3.3 Transport in Plants

Why do plants need a transport system?

• There is a large surface area to volume ratio

• The rate of diffusion into plant tissues is too slow.

• There's a high metabolic rate

What do plants need to exchange with the living environment?

It takes in CO₂ and nutrients

Waste products need to be released

Why can't large transport distances use simple diffusion?

Diffusion wouldn't be fast enough to meet the metabolic requirements of cells

How is glucose transported throughout the plant?

As sucrose

How do plants adapt to increase their SA:V ratio?

• Plants have a branching body shape

• Leaves are flat and thin

• Roots have root hairs

Metabolic activity in the plant

• Larger organisms are more physically active and contain more cells than smaller organisms

• A larger number of cells results in a higher level of metabolic activity, and as a result, the demand for oxygen and nutrients is greater, and more waste is produced

What is mass flow?

The bulk movement of materials. It is directed movement so involves some source of force.

Why have plants evolved to have specialised mass flow transport systems?

To enable the efficient transport of nutrients

What do mass transport systems help to do?

• Bring substances quickly from one exchange site to another

• Maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings

• Ensure effective cell activity by keeping the immediate fluid environment of cells within a suitable metabolic range

What 2 separate mass transport systems exist in flowering plants?

• Xylem

• Phloem

What does the xylem transport?

Water and Mineral Ions

What does the phloem transport?

Sucrose and amino acids (assimilates)

Why don't plants have a specialised transport system for oxygen and carbon dioxide?

• They have adaptations that give them a high SA:V ratio for the absorption and diffusion of gases

• The leaves and stems possess chloroplasts, which produce oxygen and use up carbon dioxide

• There is a low demand for oxygen, due to plant tissues having a low metabolic rate

Where are xylem and phloem found?

In vascular bundles

Vascular bundle in a leaf

Xylem on top

Phloem underneath

Vascular bundle in a stem

Phloem in the outer

Cambium in between

Xylem near the centre

Vascular bundle in the root

x shape in the middle is the xylem

Phloem bundles around the xylem

Dicotyledonous plant roots

Definition of transpiration

The evaporation of water from the stomata

Definition of transpiration stream

The movement of water up the xylem

Evapotranspiration Process

1. Water enters the leaves by evaporating from the xylem and passes into the mesophyll by osmosis

2. The water evaporates from the surface of the mesophyll to form water vapour

3. This water vapour gathers in the air spaces in the spongy mesophyll

4. Once the water potential inside the leaf is higher than the outside, the water vapour leaves the leaf through the open stomata

So what does transpiration involve?

• Osmosis from the xylem to the mesophyll

• Evaporation from the surface of the mesophyll into the air space in the lead

• Diffusion out of the stomata

Factors that impact transpiration

- Temperature

- Humidity

- Light intensity

- Air movement

- Number, size, and position of stomata

- Presence of waxy cuticle

- Water availability

Temperature

The higher the temperature, the more kinetic energy the water has, and therefore more evaporation of water vapour through the stomata

Humidity

The more water vapour surrounding the stomata, the less steep the diffusion gradient, and therefore less water will leave the leaf via evaporation

Light intensity

The more light, the higher the rate of photosynthesis, and therefore more gas exchange is needed.

Oxygen diffuses out of the stomata, and carbon dioxide diffuses in.

Air movement

The more wind or air movement, the less water vapour that will surround the stomata (since it will be blown away).

Therefore, the steeper the water vapour gadient, the more water that will leave the stomata via evaporation

Number, size, and position of stomata

The more stomata there, and the bigger they are, the more water that will leave through evaporation.

The more stomata present on the top of the leaf, the more water that will leave through evaporation

Presence of waxy cuticle

The waxy cuticle is waterproof, and the thicker it is, the less water that will leave through evaporation

Water availability

The more water available, the more water that will leave through evaporation

Investigating the Rate of Transpiration Method

1. Cut stem under water to avoid air bubbles entering the xylem. Cut at an angle to increase the surface area for the xylem to take up the water

2. Dry the leaves to avoid the reduction in transpiration of the plant

3. Use the same age/species of plant with the same surface area of leaves

4. Set up the potometer under water and introduce an air bubble

Why does the potometer not accurately measure the rate of water uptake?

• Some water is used in turgor pressure

• Some water is used in photosynthesis

What info do I need to calculate the rate of water uptake in the transpiration practical with a potometer?

Surface Area of a Circle x The distance the bubble has moved

Why does water move into the root hair cell via osmosis?

There is a lower water potential in the root hair cells, due to the active transport of mineral ions and salts into the root hair cells, so there is a higher concentration of solutes.

Water moves into the root hair cells by osmosis.

Name the 2 pathways the water takes to get from the root hair cell to the xylem in the vascular bundle

Apoplastic pathway

Symplastic pathway

Apoplastic Pathway

Water travels through the cell wall

How does water move in the apoplastic pathway?

Via diffusion as it is not crossing a partially permeable membrane

The water stays within the cell walls (apoplastic pathway) until it reaches what?

Casparian strip

Where is the casparian strip?

In the endodermis

What does the casparian strip do?

When the water and dissolved minerals reach the casparian strip, they must take the symplastic pathway.

Symplastic pathway

Water travels through the cytoplasm and pass from cell to cell through the plasmodesmata

Cohesion-Adhesion tension theory

There is high hydrostatic pressure in the roots

There is a low hydrostatic pressure in the leaves

As water evaporates through the stomata, this creates tension in the xylem

Water moves up the xylem in a continuous column along the hydrostatic pressure gradient by cohesion, adhesion, and capillary action.

This is by mass flow

Name some xerophytes

Cacti

Marram grass

Name a hydrophyte

Water lilies

Xerophyte adaptations to reduce water loss:

• Rolled leaves

• Hairy leaves

• Sunken stomata

• Needle like leaves

• Dense spongy mesophyll layer

• Less stomata

• Thick waxy cuticle

• Long deep root systems

Xerophyte adaptations - Rolled leaves

• Reduces surface area for evaporation

• Traps a layer of water vapour, creating a higher water potential outside the stomata, reducing the water vapour potential gradient, reducing evaporation of water from the leaves

Xerophyte adaptations -Hairy leaves

• Traps a layer of water vapour, creating a higher water potential outside the stomata, reducing the water vapour potential gradient, reducing evaporation of water from the leaves

Xerophyte adaptations -Sunken stomata

Traps a layer of water vapour (it isn’t taken away by the wind as easily), creating a higher water potential outside the stomata, reducing the water vapour potential gradient, reducing evaporation of water from the leaves

Xerophyte adaptations -Needle like leaves

Reduces the surface area of the leaf, therefore less evaporation of water vapour

Xerophyte adaptations -Dense spongy mesophyll layer

Smaller surface area for evaporation

Xerophyte adaptations -Less stomata

Less stomata which are closed in the day

Most of the stomata will be found on the lower surfaces of the leaves to reduce evaporation

Xerophyte adaptations -Thick waxy cuticle

Waterproof, prevents water leaving through evaporation

Xerophyte adaptations -Long deep root systems

Long deep roots to take up water

There are also a high solution concentration in root hair cells to draw more water in via osmosis

Hydrophyte adaptations

• Aerenchyma

• Large leaves

• Pneumatophore

• Lots of stomata

• Thin waxy cuticle

• Short root system

Hydrophyte adaptations - Aerenchyma

Allows buoyancy

Hydrophyte adaptations - Large leaves

Allows large surface area for leaves to increase the rate of photosynthesis

Hydrophyte adaptations - Pneumatophores

Roots that grow out of the water to aid with gas exchange

Increases the rate of photosynthesi

Hydrophyte adaptations - Lots of stomata

• Open most of the time

• Found on the upper surfaces of the leaves to increase gas exchange

Hydrophyte adaptations - Thin waxy cuticle

No need for a thick waxy cuticle as water loss doesn’t need to be prevented

Hydrophyte adaptations - Short root system

Prevents damage by currents

The plants can meet its requirements for water because it lives in water.

Source

Where sugars are made or released from a carbohydrate source (starch)

Sink

Anywhere in the plant where the sugars are used in respiration or converted for storage (starch in roots) and are therefore in a low concentration

Translocation Process - Active Loading at the Source

1. H⁺ ions in the companion cells are actively transported out into surrounding tissue

2. H⁺ ions move back into the companion cell with a sucrose (or an amino acid), using a cotransporter protein.

3. This process is facilitated diffusion

4. Sucrose diffuses through the plasmodesmata into the sieve tube elements

5. Mass flow hypothesis. Sucrose lowers the water potential of sieve tube elements and therefore water moves into the sieve tube elements from the xylem via osmosis

6. This causes an increase in hydrostatic pressure inside the sieve tube elements at the source

Translocation Process - Removal at the Sink

1. At the sink, sucrose leaves the sieve tube elements by diffusion and therefore the water potential of the sieve tube elements increases

2. Therefore water leaves the sieve tube elements via osmosis

3. This causes a decrease in hydrostatic pressure inside the sieve tube elements at the sink

Because there is a high hydrostatic pressure at the source and low hydrostatic pressure at the sink, what happens?

Assimilates move from source to sink down the hydrostatic pressure gradient by mass flow