3.3 Transport in plants

Why do plants need transport systems?

• They are multicellular with a low surface area to volume ratio.

• Diffusion is too slow to meet their metabolic needs.

• Substances must be moved over long distances.

Xylem tissue

transports water and mineral ions around plants. It also provides structural support. It is mostly made up of xylem vessels

adaptations of xylem vessels

• They are elongated, hollow tubes without end walls.

• They lack organelles.

• Their walls are thickened with lignin for support.

• They have non-lignified pits that allow movement of water and ions into and out of vessels.

Phloem tissue

transports sugars and amino acids (assimilates) around plants. It is mostly made up of sieve tube elements and companion cells

Adaptations of sieve tube elements

• They are connected end-to-end to form sieve tubes.

• They have sieve plates with pores at their ends to allow flow of sugars and amino acids.

• They lack nuclei and most organelles.

• They have only a thin layer of cytoplasm.

Adaptations of companion cells

• They are connected to sieve tube elements through pores (plasmodesmata).

• The cytoplasm contains a large nucleus, many mitochondria to release energy for the active transport of substances through the sieve tube elements, and many ribosomes for protein synthesis

herbaceous dicotyledonous plants

plants with two seed leaves (cotyledons) in their seeds

Distribution of xylem and phloem tissues in the roots

• Xylem forms central cylinder surrounded by phloem.

• This provides support as the root grows through soil

Distribution of xylem and phloem tissues in the stems

• Xylem and phloem are in the outer region.

• This forms 'scaffolding' to resist bending.

Distribution of xylem and phloem tissues in the leaves

• Xylem and phloem form a network of veins.

• This provides support for thin leaves.

How does water move through a plant?

1. Water enters a plant's root hair cells via osmosis.

2. It moves through the cell cytoplasm or cell walls towards the xylem.

3. The xylem transports water from the roots up to the leaves.

4. Water is used for photosynthesis.

5. Some water evaporates from leaf cells by transpiration and diffuses out of the plant.

apoplast pathway

• Water moves through spaces in the cell walls and between cells.

• This occurs due to the cohesive and adhesive properties of water

symplast pathway

• Water moves from cell to cell through the cytoplasm and plasmodesmata.

• This occurs due to water potential gradients.

What blocks the apoplast at the root endodermis?

The casparian strip

Casparian strip

a band of a waterproof substance called suberin that surrounds the endodermis cells

function of casparian strip

It forces water out of the apoplast pathway and into the symplast pathway

what happens after the xylem transports water up through a plant?

water exits the xylem into leaf cells. It travels from the xylem to photosynthesising leaf cells mainly via the apoplast pathway.

Water then evaporates from cell walls in the leaf into air spaces so it can exit the plant through its stomata.

cohesion-tension theory

explains how water moves upwards through the xylem against gravity

How does cohesion cause water to move upwards in the xylem?

Hydrogen bonding causes water molecules to stick together and move as one continuous column

How does adhesion cause water to move upwards in the xylem?

Hydrogen bonding between polar water molecules and non-polar cellulose in xylem vessel walls pulls water upwards through the xylem.

How does transpiration pull cause water to move upwards in the xylem?

Evaporation of water at leaves creates the transpiration pull, and this tension is transmitted down the whole water column due to cohesion.

why does transpiration occur?

1. Water evaporates (changes from liquid water into gaseous water vapour) from the moist surfaces of mesophyll cells.

2. Stomata open so they can absorb carbon dioxide for photosynthesis.

3. This provides a pathway for water vapour loss through the open stomata.

4. Water vapour moves down a water potential gradient from the air spaces in the leaf into the atmosphere.

how does light intensity affect transpiration rate?

At high light intensities, stomata open for maximum CO2 absorption for photosynthesis, increasing the transpiration rate.

how does temperature affect transpiration rate?

At high temperatures, evaporation of water molecules is faster due to higher kinetic energy, increasing the transpiration rate.

how does humidity affect transpiration rate?

Low humidity increases the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.

how does wind speed affect transpiration rate?

High wind speeds increase the water vapour gradient between the leaf and atmosphere, increasing the transpiration rate.

using a potometer

1. Cut the shoot underwater at slant to increase the surface area for water uptake.

2. Assemble the potometer with the shoot submerged in water.

3. Keep the capillary tube end of the potometer submerged throughout the experiment.

4. Check that the apparatus is airtight.

5. Dry the leaves, and give the shoot time to acclimatise.

6. Shut the tap, form an air bubble and record its position.

7. Measure the distance the air bubble moves and the time taken.

8. Change one variable at a time and keep everything else constant.

how to calculate the cross-sectional area of the capillary tube

surface are of a circle = pi x r²

how to calculate volume of water uptake

surface area of cross section x distance travelled by air bubble

rate of transpiration

volume of water uptake/ time taken

Is transpiration an active or passive process?

passive

Pericycle

the inner layer of meristem cells

cambium

the layer in between the xylem and phloem which has meristem cells that are involved in production of new xylem and phloem tissue

Which direction does water flow in the xylem

upwards

which direction does sucrose travel in the phloem?

upwards or downwards

structure of a leaf

• upper epidermis with waxy cuticle

• air spaces

• palisade mesophyll cells

• spongy mesophyll cells 

• stomata

• lower epidermis

• vascular tissue 

Upper epidermis with waxy cuticle

reduces water loss from the leaf surface

air spaces

interconnecting spaces that run throughout the mesophyll layer

palisade mesophyll cells

cells located beneath the upper epidermis to carry out photosynthesis

Spongy mesophyll cells

dispersed cells located beneath the palisade mesophyll layer to carry out photosynthesis

stomata

small pores surrounded by guard cells on the underside of leaves that can open and close

Lower epidermis

the bottom layer of cells in a leaf that contains the stomata and guard cells

Adaptations of leaf structures for gas exchange

1. Air spaces - These provide a network for gases to quickly diffuse in and out of the leaf and access photosynthesising cells.

2. Mesophyll cells - These are dispersed throughout the leaf, providing a large surface area across which gases can diffuse.

3. Stomata - These open when conditions are suitable for photosynthesis, allowing inward diffusion of carbon dioxide and outward diffusion of oxygen, and close to minimise water loss.

How plants can limit water loss

1. They have a waterproof waxy cuticle on their leaves.

2. They have guard cells that can close stomata when needed.

Xerophytes

plants adapted to living in dry environments with limited water availability.

How is the cuticle of a xerophyte adapted?

It is thick and waxy which reduces water loss through evaporation

How is the leaf structure of a xerophyte adapted?

They are polled up or folded which encloses the stomata on the lower surface to reduce air flow and the evaporation of water

Why do xerophytes have hairs on their leaves?

to trap moist air against the leaf surface to reduce the diffusion gradient of water vapour

Why do xerophytes have sunken stomata in pits?

they reduce air flow and evaporation of water.

why do xerophytes have small, needle-like leaves?

they reduce the surface area across which water can be lost

Why xerophytes have water storage organs?

To conserve water when it is in low supply 

What does the mass flow hypothesis propose?

translocation occurs due to pressure gradient

process of mass flow hypothesis

1. At the source, solutes like sucrose are actively loaded into sieve tube elements from companion cells.

2. This decreases the water potential in sieve tube elements.

3. Water enters the sieve tube elements from the xylem and companion cells by osmosis.

4. This increases hydrostatic pressure in the sieve tube elements at the source.

5. At the sink, solutes are actively removed from the sieve tube elements.

6. This increases the water potential in sieve tube elements at the sink.

7. Water leaves the phloem by osmosis, decreasing the hydrostatic pressure at the sink.

8. This creates a pressure gradient, pushing solutes from the source to areas of lower pressure at the sink.

What happens at the sink?

solutes are actively unloaded from the sieve tube element into companion cells. They can then move into sink cells where the solutes can be used, for example in respiration, or stored

How is a pressure gradient set up from source to sink?

assimilates like sucrose are actively loaded into the phloem

Process of active loading

1. Hydrogen ions (H⁺) are actively transported out of companion cells into surrounding source cells.

2. H⁺ is co-transported along its concentration gradient back into companion cells with sucrose.

3. Sucrose can then diffuse along its concentration gradient through plasmodesmata from companion cells to sieve tube elements.

Translocation

mass flow of assimilates from one part of a plant, the source, to another part of the same plant, the sink

Assimilates

substances that have been manufactured or modified in the plant, such as sucrose and amino acids, that are transported in the phloem sieve tubes during translocation

features of translocation

• It requires energy.

• It transports substances from sources (where they are made, like the leaves) to sinks (where they are used, like the roots).

• Water provides the medium in which these substance dissolve for transport in the phloem.

• It maintains a concentration gradient using enzymes.