Topic 1

one of the main ways substances move in and out of cells is by diffusion. diffusion is the free movement of particals in a loquid or gas down a concerntration gradient (high to low).

For a unicellular organism such as an amoeba, nutrients and oxygen diffuse directly into the cell from it external environment, and waste sustances can diffuse out.this works well because the surface area to volume ratio is large. 

the surface area to volume ratio of an organism determins wherther diffusion will allow substances to move in and out of all the cells. the bigger the organsim the smaller surface are to volume ratio. the distance from outside the organsim to the inside gets longer and there is less surface for substances to enter through so diffusion is longer.

in complex organism diffusion will not allow efficient transport of substances. so instead they have internal transport systems, in humans this transport system is the heart and circulatory system. the substances are transported in the flow of the blood. substances are delivered over short ditances by osmosis.

mass transport systems have vessels that carry the substances, a way of ensuring substances are moved in the right direction, a means of moving material fast enough to supply the needs of the organsims e.g. pumping heart, and a suitable medium.

the simple chemical formula for water is H2O which tells us that tow atoms of hydrogen are joined by one atoms of oxygen.each water molecule is slightly polarised this means it has a slightly negative part -oxygen- and a slightly positive part - hydrogen -.