Module 3.3 - Transport in Plants

Question 1

Why do some plants need transport systems?

Answer 1

> Need substances like water, minerals and sugars to live and they also need to get rid of waste substances.
Plants are multicellular - so have a small SA:vol ratio so cannot exchange substances by direct diffusion for it would be too slow to meet their metabolic needs.
They’re relatively big with a relatively high metabolic rate.
Need to move substances to and from individual cells quickly.

Question 2

What does xylem tissue transport and where to?

Answer 2

Transports water and mineral ions in solution. These substances move up the plant from the roots to the leaves.

Question 3

What does phloem tissue transport and where to?

Answer 3

Mainly transports sugars (also in solution) both up and down the plant.

Question 4

What do system do xylem and phloem make up in a plant?

Answer 4

Vascular system.

Question 5

How does the structure of xylem and phloem help support the plant in the roots?

Answer 5

In a root, the xylem is in the centre surrounded by phloem to provide support for the root as it pushes through the soil.

Question 6

What is the structure of xylem and phloem in the stems?

Answer 6

In the stems, the xylem and phloem are near the outside to provide a sort of ‘scaffolding’ that reduces bending.

Question 7

What is the structure of xylem and phloem in the leaf?

Answer 7

In a leaf, xylem and phloem make up a network of veins which support the thin leaves.

Question 8

How are xylem vessels adapted for transporting water and mineral ions?

Answer 8

> Xylem vessels are very long, tube-like structures formed from cells (vessel elements) joined end to end.
No end walls on these cells, making an uninterrupted tube that allows water to pass through the middle easily.
The cells are dead, so they contain no cytoplasm.
Their walls are thickened with a woody substance called lignin which helps support the xylem vessels and stops them collapsing inwards. Lignin can be deposited in xylem walls in different ways.
The amount of lignin increases as the cell gets older.
Water and ions move into and out of the vessels through small pits in the walls where there’s no lignin.

Question 9

How is phloem’s tissue similar and different to xylem tissue?

Answer 9

> Both is formed from cells arranged in tubes.

>But phloem is purely a transport tissue - isn’t used for support.

Question 10

What cells/structures does phloem contain?

Answer 10

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

Question 11

What is the function of sieve tube elements?

Answer 11

Living cells that form the tube for transporting solutes through the plant.

Question 12

What is the structure of sieve tube elements?

Answer 12

> Joined end to end to form sieve tubes.
The ‘sieve’ parts are the end walls, which have lots of holes in them to allow solutes to pass through.
Unusually for living cells, sieve tube elements have no nucleus, a very thin layer of cytoplasm and few organelles.
The cytoplasm of adjacent cells is connected through the holes in the sieve plates.

Question 13

What do sieve tube elements need in order to survive?

Answer 13

Due to the lack of a nucleus and other organelles in sieve tube elements it means that they can’t survive on their own. So there is a companion cell for every sieve tube element.

Question 14

What is companion cells function?

Answer 14

They carry out the living functions for both themselves and their sieve cells. For example, they provide energy for the active transport of solutes.

Question 15

Give the method of how to dissect plant stems?

Answer 15

1) Use a scalpel to cut a cross-section of the stem (tranverse/longitudinal), cutting the sections as thinly as possibly for they are better for viewing under a microscope.
2) Use tweezers to gently place the cut sections in water until you come to use them, to stop them from drying out.
3) Transfer each section to a dish containing a stain.
4) Rinse off the section in water and mount each one onto a slide.

Question 16

How does water enter/be absorbed in a plant?

Answer 16

Water is absorbed from roots through the root hair cells, passes the root cortex (including the endodermis) and into the xylem to be transported around the plant.

Question 17

What process does water travel into the roots?

Answer 17

Water is drawn into the roots via osmosis and travels down the water potential gradient.

Question 18

What keeps water moving in the right direction throughout the plant?

Answer 18

The soil around roots generally has a high water potential and leaves have a lower water potential (water constantly evaporates from them). This creates a water potential gradient that keeps water moving through the plant in the right direction, from roots (high) to leaves (low).

Question 19

Describe the symplast pathway?

Answer 19

Where it goes through the living parts of cells - the cytoplasm. The cytoplasm of neighbouring cells connect through plasmodesmata (small channels in the cell walls). Water moves through the symplast pathway via osmosis.

Question 20

Describe the apoplast pathway?

Answer 20

Goes through the non-living parts of the cells - the cell walls. The walls are very absorbent and water can simply diffuse through them, as well as pass through the spaces between them. The water can carry solutes and move from areas of high hydrostatic pressure to areas of low hydrostatic pressure. This is an example of mass flow.

Question 21

How does the water in the apoplast pathway transfer into the symplast pathway?

Answer 21

> When water in the apoplast pathway gets to the endodermis cells in the root, its path is blocked by a waxy strip in the cell walls, called the Casparian strip. Now the water has to take the symplast pathway.
The water has to go through a cell membrane which ae partially permeable and are able to control whether or not substances in the water get through.
Once past this barrier, the water moves into the xylem.

Question 22

Although both pathways are used which one is the main one and why?

Answer 22

The main one is the apoplast pathway because it provides the least resistance.

Question 23

What is the loss of water from a plant’s surface called?

Answer 23

Transpiration.

Question 24

How does water (vapour) leave the leaves into the surrounding air?

Answer 24

> Water evaporates from the cell walls into the spaces between cells in the leaf.
When the stomata open, the water vapour diffuses out of the leaf down the water potential gradient into the surrounding air.

Question 25

What are the mechanisms that move the water against gravity up a plant?

Answer 25

Cohesion, tension and adhesion.

Question 26

Describe how cohesion of water particles helps move water up plants?

Answer 26

1) Water evaporates from the leaves from the xylem (transpiration).
2) This creates a tension which pulls more water into the leaf.
3) Water molecules are cohesive (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 and through the root cortex cells.

Question 27

Describe how adhesion of water particles helps move water up plants?

Answer 27

1) As well as being attracted to each other, water molecules are attracted to the walls of the xylem vessels.
2) This helps water to rise up through the xylem vessels.

Question 28

How does transpiration link to photosynthesis?

Answer 28

A plant needs to open its stomata for gas exchange so that photosynthesis can take place. But this lets water out when there’s a higher concentration of water inside the leaf than in the air outside so water moves out down the water potential gradient when the stomata open. So transpiration is a side effect of the gas exchange needed for photosynthesis.

Question 29

What are the 4 main factors that affect transpiration?

Answer 29

Light, temperature, humidity and wind.

Question 30

How does light affect transpiration?

Answer 30

The lighter it is the faster the transpiration rate. This is because the stomata open when it gets light, so CO2 can diffuse into the leaf for photosynthesis and they’re usually closed when it’s dark -> little transpiration.

Question 31

How does temperature affect transpiration?

Answer 31

The higher the temperature the faster the transpiration rate. Warmer water molecules have more energy so they evaporate from the cells inside the leaf faster. This increases the water potential gradient between the inside and outside of the leaf, making water diffuse out of the leaf faster.

Question 32

How does humidity affect transpiration?

Answer 32

The lower the humidity, the faster the transpiration rate. If the air around the plant is dry, the water potential gradient between the leaf and the air is increased, which increases transpiration.

Question 33

How does wind affect transpiration?

Answer 33

The windier it is, the faster the transpiration rate. Lots of air movement blows away water molecules from around the stomata. This increases the water potential gradient, which increases the rate of transpiration.

Question 34

What is a potometer and what does it measure?

Answer 34

A special piece of apparatus used to estimate transpiration rates and actually measures water uptake by a plant but it’s assumed that water uptake by the plant is directly related to water loss by the leaves.

Question 35

Describe the procedure of the experiment to estimate transpiration rate?

Answer 35

1) Cut a shoot underwater to prevent air from entering the xylem. Cut it at a slant to increase the surface area available for water uptake.
2) Assemble the potometer in water and insert the shoot underwater, so no air can enter.
3) Remove the apparatus from the water but keep the end of the capillary tube submerged in a beaker of water.
4) Check the apparatus is watertight and airtight.
5) Dry the leaves, allow time for the shoot to acclimatise. Then shut the tap.
6) Remove the end of the capillary tube from the beaker of water until one air bubble has formed, then put the end of the tube back into the water.
7) Record the starting position of the air bubble.
8) Start a stopwatch and record the distance moved by the bubble per minute. The rate of air bubble movement is an estimate of the transpiration rate.
9) Only change one variable at a time and keep other conditions constant.

Question 36

What are xerophytic plants adapted to do?

Answer 36

Reduce water loss.

Question 37

Give examples of xerophytic plants?

Answer 37

Cacti and marram grass.

Question 38

Describe the adaptations of marram grass that prevent them losing too much water by transpiration?

Answer 38

> Stomata sunken in pits so they’re sheltered from the wind.
Layer of ‘hairs’ on the epidermis - traps moist air round the stomata, reduces the water potential between the leaf and air.
In hot or windy conditions marram grass roll their leaves - again this traps moist air and reduces the exposed surface area for losing water and protects the stomata from the wind.
Has a thick, waxy layer on the epidermis - waterproof.

Question 39

Describe the adaptations of cacti that prevent them losing too much water by transpiration?

Answer 39

> Thick, waxy layer on the epidermis.
Cacti have spines instead of leaves that reduces the surface area for water loss.
Close their stomata at the hottest times of the day when transpiration rates are the highest.

Question 40

What are hydrophilic/hydrophyte plants adapted to do?

Answer 40

To survive in water and how to cope with a low oxygen level.

Question 41

Describe the adaptations of hydrophytic plants?

Answer 41

> Air spaces in the tissues help the plants to float and can act as a store of oxygen for use in respiration -> allows the leaves to float on the surface, increasing the amount of light they receive.
Air spaces in the roots and stems allow oxygen to allow oxygen to move from the floating leaves down to parts of the plant that are underwater.
Stomata are usually only present on the upper surface of floating leaves which helps maximise gas exchange.
Often have flexible leaves and stems and these plants are supported by the water around them, so don’t need rigid stems for support. Flexibility helps to prevent damage by water currents.

Question 42

What is translocation?

Answer 42

Translocation is the movement of dissolved substances (e.g. sugars, amino acids) to where they’re needed in the plant.

Question 43

What can dissolved substances also be called?

Answer 43

Assimilates.

Question 44

Does translocation require energy and where does it occur?

Answer 44

It’s an energy requiring process that happens in the phloem.

Question 45

Translocation moves substances from ‘sources’ to ‘sinks’. What is a source?

Answer 45

The source of a substance is where it’s made (high concentration of assimilates there).

Question 46

What is a sink?

Answer 46

The sink is the area where it’s used up (lower concentration there).

Question 47

Describe the mass flow hypothesis in terms of phloem transport in terms of their transport between the source and the phloem?

Answer 47

1) Active transport is used to actively load the solutes into the sieve tubes of the phloem at the source.
2) This lowers the water potential inside the sieve tubes, so water enters the tubes by osmosis from the xylem and companion cells.
3) This creates a high pressure inside the sieve tubes at the source end of the phloem.

Question 48

Describe the mass flow hypothesis in terms of phloem transport in terms of their transport between the phloem and the sink and what was is the effect?

Answer 48

1) At the sink end, solutes are removed from the phloem to be used up.
2) This increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis.
3) This lowers the pressure inside the sieve tubes.

> The result is a pressure gradient from the source end to the sink end. The gradient pushes solutes along the sieve tubes to where they’re needed.

Question 49

Give an example of a source and a sink in a plant?

Answer 49

The source for sucrose is usually the leaves (where it’s made), and the sinks are the other parts of the plant especially the food storage organs and the meristems (areas of growth) in the roots, stems and leaves. During this growing season, sucrose is transported from the roots to the leaves to provide the leaves with energy for growth.

Question 50

How do enzymes aid translocation?

Answer 50

Enzymes maintain a concentration gradient from the source to the sink by changing the dissolved substances at the sink (e.g. breaking them down or making them into something else). This makes sure there’s always a lower concentration at the sink than at the source.

Question 51

What is active loading?

Answer 51

Used to move substances into the companion cells from surrounding tissues, and from the companion cells into the sieve tubes against a concentration gradient.

Question 52

Describe why active loading is needed into the companion cells?

Answer 52

The concentration of sucrose is usually higher in the companion cells than the surrounding cells, and higher in the sieve tube cells than the companion cells. Active transport and co-transporter proteins are used.

Question 53

Describe the process of active loading?

Answer 53

1) In the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into the surrounding tissues.
2) Sets up a conc. gradient - more H+ ions in the surrounding tissue than in the companion cell.
3) An H+ ion binds to a co-transport protein in the companion cell membrane and re-enters the cell ( down conc. gradient).
4) A sucrose molecule binds to the co-transport protein at the same time. The movement of the H+ ion us used to move a sucrose molecule into the cell against it’s conc. gradient.
5) Sucrose molecules ar then transported out of the companion cells and into the sieve tubes by the same process.

Question 54

What supplies the energy for active loading?

Answer 54

The breakdown of ATP supplies the initial energy needed for the active transport of the H+ ions.