Chapter 9- Transport In Plants

Question 1

Why are transport systems needed in plants- why do flowering plants have transport systems

Answer 1

These plants take in water and mineral ions through their roots and make glucose in their leaves by photosynthesis. Water, mineral ions and glucose are needed by all the cells in the plant and so need to be transported from one area to another. Flowering plants have two systems: the xylem for transporting water and mineral ions, and the phloem for transporting nutrients, such as photosynthates and amino acids.

Question 2

Why does most plants have a high surface area to volume ratio?

Answer 2

Plants have a branching body shape which helps to give a very high surface area to volume ratio. The leaves which absorb light, are thin and flat and so maximise their surface area to volume ratio. The roots have root hairs to greatly increase their surface area. Plus, the leaves and stems have chloroplasts that can generate their own oxygen and use up carbon dioxide which also help eradicate the need for a gaseous exchange transport.
Due to this, plants do not have any system for transporting oxygen and carbon dioxide.

Question 3

Why do plants have a low demand for oxygen for aerobic respiration unlike animals

Answer 3

The metabolic rates of plant tissues are low thus there is a low demand for oxygen for aerobic respiration.

Question 4

What is a vascular system

Answer 4

This is something that plants consist. It involves two systems. One transports water and mineral ions up the plants from the root through a system of tubes called xylem vessels.
A set of tubes called phloem sieve tubes carries other transport assimilates and minerals up.

Question 5

What is a xylem

Answer 5

It contains several typed of cells including vessels, tracheids, fibres and parenchyma. However, the primary cells that are responsible for the transportation are the xylem vessel elements.
These are attached end to end to form a continuous xylem vessel. The structure of these vessels are adapted to allow the xylem to fulfill its primary function- transport water and dissolved ions.
The adaptations also allow provide support and strength to the plant.

Question 6

Structure of a xylem

Answer 6

The cells that form this vessel is dead and has no cytoplasm; this allows water to flow through more easily.
The cell wall is thickened by lignin, which makes it rigid and able to give support.
There are pits which allow to move transversely (sideways) from cell to cell. Pits are areas where the cellulose cell wall is not thickened with lignin.
The end wall may be perforated to allow the free passage of water to the next cell
the pattern of lignin thickening may be in rings, coils or strips

Question 7

What is a phloem

Answer 7

Phloem is mainly made of sieve tube elements and companion cells. These cells are living unlike the cells in a xylem vessel.

The sieve tube elements form into continuous tubes and each of them had an associated companion cell. The sieve tube elements has no nucleus, few organelles and little cytoplasm despite being alive. This aids their ability to transport assimilates, but it means that they are not self-sufficient.
The main function of a companion cells is to be a sort of a ‘life support system’ providing materials to keep the sieve tube element alive. The sieve cell and the companion cell are connected by a plasmodesmata, which allows the transport of molecules.

Question 8

The definition of a plasmodesmata

Answer 8

A microscopic tunnel theough plant cell wall, connecting the cytoplasm of companion cells and sieve cells. These are lined with plasma membranes.

Question 9

What is water potential and water movement in terms of the xylem vessel

Answer 9

Water moves from a high water potential to a low water potential. So, water generally moves out of the leaves since there is a higher water potential inside the leaves compared to the surroundings of the leaves. Water enters the roots and leaves via the leaves, which creates a water potential gradient from the roots up the stem to the leaves. Roots have the high water potential and the leaves have the lowest water potential.

Question 10

How does the water reach the xylem

Answer 10

Water enters the roots from the soil down a water potential gradient assisted by the increased surface area provided by the root hair cells. The root hair cells are small in size, therefore they have a high surface area to volume ratio. This enables osmosis to occur with more ease. The water moves in the roots through the hair cells and then moves across the root cortex (which includes the epidermis cells and the casparian strip) to the xylem. The water then can be transported up the stem. This movement is down a water potential gradient and can occur via several pathways.

Question 11

What is an apoplastic pathway

Answer 11

This pathway goes through the cell walls. The cell walls are readily permeable, and so this pathway offers little resistance. Most of the water travels along this path.
The issue with this pathway is that anything can be dissolved in the water and transported along by the water- there is no selection on what can be transported via the epidermis cells.

Question 12

What is a symplastic pathway

Answer 12

This pathway goes through the cytoplasm and from cell to cell via plasmodesmata.

Question 13

What is a Casparian Strip

Answer 13

This blocks the apoplastic pathway (causes a blockage in the cell wall of the endodermal cells). This ensures that only what is needed by the plant can be transported to the xylem via the water.
At this point, nothing can pass through the apoplastic pathway; leaving only symplastic pathway. Only selected few dissolved molecules (e.g mineral ions) are able to pass through the casparian strip through the symplast pathway

Question 14

The structure of the Casparian Strip and how does it filter out the unneeded dissolved molecules

Answer 14

This is a continuous band that goes around the wall of the endodermal cells. It is made of waterproof, waxy substance called suberin, and so it blocks the apoplastic pathway because water cannot get past it.
The water and dissolved substances (such as ions) now have to enter the cytoplasm and in doing so have to go through the plasma membrane. The plasma membrane is selectively permeable and so controls what enters. The membrane of the endodermal cells also contains a number of protein carriers that can regulate active transport. Once in the cytoplasm, water can continue through the symplastic pathway or apoplastic pathway. It then continues to the xylem

Question 15

What happens when water has entered the xylem

Answer 15

Water with its dissolved ions have to travel up the plant. They need to be travelling at such a pressure that the water column can be pulled up the xylem, against the gravity. Due to this trees cannot be taller than 130m. Plus, pressures up to -600kPa have been measured in the xylem.

Question 16

How are suction pressures in xylem created?

Answer 16

Suction pressures are all due to transpiration. Evaporation of water from the leaves is responsible for pulling water up the stem (known as the transpiration stream).

Question 17

What is the transpiration stream based on

Answer 17

The cohesion-adhesion theory explains the transpiration stream. Water molecules are cohesive- they are attracted to each other. This is possible because water molecules are polar and so hydrogen bonds can form between the electronegative oxygen of one water molecule and a more positively charged hydrogen atom in the second water molecule. Because of this, when water molecules move from one place to another, it pulls other water molecules with it because of these cohesive forces. So, when water is evaporated off the surface of the leaves, they pull other water molecules behind them to the interface between water and air in the cell wall, from which they then evaporate. These water molecule pull others from the xylem from the xylem in the leaf and so on. Due to this mechanism, water is pulled all the way from the roots to the leaves.

Question 18

What is a suitable analogy to explain the cohesion-tension theory

Answer 18

Sucking on a straw- sucking the drink into your mouth causes more liquid to be pulled the straw.

Question 19

What does the tension in the title cohesion-tension theory suggest

Answer 19

This refers to the tension created by the cohesive water molecules at the top of the xylem column

Question 20

What requirements need to be met for a water transport system to work

Answer 20

There needs to be a continuous column of water with no breaks, because the molecules must be very close to each other to cohere.

Question 21

What happens if air enters the xylem

Answer 21

If air gets into the xylem, it causes an airlock-which stops water movement. This problem can normally be solved immediately since water can move laterally from one xylem vessel to another via the pits in the vessel walls so the air bubble can be bypassed.

Question 22

What can the air blockage be used to explain

Answer 22

This explains why it is so important to recut a stem underwater above the air bubble when setting up a potometer or receiving a bunch of flowers. This is because air will draw up into the xylem when a stem is cut because the tension of the water column is suddenly broken.

Question 23

What is transpiration

Answer 23

This is a consequence of gaseous exchange of carbon dioxide and oxygen. The stomata allows carbon dioxide to move out and oxygen to move in. However, water is lost when the stomata are open. This is inevitable since the surfaces of the cells inside the leaf are moist. There is relatively little water in the air around the leaf so water vapour diffuses from the air spaces in the mesophyll, where the water potential is high, through the stomata to the lower water potential outside. The internal surface area of the leaf is considerable, so large amounts of water can be lost.

Question 24

What does the rate of transpiration depend upon

Answer 24

The extent of the diffusion gradient of water vapour between inside of the leaf and the surrounding air. The larger the gradient, the faster the rate of diffusion.
The spaces in the spongy mesophyll of the leaves have a high concentration of water molecules.

Question 25

What factors affect the diffusion of water vapour out of the leaf (and thus the rate of transpiration)

Answer 25

Temperature
Humidity
Air movement
Light

Question 26

How does temperature affect the diffusion of water vapour

Answer 26

Increasing temp, increases the kinetic energy of all molecules. Therefore, the rate of transpiration would increase; assuming that the concentration of water molecules in the air is lower than in the leaf). However, very high temperatures cause the stomata to close, and transpiration then decreases.

Question 27

How does humidity affect the diffusion of water vapour

Answer 27

As the humidity of the air surrounding the plant increases, the concentration of water molecules in the air rises, and so the diffusion gradient between the inside of the leaf and the surrounding air decreases and the transpiration rate decreases. Eventually an equilibrium is reached and there is no net water vapour loss from the leaf.

Question 28

How does air movement affect the diffusion of water vapour

Answer 28

When the air is still, the water vapour is not taken away from the surface of the leaf by the wind. This causes water molecules to accumulate close to the leaf surface, giving an area of high humidity.
Due to this the concentration gradient is low or non-existent.

So, air currents move the water molecules away from the surface of leaves, causing the transpiration rate to increase.

Question 29

How does light affect the diffusion of water vapour

Answer 29

In the dark, the stomata close and the transpiration rate is drastically decreased. When the light is adequate, the stomata opens. So, increasing the light intensity will not have any effect.

Question 30

How does one measure transpiration

Answer 30

Water uptake can be measured by a potometer. It is assumed that water uptake is directly proportional to water vapour loss by transpiration.
There is always a calibrated tube connected to a plant in such a way that there is a continuous column of water.

Question 31

How so you set up a potometer?

Answer 31

It is important to ensure that no air gets into the apparatus (apart from the bubble in the capillary tubing) or else an airlock will form, preventing water movement. The stem must be cut underwater to prevent air entering the xylem, because the stream pulls water upwards and sucks air into the cut xylem if the stem is not in water.
The apparatus must be then assembled underwater, ensuring all the joints are airtight, because bubbles in the apparatus could also stop water movement.
The leaves must be dried during the set up, otherwise the water will create a humid atmosphere around the leaf, which may reduce transpiration. The potometer should be left until the rate of movement of the bubble is more or less constant before starting any experiment.

Question 32

How to calculate water uptake rates

Answer 32

It is important to have the glass tubing through which bubble moves calibrated in standard units (usually millimetres) and to know the cross-sectional area of the bore of the tubing. This allows the conversion of the distance moved in a given time into a rate of uptake, using the formula:
Volume of water absorbed (mm^3) = pi r^2 x distance moved in mm
r is the radius of the bore of the tubing

Rate of water uptake in mm^3 sec^-1 = volume of water absorbed/ time in sec

Question 33

How to reduce water loss

Answer 33

Most land plants have some some means of restricting water vapour from their leaves. These include:

The leaves have a waxy waterproof cuticle, so that water vapour loss is restricted over the whole surface of the leaf, apart from the stomata.
Stomata are mostly or only on the lower surface of the leaf, which is usually cooler because it faces away from the Sun, so that diffusion is less rapid. Whereas in the water-living plants with floating leaves, the stomata are on the upper surface of the leaves, because water vapour is not a problem and richest supply of gases is in the air rather than the water.
The stomata close in the dark, when it is not necessary to take up carbon dioxide, because there is no light for photosynthesis.

Question 34

What adaptations do plants living in dry conditions (xerophytes) have

Answer 34

The number of stomata may be reduced
The stomata may be sunken in pits or grooves. Water vapour leaving the stoma is therefore sheltered from air movement, so that high humidity builds up outside the stoma and reduces the diffusion gradient.
The stomata may be surrounded by hairs, which trap the water vapour leaving the stoma, and lowering the diffusion gradient between the inside and outside of the leaf.
The waxy cuticle is often thicker than in non-xerophytic plants, to further reduce water vapour loss through the leaf surface.
The leaf may be rolled with the lower surface inwards. This again allows high humidity to build up inside the ‘coil’, reducing the rate of diffusion from the stomata.
The whole leaf may be reduced in size (e.g spines of a cactus) or the plant may lose some or all of its leaves, so that the surface area over which water may be lost is drastically reduced.

Question 35

Give me an example of a xerophytic plant

Answer 35

Marram grass; lives in sand dunes which does not retain water

Question 36

Why do hydrophytes (water living plants) need adaptations

Answer 36

Excess water uptake can be a problem
The presence of cell wall in plants prevent their cells absorbing too much water.
However some issues remain. There is less need to transport water from the roots or to prevent water loss.
However, water contains less oxygen and carbon dioxide than air does. The structure of the hydrophytes tackle this issue.

Question 37

What adaptations do hydrophytes have

Answer 37

In floating leaves, stomata tend to be mostly or entirely on the upper surface rather than the lower. This means that gas exchange can occur with the air rather than with the water.
The leaves often have very large air spaces, which gives them buoyancy.
Floating leaves are particularly thin and flat, which also gives them greater buoyancy.
The waterproof waxy cuticle of the leaf is thin
The veins in the leaf are much reduced (especially the xylem). There is much less need for transporting water and for support, so the xylem is much less important.
Hydrophytes have a greatly reduced root system.

Question 38

What is translocation

Answer 38

This is the transport of dissolved photosynthetic assimilates in a plant. These assimilates are nutrients made by photosynthesis in the leaves, the main one being sucrose, although amino acids and a number of other substances are also transported.
The transport requires energy, and so needs living cells, unlike the transport of water by the xylem.

Question 39

What are the main sinks in a plant

Answer 39

Growing points (meristems) in the roots, stems and leaves
Roots (storage organs)