Transport in plants

Why do they need it

Plants have a low surface area to volume ratio and a fast metabolic rate so diffusion from outer surfaces would be too slow for their metabolic rates

Phloem tissue

Transports sugars in solution up and down the plant

Xylem tissue

Transports water and minerals up the plant

Location of xylem and phloem in roots

In the centre to provide support as the roots push through the soil

Xylem and phloem location in the stem

Near the outside to provide a scaffolding that reduces bending

Xylem and phloem location in leaves

Ale up a network of veins which support thin leaves

Adaptations of the xylem

Made up of dead cells so there’s no cytoplasm. Long tube like structures formed from cells joined end to end making an uninterrupted tube that allows water to pass through the middle easily. The cell walls are thickened by lignin which is deposited in a spiral or distinct rings allowing flexibility stopping them from collapsing inwards. Water and minerals ions move into the vessel via small pits in the wall where there’s no lignin

Adaptations of phloem

Contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells. The sieve tube elements are living cells that form the tube for transport. The end walls have holes in them allowing solutes to pass through. Sieve tube elements have no nucleus and a few organelles and a very thin cytoplasm to increase the volume of sugars in solution they can carry. There is a companion cell for each sieve tube element to carry out the living functions of the element as they have no nucleus and little organelles

How does waters enter a plant

The water enters the root hair cell via osmosis then has to pass through the root cortex and endodermis to reach the xylem. Then because leaves have a lower water potential than roots the water moves from the roots to the leaves

How does water travel through the roots

Can travel via the symplastic or apoplast pathway

Symplastic pathway

A pathway that goes through the living parts of a cell (cytoplasm). The cytoplasm of neighbouring cells are connected through the plasmodesmata and water can move through via osmosis

Apoplast pathway

The water travels through the cells cell wall as they are very absorbent so the water can diffuse through them and pass through the spaces between them. The water can carry solutes and move from areas with a high hydrostatic pressure to places with low hydrostatic pressure

Casparian strip

When water in the apoplast pathway gets to the endodermis layer in the roots it is blocked by the casparian strip so the water has to travel via the symplastic pathway to get though so has to go though plasma membrane which is good as it is able to filter

How does water travel through the leaves

Water leaves the xylem and moves into the cell mainly by the apoplast pathway. Water then evaporated from the cell walls into the space between cells in leaves and when the stomata in the leaves open the water evaporates out

Cohesion and tension

Water evaporates form the leaves (transpiration). This creates tension which pulls more water into the leaves. Water molecules are cohesive due to hydrogen bonding so when some are pulled into the leaves others follow so the whole water column in the xylem moves upwards. Then water enters the stem through the root cortex cells

Adhesion

Water molecules are attracted to the walls of the xylem vessel which helps them rise up

Transpiration

Evaporation of water from a plants surface .

Why does transpiration occur

Because plants have to open their stomata to let CO2 in for photosynthesis so if can create glucose however outside in the air there is a lower water potential than in the leaves so water moves out of the leaf down the concentration gradient

Winds affect on transpiration

The windier it is the faster the rate of transpiration as lots of air movement blows water molecules away from the stomata so increases the concentration gradient between the stomata and air

Humidity affect on transpiration

The lower the humidity the faster the rate of transpiration as when the air around the plant is dry there is a bigger concentration and water potential gradient between the stomata and the air

Light intensity affect on transpiration

The stomata opens when it is light so a higher transpiration rate but when it is dark the stomata closes lowering the transpiration rate

Temperatures affect on transpiration

A higher temperature means a faster transpiration rate as the water molecules have more kinetic energy so evaporate faster increasing the water potential gradient

Xerophytes

Plants adapted to live in dry conditions

Adaptations of xerophytes

Have a waxy layer on the epidermis which reduces H2O loss by transpiration as the layer becomes waterproof, have spines instead of leaves which reduces the surface area for H20 loss, have a sunken stomata that is sheltered from the wind so moist air is trapped in the pits and have hairs on epidermis to also trap moist air around stomata

Hydrophyte

Plants that live in aquatic habitats

Adaptations in hydrophytes

Have air spaces in tissue which helps them the plant float so the plant receives more light and acts as a store of oxygen for respiration, their stomata is only present on the upper surface of floating leaves helping to minimise gas exchange, have flexible leaves and stems as plants are supported by the water around them so don’t need a rigid stem and this helps prevent damage from water currents

Translocation

Movement of dissolved substances to where they need to be in a plant

Mass flow hypothesis

At the source active transport is used to load substances into the sieve tubes of the phloem lowering the water potential in the the sieve tube so water enters the tubes from the xylem and companion cells creating a high pressure at the source end. At the sink solutes are removed from the phloem to be used up via diffusion as there is a lower concentration of solutes into the surrounding tissue than the phloem. Removal of the solutes increases water potential in the sieve tubes so water leaves via osmosis lowering the pressure inside them. This results in a pressure gradient from the source end to the sink which pushes solutes along the sieve tubes towards the sink where they will be used. A higher concentration of sucrose at the source means a higher rate of translocation

Active loading

Used at the source to move substances into the companion cells from surrounding tissue and from campanile cells into the sieve tubes agains a concentration gradient

Process of active loading

In companion cells ATP is used to actively transport hydrogen ions into the surrounding tissue setting up a concentration gradient as there will be more H+ ions in the surrounding tissue than companion cells. A H+ ion then binds to a Co-transport protein in the companion cells cell membrane at the same time a sucrose molecule binds to the same Co-transport protein and movement of H+ ions down the ir concentration gradient is used to move sucrose against its concentration gradient. The sucrose molecules are then transported out of the companion cells and into the sieve tubes by the same process