Transport in plants

Question 1

What are the two types of tissue involved?

Answer 1

Xylem and phloem.

Question 2

What do xylem vessels transport?

Answer 2

Water and mineral ions.

Question 3

Which direction do xylem vessels move?

Answer 3

Roots to leaves.

Question 4

Describe the arrangement of vascular bundles in roots.

Answer 4

The xylem is in the centre surrounded by phloem to provide support for the root.

Question 5

Describe the arrangement of vascular bundles in stems.

Answer 5

The xylem and phloem are near the outisde to provide ‘scaffolding’ that reduces bending and gives the plant structure.

Question 6

Describe the arrangement of vascular bundles in leaves.

Answer 6

Xylem and phloem make up a network of veins which support the thin leaves.

Question 7

Why do plants need a transport system?

Answer 7

Plants need substances like water, minerals, and sugars to live.
They need to get rid of waste substances.
They are multicellular organisms and so have a small SA:V ratio.
High metabolic rate.

Question 8

Describe how xylem vessels are adapted to their function.

Answer 8

• Long tube-like structures joined end to end in an uninterrupted tube
• Cells are dead so have no cytoplasm
• Walls thickened with lignin to prevent vessels from collapsing

Question 9

How is phloem tissue adapted to its function?

Answer 9

It is arranged in tubes, and contains phloem fibres, parenchyma, sieve tube elements, and companion cells.

Question 10

What are sieve tube elements?

Answer 10

Living cells that form the tube for transporting solutes through the plant.
They join end to end to form sieve tubes.
The ‘sieve’ parts are the end walls which have lots of hole in them to allow solutes to pass through.
NO nucleus, v thin cytoplasm, few organelles.

Question 11

What are companion cells?

Answer 11

Theres a companion cell for every sieve tube element that carry out the living functions for both of them. They contain the nucleus.

Question 12

How does water enter a plant?

Answer 12

Water travels down a water potential gradient into the roots via osmosis. It then moves through root hair cells, and then passes through the root cortex (including the endodermis) to reach the xylem.

Question 13

What are the two routes from the root into the xylem?

Answer 13

Apoplastic and symplastic pathway.

Question 14

Describe the symplastic pathway.

Answer 14

Water moves through the living parts of cells (cytoplasm).
Cytoplasms of neighbouring cells connect via plasmodesmata.
Osmosis.

Question 15

Describe the apoplastic pathway.

Answer 15

Water moves through the non-living parts of the cell (cell walls).
Walls are very absorbent and water can diffuse through them as well as pass through the spaces between them.
Water moves from areas of high hydrostatic pressure to low hydrostatic pressure.
MASS FLOW.

Question 16

What is the casparian strip?

Answer 16

A waxy strip in the cell walls that blocks the apoplastic route once it gets to the endodermis cells. The water is then forced to go via the symplastic route.
This water has to go through a partially permeable cell membrane.

Question 17

What is meant by ‘cohesion tension’?

Answer 17

Cohesion and tension help water move up plants against gravity.
When water evaporates from leaves, a tension (suction) is created which pulls more water into the leaf to replace it. Since water molecules are polar and therefore cohesive, all molecules move when one is pulled in.

Question 18

What is adhesion in water molecules?

Answer 18

Water molecules are attracted to the walls of the xylem vessels as well as each other which helps them rise up through the vessels.

Question 19

What is transpiration?

Answer 19

The loss of water as vapour through evaporation from a plants surface (leaves).

Question 20

Why does transpiration happen?

Answer 20

When stomata open during gas exchange they allow water out as there is a higher concentration of water inside the leaf than outside.

Question 21

What are the four main factors of transpiration?

Answer 21

LIGHT- Stomata will be open more so gas exchange can happen.
TEMPERATURE- Water will evaporate faster.
HUMIDITY- The air around the plant has a higher water concentration which decreases the gradient and slows it.
WIND- Water molecules are blown away from the stomata.

Question 22

How can you measure the rate of transpiration?

Answer 22

A potometer measure water uptake by a plant but the uptake is directly correlated with water loss.

• Cut shoot and assemble underwater.
• Remove from water but keep capillary tube under.
• Check water/airtight.
• Dry the leaves, wait, then shut tap.
• Remove capillary tube until one bubble has formed then put back.
• Record distance moved in specific time.

Question 23

What are xerophytes?

Answer 23

Plants adapted to living in dry climates.

Question 24

How are xerophytes adapted?

Answer 24

• Sunken stomata
• Hairs to trap moisture
• Some can roll leaves to reduce SA exposed
• Thick waxy layer
• Some have spines instead of leaves to reduce SA
• Some can close stomata at hot times

Question 25

What are hydrophytes?

Answer 25

Plants adapted to live in aquatic habitats.

Question 26

How are hydrophytes adapted?

Answer 26

(COPE WITH LOW OXYGEN)

• Air spaces in tissue act as float, can store oxygen
• Stomata only on upper surface to maximise gas exchange
• Flexible stems n leaves to prevent damage from water currents

Question 27

What is translocation?

Answer 27

The movement of dissolved substances through a plant.

Question 28

What are assimilates?

Answer 28

The dissolved substances being transported.

Question 29

What is a sink? Give some examples.

Answer 29

A sink is a part of a plant that uses up the assimilates.

E.G. Food storage organs, meristems, roots.

Question 30

What is a source? Give some examples.

Answer 30

A source is a part of a plant where assimilates are made up.

E.G. Leaves, roots.

Question 31

Explain stage 1 of the Mass Flow hypothesis.

Answer 31

• Active transport is used to load solutes into sieve tubes at the source.
• This lowers the water potential inside the sieve tubes so water enters the tubes by osmosis from the xylem n companion cells.
• This creates a high pressure inside the sieve tubes at the source.

Question 32

Explain stage 2 of the Mass Flow hypothesis.

Answer 32

• At the sinks, solutes are unloaded.
• This increases the water potential inside the sieve tubes and so water flows out by osmosis
• This lowers the pressure inside the sieve tubes

Question 33

What is the result of the Mass Flow?

Answer 33

• The result of Mass Flow is a pressure gradient from source end to sink end.
• This gradient pushes solutes along the sieve tubes to where theyre needed.

Question 34

How do substances enter the phloem?

Answer 34

Active loading is used to move substances from the surrounding tissue into companion cells, and then from the companion cells into the sieve tubes AGAINST A CONCENTRATION GRADIENT.

Question 35

How does active loading work?

Answer 35

The concentration of sucrose is higher in the companion cells than the surroundings and higher in sieve tubes than companion cells.
Sucrose is therefore moved to where it needs to be using active transport and co-transporter proteins.

Question 36

Explain the steps of active loading.

Answer 36

• In comp cell, ATP actively transports H+ ions out of cell n into tissue.
• This sets up a conc. grad. (more H+ in tissue than comp cells)
• A H+ ion binds to a co-transport protein in comp cell membrane n re-enters cell down the con. grad.
• A sucrose molecule binds to the co-transport protein at the same time so all three move into the cell.
• Sucrose molecules then use the same process to get through the comp cells into the sieve tubes.