Notes from Feb. 19, 2004
Cellular Membranes
A. The cell Membrane
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Has a major role in reulation of what exits and enters the cell
Since some substances can pass through the cell membrane with ease, it is
said to be permeable
Most living membranes control what passes through them, which makes
them selectively-permeable
Non-living membranes were only certain substances pass through them
while others cannot, are called semi permeable
Within a cell membrane, both the phospholipids bilayer and the protein
molecules help to control the passage of materials through the cell
membrane
The general structure of living cell membranes consist of the hydrophobic
fatty acid chains making up the middle of the bilayer and the hydrophilic
phosphate heads pointing toward the liquid environments inside and
outside of the cell
Since the phospholipids heads are densely packed together forming the
bilayer, large molecules cannot penetrate the membrane
Protein molecules embedded within the cell membrane provide and entry
way for certain small molecules that cannot enter through the bilayer
portion of the membrane
Other functions of membrane proteins include:
o Structural support to the cell by binding to the protein fibres of
the cytoskeleton
o Serve as “target” sites that receive chemical messengers sent by
other cells
o Contain glucose chains, called glycocalyx, which serve as
recognition sites in cell to cell communication
The principle of diffusion
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All paricles are randomly moving even in a solid.
Therefore, it is a natural phenomenon that over time, particles tend to spread
themselves out evenly throughout any matter
The tendency of particles to move from an area of high concentration and
more random collisions, to an area of low concentration and fewer collisions
is called diffusion.
Equilibrium is established when the distribution of the particles is completely
even
When particles move from areas of high to low concentration they are moving
down a concentration gradient
Movement down the gradient is referred to as passive transport
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The driving force of the movement of many molecules through the cell
membrane like oxygen, carbon dioxide, alcohol, small lipids, is diffusion
Four factors of determines whether or not a substance can either enter or exit a
cell by diffusion:
o Particle size – small go through phospholipids bilayer, whereas large
do not
o Lipid solubility – the less soluble a molecule is with the inner fatty
acid chain part of the membrane, the less likely it will pass through it
o Size of the concentration gradient – the grater the difference across the
barrier, faster the diffusion
o The distance the molecule has to travel – if the distance is large,
diffusion is slow
The principle of Osmosis
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The diffusion of water through a selectively permeable membrane is called
osmosis
Since the phospholipids are constantly moving because of the fluid nature of the
membrane, and since water is relatively small enough to fit through the
neighbouring phospholipids, diffusion of water is passive
Regions of high water pressure, or high water content/low salt content, are
referred to as high osmotic potential
Regions of low water pressure, or low water content/high salt content, are referred
to as low osmotic potential regions
Naturally, water moves across membranes from regions of high osmotic potential
to regions of low osmotic potential
The osmotic nature of solutions surrounding a cell are:
o Hypertonic – when the fluid surrounding the cell is higher in dissolved ion
concentration than what is in a cell
o Hypotonic – when the fluid surrounding the cell is lower in dissolved ion
concentration than what is in a cell
o Isotonic – when the surrounding fluid of the cell has the same amount of
dissolve ions as the inside does.