5 Plasma Membranes Flashcards
What are membranes?
Structures that separate the contents of cell from their environnement.
They also separate different areas within cells (organelles) from each other and the cytosol.
Some organelles are divided further by internal membranes.
What is compartmentalisation?
Formation of separate membrane-bound areas in a cell
Why is compartmentalisation vital?
Metabolism includes many different and often incompatible reactions.
What does containing reactions in separate parts of the cell allow?
Allows the specific conditions required for cellular reactions, such as chemical gradients, to be maintained, and protects vital cell components.
What is the plasma membrane?
The cell surface membrane which separates the cell from its external environment.
What are membranes formed from?
A phospholipid bilayer.
What do the phosphate heads of the phospholipids form?
Both the inner and outer surface of a membrane.
What do the fatty acid tails of the phospholipids form?
A hydrophobic core inside the membrane.
Why are phospholipid bilayers perfectly suited as membranes?
Cells normally exist in aqueous environments and the inside of organelles and cells are normally aqueous. The outer surfaces of the hydrophilic phosphate heads can interact with water.
What did images of the membrane taken using electron microscopy show?
Two black parallel lines- supporting an earlier theory that membranes were composed of a lipid bilayer.
What model did Singer and Nicolson propose?
Fluid-mosaic model.
What is the fluid-mosaic model?
Where phospholipids are free to move within the layer relative to each other, giving the membrane flexibility (fluid). There are proteins embedded in the bilayer which vary in shape, size and position (like tiles in mosaic).
What are intrinsic proteins?
Transmembrane proteins that are embedded through both layers of a membrane. They have amino acids with hydrophobic R-groups on their external surfaces, which interact with the hydrophobic core of the membrane, keeping them in place.
What is the role of channel proteins?
Provide a hydrophilic channel that allows passive movement of small polar molecules and ions down a concentration gradient through membranes.
Used in facilitated diffusion.
How are channel proteins held in place?
By interactions between the hydrophobic core of the membrane and the hydrophobic R-groups on the outside of the proteins.
What do phospholipid bilayers act as a barrier to?
Water-soluble substances e.g. ions and polar molecules.
Can fat-soluble molecules dissolve in the bilayer and pass through the membrane?
Yes
What can pass through a membrane?
Non-polar and very small molecules. O2 and CO2.
What cannot pass through the membrane?
Large molecules and polar molecules.
e.g. Cl- and Na+.
What is the role of cholesterol in the membrane?
Gives membrane stability as it binds to phospholipid tails and makes them pack closely together.
More cholesterol=more stability, less fluidity.
Has hydrophobic regions to create a further barrier to polar substances.
In which membranes is cholesterol not present?
Bacterial cell membranes.
How do cholesterol molecules stop the membranes becoming too solid?
By stopping the phospholipid molecules from grouping too closely and crystallising.
How does the structure of cholesterol interact with the phospholipid bilayer?
Hydrophobic end of cholesterol interacts with tails and hydrophilic end interacts with heads, pulling them together.
What is the role of carrier proteins?
To transport larger molecules and charged particles across the membrane by active transport or facilitated diffusion.
Often involves shape of carrier protein changing.
What is a glycoprotein?
Intrinsic protein with attached carbohydrate chains of varying lengths and shapes.
What are the roles of glycoproteins?
Adhesion (cells joining together).
Stabilising membrane by forming H bonds with surrounding water molecules.
Receptors for chemical signals.
