Plant Transport

Question 1

Location of the phloem in roots?

Answer 1

On the outside of the xylem

Question 2

Location of the xylem in the roots and why?

Answer 2

In the centre surrounded by the phloem to provide support for the root as it pushes through the soil.

Question 3

Location of xylem in the stem and why?

Answer 3

The xylem and the phloem are located on the outside, the xylem is inside the phloem though, to provide a sort of scaffolding that reduces bending.

Question 4

Location of the phloem in stems?

Answer 4

On the outside of the xylem

Question 5

Location of xylem in the leaf?

Answer 5

The xylem is the upper part

Question 6

Location of the phloem in the leaf?

Answer 6

Lower part of the vascular bundle

Question 7

Structure of xylem: Function of no end plates (walls)?

Answer 7

Allows the mass flow of water and dissolved solutes as cohesive (between water molecules) and adhesive (between water and walls) forces are not impeded .

Question 8

Structure of xylem: Function of pits in walls (non-lignified sections) ?

Answer 8

Lateral movement of water, allows continual flow in case of air bubbles forming in the vessels.

Question 9

Structure of xylem: Function of lignified cell walls?

Answer 9

Adds strength to withstand the hydrostatic pressure so the vessels do not collapse, imperable to water.

Question 10

Structure of xylem: Function of small diameter of vessels?

Answer 10

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

Question 11

Main functions of xylem?

Answer 11

• Vascular tissue that transports dissolved minerals and water around the plant.
• Structural support
• Food storage

Question 12

What 4 cell types does phloem tissue contain?

Answer 12

Phloem fibres, phloem parenchyma, sieve tube elements and companion cells

Question 13

Phloem: What are sieve tube elements?

Answer 13

Living cells that form the tube for transporting solutes through the plant.

Question 14

Phloem: What forms sieve tubes?

Answer 14

Sieve tube elements that are joined end to end. The cytoplasm of adjacent cells is connected through the holes in the sieve plates.

Question 15

Phloem: What are the ‘sieves’ and what do they allow?

Answer 15

The end walls, which have lots of holes in them to allow solutes to pass through.

Question 16

Phloem: What are the structure of sieve tube elements?

Answer 16

Unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles.

Question 17

Phloem: Why does the phloem have companion cells?

Answer 17

The lack of the nucleus and other organelles in the sieve tube elements means that they can’t survive on their own. So there’s a companion cell for every sieve tube element.

Question 18

Phloem: Function of companion cells?

Answer 18

Carry out living functions for both themselves and their sieve cells. For example, they provide the energy for the active transport solutes.

Question 19

Where is the main site for photosynethesis?

Answer 19

The palisade mesophyll

Question 20

What is the symplastic pathway?

Answer 20

When water travels through the roots into the xylem by going through living parts of cells- the cytoplasm. The cytoplasm’s of neigbouring cells connect through plasmodesmata. Water moves through the symplast pathway via osmosis

Question 21

What is the apoplastic pathway?

Answer 21

When water travels through the roots into the xylem by going through non-living parts of the cells- the cell walls. The walls are very absorbent and water can simply diffuse through them, as well as pass through spaces between them.

Question 22

What blocks the apoplastic pathway?

Answer 22

When water in the pathway gets to the endodermis cells in the root, its path is blocked by a waxy strip of cells called the Casparian strip.

Question 23

What happens after the apoplastic pathway is blocked and why is this useful?

Answer 23

Water has to take the symoplastic pathway. this is useful, because water has to go through a cell membrane. Cell membranes are partially permable ans are able to control whether or not substances in water get through.

Question 24

What is the cohesion tension theory?

Answer 24

Water evaporates from the leaves at the ‘top’ of the xylem.
This creates tension (suction), which pulls more water up into the leaf.
Water molecules are cohesive (stick together) so when some are pulled into the leaf others follow. This means the whole column of water in the xylem, from the leaves down to the roots, moves upwards.

Question 25

What is adhesion partly reponsible for and how?

Answer 25

Partly responsible for the movement of water.
1) As well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels.
2) This helps water rise up through the xylem vessels.

Question 26

What are the 4 main factors that affect transpiration?

Answer 26

Light, temperature, humidity and wind

Question 27

How does temperature affect transpiration?

Answer 27

Higher the temp, the faster the rate of transpiration. Warmer water molecules have more energy and so they evaporate from the cells inside the leaf faster. This increases the water potential gradient, making water diffuse faster out of the leaf.

Question 28

How does wind affect transpiration?

Answer 28

The winder it is, the faster the rate of transpiration. Lots of air blows away water molecules from around the stomata. This increases the water potential gradient, which increases transpiration rate.

Question 29

How does humidity affect transpiration?

Answer 29

The lower the humidity, the faster the rate of transpiration. If air around the plant is dry, the water potential gradient between the leaf and air is increased, which increases transpiration rate

Question 30

Potometers: Why is the shoot cut underwater and at a slant?

Answer 30

Cut underwater- To prevent air from entering the xylem
Cut at a slant- To increase the surface area available for water uptake

Question 31

What is transpiration?

Answer 31

The evaporation of water from the plant’s surface, especially the leaves.

Question 32

How is transpiration a consquence of gas exchange?

Answer 32

1) A plant needs to open its stomata to let in carbon dioxide so that it can produce glucose (by photosynthesis).
2) But this also lets water out- there’s a higher conc of water inside the leaf than in the air outside, so water moves out of the leaf down the water potential gradient when the stomata are open.

Question 33

What are Xerophytes?

Answer 33

Plants like cacti and marram grass (which grows on sand dunes). They’re adapted to live in dry climates. Their adaptations prevent them from loosing too much water by transpiration.

Question 34

Xerophytes adaptations: Marram grass: Stomata sunk in pits?

Answer 34

So they’re sheltered from the wind. This helps to slow transpiration down.

Question 35

Xerophytes adaptations: Marram grass: A layer of ‘hairs’ on the epidermis?

Answer 35

This traps moist air round the stomata, which reduces the water potential gradient between the leaf and the air, slowing transpiration down.

Question 36

Xerophytes adaptations: Marram grass: Roll their leaves in hot or windy conditions?

Answer 36

This traps moist air, slowing transpiration. It also reduces the exposed surface area for losing water and protects the stomata from wind.

Question 37

Xerophytes adaptations: Marram grass and cacti: Thick waxy layer on the epidermis?

Answer 37

This reduces water loss by evaporation because the layer is waterproof (water can’t move through it).

Question 38

Xerophytes adaptations: Cacti: Spines instead of leaves?

Answer 38

This reduces the surface area for water loss.

Question 39

Xerophytes adaptations: Cacti: Close their stomata?

Answer 39

At the hottest times of the day when transpiration rates are highest.

Question 40

What are hydrophytes?

Answer 40

Plants like water lilies, which live in aquatic habitats. As they grow in water, they don’t need adaptations to reduce water loss (like xerophytes), but they do need adaptations to help them cope with a low oxygen level.

Question 41

Hydrophyte adaptations: Air spaces in the tissues?

Answer 41

Help the plants to float and can act as a store of oxygen for use in respiration.

Question 42

Hydrophyte adaptations: Air spaces in the tissues: Example?

Answer 42

Water lilies have large air spaces in their leaves. This allows the leaves to float on teh surface of the water, increasing the amount of light they recieve. Air spaces in the roots and stems allow oxygen to move from the floating leaves down to parts of the plant that are underwater.

Question 43

Hydrophyte adaptations: Stomata on upper surface of floating leaves?

Answer 43

This helps to maximise gas exchange

Question 44

Hydrophyte adaptations: Flexible leaves and stems?

Answer 44

These plants are supported by the water around them, so they don’t need rigid stems for support. Flexibility helps to prevent damage by water currents.

Question 45

What is translocation?

Answer 45

The movement of dissolved substances (e.g. sugars like sucrose and amino acids) to where they are needed in a plant. Dissolved substances are sometimes called assimilates.

Question 46

What does translocation require and where does it occur?

Answer 46

Its an energy-requiring process that happens in the phloem.

Question 47

What does translocation do?

Answer 47

Moves substances from ‘sources’ to ‘sinks’. The source of a substance is where it is made (so it’s at a high conc there). The sink is the area where it is used up (so it’s at a lower conc there).

Question 48

Translocation: Where is the source and sink for sucrose?

Answer 48

Source is usually in the leaves (where it’s made), and the sinks are the other parts of the plant, especially the food storage organs and the meristems (areas of growth) in the roots, stems and leaves.

Question 49

What does mass flow hypothesis best explain?

Answer 49

Phloem transport (translocation)

Question 50

Mass flow hypothesis: What happens at the source?

Answer 50

1) Active transport is used to actively load the solutes (e.g. sucrose from photosynthesis) into the sieve tubes of the phloem at the source (e.g. the leaves).
2) This lowers the water potential gradient inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.
3) This creates high pressure inside the sieve tube at the source end of the phloem.

Question 51

Mass flow hypothesis: What happens at the sink end?

Answer 51

1) At the sink end, solutes are removed from the phloem to be used up.
2) This increases the water potential gradient inside the sieve tubes, so water also leaves the tubes by osmosis.
3) This lowers the pressure inside the sieve tubes.

Question 52

Mass flow hypothesis: What is the result of what is happening at the source and sink end?

Answer 52

1) The result is a pressure gradient from the source end to the sink end.
2) This gradient pushes solutes along the sieve tubes where to they are needed.

Question 53

What is active loading used for?

Answer 53

Is used to move substances into the companion cells from surrounding tissues and from the companion cells into the sieve tubes, against a concentration gradient

Question 54

How is sucrose moved by active loading?

Answer 54

1) In the companion cell, ATP is used to actively transport hydrogen ions out of the cell and into the surrounding tissue.
2) This sets up a conc gradient- there are more hydrogen ions in the surrounding tissue than in the companion cell.
3) A hydrogen ion binds to a co-transport protein in the companion cell membrane and re-enters the cell (down the conc gradient).
4) A sucrose molecule binds to the co-transport protein at the same time. The movement of the hydrogen ion is used to move the sucrose molecule into the cell, agaisnt its conc gradient.
5) Sucrose molecules are then transported out of the companion cells an into the sieve tube elements by the same process.