cell membrane

• Cholesterol and protein molecules are embedded in the membrane
• cholesterol molecules are wedged between phospholipids and make the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
• some protein molecules extend through the membrane while others are just bound to the surface
• receptor proteins respond to changes in extracellular fluid
• channel proteins are where water soluble substances are free to pass through
• carrier proteins bind to particular molecules allowing them to pass through the membrane
• cell identity markers allow cells to recognise cells of the same tissue and foreign invaders
• a model called the fluid mosaic model is used to describe the structure of a cell membrane as it is too small to be seen
• phospholipid molecules are the main structure and are arranged in two layers known as a bilayer
• the hydrophilic heads face outwards towards the cytoplasm or outside environment, while the hydrophobic tails face inward away from the water
• cholesterol molecules are embedded within the phospholipids are important for the function integrity and stability of the membrane as they prevent the phospholipids from separating into their individual components
• cells require oxygen and glucose during cellular respiration and must have carbon dioxide and water removed
• the cell membrane has several functions which include acting as a physical barrier, regulating the passage of materials, being sensitive to change and help support the cell
• acts as a physical barrier - separates the cell contents from other cells and extracellular fluids as their compositions are very different
• regulates the passage of materials - controls what is able to enter and leave the cell
• Sensitive to change - first part of the cell affected by any changes in the extracellular fluid. It has receptors that are sensitive to particular molecules in its immediate environment
• helps support the cell - internal part of the cell membrane is attached to the microfilaments of the cell's cytoskeleton. Also connections between adjacent cells, providing support to the whole tissue
• The membrane is said to be fluid because the molecules of which it is made are constantly changing position, and it is said to be mosaic because it is composed of many different kinds of molecules.