Cells

Cytoplasm is found inside the cell and contains all the other cell structures

The large nucleus is surrounded by a nuclear membrane to separate it from the cytoplasm

The cell membrane surrounds the cell

The cell wall is made of cellulose and surrounds the cell membrane in plant cells

Mitochondria (singular: mitochondrion) are organelles found throughout the cytoplasm

Chloroplasts are organelles found in the cytoplasm that are packed with the pigment chlorophyll and so are green in colour

Ribosomes are tiny structures that can be free within the cytoplasm or attached to a system of membranes inside the cell

Vacuoles are large vesicles that take up a large part of the interior of plant cells

Specialised cells are those which have developed certain characteristics in order to perform particular functions

Cells specialise by undergoing differentiation: a process that involves the cell gaining new structures within the cell in order for it to be suited to its function

Cells can either differentiate once early on or have the ability to differentiate their whole lives (these are called stem cells) In animals, most cells only differentiate once, but in plants many cells retain the ability

Many tissues in the human body contain a small number of undifferentiated cells

These are called stem cells and their function is to divide and produce new differentiated cells within the tissue for growth and repair

Adult stem cells from bone marrow are quite useful in medicine as they can be directed to differentiate into several different cell types (unlike adult stem cells from other tissues which can only differentiate into related cell types)

All cells are surrounded by a cell membrane which is partially permeable

Molecules can move in and out of cells by diffusion or active transport

Water can move in and out of cells by osmosis

Surface Area to Volume Ratio

The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface

Many cells which are adapted for diffusion have increased surface area in some way – eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

Distance

The smaller the distance molecules have to travel the faster transport will occur

This is why blood capillaries and alveoli have walls which are only one cell thick, ensuring the rate of diffusion across them is as fast as possible

Temperature

The higher the temperature, the faster molecules move as they have more energy

This results in more collisions against the cell membrane and therefore a faster rate of movement across them

Concentration Gradient

The greater the difference in concentration either side of the membrane, the faster movement across it will occur

This is because on the side with the higher concentration, more random collisions against the membrane will occur