Transport Across the Cell
Membrane
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• ALL cells possess a cell membrane (mb)(~8 nm thick).
• Membranes function to control the passage of
materials into/out of the cell (act like “gatekeepers”).
• Membranes are described as being selectively
permeable.
-- a mb that is completely permeable will allow
anything and everything to pass through it.
-- a mb that is impermeable will allow nothing to
pass through it.
• Therefore, a selectively permeable membrane will only
allow certain molecules across.
• The ‘selection’ criteria may be based on size, polarity,
ATP (energy) availability or a combination of any of
these factors.
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IMPERMEABLE:
OIL SLICKERS
COMPLETELY
PERMEABLE
MAPLE LEAFS’ GOALTENDING
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• Other mb functions include:
-- separating the cytoplasm from the
Extracellular Fluid (ECF), in other words,
‘housing’ the organelles.
-- communication with other cells.
-- identifying the cell.
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General Structure and Function of
the Cell Membrane
• The primary structure of all membranes is a
PHOSPHOLIPID BILAYER with proteins embedded
either partially (peripheral proteins – more for support
purposes) or completely through it (integral proteins).
• The anchoring of integral proteins is discussed later;
peripheral proteins occur either on the outside or inside
of the mb anchored either by covalent bonds, or by
weaker, intermolecular interactions.
• Carbohydrates are evident as well, associating with
either the phospholipids (forming glycolipids) or with the
proteins (forming glycoproteins). These carbs only
associate on the outer edge of the mb. *more later…
• Cytoskeletal filaments associate with the inner portion of
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the mb in order to anchor it.
Fluid-Mosaic Model of a Cell
Membrane (fig 4.1 p.68)
• Phospholipids serve as the ‘grout’ or fluid portion
of the membrane (membranes possess the
consistency of a light olive oil; ie. The ‘grout’
does not ‘harden’, or else long nerve cells would
crack as you moved!).
• Proteins/glycoproteins serve as the mosaic, but
can (and do) move around within the fluid bilayer
(like wading through a pool of oil). See fig. 4.2 p. 69
• Cholesterol is also present in the cell mb and
serves to lend stability to the phospholipid
bilayer when the temperature rises.
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• Conversely, cholesterol prevents a decrease in
mb fluidity when temperatures decrease.
• Ie. Cholesterol does not let the cell mb become
too fluid at higher temperatures and also does
not let the mb become too rigid at lower
temperatures.
• A structural buffer???
• As well, cholesterol acts as a physical barrier in
the mb, disallowing larger molecules from
crossing on their own.
• Glycolipids and Glycoproteins serve as
identifiers and communicators for the cell (ie. A
driver’s licence or ‘fingerprint’).
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Fluid-Mosaic Model Picture
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Phospholipid Bilayer Structure
• Each phospholipid molecule in the mb has
a POLAR (charged or hydrophilic) head
and two NON-POLAR (uncharged or
hydrophobic) tails (see fig. 2.27 p.35).
• Hydro-philic = Water-loving
• Hydro-phobic = Water-hating
• Phospholipid molecules are referred to as
being amphipathic in that they are part
polar, and part non-polar.
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• Cells are surrounded by water (ECF) and are filled with
water (cytoplasm), so the heads face outward and inward.
• The non-polar tails bury (hide) together to stay as much
away from water as they can (hence, the double layer).
• Very little water (if any) in between the layers.
• This bilayer arrangement is spontaneous (requires no
energy)
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• Another simple diagram:
ECF
• Micelles: form when a
‘handful’ of phospholipid
molecules are placed in
water.
• Micelles are made
artificially.
CYTO.
Very little water
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• Hydrogen bonds between the polar heads of
the phospholipids and water on either side of
the mb help to keep the bilayer intact.
• So do the hydrophobic interactions (London
Forces) that exist between the tails of the
phospholipids.
• Phospholipids are able to move or shift in the
bilayer since the individual molecules are not
bound to each other. However, they can only
shift in one layer, they very rarely ‘flip’ over to
the opposite layer. Why not?
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Transporting of Substances Across Mb
• Due to the highly hydrophobic (non-polar) inner core of the
phospholipid bilayer, only certain molecules/substances are
able to move in/out of a cell without aid of some kind.
• What, therefore, is able to move across a mb on its own?
– Water! Follows a concentration gradient (moves from higher to lower
water concentrations (ie. lower to higher solute concentrations))…the
diffusion of water is known as osmosis. In general, the strength of the
‘flow’ (bulk flow) of water pushes it through the hydrophobic core
undeterred.
– Small, non-polar molecules:
•
•
•
-- relatively small hydrocarbons.
-- oxygen (O2)
-- carbon dioxide (CO2)
– One larger, non-polar molecule-type  fatty acids.
– These molecules simply diffuse across the phospholipid bilayer.
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• What molecules/substances CANNOT move across a
mb on its own (and why not)?
-- Ions/salts (Na+, K+, Ca2+, Cl-, etc.) – these are
charged, and therefore polar ions requiring
channel/carrier proteins (more on these later). As
well, they tend to be surrounded by water molecules,
making them seem bigger than they really are,
allowing the hydrophobic core of the mb to repel them
more easily.
-- Larger polar molecules (glucose, amino acids,
glycerol etc) – require carrier proteins as they are too
large for channel proteins.
-- Macromolecules (Carbohydrates, Proteins,
Fats) – they are simply too big, but can enter through
vesicle formation (endocytosis) or exit through vesicle
formation (exocytosis) (more on these later too). 14
- SEE FIG. 4.4 p. 72
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