Transport in plants Module 3

Question 1

Why do plants require transport systems?

Answer 1

Multicellular so have a small SA:V, high metabollic rate, and big so difficult for direct diffusion of substances into the cells within the plant

Question 2

What does xylem tissue to generally?

Answer 2

Transports water and mineral ions in solution, from the roots to the leaves

Question 3

What does phloem tissue do generally?

Answer 3

Transports sugars in solution up and down the plant

Question 4

What makes up the vascular system in plants?

Answer 4

Phloem and xylem vessels

Question 5

Describe the structure of the vascular system, in the root, in the stem and in the leaf?

Answer 5

In a root:
Xylem is in the centre surrounded by phloem to provide support as it pushes through the soil

In the stems:
Xylem and phloem are near the outside to provide scaffolding that reduces bending of the stem

In the leaves:
Xylem and phloem make up a network of veins

Question 6

How are xylem vessels adapted for transporting water and mineral ions?

Answer 6

Formed from long tube like cells joined end to end, there are no end walls on these cells, so water and mineral ions can pass through easily

The cells are dead so contain no cytoplasm

Their walls are thickened with a woody substance called lignin, which supports them and stops them collapsing inwards

Water and ions move from cell to cell through small pits in the wall where there is no lignin

Question 7

Phloem tissue is made up of sieve tube elements and companion cells, describe their structure?

Answer 7

Sieve tube elements:
Living cells that form the tube for transporting solutes through the plant, joined end to end to form sieve tubes
The sieve part are the end walls which have lot of holes in them to allow solutes to pass through
Have no nucleus, very few organelles, and cytoplasm is connected through holes in sieve plates of cells

Companion cells:
Sieve tube elements can’t survive on their own (lack of nucleus), so there’s a companion cell for every sieve tube element
They carry out the living functions for themselves and the sieve tube elements eg. provide the energy for active transport of solutes

Question 8

What stain do you use to stain lignin walls?

Answer 8

Toludine blue

Question 9

How does water enter the plant through it’s roots?

Answer 9

Via osmosis (water potential high in soil, low in roots), goes from soil to root hairs, then the root cortex including the endordermis to reach the xylem

Question 10

What are the 2 ways water moves through the roots to the cortex and into the xylem?

Answer 10

Through the symplast pathway, or the apoplast pathway

Question 11

Describe the symplast pathway?

Answer 11

Water goes through the living parts of cells- the cytoplasm
The cytoplasms of neighbouring cells connect through plasmodesmata (small channels in the cell walls)
So water moves through the sympoast pathway via osmosis

Question 12

Describe the apoplast pathway?

Answer 12

Water goes through the non living parts of cells - the cell walls
Water can just diffuse between the cell walls
Water moves an area of high hydrostatic pressure, to an area of low hydrostatic pressure this is an example of mass flow
However, when the water in this pathway reaches the endodermis cells, it’s pathway is blocked by a waxy Casparian strip, which forces the water to take the symplast route
Water moves into the xylem

Question 13

Why is it useful that water in the apoplast pathway will eventualy need to take the symplast route

Answer 13

Forces the water to go through a partially permeable membrane, so will control which solutes enter, so can prevent toxins entering the xylem

Question 14

Describe using cohesion, tension and adhesion how water moves up a plant against the force of gravity?

Answer 14

Water evaporates from the leaves at the “top” of the xylem
This creates tension which pulls more water into the leaf
Water molecules are cohesive (stick together), so when some are pulled into the leaf other follow

Also the water is adhesive (attracted to the walls of the xylem) which also aids it travelling up the xylem

Question 15

What 4 factors affect transpiration and describe how?

Answer 15

Light intensity: The lighter it is the faster the transpiration rate, as the stomata open when it’s light so CO2 can diffuse into the leaf for photosynthesis. So water moves out

Temperature: The higher the temperature the higher the rate of transpiration as warmer water molecules have more energy, increasing the rate at which they evaporate, and therefore increasing the water potential gradient

Humidity: The LOWER the humidity the faster the transpiration rate, as if the air is dry around the leaf, the water potential gradient between the plant and the air is decreased

Wind speed: The faster the wind speed, the faster the trasnpiration rate, as lots of air movement blows away water molecules from around the stomata, increasing the water potential gradient, which increases the transpiration rate

Question 16

Brief description of how a potometer can be used to estimate transpiration rate?

Answer 16

Cut a shoot underwater at a slant to prevent air entering xylem and slanted to increase water uptake

Assemble potometer, and measure the distance the bubble travels due to transpiration pull in a certain amount of time

(can use reservoir of water to return bubble to beginning for repeats)

Question 17

What are Xerophytes?

Answer 17

Plants that are adapted to live in dry conditions

Question 18

Describe how Xerophytes are adapted to live in dry climates?

Answer 18

Can have stomata that are sunk into pits so are shielded from the wind, decreasing their transpiration rate

Can have a layer of hairs on the epidermis, that traps moist air around the stomata, which reduces the water potential gradient between the leaf and the air, decreasing the transpiration rate

Can roll their leaves traps moist air, reduces surface area for water to be lost, and shields from the wind

Can have a thick waxy layer on the epidermis, reducing water loss by evaporation

Can close their stomata at hottest points in day, where transpiration rates would be highest

Question 19

What are hydrophilic plants?

Answer 19

Plants that area adapted to survive in water

Question 20

Describe the adaptations of hydrophillic plants so they can survive in water?

Answer 20

Air spaces in the tissues help the plant to float, and can act as a store of oxygen for use in respiration

Stomata are only present on the upper surface of floating leaves, maximises gas exchange

They have flexible leaves and stems, as already supported by water, and flexibility helps them not get damaged by strong currents

Question 21

What is translocation?

Answer 21

The movement of dissolved substance to where they are needed in the plant, it moves them from a source to a sink

Question 22

What’s a source?

Answer 22

Where the substance is made (high concentration)

eg. leaves for sucrose

Question 23

What’s a sink?

Answer 23

The area where it’s used up (by enzymes so lower concentration there)

Question 24

How can the roots be a source and a sink?

Answer 24

Are a sink, as they require sucrose for growth from the leaves, but also can store sucrose and it’s transported to other parts of the plant

Question 25

Describe the mass flow hypothesis for phloem transport?

Answer 25

Active transport is used to load the solutes into the sieve tubes of the phloem at the source

This lowers the water potential inside the sieve tube elements, so water enters the tubes by osmosis from the xylem and companion cells

This creates a high pressure inside the sieve tube tubes at the source of the phloem

At the sink end solutes are removed from the phloem to be used up, this increases the water potential inside the sieve tubes, so water also leaves the tubes by osmosis

Lowering the pressure in the sieve tube elements at the sink end

So a pressure-gradient is formed from the source end to the sink end, and this gradient pushes the solutes to where they are needed

Question 26

Describe exactly how substances enter the phloem at the source via active loading?

Answer 26

In the companion cell, ATP is used to actively transport hydrogen ions (H+) out of the cell and into the surrounding tissue cells

This sets up a conc gradient there are more H+ ions in the surrounding cells than in the companion cells

A H+ ion binds to a co-transporter protein in the companion cell membrane, and re-enters the cell down the concentration gradient

A sucrose molecule binds to the co-transporter protein at the same time. The movement of the H+ ion is used to move the sucrose into the companion cell against it’s concentration gradient

Sucrose molecules are then transported out of the companion cells and into the the sieve tube elements via the same process