| Gas Exchange Essay, Research Paper
Gas Exchange 3.1 Ø Surface
area to volume ratio Ø Exchange
of gases occurs by diffusion at surface Whereas Ø Production
of wastes and use of resources occurs in the volume Ø Therefore,
as organisms increase in size they have proportionately less surface area compared
to volume Ø Adaptations
? flat, thin , ribbed bodies increase exchange surfaces 3.2 Ø As
organisms get larger ? they must have exchange surfaces within them Ø all
are moist, thin permeable, large surface area Plants Spongy/Palisade Mesophyll Air directly contacts cells Insects Ends of tracheoles Air directly contacts cells Fish Gill Lamellae O2 absorbed by blood pigments then delivered to
cells Mammals Alveoli O2 absorbed by blood pigments then delivered to
cells Ventilation Ø Aim
? maintain concentration gradient Ø Remove
CO2 rich O2 poor air Ø Supply
O2 rich CO2 poor air Ø Move
respiratory medium over exchange surfaceØ Insects
? larger insects make pumping movements of the abdomen, which crushes the air
sacs and helps to move air Ø Fish
? move operculum out, buccal cavity up ? therefore one way flow of water over
the gill lamellae ? counter current flow of water against the direction of
blood flowØ Mammal
? Tidal flow of air ? movement of diaphragm and ribs Control of Breathing Ø Involuntary Ø Respiratory
centre is bundle of nerves in medulla oblongata Ø Impulses
are sent to the diaphragm and external intercostal muscles causing them to
contract Ø As
lungs expand stretch receptors in airway sense and send back infoØ Meeting Demand Ø CO2
levels vary according to exerciseØ As
CO2 goes up ? pH goes down Ø Chemoreceptors
sense this Ø
Receptors in the medulla oblongata Ø
Carotid bodies in the carotid arteries Ø
Aortic bodies in the aortic arch Ø
As chemoreceptors sense increase in CO2 or decrease in
pH, impulses are sent to the respiratory centre, this sends impulses to the
diaphragm and intercostal muscles increase the rate of ventilation. Oxygen/Haemoglobin
Dissociation Curves 3.7 (part)·
Red blood cells contain haemoglobin (Hb) which transports all of the oxygen around your body
and most of the CO2·
Each Hb molecule can carry up to four O2 molecules.However, ·
The relationship between O2? concentration (partial pressure of O2
- p O2 ) and how much is taken up by Hb (% saturation) is not linear, it is ‘S’ shaped (sigmoid) ·
This is because a completely ‘empty’ Hb molecule takes
up the first O2 rather ‘reluctantly’, then takes up the remaining
three rapidly, and finally it is ‘full’ and won’t take up any more. ·
Loading: In
the lungs the pO2 is very
high, so Hb is ‘filled up’ (saturated) with O2 , represented by the
flat ‘top’ of the curve·
Carrying: As
the Hb travels through arteries and arterioles, pO2 drops, but not
enough for the Hb to give up any oxygen, we are still in the flat region at the top.·
Unloading: When
the Hb reaches capillaries which are next to actively respiring cells, pO2 is much lower, due to
oxygen being consumed to make ATP. Here, Hb is ‘emptied’ of its oxygen, which
diffuses to the cells. This is represented by the steep part of the curve in
the middle of the ‘S’.·
The relationship between p O2 and Hb
saturation is not fixed, the shape of the curve alters in response to various
conditions: Condition Effect on curve Overall result Increased pCO2 Shifts to the right
(Bohr shift) At any given pO2, Hb will be less saturated, so oxygen will be
given up more easily Increased temperature Shifts to the right At any given pO2, Hb will be less saturated, so oxygen will be
given up more easily Increased pH (alkaline) Shifts to the left At any given pO2, Hb will be more saturated, so oxygen will be
given up less easily ·
This makes good sense, if cells are actively respiring
they produce CO2, heat up and become more acidic (due to dissolved
CO2, and production of lactic acid), all these things cause the
curve to move to the right, so
oxygen is given up easily. This oxygen is precisely what actively respiring
cells need!·
Other examples: Foetal Hb is to the left of its mother’s (so it can ’steal’
oxygen from her blood via the placenta). Myoglobin, in muscles has a curve to
the left of Hb (it also ’steals’
oxygen from Hb, and retains it as a store and only gives it up at very low pO2).·
Finally, Hb carries CO2 by means of a series
of reactions (catalysed by carbonic anhydrase) which result in the production
of hydrogen ions and hydrogencarbonate ions. The hydrogen ions are taken up by
Hb, meaning that Hb acts as a buffer,
absorbing excess acid. The hydrogencarbonate diffuses into the plasma, in
exchange for chloride ions (the chloride shift).CARDIAC CYCLE AND ITS CONTROL·
The heart muscle is?
myogenic ( contracts without stimulation) ·
The sino-atrial node coordinates the heart beat so that
the muscle cells contract together. ·
The SAN is in the right atrium next to the vena cava ·
Specialised muscle (Purkinje) fibres radiate out from
the node and cause atrial contracton (systole) ·
These stimulate the AVN, on the septum at the junction
of the atria & ventricles ·
The AVN causes a time delay which ensures the ventricle
contracts after the atria ·
The bundle of His (Purkinje fibres) pass down the
septum to the apex of the ventricles ·
These first cause contraction of the papillary muscles
which tension the cuspid valves ·
Ventricular systole radiates upwards from the apex ·
Once the electrical stimulation has died away the heart
chambers relax (diastole)CONTROL OF HEART RATE·
The SAN sets a resting heart rate ·
Blood O2 & CO2 levels are
detected by chemoreceptors of the Aortic & Carotid bodies ·
These send nerve impulses to the cardiovascular centre
of the medulla ·
The medulla has chemoreceptors which also detect CO2 ·
If CO2 drops the CV centre sends nerve
impulses along parasympathetic nerves to the SAN, which reduces heart rate
(vagus nerve) ·
If CO2 goes up the CV centre sends nerve
impulses along sympathetic nerves to the SAN, which increases heart rate
(accelerator nerve) ·
Adrenaline can also act directly on the SAN, mirroring
the effect of sympathetic nerves ·
CO2? dissolves
in water to release hydrogen ions which decrease the pH and increase the
acidity ·
The heart rate is controlled so that the demands of the
body are met with the minimum cardiac output.?PRESSURE &
VOLUME CHANGES & VALVES·
Valves stop the backflow of blood within the heart and
as blood exits the heart ·
Muscular contraction (systole) causes an increase in
hydrosatic pressure in the heart. ·
When the valves open the volume of the heart chamber
decreases ·
Blood always attempts to flow from high to low pressure
unless valves stop it ·
Valves open or
close when pressure lines cross (on graph) ·
The heart empties from the bottom up. ELECTRICAL ACTIVITY·
P is the trace produced by stimulation of atrial
systole ·
QRS is the trace produced by venricular systole CIRCULATION AND BLOOD VESSELS·
Blood leaves the heart in spurts when the ventricle
contacts ·
In arteries, this is first pushed along by elasticity
and then by a peristaltic pulse ·
In the tissue capillaries this is smoothed out to a
constant flow by the arterioles, in the lungs, the blood continues to flow in
pulses ·
Throughout circulation there is a pressure drop ·
Fluid leaves the arterioles and bathes the tissues,
because the hydrostatic pressure outwards exceeds the difference in water
potential (osmotic pressure) ·
Most is drawn back into the venules by the solute
potential of the blood proteins (osmotic potential), some returns via the lymph ·
Blood flow is the fastest where the total cross
sectional area is least. ·
The same volume of blood must enter and leave the heart
per minute but the pressure is different Digestion q
Mammals have a gut to digest then absorb food q
The generalised structure of the mammalian gut wall q
Epithelium q
Lumen?? q
Muscle layers?? q
How different parts of alimentary canal are adapted for
their functionq
Movement of food through peristalsis q
How ??q
Sites of production and action of Amylases? Mouth Starch to Maltose Endopeptidases Stomach/Pancreas ? Pepsin/Trypsin Polypeptides into smaller chains Exopeptidases Pancreas and intracellular (small intestine epithelial
cells) Cuts di and tripeptides into individual amino acids Lipase Pancreas Fats into monoglycerides and fatty acids Maltase intracellular (small intestine epithelial cells) Breaks maltose into glucose Bile Liver Not an enzyme ? emulsifies fats into smaller droplets q
Mechanisms for absorption in the ileumq
Structure of a liver lobuleq
Control of Digestive Secretions q
Nervous ? sight smell q
Hormonal Gastrin Presence of food in the stomach Stomach secretes pepsin and hydrochloric acid ? begins
muscular movement of stomach Cholecystokinin Presence of acidified chyme in duodenum causes cells in
the mucosa of duodenum to secrete hormone into bloodstream Pancreas secretes enzymes ? Gall bladder secretes bile Secretin Presence of acidified chyme in duodenum causes cells in
the mucosa of duodenum to secrete hormone into bloodstream Effects liver (bile) and Pancreas ? fluid ? non ? enzymic components of pancreatic
juice q
Liver q
Blood sugar q
Glycogenesis making glycogen from glucose q
Glycogenolysis breaking up glycogen into glucose q
Gluconeogenesis ? making glucose from non-carbohydrate
sources (fats and proteins) q
Roles of insulin (going down) and glucagon (going up) in controlling blood sugar levelsq Transamination ? changing one amino acid into another
? not possible to synthesis essential amino acids (must be obtained from diet3.8????????? Excretion and OsmoregulationMost
questions in the exam ask about some, or all, of the following: ·
The kidney, specifically:·
which substances
move,·
in which direction
and why, ·
how this is controlled.·
What other
animals do, particularly single-celled?
animals, fish (which both excrete ammonia
directly into water) and insects (which excrete solid uric acid) and why. ·
Deamination and the ornithine cycle (learn and
regurgitate!). The Kidney ·
Everything the kidney does is done in the nephrons (about a million per kidney) ·
First, the blood is filtered
at the glomerulus. All the components of the blood are squeezed through the
filter into Bowman’s capsule, except proteins
and cells. Reabsorption·
Glucose, amino acids and mineral ions are actively
reabsorbed into the blood in the proximal
convoluted tubule.·
Water, by osmosis is also reabsorbed to balance the
concentration.·
Varying amounts of salts and water are reabsorbed from
the distal convoluted tubule.·
Varying amounts of water are reabsorbed from the collecting ducts. ·
Some poisonous substances are secreted, actively, into
the proximal convoluted tubule.Generating
Concentrated Urine· ascending limb impermeable to water but actively pumps out sodium chloride
(salt) so the fluid in the ascending limb gets more and more dilute.·
tissue fluid
surrounding loop has sodium chloride pumped into it from ascending loop and
therefore becomes more concentrated.· descending limb loses water to the
surrounding tissue fluid, passively,
by osmosis, but is impermeable to sodium chloride, so salt doesn’t follow.·
The high sodium chloride concentration in the tissue
fluid around the loop draws water out of the nearby collecting duct, by osmosis.Antidiuretic
Hormone (ADH) controls the volume and water potential of the blood·
Osmoreceptors in hypothalamus are sensitive to water
potential of the blood· Drop
in water potential (more concentrated) results in release of ADH from pituitary gland·
ADH causes the normally impermeable collecting duct and distal
tubule walls to become more permeable resulting in more water being
reabsorbed into the blood and the urine becoming more concentrated and of a
smaller volume Aldosterone controls
the volume and sodium (Na+) content
of the blood ·
Drop in blood volume detected by cells in the kidney
(juxtaglomerular cells), which is generally associated with low blood Na+. ·
A complex chain of events causes aldosterone to be released from the cortex of the adrenal gland. ·
Aldosterone causes the distal tubule to reabsorb more
Na+ , which increases blood Na+ and volume.·
Finally, the kidney helps to control blood pH, by secreting excess acid or alkali into the
distal convoluted tubule (so the pH of urine can vary, but blood pH remains the
same).Revision notes on Xylem and Phloem (3.7 ?
part)Stem structure A
transverse section of a stem shows that the vascular tissues occur in bundles
at regular intervals around the outer part of the stem. The centre of a stem is
filled with pith. The outermost layer of the stem is waterproof with lenticels
for gas exchange. Each bundle consists
of phloem on the outside and xylem on the inside with the cambium in
between.? There may also be sclerenchyma
fibres exterior to the phloem to give extra strength.? The cambium is meristematic producing new xylem and phloem as the
stem increases in girth. At the nodes of the stem branches in the vascular
bundles occur so that the vascular bundles enter the petioles of leaves as well
as continuing up the stem. In woody plants the vascular tissue forms a complete ring
around the stem and the centre of the stem becomes filled with xylem (wood) as
the plant gets bigger. Xylem structureXylem consists of xylem
vessels and tracheids as well as parenchyma tissue.? The vessels are made from columns of cells in which the end walls
have broken down to leave a long tube.?
These cells die as they become specialised because their walls become
impregnated with lignin which is not permeable. The net result is a tube of
xylem elements in which there is no cytoplasm.?
Xylem vessels remain in contact via pits in their lateral (side)
walls.? Tracheids are also dead but each
tracheid has a pointed end and overlaps the ones above and below, the tracheids
also have connections via pits. Between the vessels and tracheids is xylem
parenchyma. Xylem functionXylem carries water and
ions from the roots to the stem, leaves, flowers and fruits. Water travels upwards in
the xylem because of the transpiration pull caused by evaporation of water from
the cells of the leaf followed by diffusion of water vapour through the stomata
i.e. transpiration (also get some transpiration through the cuticle). The continuous column of water in the xylem does not
separate due to forces of cohesion between the water molecules.? These forces are made possible because water
is a polar molecule and water molecule have hydrogen bonds between them and
they also adhere to the walls of the xylem vessel.? This is known as the COHESION TENSION THEORY of water movement. Transport in the xylem is an example
of MASS FLOW. Because
the cytoplasm has gone from the xylem and the end walls of the vessels have
disintegrated then there is no barrier to the flow of water up the xylem.? Water can leave the xylem through the pits
to move into adjacent tissues. Ions absorbed in the roots also move upward in
the xylem dissolved in the water Water
enters the xylem after it has been absorbed and has travelled across the root
to the central vascular bundle of the root.?
Capillarity and Root pressure also play a part in water movement in
plant but neither can explain how water can travel to the top of trees. EvidenceEvidence
for the cohesion tension theory of water movement comes from the fact that
water in the xylem is under tension so air enters the xylem if the xylem is
damaged and by the variation in the girth of trees at different times of the
day.? Water can be shown to move up the
xylem by allowing a stem to take up dye.?
Movement of water in the xylem is entirely passive (it continues if the
plant is poisoned so that it cannot make ATP), that means that no chemical
energy is expended in water movement through the xylem.Phloem StructureThe
phloem in a plant forms only a very this layer about the same thickness as a
piece of paper. Phloem tissue consists of sieve tubes, companions cells and
phloem parenchyma.? All phloem tissue is
living (unlike xylem) although the cytoplasm of the sieve tubes is highly
specialised and has a reduced number of cell organelles. The sieve tubes
consist of a column of cells formed end to end.? Between each cell the cell wall has a number of holes so that it
has the appearance of a sieve and this is known as the sieve plate.? The cytoplasm of the sieve tubes is modified
and contains no mitochondria.? Adjacent
to each sieve tube is a companion cell which has a very dense cytoplasm and
which supplies energy for the sieve tubes. The sieve tubes carry
sugar up and down the plant.? They are
loaded with sugars in the leaves and then the sugar moves in solution either up
or down the plant to where it is needed.Theories of Phloem Transport1. Pressure flow 2. Cytoplasmic streaming 3. Electro-osmotic flowNo one theory provides a totally satisfactory explanation to
flow. The most accepted theory is the pressure flow theory that
states that sugars are loaded into the phloem in an area of high concentration,
the source, and are then transported by mass flow to an area of low
concentration, the sink, where they are unloaded.? This theory allows for substances to move both up and down the
plant.? Movement of substances in the
phloem is an active process requiring ATP.Evidence for1.
The catchword of the phloem have a positive pressure- they
exude fluid when cut and aphid stylets exude fluid when they penetrate the
phloem. 2.
Experiments have shown a concentration in the phloem catchword
with the highest concentration near the source-analysis of exudates from aphid
stylets 3.
A physical model of this theory functions 4.
Viruses can be moved in the phloem.? This must be mass flow as they are nor in solution and are
therefore not able to move by diffusion.Evidence against1.Sugars
and amino acids have been found to move in different directions in the same
vascular bundle. 2.
Phloem transport may not occur in the direction of the deepest sink. 3.
The sieve plate is an impediment to mass flowExperiments used to investigate mass flowRadioactive tracers.? These
are introduced via radioactive carbon dioxide and photosynthesis and the path
traced by autoradiography.Ringing experiments.?
The
phloem is removed in a ring around the stem and this stops flow in the phloem. ?Shows that sugars, amino acids and salts are
transported in the phloem.Use of Aphids for sampling3.6????????? Exchange of Water and Ions in PlantsMost
questions in the exam ask about some, or all, of the following: ·
Root structure and function (particularly mineral
absorption)·
Stomata and transpiration (and factors affecting
transpiration)·
Features of xerophytes (plants that live in very dry
conditions)Root
structure and function:·
Root structure ? learn
the typical layout of tissues in roots (Support Booklet p.20) and how it
differs from stems.·
Root function:·
Water and minerals are absorbed through root hairs and pass
through the cells of the cortex. ·
These substances can move through the porous cell walls in
the cortex, rather like water soaking through paper, this is called the apoplast
pathway. ·
Water and minerals can also pass through the living part of
these cells (cell membrane, cytoplasm etc.) ? the symplast
pathway. ·
The cells of the cortex also contain large vacuoles, and
substances can pass through these (as well as the cytoplasm etc.) ? the vacuolar
pathway. ·
Between the cells of the cortex and the xylem and phloem is
a layer of cells called the endodermis. These cells have a special waterproof
layer in part of their cell walls, forming the Casparian strip.
This forces water and minerals to
take the symplast pathway through the endodermis.·
Because all cell membranes are selectively permeable, this
allows the cells of the endodermis to control
the amount of each mineral taken into the xylem: Substance Method of transport across endodermis Reason Water Osmosis Water is drawn up xylem in transpiration stream (see 3.7) Minerals at a higher
concentration in soil than plant cells Facilitated diffusion These can flow down
their concentration gradient into the plant Minerals at a lower concentration
in soil than plant cells Active transport (requires ATP) These must be moved against
their concentration gradient into the plant Toxins Transport blocked or inhibited Mechanism unknown (Water and minerals then pass
up the stem in the xylem – see 3.7 ? and enter the leaves)Stomata
and transpiration·
99% of the water that goes up the xylem evaporates into air
spaces in the leaves, and diffuses out through the stomata as
water vapour, this is transpiration. ·
Anything that affects the concentration gradient of water vapour from plant to air will
therefore affect the rate of transpiration: Factor Effect on rate of transpiration Reason Increased light intensity Increases Stomata open wider in light (see below) Increased humidity Decreases Decreased concentration gradient (humid air around leaves) Increased air movement Increases Increased concentration gradient (humid air around leaves
blown away) Increased temperature Increases More rapid evaporation from leaves Dry soil around roots or high salt
concentration (e.g. sea water) Decreases Decreased uptake of water into roots, therefore less
available in leaves (The rate of transpiration can be measured with a potometer).·
Clearly, stomata are very important in transpiration, as
most of the water vapour passes through them. They usually open in the light and close
in the dark; they also close when water supply to the roots is very poor.·
Stomatal opening is controlled by the two guard cells
which surround each stoma. The cell wall on the inner surface is much thicker
than on the outer surface. As these cells become turgid (swell) they bend outwards,
causing the stoma to open (you can demonstrate this by sticking sellotape on
one side of a sausage-shaped balloon then blowing it up, it bends away from the sellotape).·
There are two hypotheses to explain how guard cells change their shape:·
The potassium
movement hypothesis states that potassium ions (K+) are
pumped into the guard cells, by active
transport. This lowers their water potential, water flows in by osmosis,
the guard cells become turgid and stomata open. The reverse process closes
stomata. This hypothesis is the most widely accepted. ·
The starch-sugar
hypothesis states that there is a balance between sugars (soluble) and starch
(insoluble) controlled by two enzymes with different optimum pH’s. The enzyme
which converts starch into sugar has a high
optimum pH (alkaline), which is produced in the day, because acidic CO2
is used up in photosynthesis. Therefore, sugar accumulates, water potential
drops, water enters, cells become turgid, stomata open. The enzyme which
converts sugar to starch has a low
optimum pH (acidic), which is produced at night, because CO2 is
produced by respiration (no photosynthesis). Starch accumulates, but because
starch is insoluble water potential rises, water leaves, guard cells lose
turgidity, stomata close. This hypothesis is not widely accepted.Xerophytes·
These are plants that are adapted to live in very dry
conditions by having some, or all, of the following features:·
A very thick, waxy cuticle to reduce evaporation of water
through this part of the leaf (cuticular transpiration). ·
Stomata sunk into pits, which trap a layer of humid
around them, so reducing transpiration. ·
Hairs
around stomata, again trapping a layer of humid air. ·
Few, small leaves; often rolled into a tube. This reduces surface area for
transpiration, and humid air is also trapped inside the inrolled leaf. ·
Closing
stomata in the day, when it is hot, and opening them at night,
reducing evaporation. (such plants take in CO2 at night, store
it? as an organic acid and then break
the acid down in the day to release the CO2, internally, for
photosynthesis. This is called CAM
photosynthesis). ·
Storage
of water in thick stems and leaves (these plants are called succulents). ·
Deep, tap roots to draw up water from deep soil layers. ·
Roots very close to the soil surface, to absorb
condensation which forms at night.
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