Transport System In Plants

Question 1

Why do plants need a transport system?

Answer 1

All plants have a small surface area to volume ratio so transport system is necessary to:
1. To move substances from areas of absorption to where they are needed for example water is absorbed from soil and transported in xylem to other parts in the plants.
2. To move substances from where they’re produced to where they are needed for metabolism. For example sugars are produced in leaves by photosynthesis and transported to different plant parts to be used for respiration, storage or used for making structures inside the cells.
3. To move substances to a different part of the plant for storage-for example to move sugars into a potato tuber for storage in the form of starch.
Transport system is needed for transport water minerals organic nutrients as well as other substances such as plant hormone.

Question 2

Which two substances are required by the plant?

Answer 2

Oxygen obtained by diffusion from atmosphere to the leaf through stomata, or by diffusion from air space between soil particles to roots. Oxygen is necessary for respiration to release energy for active uptake of minerals against concentration gradients, cell division, protein synthesis or other metabolic reactions.
Carbon dioxide obtained by diffusion through the stomata from high concentration in the air to low concentration in the air space then to the mesophyll cells soluble in thin film in water surround the walls of cells. Most plants have evolved thin flat leaves with a large surface area to volume ratio for absorption of carbon dioxide during daylight.

Question 3

What respires at a faster rate between plants and animals?

Answer 3

Plants have much lower energy demands than animals so they respire at a much lower rate than animals.

Question 4

Are transport systems needed for gases?

Answer 4

Transport system is not needed for gases however it is needed for water minerals and organic nutrients.

Question 5

What are the two factors that the surface area for gas exchange in plants depends on?

Answer 5

Number of leaves and surface area of the leaf.

Question 6

Describe the difference in leaves of monocotyledon and dicotyledon?

Answer 6

The leaves in monocotyledon are long, narrow they have no petiole and have parallel veins.for example grass
The leaves in dicotyledon are leaves with blades, they are wide, the petiole is present and they have branched veins.

Question 7

Describe the difference in the petals of monocotyledon and dicotyledon?

Answer 7

Monocotyledon have three or their multiple petals whereas dicotyledon have four or five or their multiple petals.

Question 8

Describe the difference in the stem of monocotyledon and dicotyledon?

Answer 8

Monocotyledon have scattered vascular bundles or as dicotyledon have vascular bundles which are arranged in a ring.

Question 9

What is a tissue?

Answer 9

Tissues are a group of cells that are specialized to carry out a function. Cells in the same tissue may have similar structures such as parenchyma or different structures such as xylem and phloem.

Question 10

Describe the structure of epidermis.

Answer 10

Epidermis is a continuous layer on the outside of the plant it is one cell thick and there are no chloroplasts. upper epidermis is covered with a waxy cuticle which is the waterproof layer. In xerophytes waxy cuticles are thicker to protect plants from being dry due to the lack of water. Epidermis may contain pores called stomata which are controlled by the guard cell. Guard cells have few chloroplasts while as epidermis cells never have any chloroplasts.

Question 11

Describe the three functions of epidermis.

Answer 11

1. Epidermis provides protection against drying out and against infection
2. Epidermis has pores in the leaves (stomata) to allow gas exchange and transpiration.
3. In roots it has an extension of cytoplasm called root hair cells to increase the surface area for absorption of water and mineral salts.

Question 12

Describe the structure of parenchyma.

Answer 12

Parenchyma are thin walled cells which are very metabolically active. The parenchyma in roots form cortex whereas the parenchyma in stem form cortex and pith. The parenchyma in leaves have chloroplasts and are modified to form palisade and spongy mesophyll.

Question 13

What is a cortex?

Answer 13

a cortex is an outer layer of a stem or root in a vascular plant, lying below the epidermis but outside of the vascular bundles.

Question 14

What is pith?

Answer 14

Pith is the central region of the stem which is made of similar cells. Pits are not open pores. Pitts are non-lignified areas in xylem vessels. Pitts are crossed by permeable and unthickened cellulose cell walls. The pits in one cell link with those in neighboring cells so water can pass freely from one cell to the next cell.

Question 15

What are the functions of parenchyma?

Answer 15

1. Used as packaging tissue
2. Used for storage foods like starch
3. Support the plant when it’s fully turgid to prevent wilting.
4. May have air space to allow gas exchange
5. Water and mineral salts can be transported through the walls and through the living contents from cell to cell.

Question 16

Describe the structure of collenchyma

Answer 16

Collenchyma is a modified form of parenchyma with extra cellulose deposited at corners of the cell. Found in the midrib of the leaf.

Question 17

What is the function of collenchyma?

Answer 17

Collenchyma provides extra strength to support the tissue.

Question 18

Describe the structure of sclerenchyma.

Answer 18

Sclerenchyma is dead lignified cells.

Question 19

Describe the function of sclerenchyma.

Answer 19

Sclerenchyma is found in stem to support the plant for extra strength.

Question 20

Describe the structure of endodermis.

Answer 20

Endodermis is one cell thick and surrounds vascular tissue in stems and roots. It is covered with a thick cuticle layer of suberin to form a band called casparian strip which is an impenetrable barrier to water in the walls of endodermis. Older cells have more suberin deposits except from certain cells that are called passage cells through which water can pass freely.

Question 21

What is the function of endodermis?

Answer 21

Endodermis blocks the apoplastic halfway. It controls what minerals pass into xylem vessels as everything must pass across the cell surface membrane. It helps in generation of root pressure as it allows only the symplastic pathway. Endodermis also allows the passage of minerals to xylem against concentration gradient by active transport through carrier protein.

Question 22

Describe the structure of mesophyll cells.

Answer 22

Mesophyll cells are made of specialized parenchyma cells found between the lower and upper epidermis of the leaf. There are two types of mesophyll cells. palisade mesophyll have a columnar shape they are closely packed together and they have a large number of chloroplasts. Spongy mesophyll has many air spaces and the cells have some chloroplasts.

Question 23

Describe the function of mesophyll cells.

Answer 23

Palisade mesophyll cells are near the upper surface to trap light energy for photosynthesis. Spongy mesophyll cells help in photosynthesis and many large air spaces for gas exchange.

Question 24

Describe the structure of pericycle cells.

Answer 24

Pericycle cells are a layer of cells that are made of one to several cells thick. They are found inside the endodermis and next to vascular tissue. In roots the pericycle is one cell thick, in stems pericycle is made up of sclerenchyma.

Question 25

What is the function of pericycle cells?

Answer 25

New roots can grow from this layer.

Question 26

Describe the structure of vascular tissue

Answer 26

xylem and phloem both contain more than one type of cells. The walls of xylem are made of cellulose and lignin the walls of phloem are made of only cellulose. There are pits present in xylem whereas no piths present in phloem. The diameter is wider in xylem as compared to phloem which is narrower.

Question 27

Describe the function of vascular tissue.

Answer 27

Vascular tissue form transport systems in plants. Xylem transports water and minerals whereas phloem is used for translocation of sucrose and amino acids from the source to the sink.

Question 28

Describe the difference in xylem and phloem in terms of whether they are dead or living.

Answer 28

Xylem is dead and phloem is living.

Question 29

Describe the structure of xylem.

Answer 29

Xylem contains tubes of dead cells called xylem vessel elements. xylem walls are reinforced with a strong waterproof material called lignin.

Question 30

Describe the structure of phloem.

Answer 30

Phloem is made of sieve tubes and companion cells.

Question 31

What is the function of xylem?

Answer 31

Xylem allows long distance transport of water and mineral salts. Xylem provides mechanical support and strength. The movement in xylem is in one direction.

Question 32

What is the function of phloem?

Answer 32

Phloem does translocation of assimilates which are inorganic compounds (sucrose and amino acids) from source to sink in both directions.

Question 33

What is the location of xylem?

Answer 33

Xylem is located in stem facing the center and arranged in vascular bundles. Xylem is located in roots, it has a series of arms between which the phloem is found.

Question 34

What is the location of phloem?

Answer 34

Phloem is found in stem together with xylem forming vascular bundles that are surrounded by sclerenchyma, as phloem faces outwards. In roots phloem is found between a series of arms made of xylem vessels.

Question 35

What is the xylem vessel element?

Answer 35

A dead, lignified cell found in xylem so specialized for transporting water and for support The ends of the cells break down and join with neighboring elements to form long tubes called xylem vessels.

Question 36

What is xylem vessel?

Answer 36

A dead empty tube with lignified walls through which water is transported in plants it is formed by xylem vessel elements lined up end to end.

Question 37

What are the similarities between xylem and sclerenchyma.

Answer 37

Sclerenchyma fibers like xylem elements are long dead empty cells with lignified walls.

Question 38

What are the differences between xylem and sclerenchyma?

Answer 38

Sclerenchyma has only a mechanical function for supporting and strengthening the stem, but does not transport water while xylem has both functions.

Question 39

Describe how the thick lignified walls of xylem vessels help them be adapted to their function.

Answer 39

The thick lignified walls prevent collapse and provide support (ignored strengthened). To prevents the leakage of water as lignin is a waterproof material.

Question 40

Describe how the pits of xylem vessels help them be more adapted to their function?

Answer 40

The pits allow lateral movement and supply water to surrounding cells. (Rejected pores)

Question 41

Describe how the elongated cells of xylem vessels help them be more adapted to their function?

Answer 41

Elongated cells or vessels as cells end to end are adjacent to make tubes. This provides efficiency of water movement.

Question 42

How does xylem having no end walls help them become more adapted to their function?

Answer 42

Xylems have no end walls to form continuous tubes. The hollow tubes have no content such as nucleus or cytoplasm. This means that there is no resistance to the water flow against gravity which allows the free flow of water.

Question 43

Describe why xylems have a relatively large diameter which makes them more adapted to their function?

Answer 43

Xylems have a relatively large diameter (large cross sectional area) that makes wide lumen. This gives xylem more space and allows a greater volume to flow per unit time.

Question 44

Describe the absorption of minerals and waters by roots and root hair cells.

Answer 44

The tip of the root is covered with a hard cap which is impermeable to water. Just behind the tips some of the cells in the outer layer of epidermis are drawn out into long thin extensions called root hairs. Water is absorbed by the root hair cells from high water potential inside the soil to low water potential in root hair cells.

Question 45

Why do root hair cells have a low water potential?

Answer 45

Although some water contains some inorganic ions in a solution, it is a relatively dilute solution and so has a relatively high water potential. However the cytoplasm and the cell sup inside the root hairs have considerable quantities of inorganic ions and organic substances such as proteins and sugars dissolved in them and so have a relatively low water potential.

Question 46

Why are there a large number of very fine root hair cells?

Answer 46

Large number very fine with hair cells increase the surface area for higher rate of water absorption.

Question 47

Define Transpiration

Answer 47

The loss of water vapor from a plant to its environment it mostly takes place through the stomata in the leaves. Stomata open during the day and close at night so most transpiration takes place during the day.

Question 48

Define the stoma

Answer 48

Stoma (plural: stomata) is a pore in the epidermis of a leaf, bounded by two guard cells and needed for efficient gas exchange.

Question 49

Describe the movement of water across the roots from root hairs to xylem

Answer 49

After water enters the root it crosses the cortex of the root and enters the xylem in the center of the root. It does this because the water potential inside the xylem vessel is lower than the water potential in the root hairs. Therefore water moves down the water potential gradient across the root. Water can take two routes through the cortex: apoplastic or symplastic pathway. Individual molecules can switch randomly from one route to another.

Question 50

What happens to the movement of water from root hair cells to xylem when the water reaches the endodermis.

Answer 50

When the water reaches the endodermis the apoplastic pathway is blocked. These cells in the endodermis have a thick waterproof waxy band of suberin in their cell walls. This band is called the casparian strip and it goes right round the cell. it stops water moving through the apoplast. The only way for water coming across the cortex to cross the endodermis is through the unthickened parts of the wall into the cytoplasm of the endodermis cells. As the endodermis cells get older the suberin deposits become more extensive until no water can enter the cells except in certain cells called passage cells.

Question 51

Describe the movement of water through the xylem from the root to the leaf.

Answer 51

The removal of water(transpiration) from xylem vessels in the leaf creates a tension in the water left in the xylem vessels. (The water potential at the top of the xylem vessel becomes lower than the water potential at the bottom). This tension causes water to move up the xylem vessels. Despite the high tension xylem vessels do not collapse because of their strong lignified walls. The movement of water and mineral ions up through xylem vessels is by mass flow. Cohesion and adhesion help to keep the water in the xylem vessel moving as they continuous column. These cells are dead and empty and there's no protoplasm which is an advantage for the transport.

Question 52

During the movement of water through the xylem does the root or the leaves have a greater hydrostatic pressure?

Answer 52

Continuous absorption of water by osmosis increases the root pressure whereas continuous transpiration from leaves reduces the hydrostatic pressure in the top of the xylem vessels in leaves. Hydrostatic pressure at the root is greater than hydrostatic pressure at leaves so water moves against gravity.

Question 53

What is cohesion?

Answer 53

The attraction between water molecules due to hydrogen bonding is called cohesion.

Question 54

What is adhesion

Answer 54

The water molecules are attracted to cellulose and lignin in the walls of the xylem vessels which are hydrophilic. this attraction is called adhesion. Adhesion helps to keep the water in the xylem vessel moving as a continuous column.

Question 55

What is mass flow?

Answer 55

The movement of water and mineral ions through xylem vessels is by mass flow this means that all the water molecules plus any dissolved solutes move together at the same speed like water in a river this is different from diffusion where different types of molecules or ions move at different speeds and gradients according to their own diffusion gradients. Mass flow through xylem is helped by cohesion and adhesion.

Question 56

What is an airlock and how is it prevented?

Answer 56

If an air bubble forms in the column of water in a vessel the water stops moving upwards this is called an air lock. The small diameter of xylem vessels helps to prevent such breaks from occurring. The pits in the vessel walls allow water to move out from one vessel to a neighboring vessel and so bypass such an airlock. Air bubbles cannot pass through pits. Pits are also important because they allow water to move in and out of xylem vessels from and to surrounding living cells.

Question 57

Describe the apoplast pathway.

Answer 57

Water enters the cell wall. Water moves through the cell wall Water may move from cell wall to cell wall through the intercellular spaces. Water may move directly from cell wall to cell wall.

Question 58

Define the apoplast pathway

Answer 58

The non-living system of interconnected cell walls extending throughout the plant used as a transport pathway for the movement of water and mineral ions.

Question 59

Define the symplast pathway.

Answer 59

The living system of interconnected protoplasts extending through a plant used as a transport pathway for the movement of water and solutes. Individual protoplasts are connected via plasmodesmata.

Question 60

Describe the symplast pathway.

Answer 60

Water enters the cytoplasm by osmosis through the partially permeable cell surface membrane. Water moves into the sap in the vacuole through the tonoplast by osmosis. Water may move from cell to cell through the plasmodesmata. Water may move from cell to cell through adjacent cell surface membranes and cell walls.

Question 61

Which pathway is faster between symplast and apoplast pathway?

Answer 61

The symplast pathway is relatively slow because the pathway is obstructed by the organelles. The apoplast pathway offers much less resistance to water flow than the symplast pathway. Mark Scheme: Symplast is slower because greater resistance ; water passes across membranes ; water flows through, cytoplasm / plasmodesmata ; A ora ref. to intercellular spaces A protoplast osmosis occurs ; R through plasmodesmata

Question 62

What is plasmodesmata?

Answer 62

Plasmodesmeta are microscopic channels through the cell wall connecting the cytoplasm of cells.

Question 63

What are the external factors affecting the rate of transpiration?

Answer 63

Humidity Wind speed and temperature Light intensity Very dry conditions

Question 64

What are the internal factors affecting the weight of transpiration?

Answer 64

Surface area of the leaf Number of leaves Thickness of cuticle layer Number and distribution of stomata on leaves

Question 65

How does humidity affect transpiration?

Answer 65

Water vapor diffuses out of the leaf down the concentration gradient. That's because the concentration of water vapor outside the leaf is generally lower than inside. If the humidity outside the leaf increases then that means that there is a smaller concentration gradient between the inside of the leaf and the outside. An increase in the humidity outside of the leaf reduces the rate of transpiration.

Question 66

Describe the effect of light intensity on the rate of transpiration

Answer 66

For transpiration to take place, the stomata must be open. Stomata opens in light conditions to allow carbon dioxide to diffuse into the leaf and take part in photosynthesis. as we increase the light intensity The rate of transpiration also increases this is because increasing light intensity increases the number of open stomata allowing more water vapor to diffuse out of the leaf. However at high light intensity the rate of transpiration no longer increases almost all of the stomata will be open. At night stomata clothes to reduce unnecessary water loss.

Question 67

How does temperature affect the rate of transpiration?

Answer 67

The rate of transpiration is increased by increase of temperature and wind speed. At higher temperatures water molecules have more kinetic energy. This means there is a greater rate of evaporation of water from the internal surfaces of the leaf. Secondly at higher temperature the relative humidity of the external error decreases at high temperatures and thus increases the diffusion gradient. Increasing wind speed moves water vapor away from the leaf surface more rapidly this maintaining a steeper water potential gradient between the air spaces of the leaf and the surrounding air.

Question 68

Describe the effect of very dry conditions on the rate of transpiration?

Answer 68

In very dry conditions the water potential gradient between air space in the leaf and the surrounding atmosphere is steep, the plant may have to compromise by partially or completely closing its stomata to prevent leaves drying out even if this means less rate of transpiration.

Question 69

What is the importance of transpiration?

Question 70

What are xerophytes?

Answer 70

Xerophytes (xerophytic plants) are plants that live in places where water is in short supply. Many xerophytes have evolved special adoptions of their leaves that keep water loss down to a minimum. Examples include cacti and marram grass.

Question 71

How are cacti adapted to short supply of water

Answer 71

In many species of cacti leaves have been replaced with fibrous spines with photosynthesis taking place in this stem of the cactus. In these cacti the stomata are found on the surface of the stem. replacing the leaves with spines reduces the surface area of volume ratio of the cactus, reducing water loss. The spines also trap moist air near the cactus reducing the rate of transpiration as well as providing some shade for the stem from the heat of the sun. Cacti also have a thick waxy cuticle to reduce evaporation of water. In cacti the stomata or sunken into pits. This traps a layer of moist air around the stomata reducing water loss by transpiration. Cacti only open their stomata at night too absorb carbon dioxide when conditions are relatively cool. Coming outside is then used in the day for photosynthesis. I only opening stomata at night cacti reduce water loss during the heat of the daytime. Cacti often have extensive shallow roots. This allows the cacti to absorb water after a rain shower before the water evaporate. Cacti can also have very deep roots to access water from lower levels of the soil. Cacti can also store water in their stem

Question 72

What are succulents??

Answer 72

Plants which can store water in their stem are called succulents.

Question 73

Where is marram grass found?

Answer 73

Marram grass is found in sand dunes.m

Question 74

What are the adaptations of marram grass?

Answer 74

The leaves of the marram grass can roll into a tube with the stomata on the inside. Moist air is trapped within the tube rather than being blown away by the wind. The stomata in marram grass are found in sunken pits with fine hairs projecting inwards towards the center. These adaptations ensure that moist air is trapped around the stomata, this reduces the concentration gradient for water vapor between the air and the internal spaces between the leaf cells. This reduces the rate of diffusion of water vapor out of the stomata. Marram grass also have a very thick waxy cuticle to reduce evaporation from the surface. The roots of marram grass can be very long, extending deep into the sand to find water.

Question 75

Define Protoplasts

Answer 75

Protoplasts are all the living contents inside the cell wall surrounded by the cell surface membrane. (Including cell membrane and everything inside the cell membrane)

Question 76

What is cuticle made up of?

Answer 76

A layer covering and secreted by the epidermis, in plants it is made up of fatty substances called cutin, which helps to provide protection against water loss and infection.

Question 77

How do the cell contents in xylem vessels die?

Answer 77

As lignin builds up around the cell

Question 78

Describe assimilates

Answer 78

Assimilates are the chemical compounds made by the plants itself as a result of assimilation. Assimilation in plants is the range of processes by which the plant converts its inorganic nutrients into organic compounds. An example of assimilation is photosynthesis. During photosynthesis, inorganic carbon dioxide and water are converted using energy to organic solutes like sugar. Another example of a simulation is the use of nitrates obtained from the soil to help make amino acids sucrose and amino acids are two of the common assimilates that are transported over long distances in the phloem.

Question 79

What is a source?

Answer 79

The source is a site in a plant which provides food for another part of the plant the sink. Assimilates are transported from sources to sinks. common sources are leaves and storage organs such as tubers.m

Question 80

What is a sink?

Answer 80

A site in a plant which receives food from another part of the plant, the source. Assimilates are transported from sources to sinks. Common sinks are buds, flowers, fruits, roots and storage organs

Question 81

Is phloem alive or dead and what is the direction of movement?

Answer 81

In contrast to xylem phloem is a living tissue. The direction of movement is upwards and downwards. Phloem consists of two different types of tissue: sieve tube element and companion cells.

Question 82

What is a seive tube element?

Answer 82

A sieve tube element consists of a long line of cells arranged end to end. Inside these cells almost all of the organelles have been lost including the nucleus and vacuole. This leaves the interior of these cells almost entirely free to transport phloem sap. The end walls of these cells have been modified to contain large pores. These modified walls are called sieve plates. Sieve plates allow the phloem sap to move between the cells. As sieve tubes have lost most of their organelles, they cannot produce large amounts of essential molecules such as ATP. However next to the sieve tube element cells, there are companion cells.

Question 83

Describe companion cells

Answer 83

Companion cells are located next to the sieve tube elements. Companion cells contain a nucleus as well as large amounts of mitochondria. microscopic channels called plasmodesmata link the companion cells to seive tube element cells. Molecules such as ATP and proteins can move through the plasmodesmata into the sieve tube element cells. The role of the companion cells is to provide essential molecules to the sieve tube element cells.

Question 84

Which two organelles are found in the seive tube element?

Answer 84

Endoplasmic reticulum and reduced amount of cytoplasmforming thin layer lining the inside of cell wall and mitochondria. Cell wall containing cellulose and cell surface membrane. No nucleus and no ribosomes.

Question 85

Describe the structure of sieve pores.

Answer 85

These large pores are easily visible with a good light microscope. They allow easy flow from seive tube element to the sieve tube element and as the pores are open they form little barrier to the free flow of liquids through them.

Question 86

By which process is phloem sap transported across the seive tubes?

Answer 86

Phloem sap like xylem sap moves by mass flow. In xylem vessels the movement is passive meaning it requires no energy input from the plant (only the Sun). To create the pressure differences needed for mass flow inflow in the plant has to use energy therefore flow and transport is an active process.

Question 87

Why is the glucose produced in the leaves converted to sucrose?

Answer 87

Plants produce the monosaccharide glucose during photosynthesis in the leaves. All parts of the plant require glucose for respiration. The glucose produced in the leaves is first converted to the disaccharides sucrose. Sucrose is less reactive than glucose and is less likely to react with other molecules.

Question 88

What is translocation?

Answer 88

The transport of assimilates in the phloem is referred to as translocation. Assimilates are transported from sources to sinks.

Question 89

When do storage organs act as sinks?

Answer 89

Storage organs can also act as sinks when they are refilling their carbohydrates stores.

Question 90

What is active loading?

Answer 90

The process of loading sucrose into the sieve tube elements is called active loading.

Question 91

What is the first step in active loading?

Answer 91

The hydrogen ions are pumped out of the companion cells to the surrounding leaf tissue creating a diffusion gradient of hydrogen ions using energy from ATP(ATP hydrolyses to ADP+Pi to release energy).

Question 92

What is the second step in active loading?

Answer 92

H+ ions diffuse back into the companion cells through co-transporter proteins which act as a carrier for both hydrogen ions and sucrose.

Question 93

What is the third step in active loading?

Answer 93

High concentrations of sugar in companion cells cause sucrose to diffuse into sieve tube elements through the plasmodesmeta by symplastic pathway.

Question 94

How is sucrose unloaded from the phloem into the sink?

Answer 94

Because of active loading there is a high concentration of sucrose in the sieve tube element cells. The effect of this is to lower the water potential inside the sieve tube element. Water now moves into the sieve tube element by osmosis from nearby tissues including the xylem vessels. This now increases the hydrostatic pressure inside the seive tube element. As a result the phloem sap now moves up or down the sieve tube element towards the sink. This bulk movement of phloem sap is called mass flow.

Question 95

What happens to sucrose when it reaches the sink?

Answer 95

At the sink the sucrose moves out of the seive tube element and is converted to glucose for use in respiration. Or in the case of storage organs the sucrose is converted into starch. As the sucrose leaves this increases the water potential in the seive tube element causing water to move out of the sieve tube element by osmosis.

Question 96

What is a proton pump?

Answer 96

Proton pumps are proteins found in the cell surface membrane. Hydrogen ions are pumped out of the companion cell into its cell wall by a proton pump using ATP as an energy source. The proton pump creates a large excess of hydrogen ions in the apoplast pathway outside of the companion cell.

Question 97

What is capillary action?

Answer 97

Capillary action is the movement of liquid along a surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid. Capillary action has a role in the transport of water in xylem vessel elements. (In basic terms it means that the adhesion force is stronger than the cohesion force )

Question 98

What is cohesion tension theory?

Answer 98

The combined effect of the transpiration pull coupled with cohesion and adhesion is that water is drawn into the roots, moves out of the stem and passes out of the leaf. This whole process is called the cohesion tension theory. Cohesion tension theory affects the diameter of the stem (decreasing Diameter as cohesion tension increases.

Question 99

Give two pieces of evidence that supports the cohesion tension theory.

Answer 99

If a plant stem is cut, then air is sucked into the xylem suggesting that the xylem vessels are under tension. However the air prevents cohesion between the water molecules so water movement stops. Secondly if we measure the diameter of a tree trunk we can see that this reduces when transpiration is at its maximum. This supports the idea that transpiration pull generating a negative pressure or tension in the xylem.

Question 100

What are the xerophytic adaptations which reduce the water potential gradient between leaf surface and atmosphere?

Answer 100

The water potential gradient is affected by rolled leaves,hairy leaves/leaves with trichomes and sunken stomata. They trap moisture thereby reducing the steepness of the water potential gradient.

Question 101

What is the role of the casparian strip?

Answer 101

To prevent water moving back into the cortex from the xylem.

Question 102

Name the peice of equipment that can be used in the light microscope to work out the correct porportion of vascular tissues

Answer 102

Eyepiece graticule.