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Exchange and Transport

Why do organisms require a transport system?
For survival, living cells require a constant supply of nutrients and oxygen, and
continuous removal of waste
...

Materials that may need to be interchanged between an organism and its environment:
- Respiratory gases – O2 + CO2
- Nutrients – glucose, fatty acids, amino acids, vitamins, minerals
- Excretory products – urea and carbon dioxide
- Heat
Heat is commonly forgotten by
students Janv so make sure you
remember it!
Size + Metabolic rate of organism affects amount of
each material that needs to be exchanged, which then influences type of exchange surface
and transport system evolved to meet requirements of the organism
...

Larger, more complex organisms require a system to facilitate exchanges as their SA:V
ratio decreases
...
Mass transport accounts for the long distance transport of fluids in living
organisms
...

The exchange takes place either passively or
actively
Passively- no energy required, by diffusion +
osmosis
Actively – energy required, by active transport

To increase SA:V ratio, organisms:
-

Have a flattened shape so that no
cell is ever far from the surface
A specialised exchange surface
with large areas to increase the
SA:V ratio(e
...
lungs in mammals,
gills in fish)

Features of specialised/effective exchange
surfaces:
- Large SA:V ratio – increases rate of
exchange
- V
...
g
...
g
...
g
...

As an object becomes larger, its SA compared to its volume is smaller  diffusion no
longer an effective way to transport materials to the inside
...
g
...


NB Lowest O2 concentration in a cell
is inside the mitochondria (as O2 used
for respiration)
...


Gas exchange in single-celled organisms
Single-celled organisms (such as amoeba) are very small and have a large SA:V ratio 
can exchange gases efficiently with their surroundings through cell membrane with no
need for special gas exchange organs
...

Carbon dioxide from respiration diffuses out across body surface in the same way
...


Gas exchange in Insects:
Many insects are small terrestrial animals with a large SA:V ratio  water easily
evaporates from their body; they can become dehydrated so they need to conserve
water and thus have adapted mechanisms to conserve water
...

To reduce water loss terrestrial organisms have:
- Waterproof coverings over their body surfaces  insect has a rigid exoskeleton
covered with a waterproof cuticle (waxy outer layer) that minimises water loss
- Insect excretory systems developed to conserve water- waste products & water pass
through excretory organs into gut
...
An almost dry excretory
product (uric acid) is produced minimising water losses to the environment
...

- Tracheae are supported by strengthened rings to prevent them from collapsing
...

- These extend throughout insect’s body tissues  atmospheric air containing O2 is
bought directly to the respiring tissues
...
( see below)

How do respiratory gases move in and out of the tracheal system?
- Along a diffusion gradient – when cells are respiring, O2 is used up  it conc
...
 Creates a diffusion/concentration gradient  Causes
gaseous oxygen to diffuse from atmosphere along the trachea and tracheoles to the
cells
...


-

Ventilation – rhythmic movement of muscles in insects creates mass movements of air
in & out of tracheae  further speeds up exchange of respiratory gases
...
The end of each tube contains a small amount of fluid in which the respiratory
gases are dissolved
...
Dissolved oxygen is
delivered to muscle fibres by the fluid
...

Spiracles are opened and closed by valves that form the exit point of trachea from the
body
...

- Spiracles open periodically to allow gas exchange
...
low relative to air – O2 diffuses into water v
...
Furthermore as water temp
...

In aquatic insects, like in terrestrial insects, gases move to and from the tissues via the
trachea: the network of air-filled tubes that forms the insect’s respiratory system
...

Aquatic insect larvae rely on diffusion across the body surface (with or without gills)
...

Adult insects carry air with them when submerged
...

o A thin film of air trapped by hairs is called a plastron- it provides a source of
oxygen and acts as a non-compressible diffusion gill, into which oxygen can
diffuse from the water
...


Have a specialised internal exchange surface: Gills  membranous structures supported
by cartilaginous or bony struts
...


The gills:
- Located within body of fish, behind the head
- Made up of gill filaments stacked up in a pile
...

o  gill surfaces are v
...

Flow of water over the gill lamellae and the flow of blood within
them are in opposite directions  countercurrent flow
...


Remember this
Oxygen content of atmospheric air
is constant but amount of oxygen
dissolved in water varies
considerably
...

Polluted water often has v
...


Fish ventilation
...
In the double
pumping system, water flows into the fish's open mouth
...
Once water has passed the gills, the opercular pump
expels the 'old' water and draws ‘new’ water in
...
Fish
have a single circuit system; the blood goes directly to the body from the gills (the gas
exchange surface) and only flows once through the heart in each circulation of the body
...


Why a countercurrent flow?
By having the blood and water flow over the gill lamellae in opposite directions:
-

-

Blood that already has quite a bit of oxygen meets water which has its maximum
concentration of oxygen  diffusion of oxygen still takes places from the water to the
blood (high to lower conc
...


This means that there is a fairly constant rate of diffusion across the entire length of the
gill lamellae so that 80% of the O2 available in the water is absorbed into the blood of
the fish
...

If the water and blood flowed in the same directions, the two 2 fluids would quickly reach
equilibrium and the blood would extract much less of the oxygen available in the water 
not as efficient
...


Gas exchange in the leaf of a plant
When plant is photosynthesising, there is an overall net consumption of CO2 and a net
production of O2
...
In the same way, some O2 from photosynthesis is used in
respiration but most of it diffuses out of the plant
...
g
...
In the same way, CO2 produced
during respiration diffuses out
...

Overall therefore, there is a short, fast diffusion pathway
...

Most gaseous exchange occurs in the leaves which have the following adaptations for rapid
diffusion:
- Thin, flat shape  provides a large SA
- Many stomata (small pores) which can help control rate of gaseous exchange and
water loss (found mostly in the lower epidermis)
- Numerous interconnecting air spaces that occur throughout the mesophyll

The mesophyll tissues of a
dicot leaf are sandwiched
between epidermal layers
...


Air spaces allow gases to
move freely throughout the
leaf

Leaf epidermis covered with tiny pores called stomata- open and close to allow passage of gases into &
out of leaf
...
Stomata permit gas exchange but are also the major route for water loss by transpiration
...

Plants have to balance the conflicting needs of gas exchange and control of water loss
...

In most plant species, stomata are more numerous on the underside of the leaf than on top
...
When the guard cells take up water by osmosis, they swell and become turgid,
making the pore wider
...

The changes in turgor pressure that open & close the pore result mainly from reversible
uptake & loss of K+ ions (and thus water) by the guard cells
...

Larger organisms have a smaller SA:V ratio – needs of organism can no longer be met
by the body surface alone  specialist exchange surface is  needed to absorb
nutrients & respiratory gases & to remove excretory products
...


Features of transport systems:
- A suitable medium in which to carry materials e
...
blood
...

- A form of mass transport in which the transport medium is moved around in bulk
over large distances
- A closed system of tubular vessels that contains the transport medium and forms a
branching network to distribute it to all parts of the organism
...
This requires a pressure
difference between one part of the system and another
...
g
...

- A mechanism to maintain the mass flow movement in one direction (e
...
valves)
- A means of controlling the flow of the transport medium to suit the changing needs of
different parts of the organism
...
slow due to the low pressure
...
and hence a high rate of metabolism
...
Final exchange from blood vessels into cells
is rapid because it takes place over a large surface area, across short distances and
there is a short diffusion gradient (Fick’s law)

Superior vena cava:
receives deoxygenated
blood from the head &
body

Aorta: carries oxygenated
blood to body
...


Mesenteric artery: carries
oxygenated blood to the gut
Renal artery: carries
oxygenated blood to the
kidneys

Blood vessels:
Arteries- carry blood away from the heart and into arterioles to the capillaries within the
tissues, towards other body organs
Arterioles- smaller arteries that control blood flow from arteries into capillaries
Capillaries – tiny, permeable (‘leaky’) vessels linking arterioles to veins that allow efficient
exchange of nutrients and wastes between the blood & tissues
...
When several capillaries unite, they
form small veins called venules
...

Basic structure of arteries, arterioles and veins:
- Tough outer layer that resists pressure changes from both within and outside
- Muscle layer- can contract and  control the flow of blood
- Elastic layer- helps maintain blood pressure by stretching and springing back
- Endothelium- thin inner lining that is smooth to prevent friction and allow diffusion
- Lumen – not actually a layer but the central cavity of the blood vessel through which the
blood flows
...
However, each of these vessels have a different relative
proportion of each layer:

Thin inner layer of epithelial
cells called the endothelium

Tunica
interna
Tunica
media
Tunica
externa

Artery structure related to function:
Function of arteries is to transport blood rapidly under high pressure from the heart to
the tissues
...

o Important blood pressure in arteries is kept high if blood is to reach the
extremities of the body
...
It then springs back when the heart relaxes (diastole) (think stretched
elastic band)
...

o Arteries nearer the heart have more elastic tissue giving greater resistance to
the higher blood pressures of the blood leaving the left ventricle
o Arteries further from the heart have more muscle have more muscle to help them
maintain the blood pressure
...

Arterioles structure to related function:
Arterioles carry blood, under lower pressure than arteries from the arteries to capillaries
& control the flow of blood between the two
...
This restricts the flow of blood and so controls
its movement into the capillaries that supply the tissues with blood
...

- Resistance to blood flow is altered by contraction (vasoconstriction) or relaxation
(vasodilation) of the blood vessel walls, especially in the arterioles
...

- Muscle layer is relatively thin compared to arteries – veins carry
blood away from tissues and  their constriction & dilation cannot
control the flow of blood to the tissues
- Elastic layer is relatively thin compared to arteries- low pressure
of blood within the veins will not cause them to burst and pressure
is too low to create a recoil action
- Overall thickness of the wall is small – no needed for a thick
wall as pressure within the veins is too low to create any risk of
bursting
...
When body muscles contract, veins are compressed, pressurising the
blood within them
...

- Larger lumen than arteries to carry slower-flowing blood at a low pressure
...
g
...
Flow of blood in capillaries is slow  allows
for more time for the exchange of materials
...
Allows for rapid diffusion of
materials between the blood and the cells
...

- Lumen is narrow- means that RBC are squeezed flat against the side of a capillary
bringing them even closer to the cells to which they supply oxygen  reduces diffusion
distance (Fick’s law)
- There are spaces between the lining (endothelial) cells that allow WBC to escape in
order to deal with infections within tissues
...
Consequently, as blood flows into capillaries pressure drops and blood
flows more slowly, allowing more efficient exchange of materials across capillary walls
...

NB microscopic blood vessels in some dense organs, such as the liver, are
called sinusoids
...
Instead of the usual endothelial lining,
they are lined with phagocytic cells
...


Capillaries form branching networks where exchanges
between the blood & tissues take place
...
In
most parts of the body, there are 2 types of vessels in a
capillary bed: the true capillaries where exchange takes
place and a vessel called a vascular shunt, which
connects the arteriole and venule at either end of the
bed
...
g
...
When tissue activity increases, the entire
network fills with blood
...

- Tissue fluid supplies all of these substances to the tissues, and in return receives CO2
and other waste materials from the tissues
...

It is the immediate environment of cells and is, in effect, where they live
...
As a result, tissue fluid provides a mostly constant environment for the
cells it surrounds
...

2) This hydrostatic pressure forces water and small molecules (which form the
tissue fluid) out of the blood plasma through the permeable capillary wall when
blood enters a capillary, providing the tissues with nutrients and oxygen
...
When it
falls below the solute potential, fluid begins to drain back into the blood, taking
with it wastes such as urea and CO2
...
This outward
pressure is opposed by 2 other forces:
Hydrostatic pressure of the tissue fluid outside the capillaries prevents outward
movement of liquid
The lower water potential of the blood (due to plasma proteins) pulls water back into the
blood within the capillaries  osmotic pressure

The combined effect of all these forces creates an overall pressure that pushes tissue
fluid out of the capillaries
...
This type of filtration under
pressure is called ultrafiltration
...
Most tissue fluid returns to the blood plasma directly via
the capillaries in this order:
1) Loss of tissue fluid from the capillaries reduces the hydrostatic pressure within
them
2) As a result, by the time the blood has reached the venous end of the capillary
network its hydrostatic pressure is less than that of the tissue fluid outside of it
3)  Tissue fluid is forced back into the capillaries by the higher hydrostatic
pressure outside them
4) In addition, the osmotic forces resulting from the proteins in the blood plasma
pull water back into the capillaries
...

The tissue fluid has lost much of its oxygen and nutrients by diffusion into the cells that it
bathed, but it has gained CO2 and waste materials in return
...

Lymph is similar to tissue fluid but has more lymphocytes
...


Oedema
Oedema= accumulation of fluid within the tissues
Is due to an imbalance of filtration and reabsorption between tissue fluid and blood plasma
...


Why must plants absorb water through roots?
Most plants are terrestrial organisms   need to conserve water, so covered by a
waterproof layer which means they cannot absorb water over the general body surfaceneed to use their special exchange surface in the soil: the root hairs
...


This is what a root looks like:

Endodermis is the ‘inner
skin’- a thin layer of cells
that surrounds the vascular
(conducting) tissue
...

The vascular tissue, xylem and phloem forms a central cylinder through the root and is
surrounded by the pericycle, a ring of cells from which lateral roots arise
...

- To increase the SA for absorption, the epidermal cells grow projections called root
hairs that push their way between the soil particles
...

Plants constantly lose water through transpiration  this is water that must be reabsorbed
from the roots
...
long extensions and occur in thousands on each
of the branches of the root
They have a thin surface layer (the cell-surface membrane & cellulose cell wall) across
which materials can move easily
Root hair do not have a waterproof suberin layer, this coupled with the very thin
cuticle means they present little barrier to water entry
...

In contrast, the root (hair) cells have sugars, amino acids & mineral ions dissolved inside
them so these cells have a much lower water potential
...

Mineral ions dissolved in soil water are absorbed across the membranes of the root hairs by
a combination of facilitated diffusion and active transport
...
The root cell use ATP to secrete
hydrogen ions into the soil- results in a slight –ve charge inside the root cell that aids
absorption of positive ions (e
...
Na+ and K+)
...
in the soil, diffuse back into
the root cell
...


After being absorbed into the root hair cell, water continues its journey across the root in 3
ways:
-

The symplastic pathway – through the cytoplasm of cells
The apoplastic pathway – less resistance to flow of water through this route, about
90% of water moves through this pathway
...

Water uptake is assisted by root pressure, which arises because the soil and root tissue has
a higher water potential than other plant tissues
...

3) This creates a tension that draws water along the cell walls of the cells of the root
cortex
...


The vacuolar pathway:
A small amount of water passes into the cell vacuoles by osmosis
...

Water goes into cell through plasma membrane, into cytoplasm and then into the vacuole
...


Second cell now has a higher WP than its neighbour to the inside
 water moves from 2nd cell to 3rd cell via osmosis along WP
gradient
...


A= Apoplastic pathway
B= Symplastic pathway
C= Vacuolar pathway

Made from a waxy, waterproof
material called suberin
...
The casparian strip (a waterproof
layer) fills all the gaps between the
endodermal cells and prevents the water
from progressing further along the cell wall and
entering the xylem via the apoplast pathway
...
This is significant because, with the symplast
pathway, water and ions must pass through living material and membranes thus the
plant has a degree of control over what passes into the xylem
...
Endodermal cells actively transport salts into the xylem
...
It takes place along carrier proteins in the cellsurface membrane
...


The active transport of mineral ions into the xylem by the endodermal cell creates a
lower WP in the xylem  water moves into xylem now by osmosis along WP gradient 
result of active transport of salts into the xylem from the endodermal cells
...


Evidence for the existence of root pressure due to the active pumping of salts into the xylem:
-

Pressure increases with a rise in temp
...

Metabolic inhibitors such as cyanide prevent most energy release by respiration and also
cause root pressure to cease
A decrease in the availability of oxygen or respiratory substrates causes a reduction in
root pressure
...
g
...
Most of the water a plant absorbs from the soil is lost by evaporation from the leaves
and stem
...

Plants rely on a gradient in WP from the roots to the air to move water through their
cells

Water flows passively from soil to air along a gradient of decreasing water potential
...
A number of
processes contribute to water movement up a plant:
o
o
o

Transpiration pull
Cohesion
Root pressure

Although transpiration seems wasteful, it has benefits:
-

Evaporative water loss cools the plant
The transpiration stream helps the plant to maintain an adequate mineral uptake, as
many essential minerals occur in low concentrations in the soil- not high enough to move
around plant themselves
...
Without
transpiration, water would not be so plentiful and so the transport of materials would not
be as rapid
...
Mesophyll cells now have a lower WP so water
control their rate of
enters by osmosis from neighbouring cells, now neighbouring
transpiration
cells have a lower WP so in turn take water from their
neighbouring cells by osmosis etc
...
Water is pulled through the plant down a decreasing
gradient in water potential
...
Water molecules form hydrogen bonds between one
another and hence cling together as they are pulled through the plant – cohesion
...
This creates one unbroken column of water through the
plant
...
This
facilitates water uptake and movement through the plant
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The transpiration pull puts the xylem under tension, i
...
there is negative pressure within the
xylem (hence the name cohesion-tension theory)
...
The cellulose forms strong fibres, while the lignin
forms an interlocking network in between the cellulose
...


Root pressure: Water entering the stele (central core of the stem & root of a vascular plant
consisting of the vascular tissue [xylem and phloem] and associated supporting tissue) from
the soil creates a root pressure – a weak ‘push’ effect for the water’s upward movement
through the plant
...
Root pressure can force water droplets from some small plants under
certain conditions, but generally it plays a minor part in the ascent of water
...


Figure: Summary of water movement from the soil, through the plant and into the atmosphere

Factors affecting transpiration
Light- Stomata usually open when light (for gases to enter and leave as plant can
photosynthesise)  water moves out of leaf into atmosphere  increase in light intensity
causes an increase in transpiration rate
...
changes affect how much water the air can hold (the WP of air) and
the speed at which molecules move
...
e
...


Humidity- affects the water potential gradient between the air spaces in the leaf and the
atmosphere
...
Low humidity increases transpiration rate
...
More air movement causes an increase in transpiration rate,
less air movement causes a decrease in transpiration rate
...

Some of the physical conditions investigated are:
-

Humidity or vapour pressure (high or low)
Temperature (high or low)
Air movement (still or windy)
Light level (high or low)
Water supply

It is possible to compare the transpiration rate of plants with different adaptations e
...

comparing transpiration rates in plants with rolled leaves vs rates in plants with broad leaves
...
(As xylem is under tension, cutting the shoot in air
would lead to air being drawn into the stem, which would strop transport of water up
the shoot
...
)
2) Potometer filled completely with water, making sure there are no air bubbles
3) Using a rubber tube, leafy shoot is fitted to the photometer under water
4) The photometer is removed from under the water and all joins are sealed with
waterproof jelly
5) An air bubble is introduced into the capillary tube
6) Distance moved by air bubble in a given time is measured a number of times – mean
calculated
7) Using mean value, volume of water lost is calculated
8) Once air bubble nears junction of the reservoir tube and the capillary tube, the tap on
the reservoir is opened and the syringe is pushed down until the bubble is pushed
back to the start of the scale on the capillary tube
...


Experiments like this should be conducted simultaneously using replicate equipment
...


Adaptations of xerophytes
Xerophytes- plants adapted to dry conditions

Adaptation for water conservation

Effect of adaptation

Thick, waxy cuticle to stems and leaves
Reduced no
...
of pores through which water loss can
occur

Stomata sunken in pits, grooves or
depressions
...
Massing of leaves into a rosette
at ground level

Moist air is trapped close to the area of water loss,
reducing the WP gradient and therefore the rate of
water loss & transpiration
...


Leaves reduced to scales, leaves
curled/rolled/folded when flaccid

Reduction in SA from which transpiration can occur
...
This region becomes saturated
with water vapour and so no WP gradient between
inside & outside leaf  reduces transpiration
...
Fleshy or
succulent leaves
...


Deep root system below the water table
Shallow root system absorbing surface
moisture

Roots tap into the lower table
Roots absorb overnight condensation

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