Plasma Membranes Flashcards
Membranes
- Partially permeable barriers that separate contents of cells from their environment
- Also separate organelles from each other and the cytosol- some organelles divided further by internal membranes
- Sites of chemical reactions
- Sites of cell communication (Cell signalling)
Compartmentalisation
- The formation of separate membrane-bound areas in a cell
- Vital to a cell as metabolism includes many different and often incompatible reactions
- Containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions to be maintained and provides vital cell components
Plasma membrane
- Cell surface membrane which separates the cell from its external environment
Phospholipid bilayer
- Hydrophilic phosphate heads of the phospholipids form both inner and outer surface of a membrane, sandwiching fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane
- Cells normally exist in aqueous environments, as are the inside of cells and organelles
Phospholipid bilayers= perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.
Fluid - mosaic model
- Phospholipids are free to move within the layer (they are fluid)-gives membrane flexibility
- Proteins embedded in the bilayer vary in shape, size and position-like the tiles of a mosaic
Components of the cell membrane
- Glycoproteins
- Glycolipid
- Cholesterol
- Protein molecule partly embedded (Extrinsic protein)
- Hydrophobic tails of phospholipid molecules-point inwards
- Protein molecule spanning the phospholipid layer (intrinsic protein)
- Protein molecule lying on the surface (extrinsic protein)
- Hydrophilic heads of phospholipids molecules- point outwards
Membrane proteins
Two types of proteins in the cell-surface membranes- intrinsic + extrinsic
Intrinsic proteins
- Transmembrane proteins embedded through both layers of a membrane
- They have amino acids with hydrophobic R groups on their external structures-interact with the hydrophobic core of the membrane keeping them in place
- Includes: channel and carrier proteins, glycoproteins
Channel proteins
- Provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a conc gradient through membranes
- Held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on outside of proteins
Carrier proteins
- Important role in both passive transport, down a conc gradient, and active transport, against a conc gradient into cells- often involves the shape of the protein changing.
Glycoproteins
- Branching carbohydrate portion of a protein
- Embedded in the cell-surface membrane with attached carb chains
- Play a role in cell adhesion and as RECEPTORS for chemical signals
- Recognition site for chemicals, eg hormones
Cell signalling
- When a chemical binds to the receptor, it elicits a response from the cell- causing a direct response or setting off a cascade of events inside the cell
- Receptors for neurotransmitters- binding of neurotransmitters triggers or prevents an impulse in the next neurone
- Receptors for peptide hormones-including insulin and glucagon, affect uptake and storage of glucose by cells
Some drugs eg beta blockers act by binding to cell receptors
Glycolipids
- Lipids with attached carb chains- these are called antigens, recognised by cells of the immune system as self or non-self
- Acts a recognition site eg. cholera toxins
Extrinsic proteins
Present in one side of the bilayer
Hydrophilic R groups on their outer surfaces; interact with the polar heads of phospholipids or with intrinsic proteins
Can be present in either layer and some move between layers
Cholesterol
- Lipid with hydrophilic end and a hydrophobic end, like a phospholipid
- Regulates fluidity of membranes
Molecules positioned between phospholipids in a membrane bilayer- hydrophilic end interacting with the heads and hydrophobic end interacting with the tails–pulls them together.
Cholesterol adds stability without making them too rigid
Prevent membranes becoming too solid by stopping phospholipid molecules from grouping too closely and crystallising.
