Transport in plants 3.1.3

what are the key substances which plants need in order to survive?

plants need water, minerals and sugar to live

why do plants require exchange surfaces to meet their metabolic demands?

exchanging substances by direct diffusion would be too slow to meet their metabolic demands so they need transport systems to move substances to and from individual cells quickly

why do plants require exchange surfaces to increase their sa:v ratio?

plants are multicellular so have a small surface area to volume ratio

where is the xylem positioned in the roots?

• it is positioned in the centre

where are the xylem and phloem positioned in the stem?

• near the outside to prevent bending

what do the xylem and phloem make up in the leaves?

make up a network of veins which support thin leaves

define transverse cross section?

• sections cut through each structure at right angles to its length

define longitudinal cross section?

taken along the length of a structure

what stain can be used to stain the cross section of a plant?

toluidine blue

what colour does toluidine blue stain lignin in the walls of xylem vessels?

blue-green

define dicotyledonous plants?

plants that produce seeds that contain 2 cotoyledons

what system are dicotyledonous plants made of?

a vascular system made up of xylem and phloem

state the advantage of xylem vessels having no end walls?

makes an uninterrupted tube which allows water to pass upwards easily

are xylem vessels dead or living?

They are dead therefore contain no cytoplasm

What are the walls of xylem vessels made up of?

Lignin

State the function off lignin?

Arranged in spirals or small rings which helps support the xylem vessels and prevents them from collapsing inwards

How does water and ions move out the xylem?

Through small pits in walls where there is no lignin

State the for components which phloem tissue is made up of?

• Phloem fibres

• phloem parenchyma

• sieve tube elements

• companion cells

State properties of sieve tube elements?

• Joined end to end to form sieve tubes

• have holes to allow solutes to pass through

• have no nucleus

Why do sieve tube elements also require companion cells too assist them?

Because they do not contain a nucleus or other organelles therefore they are unable to survive on their own

State the function of companion cells?

Have large nucleus and carry out the living functions for both themselves and the sieve tube elements

How can water be transported around the plant?

Water enters from the soil through the root and into the xylem to be transported around the plant

State the process by which water is drawn into the roots?

Osmosis

Define how water goes down the concentration gradient?

Water moves from area of higher water potential to areas of lower water potential

does the soil around the roots have a high or low water potential?

It has high water potential

Do the leaves have a high or low water potential?

They have low water potential as water constantly evaporates from them

State the 2 pathways which water can travel through the roots?

• The symplast pathway

• The apoplast pathway

define plasmodesmata?

small channels in the cell walls

by what process does water move through the symplast pathway?

by osmosis

Where does the symplast pathway travel through?

Through the living parts of cells

Where does the apoplast pathway travel through?

Through the non living parts of the cell

by what process does water go through the apoplast pathway?

by diffusion

at the leaves how does water leave the xylem and move into cells?

by the apoplast pathway

what is the movement of water from the roots to the leaves called?

the transpiration stream

state the mechanisms that move water?

• cohesion

• adhesion

• tension

how does cohesion and tension help water move up the plant?

help water move up the plant from roots to leaves against the force of gravity

how does adhesion help water move up the xylem?

water molecules are attracted to the walls of xylem vessels so helps them rise up the xylem

define transpiration?

the evaporation of water form a plants surface

state the 4 main factors which effect transpiration rate?

• light

• temperature

• wind

• humidity

how does light affect transpiration rate?

• the brighter it is the faster the transpiration rate

• this is because the stomata open when it is light so CO2 can diffuse into the leaf for photosynthesis

• when it is dark stomata usually are closed so there is little transpiration

how does temperature affect tanspiration?

• the higher the temperature the faster the transpiration rate

• warmer water molecules have more energy so evaporate from cells in the leaf faster

• this increases the water potential gradient between the inside and outside of the leaf making water diffuse out the leaf faster

how does humidity affect transpiration?

• the lower the humidity the faster the transpiration rate

• if air around the plant is dry the water potential gradient between the leaf and the air is increased which increases transpiration

how does wind affect the transpiration rate?

• the windier it is the faster the transpiration rate

• increased air movement blows away water molecules from around the stomata

• this increases the water potential gradient which increases the rate of transpiration

what apparatus is used to measure transpiration rates?

• a potometer

What assumption is made when using potometers?

• that water uptake by a plant is directly related to water loss by the leaves

why should the shoot be cut underwater?

• to prevent air from entering the xylem

state examples of xerophytes?

• cacti and marram grass

in what climates are xerophytes adapted to live in?

dry climates, their adaptations prevent them from losing too much water by transpiration

why do cacti have spines instead of leaves?

to reduce surface area for water loss

why do most xerophytes have rolled leaves?

• to trap moist air to slow down transpiration

• also protects stomata from wind

why do xerophytes have a thick waxy layer on their epidermis?

reduces water loss by evaporation as the layer is waterproof

why do xerophytes have a layer of hairs on their epidermis?

• traps moist air around the stomata which reduces the water potential gradient between the leaf and the air slowing transpiration down

where are hydrophytes adapted to survive?

plants adapted to survive in water

why do hydrophytes need adaptations?

to cope with a low oxygen level

how do air spaces help hydrophytes?

air spaces help the plant to float and can act as an oxygen store for use in respiration

why do floating leaves have their stomata on the upper surface?

• floating leaves have their stomata on the upper surface to help maximise gas exchange

why do hydrophytes have no supporting structures?

as water already supports the leaf

define translocation?

the movement of dissolved substances to where they are needed in a plant

define assimilates?

dissolved substances

in which vessel of the vascular bundle system does translocation occur?

the phloem

where does translocation move substances from?

• from source to sink

define the source of a substance?

where its made, has a higher concentration there

define the sink of a substance?

the area it is used up, has a lower concentration there

what theory is used to explain transpiration?

the mass flow hypothesis

how is a high pressure created inside the sieve tubes at the source of the phloem according to the mass flow hypothesis?

• active transport is used to actively load the solutes into the sieve tubes

• this lowers the water potential inside the the sieve tubes so water enters the tubes by osmosis from the xylem and companion cells

• this creates a high pressure inside the sieve tubes at the source end of the phloem

how is pressure lowered inside the sieve tubes?

• at the sink end solutes are removed from the phloem to be used up

• this increases the water potential onside the sieve tube so water leaves the tube by osmosis

• this lowers the pressure inside the sieve tubes

what occurs after the pressure inside the sieve tubes is lowered?

• this results in a pressure gradient from the source end to the sink end

• this gradient pushes solutes along the sieve tubes to where they are needed

what is active loading used for?

used to move substances into the companion cells from surrounding tissues and from the companion cells into the sieve tubes against a concentration gradient

by what process is sucrose moved to where it is needed?

by active transport and co- transporter proteins

what is the function of atp in the companion cell?

• it is used to actively transport hydrogen ions out of the cell and into surrounding tissue

• this creates a concentration gradient as there are more H+ ions in the surrounding tissue than the companion cellqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqqq¬|`

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