The need for transport systems

The need for transport systems
Why do large organisms have specialised transport systems while smaller ones do not?
With large size comes a small SA:vol ratio. The consequence of this is specialised organs of exchange and with these come a need to transport materials between the cells and these sites of exchange which is met with a transport system.
Achievements of plant transport
Water transport
From roots to leaves
Achievements of plant transport
Water transport
From roots to leaves
Nutrient (sucrose / amino acid) transport
From leaves throughout the plant
Achievements of plant transport
Water transport
From roots to leaves
Nutrient (sucrose / amino acid) transport
From leaves throughout the plant
Ion transport
From roots to growing points
Xylem
The term ‘xylem’ refers to a tissue made up from the following cell types: vessels, fibres, tracheids and xylem parenchyma. 
Xylem vessels transport water (the ‘transpiration stream’) and mineral ions absorbed by the roots.
Flow is unidirectional (upward) ‘pulled’ by evaporation from the leaves

Specialisations of xylem vessels
Specialisations of xylem vessels
Specialisations of xylem vessels
Specialisations of xylem vessels
Specialisations of xylem vessels
Specialisations of xylem vessels
Specialisations of xylem vessels
Exchanging water between adjacent cells
s=-2.0
Cell A
s=-2.0
Cell B
s=-2.0
Cell C
Evaporation of water
Evaporation of water from Cell C results in the movement of water from B to C and then from A to B. How is this water moved along?
A closer look at the cell boundaries…
Cellulose cell wall
Cell membrane
Cytoplasm
Vacuole membrane (tonoplast)
Vacuole
Plasmodesmata
Middle lamella (rich in pectates)
Plasmodesmata
Small holes in the cell walls between adjacent cells
Membrane flows through from cell to cell
Cytoplasm is continuous between the cells
Some have threads of smooth ER passing through increasing communication further

Three modes of water transfer between cells…
Apoplast route

Three modes of water transfer between cells…
Apoplast route
Water flows within the cell walls.
Cellulose if freely permeable to water.
Relies on the cohesive forces of water (H-bonds).
cell has no control over water movement since the water never enters the living contents of the cell (protoplasm).
By far the most significant route (accounts for ~80% of water flow between cells)
The Apoplast Route
Cellulose cell wall
Cell membrane
Cytoplasm
Vacuole membrane (tonoplast)
Vacuole
Plasmodesmata
Middle lamella (rich in pectates)
Three modes of water transfer between cells…
Apoplast route
Symplast route


Three modes of water transfer between cells…
Apoplast route
Symplast route
Water moves between cells via plasmodesmata.
Water stays in cytoplasm and moves by diffusion
The Symplast Route
Cellulose cell wall
Cell membrane
Cytoplasm
Vacuole membrane (tonoplast)
Vacuole
Plasmodesmata
Middle lamella (rich in pectates)
Three modes of water transfer between cells…
Apoplast route
Symplast route
Vacuolar route

Three modes of water transfer between cells…
Apoplast route
Symplast route
Vacuolar route
Water moves by osmosis
Movement is between adjacent vacuoles
The Vacuolar Route
Cellulose cell wall
Cell membrane
Cytoplasm
Vacuole membrane (tonoplast)
Vacuole
Plasmodesmata
Middle lamella (rich in pectates)
The transpiration stream
Evaporation of water from the spongy mesophyll tissue…
…draws water across the leaf by apoplast, symplast and vacuolar routes…
…and out of the xylem in the leaves.
This pulls water up the continuous column of water in the xylem (cohesive forces stop the column from breaking).
In the roots water is withdrawn from the cells which surround the xylem…
…this draws water across the root tissue…
…and into the root hair cells from the soil.

The endodermis
Root cortex
Endodermis
Xylem vessel
The stele
The endodermis
Endodermal cells have a waterproof layer (made of suberin) impregnated into their cell wall called the Casparian strip.
Band of suberin
The endodermis
The Casparian strip blocks the apoplast route and so any water passing though the endodermis must pass through symplast route.
This gives the plant control of water and mineral uptake since both must pass across a membrane
Stomata
Stomata exist to allow CO2 into leaves for photosynthesis
Opening and closure is controlled by the plant
Water is lost by evaporation and then diffusion

Major water loss is an unavoidable consequence of leaf function
Gas exchange is critical for photosynthesis
Gas exchange requires a huge, moist surface area (provided by the spongy mesophyll)
The gas exchange surface must be ventilated (achieved by diffusion though air spaces and open stomata)
Major water loss is an unavoidable consequence of leaf function
Gas exchange is critical for photosynthesis
Gas exchange requires a huge, moist surface area (provided by the spongy mesophyll)
The gas exchange surface must be ventilated (achieved by diffusion though air spaces and open stomata)
Major water loss is an unavoidable consequence of leaf function
Gas exchange is critical for photosynthesis
Gas exchange requires a huge, moist surface area (provided by the spongy mesophyll)
The gas exchange surface must be ventilated (achieved by diffusion though air spaces and open stomata)

Measuring transpiration – the potometer
Measures uptake of water into a plant
Makes the assumption that uptake is the same as evaporative loss
Some have syringes, used to calibrate or reset the apparatus
Measuring transpiration – the potometer
Precautions:
Stem must be cut under water
Apparatus is assembled under water
All joins must be air/water tight
Bubbles are eliminated from capillary tube
Leaves must be dry
Here’s one from an exam paper…
Environmental factors affecting transpiration
Temperature
Wind speed
Relative humidity
Light intensity
Blocking of stomata

Can you make predictions on a graph like this?
What’s the point in this?
Time
Volume of water lost
Translocation
Movement of organic solutes (sucrose, amino acids etc)
Occurs in phloem sieve tubes (not ‘phloem’)
Bi-directional movement
Selective and active process
Phloem
A tissue made from several cell types
Sieve tubes
Companion cells
Transfer cells
Phloem parenchyma
Phloem – transverse section
Phloem within the vascular bundle
Sieve tube
Companion cell
Phloem parenchyma
Phloem – longitudinal section
Most of the cytoplasmic contents of the sieve tube are removed and its metabolic demands are met by the companion cells associated with it
The sieve plates are responsible for pumping materials from cell to cell

This process is active and selective and controlled by threads of protein which pass through the sieves’ pores
Phloem – longitudinal section
The transfer cell is a key cell in loading sieve tubes with sucrose

It has a highly folded membrane and many mitochondria to aid active transport into the sieve tube
Phloem loading
Mineral uptake and transport
Uptake
Most minerals are taken up from the soil by active transport by root hair cells
These cells have a large surface area for uptake
In certain soils some ions are taken up passively (e.g. Ca2+ in lime soils)
Mineral uptake and transport
Transport
The ions move in solution via the apoplast route to the endodermis
Here they are pumped into the stele – this accounts for root pressure


Mineral uptake and transport
Transport
The ions move in solution via the apoplast route to the endodermis
Here they are pumped into the stele – this accounts for root pressure


Mineral uptake and transport
Transport
The ions move in solution via the apoplast route to the endodermis
Here they are pumped into the stele – this accounts for root pressure
They then are taken into the xylem vessels and are transported in the transpiration stream

Mineral uptake and transport
Transport
The ions move in solution via the apoplast route to the endodermis
Here they are pumped into the stele – this accounts for root pressure
They then are taken into the xylem vessels and are transported in the transpiration stream
Transfer cells can pass specific ions into phloem (lateral movement) for more selective transport

Analysing data - absorption of mineral ions
Root tissue from trees can be extracted and cultured in an ion-rich medium.
Its mineral absorbing activity can be measured by analysing the contents of the cytoplasm over time.
Two such groups of such tissue were cultured and their potassium (K+) content recorded over a 150 minute period.
Analysing data - absorption of mineral ions
One group was cultured as above in air, a second with an atmosphere of pure nitrogen
Sealed chamber with controlled atmosphere
Root tissue blocks in culture solution
Results
Questions
Compare the results of the two groups [3]
What is the K+ concentration of the culture solution? Explain your answer [3]
List three factors, other than K+ concentration, which must be controlled in this experiment [3]
Explain the differences in K+ absorption between the two groups [4]