineral ions to enter.

Question 32

Function of the casparian strip

Answer 32

Helps to control which substances reach the xylem vessels
Plays a part in increasing root pressure

Question 33

How does active transport allow water to enter the endodermis

Answer 33

Active transport reduces the water potential of endodermal cells
Water moves by osmosis, down a water potential gradient

Question 34

How does water leave the leaf once it has been transported through the xylem

Answer 34

Xylem vessels transport the water all round the plant
At the leaves, water leaves the xylem and moves into the cells mainly by the apoplast pathway
Water evaporates from the cell walls into the spaces between cells in the leaf
When the stomata (tiny pores in the surface of the leaf) open, the water diffuses out of the leaf (down the water potential gradient) into the surrounding air
The loss of water from a plant’s surface is called transpiration

Question 35

How is water transported by cohesion

Answer 35

Water molecules are cohesive (they stick together) so when some are pulled into the leaf others follow (due to hydrogen bonds)
This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards
Water enters the stem through the root cortex cells

Question 36

How is water transported by adhesion

Answer 36

Water molecules are attracted to the walls of the xylem vessels as they are polar
This helps water to rise up through the xylem vessels
This is done through capillary action

Question 37

What is translocation

Answer 37

Translocation is the movement of assimilates within phloem sieve tubes (e.g. sucrose/amino acids, hormones etc) from where they are made (source) to where they are required (sink)
It is an active process

Question 38

Features of translocation

Answer 38

Translocation occurs in phloem vessels.
Requires ATP energy to create a pressure difference.
Movement is bidirectional (from source to sink).
Liquid being transported is called ‘phloem sap’
Glucose is transported as sucrose in the phloem sap (20-30%)

Question 39

What is a source

Answer 39

This is the site where sucrose/assimilates are made and loaded into the phloem
This has a high concentration

Question 40

Examples of sources in plants

Answer 40

Green leaves and green stem
Storage organs e.g. tubers and tap roots
Food stores in seeds (which are germinating)

Question 41

How is glucose transported in a plant and why is this

Answer 41

Glucose is transported as sucrose
Sucrose has less of an osmotic effect

Question 42

What is a sink

Answer 42

Site where sucrose/assimilates are unloaded from the phloem for use or storage

Question 43

Examples of sinks in plants

Answer 43

Meristems (apical or lateral) that are actively dividing
Roots that are growing and/ or actively absorbing mineral ions
Parts of the plant where the assimilates are being stored (e.g. developing seeds, fruits or storage organs)

Question 44

3 stages or translocation

Answer 44

Active loading at the source into the phloem sieve tube
Mass flow of sucrose through the sieve tube elements (involves water from xylem)
Active unloading of sucrose at the sink

Question 45

Pathways of loading of assimilates in translocation

Answer 45

Symplastic pathway (through the cytoplasm and plasmodesmata) which is a passive process as the sucrose molecules move by diffusion.

Apoplastic pathway (through the cell walls) which is an active process.

Question 46

Process of active loading at the source

Answer 46

1. Hydrogen ions (H+) are actively pumped out of the cytoplasm of companion cells via a proton pumps into their cell walls (involves the hydrolysis of ATP – active process).
2. This increases the hydrogen ion concentration in the cell walls of the companion cells compared to the inside. Creating a concentration gradient.
3. Hydrogen ions, re-enter the cytoplasm of the companion cell, down their concentration gradient via a cotransporter protein.
4. While transporting the hydrogen ions this co-transporter protein also carries sucrose molecules (at a different binding site) into the companion cell against the concentration gradient for sucrose (by facilitated diffusion).
5. The sucrose molecules then diffuse into the phloem sieve tubes via the plasmodesmata from the companion cells.

Question 47

Adaptations of companion cells for active loading

Answer 47

They have infoldings in their cell surface membrane which increases the available surface area for the active transport of solutes
Many mitochondria to provide the energy for the proton pump
This mechanism permits some plants to build up the sucrose in the phloem to up to three times the concentration of that in the mesophyll.

Question 48

Process of mass flow through phloem sieve tubes

Answer 48

Sugars/sucrose/assimilates enter the sieve tube element (at the source), this lowers the water potential in the sieve tube.
Water enters the sieve tube by osmosis from the xylem.
This raises the hydrostatic pressure at the source.
When assimilates leave the sieve tube at the sink, this increases the water potential inside the sieve tube.
Water leaves the sieve tube by osmosis, down a water potential gradient and lowers the hydrostatic pressure (at the sink).
Water moves down the hydrostatic pressure gradient (from high to low) towards the sink, also moving sucrose (and other assimilates) along the phloem.
This is called mass flow.

Question 49

Active unloading of sucrose at the sink

Answer 49

Sucrose is actively transported out of the companion cells and then moves out of the phloem sieve tubes into the sinks via the apoplastic or symplastic pathways.
In the sink sucrose is converted into other molecules e.g. starch. This helps to maintain a concentration gradient.
When sucrose diffuses out of the sieve tubes, this increases the water potential of the tube.
Water therefore moves out of the sieve tube (back into the xylem vessels) by osmosis.
This creates a low hydrostatic pressure at the sink, compared to the higher hydrostatic pressure at the source.

Question 50

Uses of glucose in plants

Answer 50

Raw material for growth, repair and replacement of damaged parts
Used to release energy in respiration - energy then used make amino acids then proteins
Used to make cellulose
Energy stored as starch
Energy stored as sucrose in fruit
Makes fats and oils

Question 51

Evidence for mass flow

Answer 51

Translocation can be slowed down or even stopped at high temperatures or by respiratory inhibitors
Collecting and studying the sap from plants with ‘clotting’ sap (eg. castor oil plants).
Using radioactively labelled metabolites (eg. Carbon-14 labelled sugars) which can be traced during translocation.
Advances in microscopes enabling the adaptations of companion cells to be seen.
Observations about the importance of mitochondria to the process of translocation

Question 52

Explain the cohesion tension theory

Answer 52

Water vapour diffuses/evaporates out of the leaf via the stomata (transpiration) from an area of high ψ to an area of low ψ.
This loss of water vapour creates a low hydrostatic pressure at the top of the xylem (i.e. in the leaf).
Water is drawn into the xylem in the root (higher hydrostatic pressure). Pressure gradient is created.
This creates a tension (suction) in the xylem which pulls up water in a continuous column.
Within the xylem vessels the columns of water are held together by cohesion and by adhesion
Column (of water) is pulled up by tension

Question 53

Evidence for the cohesion tension theory

Answer 53

Changes in tree diameter – at high transpiration rates (during the day) diameter decreases due to the tension. At night, during low transpiration rate diameter increases

Cut flowers – often they draw air in rather than leaking water out, as water continues to move up the cut stem

Broken xylems – broken or cut xylems stops drawing up water as the air drawn in breaks the transpiration stream – cohesion between water molecules

Question 54

How do the stomata open and close in the leaf

Answer 54

When guard cells are turgid: stomata open
When guard cells are flaccid: stomata closed

Question 55

How do guard cells open the stomata

Answer 55

Guard cells are turgid – Stomata Open
Water moves into the vacuoles by osmosis.
Outer wall is more flexible than the inner wall, so to cell bends and opens the stoma

Question 56

How do guard cells close the stomata

Answer 56

Guard cells are flaccid – Stomata closed
Water moves out of the vacuoles by osmosis.
Outer wall is more flexible than the inner wall, so to cell bends back and closes the stoma

Question 57

What conditions are stomata open

Answer 57

Low CO2 concentration inside the leaf
High light intensity

Question 58

What conditions are stomata closed

Answer 58

High CO2 concentration inside the leaf
Low light intensity (e.g. darkness)

Question 59

What is transpiration

Answer 59

Transpiration is the loss of water vapour (by evaporation and diffusion) from the surface of leaves and stems of a plant

Question 60

How does the transpiration stream work

Answer 60

Roots take up water from the soil
Water is drawn up the stem to the leaves
Veins carry water into the leaves
Water evaporates from the leaves

Question 61

How does transpiration relate to gas exchange in the leaf

Answer 61

Happens as a consequence of gas exchange
Stomata need to be open already to allow CO2 in and O2 out
So water moves out of the leaf from an area of high WP to low WP

Question 62

How is the rate of transpiration controlled

Answer 62

Waxy cuticle (waterproof layer)
Guard cells can open or close stomata
Very few stomata on the upper surface of the leaf

Question 63

Difference between transpiration and the transpiration stream

Answer 63

Transpiration: the loss of water vapour/evaporation of water from the aerial parts of a plant (leaves, stem, stomata)

Transpiration stream: the flow of water (in continuous columns), up the xylem vessels from roots to leaves

Question 64

Function of stomata

Answer 64

When stomata are open they allow gas exchange between the leaf and the outside environment.
Carbon dioxide can enter the leaf through the stomata and oxygen and water vapour can diffuse out of the stomata.
Transpiration is mainly controlled by the opening and closing of the stomata.

Question 65

Conditions for stomata opening

Answer 65

Open during the day and close at night
High water potential outside stomata

Low concentrations of CO2 inside the leaf cause stomata to open
High CO2 in leaf causes stomata to close.

Question 66

Adaptations of guard cells

Answer 66

Unevenly thickened (cell) wall – wall beside the pore is thicker.
Able to change shape/bend
Transport proteins/ion channels in the plasma membrane. Absorption of K+ ions by the guard cells.
K+ ions decrease the water potential hence water enters by osmosis and guard cells can become turgid.
Presence of chloroplasts & mitochondria to provide ATP energy

Question 67

What is the rate of transpiration

Answer 67

The rate at which transpiration occurs refers to the amount of water lost by plants over a given time period

Question 68

What factors affect the rate of transpiration

Answer 68

Temperature
Light intensity
Humidity
Wind

Question 69

How does temperature affect transpiration

Answer 69

Temperature increase causes an increase in kinetic energy of molecules
This means that water molecules will have more kinetic energy and will move down a concentration gradient at a faster rate
If the temperature gets too high the stomata close to prevent excess water loss, this decreases the rate of transpiration

Question 70

How does light intensity affect the rate of transpiration

Answer 70

Stomata close in the dark
This greatly reduces the rate of transpiration
When light is sufficient enough for the stomata to open, the rate of transpiration increases
Once the stomata are open, any increase in light intensity has no effect on the rate of transpiration
Stomata will remain open at relatively low light intensities

Question 71

Why do stomata open at high light intensity

Answer 71

At high light intensity, the rate of photosynthesis increases.
As photosynthesis increases, the amount of stored glucose in the guard cells
increases.
This lowers the water potential of the leaf (i.e. the contents of the leaf are less dilute).
As the water potential decreases, more water enters the guard cells making them more turgid.
The turgor pressure of the guard cells leads to an opening up of stomata resulting in transpiration

Question 72

How does air movement affect transpiration

Answer 72

Usually a lower concentration of water molecules in the air outside the leaf
When the air is still water molecules accumulate near the leaf surface
This creates a small area of high humidity which lowers the concentration gradient and the rate of transpiration
Air currents can sweep water molecules away from the lead surface, maintaining the concentration gradient and increasing the rate of transpiration

Question 73

How does humidity affect transpiration

Answer 73

High humidity means there’s a large concentration of water molecules in the air surrounding the leaf surface
This reduces the concentration gradient between inside the lead and the outside air which causes the rate of transpiration to decrease
At a certain level of humidity, an equilibrium is reached; there is no concentration gradient and so there is no net loss of water vapour from the leaves

Question 74

What is a potometer

Answer 74

Instrument used to measure transpiration rates
Measures the rate of water uptake by a plant
Assumed that water uptake by the plant is directly related to water loss by the leaves

Question 75

What can potomer be used to measure

Answer 75

Used to measure rate of transpiration
Can use it to estimate how different factors effect the rate of transpiration

Question 76

How to use a potometer to measure rate of transpiration

Answer 76

Cut a shoot at a slant under water to prevent air from entering the xylem
Slanted cut increases surface area for water uptake
Assemble potometer under water and inset shoot underwater so no air can enter
Remove aparatus from the water but keep the end of the capillary tubing submerged in water
Check apparatus is water tight and airtight (can use Vaseline at joints to achieve this)
Dry leaves and allow time for shoot to acclimatise before shutting the tap
Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put end of the tube back in the water
Record starting position of air bubble
Start a stopwatch and record the distance move by the bubble per unit of time (e.g. per hour)
Rate of air bubble movement is an estimate for the rate of transpiration
Only change one variable at a time (e.g. temperature)
All other conditions (e.g, light and humidity) must be kept constant

Question 77

What are the different types of plants and water

Answer 77

Mesophytes
Hydrophytes
Xerophytes

Question 78

What are mesophytes

Answer 78

Plants that are able to take up sufficient water to replace transpiration (most plants)

Question 79

Where are hydrophytes found

Answer 79

Plants that live either partially or completely submerged in water
This causes problems with O2 uptake

Question 80

Where are xerophytes found

Answer 80

Plants that live in areas where water lost via transpiration is greater than taken up by roots

Question 81

What are xerophytes

Answer 81

Xerophytes are plants with structural and physiological adaptations that enable them to survive in hot, dry conditions

Question 82

What are the differences in xerophytes transpiration rates depending on conditions

Answer 82

When water is abundant, their rate of transpiration is about the same as other plants.
However, in prolonged drought, they have several adaptations, which make them successful

Question 83

What are the adaptations of xerophytes

Answer 83

Smaller leaves which reduce the surface area for water loss.

Densely packed mesophyll and thick waxy cuticles prevent water loss via evaporation

Xerophytes respond to low water availability by closing the stomata (which are found in pits to shelter from wind) to prevent water loss

Contain hairs and pits which serve as a means of trapping moist air above the leaf thus reducing the water vapour potential.
Stomata may be found only on upper epidermis so they can open into the humid space created by the hairs

Xerophytes also roll the leaves when flaccid to reduce the exposure of lower epidermis to the atmosphere, thus trapping air e.g. Maram grass

Leaves reduced to scales, spines or needles to reduce surface area for transpiration

Question 84

How do xerophytes reduce their surface area : volume ratio

Answer 84

Have thick leaves rather than thin, broad leaves
Leaves are reduced to spines
Spines can help protect plant from animals which would otherwise remove water from the plant

Question 85

What are hydrophytes

Answer 85

Plants that live in water
E.g. water lilies

Question 86

Adaptations of hydrophytes

Answer 86

Very thin or no waxy cuticle as they don’t need to conserve water

Many constantly open stomata are found on the upper surfaces of leaves to maximise gas exchange

Wide, flat leaves give a large surface area for light absorption

Air sacs are found in some hydrophytes to enable leaves to stay afloat

Reduced root system as they extract nutrients from the water through their tissues

Reduced veins in leaves as xylem is not needed to transport water through the plant

Question 87

Where else can water be lost from a leaf apart from stomata

Answer 87

Waxy cuticle or epidermis

Question 88

How to reset potometer

Answer 88

Use syringe to suck the bubble back to the starting place or open reservoir
Bubble is reused
Bubble must not reach stem

Question 89

Labelled potometer with how each component contributes to measuring rate of transpiration

Answer 89

Question 90

Translocation diagram

Answer 90