Transport In Plants

Question 1

Define transpiration

Answer 1

This is the process of water loss in form of water vapour to the atmosphere from the plant.

Question 2

What are the types of transpiration?

Answer 2

i. Stomatal transpiration
ii. Cuticular transpiration
iii. Lenticular transpiration

Question 3

Describe the three types of transpiration

Answer 3

Stomatal
- This is the loss of water vapour to the atmosphere through the stomatal pores of the leaves.
- This contributes 90% of the total water loss from a leafy shoot.
- This is because leaves contain a large number of stomata for gaseous exchange where this water vapour can pass and also there’s little resistance to the movement of water vapour through the stomatal pores.
- In addition, leaves also have a large surface area over which water vapour can evaporate rapidly to the atmosphere.

Cuticular transpiration
- This is the loss of water vapour to the atmosphere directly through the epidermis coated with a cuticle layer.
- It contributes 5% to the total water loss from the leafy shoot.
- This is because the cuticle is hard, waxy and less permeable to most diffusing molecules including water vapour molecules.

Lenticular transpiration
- This is the loss of water vapour through a mass of loosely packed cells known as lenticels found scattered on the stems.
- It also contributes 5% of the total water loss to the atmosphere in a leafy shoot.
- It is because the lenticels are usually few in number and not directly exposed to environmental conditions.
- Lenticular transpiration is the main source of water loss from deciduous plants after shading off their leaves. Because there are more stomata on the leaves than elsewhere in the shoot system, it is evidence that most of the water vapour is lost from the leaves.

Question 4

Describe the structure of a lenticel

Answer 4

• Loosely packed epidermal cells of the stem, non-lignified, capable of losing water by evaporation

Question 5

What are hydathodes?

Answer 5

Modified stomata found in flowers and leaf margins capable of guttation

Question 6

What is guttation?

Answer 6

The loss of water from a plant in liquid form.

Question 7

Describe the structure of a stoma

Answer 7

• A pore, bordered by a pair of bean shaped cells called guard cells that control its opening and closing. The inner layer of the guard cell is thicker and less elastic while the outer is thinner and more elastic.

Question 8

What assumption is made when using weighing method to determine rate of transpiration?

Answer 8

• Assume mass loss is only due to water loss by transpiration

Question 9

What assumption is made when using potometer method to determine rate of transpiration?

Answer 9

• Assume water uptake is equal to water loss

Question 10

What precautions are taken when using a potometer?

Answer 10

1. The leafy shoot used should have a significant water loss by having very many leaves
2. The stem of the leaf shoot must be cut under water to prevent air from entering and blocking the xylem vessels
3. The setup must have plenty of water
4. Ensure that only one bubble is present in the capillary tube
5. A well graduated scale must be used e.g. a ruler, so that clear readings are taken
6. The air bubble should always be reset to zero mark before the potometer is used again under different conditions
7. The water reservoir should be filled with water when setting the air bubble at the zero mark
8. The cut leafy shoot must be in contact with water in the sealed vessel

Question 11

What is the ecological significance of transpiration?

Answer 11

• Water moves up a stem, makes it turgid, support the plant parts
• Absorption of water for photosynthesis, primary productivity

Question 12

What is a criticism of the photosynthetic product theory of stomatal opening and closing?

Answer 12

The theory does not explain how the low rate of glucose formation can account for the rapid opening of stomata

Question 13

What are the advantages of transpiration?

Answer 13

• It allows the uptake of water from the roots to leaves in form of a transpiration stream. This is due to a transpiration pull created in the leaves. This ensures proper distribution of water throughout the plant to keep it alive.
• It facilitates the uptake of the absorbed mineral salts within the xylem vessels from roots to leaves
• It brings about the cooling of the plant since as water evaporates to the atmosphere, excessive heat is also lost as heat of vaporization, which results into the cooling of the plant
• It brings about mechanical support in non- woody or herbaceous plants, due to water uptake which provides turgidity to the parenchyma cells of the stem and leaves
• It is important for cloud formation via evapotranspiration hence resulting into rainfall

Question 14

What are the disadvantages of transpiration?

Answer 14

• It causes wilting of plants in case of excessive transpiration
• It may eventually cause death of the plant, when the plant looses water excessively due to excessive transpiration

Question 15

What are the effects of wilting?

Answer 15

• There is reduction in turgor pressure and drooping of the plant.
• Causes closure of the stomata which cuts off gaseous exchange and therefore may cause death if it persists.

Question 16

What is the ecological significance of transpiration?

Answer 16

• Plants take up water, plant cells become turgid giving support to plants
• Leaves spread out when cells are turgid and absorb more sunlight
• Absorption of water for photosynthesis which increases primary productivity

Question 17

What is the physiological significance of transpiration?

Answer 17

• Cools the plant enabling enzyme activity to occur at optimum temperature

Question 18

What factors affect transpiration?

Answer 18

Environmental:
- Temperature
- Humidity
- Wind/ air currents
- Light intensity

Non-environmental:
- Number of stomata
- Leaf area
- Cuticle
- Leaf arrangement
(Check book for explanations)

Question 19

What conditions favor stomatal opening?

Answer 19

• High light intensity
• Low mesophyll carbon dioxide levels

Question 20

Describe the photosynthetic product theory of stomatal openings and closing

Answer 20

• Guard cells have chloroplasts.
• During day light, they carry out photosynthesis producing sugar.
• The sugar lowers the water potential of the cell sap.
• This causes water to move into the guard cells from nighbouring epidermal cells by osmosis.
• The result is an expansion and increase in turgidity of the guard cells containing the stomata to open.
• In darkness, photosynthesis stops and the sugar in the guard cells is converted to starch.
• This raises the water potential of guard cells causing them to lose water to neighboring cells by osmosis.
• The guard cells become flaccid and the stomata close.

Question 21

Describe the active potassium theory of stomatal opening and closing in plants

Answer 21

In light:
- Starch is converted to malic acid in guard cells
- Malic acid dissociates into malate ions and hydrogen ions
- Blue light activates ATPase and hydrogen ions are actively pumped out of guard cells into neighboring epidermal cells as potassium ions are actively pumped into the guard cells.
- Accumulation of malate ions and potassium ions in the guard cells lowers the water potential of the guard cells.
- However in some plants such as onions where the guard cells have no starch, malate does not accumulate and instead chloride ions are taken up with positive ions to maintain the electrochemical neutrality.
- Water enters into the guard cells by osmosis and guard cells become turgid, stomata open

In darkness:
- Potassium ions diffusion out of the guard cells into neighboring epidermal cells
- Malic acid converts back into starch
- Hydrogen ions re-enter the guard cells via carrier proteins, lowering the pH of guard cells
- Water potential of the guard cell sap increases above that of neighboring epidermal cells
- Water moves out of guard cells into neighboring epidermal cells, they become flaccid and the stomata close

Question 22

Describe the starch-sugar interconversion theory of stomatal opening and closing

Answer 22

In the day:
- Guard cells photosynthesize
- Decreased concentration of CO2 in guard cells
- Increased pH (alkaline)
- Starch is converted to sugar
- The cell sap concentration increases thus the water potential decreases
- Water enters the guard cells by endosmosis
- Increased turgor in the guard cell
- Stomata open

At night:
- Photosynthesis in guard cells ceases
- Increased concentration of CO2 in guard cells
- Decreased pH (acidic)
- Sugar is converted to starch
- The cell sap concentration decreases thus the water potential increases
- Water leaves the guard cells by exosmosis
- Decreased turgor in the guard cell
- Stomata close

Question 23

What is the sequence of layers in the root?

Answer 23

Epidermis > Cortex > Endodermis (which has suberin) > xylem vessel

Question 24

Describe the mechanism of water uptake by plants (ie into the endodermal cells)

Answer 24

Water flows from root hairs endodermal cells using three pathways, namely;

a) Apoplast (cell wall) pathway
b) Symplast (cytoplasm) pathway
c) Vacuolar pathway

Apoplast pathway
- This is the pathway in which water moves through the spaces between the cellulose fibres in the cell wall of one cell to the cell wall of the adjacent cells.

• However, this movement does not occur within the endodermal cells because they possess the impermeable casparian strip which prevents water and solutes flow through the cell walls of the endodermal cells.
• This means that water and solutes flow through the cell walls of the endodermal cells via the Symplast and the vacuolar pathways only.

Symplast pathway
- This is the movement of water through the cytoplasm of one cell to the cytoplasm of the adjacent cell via plasmodesmata.

• Water leaving the pericycle cells to enter the xylem causes the water potential of these cells to become more negative (more dilute).
• This facilitates the flow of water by osmosis from the adjacent cells into these cells.
• In this way the water potential gradient from the root hairs to the xylem is established and maintained across the root.

Vacuolar pathway
- This is the movement of water from the sap vacuole of one cell to the sap vacuole of the adjacent cell following a water potential gradient.

• This is achieved by maintaining a steep water potential gradient.

Question 25

What adaptations do root hairs in plants have to ensure maximum absorption of water?

Answer 25

a. They are numerous in number so as to provide a large surface area for the maximum absorption of water by osmosis.
b. They are slender and flexible for easy penetration between the soil particles so as to absorb water.
c. The lack a cuticle and this enhances the passive osmotic absorption of water without any resistance
d. They have a thin and permeable membrane which allows the absorption of water by osmosis.
e. They have a water potential lower than that of the soil solution which facilitates a net osmotic flow of water from the soil

Question 26

By what means does water move up the stem from the roots to the leaves?

Answer 26

• Root pressure
• Transpiration
• Capillarity

Question 27

Describe the root pressure mechanism

Answer 27

• Root pressure is the force developed by cells of the roots which forces water from the endodermal cells into the xylem vessels of the root and constantly forces water upwards through the stem to leaves.
• This process is active and involves utilization of many ATP molecules.
• Root pressure occurs as a result of endodermal cells actively secreting salts into the xylem sap from their cytoplasm, which greatly lowers the water potential in the xylem.
• In some plants, root pressure maybe large enough to force liquid water through pores called hydathodes of the leaves in a process called guttation

Question 28

What evidence supports root pressure mechanism of water uptake from the endodermis into the xylem vessel as an active process?

Answer 28

a. There are numerous starch grains in endodermal cells which could act as an energy source for active transport.

b.Lowering the temperature reduces the rate of water exudation (given out) from the cut stem as it prevents root pressure, an active process.

c. Treating the roots with metabolic poisons e.g. potassium cyanide also prevents water from being exuded from the cut stems. This is because the poisons kill the cells thereby preventing aerobic respiration, a source of ATP molecules.

d.Depriving roots of oxygen prevents water from being exuded from the cut stems. This shows that water was being pushed upwards in the cut stem by root pressure, an active pressure.

Question 29

What evidence shows that water moves by root pressure in a plant?

Answer 29

• When the stem of a plant is cut water continues to exude from the xylem vessels of the plant stem.
• The continuous exudation of water from the xylem vessels of the cut stem is due to root pressure because the leafy shoot is cut off, meaning that water not only moves upwards by transpiration pull, but also due to pressure and other forces.

Question 30

What is a shortcoming of root pressure?

Answer 30

Not sufficient to push water to the leaves

Question 31

Describe the transpiration pull (cohesion-tension theory) mechanism

Answer 31

• Water is removed from the plant leaves by transpiration which creates a tension within the leaf xylem vessels that pulls water in the xylem tubes upwards in a single unbroken column or string held together by the cohesive forces of attraction between water molecules.
• According to the cohesion-tension theory, evaporation of water from the mesophyll cells of the leaf to the sub-stomatal air chamber and eventually to the atmosphere via the stomata by transpiration, is responsible for the rising of water from the roots to the leaves.
• This is because the evaporated water molecules get replaced by neighbouring water molecules which in turn attract their other neighbours and this attraction continues until the root is reached.
• Evaporation of water results in a reduced water potential in the cells next to the leaf xylem.
• Water therefore enters these mesophyll cells by osmosis from the xylem sap which has the higher water potential.
• Once in the mesophyll cells water moves using the three pathways namely; apoplast, Symplast and vacuolar pathways from one cell to another by osmosis across a water gradient.
• When water leaves the leaf xylem to the mesophyll cells by osmosis, a tension is developed within the xylem tubes of water which is transmitted to the roots by cohesive forces of water molecules.
• The tension develops in the xylem vessels and builds up to a force capable of pulling the whole column of water molecules upwards by means of mass flow and water enters the base of these columns from neighbouring root cells.
• Because such a force is due to water loss by osmosis by transpiration, it is referred to as transpiration pull.
• The upward movement of water through the xylem tissue from the roots to leaves is also facilitated by the cohesive forces of attraction which holds the water molecules firmly together, due to the hydrogen bonds which exist between them.
• This enables water to have a high tensile strength which enables it to move upwards in a continuous stream without breaking.
• In addition, the upward movement of water from roots to leaves is also facilitated by adhesive forces which hold the water molecules on the xylem walls so that it continues moving upwards.

Question 32

Which mechanism offers an explanation for the continuous flow of water upwards through the xylem of the plant?

Answer 32

Transpiration pull

Question 33

Describe the capillarity mechanism of water movement up the xylem

Answer 33

• Since the water rises upwards through narrow tubes, it is also facilitated by capillarity through the stem.
• This is because the xylem vessels are too narrow and the flow of water is maintained without breaking by both the cohesive and adhesive forces.

Question 34

What is a shortcoming of capillarity?

Answer 34

• Maximum height water can travel is only 3 meters even in the tallest of trees

Question 35

Differentiate between cohesion and adhesion forces

Answer 35

Adhesion is the force of attraction between molecules of different substances while cohesion is the force of attraction between molecules of the same substance.

Question 36

Define translocation

Answer 36

Translocation is the movement of mineral salts and chemical compounds within a plant.

Question 37

What are the two main processes of translocation?

Answer 37

a. The uptake of soluble minerals from the soil and their passage upwards from the roots to the various organs via the xylem tubes.
b. The transfer of organic compounds synthesized by the leaves both upwards and downwards to various organs via the phloem tubes

Question 38

Describe the mechanism of mineral ion uptake

Answer 38

Minerals such as nitrates, phosphates, sulphates e.t.c. may be absorbed either actively or passively.

1. Active absorption of minerals
   - Most minerals are absorbed from the soil solution having the less mineral concentration into the root hairs with the higher mineral concentration, selectively by using active transport which uses a lot of energy.
   - The rate of active absorption of minerals into the root hairs depends on the rate of root respiration.

Factors such as oxygen supply and temperature will affect the rate of ion uptake. The addition of respiratory poison has shown to inhibit uptake of mineral ions.

2. Passive absorption
   - If the concentration of a mineral in a soil solution is greater than its concentration in the root hair cell, the mineral may enter the root hair cell by diffusion.
   - Mass flow or diffusion occurs once the minerals are absorbed by the root hairs so that they move along cell walls (apoplast pathway).
   - In mass flow, the mineral ions are carried along in solution by water being pulled upwards in the plant in the transpiration stream, due to the transpiration pull i.e. the mineral ions dissolve in water and move within the water columns being pulled upwards.
   - The mineral ions can also move from one cell of the root to another against the concentration gradient by using energy inform of ATP.
   - The mineral ions can also move through the Symplast pathway i.e. from one cell cytoplasm to another.
   - When the minerals reach the endodermis of the root, the Casparian strip prevents their further movement along the cell walls (apoplast pathway).
   - Instead the mineral ions enter the cytoplasm of the cell (Symplast pathway) where they are mainly pumped by active transport into the xylem tissues and also by diffusion to the xylem tissues.
   - Once in the xylem, the minerals are carried up the plant by means of the transpiration stream.
   - From the xylem tissues, minerals reach the places where they are utilised called sinks by diffusion and active transport i.e. the minerals move laterally (sideways) through pits in the xylem tissue to the sinks by diffusion and active transport.

Question 39

What evidence shows that most mineral ions are absorbed actively by the root hairs?

Answer 39

a. Increase in temperature around the plant increases the rate of mineral ion uptake from the soil as it increases respiration that can provide energy for active transport
b. Treating the root with respiratory inhibitors such as potassium cyanide prevents active mineral ion uptake leaving only absorption by diffusion. This is because the rate of mineral ion uptake greatly reduces when potassium cyanide is applied to the plant.
c. Depriving the root hairs of oxygen prevents active uptake of minerals by the roots and as a result very few ions enter the plant by diffusion.

Question 40

What is the evidence supporting the role of the xylem in transporting minerals?

Answer 40

a. The presence of mineral ions in the xylem sap i.e. many mineral ions have been found to be present in the xylem sap.
b. There’s a similarity between the rate of mineral ion transport and the rate of transpiration i.e. if there’s no transpiration, then there’s no mineral ion transport and if transpiration increases, the rate of mineral ion transport also increases.
c. There’s evidence that other solutes e.g. the dye, eosin, when applied to the plant roots, it is carried in the xylem vessels
d. By using radioactive tracers e.g. phosphorous- 32. When a plant is grown into a culture solution containing radioactive phosphorous-32, phosphorous -32 is found to have reached all the xylem vessels but not the phloem tubes. (The interpretation of these elements is that where lateral transfer of minerals can take place minerals pass from the xylem to the phloem and where lateral transfer is prevented, the transport of minerals takes place in the xylem)

Question 41

What evidence supports that organic molecules of photosynthesis are transported in the phloem?

Answer 41

a. When the phloem is cut, the sap which exudes out of it is rich in organic food materials especially sucrose and amino acids.
b. The sugar content of the phloem varies in relation to environmental conditions. When the conditions favor photosynthesis, the concentration of the sugar in the phloem increases and when they not favor photosynthesis and concentration of the sugar in the phloem reduces.
c. Removal of a complete ring of phloem around the phloem causes an accumulation of sugar around the ring, which results into the swelling of the stem above the ring. This indicates that the downward movement of the sugars has been interrupted and results into the part below the ring failing to grow and may dry out. This is called the ringing experiment.
d. The use of radioactive tracers. If radioactive carbon dioxide-14 is given to plants as a photosynthetic substrate, the sugars later found in the phloem contain carbon-14. When the phloem and the xylem are separated by waxed paper, the carbon-14 is found to be almost entirely in the phloem.
e. Aphids have needle like proboscis with which they penetrate the phloem so as to suck the sugars. If a feeding aphid is anaesthetized using carbon dioxide or any other chemical e.g. chloroform and then its mouth parts cut from the main body, some tiny tubes called the proboscis remain fixed within the phloem sieve tubes from which samples of the phloem content exudes.

Question 42

State the three theories of mechanisms of translocation of organic materials in the phloem

Answer 42

1. The mass flow or pressure flow hypothesis (i.e. Much’s hypothesis)
2. Electro-osmosis
3. Cytoplasmic streaming

Question 43

What is mass flow?

Answer 43

Mass flow is the movement of large quantities of water and solutes in the same directions.

Question 44

Under what conditions does mass flow occur?

Answer 44

• Solute addition at the source and solute removal at the sink
• Hydrostatic pressure gradient between the two points
• Continuous pathway for flow of solutes and water

Question 45

Is mass flow active or passive?

Answer 45

Passive

Question 46

Describe the mass flow or pressure flow hypothesis

Answer 46

• Leaves make sucrose during photosynthesis which accumulates and lowers the water potential of the photosynthesizing cells in the leaf
• Sucrose is actively pumped into the phloem sieve cells of the leaf.
• As a result, water brought to the leaf via the xylem enters by osmosis which increases the pressure potential of the leaf cells.
• The food solution in the sieve tubes then moves by mass flow from a region of higher water potential in the leaves to that of lower water potential in the sink such as roots following a hydrostatic pressure gradient.
• At the other parts of the plant which form the sink sucrose is either being utilized as a respiratory substrate
• The soluble content of the sink cells therefore is low and this gives them a higher water potential and consequently lower pressure potential exists between the source (leaves) and the sink such as roots and other tissues
• The sink and the source are linked by the phloem sieve tubes and as a result the solution flows from the leaves to other tissues (sinks) along the sieve tube elements.

Question 47

What evidence supports the mass flow theory?

Answer 47

1. When the phloem is cut, the sap exudes out of it by mass flow
2. There’s rapid and confirmed exudation of the phloem’s sap from the cut mouth parts of the aphids which shows that the content of the sieve tubes move out at high pressure.
3. Most researchers have observed mass flow in microscopic sections of the sieve tube elements.
4. There’s some evidence of concentration gradient of sucrose and other materials with high concentration in the leaves and lower concentration in the roots.
5. Any process that can reduce the rate of photosynthesis indirectly reduces the rate of translocation of food.
6. Certain viruses are removed from the phloem in the phloem translocation stream indicating that mass flow rather than diffusion, since the virus is incapable of locomotion.

Question 48

What are the criticisms of mass flow?

Answer 48

1. By this method all organic solutes would be expected to move in the same direction and at the same speed. It was however observed that the organic solutes move in different directions and at different speeds.
2. The phloem has a relatively high rate of oxygen consumption which this theory does not explain.
3. When a metabolic poison such as potassium cyanide enters the phloem, the rate of translocation is greatly reduced, implying that translocation is not a passive process, but an active one.
4. The mass flow hypothesis does not mention any translocation of solutes with influence of transfer cells and Indole Acetic Acid (IAA) hormone that loads the sugars or solutes into the sieve tubes and also unload it into the cells of the sink.
5. The sieve plates offer a resistance which is greater than what could be overcome by the pressure potential of the phloem sap. This implies that the pressure would sweep away the sieve plates during this transport.
6. Higher pressure potential is required to squeeze the sap through the partially blocked pores in the sieve plates than the pressure which has been found in the sieve tubes

Question 49

Define Electro-Osmosis

Answer 49

This is the passage of water across a charged membrane.

Question 50

Describe electro-osmosis mechanism of movement of organic compounds in plants

Answer 50

• The sieve plate is charged because positively charged ions e.g. K+ , actively pumped by the companion cells across the sieve plate into the sieve tube element using energy from ATP of the companion cells.
• Potassium ions accumulate on the upper side of the sieve plate thereby making it positively charged.
• Negatively charged ions accumulate on the lower sides of the sieve plate thereby making it negatively charged.
• The positive potential above the sieve plate is further increased by hydrogen ions, actively pumped from the wall to the upper sieve tube element into its cytoplasm.
• Organic solutes such as sucrose are transported across the sieve plates due to an electrical potential difference between the upper and the lower side of the sieve plate whereby the lower side is more negative than the upper side
• solutes move from the upper sieve tube element which is positively charged to the lower sieve element which is negatively charged.
• The electrical potential difference is maintained across the plate by active pumping of positive ions, mainly potassium ions, in an upward direction.
• The energy used is produced by the companion cells.
• The movement of K+ ions through the pores of the sieve plates rapidly draws molecules of water and dissolved solutes through the sieve pores, to enter the lower cell.

Question 51

What evidence supports the electro-osmosis theory?

Answer 51

1. K+ ions stimulate the loading of the phloem in the leaves with sugars during photosynthesis.
2. Numerous mitochondria produce a lot of energy for translocation, an indicator that translocation is an active process. If however, the phloem tissues are treated with a metabolic poison, the rate of translocation reduces.
3. Presence of a sieve plate where a potential difference can be developed across the plate

Question 52

Describe the cytoplasmic streaming theory

Answer 52

• This suggests that the protoplasm circulates using energy from sieve tubes elements or companion cells through the sieve tube elements from cell to cell via the sieve pores of the sieve plates.
• As the protoplasm circulates, it carries the whole range of the transported organic materials with it. - The solutes are moved in both directions along the trans-cellular strands by peristaltic waves of contraction, such that they move from one sieve tube element to another using energy in from of ATP.
• The proteins in the strands contract in a wave form, pushing the solutes from one sieve tube element to another, using energy in form of ATP.

Question 53

What evidence supports the cytoplasmic streaming theory?

Answer 53

• It has been found that the solute materials move in both directions in the phloem tissue
• The theory explains the existence of the trans- cellular strands in the phloem tissue as well as many mitochondria in the companion cells

Question 54

What are the criticisms of the Cytoplasmic Streaming Theory?

Answer 54

1. Cytoplasmic streaming has not been reported in mature sieve tube elements but only in young sieve tubes.
2. The rate at which the protoplasm streams is far slower than the rate of translocation

Question 55

What is the significance of the casparian strip?

Answer 55

• It actively pump salts (ions) from the cytoplasm of the endodermal cells into the xylem vessels which creates a high solute concentration in the xylem, thereby greatly lowering the water potential in the xylem than in the endodermis.
• This makes the water potential of the xylem vessels more negative (very low) and results into rapid osmotic flow of water from the endodermal cells to the xylem vessels, due to the steep water potential gradient between the endodermal cells and the xylem vessels.
• The casparian strip facilitates the pushing of water upwards through the xylem vessels by root pressure up to the leaves due to its active pumping of the salts.
• In addition, this active pumping of the salts into the xylem vessels prevents leakage of slats (ions) out of the xylem vessels so as to maintain a low water potential in this vessel.

Question 56

What is the main transport carbohydrate in plants?

Answer 56

Sucrose

Question 57

What are the adaptations of the xylem to their function?

Answer 57

• Vessels have narrow lumen; increasing capillarity forces
• (Vessels)Bordered pits; lateral flow of water
• (Tracheids) bordered pits; reduce resistance to flow of water where end walls are present
• Joined end to end; continuous flow of water
• (Vessels) End walls broken down; uninterrupted flow of water
• (Tracheids & vessels) Long; transport of water over long distances
• Lignified; prevent escape/ loss of water
• Lignified; rigid to prevent collapsing under tension forces
• Xylem vessels hollow; minimize resistance as water moves

Question 58

What are the three patterns of thickening?

Answer 58

• Annular
• Spiral
• Reticulate

Question 59

What are the adaptations of the phloem to their function?

Answer 59

• Sieve plates; perforated to enable continuous movement of food
• Plasmodesmata; lateral movement of organic substances in sieve tubes
• (Companion cells) Numerous mitochondria; energy for loading food onto phloem
• (Mature sieve tubes) No nucleus; space for translocation
• Elongated; transport food over long distances
• Fibres; mechanical support

Question 60

Give similarities between xylem and phloem

Answer 60

• Both conducting tissues
• Both elongated
• Both have parenchyma
• Perforated
• Fibres
• Tubular

Question 61

What is a bordered pit?

Answer 61

• An area in the walls of xylem vessels and tracheids that is unlignified and has numerous plasmodesmata

Question 62

Describe the structure of a bordered pit

Answer 62

• Many plasmodesmata
• Unlignified
• Perforated

Question 63

What is the use of the casparian strip?

Answer 63

• Prevents movement of water by apoplast pathway
• Prevents mineral salts from leaking into the endodermis

Question 64

Describe the process of water uptake in root hair cells

Answer 64

• Active transport
• Water potential of root hair cell sap lowers below that of the soil solution
• Water moves by osmosis
• Water entering the root hair cells increases the water potential of root hair cell sap above that of the cells of the cortex
• Water moves into the cortex by apoplast, symplast and vacuolar pathways
• Water entering the cortex cells increases the water potential of cortex cell sap above that of the cells of the endodermal cells
• Water moves into the endodermis by the same three means
• Endodermal cells secrete mineral salts into the xylem by active transport thus water potential of xylem cells lowers below that of the endodermal cells
• Endodermal cells have suberin deposits that form the casperian strip which prevents apoplast as well as diffusion of mineral salts back into endodermal cells
• Therefore water moves into xylem by symplast and vacuolar only

Question 65

What are the adaptations of xerophytes to reducing transpiration?

Answer 65

• Waxy cuticle (prevents water loss)
• Shiny cuticle (reflects light)
• Few stomata on upper epidermis(reduced exposure to direct light)
• Sunken stomata(high humidity build up to reduce water vapor pressure)
• Reversal of stomatal opening( at night)
• Hairy leaves (trap moisture around leaves)
• Curled leaves (reduce surface area for transpiration)
• Absence of leaves (reduced surface area )
• Leaves reduced/ modified to thorns
• Shedding off leaves
• High abscissic acid concentration in leaves
• Tolerance of tissues to high temperatures

Question 66

What are the adaptations of xerophytes to conservation of water?

Answer 66

• Succulent leaves
• Succulent stems
• Fleshy underground tuber

Question 67

What are the adaptations of xerophytes to water uptake?

Answer 67

• Deep roots for absorption of water from deeper layers of soil
• Shallow roots for absorbing surface moisture

Question 68

What are the ecological adaptations of xerophytes to their habitat?

Answer 68

• Deep roots; more water, competitive advantage
• Shallow roots

Question 69

What are the behavioral adaptations of xerophytes to their habitat?

Answer 69

• Shedding leaves during the dry seasons
• Opening stomata during night and closing during day

Question 70

What are the physiological adaptations of xerophytes to their habitat?

Answer 70

• Secrete thick layers of wax for cuticle
• Produce large amounts of ABA

Question 71

Define water stress

Answer 71

A condition when water taken up by a plant from soil is less than water lost by evaporation from leaves

Question 72

What is the effect of water stress on productivity of a plant?

Answer 72

• Reduces productivity
• Stomata close to reduce water loss, CO2 uptake decreases, rate of photosynthesis reduces