5.1 Structure and Function of Membranes Flashcards
What are membranes?
Structures that separate the contents of cells from their environment.
What are the functions of membranes?
-Separate the different organelles within cells
-Cell communications
-Acts as a partially permeable barrier
-Site of chemical reactions
-Compartmentalisation.
Describe the features of compartmentalisation.
-The formation of separate membrane-bound areas in a cell
-Contains reactions in separate parts of the cell
-Allows the specific conditions required for cellular reactions to be maintained
-Protects vital cell components
Describe the basic membrane structure
-Membranes are formed from a phospholipid bilayer
-The hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids, to form a hydrophobic core inside the membrane.
What is a plasma membrane?
The cell surface membrane that separates the cell from its external environment
What kinds of environments do cells normally exist in?
Aqueous environments
Why are phospholipid bilayers perfectly suited as membranes?
Because the outer surfaces of the hydrophilic phosphate heads can interact with water.
What is the name of the current model of the plasma membrane, and why?
-Fluid-mosaic model
-Because the phospholipids are free to move within the layer, and because the proteins embedded in the bilayer vary in shape, size and position.
What are membrane proteins?
Proteins that have important roles in the various functions of membranes.
What are the two types of proteins in the cell surface membrane?
Intrinsic and extrinsic proteins
What are intrinsic proteins?
-Transmembrane proteins that are embedded through BOTH layers of a membrane
What is the key structure of an intrinsic protein, and why?
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 type of protein is a carrier protein?
Intrinsic protein
What type of protein is a channel protein?
Intrinsic protein
What do channel proteins do?
-Provide a hydrophilic channel that allows the passive movement of polar molecules and ions down a concentration gradient, through membranes.
How is a channel protein held in place?
Because it is an intrinsic protein, by interactions between the hydrophobic core of the membrane, and the hydrophobic R-groups on the outside of the proteins.
What do carrier proteins do?
Have an important role in both passive and active transport into cells, often involving the shape of the protein changing.
What type of proteins are glycoproteins?
Intrinsic proteins.
How are glycoproteins connected to the membrane?
Embedded in the cell-surface membrane, with attached carbohydrate chains of varying lengths and shapes
What is the function of a glycoprotein?
-Cell adhesion
-Cell recognition
-Cell receptors
Describe the structure of a glycolipid.
Similar to a glycoprotein, a lipid with an attached carbohydrate chain.
Are glycolipids intrinsic or extrinsic proteins?
Neither, because they are not proteins.
What are alternate names for glycolipids?
Cell markers and antigens
What are extrinsic proteins?
Present in only one side of the bilayer
Describe the structure of extrinsic proteins.
Normally have HYDROPHILIC R-groups on their outer surfaces, and interact with the polar heads or intrinsic proteins of the phospholipids.
Describe the position and movement of extrinsic proteins.
Can be present in either layer, and some move between layers.
Is cholesterol intrinsic or extrinsic?
It is neither, because it is not a protein.
Describe the structure of cholesterol.
A lipid with a hydrophilic end and a hydrophobic end
Describe the functions of cholesterol.
-Regulates the fluidity of membranes
-Provide stability to membranes
Where is cholesterol positioned in the membrane?
Positioned between phospholipids in a membrane bilayer. Hydrophobic ends interact with the tails, hydrophilic ends interact with the heads, pulling them together.
How does cholesterol add stability to membranes?
By their hydrophobic heads interacting with the phospholipid tails, and the hydrophilic tails interacting with the phospholipid heads. These interactions pull the membrane together, giving it stability without making it too rigid.
True or False? A phospholipid bilayer allows water-soluble substances to pass through.
False. Phospholipid bilayers allow lipid-soluble substances through, not water-soluble substances.
Give three cell processes involving glycoproteins or glycolipids.
-Cell adhesion
-Cell recognition
-Cell signalling
