Cells - Transport Across Membranes: Cell Membranes Flashcards
What is common between all plasma membranes?
Phospholipid bilayer structure
Describe the structure of the phospholipid bilayer
Two layers of phospholipids facing each other. Hydrophilic heads point out, attracted by water on both sides. Hydrophobic tails point into the centre of the membrane, repeller by the water.
Functions of phospholipids in the membrane
Allow lipid soluble substances to enter and leave the cell.
Prevent water-soluble substances entering and leaving the cell.
Make the membrane flexible and self-sealing.
What gives different membranes different properties?
The different substances they contain.
What 4 substances to cell-surface membranes contain?
Proteins
Cholesterol
Glycoproteins
Glycolipids
Where are proteins in the cell-surface membrane?
They may move freely or be anchored by the cytoskeleton.
Which type of proteins move freely in the cell-surface membrane?
Intrinsic proteins
Which type of proteins are anchored in the cell -surface membrane by the cytoskeleton?
Extrinsic proteins.
How are intrinsic proteins positioned in the phospholipid bilayer?
They span the bilayer
Where are extrinsic proteins positioned in the phospholipid bilayer?
They are partially embedded in the inside or outside of the bilayer.
Give an example of an intrinsic protein
Transport proteins
What are two different types of transport protein?
Channel protein
Carrier protein
What do transport proteins do in the phospholipid bilayer?
They control the movement of polar ions/molecules across the membrane.
Give 4 functions of proteins in the cell surface membrane.
Structural support
Transport of water-soluble substances across the membrane
Allows active transport across the membrane
Act as receptors allowing cell signalling
Where is cholesterol positioned in the cell-surface membrane?
Fits between the fatty acids tails within the phospholipid bilayer.
Give 3 functions of cholesterol in the cell-surface membrane.
Reduces lateral movement of other molecules, stabilising the membrane.
Prevents loss of water and dissolved ions.
Regulates fluidity.
What are the 3 factors that change membrane fluidity?
Cholesterol
Temperature
Saturated or unsaturated fatty acid tails in the bilayer
What are glycoproteins?
Carbohydrate chains attached to extrinsic proteins on the outer surface of the membrane.
What are glycolipids?
Carbohydrates covalent oh bonded with lipids on the outer surface of the membrane.
What are glycoproteins receptors for?
Hormones and neurotransmitters
What are glycolipids receptors for?
Specific chemicals
Why do glycoproteins act as antigens?
To allow recognition of ‘self’ by lymphocytes in the immune system.
Give 4 functions of glycoproteins and glycolipids in the cell-surface membrane.
Act as recognition sites
Form glycolax on the outer surface
Help cell adhesion to form tissues
Used in cell signalling
What is the fluid mosaic model?
The arrangement of the various molecules in the structure of the cell surface membrane.
Why is it called the fluid mosaic model?
Fluid- flexible structure, constantly changing shape, phospholipid molecules can move relative to one another.
Mosaic - proteins vary in shape, size and pattern.
Model - representation that works for all current evidence.
How does the cell surface membrane control the movement of substances in and out of the cell?
By being selectively permeable
Give 3 functions of the cell-surface membrane.
Controls the movement of substances in and out of the cell.
Separates cell contents from external environments, allowing different conditions inside and out.
Cell recognition and signalling.
Where is the modification of carbohydrates into glycoproteins and glycolipids carried out?
The Golgi body
Give 2 functions of plasma membranes inside cells.
Separates organelle contents from cytoplasm.
Holds components of metabolic pathways in place.
Give 4 reasons why most molecules cannot freely diffuse through the cell-surface membrane?
Not soluble in lipids
Too large to pass through channel proteins
Of the same charge as the protein channels
Polar
Why cant molecules which are not soluble in lipids freely diffuse through a cell-surface membrane?
They can’t get through the phospholipid bilayer.
Why can’t molecules of the same charge as channels proteins freely diffuse through a cell-surface membrane?
They are repealed by the channel proteins.
Why can’t polar molecules freely diffuse through a cell-surface membrane?
They can’t pass through the non-polar hydrophobic tails in the phospholipid bilayer.
