S-Cool Revision Summary

S-Cool Revision Summary

If an organism is small and has a large surface area to volume ratio, all the nutrients and respiratory gases can be taken in by diffusion across the body surface.

Most multicellular plants and animals have too small a surface area to volume ratio so diffusion would be too slow to provide the necessary molecules. Therefore, they require a system to transport nutrients and waste products around the organism.

The needs of a plant and animal are similar in some aspects and very different in others.

Both need to transport food molecules around the organism but plants, for instance, do not use the transport system to fight disease.

Two main types of plant tissue are used in transport - xylem and phloem. Xylem transports water and minerals. Phloem transports organic molecules such as the products of photosynthesis.


There are four types of xylem cells:

Xylem vessels





There are four types of phloem cells:

Sieve tube elements

Companion cell



In mammals, the pump is the heart. Substances are carried in a transport medium of the blood. The blood is contained within vessels, with substances being released out of, or into the blood as it flows through certain vessels called capillaries.

Blood is carried within a closed transport system that is made up of three types of vessel:

Arteries carry blood away from the heart.

Capillaries are the site of the exchange of materials between the blood and tissues.

Veins take blood back into the heart.

Just over half of the blood volume is made up of a pale yellow fluid called plasma. The rest of the blood is made up of cells (red blood cells and white blood cells) and platelets.

Blood has several vital functions:



Formation of lymph and tissue fluid.


Also known as erythrocytes. These contain a pigment, haemoglobin, which gives them their colour.

Red blood cells are made in the bone marrow (the liver in a foetus) of many bones.

Being like a biconcave disc in shape, the surface area to volume ratio is very large. Oxygen can therefore diffuse very quickly into the cell and because the cell is so small, quickly bind to a haemoglobin molecule.

These cells all have a nucleus, most are much larger than red blood cells and are spherical or irregular in shape.

There are two basic types of white blood cells; the granulocytes (they have granular cytoplasm and lobed nuclei) and agranulocytes (the cytoplasm appears smooth and the nucleus is rounded or horseshoe in shape).

These are formed in the bone marrow and are fragments of larger cells. They have no nucleus but reactions do take place in the cytoplasm.

They have a variety of role such as blood clotting and the production of prostaglandins that regulate the degree of constriction or dilation in blood vessels.

The most commonly required blood-grouping system is the ABO system. It concerns two antigens that can occur on the surface of red blood cells. The antigens are called agglutinogens in this case and are: agglutinogen A and agglutinogen B.

Plasma also contains antigens, called agglutinins in this case, and they are agglutinin A and agglutinin B.

It is important to match blood correctly so that agglutinins in the recipient don't clump the red blood cells of the donor.

In transfusions it is important to remember that the volume of blood donated is relatively small compared to the volume of the recipients blood. The agglutinins in the plasma from the donor are so diluted that no harm is done. However the aggluinogens on the red blood cells are not so diluted so harm can be done.

Oxygen does dissolve in plasma but the solubility is low and decreases further if the temperature increases. The amount that could be carried by the plasma therefore would be completely insufficient to supply all cells.

The structure is closely related to its function.

Mammals have a double circulation, which means that the right hand side of the heart pumps deoxygenated blood to the lungs in the pulmonary artery to pick up oxygen and release carbon dioxide. The oxygenated blood then returns to the left hand side of the heart in the pulmonary vein.

From there the blood is pumped to the body in the aorta, eventually returning to the right hand side of the heart in the vena cava to start the cycle again.

Since the right side pumps to the lungs which are situated close to the heart, the walls are much thinner than the left side which has to pump blood out of the heart to the body.

The heart has 4 chambers:

2 on the left hand side

2 on the right hand side

Atrium: the top chamber on each side

Ventricle: the bottom chamber on each side

The muscle of the heart is called cardiac muscle and is made of tightly connecting cells.

One cardiac cycle consists of the atria and then the ventricles contracting so that the blood that has entered the heart is pumped out. This occurs about 70 times every minute and is continuous. The periods of contraction are called systole. The periods of relaxation are called diastole.

The heartbeat is initiated in a specialised area of muscle in the right atrium called the sinoatrial node (SAN) or the pacemaker. The SAN starts the waves of depolarisation, which results in contraction.