The cell surface membrane

What are the 4 roles of membranes in cells and at their surfaces?

• Act as partially permeable barriers/ allows selected molecules to move in and out of the cell: between the cell and its external environment, between cytoplasm and organelles, and within organelles (controlling exchange of substances).

• Form compartments (intracellular membranes) that allow organelles to have specialised internal environments and allowing cellular processes to occur separately (e.g. mitochondria, RER, nucleus).

• a site for biochemical reactions

• Serve as interfaces for cell signalling: contain receptors (e.g. membrane-bound receptors for hormones, neurotransmitters).

• allows a cell to change shape

why is compartmentalisation important?

because organelles have different functions and so have slightly different pH and temps, so this allows these environmental conditions to be controlled to avoid enzyme denaturation

Why is the cell membrane described as “fluid” in the fluid mosaic model?

• Because the components of the cell can move

• Phospholipids and proteins can move laterally (diffuse) within the membrane plane.

• Some membrane proteins are free to move; others are anchored or fixed.

Why is the membrane described as a “mosaic”?

• Because proteins of varying sizes and shapes (intrinsic and extrinsic), glycoproteins, glycolipids, and cholesterol are interspersed among the phospholipid molecules in a scattered pattern, like tiles in a mosaic.

what are extrinsic proteins and their general structure, their purpose and how are they held in place?

• proteins partially embedded in the phospholipid bilayer

• 30 % of membrane proteins are extrinsic, they only go through one layer of the membrane

• they have amino acids with hydrophillic amino R groups on their surface

• those on the outside are involved with cell to cell signalling

• those on the inside are associated with the cytoskeleton

what do extrinisc proteins on the extracellular side of the membrane do

• they act as receptors for hormones or neurotransmitters or are involved in cell recognition

• many are glycoproteins

what do extrinisic proteins on the cytosolic side of the membrane do

• involved in cell signalling or chemical reactions

• they can dissociate from the membrane and move into the cytoplasm

what are intrinsic proteins and their general structure, their purpose and how are they held in place?

• these proteins are fully embedded and span the phospholipid bilayer

• many intrinsic proteins are carrier molecules or channels

• other intrinsic proteins are receptors for hormones, enzymes etc

• 70% of proteins in the membrane are intrinsic

• they have amino acids with hydrophobic R groups on their surface

• they’re held in place because these groups are able to interact with the hydrophobic fatty acids

what are channel proteins?

• allow specific substances to move across the membrane, they have a hydrophillic core to allow charged and polar substances to move through passively ( no energy needed)

• form hydrophillic pores that allow ions or polar molecules to diffuse across

what are carrier proteins

• change shape to carry substances across the membrane

• have a role in both passive and active transport, they need ATP in order to work whereas channel proteins don’t as they transport things passively

what role do glycoproteins play in the membrane

• proteins with carb chains attached

• can be receptors for messages received by the cell ( cell signalling)

• can be involved in joining cells together ( cell adhesion)

• can act as antigens to allow cells to be recognised as self or non self

• stabilise cell positions

what is cell signalling?

• receptors in the membrane are vital for communication between cells and therefore parts of the body

• receptors have a specific shape which is complimentary to the shape of the molecule that binds to it, different cells have different types of receptors that respond to different messenger molecules

how does cell signalling work

• messenger molecules bind to receptors on the cell membrane of the target cell, the binding triggers a change inside the cell.

what is the role of glycolipids in the membrane?

• carb chains directly attached to lipids, used as cell markers recognised by immune system ( antigens)

what is the role of cholesterol in cell membranes?

• a type of lipid with the molecular formula C₂₇H₄₆O

• very important in controlling membrane fluidity embedded between the cells, the more cholesterol the less fluid and less permeable the membrane is

• without it cells would burst open

• adds mechanical stability and strength to the membrane

• they fit between the phospholipids

explain how cholesterol controls cell membrane fluidity

• at higher temperatures they bind to the hydrophobic fatty acid tails of the phospholipids causing them to pack more closely together, this makes the membrane less rigid and more fluid

• at lower temperatures cholesterol prevents phospholipids from packing too closely together and so increases membrane fluidity, cholesterol also has hydrophobic regions so is able to create a further barrier to polar substances moving through the membrane

what role do phospholipids play in the membrane?

• form the bilayer with hydrophilic heads facing aqueous environments and hydrophobic tails forming a nonpolar core — act as barriers to water-soluble molecules.

What special properties arise from phospholipid bilayers?

• The hydrophobic (nonpolar) tails create a core that prevents passage of water-soluble/polar/ionic substances.

• Hydrophilic heads interact with the aqueous environments on each side, helping stability.

• Phospholipids can act in cell signalling (via movement, hydrolysis to produce signalling molecules).

what are the two main types of cell membrane?

1. Cell-surface membranes - These surround cells to act as a barrier between the cell and its environment, controlling which substances enter and leave the cell. 

2. Membranes around organelles - These surround organelles (e.g. mitochondria) to act as a barrier between the organelle and the cytoplasm, dividing the cell into different compartments (this is sometimes called compartmentalisation).