transport in plants

the need for a transport system

Water and mineral ions are taken through roots and are needed by all the cells in the body, so they need to be transported from one area to another- xylem transports water and mineral ions

Glucose is also needed by all cells in the body, transported as sucrose through the phloem.

Plants have a high SA: V ratio

Don’t have a system for transporting oxygen and carbon dioxide as they have a low metabolic demand so low demand for oxygen for aerobic respiration- chloroplasts generate their own oxygen

describe the xylem

no end walls or cytoplasm- free passage of water to the next cell

Dead cells- water moves through more easily

cell wall thickened with lignin- makes it rigid and provides support

pits allow water to move transversely from cell to cell- pits are areas where cell wall is not thickened with lignin

describe the phloem

sieve tube elements-alive, no nucleus, few organelles, little cytoplasm-helps their ability to transport assimilates
companion cell-dense cytoplasm and many organelles

plasmodesmata-allow molecules to pass between plant cells through the cytoplasm

perforated end walls- forming a sieve plate

absorbing water

water enters the roots from the soil down a water potential gradient, assisted by the increase in surface area provided by root hair cells.

Then it needs to move across the root cortex and through the endodermis to reach xylem where it can be transported up the cell

What are the two pathways where water transported to stem?

apoplastic pathway- goes through the cell walls. Cell walls are readily permeable, so this pathway offers little resistance;hw most water travels along this path.

symplatic pathway goes through the cytoplasm and from cell to cell via thw plasmodesmata

what is the casparian strip?

The Casparian strip is a continuous band that goes around the wall of endodermal cells

Made of a waterproof waxy substance called Suberin, which blocks the apoplastic pathway because water cannot get past it.

The water and dissolved substances, such as ions, then go through the cytoplasm and therefore through the plasma membrane, which is selectively permeable and controls what enters.

Once is cytoplasm the water can continue through the symplastic pathway or return to apoplastic pathway, then continues into the xylem

what is the problem with the apoplastic pathway?

no opportunity to select what comes into the plant and what is kept out- everything dissolved in the water travels with it- problem resolved by the presence of a casparian strip in the walls of endodermal cells.

what is a theory that explains how the transpiration stream works?

cohesion tension theory- based on the principle that water molecules and cohesive (stick together) as water is a polar molecule

The more negatively charged oxygen of one water molecule forms a hydrogen bond with the positively charged hydrogen atom in the second water molecule

If water moves from one place to another, it pulls the other water molecules with it because of these cohesive forces- so when water molecules evaporate from the cells inside the leaf, they pull other water molecules behind them and then evaporate

If air gets into xylem, it stops the continuous stream, which stops water movement

how does adhesion help in transpiration

Water molecules adhere to the walls of the xylem, which helps to maintain the water column in position even when transpiration is not taking place.

what is transpiration?

Carbon dioxide and oxygen move in and out via the stomata.

When stomata is open, the water diffuses out as the water potential is higher in the leaf than outside- this is called transpiration

Water loss is an inevitable side effect of opening the stomata to allow for gas exchange.

Stomata close during darkness when carbon dioxide is not needed for photosynthesis so reducing water loss

factors affecting transpiration

temperature- increases the kinetic energy of all molecules, so increases the transcription rate(very high temperatures cause stomata to close, and transpiration then decreases)

humidity-concentration of water molecules in the air rises, so the diffusion gradient between inside and outside of the leaf decreases and transpiration decreases, eventually equilibrium is reached and there’s no net water vapour loss from the leaf.

air movement- air currents move the water molecules away from the surface of the leaf and maintain a concentration gradient- increasing transpiration rate.

light- in the dark, stomata close and the transpiration rate decreases. When there is light, stomata open, increasing transpiration rate.

measuring transpiration

measure uptake using a photometer

assumed that water uptake is directionally proportional to the water vapour loss by transpiration

The plant stem must be cut underwater to prevent an airlock from forming in the xylem

The apparatus must be set up underwater and ensure all joints are airtight because bubbles in the apparatus will stop water movement

Leaves should be dried before taking readings other wise this can create a humid atmosphere and reduce transpiration.

ways that land plants restrict water vapour loss from their leaves

The leaves have a waxy, waterproof cuticle, so water vapour loss is restricted over the whole surface of the leaf apart from the stomata.

stomata are mostly on the lower surface of the leaf, which is cooler as it faces away from the sun, so diffusion is less rapid

Stomata close in the dark when it’s not necessary to take up carbon dioxide, as there is no light for photosynthesis

adaptations in xerophytes to reduce water vapour loss

The number of stomata may be reduced

Stomata may be sunken in pits or grooves, so high humidity builds up outside the stomata and reduces the diffusion gradient

stomata may be surrounded by hairs to trap water vapour, leaving the stomata, which lowers the diffusion gradient

waxy cuticle is much. thicker to further reduce water vapour loss

The leaf may be rolled with the lower surface inwards to allow high humidity to build up inside the coil

The whole leaf may be reduced in size or lose some or all of its leaves to reduce the surface area, through which water vapour may be lost

adaptations of hydrophytes to prevent excessive water uptake

Stomata tend to be entirely on upper surface rather than the lower, so gas exchange can occur with air rather than with water

The leaves have very large air spaces, which gives them buoyancy

Floating leaves are thin and flat, which gives them greater buoyancy

The waterproof waxy cuticle of the leaf is thin

greatly reduced root system

what is translocation?

transport of dissolved photosynthetic assimilates in a plant

These assimilates are nutrients made by photosynthesis in the leaves, the main one being sucrose

In translocation, the movement is from areas called sources to other areas called sinks.

Sources of sucrose are the leaves, as they produce sucrose in photosynthesis

A sink is any area that requires sucrose

Main sinks are meristems in the roots, stems and leaves

Sources have a high concentration of sucrose, and sinks have a low concentration- sucrose moves down a concentration gradient from source sinks.

mechanism of translocation

mass flow hypothesis- involves loading of sucrose into the phloem in the leaves and then unloading it again in the sinks.

Loading of sucrose into the phloem in the leaves involves active transport, although it seems that some sucrose may be transported passively through the cytoplasm and plasmodesmata

ATP needed for active transport into the phloem is supplied by the companion cells

As sucrose concentrations build up in the sieve tube elements, the reduced water potential draws in water from the neighbouring xylem, which creates a hydrostatic pressure in the phloem, which forcessucrose away from the source towards sink areas

In the sinks, sucrose is moved out of the phloem into the cells that need it

As a result, the water potential in the phloem is increased, and water leaves the phloem by osmosis- some of this water reenters the xylem

The pressure in the phloem is reduced, creating a gradient from the sources where it’s high to the sinks where it’s low.