Chapter 9 Transport in plants

What are the three main reasons for plant transport systems?

1. Metabolic demands- the cells of green parts of the plant make their own glucose and oxygen by photosynthesis, however roots and internal parts of the plant do not photosynthesise so need oxygen transported to them and waste products removed.

2. Size- because plants continue to grow throughout their lives, many plants are very large and so need very effective transport systems to move substances both up and down the plant.

3. Surface area : volume ratio (SA:V)- leaves are adapted to have a relatively large SA:V ratio, however when the size and complexity of these plants is taken into account, they still have a relatively small SA:V ratio. This means they cannot rely on diffusion alone to supply their cells.

What is the vascular bundle in plants?

• A series of transport vessels running through the stem, roots and leaves, made up of the xylem and the phloem.

Describe the structure of the roots in a herbaceous plant.

• Organisation of vascular bundles is different to the leaves and stem in that the xylem is found in the centre resembling a star shape.

• The phloem is found between each of the points of the star shape.

Describe the structure of the stem in a herbaceous plant.

• There are several vascular bundles with each of them containing xylem and phloem tissues.

• The xylem is found on the inner edge closer to the centre of the stem and the phloem on the outer edge closest to the surface of the stem.

• In between the xylem and phloem is a layer of cambium (meristematic tissue).

What is cambium and what does it contain?

• Cambium is a meristematic tissue and it contains actively dividing pluripotent cells.

Describe the structure of the leaf in a herbaceous plant.

• The vascular bundle runs down the centre of the leaf as a vein and contains both the xylem and the phloem tissues.

• The xylem is towards the top of the leaf and the phloem is towards the bottom.

Describe the structure of the xylem vessel.

• They are hollow and dead cells, that are made of several columns fused together.

• They do not contain any organelles or end walls, so this makes a continuous column for transporting water and mineral ions.

• The xylem walls contain a chemical called lignin that provides mechanical strength.

What are xylem parenchyma?

• Xylem parenchyma packs are located around the xylem vessel, they store food and contain tannin deposits (bitter chemical that protects the plant from herbivores).

What is the function of the pits in the xylem vessel walls?

• The bordered pits allow water to leave the xylem vessel.

What are the two main cells that the phloem tissue is made up of?

1. Companion cells

2. Sieve tube elements

Describe the structure of the sieve tube elements in the phloem.

• They are living cells but contain no nucleus and have very few organelles.

• They have perforated sieve plates at the ends of the cells, which allow the phloem contents to pass through.

Describe the structure and function of the companion cells in the phloem.

• They are linked to the sieve tube elements by many plasmodesmata (channels through the cellulose cell walls).

• They maintain their nucleus and organelles.

• The companion cells are through to act as the support for the sieve tube elements as they have lost all their organelles.

How is water transported into the plant?

• Water is absorbed into plants through the root hair cells by osmosis.

In what ways is water key to the structure and metabolism of plants?

1. Turgor pressure as a result of osmosis provides a hydrostatic skeleton to support the stems and leaves.

2. Turgor drives cell expansion- the force that enables plant roots to force their way through tarmac/ concrete.

3. Loss of water by evaporation helps keep plants cool.

4. Mineral ions and the products of photosynthesis are transported in aqueous solutions.

5. Water is a raw material for photosynthesis.

How are root hair cells adapted as exchange surfaces?

1. Microscopic size means they can penetrate easily between soil particles.

2. Large SA:V ratio with thousands growing on each root tip.

3. Each hair has a thin surface layer (just cell wall and membrane), through which diffusion and osmosis can take place quickly.

4. Concentration of solutes in the cytoplasm maintains a water potential gradient between the soil and the cell.

5. The cytoplasm and vacuolar sap contain many solvents including sugars, mineral ions and amino acids so the water potential in the cell is lower. As a result water moves into the root hair cells by osmosis.

What are the two ways water moves across the root into the xylem?

1. Symplast pathway

2. Apoplast pathway

Explain how water moves through the symplast pathway.

1. Water moves through the continuous cytoplasm of the cells that are connected through the plasmodesmata by osmosis.

2. The root hair cells has a higher water potential than the next cell along so the water can move continuously through the cells by osmosis.

3. This process continues across the root until the xylem is reached.

How is a steep water potential gradient maintained in the root hair cells?

• As water leaves the root hair cells by osmosis, the water potential of the cytoplasm falls, this maintain the gradient so that as much water as possible continues to move through the cell.

Explain how water moves through the symplast pathway.

1. The apoplast is the movement of water through the cell walls and intercellular spaces.

2. The water fills the spaces between the loose open network of fibres in the cellulose cell wall.

3. The cohesive forces between the water molecules cause them to stick together in a continuous stream, this creates tension so there is little resistance through the open structure.

How is water moved from the root to the xylem?

1. Water reaches the endodermis that surrounds the vascular tissue.

2. Water in the apoplast pathway is forced into the cytoplasm, joining the symplast pathway due to the waterproof Casparian strip.

3. Water passes through the selectively permeable cell surface membranes to get there which excludes any toxic solutes.

4. Water potential of the xylem vessel is much lower, so water moves into the vessel from the endodermal cells by osmosis down a water potential gradient.

5. Once inside the vascular bundle, water returns to the apoplast pathway to move into the xylem.

Explain the process of transpiration.

1. Carbon dioxide moves from the air into the leaf and oxygen moves out of the leaf by diffusion down concentration gradients through the stomata.

2. The stomata are opened and closed by guard cells which surround the stomatal opening.

3. When open to exchange gases, water vapour also moves out by diffusion and is lost. This lost of water vapour from leaves and stems is called transpiration.

4. Stomata open and close to control the amount of water lost.

Explain the transpiration stream.

1. Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata down a concentration gradient.

2. Loss of water from the mesophyll cells lowers the water potential of the cell, so water moves into the cell from an adjacent cell by osmosis along both pathways.

3. Repeated across the leaf to the xylem, water moves out of the xylem by osmosis into the cells of the leaf.

4. Water molecules form hydrogen bonds with the carbohydrates in the walls of the xylem vessel (adhesion), they also form hydrogen bonds with each other (cohesion).

5. The combination of these results in capillary action which draws the water up in a continuous stream known as the transpiration pull.

What is the transpiration pull?

• Water being drawn up the xylem in a continuous stream to replace water lost by evaporation.

How do stomata control the rate of transpiration?

• A turgor-driven process by which the stomata open and close.

• When turgor is low the guard cell walls close the pore, when conditions are favourable guard cells pump solutes in by active transport increasing their turgor.

• Cellulose hoops prevent the guard cell from swelling in width, so they extent lengthways.

• Because the inner wall is less flexible than the outer wall, the guard cells become bean-shaped and open the pore.

• When water is scarce, hormonal signals from the roots can trigger turgor loss from the guard cells, which close the stomatal pore and conserve water.

How does light intensity affect the rate of transpiration?

• Light is required for photosynthesis so increasing light intensity will mean more stomata will be open for gas exchange, in the dark most of the stomata will be closed.

• Increasing light intensity increases the number of open stomata, which increases the rate of water vapour diffusing out and therefore increasing the evaporation from the surface of the leaf.

• Therefore, increasing light intensity increases the rate of transpiration.

How does relative humidity affect the rate of transpiration?

• A measure of the amount of water vapour in the air compared to the total concentration of water the air can hold.

• A high relative humidity will lower the rate of transpiration because of the reduced water potential gradient between the inside of the leaf and the outside air.

• Dry air has the opposite effect.

What are the two ways in which temperature affects the rate of transpiration?

• Increase in temperature increases the kinetic energy of the water molecules, therefore increasing the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf.

• Increase in temperature increases the concentration of water vapour that the external air can hold before it becomes saturated (so decreases its relative humidity and water potential).

How does air movement affect the rate of transpiration?

• Each leaf has a layer of still air around it trapped by the shape of the leaf and features like hairs on the surface which decrease air movement close to the leaf.

• Water vapour that diffuses out accumulates here and so the water potential around the stomata increases in turn reducing the concentration gradient.

• Conversely a long period of still air will reduce transpiration.

How does soil-water availability affect the rate of transpiration?

• Amount of water available in the soil can affect transpiration rate, if it is very dry the plant will be under water stress and the rate of transpiration will be reduced.

What is translocation?

• The transport of organic compounds in the phloem from sources (where they are produced), to sinks (where they are used or stored).

What are sources and sinks?

• In plants, a source is an area where substances are produced and a sink is an area where substances are used or stored.

Translocation is an active process, what is meant by this?

• Translocation requires energy (ATP) to transport substances up and down the plant.

What are the products of photosynthesis that are transported by translocation?

• The products of photosynthesis are known as assimilates.

What is the main assimilate transported around the plant?

• The main assimilate transported around the plant is sucrose.

What are the main sources of assimilates in plants?

1. Green leaves and green stems.

2. Storage organs like tubers and tap roots that are unloading their stores at the beginning of a growth period.

3. Food stores in seeds when they germinate.

What are the main sinks in plants?

1. Roots that are growing and/or actively absorbing mineral ions.

2. Meristems that are actively dividing.

3. Any parts that are laying down food stores, like developing seeds. fruits or storage organs.

Why is sucrose the main carbohydrate transported?

• As it is not used in metabolism as readily as glucose, it is less likely to be metabolised during the transport process.

Describe the active process of phloem loading through the apoplast route.

1. Sucrose is actively moved into the cytoplasm of the companion cells.

2. Hydrogen ions are actively pumped out of the companion cells into the surrounding tissue using ATP.

3. Hydrogen ions return to the companion cell down a concentration gradient via a co-transport protein.

4. Sucrose is co-transported, which increases the sucrose concentration in the companion cells and in the sieve tube elements.

5. As a result of the sucrose build up, water also moves into by osmosis which leads to a build up of turgor pressure due to the rigid cell walls.

6. The water carrying assimilates moves into the tubes of the sieve elements, this reduces the pressure of the companion cells and moves up or down the plant by mass flow.

Describe the process of phloem unloading.

1. Sucrose is unloaded from the phloem at any point into the cells that need it.

2. Main mechanism of phloem unloading is by diffusion of the sucrose from the phloem into surrounding cells.

3. It moves rapidly by diffusion into other cells or is converted into another substance, so that a concentration gradient of sucrose is maintained between the phloem contents and surrounding cells.

4. Loss of solutes from the phloem leads to a rise in the water potential of the phloem, water moves out into the surrounding cells by osmosis.

5. Some water is drawn into the transpiration stream in the xylem.

What are xerophytes?

• Xerophytes are plants that live in hot, dry and breezy habitats where water evaporates from leaf surfaces very rapidly.

• Examples include marram grass and cacti.

In what ways are xerophytes adapted to conserve water?

1. A thick waxy cuticle- helps to minimise water loss through the cuticle, most common in evergreen plants to help survive hot dry summers and cold winters when water can be hard to absorb from the frozen ground.

2. Sunken stomata- stomata located in pits, which reduces air movement, producing a microclimate of still, humid air that reduces the water vapour potential gradient and so reduces transpiration. Most common in marram grass and cacti.

3. Reduced number of stomata- reduces water loss by transpiration and reduce their gas exchange capabilities.

4. Reduced leaves- reducing leaf area can greatly reduce water loss, conifers have almost circular in cross section, thin needles as leaves. These have a greatly reduced SA:V ratio, which minimises water lost by transpiration.

What are hydrophytes?

• Plants that actually live in water, submerged, on the surface or on the edges of bodies of water. They all need special adaptations to cope with growing in water or permanently saturated soil.

• An example of a hydrophyte is a water lily which grows at the surface.

What adaptations do hydrophytes have to live in water?

1. Very thin or no waxy cuticle- do not need to conserve water as there is always plenty available so water loss by transpiration is not an issue.

2. Many always-open stomata on the upper surfaces- maximises gaseous exchange, there is no risk of loss of turgor as there is an abundance of water so stomata are usually open all the time. In water lilies, the stomata need to be on the upper surface of the leaf so they are in contact with the air.

3. Wide, flat leaves- water lilies have wide, flats leaves that spread across the surface of the water to capture as much light as possible.

4. Small roots- water diffuses directly into stem and leaf tissue so less need for uptake by roots.

5. Large SA of stems and roots under water- maximises the area for photosynthesis and for oxygen to diffuse into submerged plants.

6. Air sacs- enable the leaves and/or flowers to float on the surface of the water.