Gas Exchange Essay Research Paper Gas Exchange

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 &


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


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


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 contents 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 contents

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.?


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


Stomata and transpiration (and factors affecting


Features of xerophytes (plants that live in very dry


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. ·


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. ·


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). ·


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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