C6 - Transport Systems in Plants

Question 1

What is the structure of the xylem tissue?

Answer 1

It’s made of different cell types - the main is the xylem vessel element which is long, thin, waterproof and has walls thickened with lignin.

Lignin is impermeable to water so the cell contents die. The cells are then arranged one above the other in columns and the walls between them have broken down.
This leaves a long, thin column called a ‘xylem vessel’ which offers little resistance to the passage of water.

Question 2

What causes the xylem’s structure to form long tubes?

Answer 2

Lignin in the xylem wall is impermeable to water so the tonoplast breaks down causing autolysis of the cell contents.

The cells then die and are arranged one above the other in columns and the walls between them have broken down.

This leaves a long, thin column called a ‘xylem vessel’ which offers little resistance to the passage of water.

Question 3

How is lignin arranged within the xylem?

Answer 3

In spirals, rings (annular) or patchwork (reticulum).

Question 4

What does lignin do within the xylem?

Answer 4

It provides strength and support, preventing the vessels collapsing inwards during droughts.

Question 5

Why do xylem have pores (bordered pits)?

Answer 5

To allow water to move horizontally across other xylem vessels, in case of a blockage in any of the columns.

Question 6

What does phloem tissue consist of?

Answer 6

Sieve tubes and companion cells

Question 7

How are sieve tubes arranged (in phloem tissue)?

Answer 7

Sieve tubes are cells laid end to end with perforations in the ends of cell walls to allow sugar sap to flow (sieve plates).

They have no nucleus or tonoplast as these would impede flow.

Question 8

How are companion cells arranged (within phloem tissue)?

Answer 8

They’re next to the sieve tubes and contain a large nucleus and many mitochondria.

Question 9

What do companion cells do?

Answer 9

They provide the sieve tubes (in phloem tissue) with ATP and other materials for metabolic reactions.

Question 10

What are plasmodesmata?

Answer 10

Gaps between sieve tubes and companion cells where fluid moves.

Question 11

What are sclerenchyma fibres?

Answer 11

Columns of cells (in plants) which also have stiffened cell walls for support

Question 12

Where’s the cambium found and what does it do?

Answer 12

Found in vascular bundles and meristemic cells, it produces new xylem, phloem or cork (via cell division).

Question 13

What is transpiration?

Answer 13

The loss of water vapour from aerial parts of a plant.

Most water vapour is lost by diffusion via open stomata.

Question 14

How is water lost from leaves during transpiration?

Answer 14

Water evaporates from the surfaces of the mesophyll cells in the air spaces within the leaf, lowering the water potential in the mesophyll cells.

This causes water to move across the leaf from the xylem, through osmosis down a water potential gradient, to replace the water lost.

Question 15

What occurs during transpiration?

Answer 15

1) Water moves via osmosis into the epidermis of the root (via root hair cells).
2) The water moves through the cortex (main bulk or the root) to reach the xylem (via the symplastic, apoplastic or vascular pathways).
3) The endodermis separates the root cortex from the stele (containing xylem). The apoplastic pathway is blocked by the Casparian strip (a coating of Suberin).
4) The Casparian strip gives additional control over what enters the xylem, as water must go through the cell membrane of endodermal cells.
5) Water enters the xylem and rises up the roots towards the stem (via cohesion and adhesion).
6) When reaching the leaf, water leaves the xylem via osmosis and goes to surrounding cells. Water vapour enter air spaces in the spongey mesophyll.
7) Water vapour finally diffuses out the plant from stomata.

Question 16

What happens in the transpiration stream?

Answer 16

1) Water moves via osmosis into the epidermis of the root (via root hair cells).
2) The water moves through the cortex (main bulk or the root) to reach the xylem (via the symplastic, apoplastic or vascular pathways).
3) The endodermis separates the root cortex from the stele (containing xylem). The apoplastic pathway is blocked by the Casparian strip (a coating of Suberin).
4) The Casparian strip gives additional control over what enters the xylem, as water must go through the cell membrane of endodermal cells.
5) Water enters the xylem and rises up the roots towards the stem (via cohesion and adhesion).
6) When reaching the leaf, water leaves the xylem via osmosis and goes to surrounding cells. Water vapour enter air spaces in the spongey mesophyll.
7) Water vapour finally diffuses out the plant from stomata.

Question 17

What is the ‘cohesion-tension’ theory?

Answer 17

Water moves up the stem to replace the water leaving the xylem in the leaf.

The force of cohesion between water molecules is strong enough to allow a continuous column of water to move up in the xylem vessels.

The pulling force created is called the ‘transpiration pull’ and it causes a negative pressure/tension within the xylem.

This mechanism for water movement in the xylem is called ‘cohesion-tension theory’.

Question 18

What are the 3 methods of movement of water between plant cells?

Answer 18

The apoplastic pathway

The symplastic pathway

The vacuolar pathway

Question 19

What occurs during the apoplastic pathway?

Answer 19

A method of movement of water between plant cells where water moves into the space outside the plasma membrane and through the cell walls and extra cellular spaces.

Question 20

What’s the symplastic pathway?

Answer 20

A method of movement of water between plant cells where water moves through the plasma membrane into the cytoplasm and can move from one cell to another through the plasmodesmata.

A lower water potential in one cell caused by loss of water causes water to enter by osmosis from the neighbouring cells.
This in turn lowers their water potential so water moves from cell to cell through the continuous link of the cytoplasm down the water potential gradient.

Question 21

What is the vacuolar pathway?

Answer 21

A method of movement of water between plant cells where water enters the cytoplasm and the vacuoles of the cells and moves down the water potential gradient from cell to cell in a similar way to the symplastic route.

Question 22

How does hydrostatic pressure differ within a plant?

Answer 22

Hydrostatic pressure is lower at the top of the plant/vessel than the bottom as water moves out of the leaf cells and is replaced by water in the xylem vessels.

Question 23

How is root pressure produced?

Answer 23

Due to the Suberin and active transport in the endodermis.

Question 24

What causes the stomata (of plants) to open?

Answer 24

It’s stimulated by an environmental trigger e.g. light, temp, humidity.
K+ ions are then pumped into guard cells via active transport, producing a concentration gradient.

Water moves into guard cells via osmosis, causing them to swell, become turgid and open.

Question 25

What causes stomata to close?

Answer 25

It’s stimulated by an environmental trigger e.g. light, temp, humidity.
K+ ions then leave guard cells and water follows.

Guard cells become flaccid so the stomata close.

Question 26

What are parenchyma cells?

Answer 26

Packing cells found in vascular bundles making up a thin layer.

Question 27

What’s the pith?

Answer 27

The centre of the vascular bundle, surrounded by spongey tissue.

Question 28

What’s the endodermis?

Answer 28

The inner layer of cells separating the vascular bundle from the cortex.

Question 29

What’s the cortex?

Answer 29

The layer of cells below the epidermis, made of parenchyma.

Question 30

What’s the purpose of lignin in the xylem?

Answer 30

It strengthens and thickens the xylem walls.

Used for waterproofing the walls.

It improves the adhesion of water (molecules).

It’s (spiral) pattern allows flexibility and movement.

Question 31

What helps the structure of plant cell walls provide the plant with strength?

Answer 31

Ligninification makes walls stiffer

Hydrogen bonding

Cellulose walls which are thick

Question 32

What are monocotyledons?

Answer 32

Flowering plants whose seeds typically contain only one embryonic leaf, or cotyledon.

Question 33

What are dicotyledons?

Answer 33

Flowering plants whose seeds typically contain only two embryonic leaves, or cotyledons.

Question 34

How do monocotyledon root cells appear under a microscope?

Answer 34

Vascular tissue is found towards the centre of the root, arranges as a ring of xylem and phloem tissue.

Question 35

How do monocotyledon stem cells appear under a microscope?

Answer 35

They have scattered vascular bundles.

Question 36

How do monocotyledon leaf cells appear under a microscope?

Answer 36

Palisade cells cannot be seen and it all appears like a spongey layer.

There are rarely any midribs - veins run parallel along the leaf length.

Question 37

How do dicotyledon root cells appear under a microscope?

Answer 37

The vascular tissue is found in a central core.

Xylem tissue generally forms an ‘X’ shape and the phloem is found as patches between the ‘X’.

This is surrounded by the endodermis which is necessary for water passage.

Question 38

How do dicotyledon cells appear under a microscope?

Answer 38

The outermost layer is the endodermis.
Beneath is the cortex (thin layer of parenchyma cells tightly packed).

Below are the vascular bundles in circles around the large central region/pith.

The xylem is inside the bundle, the phloem is outside and the cambium is in between.

Question 39

How does water move into roots?

Answer 39

Roots are covered with root hair cells which increase S.A.
Water then enters by osmosis.

Water will then move across cells in the root cortex by the apoplastic, symplastic and vacuolar pathway.

Question 40

How does water move within the roots?

Answer 40

Once water has entered the root via osmosis, it will then move across cells in the root cortex by the apoplastic, symplastic and vacuolar pathway.

Question 41

What does the endodermis do to the movement of water in roots?

Answer 41

The endodermis surrounds the vascular tissue within the root, and is impregnated with a layer of Suberin - a waterproof material - which forms the Casparian strip.

The Casparian strip blocks the apoplastic pathway (blocking extra cellular space) which forces water into the cells via the symplastic pathway to the endodermis.

Question 42

What’s the Casparian strip?

Answer 42

A structure formed in plant cells due to a layer of waterproof material (Suberin) impregnated in the walls of root cells.

Question 43

What does the Casparian strip do?

Answer 43

Being waterproof, the (Suberin) Casparian strip blocks the apoplastic pathway (blocking extra cellular space) which forces water into the cells via the symplastic pathway to the endodermis.

Question 44

What do endodermal cells do?

Answer 44

Actively transport mineral salts into the xylem and regulate water potential, controlling the flow of water via the symplastic pathway.

Question 45

How do endodermal cells affect root pressure?

Answer 45

Root pressure is a force responsible for pushing water up the xylem.

The plasma membrane of endodermal cells has many transport proteins to move molecules via active transport.

Question 46

What features of a plant affect transpiration?

Answer 46

Number of leaves on the plant

Number and size of stomata

Presence and thickness of a cuticle

Question 47

How does the number of leaves on a plant affect transpiration?

Answer 47

The greater the number of leaves, the greater the surface area for the loss of vapour, and so the increase in transpiration.

Question 48

How does the number and size of stomata affect transpiration?

Answer 48

This affects the diffusion of water from air spaces in the mesophyll.

The greater the number or size of stomata, the more diffusion of water from the air spaces, causing increased evaporation from the surface of mesophyll cells so transpiration increases.

Question 49

How does the presence and thickness of a cuticle affect transpiration?

Answer 49

The waxy cuticle reduces water loss. A thicker cuticle will increase this effect.

The thinner the the cuticle / absence of the cuticle, the greater the transpiration.

Question 50

What environmental factors affect the rate of transpiration?

Answer 50

Light

Temperature

Humidity

Air movement

Question 51

How does light affect the rate of transpiration?

Answer 51

Stomata open in the light and close in the dark.

The brighter the light, the greater the rate of transpiration.

Question 52

How does temperature affect the rate of transpiration?

Answer 52

It changes the kinetic energy of the water molecules and the relative humidity of the air.

The greater the temperature, the greater the rate of transpiration.

Question 53

How does humidity affect the rate of transpiration?

Answer 53

It affects the water potential gradient between the leaf’s air spaces and the atmosphere.

The lower the humidity, the greater the rate of transpiration.

Question 54

How does air movement affect transpiration?

Answer 54

Changes the water potential gradient by altering the rate at which moist air is removed from around the leaf.

The greater the air movement, the greater the rate of transpiration.

Question 55

What’s translocation?

Answer 55

The movement of assimilates, nutrients and sugars around the plant via the phloem.

Question 56

In what direction does translocation occur?

Answer 56

It is bidirectional (and active).

It transports assimilates from a source (where there’s a surplus) to the sink (wherever there’s a demand).

Question 57

What’s phloem loading?

Answer 57

The active loading of sucrose into the phloem at a source.

Question 58

What does translocation depend on?

Answer 58

Pressure flow hypothesis - the flow relies on hydrostatic pressure gradients between source and sink.

Question 59

What occurs during phloem loading?

Answer 59

Companion cells alongside the phloem sieve tubes use ATP to actively transport protons out of the companion cell cytoplasm and into surrounding tissue / mesophyll cells.

This produces a concentration gradient. A co-transporter protein allows the hydrogen ions to flow back, along with sucrose.

Question 60

What happens during translocation?

Answer 60

1) Plants produce glucose from photosynthesis and turn it into sucrose.

2) Sucrose is then moved into the phloem via active/phloem loading: companion cells alongside the phloem sieve tubes use ATP to actively transport protons out of the companion cell cytoplasm and into surrounding tissue / mesophyll cells.
This produces a concentration gradient. A co-transporter protein allows the hydrogen ions to flow back, along with sucrose.

3) Water potential is reduced, causing water to enter sieve tubes via osmosis.
As water enters the phloem, hydrostatic pressure increases which forces phloem sap through vessels towards regions of lower pressure (mass flow).

4) Sucrose is actively unloaded from the phloem wherever it’s needed for growth, or into storage organs e.g. tubers, where it’s converted into starch.
5) A sink removes sugar from the phloem, increasing water potential. Water leaves sieve tubes via osmosis, keep in hydrostatic pressure low so that sap continuously flows from source to sink.

Question 61

How is sucrose transported within the phloem?

Answer 61

Once glucose is converted to sucrose, it enters the phloem via active/phloem loading.

Water potential is reduced, causing water to enter sieve tubes via osmosis.

As water enters the phloem, hydrostatic pressure increases which forces phloem sap through vessels towards regions of lower pressure (mass flow).

Sucrose is actively unloaded from the phloem wherever it’s needed for growth, or into storage organs e.g. tubers, where it’s converted into starch.

A sink removes sugar from the phloem, increasing water potential. Water leaves sieve tubes via osmosis, keep in hydrostatic pressure low so that sap continuously flows from source to sink.

Question 62

What’s mass flow (in plants)?

Answer 62

The large movement of fluid down a pressure gradient.

When water enters the phloem, hydrostatic pressure increases. This forces phloem sap through vessels towards regions of lower hydrostatic pressure (mass flow).

Question 63

Explain the role of a ‘sink’ in the mechanism of translocation:

Answer 63

Sucrose gets removed from the phloem.
This causes water potential in the phloem to increase.

Hydrostatic pressure at the sink (of sieve tube cells) decreases.

Sucrose concentration decreases as the sucrose is used for respiration.