Transport Across the Cell Membrane

1
Transport Across the Cell Membrane
2

• 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.

3
IMPERMEABLE OIL SLICKERS
COMPLETELY PERMEABLE
MAPLE LEAFS GOALTENDING
4

• 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.

5
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 the mb in order to anchor it.

6
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.

7

• 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
  drivers licence or fingerprint).

8
Fluid-Mosaic Model Picture
9
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.

10

• 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)

11

• Another simple diagram
• Micelles form when a handful of phospholipid
  molecules are placed in water.
• Micelles are made artificially.

ECF
CYTO.
Very little water
12

• 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?

13
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.

14

• 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).
• - SEE FIG. 4.4 p. 72

15
(No Transcript)
16
(No Transcript)