Download transport across the membrane

BIOLOGY 12 - Cell Membrane Notes
TRANSPORT ACROSS THE MEMBRANE
Please Label the Parts of the Cell Membrane
THERE ARE TWO GENERAL MEANS BY WHICH
SUBSTANCES CAN ENTER AND EXIT CELLS:
I. Passive Transport
II. Active Transport
Requires no energy and follows the
concentration gradient.
Requires energy and goes against
the concentration gradient.
1. DIFFUSION
2. OSMOSIS
3. FACILITATED TRANSPORT
1. ACTIVE TRANSPORT
2. ENDOCYTOSIS
3. EXOCYTOSIS
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1. DIFFUSION
BEFORE
AFTER
• Diffusion is a physical process that can be observed
with any type of particle.
• Law of Diffusion: molecules MOVE FROM AN
AREA OF HIGH CONCENTRATION TO AN AREA
OF LOW CONCENTRATION UNTIL EQUALLY
DISTRIBUTED.
• movement of molecules by diffusion requires no
energy to be added (although adding energy (i.e.
heat) will speed it up).
• is a slow process
- diffusion in liquid is slower than in gas
Three Ways of increasing the rate of diffusion:
1. increase the temperature
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2. increase the concentration gradient
3. decrease the size of the diffusing molecules
• properties of the cell membrane allow few types of
molecules to pass by diffusion: Lipid-soluble
molecules like steroids and alcohols can diffuse
directly across because the membrane itself is made
of lipids
2. Osmosis
• is the movement of water from an area of high water
conc. to an area of low water conc. across a
selectively permeable membrane
• Water diffuses readily across membrane, through
charged, protein-lined pores in the membrane
(remember, water is not lipid-soluble) that will not
allow anything else but water through.
Solute: particles which are dissolved in water.
Solvent: liquid which dissolves the solute. This is
water when we are talking about osmosis.
Solution: combination of solute and solvent.
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Osmotic Pressure: the pressure due to the flow of
water from the area of greater concentration to the
area of lesser concentration. The greater the
concentrations difference across the membrane,
the greater the osmotic pressure.
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Summary of what happens to ANIMAL CELLS
placed in different tonicities of solution:
Tonicity of Solution
Cell is Put Into
Net Movement of
Water
Effect on Cell
Isotonic
No net movement
Remains the same
Hypotonic
Cell gains water
Cell Swells & May
Burst
H2O
“lyses”
Hypertonic
Cell loses water
Cell Shrinks
H2O
“crenates” (animals)
“plasmolysis”
(plants)
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3. FACILITATED TRANSPORT
• Utilizes PROTEIN CHANNELS in cell membrane to
control passage of molecules in and out of cell.
• are highly specific - each carrier passes only one
type molecule
• Molecules only pass along concentration gradient.
• REQUIRES NO ENERGY - is like diffusion in this
sense
• Explains how lipid-insoluble molecules like
GLUCOSE and AMINO ACIDS cross the cell
membrane.
FACILITATED TRANSPORT
BEFORE
OUTSIDE CELL
INSIDE CELL
OUTSIDE CELL
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INSIDE CELL
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II ACTIVE TRANSPORT:
Active Transport:
• performed by protein carriers
• REQUIRES ENERGY (ATP)
• moves molecules against the conc. gradient
(moving from an area of low conc. to an area of high
conc).
ACTIVE TRANSPORT
BEFORE
OUTSIDE CELL
INSIDE CELL
OUTSIDE CELL
INSIDE CELL
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ENDOCYTOSIS AND EXOCYTOSIS
• are both forms of Active Transport and require ATP
energy
- energy required because they are changing the
shape of the cell membrane. It requires a great
deal of energy to fuse a vacuole with the cell
membrane or create a vacuole from the cell
membrane. The vacuoles are made from a
phospholipid bilayer that is the same as the cell
membrane’s bilayer. This is why they can fuse
together.
• ENDOCYTOSIS: cell membrane forms a vesicle
around the substance to be taken in.
• Phagocytosis: what you call endocytosis if
particles taken in are really large (like other cells e.g. human macrophages). Can be seen with light
microscope.
• Pinocytosis: (= cell drinking) - same idea as
phagocytosis, except smaller particles taken in
(requires electron microscope to see).
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• EXOCYTOSIS: Reverse of endocytosis. Vacuole
within cell fuses with cell membrane and the vacuole
contents are deposited on the outside. Important in
secretion and excretion in cells.
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