Biology Edexcel Topic 6 (Transport in plants)

6.7 to 6.14B

Why do plants require water

Photosynthesis, maintenance of structure (turgidity), and a cooling effect

Why do plants require mineral ions

For growth (e.g. nitrates are required to produce proteins)

Which structure in plants is adapted for the uptake of water and minerals

Root hairs

How is water transported into root hairs

Lower concentration of water in root hair cells than in the soil so the water diffuses down its concentration gradient into root hair cells by osmosis

How are minerals transported into root hairs

Lower concentration of mineral ions in the soil than in the root so root hair cells take up mineral ions by active transport

Outline how plant roots are adapted for the absorption of water and minerals

Plant roots are composed of millions of root hair cells which have long hairs that extend from the cell body which increases the surface area for absorption and many mitochondria which produce ATP for active transport of mineral ions

Name the two plant transport tissues

Xylem and phloem

How many xylem and phloem are there in the plant

Hundreds - some phloem go up and some go down

What is the function of the xylem

It transports water and mineral ions up the plant from the roots to the laves via the transpiration stream

Where does the xylem flow from

Roots to leaves

Describe how the xylem is adapted to its function

It is composed of dead cells laid end to end to form a long, hollow, continuous column, there are no end walls, there is a thick cell wall strengthened with lignin, and there are tiny pores in the cell walls

How do xylem cells become dead

Differentiation in the roots - zone of elongation - losing all the cellular structures inside it and their end cell walls

Why would the cells want to become dead

So there is little resistance to the passage of water

What does lignin do

It strengthens the cells to prevent them from collapsing, and it makes the cells waterproof

Why would the cells want to be waterproof

This kills the xylem cells as there is no water coming into them, and it stops the water from escaping when moving up the xylem

What do the pores do

They allow some water to escape the xylem and get passed to cells that need it

What is the function of the phloem

It transports sugars (made during photosynthesis) up and down the stem from sources (e.g. leaves in the sun) to sinks (e.g. roots, fruit, seeds, and leaves in the shade) via translocation

Why does the plant divert sugars to leaves in the shade

This makes the leaves in the shade larger as it is looking for light so the leaves in the shade get the same amount of light as the leaves in the sun

What are the two alive cells that make up the phloem

Sieve tube elements and companion cells

Describe how the phloem is adapted to its function

Sieve tube elements are long, thin cells, laid end-to-end with end plates with holes in, which enables the flow of sugars, they contain no nucleus and little cytoplasm to allow sugars to flow easily, and companion cells (adjacent to sieve tube elements) contain a dense cytoplasm, nucleus and lots of mitochondria, and they provide energy for processes in both cell types

Is there lignin in phloem cell walls

No, because the cells are not waterproof, as they would die, but the cells in the phloem are alive

What is the concentration gradient in the phloem

There is a high gradient in the phloem

What do companion cells do

Companion cells use active transport to move sugars into the sieve tube elements

What does the increase of sugars actively transporting into the sieve tube elements do

It increases the pressure in the phloem which helps the sugars get transported up and down the plant

What is transpiration

The movement of water from the roots to the leaves and the loss of water vapour from parts of a plant exposed to air due to evaporation and diffusion

Where does the majority of evaporation take place

Leaves

Describe the process of transpiration

Water evaporates from the mesophyll cell surfaces and diffuses out of the stomata, which draws up the water molecules in the xylem vessels, as they are attracted to each other by weak hydrogen bonds - this replaces the water that has been lost, which causes more water molecules to be absorbed form the soil into root hair cells

How does the transpiration stream transport mineral ions

Mineral ions are dissolved in the water that is carried by the transpiration stream

What are stomata

They are pores found in the lower epidermis of a leaf which allow gas exchange

What are guard cells

Specialised cells surrounding the stomata that change shape to control the size of the pore

How do guard cells control the size of stomata

To open the stomata, water enters guard cells, making them swell and become turgid, they bend and draw away from each other, opening the stomata
To close the stomata,
water leaves guard cells, making them become flaccid, closing the stomata

What factors affect the rate of transpiration

Light intensity, temperature and air movement

Describe how high light intensity affects the rate of transpiration

High light intensity means a greater number of stomata will open to allow gas exchange so the rate of photosynthesis increases, and more water is taken up from the soil, which pushes water up the xylem and so more water vapour diffuses out of the stomata and rate of transpiration increases

Describe how temperature affects the rate of transpiration

As temperature increases, the water molecules have more kinetic energy so the rate of diffusion increases, meaning more water needs to be taken up from the soil so more water vapour diffuses out of the stomata and rate of transpiration increases

Describe how air movement affects the rate of transpiration

As air movement increases, there is a high water concentration gradient between the air spaces in the leaf and the atmosphere, which leads to an increased rate of diffusion of water molecules out of the stomata, increasing the rate of transpiration

What apparatus is used to measure the rate of transpiration

Potometer

What is assumed when measuring the rate of transpiration using a potometer

Rate of water intake is equal to the rate of transpiration

How can the rate of transpiration be calculated using a potometer

Rate of transpiration = distance moved by bubble/time taken

What is translocation

The movement of sugars (e.g. sucrose, amino acids) up and down a plant via the phloem (requires ATP - releases energy for active transport)

Identify the structures of the leaf labelled in the diagram

A - waxy cuticle
B - vascular bundle (xylem and phloem)
C - upper epidermis
D - palisade mesophyll tissue
E - spongy mesophyll tissue
F - lower epidermis
G - air space
H - stoma
I - guard cell

Describe how leaves are adapted for photosynthesis

They are broad, have a large surface area for light absorption, thin, have a short diffusion distance for gases, allows light to reach all cells, it has vascular bundles (xylem and phloem) which form a network to deliver water and remove glucose, and also provide support, and have photosynthetic pigments (e.g. chlorophyll) to absorb light

Describe how tissues of the leaves are adapted for photosynthesis and gas exchange

Palisade mesophyll layer receives the most light so contains the greatest concentration of chloroplast, the upper epidermis is transparent, allowing light to reach the palisade layer, the spongy mesophyll layer contains many air spaces to increase the rate of diffusion and gas exchange, and the lower epidermis contains many stomata for gas exchange and diffusion

Describe how plants are adapted to live in hot, dry conditions

They have small leaves/spines to reduce their surface area for less water loss, have a thick waxy cuticle to reduce evaporation, have a thick stem to provide a storage of water, have shallow but widespread roots, giving it a large surface area to absorb water, its stomata are sunken in pits and its leaves are curled, reducing air flow, lowering the diffusion gradient and reducing water loss by evaporation, and it closes its stomata a lot, reducing water loss