Adaptation for Transport: Plants

Question 1

How are water and mineral ions transported from the roots to the leaves?

Answer 1

Upwards through xylem tissue (hollow tubes formed by dead cells)

Question 2

How are the products of photosynthesis transported from leaves to other organs of the plant?

Answer 2

Upwards and downwards (bi-directional) in phloem tissue

Question 3

Draw and label the stem

Question 4

Draw and label the root

Question 5

How are mineral ions transported from soil into root hair cells?

Answer 5

Active transport

Question 6

How does water enter the root hair cell?

Answer 6

Mineral ions have been actively transported into the root hair cell which lowers the water potential inside the cell so water moves in by osmosis

Question 7

What happens when plants are in waterlogged soil?

Answer 7

Oxygen enters the roots from the soil to be used in aerobic respiration, providing ATP for active transport.
Waterlogged soils lack oxygen and so plants struggle with uptake of ions

Question 8

Transport of water across the root occurs by which 3 pathways?

Answer 8

Symplast, vacuolar and appoplast

Question 9

Define the appoplast pathway

Answer 9

Water is taken up by the root hair cell and moves across the cortex by cohesion via cell walls

Question 10

Define the symplast pathway

Answer 10

Water moves from the cytoplasm of one cell to the next by osmosis via plasmodesmata

Question 11

Define the vacuolar pathway

Answer 11

Water can move via the cytoplasm and vacuoles

Question 12

What does the cell wall of all endodermal cells contain?

Answer 12

Casparian strip (impermeable to water molecules)

Question 13

Water and mineral ions from the apoplast pathway are forced across the cell membrane into the _____________ pathway

Answer 13

Symplast

Question 14

What is needed to move ions into the cytoplasm of the endodermal cells?

Answer 14

Active transport

Question 15

What is the role of the casparian strip?

Answer 15

To give the plant greater control over which ions enter the xylem and are transported to the rest of the plant

Question 16

Define root pressure

Answer 16

When water moves from the endodermal cells of the root and into the xylem by osmosis, this generates hydrostatic pressure and forces water a small distance up the xylem

Question 17

With reference to the role of the Casparian strip, explain how cyanide would result in the reduction in root pressure

Answer 17

• The casparian strip stops the apoplast pathway so forces ions into the symplast pathway
• Movement of ions into the xylem requires active transport
• Cyanide is a respiratory inhibitor so prevents cells respiring and stops ATP synthesis
• So lower water potential gradient reduces movement of water in by osmosis thus lowering root pressure

Question 18

Why does the plant need to control entry of mineral ions into the xylem?

Answer 18

Some mineral ions are toxic if they’re absorbed and accumulate inside plant tissues

Question 19

How does the plant ensure toxic ions cannot enter its cells?

Answer 19

There are no carrier proteins specific to these ions on the membrane so these ions cannot enter by facilitated diffusion

Question 20

Why do plants need to absorb nitrates from the soil?

Answer 20

Nitrogen is a constituent of amino acids, proteins, nuclei acids, nitrogenous basis - overall growth

Question 21

Give 3 features of root hair cells that are adaptations for uptake of water and mineral ions

Answer 21

• Large surface area
• Thin cell walls
• High number of mitochondria to provide the energy needed for active transport

Question 22

Define transpiration

Answer 22

The evaporation of water from inside the leaves, through the stomata, and into the atmosphere

Question 23

Describe the transpiration stream

Answer 23

• Water is absorbed by the root hair cells
• Water moves through the root tissue into the xylem, and is transported up the xylem in the plant stem to the leaf
• Water is transported by osmosis from the xylem in the leaf to the cells of the spongy mesophyll, where it evaporates from the surface of the cells into the air spaces
• Water vapour, then diffuses from the air spaces out of the leaf through the stomata down a water potential gradient

Question 24

Define cohesion

Answer 24

Water molecules are attracted to each other by hydrogen bonds

Question 25

Define adhesion

Answer 25

Water molecules are attracted to the hydrophilic lining of the lignified xylem vessel walls

Question 26

How does the water travel up the xylem to the leaves of the plant?

Answer 26

The Cohesion-Tension Theory (Transpiration Pull)
As water vapour diffuses out of the stomata of the leaf by transpiration water molecules are drawn up from behind to replace those lost. Water molecules are drawn across the leaf and up the xylem. This is possible because of cohesion between water molecules due to hydrogen bonds and adhesion between water molecules and the xylem vessel walls. This upward movement of water creates tension on the xylem vessel walls .

Question 27

What are the 2 other processes that help water move up the xylem a small amount?

Answer 27

Capillarity
The forces of adhesion and cohesion allow water molecules to rise up narrow tubes for a short distance
Root pressure

Question 28

Why is capillarity not useful in large trees?

Answer 28

After a short distance, capillary action is opposed by gravity

Question 29

What are the 4 factors that affect the rate of transpiration?

Answer 29

1. Temperature
2. Wind speed
3. Humidity
4. Light intensity

Question 30

How does an increase in temperature increase the rate of transpiration?

Answer 30

A rise in temperature increases the kinetic energy of the water molecules and increases the rate of evaporation and diffusion of water vapour into the atmosphere.
The water potential of the atmosphere is lower in higher temperatures - this increases the water potential gradient

Question 31

How does an increase in wind speed increase the rate of transpiration?

Answer 31

Air movement blows away the diffusion shell and increases the water potential gradient

Question 32

How does increased humidity reduce the rate of transpiration?

Answer 32

The water potential gradient between the inside and the outside of the leaf decreases

Question 33

How does increased light intensity increase the rate of transpiration?

Answer 33

Light causes stomata to open to allow gas exchange for photosynthesis

Question 34

Why is the rate of water uptake only man estimate of transpiration rate?

Answer 34

Some water is used by the plant as a reactant in photosynthesis

Question 35

Describe how you would set up a potometer?

Answer 35

1. Cut a leafy shoot under water to prevent entry of air bubbles into the xylem vessels as these would break hydrogen bonds, affecting cohesion
2. Completely fill the apparatus with water to avoid introducing air bubbles into the glassware
3. Fit leafy shoot and seal all joints with Vaseline so apparatus is airtight
4. Pat the leaves dry otherwise the water potential gradient will be reduced and this could affect the results
5. Introduce one air bubble into the capillary tube (open the tap)
6. Measure the distance the air bubble moves along the scale in a specific time

Question 36

Describe how to calculate standard deviation

Answer 36

1. Calculate the mean for the data set
2. Subtract the mean from each value
3. Square each answer
4. Add up all the squares
5. Divide this total by the number of pieces of data minus 1
6. Find the square root of that value

Question 37

What are the 4 different types of cell that make up the xylem tissue?

Answer 37

1. Vessels
2. Tracheids
3. Fibres (support only)
4. Xylem parenchyma (living tissue)

Question 38

Why is it important that vessel walls are impermeable to water and solutes?

Answer 38

Vessels transport water - dont want water to leave the vessel until it reaches the leaf

Question 39

What are the 4 different types of cell that make up the phloem tissue?

Answer 39

1. Sieve tubes - transport sucrose and amino acids up/down plant stem
2. Companion cells - connect to sieve tubes via plasmodesmata
3. Phloem fibres - for support
4. Phloem parenchyma - living tissue

Question 40

The sieve tubes are formed from cells called ______ ____________

Answer 40

Sieve elements

Question 41

What is the function of companion cells?

Answer 41

Provide ATP for the active transport of sugars into/out of the sieve tubes

Question 42

Give 3 ways in which sieve tube elements are adapted to their function

Answer 42

• Organelles and nucleus disintegrate during development. Less resistance to flow in phloem
• Phloem sieve tubes are associated and linked by plasmodesmata with companion cells which provide energy in the form of ATP
• Phloem sieve plate pores allow cytoplasm and phloem protein to move between cells

Question 43

Define translocation

Answer 43

The transport of soluble organic materials produced by photosynthesis in the phloem

Question 44

What is the source?

Answer 44

The region where the products of photosynthesis are produced and exported eg leaf

Question 45

What is the sink?

Answer 45

The region where the products of photosynthesis are stored or used for growth eg root, shoot tips, flower

Question 46

What are the 3 pieces of evidence for translocation in the phloem?

Answer 46

• Ringing experiments
• Aphid experiments
• Radioisotope labelling

Question 47

Describe ringing experiments

Answer 47

Removal of a ring of outer bark tissue from a woody stem removes the phloem. Bulge seen due to accumulated phloem sap that cannot move down any further. It shows that transport of sucrose must be from above.

Question 48

Describe aphid experiments

Answer 48

Anaesthetise aphids with carbon dioxide, the stylet can be cut off and left in the stem. Pure phloem sap can be collected through the stylet for analysis. Aphid’s enzymes ensure that the stylet doesn’t get blocked.

Question 49

Describe radioisotope labelling

Answer 49

Supply the plant with radioactive carbon. After 20 minutes the whole plant is placed on photographic film. Dark areas on the film show areas containing radioactivity. The results show that the radioactive carbon is fixed into the sugar at the source and is then translocated to sink parts of the plant. This technique shows that the sugar is transported bidirectionally.

Question 50

How does the mass flow hypothesis work?

Answer 50

• Sucrose made in photosynthesis is loaded by active transport into the sieve tubes using ATP.
• Water enters the sieve tubes along a water potential gradient by osmosis
• The pressure in the sieve tubes increases, and the sucrose moves down a pressure gradient through the phloem towards the sinks
• Sucrose is unloaded by active transport into the cells at the sinks
• Water moves by osmosis out of the phloem as the sucrose is removed and the pressure in the phloem tissue becomes lower at the sink

Question 51

What are 4 arguments against the mass flow theory?

Answer 51

• No explanation of sieve plates, which seem to act as barriers to flow
• Sucrose and amino acids have been observed moving at different rates and in different directions
• Sieve tubes have a high rate of ATP consumption and translocation is slowed or stopped if respiratory inhibitors, such as cyanide are added
• The companion cells are found all the way along the sieve tubes and contain numerous mitochondria for production of ATP. If companion cells purely load and unload photosynthates from the sieve tubes, they would not be needed anywhere but sources and sinks.

Question 52

Name alternative theories for the mass flow theory

Answer 52

1. Streaming in the cytoplasm of sieve tubes could be responsible for bidirectional movements.
2. Protein filaments have been observed passing through the sieve pores suggesting different solutes are transported by different filaments.

Question 53

What plant behaviours allow mesophytes to survive at unfavourable times of the year?

Answer 53

1. Deciduous trees shed leaves in autumn to survive unfavourable conditions over winter.
2. Bulbs and corms are produced by nonwoody plants to survive under ground over winter.
3. Annual plants produce seeds and die in the same year, seeds survive winter frost and germinate the next spring.

Question 54

What are hydrophytes?

Answer 54

Water plants that live submerged or partially submerged in water, e.g. a water lily

Question 55

State and explain five features of a hydrophyte

Answer 55

1. Stomata on upper epidermis - to allow gas exchange with the air above.
2. Large air spaces - to provide buoyancy for the leaves and act as a reservoir of oxygen and carbon dioxide.
3. Thin or no waxy cuticle - no need to reduce water loss as they live in or on water.
4. Poorly developed xylem tissue - no need to transport large quantities of water as plant is aquatic.
5. Little lignin - water is a supportive medium and so little lignin is required to support the xylem tissue.

Question 56

What is a xerophyte?

Answer 56

Xerophytes are adapted to conditions of low-water availability

Question 57

State and explain five features of Maram grass

Answer 57

1. Sunken stomata - water vapour is trapped in the pits. This decreases the water potential gradient between the inside and outside of the leaf. Therefore less water is lost by transpiration.
2. Hairs on leaf surface - water vapour is trapped between the hairs. This decreases the water potential gradient between the inside and outside of the leaf. Therefore less water is lost by transpiration.
3. Thick cuticle - thick, waxy cuticle reduces water loss from the epidermis
4. Rolled leaves - stomata are less exposed to the atmosphere. Water vapour is also trapped and so the water potential gradient between the inside and outside of the leaf decreases. Less water is therefore lost by transpiration.
5. Reduced leaf size/spines - reduces the surface area from which transpiration can occur