Cell Membranes

• Cell membranes keeps all cellular components inside the cell.
• It isolates the processes in the organelle from the rest of the cytoplasm.
• A site for biochemical reactions.
• Allows a cell to change shape.
• Allows selected molecules to move in and out of the cell. 

MEMBRANES ARE PARTIALLY PERMEABLE!!!!!

• Non-polar and water soluble molecules can diffuse through them.

Phospholipid bilayer =  forms the structural basis of membranes. It acts a barrier to some molecules whilst enabling others to freely cross it.

• Phospholipids have a polar head which is hydrophilic (phosphate) and lipid tails which are non-polar and hydrophobic. (There are saturated and un-saturated tails)

Cholesterol =  binds the fatty acid chains together and makes it less fluid and more stable. Without it, membranes could collapse. 

Peripheral proteins =  do not span through the entire membrane. Many are on the outside face and are glycoproteins.

Integral proteins =    span the entire width of the membrane. Important in cell signalling and the transport of substances across the membrane. 

Glycolipid =  carbohydrate attached to the lipid of a phospholipid. Act as receptors and are involved in cell recognition. May also be involved in cell-to-cell attachment and membrane stability.

Glycoprotein =  carbohydrate attached to protein. Involved in cell recognition, receptor molecules and membrane stabilisation. (INTRINSIC PROTEINS)

The more saturated molecules present, the more fluid. Straight tails makes it more permeable. 

Glycoproteins and lipids have a role in cell adhesion.

• The carbs can form bonds of hydrogen with water molecules to increase stability. 
• Act as receptors to bind with substances such as hormones. Some act as antigens to enable cell recognition.

Two types of proteins: carrier and channel proteins: these facilitate the movement of water, soluble ions and other molecules across the membrane. 

Channel proteins: 

• Hydrophobic regions to anchor them in the membrane.
• Hydrophobic tails attract to the hydrophobic region and the hydrophilic phosphate heads attract to the hydrophilic region, creating a channel. 
• Allowing the movement of soluble ions and water. 

Carrier proteins: 

• Carry substances across the membrane. 
• Molecules bind tto the carrier proteins, the carrier proteins changes shape and moves the molecule out or in the cell. involved in facilitated diffusion and active transport of substances.

• Increased vibrations widen the gap between phospholipids making the cell more permeable.
• Increasing the cholesterol in the membrane can reduce the fluidity and can reduce this affect for organisms living at high temps. and reducing it can help those that live in cold climates.
• The higher the temp, the faster molecules vibrate and also the more proteins denature 
• Cell signalling = the process by which cells communicate with each other.

Cells have receptors on their membranes. These tell other cells what they are or sites to recieve signals from other cells.

Glycolipids and proteins act for recognition and as receptors. 

Receptors = messages are often sent out as hormones from glands into the body.

Hormones bind to the receptors due to complementary shapes and this causes the cell to act in a certain way.