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School biology revision notes: Organs and their exchange surface structure

Examples of surfaces for the exchange of substances in animal organisms transport systems

For plants see Transport in plants notes

BUT first read Diffusion, osmosis and active transport and then this page

 Doc Brown's school biology revision notes: GCSE biology, IGCSE  biology, O level biology,  ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old students of biology

 This page will help you answer questions such as ...  Why do exchanges surfaces need to be large?  In what way are organs designed to have large surface areas?

Sub-index for this page

1. A mathematical 'extra' on surface area to volume ratio calculations and implications for exchange surfaces in living organisms

2. Introduction to exchange surfaces in cells and organs

3. Gas exchange in the lungs by diffusion - comment on COPD and ventilators

4. Gas exchange and the structure of fish gills

5. The function of villi in the exchange surface of the small intestine

6. Exchanges surface structure adaptations in other animals

See also The human circulatory system - heart, lungs, blood, blood vessels

and for plants see Transport in plants - gas exchanges, root absorption etc.



1. First - a mathematical 'extra' on surface area to volume ratio calculations and implications for exchange surfaces in living organisms

The greater the surface area the greater the possible rate of material transfer.

The most compact shape to give the lowest surface area/volume ratio is a sphere, but that's not very practical for the working of many specialised cells, tissues or organs - but very good for single-celled organisms!

However, systems in living organisms that involve transfer of substances, do need as large a surface area as possible within the volume the 'system' occupies.

To this end, many organs have evolved to give the maximum surface area as possible within the volume the 'system' occupies.

A bit of area/volume maths to illustrate this idea with cubes of various sizes (6 faces):

A 1 cm cube has a volume of 1 cm3 (1 x 1 x 1), a total surface are of 6 x 1 x 1 = 6 cm2

So the surface area / volume ratio = 6 / 1 = 6.0 cm-1  (6 : 1 ratio)

A 2 cm cube has a volume of 8 cm3 (2 x 2 x 2), a total surface are of 6 x 2 x 2 = 24 cm2

So the surface area / volume ratio = 24 / 8 = 3.0 cm-1  (3 : 1 ratio)

A 3 cm cube has a volume of 27 cm3 (3 x 3 x 3), a total surface are of 6 x 3 x 3 = 54 cm2

So the surface area / volume ratio = 54 / 27 = 2.0 cm-1  (2 : 1 ratio)

A 4 cm cube has a volume of 64 cm3 (4 x 4 x 4), a total surface area of 6 x 4 x 4 = 96 cm2

So the surface area / volume ratio = 96 / 54 = 1.5 cm-1  (1.5 : 1 ratio)

You can see clearly that the smaller (thinner etc.) the 'system' or parts of the 'system' the greater the surface to volume ratio - potentially increasing the rate of transfer of substances.

Good examples of this are the millions of tiny air sacs (alveoli) in the lungs and the thin multi-layered sections of gills in fishes - both of which are to do with animal respiration.

Another good example is the fine and numerous villi in the intestine where their large surface area is very efficient for absorbing nutrients from absorbed food.

The villi can be envisaged as tall thin rectangular blocks in shape to maximise surface area.

An extra calculation based on a volume of 8 units. to make the point about villi.

A 2 x 2 x 2 block has a surface to volume ratio of 3 : 1 (see above).

A 1 x 2 x 4 block has a surface area to volume ratio of 3.5 : 1 (see adaptations)

0.1 x 0.1 x 800 block has a surface area of (2 x 0.12) + (4 x 0.1 x 800) = 0.02 + 320 = 320.02 = ~320 (3 sf)

This gives a surface to volume ratio of 320 / 8 = 40 : 1, much higher than the blocks above, over 10 x higher in fact.

Just think about the very fine capillaries in the blood system too.

 

In any surface area : volume calculations, make sure all measurements and calculations are quoted with the same length units!

 

The implications of these calculations for transfer of substances

This is the mathematics behind why for small cells in single or multicellular organisms, the transfer of nutrients, oxygen and waste products, diffusion rates are high - substances can be moved quickly in and out of cells.

As the volume of a cell increases, the distance from the outer cell membrane through the cytoplasm to the centre of the cell increases.

This slows down the rate of exchange of substances in or out of the cell from or to the environment.

Cells larger than 1 mm in diameter may not be viable because the rate of diffusion is too slow to supply nutrients and oxygen sustain the cell's life-supporting biochemistry.

Multicellular organisms, with many layers of cells, tend to have a smaller surface to volume ratio and therefore need specialised organ systems with large surface areas for the efficient transfer of substances and also thermal energy to avoid heating.

Because multicellular organisms have many layers of cells, this increases the time needed for nutrients and oxygen to diffuse in and reach the inner cells.

Therefore the cells of the outer layers would tend to use up the resources first and faster, depriving inner cells life-supporting resources.

Therefore adaptations have evolved to enable complex multicellular organisms to overcome this problem.

Examples of surface : volume ratio in various organisms

(based on the same length units)

Single cell bacterium 6 x 106 : 1;  single celled amoeba6 x 104 : 1;   fly 6 x 102 : 1dog 6 : 1whale 0.06 : 1

You can see there is quite a contrast between microscopic single celled and large multicellular organisms!

 

This mathematical 'extra' was 'adapted' from the page on Structural adaptations of plants and animals

and is also appropriate to various points in Diffusion, osmosis and active transport


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2. Introduction to exchange surfaces in cells and organs

Living organisms must be to exchange substances with their surroundings in order to survive - grow, mature and reproduce.

The size (volume) of an organism or a specific organ, and its surface area, greatly affects how efficient this exchange process is. The rate of transfer is often governed by the surface area : volume ratio.

Diffusion is used by cells to take in useful substances and remove waste products.

Why do we need exchange surfaces?

Exchange or transfer of substance usually involves diffusion through a membrane (permeable, partially permeable), water movement by osmosis and also active transport e.g. the transfer needs of organisms include:

(i) Useful nutrient substances e.g. food molecules from digestion like amino acids and sugars, mineral ions, water taken up by  cells by osmosis,

(ii) Removal of waste products e.g. carbon dioxide from respiration, urea (poisonous) from breakdown of proteins in animals - diffuses from cells into blood plasma and transferred to be absorbed by the kidneys prior to excretion.

(iii) Gas exchange usually involves taking oxygen into cells for aerobic respiration and passing out carbon dioxide to the environment.

Know and understand that many organ systems are specialised for exchanging materials.

The ease with which an organism can exchange substances with the environment depends on the organisms surface area to volume ratio AND you can extend this idea to an organ itself e.g. the lungs.

In single-celled microorganisms gases and dissolved substances can often diffuse directly into and out of the cell through the cell membrane.

This is very efficient because a single cell has a large surface area to volume ratio membrane - large surface area relative to the volume of the cell.

Therefore the single-celled organism has no trouble in exchanging sufficient materials with its environment.

Know that the size and complexity of an organism increases the difficulty of exchanging materials.

One reason for this increased difficulty in exchanging materials is that the distance from the exchange surface is getting further away from where the nutrients and oxygen are needed and the waste to be removed.

Know that gas and solute exchange surfaces in humans and other multi-cellular organisms are adapted to maximise effectiveness - they don't have the obvious surface/volume ratio single-celled organisms have.

Multicellular organisms have a smaller surface area to volume ratios compared to a single celled organism.

This surface area is NOT sufficient to provide efficient rates of diffusion of substances in and out of the organism without significant adaptation through evolution - some examples are described and explained on this page.

It is essential that the transfer processes of moving sugars, amino acids, oxygen etc. into cells and the removal of waste products, can happen as efficiently as possible.

Therefore exchange surfaces have evolved to maximise the rate of transfer of wanted substances into, and unwanted chemicals out of, multicellular organisms.

To increase and maximise the efficiency of transfer the exchange system needs to have/be ...

(i) a large surface area to increase diffusion rate eg alveoli in lungs, villi in intestine,

(ii) thin permeable cell membranes are usually quite thin to provide a short diffusion distance (part of thin layers of cell tissue, so diffusion distance and times are short over a wide area),

(iii) a moist exchange surface - gases can dissolve into and diffuse through.

Animals have lots of thin blood vessels to bring in essential nutrient molecules and ions for life and carry waste molecules away e.g. the thin bronchiole tubes in the lungs,

Thin capillaries which have a particularly large surface to volume ratio - this allows fast diffusion in either direction,

Animals need an efficient gaseous exchange ventilation system to take in air for oxygen and give out air including waste carbon dioxide,

in the lungs the tiny pockets called alveoli greatly increase the gas exchange surface area : volume ratio.

Substance exchange problems for multicellular organisms

The larger a multicellular organism, the more difficult it is to exchange substances.

Cells deep in the body are some distance to the surrounding environment - air or water.

Larger organisms have low surface to volume ratio reducing exchange efficiency.

Therefore, through evolution, instead of exchange through an outer membrane ('skin') multicellular organisms have developed specialised exchange organs including an equally specialised exchange surface.

BUT, specialised organs are not enough on their own to serve a relatively large body, you also need specialised transport systems to convey substances to and from the body cells e.g. to provide nutrients or remove waste products.

In animals the transport system is the circulatory system - blood vessels etc.

and also gaseous exchange in lungs, the lengthy digestive system and the excretory system - and all systems must work in harmony with each other!

See The human circulatory system - heart, lungs, blood and blood vessels

In plants, transport is effected through the xylem and phloem vessels.

See Transport and gas exchange in plants, transpiration, absorption of nutrients etc.


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Examples of exchange systems are now described in detail with diagrams.


3. Gas exchange in the lungs by diffusion - plus comments on COPD and ventilators

The lungs are the means of transferring oxygen from air to the blood stream (blood plasma) and to remove the waste gas carbon dioxide.

Know and understand that in humans:

The surface area of the lungs is greatly increased by the alveoli - millions of tiny air sacs of the end of the tiny bronchiole tubes in the lungs where the gas exchange by diffusion takes place, but some basic stuff before we reach the alveoli.

Know and understand that the lungs are in the upper part of the body (thorax), protected by the ribcage and separated from the lower part of the body (abdomen) by the diaphragm.

You should be able to recognise these structures of the lungs in the diagram on the right - the lungs are in the thorax.

The ribcage physically protects the lungs from being easily crushed and damaged.

In between the ribs are the intercostal muscles which help to move air in an out of the lungs (ventilate).

To increase the efficiency of gas exchange in the lungs the bronchus divides in two (the bronchi), so each lung gets a good supply of air.

Each bronchus divides and divides into many bronchioles with a tiny sac at the end of each one - the alveoli - tiny sacs that considerably increases the area for oxygen and carbon dioxide gas exchange.

The 'ventilation' pathway for air (including oxygen) is:

inhaled air ==> trachea ==> bronchus ==> bronchiole ==> alveoli ==> individual alveolus (air sac)

The mouth and nasal passages filter, warm and moisten the inhaled air before finally reaching .

Know and understand that the breathing system takes air into and out of the body so that oxygen from the air can diffuse into the bloodstream for respiration, and waste carbon dioxide from respiration, can diffuse out of the bloodstream into the air.

This gas exchange happens in the lungs which has millions of tiny air sacs called alveoli at the ends of the finest bronchiole tubes - a large surface area for gas exchange.

Surrounding the alveoli are many small arteries (fine capillaries) bringing a good supply of 'dark red' deoxygenated blood to the lungs - the thin walls of the fine capillaries of the small arteries mean a short distance to enable faster diffusion rates for the gases and they form a large surface area for gas exchange.

The gas exchange occurs on the specialised moist thin membrane surfaces of the alveoli and the fine blood vessels - the moisture in the membranes is good for dissolving gases and increases the rate of gaseous diffusion.

When the blood from the rest of the body arrives at the alveoli in the lungs it contains a relatively high concentration of carbon dioxide and low concentration of oxygen.

This maximises the diffusion concentration gradients for the gas exchange i.e. the blood to absorb fresh oxygen from the alveoli and the expulsion of carbon dioxide from the blood in breathing out.

Direction of diffusion gradients - from high to low concentration:

When air enters the alveoli it has a greater concentration than the deoxygenated blood.

The steep concentration gradient produces very efficient diffusion of oxygen into the blood.

O2 air in lungs ==> alveoli ==> blood, favours oxygen transfer by diffusion through the alveoli membranes

Deoxygenated blood has a greater concentration of carbon dioxide than the external air, so it will diffuse out of the blood.

CO2 blood ==> alveoli ==> air in lungs, favours carbon dioxide transfer by diffusion through alveoli membranes

Therefore the oxygen diffuses out of the air into the blood capillaries of the alveoli (from high to low concentration) and carbon dioxide diffuses out in the opposite direction from the blood to the air in the lungs (again, from high to low concentration).

So, oxygen, from breathing in, is transferred from the air in the alveoli into the fine veins which carry the 'bright red' oxygenated blood away to where it is needed in the rest of the body. Simultaneously carbon dioxide diffuses in the opposite direction, from the deoxygenated blood into the alveoli and breathed out.

The alveoli are well designed by evolution to perform this gas exchange efficiently - refer to repeated diagrams above.

Alveoli are very efficient exchange surfaces and the adaptations to increase the rate of transfer of gas molecules are:

(i) The alveoli have a huge surface area because of their tiny spherical sac like structure,

 (smaller spheres have a larger surface area : volume ratio than larger spheres. For a given radius: surface area / volume = 3 / r. For more on this see adaptations page.)

(ii) The sac walls are very thin, only one cell thick, to reduce diffusion distance and hence reduce diffusion time - giving a faster rate of gas exchange,

(iii) The cell membrane lining is moist to dissolve gases which can diffuse down their concentration gradients across the exchange surface.

(iv) The alveoli have an excellent blood supply from numerous tiny blood vessels - vein and artery capillaries. Each alveolus is surrounded by blood capillaries that ensure efficient transfer and the gas exchange can function down the steepest concentration gradients.

 

Know and understand that to make air move into the lungs the ribcage moves out and up and the diaphragm becomes flatter.

Know these changes are reversed to make air move out of the lungs.

Know the movement of air into and out of the lungs is known as ventilation.

You should be able to describe the mechanism by which ventilation takes place, including the relaxation and contraction of muscles leading to changes in pressure in the thorax.

As you breathe in, the intercostal muscles contract expanding the rib cage, and the diaphragm also contracts making it flatter, both of which increase the volume of the thorax.

This has the effect of decreasing the pressure in the lungs and allowing air to be easily drawn in, the air will flow in naturally, due to the pressure difference between the air in the lungs (lower pressure) and the 'outside' air (higher pressure).

In breathing out, the intercostal muscles relax (ribcage contracts), the diaphragm relaxes and moves up, so the combined effect is to increase the air pressure in the lungs and air is expelled.

 

Chronic obstructive pulmonary disease (COPD)

Chronic obstructive pulmonary disease (COPD) is the name for a group of lung conditions that cause breathing difficulties.

They include emphysema damage to the air sacs in the lungs and chronic bronchitis long-term inflammation of the airways

COPD is a common condition that mainly affects middle-aged or older adults who smoke.

The problems are often caused by long-term exposure to irritants (particles in tobacco smoke, any kind of fine dust e.g. mineral or coal, atmospheric particles from vehicle exhaust) which damage and destroy the walls of the alveoli.

This means the gas exchange in the lungs (O2 <=> CO2) is not as efficient as it needs to be.

For COPD sufferers, the breathing problems tend to get gradually worse over time and can limit your normal activities (readily become 'out of breath'), although medication treatment can help keep the condition under control.

Pandemic footnote in June 2020

As I'm adding this section on COPD, we are in the middle of the Covid-19 coronavirus pandemic.

In the more serious cases, people are suffering from breathing problems due to the virus causing inflammation in the lungs - and this is where artificial ventilation systems using oxygen are used to save lives.

 

Artificial ventilators move air into and out of a persons lungs, where they cannot work unaided. This may be because some injury or medical condition or undergoing an operation, which prevents them from breathing normally.

This used to be done by a large 'capsule' called an 'iron lung' which encased the whole body of the patient except for the head.

The pressure in the capsule is mechanically lowered to allow the lungs to expand and take in air and then raised to make the lungs contract and expel air.

However the blood flow in the lower part of the body can be poor and giving rise to poor circulation side effects.

Modern ventilators work by pumping air in a go/stop cycle, using a mouth piece connection, directly into the lungs to expand them and push out the ribcage.

When the pump temporarily stops, the ribcage relaxes, contracting the lungs and expelling the air.

This is a much more convenient method with a wide range of applications, and, it doesn't interfere with the body's blood supply, but there can be problems if the alveoli (may burst) can't cope with the artificially increased air supply.

 

See also

The human circulatory system - heart, lungs, blood, blood vessels, causes/treatment of cardiovascular disease

 

Possible practical work

You can use sensors, eg spirometers, to measure air flow and lung volume 


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4. Gas exchange and the structure of fish gills

gcse biology single circulatory system of a fish heart gill capillaries body capillaries blood circulation Fish have a single circulatory system in which deoxygenated blood from the fish's body is pumped to the heart, which then pumps it through the gills to absorb oxygen from the water and round through the rest of the body in one continuous loop - just one circuit in operation (unlike the double circulatory system of mammals).

Fish have very thin gills covered in protective muscular flaps. Water is continuously through the mouth and forced over the gill surfaces and ejected out through the flaps.

Gills are the gas exchange system in fishes and the structure provides a large surface area for oxygen to be absorbed into the blood stream and waste carbon dioxide passed out.

Water, containing dissolved oxygen, enters the fish through its mouth and passes out through the gills facilitating gas exchange.

In the gills, oxygen diffuses from the water to the blood, simultaneously, carbon dioxide diffuses from the blood into the water.

To make the gas exchange process as efficient as possible, the surface area of the gills is greatly increased by the presence of lots of thin plates called gill filaments - diagram on the right.

The surface area is increased even more by lots of tiny thin tissues called lamellae (plural of lamella).

The lamellae of lots of blood capillaries, increasing the contact area to speed up the diffusion of gases - oxygen or carbon dioxide.

The lamellae also have a thin layer of surface cells to minimise the gas diffusion distance and shorten diffusion times.

The blood flows through the lamellae in one direction and water flows over them in the other direction and this produces a continuous high concentration gradient between the blood and water.

The concentration of oxygen in the water is always higher than its concentration in the blood so maintaining a good supply of oxygen to the blood by diffusion from the water.

I presume the concentration of carbon dioxide is higher in the blood than in the water, so the waste gas is continually diffusing out of the blood?


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5. The function of villi in the exchange surface of the small intestine

The small intestine is about 7 m long, and is where dissolved digested food particles are absorbed from the digestive system into the bloodstream to supply the cells with the necessary nutrients.

The long length and large surface gives plenty of time for the soluble food molecules to absorbed into the bloodstream as the food moves slowly along. It takes at least 6-8 hours to travel through the small intestine.

The transfer through the partially permeable membrane might be by 'natural' diffusion down a diffusion gradient or by active transport against a diffusion gradient.

The partially permeable membrane regulates the transfer of substances.

The efficiency of the process is considerably increased by the structure of the small intestine  - adaptations:

(i) a single layer of surface cells - short diffusion time and distance - fast diffusion through the permeable membrane,

(ii) long length - increase contact time for breakdown and absorption of food molecules.

(iii) a large surface area for absorption - result of many small projections called villi which have microvilli to increase the surface area even more,

(iv) and a good blood supply from numerous capillaries that transport the nutrients away efficiently and maintain the concentration gradient in the direction of absorption.

all of which speed up the process, so read on for the detail ...

Know and understand that the villi in the small intestine provide a large surface area with an extensive network of thin blood capillaries to absorb the products of digestion by diffusion and active transport.

The tissue lining in the small intestine is covered with millions of protuberances called villi, which poke up from the intestine surface into the partially or wholly digested food /mush'.

The villi consist of a single layer of cells (thin) on the very large surface area of the intestine.

Both factors considerably speeds up the food absorption process.

Each villus (of the millions of villi) has single layer of surface cells and each villus contains a multitude of fine blood capillaries into which the small digested food molecules can rapidly diffuse and be absorbed into the body.

A good blood supply is needed to efficiently carry the digested food away to where they are needed.

The food molecules can diffuse into the bloodstream down a normal concentration gradient, but sometimes active transport is required.

For example ...

After a meal has been digested, the concentration of food molecules in the blood can be higher than in the intestine. In this situation, molecules are conveyed into the blood by active transport e.g.

When there is a higher concentration of glucose in the intestine than in the bloodstream, glucose molecules will naturally diffuse into the blood stream down the diffusion gradient (concentration gradient from higher to lower concentration).

However, if there is a lower concentration of glucose in the intestine, your body still needs glucose for respiration, therefore active transport must be deployed. This uses energy in such a way as to transfer glucose molecules from the intestine against the natural concentration (diffusion) gradient.

For more on active transport see Diffusion, osmosis and active transport

 

See also Enzymes - structure, functions, optimum conditions, investigation experiments


6. Exchanges surface structure adaptations in other animals

Tadpoles and aquatic worms

For their gas exchange, tadpoles and aquatic worms absorb air through their skin and external gills.

The gills are feather-like projections that produce a large surface area for gas exchange with the water - so oxygen can be absorbed for cellular respiration and waste carbon dioxide removed.

With adult amphibians the gas exchange takes place mainly through their skin and lungs.

Insects  (need diagram?)

Insects do not have a transport system and gases cannot be directly exchanged with respiring cell tissue and the external air.

Insects have tiny holes called spiracles all along the side of its body.

The spiracles open out into tiny tubes called trachea - with a moist surface, through which insects pump air in and out.

The trachea are stiffened to prevent the minute 'tubes' collapsing.

The trachea have many minute branches called tracheoles which connect to cells - this increases surface area and shortens gas diffusion distance and time.

At the end of the trachea is a tiny drop of water that connects it to the cells.

So, gases can diffuse through the trachea and water into the cells - providing the cells with oxygen for respiration.

The spiracles can close to prevent evaporation and keep the exchange surfaces moist.

Slug

The slug absorbs air through its skin and has a moderately large surface area to volume ratio.

The gas exchange surface is moist to dissolve gases.

The membrane thin for a short diffusion time.

-


Gas exchange in plant leaves

For plants see Transport in plants notes


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