Module 3: Transport in Plants

Question 1

Why do multicellular plants need transport systems?

Answer 1

Plants are multicellular -have a small surface area to volume ratio.

They are relatively big so have a high metabolic rate.

Diffusion would take place too slowly so plants have mass transport systems.

Question 2

What are the two types of tissue involved in transport in plants?

Answer 2

• Xylem tissue

-Phloem tissue

Question 3

What are the functions of both Xylem and Phloem tissue?

Answer 3

Xylem tissue: Transports water and minerals in solution from the roots to the rest of the plant.. It also provides structural support.

Phloem tissue: transports mainly sugars in solution and dissolved both up and down the plant.

Question 4

What is a vascular bundle?

Answer 4

Xylem and phloem vessels are grouped together within the plant stem and form vascular bundles.

Sclerenchyma fibres are also found within vascular bundles and provide support to the stem.

Question 5

What is the structure/arrangement of a transverse cross-section root?

Answer 5

Xylem in the middle and phloem around it.

-The Xylem and Phloem are in the centre to provide support for the root as it pushes through the soil.

Xylem vessels are mechanically strong, and because they are grouped together in the centre of the root, this helps to prevent the root from being pulled out of the soil.

Question 6

What is the structure/arrangement of a transverse cross-section stem?

Answer 6

The vascular bundles are arranged in a ring around the edges of the stem.

Xylem vessels are found closer to the centre whereas the phloem vessels are located at the edge of the stem.
This provides a sort of support and scaffolding that reduces bending.

Question 7

What is the structure/arrangement of a transverse cross-section leaf?

Answer 7

Within the leaf, the xylem vessels are found towards the top of the vascular bundle with the phloem vessels found underneath.

The main vascular bundle is in the middle.

Question 8

Describe the structure/adaptations of Xylem vessels?

Answer 8

very long, tube-like structures formed from cells (vessel elements) joined end to end.

There are no end walls on these cells, making an uninterrupted tube that allows water to pass through the middle easily.

The cells are dead and hollow- have no cytoplasm.

The cell walls are thickened with a woody substance called lignin, which helps to support the walls and stops them collapsing inwards.

Lignin can be deposited in xylem walls in different ways such as in spirals or rings and these patterns allows flexibility and prevents the stem from breaking. The amount of lignin increases as the cell gets older.

Water and minerals ions move into and out of the vessels through small pits in the walls where there is no lignin and this is how other types of cells are supplied with water.

Question 9

Describe the structure/adaptations of Phloem vessels?

Answer 9

Transports solutes (dissolved substances), mainly sugars like sucrose.

It is formed from cells arranged in tubes.

Purely used for transport - It is not used as support.

Contains phloem fibres, parenchyma, sieve tube elements and companion cells.

Question 10

What are sieve tube elements?

Answer 10

They are living cells that form the tube for transporting sugars through the plant.

They are joined end to end to form sieve tubes.

The ends of the tubes form sieve plates which have lots of holes in them to allow solutes pass through.

They have no nucleus, a very thin layer of cytoplasm and few organelles to create more space for solutes to be transported.

The absence of a nucleus and other organelles means that these cells cannot survive on their own, so each sieve tube element is associated with a companion cell

Question 11

What is a companion cell?

Answer 11

There is a companion cell for every sieve tube element.

They have a nucleus and is packed full of mitochondria.

The mitochondria provide lots of energy for the active loading of sucrose into the sieve tube element.

The sieve tube element and the companion cell are connected through plasmodesmata (channels in the cell wall) which allow the two cells to communicate.

Question 12

Describe the practical for dissecting plant stems.

Answer 12

Cut a thin section of the plant stem using a scalpel. Remember to cut away from you.

Place the tissue sample into water to prevent it from drying out.

Place the tissue sample into a small dish containing the stain. A common stain that is used to view vascular bundles is toluidine blue O (TBO) which stains lignin blue/green which will enable you to visualise the xylem and sclerenchyma fibres. The phloem cells and remaining tissue will appear a pink/purple colour.

Rinse the tissue samples in water and place each one onto a microscope slide.

Question 13

How does water enter a plant?

Answer 13

Water enters through root hair cells and then passes through the root cortex, including the endodermis, to reach the xylem. This happens via osmosis.

The soil around the roots has a high water potential and the leaves have a low water potential, as water constantly evaporates from them.

This creates a water potential gradient that keeps water moving through the plant in the right direction, from the roots to leaves - high to low.

Question 14

What are the two pathways water travels through the roots?

Answer 14

The Symplast pathway

The Apoplast pathway.

Question 15

What is the symplast pathway?

Answer 15

Water goes through the living parts of the cells such as the cytoplasm.

The cytoplasm of neighbouring cells connects through plasmodesmata - small channels in the cell walls.

Water moves through the symplast pathway via osmosis.

Question 16

What is the apoplast pathway?

Answer 16

Goes through the non-living parts of the cells- the cell walls.

The walls are very absorbent and the water can simply diffuse through them, as well as passing through the spaces between them.

The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure along a pressure gradient.

The problem with the apoplast pathway, is that the water (and the substances dissolved in it) bypasses the cell membrane, which controls what is going into the cell and prevents entry of toxic substances.

The root, therefore, has something called the Casparian strip which is a waxy strip within the cell walls which is impermeable to water.

This forces the water to go through a cell membrane which can then control which substances are allowed to enter the plant. Once water has passed through the Casparian strip, it can then reach the xylem.

Question 17

How does water move through the leaves?

Answer 17

Xylem vessels transport the water all around the plant. At the leaves, water leaves the xylem and then enters into cells via the apoplast pathway.

Water evaporates from the cell walls into the spaces between cells in the leaf.

when the stomata- tiny pores in the surface of the leaf) open, water evaporates- it diffuses out of the leaf - down a water potential gradient- into the surrounding air. The evaporation of water from the plants surface is called transpiration.

Question 18

What is a transpiration stream?

Answer 18

The movement of water from roots to leaves is called the transpiration stream.

The mechanisms that move the water are cohesion, adhesion and tension.

Question 19

How does cohesion and adhesion help water move up the plants?

Answer 19

1) Water evaporates from the leaves at the ‘top’ of the xylem- transpiration.

2) This creates a tension (suction) which pulls more water into the leaf.

3) Water molecules are cohesive meaning they 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.

4) Water enters the stem through the root cortex cells.

Question 20

How does Adhesion also help with the transport of water molecules up the plant?

Answer 20

Adhesion describes the attraction of water to non-water molecules (such as the molecules which make up the xylem walls).

The attraction of water to the walls of the xylem helps water to rise up through the vessel.

Question 21

What is transpiration?

Answer 21

Transpiration is the loss of water vapour through evaporation from a plant’s surface.

Question 22

What happens in transpiration?

Answer 22

It mainly happens through gaps in the leaf called the stomata, which need to open during the daytime to allow gas exchange.

Plants need to take in carbon dioxide for photosynthesis and get rid of oxygen, which happens through the stomata.

A side-effect of this is that water vapour can also diffuse out of the leaf through the stomata - this is known as transpiration.

Plants will close their stomata at night (because they are not photosynthesising so gas exchange does not need to take place) which minimises transpiration.

Question 23

What are the 4 main factors that affect transpiration rate?

Answer 23

1) Light intensity

2) temperature

3) Humidity

4) Wind

Question 24

How does light intensity affect transpiration rate?

Answer 24

The more light intensity, the faster the rate of transpiration.

This is because the stomata open when it gets lights- the lighter it gets, the wider they open.

When it is dark the stomata are closed, so there is little transpiration.

Question 25

How does temperature affect the rate of transpiration?

Answer 25

Increasing temperature increases the rate of transpiration because more heat energy means that the water molecules have more kinetic energy and will diffuse faster out of the stomata.

This increases the water potential gradient between the inside and outside of the leaf.

Question 26

How does humidity affect the rate of transpiration?

Answer 26

More humid conditions decreases the rate of transpiration. Humidity is a measure of the level of moisture in the air.

The more humid the air surrounding a leaf, the lower the water potential gradient between the inside of the leaf and the outside.

Question 27

How does wind affect the rate of transpiration?

Answer 27

windier conditions increases the rate of transpiration.

This is because the wind will immediately blow away any water molecules that have just diffuses out of the leaf and increase the water potential gradient between the inside of the leaf and the outside.

Question 28

What does a potometer measure?

Answer 28

It measures the amount of water uptake by a plant.

Question 29

Describe the practical measuring the transpiration rate using a potometer.

Answer 29

Take a plant and cut off a shoot. You need to do this underwater to prevent air from entering the xylem.

You should also cut the shoot at a slant to increase the amount of surface area for water uptake.

Keeping the shoot underwater, insert it into the potometer.

Take the potometer out of the water, but ensure that the capillary tube is submerged in a beaker of water.

Dry the leaves and leave the apparatus for 30 mins to enable the shoot to acclimatise.

Ensure that all other variables are controlled - e.g. light intensity and humidity.

Shut the tap so that the capillary tube system is closed them remove the end of the capillary tube from the beaker of water until one air bubble has formed. Place the capillary tube back into the beaker of water.

Record the starting position of the air bubble then time how long it takes for the air bubble to move along the capillary tube. Record the distance moved by the bubble and calculate the speed (distance moved per unit time).

The speed of the bubble can be used to estimate the rate of transpiration of the plant shoot.

Question 30

What are Xerophytes and give examples

Answer 30

Xerophytes are plants which are adapted to living in regions where water is scarce.

Examples include cacti, pineapple and marram grass.

Question 31

What are the adaptations of xerophytes that enables them to survive in harsh conditions?

Answer 31

CACTI + MARRAM GRASS: Waxy layer on the epidermis - this waterproof outer layer reduces evaporation from the surface because water cannot easily pass through (it is impermeable).

MARRAM GRASS: Sunken stomata - xerophytes have stomata which are sunken in pits. The pits shelter the stomata from the wind, reducing the water potential gradient between the inside of the leaf and the outside.

MARRAM GRASS: Hairs on epidermis - hairs on the epidermis trap water vapour, reducing the water potential gradient between the inside and outside of the leaf.

CACTI: Spines - many xerophytes have spines instead of leaves which reduces the surface area for water loss.

MARRAM GRASS: Rolled leaves - curled leaves traps water vapour, reducing the water potential gradient for transpiration. It also reduces the surface area of the leaf for water loss.

CACTI: Closure of stomata - xerophytes can close their stomata during conditions of particularly high temperature or light intensity. This reduces transpiration at times when the rate of transpiration would be very high.

Question 32

What are Hydrophytes?

Answer 32

Hydrophytes are plants which live on water, such as water lilies. Because oxygen does not dissolve well in water, hydrophytes need adaptation to enable them to cope with low oxygen levels.

Question 33

In what ways are hydrophytes adapted to their functions?

Answer 33

Stomata on the upper surface - usually stomata are found on the underside of plant leaves but for hydrophytes, this side will be submerged in water. Instead, hydrophytes have their stomata on the upper surface of their floating leaves to maximise gas exchange.

Air spaces - pockets of air in the plant tissue help the plant to float and can be used to store oxygen for aerobic respiration.

Flexible leaves and stems - the flexibility of the leaves and stems helps to prevent damage from water currents. Unlike land plants, which need a sturdy stem to keep upright, hydrophytes are supported by the water around them.

Question 34

What is Translocation?

Answer 34

Translocation is the movement of dissolved substances, such as sucrose and amino acids, from parts of the plant where the substances are made to other parts of the plant where they’re needed.

Question 35

What happens in Translocation?

Answer 35

Translocation takes place in the phloem - transport vessels made up of two types of cell, sieve tube elements and companion cells.

The parts of the plant which make these substances are referred to as sources (e.g. the leaves) and the parts of the plant which store or use the substances are called sinks (examples include bulbs and roots). When sucrose reaches a sink, it is converted into starch for carbohydrate storage.

This maintains a concentration gradient between the source and the sink, so that more sucrose moves into the source. Translocation is an active process, so if respiration is reduced or inhibited (e.g. using a respiratory toxin), translocation will be impaired.

Question 36

What is the mass flow hypothesis?

Answer 36

The mass flow hypothesis is a theory which attempts to explain how solutes are transported from source cells into sinks through the phloem.

Question 37

What are the stages of the mass flow theory?

Answer 37

Sucrose moves from companion cells into sieve tube elements by active transport.

This reduces the water potential of the sieve tube element.

Water moves into the phloem by osmosis, which increases the hydrostatic pressure.

There is a pressure gradient with high hydrostatic pressure near the source cell and lower hydrostatic pressure near the sink cells.

Solutes move down the pressure gradient towards the sink end of the phloem.

Solutes move into sink cells and are converted into other molecules (e.g. starch).

The removal of solutes increases the water potential at the sink end, causing water to move out of the phloem by osmosis. This maintains the hydrostatic pressure gradient between the source and the sink.

Question 38

What is Active loading?

Answer 38

Active loading is used at the source to move substances into the companion cells from surrounding tissues, and from the companion cells into the sieve tube, against a concentration gradient.

Question 39

Describe the movement of sucrose.

Answer 39

Sucrose from surrounding tissues move into companion cell and then into the sieve tubes.

The concentration of sucrose is usually higher in the companion cells than the surrounding tissue cells, and higher in the sieve tube cells than in the companion cells.

Question 40

Describe the process of active loading.

Answer 40

1) in the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and inti surrounding tissue cells. This sets up a concentration gradient- there are more H+ ions in the surrounding tissue than in the companion cell.

2) An H+ ion binds to a co-transport protein in the companion cell membrane and re enters the cell down the concentration gradient.
A sucrose molecule binds to the co-transport protein at the same time. The movement of H+ ions is used to move the sucrose molecule into the cell, against its concentration gradient.

3) Sucrose molecules are then transported out of companion cells and into the sieve tubes by the same process.