3.3 - Transport in Plants

Question 1

Why do plants need transport systems?

Answer 1

• Plants need substances like water and minerals and sugars to live. They also need to get rid of waste substances.
• Like animals,plants are multicellular- they have a small surface area to voume ratio. They are also relatively big with a relatively high metabolic rate.
• Exchangind substances by direct diffusion would be too slow to meet their metabolic needs.
• So plants transport systems to move sunstances to and from individual cells quickly.

Question 2

What are the two main types of transport vessels found in dicotyledonous plants?

Answer 2

Xylem and phloem

Question 3

What is a dicotyledonous plant ?

Answer 3

A plant which has seeds made of two parts.

Question 4

How is the plant tissue arranged?

Answer 4

-Transport tissues are arranged together in the stems, leaves and roots of dicotyledonous plants in vascular bundles.

Question 5

How are the vascular tissues arranged in the stem of dicotyledonous plants?

Answer 5

The vascular tissues are arranged in a ring around the edge of the stem.

Question 6

How are the vascular tissues arranged in the roots of dicotyledonous plants?

Answer 6

The vascular tissues are arranged in a bundle in the centre of the root.

Question 7

What is the function of the xylem tissue ?

Answer 7

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

Question 8

Describe the structure of the xylem tissue

Answer 8

• Xylem vessels are very long tube-like structures formed from cells joined end to end
• No end walls
• Cells are dead- contain no cytoplasm
• Walls are thickened by lignin which helps support the xylem vessels and stop them from collapsing inwards.
• Lignin can be deposited xylem walls in different ways , e.g in a spiral or as distinct rings.
• The amount of lignin increases as the cell gets older
• Water and ions move into and out of vessels through small pits in the walls when theres no lignin.

Question 9

What is the function of phloem tissue?

Answer 9

• It transport solutes , mainly sugars like sucrose
• It is also arranged in tubes
• ## But unlike xylem it is purely a transport tissue

Question 10

Describe the structure of the phloem.

Answer 10

• Phloem tissue contains phloem fibres , phloem parnchyma , sieve tube elements and companion cells

Question 11

What are the most important cell types in phloem for transport ?

Answer 11

• Sieve tube elements
• Companion cells

Question 12

What is the function of seive tube elements ?

Answer 12

• These are living cells that form the tube for transporting solutes through the plant.
• The are 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.
• Have no nucleus, a very thin layer of cytoplams and very few organelles
• Th cytoplasm of adjacent cells is connected through the holes in the seive tube plates.

Question 13

What are companion cells ?

Answer 13

• The lack of nucleus and other organelles in seive tube elements mean thye cant survive on their own .
• So they become companion cells for every sieve tube element.
• They provide the energy for the active transport of solutes.

Question 14

How does water enter the plant ?

Answer 14

1) Water to get from the soil through the root and into the xylem to be transported around the plant
2) Water enters through root hair cells then passes through root cortex , including the endothermis,to reach the xylem.
3) The soil around roots generally has a high water potential and leaves have a lower water potential as water evaporates from them.
4) This creates a water potential gradient that keeps water moving through the plant in the right direction from roots to leaves.

Question 15

How does water travel through the roots to the xylem?

Answer 15

• Symplast pathway
• Apoplast pathway

Question 16

Explain the symplast pathway

Answer 16

• It goes through living parts of the cell- the cytoplasm.
• The cytoplasm of neighboring cells connect through plasmodesmata. Water moves through this pathway through osmosis.

Question 17

Explain the apoplast pathway

Answer 17

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

Question 18

What happens when water gets to endothermis cells in root (apoplast pathway continued)?

Answer 18

• Its path is blocked by waxy strip in the cell walls , called the casparian strip .
• Then water is taken to symplast pathway
• This is useful as water goes through cell membrane, due to them being partially permeable and are able to control whether substances get through or not.
• Once past this barrier, the water moves out of the xylem

Question 19

What is the transpiration stream ?

Answer 19

• The movement of water from from roots to leaves.

Question 20

Explain cohesion and tension

Answer 20

1) Water evaporates from the leaves at the ‘top’ of the xylem (transpiration)
2) This creates a tension , which pulls more water into the leaf
3) Water molecules are cohesive (they 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 through root cortex cells.

Question 21

Explain adhesion

Answer 21

• Adhesion is also partly responsible for the movement of water.
  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 xylem vessels

Question 22

What is transpiration ?

Answer 22

• Transpiration is a consequence of gas exchange.
• It is evaporation of water from plant’s leaves.

Question 23

How does transpiration take place ?

Answer 23

• The plant opens its stomata to let in carbon dioxide so that it can produce glucose (photosynthesis)
• ## But this also lets out water - there’s a high concentration of water inside the leaf than in the air outside so water moves out of leaf down the water potential gradient when the stomata opens.

Question 24

What are the factors of transpiration?

Answer 24

• Light
• Temperature
• Humidity
• Wind

Question 25

Explain light as a factor affecting transpiration

Answer 25

Light - the lighter it is faster the transpiration rate , as the stomata is open when it gets light , and co2 can diffuse into the leaf for photosynthesis.
When its dark , the stomata are usually closed , so there’s little transpiration

Question 26

Explain temperature as a factor affecting transpiration

Answer 26

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

Question 27

Explain wind as a factor affecting transpiration

Answer 27

• 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 28

Explain humidity as a factor affecting transpiration.

Answer 28

• 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 29

What are xerophytic plants ?

Answer 29

• Xerophytes are plants that have adapted to living in dry and arid environments.

Question 30

How are xerophytic plants adapted ?

Answer 30

• Waxy layer on the epidermis - this waterproof outer layer reduces evaporation from the surface because water cannot easily pass through (it is impermeable).
• Sunken stomata - xerophytes have stomata which are sunken in pits. The pits shelter the stomata from the wind, reducing the water potential gradient between the inside of the leaf and the outside.
• Hairs on epidermis - hairs on the epidermis trap water vapour, reducing the water potential gradient between the inside and outside of the leaf.
  -Spines - many xerophytes have spines instead of leaves which reduces the surface area for water loss.
  -Rolled leaves - curled leaves traps water vapour, reducing the water potential gradient for transpiration. It also reduces the surface area of the leaf for water loss.
  -Closure of stomata - xerophytes can close their stomata during conditions of particularly high temperature or light intensity. This reduces transpiration at times when the rate of transpiration would be very high.

Question 31

How are xerophytic plants adapted (Marram grass)?

Answer 31

• Marram grass has stomata that are sunk in pits , so they’re sheltered frrom the wind.This helps to slow transpiration down.
• It also has layers of hairs on the epidermis - this traps moist air round the stomata, which reduces the water potential gradient between the leaf and the air, slowing transpiration down.
• In hot or windy conditions maramm grass plants roll their leaves - again this traps moist air , slowing down transpiration .

Question 32

What are hydrophytes and how are they adapted?

Answer 32

-Hydrophytes are plants which live on water, such as water lilies. Because oxygen does not dissolve well in water, hydrophytes need adaptation to enable them to cope with low oxygen levels. They are adapted in the following ways:
-Stomata on the upper surface - usually stomata are found on the underside of plant leaves but for hydrophytes, this side will be submerged in water. Instead, hydrophytes have their stomata on the upper surface of their floating leaves to maximise gas exchange.
-Air spaces - pockets of air in the plant tissue help the plant to float and can be used to store oxygen for aerobic respiration.
-Flexible leaves and stems - the flexibility of the leaves and stems helps to prevent damage from water currents. Unlike land plants, which need a sturdy stem to keep upright, hydrophytes are supported by the water around them.

Question 33

What are plasmodesmata?

Answer 33

• Plasmodesmata are small channels connecting the cytoplasm of adjacent plant cells.

Question 34

What is the name of the pathway by which water moves through the cellulose fibres in the cell walls of connecting cells?

Answer 34

• The apoplast pathway