Cell Membranes

The Roles of Membranes within cells and at the Surface of Cells.

• Seperate cell contents and components from the outside environment. 
• Cell recognition and cell signalling.
• Holding the components of some metabolic pathways in place.
• Regulating the transport of materials into and out of cells.

There is a bilayer of phospholipid molecules that form the main structure. Various proteins are studded in the bilayer. Some are partially embedded (extrinsic) where as some completely span the membrane (intrinsic) such as the channel protein.

• Phospholipids: Have a hydrophilic (attracts water) head and a fatty hydrophobic (repels water) acid tail.
• The molecules automatically form into a bilayer - the head faces out towards the water on either side of the membrane.
• They are fluid so components can movearound freely. 
• The centre of the bilayer is hydrophobic (water repelling) so the membrane doesn't allow water-soluble substances through (like ions) through it - though it acts like a barrier to these disolved substances.

Cholesterol gives the membrane stability. 

• It is a type of lipid (fat).
• It's present in all cell membranes (except bacterial (prokaryotic) cell membranes). 
• Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This makes the membrane less fluid and more rigid. 

Proteins control what enters and leaves the cell. 

• Some proteins form channels in the membrane - these allow small or charged particles through. 
• Carrier proteins transport molecules and ions across the membrane by active transport and faciliated diffusion. 
• Proteins also act as receptors for molecules (eg hormones) in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell. 

Glycolipids and Glycoproteins act as receptors for messenger molecules. 

• They stabilise the membrane by forming hydrogen bonds with surrounding water molecules. 
• They're also the site where drugs, hormones and antibodies bind. 
• They act as receptors for cell signalling. 
• They're also antigens - cell surface molecules involved in the immune response. 

The Fluid Mosaic Structure.

• In 1972, the fluid mosaic structure was suggested to describe the arrangement of molecules in the membrane. 
• Phospholipd molecules form a continuous double layer (bilayer) that is 7nm thick.
• The bilayer is 'fluid' because the phospholipds are constantly moving. 
• Cholesterol molecules are present within the bilayer. 
• Protein molecules are scattered through the bilayer, like tiles in a mosaic.
• Some proteins have a polysaacharide (carbohydrate) chain attached - these are glycoproteins. 
• Some lipids also have a polysaacharide chain attached - these are glycolipids.

Membranes at the Surface of Cells. 

• They control which substances enter and leave the cell. They're partially permeable - letting some molecules through but not others. Substances can move across the membrane by diffusion, osmosis (water) or active transport. 
• They allow recognition by other cells. E.g, the cells of the immune system. 
• They allow cell communication. 

Membranes Within Cells. 

• The membranes around organelles divide the cell into different compartments. This makes different functions more effiecient e.g, the substances needed for respiration (enzymes) are kept together inside the mitochondria. 
• The membranes of some organelles are folded, increasing their surface area. 

Cells communicate with each other using messenger molecules.

• One cell releases a messenger molecule, eg a hormone. 
• This molecule travels to another cell eg in the blood.
• The messenger molecule is detected by the cell because it binds to a receptor on it's cell membrane. 

The cell membrane is important for the signalling process. 

• Membrane bound proteins act as receptors for messenger molecules. 
•  Receptor proteins have specific shapes - only messenger molecules with a complementary shape can bind to them. 
• Different cells have different types of receptors - they respond to different messenger molecules. 
• A cell that responds to a particular messenger molecule is called a target cell. 

EXAMPLE: Glucagon is a hormone that's released when there isn't enough glucose in the blood. It binds to receptors on liver cells, causing the liver to break down stores of glycogen to glucose. 

Temperatures below 0 degrees.

Phospholipids don't have much energy, so they can't move very much. Channel proteins and carrier proteins in the membrane denature increasing the permeability of the membrane. Ice crystals may form and peirce the membrane, making it highly permeable when it thaws. 

Temperature between 0 and 45 degrees.

The phospholipids can move around and aren't packed as tightly together - the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane. 

Temperatures above 45 degrees.

The phospholipid bilayer starts to melt (break down) and the membrane becomes more permeable. Water inside of the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins in the membrane denature so they can't control what enters or leaves the cell - this increases the permeability of the membrane. 

Diffusion is the passive movement of particles. 

• Diffusion is the net movement of particles (molecules or ions) from an area of higher concentration to an area of lower concentration.
• Molecules will diffuse both ways, but the net movement will be to the area of lower concentration. This continues until particles are evenly distributed throughout the liquid or gas. 
• The concentration gradient is the path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration gradient. 
• Diffusion is a passive process - no energy is needed for it to happen. 
• Particles can diffuse across plasma membranes, as long as they can move freely through the membrane, are small and do not have an electrical charge. Eg, oxegen and carbon dioxide molecules are small enough to pass easily through spaces between the phospholipid bilayer. 

The rate of diffusion depends on several factors. 

1. The higher the concentration gradient. 2. The thinner the exchange surface, the faster the rate of diffusion. 3. The larger the surface area, the faster the rate of diffusion. 

Osmosis is Diffusion of Water Molecules.

• Osmosis is the diffusion of water molecules across a partially permeable membrane, from an area of high water potential to an area of lower water potential. 
• Water potential is the potential of water molecules to diffuse out of or into a solution.
• Pure water has the highest water potential. All solutions have a lower water potential than pure water. 

Animal Cell: 

• Solution with a higher water potential than the cell (hypotonic solution) concludes in the net movement of water molecules going into the cell, and the cell bursts. 
• Solution with the same water potential as the cell (isotonic solution) concludes in water molecules passing into and out of the cell in equal amounts. The cell stays the same.
• Solution with a lower water potential than the cell (hypertonic solution) concludes in the net movement of molecules moving out of the cell. The cell shrinks (crenated).

• Some larger molecules (eg. amino acids, glucose) and charged atoms (eg chloride ions) can't diffuse directly through the phospholipid bilayer.
• Instead they diffuse through carrier proteins or channel proteins, in the cell membrane.
• Like diffusion, faciliated diffusion moves particles down a concentration gradient, from a higher to a lower concentration. 
• It is also a passive process - it doesn't use energy. 

Carrier proteins move large molecules into or out of the cell. Different carrier proteins faciliate the diffusion of different molecules. 

• First, a large molecule attaches to a carrier protein in the membrane. 
• Then, the protein changes shape.
• This releases the molecule on the opposite side of the membrane. 

Channel Proteins form pores in the membrane for their charged particles to diffuse through (down their concentration gradient). Different channel proteins faciliate the diffusion of different charged particles.

Active transport uses energy to move molecules and ions across membranes, against a concentration gradient. This process involves carrier proteins.

• The process is pretty similar to faciliated diffusion - a molecule attaches to a carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side.
• The only difference to faciliated diffusion is that energy is used (ATP) to move the solute against the concentration gradient. (From an area of low concentration to high concentration, instead of high concentration to low concentration). 

Endocytosis

• Some molecules are too large to be taken into a cell by carrier proteins, eg proteins, lipids and some carbohydrates.
• Instead a cell can surround a substance with a section of it's plasma membrane.
• The membrane then pinches off to create a vesicle inside the cell containing the ingested substance. This is endocytosis.