Chapter 9 Transport in Plants

Question 1

What are dicotyledonous plants?

Answer 1

These are plants which makes seeds with 2 cotyledons (organs that are food stores).

Question 2

Describe the arrangement of vascular bundles in the stem of a plant.

Answer 2

The outermost layer is called the epidermis. The space of cells from the epidermis to the centre with the vascular bundles is called the cortex. The vascular bundles are arranged in a circle with a space called the parenchyma in the middle of the circle. In each vascular bundle, the interior half is the xylem, while the exterior half is the phloem.

Question 3

Describe the arrangement of vascular bundles in the roots of a plants.

Answer 3

The outermost layer with root hair extensions is called the exodermis. The next inner layer is called the epidermis. The space of cells from the epidermis and the next layer is called the cortex. The next said layer is called the endodermis. Within the endodermis, there is the vascular bundle in the arrangement where the xylem is in the middle in the shape of an ‘x’. The phloem then surrounds the xylem within the endodermis.

Question 4

Describe the arrangement of vascular bundles in the leaf of a plant.

Answer 4

The vascular bundle is located in the centre is the stem of the leaf. The xylem is located at the top of the vascular bundle at the upper side of the leaf. The phloem is located towards the bottom of the leaf.

Question 5

What is the function of the xylem?

Answer 5

The job of the xylem is to transport water and minerals up the plant. It also provides structural support to the plant.

Question 6

How is the xylem adapted to its function?

Answer 6

The xylem is dead tissue and is merely a continuous tube. This means the cell walls between adjacent cells are broken down, and that these cells have no organelles. There is not a lot (organelles) that hinders the flow of water, so water flows more easily.
A chemical called lignin (in the form of rings and spirals) is deposited in the cell walls of the xylem vessel. This provides the mechanical strength to the cells and helps with the plant staying upright.
Parts of the xylem wall that is not lignified is called non-lignified pits (they are gaps). These gaps allows water and minerals to flow to parts of the plant where water is needed (or another xylem vessel).
Between xylem vessels are cells called xylem parenchyma, and they act as a food store for the xylem vessels. A bitter tasting chemical called tannin is also deposited in these cells, which drives way any herbivores due to its bitter taste.

Question 7

What is the function of the phloem?

Answer 7

The phloem transports sugars (sucrose) up and down the plant (from source to sink).

Question 8

How is the phloem adapted to its function?

Answer 8

The phloem vessel has sieve plates which are perforated end cell walls. These gaps gives an easier flow of sugars.
Each sieve tube element has at least one or more companion cell. The phloem vessel and companion cell shares organelles- with less organelles in the sieve tube element, there will be an easier flow of sugars. The companion cell also plays a vital part in producing ATP to help with the loading of sugars.
The plasmodesmata between companion cell and phloem vessel ensures there is an easier flow of substances between the two cells.

Question 9

Why is it important that the phloem vessel is made of live tissue?

Answer 9

In the phloem vessel, ATP is required for translocation (more specifically the loading of sugars into the phloem). If the tissue is dead, no ATP can be used to actively transport the sugars.

Question 10

Why is the sugars transported in the form of sucrose and not glucose?

Answer 10

In comparison to sucrose, glucose is more reactive and would be more readily used up to produce ATP. So instead of being transported to parts of the plant where the glucose is needed, it may be used to produce ATP to actively transport the sugars itself. This is why sugars are transported in the form of sucrose.

Question 11

Describe the structure of the xylem.

Answer 11

The xylem is a continuous hollow tube with no organelles and broken down end walls. The side walls of the xylem vessel is lignified, and parts of the vessel that is not lignified is called pits, for water to flow through. Between xylem vessels are cells called xylem parenchyma.

Question 12

Describe the structure of the phloem.

Answer 12

Each cell that makes up the phloem vessel is referred to as a ‘sieve tube element’. These sieve tube elements are connected with sieve plates, which are perforated end walls. Each sieve tube element has at least one or more companion cell, connected together via a plasmodesmata.

Question 13

Why is water a key component to a plant?

Answer 13

More water increases the turgor pressure in the cells of plants. This provides a hydrostatic skeleton to support the stem and leaves.
This increase in turgor pressure also drives cell expansion. This allows cells in the root of the plant to grow and force their way through concrete.
Loss of water by transpiration cool plants down.
Many mineral ions and products of photosynthesis are transported in aqueous solutions.
Water is a raw material for photosynthesis to occur.

Question 14

Describe the movement of water from the soil to roots.

Answer 14

Mineral ions and solutes in the soil move into root hair cell by active transport.
The root hair cells now have a lowered water potential and the surrounding soil has a relatively higher water potential.
Water moves from soil to root hair cells down the water potential gradient by osmosis.

Question 15

Describe the movement of water through exodermis, epidermis and cortex.

Answer 15

Water moves through these space of cells by two pathways: apoplast and symplast pathway. The symplast pathway is the movement of water through the cytoplasm (diffusion) and plasmodesmata (osmosis). The apoplast pathway is the movement of water through the cell wall of cells. The apoplast pathway would be relatively faster to the symplast pathway. This is because in the symplast pathway there are more obstacles to the flow of water (various organelles). The movement of water from a cell decreases the water potential of the cell, and this urges water from a neighbouring cell to move in down the water potential gradient. This continuous process in many cells around the plant is what maintains the transpiration stream.

Question 16

Describe the movement of water from the cortex across the endodermis.

Answer 16

In the cortex, water was moving via the apoplast and symplast pathway. However, the cell walls of the endodermal cells are wrapped in a waxy material called suberin- this is referred to as the Casparian strip. The Casparian strip is impermeable to water (or waterproof). For this reason, water travelling in the cell wall (apoplast pathway) has to move into the cytoplasm of cells to get through the endodermis (as the Casparian strip only exists in the cell wall).

Question 17

What is essential about water moving from the apoplast pathway to the symplast pathway across the endodermis?

Answer 17

If water is moving from the apoplast pathway to the symplast, water must cross the plasma membrane to enter the cytoplasm of cell. A key function of the plasma membrane is that it is selectively permeable so can control what enters and leaves the cell. For this reason, the plasma membrane filters out any toxins and pathogens in the water, and only allows water to pass through the endodermis. This acts as a type of defence for plants.

Question 18

Describe the movement of water into the xylem.

Answer 18

Once passing the endodermis, water can once again travel through the symplast and apoplast pathway to reach the xylem.
To enter the xylem, mineral ions first move into the xylem by active transport- this decreases the water potential in the xylem. The xylem now has a relatively lower water potential compared to outside- now water can travel down the water potential gradient by osmosis into the xylem.

Question 19

Define transpiration.

Answer 19

The loss of water vapour by evaporation through the stomata.

Question 20

What are the advantages of transpiration?

Answer 20

Transpiration helps to cool the plant down. Like mammals, becoming too warm can cause enzymes to denature.
Loss of water by evaporation lowers the water potential gradient nearer to the leaves. This steep water potential gradient maintains the transpiration stream, as water keeps travelling up to replace water lost by evaporation.

Question 21

What are the disadvantages of transpiration?

Answer 21

If water is lost by evaporation, and there is no replacement of water from the roots, the plant will be under water stress. Lower volume of water can cause cells to lose their turgor pressure, some cells may start to undergo plasmolysis, and the plant can ever start to wilt. The change in structure in the plant may cause it to be outcompeted by other plants, and may not be able to photosynthesise properly.

Question 22

What help water move up the xylem in plants?

Answer 22

The cohesion (hydrogen bonding between water molecules) and adhesion (hydrogen bonding between water molecules and xylem wall) is what pulls the water molecules up and maintains a continuous transpiration stream.
Water is also pushed up at the bottom of the xylem by root pressure, which is generated by the movement of water from the endodermis into xylem vessel.

Question 23

Why is transpiration described as a side effect of photosynthesis?

Answer 23

For photosynthesis to occur, the plant needs to carbon dioxide. It gains carbon dioxide by opening the stomata and allowing it to diffuse through. The opening of stomata also allows water vapour to evaporate out of the stomata.

Question 24

The rate of transpiration increases if temperature increases. Why is this?

Answer 24

Increase in temperature means more thermal energy in the system.
Thermal energy converted to kinetic energy.
Increase in kinetic energy means water vapour moves out of the stomata at a faster rate.

Question 25

The rate of transpiration increases with what respect to humidity? Explain why.

Answer 25

If humidity decreases, the rate of transpiration increases.
The concentration of water vapour right outside the plant is low. This creates a steep water vapour concentration gradient.
Hence, water vapour is more likely to move out of the stomata down this concentration gradient via diffusion.

Question 26

The rate of transpiration increases if air movement increases. Why is this?

Answer 26

An increased air movement means water vapour molecules are constantly being pushed away from right outside the plant near the stomata.
This means the water vapour concentration outside the stomata is low and this creates a steep concentration gradient.
More water vapour molecules are able to move out via diffusion.

Question 27

The rate of transpiration increases if the light intensity increases. Why is this?

Answer 27

An increased light intensity would give the plant a sign to open the stomata to prepare for photosynthesis (absorption of carbon dioxide). If the stomata is open, water vapour can be lost by diffusion.

Question 28

Describe the set up of a potometer, in the measurement of water uptake of in a plant.

Answer 28

A potometer is a capillary tube that has an opening to a reservoir and a beaker containing a plant. The stem of the plant should be in contact with the water underneath, as well as the top of the beaker should be covered so the beaker is air tight. This ensures no water directly from the beaker is lost by evaporation. A ruler will be needed to measure the distance moved by the air bubble. The reservoir is needed to reset the position of the air bubble in the case the experiment should be carried out again.

Question 29

How can a potometer be used to measure water uptake in different conditions? What conditions could they be?

Answer 29

In different conditions, the water uptake of a plant will differ as the transpiration rate would vary. Conditions include placing the plant in varying temperatures, or varying light intensity or more.

Question 30

How can volume of water uptake in a plant be measured using a potometer?

Answer 30

First, make sure to take an initial reading of the capillary tube of where the air bubble is using a ruler. After a set time, using a rule again measure the distance travelled. As the capillary tube is a cylinder, measuring the volume of water uptake would be equal to the volume of the cylinder in that set distance. So to do that:
Volume of water uptake= Pi x radius^2 x distance travelled

Question 31

Why is the reading taken by a potometer into an investigation of transpiration rate, an estimate rather than a measurement of transpiration rate?

Answer 31

With a potometer, the volume of water taken up is measured. However, we cannot solely say the volume of water taken up wad due to transpiration. The water taken up may be used by cells to maintain turgor pressure or for photosynthesis. Due to these other factors, we say water taken up gives an estimate of transpiration rate.

Question 32

Define translocation.

Answer 32

Transporting assimilates (sugars) from source to sink in a plant.

Question 33

Describe the process of phloem loading (moving sugars from parts of the plant into phloem vessel).

Answer 33

First, there are proton pumps on the membranes of the companion cells. These proton pumps actively transports (requires ATP) hydrogen ions outside of the companion cell by pumping them through. Due to this, a proton gradient is established.
This proton gradient makes the hydrogen ions want to diffuse back into the companion cell. They do this through facilitated diffusion, where they are co-transported with sucrose (that currently exists outside the companion cell) through a co-transport protein in the plasma membrane of the companion cell.
Now the concentration of sucrose in the companion cell has now increased. Relative to the sieve tube element (phloem vessel), the sucrose concentration in the companion cell is high. This makes the sucrose diffuse from the companion cell to the sieve tube element via the plasmodesmata.
The movement of sucrose into the sieve tube element decreases the water potential in the sieve tube element. Water is more likely to move across the plasmodesmata via osmosis from the companion cell to sieve tube element down the water potential gradient.

Question 34

Why is the phloem loading of translocation an active process?

Answer 34

In order to transport hydrogen ions from inside the companion cell to outside, proton pumps are needed in the membrane to pump the hydrogen ions across. However, in order for the proton pumps to function, they require ATP. Without ATP, the proton pumps would not be able to pump the hydrogen ions across.

Question 35

During phloem unloading of translocation, how is a sucrose concentration gradient maintained to give a constant movement of sugars throughout the plant?

Answer 35

The reason sugars are being transported to the sink is because the sink needs to use these sugars. For example, if root hair cells were the ‘sink’, the root hair cells would be converting the sucrose to glucose, then to ATP to help with actively pumping mineral ions from the soil to roots. If the sink is constantly using the sugars, the concentration of sugars in the sink would be relatively low compared to the phloem. This creates a sucrose concentration gradient and helps with the constant flow of sugars around the plant.

Question 36

How does phloem unloading help with the movement of water into parts of the plants?

Answer 36

Once sucrose is moved from the phloem to the sink, the concentration of sucrose in the sink in now high. This also means the water potential in the sink is now low. Water is able to move down the water potential gradient from the xylem vessel to the sink.