Transport in Plants

Question 1

What is a dicotyledonous plants

Answer 1

Plants with two cotyledons and has a network of veins

Question 2

What is the meristem

Answer 2

A layer of dividing cells, here it is called a pericycle

Question 3

What is the need for transport systems in plants

Answer 3

All living things need to take substances from, and return waste to, their environment. Larger plants typically have a smaller surface area to volume ratio. Plants have a low metabolic rate so the demand for oxygen can be met by diffusion. However the demand for water and sugars is still high.

Question 4

What does the xylem and phloem look like in a transverse section of a root

Answer 4

They xylem is a star shaped in the centre with phloem’s in between the arms. Surrounding the vascular bundle is the endodermis which helps water get into the xylem vessels.
Inside the endodermis is a layer of meristematic cells called the pericycle

Question 5

What does the xylem and phloem look like in a transverse section of a stem

Answer 5

The vascular bundles are distributed around the edge of the stem. The phloem is the most outer layer, then the cambium then the xylem.
Aphids insert their stylet into sieve tube elements to obtain the sucrose

Question 6

What does the xylem and phloem look like in a leaf

Answer 6

The xylem is on top then the cambium then the phloem at the bottom

Question 7

What is the xylem

Answer 7

It’s a tissue used to transport water and mineral ions from the roots to the leaves and other parts of the plants

Question 8

Describe how a xylem is formed

Answer 8

As xylem vessels develop lignin is deposited which impregnates the walls of the cells. This makes the cells waterproof and kills the cells.
End walls decay and fuse providing a continuous column, this also helps provide little resistance
The lignin helps prevent the vessel from collapsing under the tension
The cellulose cell walls allows adhesion of water molecules

Question 9

Why is lignin deposited in patterns - spiral , annular or reticulate
What parts aren’t lignified?

Answer 9

This prevents the vessel from being too rigid and allows some flexibility
Some gaps aren’t lignified, providing bordered pits allowing lateral flow of the movement of water

Question 10

What is the phloem

Answer 10

Tissue that transports assimilates from the source to the sink
The main types are sieve tube elements and the companion cells
Sucrose is able to be transported as it dissolves in the water to form sap

Question 11

What is the structure of sieve tube elements

Answer 11

Elongated sieve tube elements lined up end to end that forms sieve tubes. They contain a small number of organelles in order to decrease resistance when mass flow occurs.
At the end of each sieve tube elements are sieve plates.

Question 12

How can the sieve plates help with infection or damage

Answer 12

They contain sieve pores that can become blocked by deposition of callose
This prevents the loss of sap and inhibits transport of pathogens around the plant

Question 13

What are companion cells

Answer 13

They are near to sieve tubes and involved in translocation. They contain ribosomes nucleus and many mitochondria for active transport.
The plasmodesmata allow communicational and flow of substances between cells

Question 14

How are roots hair adapted to their function

Answer 14

Large surface area to absorb more water
Many mitochondria for active transport
Root hairs are permeable to water

Question 15

How does water enter the roots from soil

Answer 15

Ions are pumped into the roots using ATP and this lowers the water potential. Water diffuses down the water potential gradient by osmosis into the roots.
This happens across a partially permeable membrane

Question 16

How does water move from the root hairs to the cortex

what 3 routes can water take

Answer 16

There’s a water potential gradient again and this time water can choose to move through 3 different pathways (ions can dissolve in the water and get carried by it)
Apoplast - Water moves through the well wall between cellulose fibres
Symplast - Water moves through they cytoplasm and across plasmodesmata from one cell
to the next
Vacuolar - Water takes the symplast route but can move diffuse in and out of the vacuoles

Question 17

What does the endodermis contain that is made of suberin

Answer 17

Casparian strip this a waxy, waterproof layer of suberin. It forces water to take the symplast pathway.
It’s though that as water has to pass the cell membrane that ion concentration can be controlled

Question 18

How does water move from the endodermis to the xylem

Answer 18

Down the water potential gradient, water has to pass the pericycle then move through the unlignified pits in the xylem

Question 19

How does root pressure arise

Answer 19

The action of the endodermis moving ions into the medulla and xylem by active transport draws water in.
This builds up hydrostatic pressure and pushes water a few meters up the stem

Question 20

What is the cohesion tension theory

Answer 20

The lass of water through evaporation must be replaced by water leaving the xylem so this draws water up from the roots. creating a transpiration pull
Waters cohesive nature means it can form hydrogen bonds with other water molecules.
As water is pulled up this creates tension (negative pressure)
The lignin helps prevent the xylem vessel from collapsing
If the column is broken is one vessel this can be maintained by the bordered pits

Question 21

What is capillary action

Answer 21

Water molecules are attracted to the sides of the xylem these forces of attraction can pull water up the sides of the vessel

Question 22

What is transpiration

Answer 22

Loss of water vapour by evaporation

Water evaporates from the cell walls of spongy mesophyll then water vapour in the air spaces diffuse out of stomata

Question 23

What are the two ways water can leave the leaf

Answer 23

Stomata - Diffusion of water from the air space into the atmosphere, stomata are open during gas exchange

Cuticle - A small amount of water is lost through the waxy cuticle

Question 24

What factors increase the rate of transpiration

Answer 24

Humidity (conc of water vapour in atmosphere) - A lower humidity maintains a steep water potential gradient

Temperature - A high temperature gives water molecules a higher kinetic energy

Light intensity - A high light intensity means photolysis in light dependant stage of photosynthesis is occurring faster and that co2 is needed in Calvin cycle so stomata are open for gas exchange

Stomatal aperture - The wider the stomata the more water vapour that can diffuse out

Question 25

How can we measure rate of transpiration what do you have to be wary of when setting up the practical

Answer 25

You can use a potometer
Set the practical underwater to ensure no air bubbles block the xylem
Use petroleum jelly to make sure it’s air tight
Make sure leaves are dry so rate isn’t affected
Cut plant stem at a slant to increase surface area
Ensure your shoot is healthy

Question 26

What is assumed when using a potometer why isn’t this assumption correct

Answer 26

It’s assumed
water uptake = rate of transpiration
Some water taken up by the plant is used in photosynthesis

Question 27

How could you measure the volume of water taken in by the plant and rate of transpiration

Answer 27

You would need to know the radius of the internal lumen of the tube and use the formula πr^2l
Rate = volume/time

Question 28

What is transpiration stream

Answer 28

The movement of water from the roots all the way to the air surrounding the leaves

Question 29

What is a xerophyte and what are some adaptations

Answer 29

Adapted to living in conditions with little water
eg Cactus, Marram Grass
Curled leaves to reduce water potential gradient
Sunken stomata or reduced number of stomata or only on underside of leaf
spineless/thin leaves
Hairs to trap water
Thick waxy cuticle

Question 30

What is a hydraphyte and what are some adaptations

Answer 30

Adapted to living in conditions with a lot of water
eg Water Lilly
Stomata on top
Thin waxy cuticle
Large air spaces to keep leaves afloat, buoyancy

Question 31

Describe the process of sucrose loading and unloading

Answer 31

Protons are pumped out of the companion cells using ATP against it’s conc gradient
The protons diffuse back down the conc gradient into the companion cells and are co-transported with sucrose
The sucrose diffuses across the plasmodesmata into the sieve tube element
This lowers the water potential and so water diffuse in from the xylem
High hydrostatic pressure is built up so mass flow occurs to the sink
Sucrose diffuses out the water potential increases so water returns to the xylem

A question could be asked about the use of cyanide which inhibits ATP production. As a result there is no translocation