All animals and plants are made of cells. Animal cells and plant cells have features in common, such as a nucleus (except red blood cells), cytoplasm, cell membrane, mitochondria and ribosomes. Plant cells also have a cells wall, and they often also have chloroplasts and a permanent vacuole. Cells may be specialised to carry out a particular function.
Dissolved substances pass into and out of cells by diffusion. Water passes into and out of cells by osmosis.
Dissolved substances have to pass through the cell membrane to get into or out of a cell. Diffusion is one of the processes that lets this happen.
Diffusion happens when gas particles or dissolved particles in a solution spread. They move from a region where they are in high concentration to a region where they are in low concentration. Particles diffuse down a concentration gradient, from an area of high concentration to an area of low concentration.
In the lungs, the blood will continue to take in oxygen from the alveolar air spaces, provided that the concentration of oxygen there is greater than in the blood. Oxygen diffuses across the alveolar walls into the blood, and the circulation takes the oxygen-rich blood away.
DNA, genes and chromosomes
DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing.
Except for identical twins, each person’s DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, forming a sort of “bar code” that is different from one person to the next.
A gene is a short section of DNA. Each one codes for a specific protein byspecifying the order in which amino acids must be joined together.
The cell’s nucleus contains chromosomes. These are made from long DNA molecules.
The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus and genes.