Transport in Plants Pack 7

Give an overview for the phloem

- Transport of organic substances such as sugars to other parts of the plant (amino acids, sucrose)

- Sucrose solution

- Translocation

Give an overview to the xylem

- Process of water movement through a plant

- Water moves in continuous columns

- Transpiration stream

Describe the movement of water through a leaf

- When stomata are open water vapour diffuses out from the air spaces through the stomata down a wp gradient. The loss of water is transpiration

- The water is replaced from the walls of the mesophyll cells into the air spaces forming water vapour which builds up into the air spaces again

- The water in mesophyll cells is replaced by water from xylem vessels in the leaf which are continuous in the stem

- This causes a wp gradient across the leaf from the xylem to the mesophyll cells

-Water is drawn from the xylem vessels and diffuses out into the mesophyll cells and replaces the lost water

- negative pressure

Explain the benefit in xylem for -

Cell walls contain lignin

- helps strengthen xylem walls against tension within them and makes them waterproof

Explain the benefit in xylem for -

Lignified vessel walls causing cell contents to die

- leaves hollow lumen with no cytoplasm which offers little resistance to mass flow of water and minerals

Explain the benefit in xylem for -

walls of xylem vessels contain tiny pits

- If a vessel becomes blocked or damaged the water can be diverted laterally, so upward movement of water can continue in an adjacent vessel

Explain the benefit in xylem for -

vessels lose their wall ends

form a continuous column for water movement from roots to leaves

Give 4 structural features of xylem vessels

- cell walls contain lignin

- lignified vessel walls causes the cell contents to die

- xylem vessels contain pits

- vessels lose wall ends

Describe cohesion

the hydrogen bonds between dipolar water molecules makes them stick together

Describe adhesion

water molecules form hydrogen bonds with the xylem vessel walls

What happens when the stomata are open?

transpiration occurs

water evaporates from the cell walls of mesophyll cells and is replaces by water from the xylem due to cohesive forces between water molecules

What happens to the xylem when water from the vessel replaces lost water in the cell walls of mesophyll cells?

- This creates negative pressure/ tension at the top of the xylem

- This means the water under tension in the stem is pulled up towards the leaves

Describe how water remains in continuous columns

- maintained by cohesive forces between water molecules

- adhesion of water molecules to walls of xylem vessels also ; this creates inward pressure on vessel walls as water is pulled up decreasing the diameter of the xylem vessels

Describe the energy needs in transpiration

- Passive process (no ATP)

- energy needed is from heat to evaporate water in the leaves

Give evidence for the pull on xylem vessels during transpiration

- Diameter of trees decrease when transpiring when temperature and light intensities are higher

- This can be measured using a dendrometer

When a xylem vessel is broken, water doesn't leak out but air is drawn in

How does this prove the cohesion tension theory

- The fact that air is drawn in when the xylem is broken means that the water must be under tension in the xylem (pulled from the top) rather than under pressure (pushed from the bottom)

Why is trunk diameter lowest at noon?

Stomata are open, transpiration rates are high, increased tension, so water column is pulled up faster, so xylem walls pulled in which decreases diameter

Why is trunk diameter highest at night?

Stomata are closed, low transpiration rate, little tension, so little inwards pull so diameter is greatest

Why would the diameter change happen first higher up a tree?

transpiration pull is from the top of the plant where water evaporates from the leaf

How do normal plants reduce water loss?

- Waxy cuticle

- Close stomata

What are xerophytes?

- Plants that live in areas of water scarcity

- They have structural adaptations to live in dry conditions by reducing transpiration or storing water

How do curled leaves reduce transpiration?

- Trap humid air with high wp

- reduces wp gradient between air spaces in the leaf and the atmosphere

- Lower rate of diffusion occurs from the stomata

How do hairs on lower epidermis of the leaf reduce transpiration?

- Water vapour trapped between hairs reducing wp gradient between air spaces and atmosphere

- Lower rate of diffusion occurs from the stomata and less transpiration

How do stomata sunken in pits reduce transpiration?

- water vapour held above the stomatal pore reducing wp gradient between air spaces and atmosphere

- Lower rate of diffusion and less transpiration occurs

How does a thick waxy cuticle reduce transpiration?

- reduces water loss from the epidermis

- greater thickness increases length of diffusion pathway to reach atmosphere

- decreases rate of diffusion through the cuticle

Name ways in which transpiration rate can be reduced

- curled leaves

- hairs on lower epidermis

- stomata sunken in pits

- thick waxy cuticle

- leaves with small SA: vol eg spines

- stomata on underside of leaf

- daylight stomata closure

- succulent stems or leaves

How can reduction in SA: vol of leaves reduce transpiration?

- Smaller SA:vol means slower rate of diffusion

- Can have spines instead of leaves

How can stomata on underside of leaf reduce transpiration?

- Underside of leaves is usually cooler so less heat energy to evaporate water

How can daylight closure of stomata reduce transpiration?

- reduces transpiration during hottest part of the day

How can succulent stems or leaves be helpful?

storage of water

Why is it difficult to obtain water on sand even during rainfall?

- Rain rapidly drains through the sand

Define translocation

process by which soluble organic molecules (sucrose and amino acids) and some mineral ions (K+, Cl-) are transported around the plant as sap

Why are non reducing sugars such as sucrose transported around the plant rather than reducing sugars such as glucose?

- Reducing sugars are reactive and would be chemically altered before arriving at their destination

Give an overview to translocation in the phloem

- Phloem made of sieve tube elements which are long thin cells

- Associated with companion cells

Describe the structure of phloem tissue

- Sieve tube elements link to the next ones via sieve plates which have holes/ perforated

- Sieve tube has little cytoplasm, no nucleus, no vacuole, and few organelles other than mitochondria

- Sieve tubes are alive due to cytoplasmic connections called plasmodesmata with the companion cells

- Each companion cell has a nucleus, many mitochondria and other organelles

What organelles does/doesn't the phloem have?

- No nucleus

- No vacuole

- Little cytoplasm

- Has mitochondria in small amounts

Describe the directions of travel in the phloem and xylem

Phloem - translocation - bidirectional

Xylem - transpiration - unidirectional

Describe the sources

- organic solutes are produced here so are at high concentrations

- sucrose source is usually mesophyll cells of leaves where it is formed by condensation of glucose and fructose

Describe the sinks

- organic solutes are used up here

- sinks are other parts of the plant such as growing regions (meristems) of roots, flowers, leaves and stems where sucrose is hydrolysed to glucose and fructose then respired to provide ATP (release energy)

- Fruits, seeds, roots and other storage organs act as sinks where sucrose is converted to starch and stored

Describe the mass flow theory

1) Sucrose from photosynthesising tissue to sieve tubes

- Sucrose is produced in mesophyll cells

- Active transports pumps the sucrose from the mesophyll cells of the leaf into the companion cells then into the sieve tube elements of the phloem

- This requires ATP and special carrier proteins in the cell surface membranes of companion cells

- This increases the conc of sucrose (other solutes such as amino acids) into the sieve tube at the source region

Describe the mass flow theory

2) Mass flow of sucrose through sieve tube elements

Mass flow is bulk movement of substances from high to low pressure

- Increased sucrose conc (and other solutes such as amino acids) in the sieve tubes lowers the water potential of the sieve tubes so water enters by osmosis from the xylem (and companion cells)

- This increases the volume within the sieve tubes around the source which increases hydrostatic pressure

- Sucrose solution travels from high to low pressure

- At sinks sucrose is either used up or converted to starch as storage so sucrose conc is low in this region

- Sucrose is actively transported from sieve tube elements through companion cells into sink cells

- This increases water potential of sieve tube cells so water leaves the sieve tube cells by osmosis into sink cells and the xylem

- This decreases the volume within the sieve tubes near the sink which decreases hydrostatic pressure

- As water enters the sieve tube elements at the sources and leaves at the sinks this creates a hydrostatic pressure gradient between the source and sink

- This means there is a mass flow of sucrose down the pressure gradient in the sieve tubes

Describe the mass flow theory

3) Transfer of sucrose from sieve tube elements into sink cells

- Sucrose is unloaded from the phloem by active transport from sieve tubes into sink cells via companion cells

Compare for the xylem and phloem:

what they transfer

- Xylem - water and mineral ions

- Phloem - organic solutes such as sucrose and amino acids

Compare for the xylem and phloem:

tubes friction

- Xylem - no sieve plates (hollow end to end)

- Phloem - sieve plates with perforated walls

Compare for the xylem and phloem:

walls

- Xylem - lignin

- Phloem - no additional support

Compare for the xylem and phloem:

pits

- Xylem - vessel walls contain pits which allow water to pass through adjacent vessels

- Phloem - gaps in cell walls between companion cells and sieve tube called plasmodesmata

Compare for the xylem and phloem:

living

- Xylem - dead

- Phloem - living

Give evidence for the mass flow theory

- High hydrostatic pressure in phloem shown by release of sap when cut

- Conc of sucrose is higher in leaves (source) then roots (sink)

- Downward flow in phloem during day ceases at night/ shade

- Increase in sucrose levels in the leaf are followed by increase in the phloem later

- metabolic inhibitors/lack of O2 inhibit translocation of sucrose

- Companion cells have many mitochondria

Give evidence against mass flow theory

- Function of sieve plates are unclear as they reduce flow of sucrose - could have structural function to prevent bursting

- Not all solutes move at same speed - they should by mass flow

- Sucrose delivered at same rate to all sinks; not going faster to areas of low sucrose conc as mass flow suggests

Explain in terms of mass flow theory why downward flow in the phloem ceases at night or when leaves are shaded

- No photosynthesis taking place so the production of sucrose also stops

- No active transport of sucrose into the sieve tubes via companion cells from mesophyll cells

- This means the water potential doesn't decrease and sucrose conc remains low

- No osmosis of water from xylem into phloem

- No hydrostatic pressure gradient created

Explain in terms of mass flow theory why metabolic inhibitors/ lack of O2 inhibit translocation of sucrose in the phloem

- cells can't respire to produce ATP

- No ATP which can be used in active transport of sucrose into the sieve tubes

- High water potential potential maintained at the sieve tubes at the source so no water enters from the xylem by osmosis

- No hydrostatic pressure gradient formed

How can sucrose be tracked?

- Autoradiography

- use radioactive carbon 14 which can be traces

- introduce 14CO2 which is converted to radioactive sucrose

explaining transpiration

use loss of WATER VAPOUR not loss of WATER