Transport In Plants

Question 1

What are the main transport tissues and where are they found?

Answer 1

The xylem and phloem are found associated together in vascular bundles through the plant

Question 2

What does the Xylem tissue do?

Answer 2

• Carries water and dissolved minerals from the roots to the photosynthetic parts of the plant
• the movement in the xylem is always upwards

Question 3

What does the phloem tissue do?

Answer 3

Transport the dissolver product of photosynthesis from the leaves to where it is needed for growth or storage as starch
• the flow through phloem can go both up and down within a plant

Question 4

What is the cambium?

Answer 4

A layer of unspecialised cells that divide, giving rise to more specialised cells that in turn form both the xylem and phloem

Question 5

What is the first xylem to form called and describe its structure

Answer 5

• the protoxylem
• it is capable of stretching and growing because the walls are not fully lignified
• the cellulose microfibrils in the walls of the xylem vessels are laid down more or less vertically in the stem which increases the strength of the tube and allows it to withstand the compression forces from the weight of the plant pressing down on it

Question 6

What is the xylem called after it is the protoxylem and how does this form and what is its structure?

Answer 6

• the metaxylem
• as the stem ages and the cells stop growing increasing amounts of lignin are laid down in the cell walls
• as a result the cells become impermeable to water and other substances
• the tissue becomes stronger and more supportive but the contents of the cells die
• the end walls between the cells largely breakdown so the xylem forms hollow tubes running from the roots to the tip of the steme

Question 7

What does the support come from in smaller non-woody plants?

Answer 7

From the turgid parenchyma

Question 8

In what provess are water and minerals transporter from the roots to the leaves?

Answer 8

The transpiration stream

Question 9

Water moves out of the xylem into surrounding cells. What does it go through to do this?

Answer 9

• Unlignified areas

* Specialised pits (holes) in the walls of the xylem vessels

Question 10

What happens as woody plants grow older?

Answer 10

More xylem tissue is lignified to increase support

Question 11

What are the three experiments you can do to act as evidence for the movement of water through the xylem and explain them:

Answer 11

• put the cut end of a shoot in a solution of eosin dye, the dye can then be seen carried into the transport system and through to the vascular tissue of the leaves. So you can see a red column moving upwards as the dye moves up the xylem with the water
• Ringing experiments: involve removing a complete ring of bark of bark or killing a complete ring with a steam jet. This destroys the living phloem but not xylem cells. Therefore when you see the upwards movement of the eosin dye you know it must be due to the xylem
• autoradiography:
- the plant is given a radioactively labelled version of the substance being studied
- the radioactive substance is taken up the same way by the llant as the normal isotope
- the substance can then be tracked by placing the plant against photographic film for a while to produce an autoradiograph
So you can observe the movement of water up the xylem

Question 12

What are the phloem seive tubes made up of?

Answer 12

• many cells joined together to make very long tubes that run from the highest shoots to the end of the roots
• the phloem cells dont become lignified and so the contents remain living
• the walls between the cells become perforated to form specialised sejve plates and the phloem contents flow through the holes in these plates
• as the gaps in the seive plates form, the nucleus, the tonoplast and some of the other organelles break down

Question 13

How do the phloem cells survive?

Answer 13

• Because they have closely associated cells called companion cells
• The companion cells are very active cells that have all the normal organelles and are linked to the seive tube elements by many plasmodesmata
• the cells membranes of the companion cells have many infoldings that increase the surface area over which they can transport sucrose into the cell cytoplasm
• companion cells have many mitochondria to supply the ATP needed for ative transport

Question 14

Where is water mainly absorbed?

Answer 14

In the younger parrs of the roots where the majority of the root hairs are found

These microscopic hairs are extensions of the membranes of the outer cells of the root and they greatly increase the surface area for absorption

Question 15

There is a concentration gradient across the root from the root hair cells to the cells closest to the xylem. What is this the result of?

Answer 15

• water is continuously mover up the xylem by transpiration

* the solute concentration increases in the cells across the root towards the xylen

Question 16

What are the alternative roots to rhe xylem vessel and how do they work?

Answer 16

• the symplast pathway: water moves bg diffusion down the concentration gradienr from the root hair cells to the xylem through the interconnected cytoplasm (symplast) of the cells of the root system. It moves through the plasmodesmata, gaps in the cellulose cell wall that allow stands of cytoplasm to pass through tbem
• apoplast pathway: water is pulled by the attraction between water molecules across adjacent cells walls (the apoplast) from the rootnhair cell tonthe xylem. Because of the loose open network structure ofncellulose up to half of the volume of the cell wall can be filled with water. As water is drawn into the xylem, attraction between the molecules ensires that more water is pulled across from the adjacent cell wall and so on. Water enterjng the root hair from the soil has mineral ions dissolved in it, and they are drawn through the apoplast pathway too. The water moves across the cells of the root in the cell walls until it reaches the endodermis which contains a waterproof layer called the Casparian strip
• vacuoler pathway. Water moves through the vacoules of the root hair cells

Question 17

How do water and minerals enter the waterproof Casparian strip?

Answer 17

• They enter the cytoplasm of the cell temporarily.
• Minerals may eed to enter the cytoplasm up a concentration gradient involving active transport
• this is a way by which the cells control the amount of water and mineeals moving from the soil into the xylem

Question 18

What is translocation?

Answer 18

The movement of substances around a plant

Question 19

Are translocation in the xylem and phloem active or passive

Answer 19

Xylem: passive

Phloem: active

Question 20

What is transpiration?

Answer 20

The loss of water vapour from the surface of the plant, mainly from the leaves

Question 21

What happens when water reaches the leaves?

Answer 21

• moves from the xylem jn the veins of the leaves to the spongy mesophyll cells by osmosis
• water then evaporates from the cellulose cell walls of the spongy mesophyll cells into the air spaces
• the water vapour moves through open stomata intonthe external air along a diffusion gradient.
• there is a layer of still air which the water vapour diffuses through before it is swept away by the mass of moving air

Question 22

What is the cohesion- tension theory of transpiration and what does this make possible?

Answer 22

• when water is lost by transpiration from the leaves it moves by osmosis across the leaf from cell to cell all the way from the xylem
• when molecules of water leave the xylem to enter a cell by osmosis this creates tension in the column of water in the xylem and this tension is transmitted all the way down to the roots
• this is due to the cohesion of the water molecules. Because of their polar nature and the hydrogen bonds that form between them, water molecules stick together giving the column of water a high tensile strength - it is not likely to break

Question 23

What us Adhesion?

Answer 23

• the water molecules adhere strongly to the walls of the narrow xylem vessel qnr to the millions of channels and pores within the cellulose cell walls of the leaf
• adhesion is the attraction between unlike molecules and it is sufficient to support the entire column of water in the xylem

Question 24

What is it that pulls the whole volume of water upwards jn the xylem?

Answer 24

The combunation of ahesive and cohesive forces

Question 25

What is the transpiration stream?

Answer 25

Water continuously being moved into the roots by osmosis from the soil to replace that lost from the leaves by transpiration

Question 26

How can you demonstrate the loss of water from the surface of a plant?

Answer 26

• first seal the pot of a potted plant in a plastic bag to prevent evaporation of water from the soil surface interfering wjth the experiment
• then seal the plant in a bell jar
• as water is lost a colourless liquid collects on the glass of the belm jar
• you can show this contains water by using cobalt chloride or copper sulfate paper

Question 27

To measure the amount of transpiration you can measure the uptake of water of a plant. How do you do this?

Answer 27

• use a potometer:

• have a leafy shoot at then end: it must be fresh with stem cut under water and then transferred to apparatus undetwater to avoid air bubbles
• the seal must be airtight: the use of petroleum jelly helps to ensure this
• there is a reservoir of water to reset the air bubble
• there is a capillary tube of known diameter in which you can measure the sir bubbke miveing through
• bubble at the start of the scale- this must be the only bubble in the aparatus
• beaker of water

Question 28

What are the four factors affecting transpiration?

Answer 28

• Light
• Temperature
• Air movement or wind
• Humidity

Question 29

How does light affect transpiration?

Answer 29

Stomata open in the light for photosynthetic gas exchange and most are closed in the dark. As a result transpiration rates increase with light intensity until all of the stomata are open

Question 30

How does Temperature affect transpiration?

Answer 30

• an increase in temperature will increase the amount of evaportation from the surfaces of the spongy mesophyll cells
• it will also increase the amount of water vapour the air can take in before it gets saturated
These both increase the concentration gradient between the air inside and out of the leaf
• an increase in temperature will also increase the rate of movement of the molecules which in turn increases the rate of diffusion of water vapour out of the leaf increasing the rate of transpiration

Question 31

How does wind speed affect transpiration?

Answer 31

High wind speed increases the rate of transpiration because it reduces rhe shell of still air around the stomata. This in turn increases the diffusion gradient between the inside and outside of the leaf, increasing the rate of transpiration

Question 32

How does air humidity affect transpiration?

Answer 32

• it is the concentration of water vapour in the air
• a high air humidity lowers the rate of transpiration because of the reduced concentration gradient between the inside of the leaf and the air. Very dry with low humidity has the opposite effect

Question 33

What adaptations do plants have that minimise water loss?

Answer 33

Curled, hairy and grooved leaves that trap still, mosut air aroynd the stomata and so reduce water loss by transpiration

Question 34

In situations when transpiration rates can be very low such as at night what helps?

Answer 34

• guttation: where drops of water are forced put of the leaves
• root pressure: based on active transport. Produced by the active secretion of salts from the root cells into the xylem sap increasing the concentration gradient across the root. This increases the movement of water into cells by osmosis

Question 35

How is glucose transported in the phloem?

Answer 35

• it is converted to sucrose because sucrose has less of an osmotic effect than glucose
• it is then converted beack to glucose or starch for storage in the target cells or used to make other molecules such as amino acids or lipids

Question 36

What are the substances transported in the phloem called and what is the main one?

Answer 36

They are called assimilates and the main assimilate is sucrose

Question 37

The transport of assimilates in the phloem is from sources to sinks. What are sources and what are sinks?

Answer 37

• Sources of sucrose in plants are the green parts, storage organs such as tubers and the food stores of seed when they germinate
• sinks are any plant tissue that needs sucrose for example the actively dividng cells in the meristems, fruits, seeds and storage organs as they build up food stores and roots that are growing or actively absorbing mineral ions from the soil

Question 38

What are the two main routes of phloem loading?

Answer 38

• the symplast pathway: sucrose moves by diffusion down a concentration gradient from the source cells through the cytoplasm of a number of cells into the companion cells and on into the phloem sieve tubes. This is a largely passive process. The high sucrose concentration in the phloem means water then moves into the seive tubes by osmosis, causing a positive hydrostatic pressure that moves the phloem sap towards the sinks. Because the sucrose is moved in the phloem sap to the sinks, a constant diffusion gradient is maintained
• the apoplast pathway: sucrose moves by diffusion down a concentration gradient through the cellulose cell walls and cell spaces to the companion cells. In the companion cells sucrose is moved from the wall spaces across the membrane into the cytoplasm by active transport using AT, producing a high sucrose concentration in the cytoplasm of the companion cells and so in the seive tubes- the sucrose passes into the seive tubes through the many plasmodesmata. Water moves into the companion cells by osmosis as a result of the high sucrose concentration, producing a positive hydrostatic pressure that moves assimilares and water into and through the phloem seive tubes. This hydrostatic pressure moves the sap towards the sucrose sinks where the hydrostatic pressure is lower as there is less sucrose in the companion cell sap and sk less movement of water into the sieve tubes by osmosis. The diffusion gradient for sucrose into the companion cells and sieve tubes is maintained because sucrose is removed by mass flow of the cell sap to the sucrose sinks

Question 39

How are assimilates moved many metres up or down a plant?

Answer 39

The accumulation of sucrose in the source phloem leads to an increase in turgour pressure that in turn forces the sap to regions of lower pressure in the sucrose sinks. This pressure can be very high

Question 40

How is the phloem unloaded?

Answer 40

• by a process of diffusion down a concentration gradient from the sieve tubes to the surrounding cells
• the sucrose then moves rapidly on into other cells by diffusion or may be converted into other compounds.
• this maintains the sucrose diffusion gradient between the phloem and the sink cells
• as the sucrose moves out of the sieve tubes, the water potential of the sap rises and so water also moves out by osmosis down a concentration gradient. Some of this water then moves into the xylem

Question 41

What are the experiments you can do to test for evidence for translocation?

Answer 41

• radioative isotopes of carbon can be made available to the leaves of a plant so the glucose is radioactively labelled. The sucrose made from this glucose is also labelled and the path it takes in the plant can be traced using autoradiography. This labelled sucrose is found in the phloem
• if a jet of steam is used to kill a complete outer ring of bark in a young shoot, just below a leaf, the movement of solutes from the leaf through the phloem to areas below the region of dead tissue stops. The xylem is not affected
• Aphids penetrate the phloem with their mouthparts. Sometimes the pressure of the fluid in the phloem is so great that it moves right through the digestive system of the aphid and appears as a droplet at the end of the body. If the insect is removed from the plant, the contents kf the phloem ooze out of the mouthparts and can be analysed, both for their content and the rate of flow

Question 42

Mass transport systems involve the transport of materials from one point to another in a transport system with a transport medium and a pressure or force to bring about movement. How does translocation fulfil these requirements?

Answer 42

• the phloem sap is the transport medium
• the phloem seive tubes are the system
• sucrose and other assimilates are to be moved and turgour pressure (hydrostatic pressure) in the system provides the force for movement

Question 43

What was Ernst Münch’s simple mass flow hypothesis and what does it explain?

Answer 43

• he had two containers. A and C. A is a partially permeable membrane containing conentrated sugar solution. C is a partially permeable membrane containing a dilute sugar solution with tube B joining them. They are both surrounded in water
• initially water moves into both containers by osmosis
• As A contains a much higher concentration of sugar solution than C water will move into A more rapidly and there will be a flow of solution through the tube from A to C
• the hydrostatic pressure this creates forces water out of C. This flow continues until the concentration of the solutions in A and C is the same
• this explains the movement of solutes in the phloem of plants

Question 44

What are the main limitations of a passi e mass flow model?

Answer 44

• it doesnt take into account the active loading of sucrose into the phloem sieve tube elements by the companion cell at the source. This changes the concentration gradient and so the rate of osmosis and can even change the rate of flow
• translocation is continuous- It does not end with equal concentrations - but the model does not take into account the continuous loading of sucrose at the sources and removal at the sinks to make this possible
• water can move into the tubes at any point by osmosis
• the return route for water to the cells is through the xylem?

Question 45

What is the modified version of the mass flow hypothesis that gives us a very good model of how assimates are transported in the phloem called?

Answer 45

The pressure flow hypothesis