A cell is in similitude with a fenced house that has a gate. The cell membrane serves to a cell what function does to a house. Just like a gate allows for passage to and fro the house, the cell membrane has adaptions that allow communication between the internal environment with the external environment.

1. TRANSPORT ACROSS
MEMBRANES (BCH 857)

2. OUTLINE
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Introduction
The Cell Membrane
Types of Transport
Tonicity of Solutions
Diffusion
Facilitated Transport
Osmosis
Active Transport
Factors Affecting Transport Across a Membrane
Endocytosis and Exocytosis
Summary and Conclusion

3. INTRODUCTION
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4. THE CELL MEMBRANE
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Comprises phospholipids and
proteins in ratio 3:2;
Contains carbohydrate bound to
protein as glycoprotein;
Measures about 6.5-10nm in
thickness;
Has glycolipids and glycoprotein
projecting outwards as glycocalyx;
Contains pores measuring between
0.7-1.0μ in diameter;
Is selectively permeable.

5. Selective Permeability
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6. 6
PASSIVE
ACTIVE
Passive diffusion is a spontaneous phenomenon that
increases the entropy of a system and decreases the free
energy. The transport process is influenced by the
characteristics of the transport substance and the nature
of the bilayer.
In active transport a solute is moved against a
concentration or electrochemical gradient, in doing so
the transport proteins involved consume metabolic
energy, usually ATP.
TYPES OF TRANSPORT

7. TRANSPORT PROTEINS
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Channel proteins are embedded in the
cell membrane & have a pore for materials
to cross.
Carrier proteins can change shape to
move material from one side of the
membrane to the other.

8. TONICITY OF SOLUTIONS
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The tonicity of a solution will affect the size & shape of cells:
ISOTONIC SOLUTION:
the solution concentration is equal on both sides of the membrane.
HYPERTONIC SOLUTION:
The solution outside the cell is more concentrated than inside.
HYPOTONIC SOLUTION:
The concentration inside the cell is more concentrated than outside.

9. TONICITY OF SOLUTIONS
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cytolysis
plasmolysis

10. So Should You Drink
Sea Water???
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11. DIFFUSION
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Requires NO energy.
Molecules move from
area of HIGH to LOW
concentration.
They have a natural
KINETIC ENERGY.

12. DIFFUSION
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Molecules diffuse until
they are evenly
distributed.
EXAMPLES of molecules that
easily cross cell membranes by
simple diffusion are: oxygen,
carbon dioxide, alcohols, fatty
acids, glycerol, and urea.

13. FACILITATED TRANSPORT
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Molecules will randomly
move through the opening
like pore, by diffusion.
This requires no energy, it
is a PASSIVE process.
Molecules move from an
area of high concentration
to an area of low conc.

14. FACILITATED TRANSPORT
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Each protein
channel or protein
carrier will allow
only ONE TYPE OF
MOLECULE to pass
through it.
Many channels contain a
“gate” which control the
channel's permeability.

15. OSMOSIS
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Diffusion of water across a
membrane.
Moves from HIGH water potential
(low solute) to LOW water potential
(high solute).

16. OSMOSIS
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•To cross the membrane, water
must move through a protein ion
channel.
•In certain cellular conditions,
these protein channels can be
opened or closed (i.e. in the
kidneys, large intestines)
depending on how much water is
needed by the body.

17. ACTIVE TRANSPORT
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Molecules move against the concentration
gradient.
Requiresenergy
1. Solute Pumps (Na+-K+-ATPase pump)
mostly in nerve membranes.
2. Bulk Transport.

18. FACTORS AFFECTING MOVEMENT ACROSS MEMBRANES
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The rate of diffusion will be increased when there is :
1. Concentration: the difference in concentration between two areas (the
concentration-gradient) causes diffusion. The greater the difference in
concentration, the faster the diffusion.
2. Molecular size: smaller substances diffuse more quickly. Large
molecules (such as starches and proteins) simply cannot diffuse
through.
3. Shape of Ion/Molecule: a substance’s shape may prevent it from
diffusing rapidly, where others may have a shape that aids their
diffusion.

19. FACTORS AFFECTING MOVEMENT ACROSS MEMBRANES
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4. Viscosity of the Medium: the lower the viscosity, the more slowly molecules
can move through it.
5. Movement of the Medium: currents will aid diffusion. Like the wind in air,
cytoplasmic steaming (constant movement of the cytoplasm) will aid
diffusion in the cell.
6. Solubility: lipid - soluble molecules will dissolve through the phospholipid
bilayer easily, as will gases like CO2 and O2.
7. Polarity: water will diffuse, but because of its polarity, it will not pass
through the non-polar phospholipids. Instead, water passes though
specialized protein ion channels.

20. BULK TRANSPORT
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Endocytosis (Pinocytosis)
Pinocytosis (cell drinking): small molecules
are ingested and a vesicle is immediately
formed. This is seen in small intestine cells
(villi)

21. BULK TRANSPORT
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Endocytosis (Phagocytosis)
phagocytosis (cell eating): large particles,
(visible with light microscope) are invaginated
into the cell (e.g. white blood cells ‘eat’
bacteria).

22. BULK TRANSPORT
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Endocytosis (Receptor-Mediated)
Some integral proteins have receptors
on their surface to recognize & take in
hormones, cholesterol, etc.

23. BULK TRANSPORT
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Exocytosis
Inside Cell
Cell
Environment

24. SUMMARY &
CONCLUSION
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