Transport in Plants

Xylem n phloem

whats mass transport

Mass transport is the movement of materials over large distances, due to pressure differences.

describe the Movement of water through the leaf

1. When stomata are open, water vapour diffuses from the air spaces in the leaf, out through the stomata, down a water potential gradient. This loss of water is called transpiration.

2. To replace this, water evaporates from the walls of mesophyll cells into the air spaces forming water vapour, which builds up in the air spaces

. 3. The water in the mesophyll cells is replaced by water from the xylem vessels in the leaf which are continuous colume of water with xylem in the stem.

whats the phloem tissue transporting

Phloem tissue is the transport of organic substances such as sugars and other soluble products of photosynthesis from the leaves to other parts of the plant.

what does the xylem vessel transport

Xylem tissue is concerned with the transport of water and dissolved minerals, from the soil, through the roots, the stem and to the leaves.

how and where is water carried from

Water is carried in xylem tissue. This is a series of continuous tubes called xylem vessels. These are modified dead cells arranged end-to-end.

what is on the xylem tissues walls that helps it function efficiently and whats the function of this structure

Their cell walls contain lignin-lignin strengthens the xylem walls against the tension within them, and makes them waterproof

what does the lignified walls allow to happen

The lignified vessel walls cause the cell contents to die - this leaves a hollow lumen with no cytoplasm that offers little resistance to the mass flow of water and minerals.

what’s transpiration

Transpiration is the process of water movement through a plant and its evaporation from leaves. The continuous columns of water that move from the soil through the roots, stems and leaves to the air are known as the transpiration stream.

what do the walls of xylem vessel s contain and whats the function of this structure

The walls of xylem vessels contain tiny holes called pits -If a vessel becomes blocked or damaged, the water can be diverted laterally, so the upwards movement of water can continue in an adjacent vessel.

what happens to the end walls of the vessels

The vessels also lose their end walls - they form a continuous column for water movement from root to leaves

what percentage of water do plants absorb during photosynthesis

Plants only absorb about 1% of the water entering their roots. This is used in photosynthesis and in maintaining turgid cells. The remaining 99% evaporates from the leaves and is lost from the plant. Transpiration rates in large trees can be as high as 1 dm3 min-1.

Suggest a feature of lignin other than strength that would be useful in ensuring that water was carried up the plant.

waterproof

whats the name of the tallest tree and whats the height

The tallest known living tree is a coast redwood (Sequoia sempervirens) is found in California and is 115.6 m (379 feet) high.

Suggest one advantage of xylem vessels being dead cells in order to carry out their function

effectively.

Living cells have cell membranes and cytoplasm. Water movement( osmosis) would be slower if it had to pass through these;

The thickening of the cell wall in xylem vessels is often spiral (see right).

Q.5. Suggest two advantages to the plant of having this arrangement rather than continuous thickening.

1-Uses less material is therefore less wasteful; 2- Uses less material therefore allows xylem to be flexible/have lower mass;

explain the cohesion theory

As transpiration occurs through open stomata, water is evaporated from the cell walls of mesophyll cells of the leaf and is replaced by water from the xylem due to the cohesive forces between water molecules.

- This creates low pressure at the top of the xylem in the stem

- Water in the xylem is under tension and is pulled up towards the leaves.

-Continuous columns of water are maintained due to cohesion between water molecules

·-There is also adhesion of water molecules to the walls of the xylem vessels. This creates an inward pull on the vessel walls as the water is pulled up, causing the xylem vessels to decrease in diameter.

describe the property of water molecules

water molecules are polar and so very many weak hydrogen bonds form between them making them stick together. This is called cohesion. Water molecules also form hydrogen bonds with the walls of the xylem. This is called adhesion.

does the entire cohesion theory process require ATP or NO ATP

This process is entirely passive as no ATP (metabolic energy) is required for this to occur. The energy is needed is the heat from sun/solar required to evaporate water from the leaves.

what do pits allow in xylem vessel

they allow water diversion incase the xylem vessel is blocked

what happens when the column of water is broken in the xylem vessel

If a column of water in the xylem is broken, air bubbles in the xylem form and this stops any further upward movement of water in that xylem vessel as the air bubbles prevent cohesion

Why do xylem vessels lose their end walls?

to form a continuous column for water movement

How does the leaf minimize water loss?

stomata mainly on underside ,thick cuticle

Suggest a feature of lignin other than strength that would be useful in ensuring that water was carried up the plant.

Waterproof;

Suggest one advantage of xylem vessels being dead cells in order to carry out their function effectively.

Living cells have cell membranes and cytoplasm. Water movement would be slower if it had to pass through these

explain the process of transpiration

• As transpiration occurs through open stomata, water is evaporated from the cell walls of mesophyll cells of the leaf and is replaced by water from the xylem due to the cohesive forces between water molecules· This creates low pressure at the top of the xylem in the stem.  
  

• · Water in the xylem is under tension and is pulled up towards the leaves.  
  

• · Continuous columns of water are maintained due to cohesion between water molecules  
  

• · There is also adhesion of water molecules to the walls of the xylem vessels. This creates an inward pull on the vessel walls as the water is pulled up, causing the xylem vessels to decrease in diameter.  

Give examples of respiratory inhibitor

Respiratory inhibitors, such as cyanide or a lack of oxygen, do not inhibit this process.

What happens to the diameter of trees when they’re transpiring

The diameter of trees decrease when they are transpiring, (tension pulls xylem walls in), and more so when temperatures and light intensities are higher. This can be measured using a dendrometer.

When a xylem vessel is broken, water does not leak out, instead air is drawn in.  
Q.7. Explain how this observation provides evidence in support of the cohesion-tension theory.

The fact that air is drawn into the xylem when it is broken means that the water in the xylem must be under tension (pulled from the top) rather than under pressure (pushed from the bottom) as predicted by the cohesion-tension theory.

Why is the trunk diameter lowest at noon?

Stomata are open, transpiration rates are high, increased tension, so water column pulled up xylem faster, so walls of xylem pulled in and xylem vessels decrease in diameter

Why is the trunk diameter highest at midnight

Stomata are closed, transpiration rates are low, little tension so little inward pull on walls of xylem vessels

What plants have additional adaptations to further reduce water loss through transpiration.

xerophytes

How does the trapped humid air with high water potential help limit water loss

Water vapour is trapped within the rolled leaf, this reduces the water potential gradient between air spaces inside leaf and the atmosphere. A lower rate of diffusion from the stomata occurs

How does the hairs on lower epidermis of leaf help reduce/ limit water loss

Water vapour is trapped between the hairs so reducing the water potential gradient between air spaces inside leaf and the atmosphere. A lower rate of diffusion and less transpiration occurs

How does stomata sunken in pits help reduce/limit water loss

Water vapour is held above the stomatal pore so reducing the water potential gradient between air spaces inside leaf and the atmosphere. A lower rate of diffusion and less transpiration occurs

How does thick waxy cuticle help limit/reduce water loss

The cuticle is waxy so reduces water loss (from the epidermis). Greater thickness increases the length of the diffusion pathway for water to reach the atmosphere, and so decreases rate of diffusion of water through the cuticle.

Xerophyctes -how does reduction in the surface area to volume ratio of leaves limit water loss

The smaller the SA:VOLUME ratio the slower the rate of diffusion. Some xerophytes like cacti have spines rather than broad, flat leaves

Xerophytes- how does the stomata confined to underside of leaf help limit water loss

Underside of leaves is often cooler so less heat energy to evaporate water.

How does the daylight closure of stomata help limit water loss

reduces transpiration during the hottest part of the day

How does the succulent(moist )stems or leaves help limit water loss

Storage of water

Give one reason why plants growing on sand dunes need to have xerophytic features even though there is plentiful rain.

Rain rapidly drains through the sand out of the reach of roots

Sand dunes are in windy situations, this reduces the water potential of the air and so increases the water potential gradient leading to increased water loss from the soil (wind dries sand out)

Describe the similarities in preventing water loss seen in xerophytes and insects.

Stomata and spiracles can be closed to prevent water loss .

Waterproofing of cuticles