M3 Transport in Plants

Question 1

Why do metabolic demands mean plants need transport systems?

Answer 1

• The cells of the green parts of the plant make their own glucose and oxygen by photosynthesis, but many internal and underground parts of the plant so not photosynthesise.
• They need oxygen and glucose transported to them and the waste products of cell metabolism removed.
• Hormones made in one part in of a plant need transporting to the areas where they have an effect.
• Mineral ions absorbed by the roots need to be transported to all cells to make the proteins required for enzymes and the structure of the cell.

Question 2

Why does size mean that plants need transport systems?

Answer 2

• Some plants are very small but because plants continue to grow throughout their lives, many perennial plants (plants that live a long time and reproduce year after year) are very large.
• This means that plants need very effective transport systems to move substances up and down from the tip of the roots to the topmost leaves and stems.

Question 3

Why does surface area : volume ratio mean that plants need transport systems?

Answer 3

• Leaves are adapted to have a large SA:V ratio for the exchange of gases with the air.
• However, the size of trunks and complexity of multicellular plants means that when the stems, trunks and roots are taken into account they have a small SA:V ratio.
• Therefore they cannot rely on diffusion alone to supply their cells with everything that they need.

Question 4

Describe the transport system in dicotyledonous plants

Answer 4

• Dicotyledonous plants (dicots) make seeds that contain two cotyledons (organs that act as food stores for developing embryo plant, and form the first leaves when the seeds germinates).
• There are herbaceous dicots, with soft tissues and a short life cycle (leaves and stems that die down at the end of the growing season), and woody dicots which have hard lignificad tissues and a long life cycle.
• Dicotyledonous plants have a series of transport vessels running through the stem, roots and leaves. This is known as the vascular system.
• In herbaceous dicots this is made up of two main types of transport vessels, the xylem and the phloem. These transport tissue are arranged in vascular bundles in the leaves, stems and roots of herbaceous dicots.

Question 5

Structure of vascular bundles in the stem

Answer 5

Vascular bundles are around the edge to give strength and support

Question 6

Structure of vascular bundles in the roots

Answer 6

Vascular bundles are in the middle to help the plant withstand the tugging strains that result as the stems and leaves are blown in the wind

Question 7

Structure of vascular bundles in the leaf

Answer 7

The midrib of a dicot leaf is the main vein carrying the vascular tissue through the organ. It also helps support the structure of the leaf. Many small, branching veins spread through the leaf functioning both in transport and support.

Question 8

Describe the structure and functions of the xylem

Answer 8

• The xylem is a non-living tissue that has two main functions in a plant - transport of water and mineral ions, and support.
• The flow of minerals in the xylem is up from the roots to the shoots and leaves.
• Xylem is made up of several types of cells, most of which are dead when they are functioning in the plant.
• The xylem vessels are the main structures. They are long, hollow structures made by several columns of cells fusing together end to end.
• There are two other tissues associated with xylem in herbaceous dicots. Thick-walled xylem parenchyma packs around the xylem vessels, storing food and containing tannin deposits (chemical protecting plant tissues from herbivores).
• Xylem fibres are long cells with lignificad secondary walls that provide extra mechanical strength but do not transport water. Lignin can be laid down in the walls of xylem vessels in different ways. It can form rings, spirals or solid tubes with lots of small unlignificad areas called bordered pits, where water leaves the xylem and moves into other cells of the plant.

Question 9

Describe the structure and functions of the phloem

Answer 9

• Phloem is a living tissue that transports food in the form of organic solutes around the plant from the leaves where they are made by photosynthesis.
• The phloem supplies the cells with the sugars and amino acids needed for cellular respiration and for the synthesis of all other useful molecules.
• The flow of materials in the phloem can go both up and down the plant.
• The main transporting vessels of the phloem are the sieve tube elements. Sieve tubes are made up of many cells joined end to end to form a long, hollow structure.
• Unlike xylem tissue, the phloem tubes are not lignified. In the areas between the cells, the walls become perforated to form sieve plates, which look like sieves and let the phloem contents flow through.
• As the large pores appear in these cells, the tonoplast (vacuole membrane) the nucleus and some of the other organelles break down. The phloem becomes a tube filled with phloem sap and the mature phloem cells have no nucleus.
• Closely linked to the sieve tube elements are companion cells, which form with them. These cells are linked to the sieve tubes by many plasmodesmata (microscopic channels through the cellulose cell walls linking the cytoplasm of adjacent cells). They maintain their nucleus and all their organelles. The companion cells are very active cells and it is thought that they function as a ‘life support system’ for the sieve tube cells, which have lost most of their normal cell functions.
• Phloem tissue also contains supporting tissues including fibres and sclereids (cells with extremely thick cell walls).

Question 10

Why is water important in the structure and metabolism of plants?

Answer 10

• Turgor pressure as a result of osmosis in plant cells provides a hydrostatic skeleton to support the stems and leaves.
• Turgor also drives cell expansion - it is the force that enables plant roots to force their way through tarmac and concrete.
• The loss of water by evaporation helps to keep plants cool.
• Mineral ions and the products of photosynthesis are transported in aqueous solutions.
• Water is a raw material for photosynthesis.

Question 11

How does water move into the root?

Answer 11

• Root hair cells are the exchange surface in plants where water is taken into the body of the plant from the soil.
• Soil water has a very low concentration of dissolved minerals so it has a very high water potential.
• The cytoplasm and vacuolar sap of the root hair cell contain many different 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.

Question 12

How are root hair cells adapted as exchange surfaces?

Answer 12

• Their microscopic size means they can penetrate easily between soil particles.
• Each microscopic hair has a large SA:V ratio and there are thousands on each growing root tip.
• Each hair has a thin surface layer (just the cell wall and cell surface membrane) through which diffusion and osmosis can take place quickly.
• The concentration of solutes in the cytoplasm in the cytoplasm of root hair cells maintains a water potential gradient between the soil water and the cell.

Question 13

In which two roots does water move across the root?

Answer 13

The symplast pathway and the apoplast pathway

Question 14

Describe the apoplast pathway

Answer 14

• Most water travels via the apoplastic pathway (when transpiration rates are high), which is the series of spaces running through the cellulose cell walls, dead cells, and the hollow tubes of the xylem. apoplast = cell walls and intercellular spaces
• The water moves by diffusion, water can move from cell wall to cell wall directly or through the intercellular spaces and the movement of water through the apoplast occurs more quickly than the symplast pathway.
• The water can move from cell wall to cell wall directly or through the intercellular spaces.
• When the water reaches the endodermis the presence of a thick, waterproof, waxy band of suberin within the cell wall blocks the apoplastic pathway.
• This band is called the Casparian strip and forms an impassable barrier for the water.
• When the water and dissolved minerals reach the Casparian strip they must take the symplastic pathway. The presence of this strip is not fully understood but it is thought that this may help the plant control which mineral ions reach the xylem and generate root pressure.
• As the plant ages the Casparian strip thickens (as more suberin is deposited) except in cells called the passage cells, allowing for further control of the mineral ions.

Question 15

Describe the symplast pathway

Answer 15

• A smaller amount of water travels via the symplastic pathway, which is the cytoplasm and plasmodesmata or vacuole of the cells. symplast = continuous cytoplasm of the living plant cells
• The water moves by osmosis into the cell (across the partially permeable cell surface membrane), possibly into the vacuole (through the tonoplast by osmosis) and between cells through the plasmodesmata - because the next root hair cell has a higher water potential.
• The movement of water in the symplastic pathway is slower than the apoplastic pathway.

Question 16

How does water move into the xylem?

Answer 16

• Water moves across the root in the apoplast and symplast pathways until it reaches the endodermis.
• At this point, water in the apoplast pathway can go no further and it is forced into the cytoplasm of the cell, joining the water in the symplast pathway.
• This diversion to the cytoplasm is significant as to get there, water must pass through the selectively permeable cell surface membranes, as membranes would have no carrier proteins to admit them.
• Once forced into the cytoplasm the water joins the symplast pathway.
• The solute concentration in the cytoplasm of the endodermal cells is dilute compared to the cells in the xylem. In addition, the endodermal cells move mineral ions into the xylem by active transport.
• As a result the water potential of the xylem cells is much slower than the water potential of the endodermal cells. This increases the rate of water moving into the xylem by osmosis down a water potential gradient from the endodermis through the symplast pathway.
• Once inside the vascular bundle, water returns to the apoplast pathway to enter the xylem itself and move up the plant. The active pumping of minerals into the xylem to produce movement of water by osmosis results in root pressure and it is independent to any effects of transpiration. Root pressure gives water a push up the xylem, but under most circumstances it is not the major factor in the movement of water up from roots to the leaves.

Question 17

What is the endodermis?

Answer 17

• The later if cells surrounding the vascular tissue (xylem and phloem) of the roots.
• The endodermis is particularly noticeable in the roots because of the Casparian strip.
• The Casparian strip is a band of waxy material called suberin that runs around each of the endodermal cells forming a waterproof layer.

Question 18

What is the evidence for the role of active transport in root pressure?

Answer 18

• Some poisons, such as cyanide, affect mitochondria and prevent the production of ATP. If cyanide is applied to root cells so there is no energy supply, the root pressure disappears.
• Root pressure increases with a rise in temperature and falls with a fall in temperature, suggesting chemical reactions are involved.
• If levels of oxygen or respiratory substrates fall, root pressure falls.
• Xylem sap may exude from the cut end of stems at certain times. In the natural word, xylem sap is forced out of special pores at the ends of leaves in some conditions, for example overnight, when transpiration is low. This is known as guttation.

Question 19

Describe the process of transpiration

Answer 19

• Carbon dioxide moves from the air into the leaf and oxygen moves out of the leaf by diffusion down a concentration gradient through stomata.
• The stomata can be opened and closed by guard cells, which surround the stomatal opening.
• When the stomata are open to allow an exchange of carbon dioxide and oxygen between the air inside the leaf and the external air, water vapour also moves out by diffusion and is lost.
• This loss of water vapour from the leaves and stems of plants is called transpiration. Transpiration is an inevitable consequence of gaseous exchange.
• Stomata open and close to control the amount of water lost by a plant, but during the day a plant needs to take in carbon dioxide for photosynthesis, and at night no oxygen is being produced by photosynthesis so it needs to take in oxygen for cellular respiration. So at least some stomata need to be open all the time.

Question 20

How are leaves adapted for gaseous exchange/transpiration?

Answer 20

• Leaves have a very large surface area for capturing sunlight and carrying out photosynthesis.
• Their surfaces are covered in a waxy cuticle that makes them waterproof, preventing leaf cells loosing water rapidly and constantly by evaporation from their surfaces.
• However it is also important that gases can move into and out of the air spaces of the lead so that photosynthesis is possible.

Question 21

Describe the transpiration stream

Answer 21

1. Water molecules evaporate from the surface of mesophyll cells into the air spaces in the leaf and move out of the stomata into the surrounding air by diffusion down a concentration gradient.
2. The loss of water by evaporation from a mesophyll cell lowers the water potential if the cell, so water moves into the cell from an adjacent cell by osmosis, along both the apoplast and symplast pathways.
3. This is 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 of the narrow xylem vessels (adhesion). Water molecules also form hydrogen bonds with each other and tend to stick together (cohesion). The combined effects of adhesion and cohesion result in water exhibiting capillary action. Water can rise up a narrow tube against the force of gravity. Water is drawn up the xylem but a continuous stream to replace the water lost by evaporation. This is known as the transpiration pull.
5. The transpiration pull results in a tension in the xylem, which in turn helps to move water across the roots from the soil.

Question 22

What is the model of water moving from soil in a continuous stream up the xylem and across the leaf known as?

Answer 22

Cohesion-tension theory

Question 23

What is the evidence for the cohesion-tension theory?

Answer 23

1. Changes in diameter of trees - when transpiration is at its height during the day, the tension in the xylem vessels is at its highest too. As a result the tree shrinks in diameter. At night, when transpiration is at its lowest, the tension in xylem vessels is at its lowest and the diameter of the tree increases. This can be tested by measuring the circumference of a suitably sized tree at different times of the day.
2. When a xylem vessel is broken - eg. when you cut flower stems to put them in water, in most circumstances air is drawn in to the xylem rather than water leaking out.
3. If a xylem vessel is broken and air is pulled in, the plant can no longer move water up the stem as the continuous stream of water molecules held together by cohesive forces has been broken.

Question 24

Why is transpiration necessary?

Answer 24

• Transpiration delivers water, and mineral ions dissolved in water, to the cells where they are needed.
• Evaporation of water from the leaf cell surfaces also helps to cool the leaf down and prevent heat damage.
• However, transpiration is also a problem for a plant because the amount of water available is often limited. In high intensity sunlight, when the plant is photosynthesising rapidly, there will be a high rate of gaseous exchange, stomata will be open and the plant may loose so much water through transpiration that the supply cannot meet demand.

Question 25

How can transpiration be measured?

Answer 25

Using a potometer
Rate of water uptake = distance moved by air bubble/time taken for air bubble to move that distance (cm/s)

Question 26

How do stomata control the rate of transpiration?

Answer 26

• The main way in which the rate of transpiration is controlled is the opening and closing of stomatal pores. This is a turgor-driven process.
• When turgor is low the asymmetric configuration of the guard cells walls closes the pore.
• When the environmental conditions are favourable guard cells pump in solutes by active transport, increasing their turgor.
• Cellulose hoops prevent the cells from swelling in width, so they extend lengthways. Because the inner wall of the guard cell is less flexible than the outer wall, the cells become bean-shaped and open the pore.
• When the water becomes scarce, hormonal signals from the roots can trigger turgor loss from the guard cells, which close the stomatal pore and so conserve water.

Question 27

What are the factors affecting transpiration?

Answer 27

• Light
• Humidity
• Temperature
• Air movement
• Soil-water availability

Question 28

How does light affect the rate of transpiration?

Answer 28

• Light is required for photosynthesis and in the light the stomata open for gas exchange needed. In the dark, most of the stomata will close.
• Increasing light intensity gives increasing numbers of open stomata, increasing the rate of water vapour diffusing out and therefore increasing the evaporation from the surfaces of the leaf.
• So, increasing light intensity increases the rate of transpiration.

Question 29

How does relative humidity affect the rate of transpiration?

Answer 29

• Relative humidity is 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 vapour potential gradient between the inside of the leaf and the outside air.
• Very dry air has the opposite effect and increases the in rate of transpiration.

Question 30

How does temperature affect the rate of transpiration?

Answer 30

1. An increase in temperature increases the kinetic energy of the water molecules, and therefore increases the rate of evaporation from the spongy mesophyll cells into the air spaces of the leaf.
2. An increase in temperature increases the concentration of water vapour that external air can hold before it becomes saturated (so decreases its relative humidity and its water potential).
   - Both factors increase the diffusion gradient between the air inside and outside of the leaf, therefore increasing the rate of transpiration.

Question 31

How does air movement affect the rate of transpiration?

Answer 31

• Each leaf has a layer of still air around it trapped by the shape of the leaf and features such as hairs on the surface of the leaf decrease air movement close to the leaf.
• The water vapour that diffuses out of the leaf accumulates here and so the water vapour potential around the stomata increases, in turn reducing the diffusion gradient.
• Anything that increases the diffusion gradient will increase the rate of transpiration. So air movement or wind will increase the rate of transpiration, and conversely a long period of still air will reduce transpiration.

Question 32

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

Answer 32

• The amount of water available in the soil can affect the transpiration rate.
• If it is very dry the plant will be under water stress and the rate of transpiration will be reduced.

Question 33

Describe translocation

Answer 33

• Translocation is a transport of organic compounds in the phloem from sources to sinks (the tissues that need them).
• Translocation is mostly an active process that requires energy, and substances can be transported up or down the plant.
• The products of photosynthesis that are transported are known as assimilates. Although glucose is made in the process of photosynthesis, the main assimilate transported around the plant is sucrose.

Question 34

What are the main sources of assimilates (products of photosynthesis) in the plant?

Answer 34

• Green leaves and green stems
• Storage organs such as tubers and tap roots that are unloading their stores at the beginning of a growth period.
• Food stores in seeds when they germinate.

Question 35

What are the main sinks (tissues that need the products of photosynthesis) in a plant?

Answer 35

• Roots that are growing and/or actively absorbing mineral ions.
• Meristems that are actively dividing.
• Any parts of the plant that are laying down food stores, such as developing seeds, fruits or storage organs.

Question 36

Why is sucrose the main carbohydrate transported in translocation?

Answer 36

It is not used in metabolism as readily as glucose, and is therefore less likely to be metabolised during the transport process.

Question 37

What are the two main ways the plant loads assimilates into the phloem for transport?

Answer 37

• Active phloem loading by the apoplast route
• Passive phloem loading by the symplast route

Question 38

Describe the apoplast route of phloem loading

Answer 38

• In many plant species sucrose from the source travels through the cell walls and inter-cell spaces to the companion cells and sieve elements by diffusion down a concentration gradient, maintained by the removal of sucrose into the phloem vessels.
• In the companion cells sucrose is moved into the cytoplasm across the cell membrane in an active process. Hydrogen ions (H+) are actively pumped out of the companion cells into the surrounding tissue using ATP. The hydrogen ions return to the companion cell down a concentration gradient via a co-transport protein.
• Sucrose is the molecule that is co-transported. This increases the sucrose concentration in the companion cells and in the sieve elements through the many plasmodesmata between the two linked cells.
• As a result of the build up of sucrose in the companion cell and sieve tube element, water also moves in by osmosis. This leads to a build up of turgor pressure due to the rigid cell walls. The water carrying the assimilates moves into the tubes of the sieve elements, reducing the pressure in the companion cells, and moves up or down the plant by mass flow to areas of lower pressure (sinks).
• Solute accumulation in source phloem leads to an increase in turgor pressure that forces sap to regions of lower pressure in the sinks. The pressure differences in plants can transport solutes and water rapidly over many meters. Solutes are translocated up or down the plant, depending on the positions of the source.

Question 39

How are companion cells (cells in the phloem that carry out the same functions as sieve elements) adapted?

Answer 39

• Companion cells have many in foldings in their cell membranes to give an increased surface area for the active transport of sucrose into the cell cytoplasm.
• They also have many mitochondria to supply the ATP needed for the transport pumps.

Question 40

Describe the symplast route of phloem loading

Answer 40

• Sucrose from the source moves through the cytoplasm of the mesophyll cells and in into the sieve tubes by diffusion through the plasmodesmata.
• This route is largely passive. The sucrose ends up in sirve elements and water follows by osmosis. This creates a pressure of water that moves the sucrose through the phloem by mass flow.

Question 41

What is phloem loading?

Answer 41

Loading assimilates into the phloem

Question 42

Describe phloem unloading

Answer 42

• The sucrose is unloaded from the phloem at any point into the cells that need it.
• The main mechanism of phloem unloading seems to be by diffusion of sucrose from the phloem into surrounding cells.
• The sucrose rapidly moves on into other cells by diffusion or is converted into another substance (eg. glucose for respiration) so that a concentration gradient of sucrose is maintained between the contents of the phloem and the surrounding cells.
• The loss of the solutes from the phloem leads to a rise in the water potential of the phloem. Water moves out into the surrounding cells by osmosis.
• Some of the water that carried the solute to the sink is drawn into the transpiration stream in the xylem.

Question 43

What evidence supports translocation?

Answer 43

• Advancements in microscopy allow us to see the adaptations of companion cells for active transport.
• If the mitochondria of the companion cells are poisoned, translocation stops.
• The flow of sugars in the phloem is about 10,000 times faster than it would be by diffusion alone, suggesting an active process is driving the flow.
• Aphids can be used to demonstrate the translocation of organic solutes in the phloem. It has been shown that there is a positive pressure in the phloem that forces the sap out through the stylet. The pressure and therefore the flow rate in the phloem is lower closer to the sink than it is to the source. The concentration is sucrose in the phloem sap is also higher near to the source than near the sink.

Question 44

What are xerophytes?

Answer 44

• Xerophytes are plants in dry conditions that have evolved a wide range of adaptations that enable them to live and reproduce in places where water availability is low.
• They have a waxy cuticle to reduce transpiration from the leaf surfaces, stomata found mainly on the underside of the leaf that can be closed to prevent the loss of water vapour, and roots that grow down to the water in the soil.
• Conifers and marram grass (a plant found on sand dunes and coastal areas) are xerophytes. Many plants that survive in very cold and icy conditions are also xerophytes, water in the ground is not freely available to them because it is frozen.

Question 45

How do xerophytes conserve water?

Answer 45

• Thick waxy cuticle to reduce water loss.
• Sunken stomata to reduce air movement, producing a microclimate of still, humid air that reduces the water vapour potential gradient so reduces transpiration.
• Reduced number of stomata to reduce water loss by transpiration and reduce their gas exchange capabilities.
• Reduced leaf surface area to reduce water loss, narrow leaves (needles) have a greatly reduced SA:V ratio, minimising the amount of water lost in transpiration.
• Hairy leaves to create a microclimate of still, humid air reducing the water vapour potential gradient and minimising the loss of water by transpiration from the surface of the leaf.
• Curled leaves, confining all the stomata within a micro environment of still, humid air to reduce diffusion of water vapour from the stomata.
• Succulents, succulent plants store water in specialised tissue in their stems and roots. Water is stored when it in plentiful supply and then used in times of drought.
• Leaf loss, some plants prevent water loss by loosing their leaves when water is not available.
• Root adaptations, many xerophytes have root adaptations that help them to get as much water as possible from the soil, including long roots growing deep into the ground so they can access water that is a long way below the surface, and a mass of widespread, shallow roots with a large surface area able to absorb any available water before a rain shower evaporates.
• Avoiding the problem, plants may loose their leaves and become dormant, or die completely, leaving seeds behind to germinate and grow rapidly when rain falls again. Others survive as storage organs such as bulbs. A few plants can withstand complete dehydration and recover, appearing dead until it rains and cells recover.

Question 46

What are hydrophytes?

Answer 46

• Hydrophytes are plants that live in water, that require special adaptations to cope with growing in water or permanently saturated soil.
• Examples include water lilies, water cress, duckweeds, yellow iris and marginals.
• Water-logging is a problem for hydrophytes, the air spaces of the plant need to be full of air, not water for the plant to survive.

Question 47

What are the adaptations of hydrophytes?

Answer 47

• Very thin or no waxy cuticle as hydrophytes do not need to conserve water as there is always plenty available so water loss by transpiration is not an issue.
• Many always-open stomata on the upper surfaces, maximising gaseous exchange. There is no risk of loss of turgor so stoma are usually open all the time and guard cells are inactive. Stomata need to be on the upper surface so they are in contact with the air.
• Reduced structure to the plant, water supports the leaves and flowers so there is no need for strong supporting structures.
• Wide, flat leaves to increase surface area to capture as much light as possible.
• Small roots as water can diffuse directly into the stem and lead tissue so there is less need for uptake by roots.
• Large surface areas of stems and roots under water, to maximise the area for photosynthesis and for oxygen to diffuse into submerged plants.
• Air sacs to enable the leaves/flowers to float to the surface of the water.
• Aerenchyma (specialised packing tissue) forms in the leaves, stem and roots of hydrophytes. It has many large air spaces, making the leaves and stems more buoyant and forming low-resistance internal pathways for the movement of substances such as oxygen and tissues below water, helping the plant cope with the anoxic (low oxygen) conditions in the mud, by transporting oxygen to the tissues.