Transport in Plants

Question 1

How is water taken up into plants?

Answer 1

Water moves by osmosis from the soil through root hair cells

The soil has a very dilute solution of mineral ions meaning it has a high water potential and the vacuole in the root hair call contains cell sap of ions and sugars lowering water potential.

Question 2

How are root hair cells adapted to their function?

Answer 2

The root hair cell has root hairs which increase the surface area massively meaning water can be taken up quickly.

Root hair cells have thin cell walls which decreases the diffusion pathway and increase the rate of water uptake.

Question 3

What is the apoplast pathway?

Answer 3

Water moves in the cell walls. Cellulose fibres in the cell wall are separated by spaces through which the water moves.

Question 4

What is the symplast pathway?

Answer 4

Water moves through the cytoplasm and plasmodesmata.

Question 5

What is the vacuolar pathway?

Answer 5

Where water moves from vacuole to vacuole

Question 6

Why does water move across the root?

Answer 6

There is a water potential gradient across the root cortex. It is highest in the root hair cells and lowest in the xylem, so water moves down the water potential gradient, across the root.

Question 7

Describe the structure of the xylem in the roots

Answer 7

In roots, the xylem is central and star-shaped with phloem between groups of xylem cells.

Question 8

Why are the xylem and phloem arranges the way they are in the root?

Answer 8

It resists vertical stress (pull) and anchors the plant in the soil

Question 9

Describe the arrangement of tissues in the stem of a plant

Answer 9

The vascular bundles are in a ring at the periphery, with xylem towards the centre and phloem towards the outside .

Question 10

Why are the xylem and phloem arranges the way they are in the stem?

Answer 10

This gives flexible support and resists bending

Question 11

How are tracheids adapted to their function?

Answer 11

Their cell walls contain lignin, which is hard, strong and waterproof.

Question 12

How does water travel through tracheids?

Answer 12

The walls have gaps, called pits, through which the water travels.

Tracheids are spindle-shaped so water takes a twisting rather than a straight path up the plant.

Question 13

What plants do tracheids occur in?

Answer 13

Ferns, conifers and angiosperms (flowering plants)

Question 14

Why can mosses not grow as tall as other plants?

Answer 14

They have no water-conducting tissue and are therefore poorer at transporting water.

Question 15

What are the two types of conducting cell in the xylem?

Answer 15

Tracheids and vessels

Question 16

What are the two main functions of the xylem?

Answer 16

1) Transport of water and dissolved minerals

2) Providing mechanical strength and support

Question 17

What conducting tissue only occurs in angiosperms?

Answer 17

Vessels

Question 18

How do vessels get the shape of a long hollow tube?

Answer 18

As lignin builds up in their cell walls, the contents die, leaving an empty space, the lumen. As the tissue develops, the end walls of the cells break down.

Question 19

How does water climb in vessels?

Answer 19

Due to it having a long hollow fine, like a drainpipe, water climbs straight up the plant.

Question 20

Why are angiosperms the most dominant plant on earth?

Answer 20

Water moving through vessels is so much more efficient than the twisting path through tracheids.

Question 21

How do you identify xylem in microscope sections?

Answer 21

Unlike cellulose of phloem cell walls, the lignin in the xylem stains red.

Question 22

Why cannot water enter the xylem from the apoplast pathway ?

Answer 22

Because lignin makes xylem walls waterproof, water can only pass from the symplast or vacuolar pathways.

Question 23

How does water leave the apoplast pathway?

Answer 23

The endodermis walls are impregnated with a waxy, suberin, forming a distinctive band called the casparian strip. Suberin is waterproof so the Casparian strip prevents water moving further in the apoplast and drives it into the cytoplasm.

Question 24

How does the plant achieve a lower water potential in the xylem than the endodermis cells?

Answer 24

1) The water potential of the endodermis cells is raised by water being driven in by the casparian strip
2) The water potential of the xylem is decreased by active transport of mineral salts, mainly sodium ions from the endodermis and pericycle into the xylem

Question 25

How do minerals move into the root hair cells?

Answer 25

In the soil minerals are present in very low concentrations, so generally, minerals are absorbed into the cytoplasm by active transport.

Question 26

How do mineral ions move through the root cortex?

Answer 26

Along the apoplast pathway, in solution

Question 27

How do mineral ions enter the xylem from the apoplast pathway?

Answer 27

When the minerals reach the endodermis, the casparian strip prevents further movement in the cell walls, so they enter the cytoplasm by active transport, and then diffuse or are actively transported into the xylem.

Question 28

How is active transport important at the endodermis?

Answer 28

It allows the plant to absorb the ions selectively

Question 29

Describe the cohesion tension theory

Answer 29

Water has hydrogen bonding and a dipole structure, therefore water molecules are attracted towards each other which is cohesion.

As water is drawn up in the root, it evaporated from the leaf pulling up the other water molecules behind it which is called tension.

Adhesion is when water molecules are attracted to the hydrophilic lining of the xylem

These all help water move up the xylem

Question 30

What is root pressure?

Answer 30

A consequence of osmotic movement of water going down a concentration gradient at the base of the xylem due to uptake of ions pushing water already there further up.

Question 31

What other mechanisms move water up plant?

Answer 31

Capillarity

Question 32

What is a harmful aspect of transpiration?

Answer 32

Water loss

Question 33

What are the useful aspects of transpiration?

Answer 33

1) water uptake
2) water distribution
3) ion distribution
4) evaporative cooling

Question 34

What factors does transpiration rate depend on?

Answer 34

Genetic factors e.g distribution ad size of stomata

Environmental factors e.g temperature, humidity and air movement - affect the water potential gradient between water vapour in leaf and atmosphere

Light intensity

Question 35

How does temperature increase effect transpiration rate ?

Answer 35

It increases the kinetic energy of water molecules, meaning they move faster and accelerates their rate of evaporation. The higher temperature causes the water molecules to diffuse away from the leaf more quickly, reducing the water potential around the leaf.

Question 36

How does high humidity effect transpiration rate ?

Answer 36

Transpiration leaves a layer of saturated air at the surface of leaves surface. The higher the humidity the smaller the difference in water potential and therefore water will not diffuse down a concentration gradient as quickly so transpiration rate will decrease.

Question 37

What does an increase in air movement do to the transpiration rate?

Answer 37

The faster the movement of the surrounding air the faster the concentric shells of water vapour get blown away. This increases the water potential gradient between the inside and the outside of the leaf meaning transpiration rate increases.

Question 38

How does a higher light intensity effect the transpiration rate?

Answer 38

As light intensity this increases the width of which the stomata open, increasing the rate of transpiration. So stomata tend to open the widest in the middle of the day increasing transpiration rate.

Question 39

What do you have to make sure you do when you set up a potometer?

Answer 39

Cut the leafy shoot/assemble the equipment under water - to make sure no air bubbles enter the xylem as this could block transportation stream

Seal joints with Vaseline to make sure no air enters the equipment

Dry leaves to remove and water from around the leaf which would reduce transpiration rate

Question 40

What is a mesosphyte?

Answer 40

A plant living in conditions of adequate water supplies

Question 41

What happens if a mesophyte loses too much water?

Answer 41

It wilts and it’s leaves droop. The stomata close and the leaf surface are available for absorbing light is reduced so photosynthesis becomes less efficient.

Question 42

What do mesophytes do to survive unfavourable times of the year when the ground is frozen and liquid water is not available?

Answer 42

1) Shed their leaves before winter, so that they do not lose water by transpiration.
2) The aerial parts of many non-Woody plants die off in winter so they are not exposed to cold winds, but their underground organs survive.
3) most annual mesophutes lay over-winter as dormant seeds, with such a low metabolic rate that almost not water is required

Question 43

What are xerophytes?

Answer 43

They are plants that live in conditions where water is scarce e.g hot or dry deserts or cold regions where the soil water is frozen, windy locations

Question 44

How are the stomata of marram grass adapted to their environment?

Answer 44

Stomata occurs on the upper surface, but not the outer surface of the leaf.

They are in pits and humid air is trapped in the pit, outside the stomata.

Question 45

How do the adaptations of the stomata in marram grass help the plant survive?

Answer 45

This reduces the water potential gradient between the inside of the leaf and the outside and so reduces the rate of diffusion of water out through the stomata.

Question 46

How does the adaptations of the shape of marram grass leaves help the plant survive?

Answer 46

This reduces the leaf area exposed to air so reduces transpiration.

Question 47

How is the surface of the leaf of a marram grass plant adapted to its surroundings? And how does it help them to survive?

Answer 47

It has stiff, interlocking hairs trap water vapour and reduce the water potential gradient between the inside of the leaf and the outside.

Question 48

How has the thickness of the layers in a marram grass plant cell adapted to their surroundings? And how does this help the plant to survive?

Answer 48

The plant cells have a thick cuticle and the cuticle is a waxy covering over the outer surface of the leaf. Wax is waterproof and so reduces water loss. The thicker this cuticle, the lower the rate of transpiration through the cuticle

Question 49

What are hydrophytes?

Answer 49

They are plants which grow partially or wholly submerged in water.

Question 50

What tissues do hydrophytes not have and why?

Answer 50

They have little or no lignified support tissues because water is a supportive medium.

There is little need for transport tissue (xylem is poorly developed) because they are surrounded by water

Question 51

How are hydrophytes’ leaves and stems adapted to their surroundings?

Answer 51

Leaves have little or no cuticle, because there is no need to prevent water loss.

Stems and leaves have large air spaces, continue down to their leaves, forming a reservoir of o2 and co2, which provide buoyancy.

Question 52

How are the stomata in hydrophytes adapted to their function?

Answer 52

Stomata are on the upper surface of floating leaves, because the lower surface is on the water.

Question 53

How are the end walls of the phloem different than in the xylem?

Answer 53

The end walls do not break down. Instead, the end walls and sometimes parts of the side walls are perforated in areas called sieve plates.

Question 54

What types of cells and tissue is the phloem made up of?

Answer 54

It is a living tissue and consists of several types of cells, including sieve tubes and companion cells.

Question 55

What adaptation do sieve tube elements have to make them suited to their function?

Answer 55

They lose their nucleus and most of their other organelles during development, allowing space for transporting materials.

Question 56

How is the metabolism of a sieve tube element controlled?

Answer 56

It is controlled by at least one neighbouring companion cell.

Question 57

What features of a companion cell suggest they are very biochemically active?

Answer 57

This is indicated by the large nucleus, dense cytoplasm containing much RER and many mitochondria.

Question 58

How are companion cells and sieve tube elements connected?

Answer 58

Plasmodesmata

Question 59

How is transport in the xylem different to in the phloem?

Answer 59

Products of photosynthesis are translocated from “source” to “sink”. Means unlike xylem which can only transport upwards, phloem can translocate up, down and sideways.

Question 60

What main materials does the phloem transport?

Answer 60

Sucrose and amino acids

Question 61

What is the technique for a ringing experiment?

Answer 61

Cylinders of outer bark tissue are removed from all the way around a Woody stem. This would remove the phloem. The plant would then be left some time, while it photosynthesised, then the contents above and below the ring would be analysed.

Question 62

What are the results of a ringing experiment?

Answer 62

Above the ring, there was a lot of sucrose, suggesting it had been translocated in the phloem from source to sink. Below the ring, there was no sucrose, suggesting it had been used by plant tissues but not replaced, because the ring prevented it moving downwards.

Question 63

How is a radioactive tracer and autoradiography experiment set up to investigate organic substances translocated in phloem?

Answer 63

A stem section is placed on a photographic film which is exposed if there is a radiation source, producing an autoradiograph.

Question 63

What are the results of the autoradiography experiment?

Answer 63

A plant photosynthesis in the presence of 14C in carbon dioxide 14CO2. The position of exposure and therefore the radioactivity, coincides with the position of the phloem, because it is the phloem that translocates the sucrose made of 14CO2 in photosynthesis.

Question 64

What is the technique for using aphids to investigate the organic substances transported in phloem.

Answer 64

An aphid has a styler, this is inserted into a sieve tube and the phloem contents exudes under pressure into the stylet. The aphid was anaesthetised and removed.

Question 65

What are the results of the aphid experiment?

Answer 65

As the sap in the phloem is under pressure it exuded from the styles and was collected and analysis showed the presence of sucrose.

Question 66

What did aphid and radioactive tracer experiments show?

Answer 66

The radioactivity and therefore, the sucrose made in photosynthesis, moved at a speed of 0.5-1 m h^-1.

Question 67

Why were the results of the aphid and radioactive tracer experiment unusual?

Answer 67

The speed that the sucrose moves at is much faster than the rate of diffusion alone so some additional mechanism had to be considered.

Question 68

What is the mass flow hypothesis?

Answer 68

Suggests a passive flow of sugars from the phloem of the leaf, where there is the highest concentration (the source), to growing tissues, where there is a lower concentration (the sink).

Question 69

Describe process of mass flow?

Answer 69

In leaf cells, sucrose is made by photosynthesis. The sucrose makes the water potential very negative and water passes into the cell by osmosis.

As water enters the leaf hydrostatic pressure builds up, forcing sucrose in solution into the phloem joining source to sink.

The pressure pushes the sucrose solution down the phloem and this movement is called mass flow.

In the sink the sucrose is converted to insoluble starch so water potential is higher and then water is forced up the xylem back to the source.

Question 70

What aspects of translocation does mass flow not explain?

Answer 70

1) Rate of phloem transport is 10000 times faster than diffusion.
2) Does not take into account sieve plates
3) Sucrose and amino acids move at different rates in different directions
4) Phloem has a high oxygen consumption, translocation is slowed at low temperatures or when cyanide is applied.
5) Companion cells are very active but mass flow does not suggest a role for them.

Question 71

What do other theories suggest instead of mass flow?

Answer 71

1) Active process - cyanide and low temperatures inhibit translocation - indicates energy generated by respiration is used.
2) Protein filaments - pass through sieve pores so different solutes are carried along different routes though same sieve tube.
3) Cytoplasmic streaming - could be responsible for movement in different directions.