Cell Integrity
Inquiry Question: To describe the structure of cell membranes and their function in cell
integrity.
1.0
The Cell Membrane
Membranes:
- Form the boundary between life (a cell) and its surroundings
- Selectively permeable  letting some molecule across and not others
Permeability:
- Semi-permeable
- Pass through  small, non-polar, no charge
o E.g. O2 and CO2
- Cannot pass
o Large molecules
 E.g. Glucose (C6H12O6)
o Ions
Proteins in membranes:
- Most are amphipathic
o Hydrophilic portions protruding
o Hydrophobic portion within the membrane
- Different membranes have different proteins (amount depends on function)
1.1
Fluid Mosaic Model
Fluid mosaic model:
- Explains how molecules are spatially arranged in the membrane
- Proteins are not randomly located (e.g. may group in areas)
Sidedness:
- Asymmetrical distribution of proteins  CHO and lipids on its two sides
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Position of protein domains relative to the membrane is fixed after the protein is inserted into the
plasma membrane
Plasma membrane has distinct cytoplasmic and extracellular faces
o Extracellular face is topologically equivalent to the inside face of the ER, Golgi, lysosome and
vesicle membranes
Proteins and lipids do not easily “flip” from one side of the membrane to the other
Some components:
- Integral membrane proteins
- Peripheral membrane proteins
- Cholesterol
- CHO
- Cytoskeleton
Membrane protein fluidity:
- Rapid movement of lipids and proteins laterally in the plane of the membrane
Experiment:
- Larry Frye and Michael Edidin labelled plasma membrane proteins of a mouse cell and human cell
and fused the cells
- Indicated that at least some membrane proteins move sideways within the plane of the plasma
membrane
Inquiry Question: To describe the different components of the cell membrane that are
important in maintaining cell integrity.
2.0
Cell Membrane and Maintaining Cell Integrity
Membrane composition:
Lipids:
- Phospholipids and cholesterol (0-25%)
Proteins:
- Peripheral and integral
o Span the membrane (different domains at each end)
CHO:
- Glycolipids and glycoproteins for recognition
o E.g. ABO blood types
Maintenance of cell integrity:
Phospholipid bilayer:
- Permeability barrier to most molecules
Will cross:
- Hydrophobic molecules will dissolve in the hydrophobic core and diffuse across the membrane
- Small molecules (e.g. O2, CO2 and H2O) will cross membranes by diffusion
Will not cross:
- Ionised, polar and large molecules will not cross membranes unless specific protein transporters are
present
Membrane proteins:
- Red blood cells deliver oxygen to the body tissue via the blood flow through the circulatory system
o Take up oxygen in lungs and realise it while squeezing through the body’s capillaries
- Can sustain substantial mechanical forces
o Proteins of the membrane skeleton are responsible for the deformability, flexibility and
durability of the red blood cell
Inquiry Question: To explain the non-selective diffusion of some small molecules across cell
membranes and osmosis.
3.0
Diffusion
Diffusion:
- Transport across the membrane can affect the cell integrity
- Molecules can diffuse across a membrane in both directions
o Difference in concentration between two sides of the membrane  net movement down
the concentration gradient until equilibrium is reached
 Passive transport (cell does not expend energy in process)
3.1
Osmosis
Osmosis:
- Passive transport of water
Animal cells:
- No cell wall  require an isotonic environment to maintain stable cell volume
- Hypotonicity may result in cell lysis
Plant cells (other cells with rigid cell walls):
- Cell wall  requires a hypotonic environment
- Hypertonicity causes plasmolysis  usually fatal