B1.2.3 Cell Membrane Structure and Function
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All membranes around and within cells (including those surrounding organelles) are called…
plasma membranes
What do we call the plasma membrane that surrounds cells?
Cell-surface membrane
What is the function of a cell surface membrane?
• To separate the internal and external environments of the cell. • To control the movement of substances in and out of the cell.
Name the components of a cell surface membrane.
• Phospholipids • Proteins (intrinsic and extrinsic) • Glycoproteins • Glycolipids • Cholesterol
Phospholipids have polar and non-polar regions. A molecules that exhibits both of these properties is said to be…
Amphipathic
What is a polar molecule?
A molecule that does not have an even distriibution of electrons making some regions more negative than others.
What is a non-polar molecule?
A moelucle that has an even distribution of electrons, meaning it does not have areas that are more negative than other areas.
Phospholipids are made form which 3 molecules?
Phosphate Glycerol Fatty acid ‘tails’ x 2
Describe the phosphate head of a phospholipid.
• Polar • Hydrophillic - water loving
Describe the fatty acid ‘tails’ of a phospholipid.
• Non-polar • Hydrophobic - ‘water hating’
What are the functions of the phospholipids in the cell membrane?
• Allow lipid-soluble (non-polar) substances to enter/exit the cell. • Prevent water soluble (polar) substances entering/exiting the cell. • Makes the membrane flexible and self-sealing.
Proteins in a cell membrane can either be __________ or _________.
Intrinsic or extrinsic
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer Cholesterol molecule (green)
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer 2 x extrinsic proteins (carriers)
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer Glycolipid molecules x 2
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer Glycoproteins x 2
Which components of the cell-surface membrane are visible in this diagram?
Phospholipid bilayer Intrinsic proteins x 2 (channel and carrier)
What are the funcitons of proteins in the cell membrane?
• Provide structural support • Act as channels to transport polar molecules (facillitated diffusion) • Allow active transport (carrier proteins)
What is the function of cholesterol in cell-surface membranes?
• Reduces lateral movement of molecules that make up the membrane. • Acts as a buffer against temp increases/decreases to maintain membrane fluidity. • Prevents leakage of water and dissolved ions from the cell.
What is a glycolipid?
A carbohydrate (polysaccharide) covalently bonded to a lipid.
What are the functions of glycolipids?
• Act as cell recognition sites (ABO blood group antigens are glycolipids) • Maintain stability of cell surface membrane. • Help cells to attach to one another and form tissues.
What is a glycoprotein?
An extrinsic cell surface membrane prtoein with a carbohydrate covalently bonded to it.
What are the funcitons of glycoproteins in the cell surface membrane?
• Act as recognition sites (for hormones and neurotransmitters) • Help cells to attach and form tissues.
Substances that can readily diffuse through the cell-surface membrane are small polar molecules and non polar molecules such as…
• Gases like oxygen and CO2 • Water • Lipid-based hormones (steroids) e.g. oestrogen