Plasma Membranes & Diffusion Flashcards
What is meant by compartmentalisation?
Membranes formed by phospholipid bilayers can compartmentalise different regions within the cell as well as forming the cell surface membrane.
This is vital in allowing parts of the cell to carry out specific functions under optimum conditions.
What is the role of the cell surface membrane?
- creates an enclosed space separating the cell’s internal environment from the external environment
- partially permeable - controls exchange of materials
- acts as a barrier between cells and the environment, between organelles and the cytoplasm and within organelles
- roll in cell signalling - interface for communication between cells
What are the main components of the phospholipid bilayer?
- phospholipids
- cholesterol
- proteins (intrinsic and extrinsic)
- transport proteins
- glycoproteins
- glycolipids
- receptor sites
What does the fluid mosaic model explain?
- passive and active movement between cells and surroundings
- cell to cell interactions
- cell signalling
How does the fluid mosaic model describe membranes as fluid?
- phospholipids and proteins can move around by diffusion
- phospholipids mainly move sideways within their own layers
- proteins interspersed throughout bilayer can move around. Some may be fixed
How does the fluid mosaic model describe membranes as mosaics?
The proteins’ scattered pattern resembles a mosaic.
What is the function of phospholipids in a plasma membrane?
Water soluble molecules do not leak out of the cell, and unwanted soluble molecules cannot enter.
They can be chemically modified to be signalling molecules by:
- being hydrolysed to release soluble molecules that bind to receptors in the cytoplasm
- moving within the bilayer to activate molecules such as enzymes
What is the function of glycoproteins in a plasma membrane?
They are intrinsic proteins with external carbohydrate chains. Important in cell adhesion and as receptors for chemical signals.
They also stabilise membranes by forming hydrogen bonds with surrounding water molecules.
What is the function of glycolipids in a plasma membrane?
Glycolipids act as receptor molecules (cell markers / antigens) and are recognised by the immune system as self or non-self.
What is the function of cholesterol in a plasma membrane?
It controls membrane fluidity. More cholesterol = less fluidity, less permeability.
They are located randomly between the tails and prevent rigidity at low temperatures by stopping the tails from packing too closely together.
What are intrinsic proteins?
Proteins embedded through both layers of a membrane. Their external hydrophobic R-groups interact with the hydrophobic membrane core.
What are extrinsic proteins?
Proteins present in one side of the bilayer. They normally have hydrophilic R-groups on outer surfaces and interact with the polar heads of the phospholipids or intrinsic proteins.
Some extrinsic proteins move between layers.
What is meant by a concentration gradient?
A difference in the concentration of particles between 2 areas. Can be steep (faster) or shallow (slower).
What is the definition of simple diffusion?
The net random movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient due to the random movement and collisions of particles which have kinetic energy.
It is a passive process (no ATP energy is required) and continues until a concentration equilibrium is reached.
What molecules can move by simple diffusion?
Small, non-polar molecules can diffuse easily as they are very small and can pass through phospholipids.
