Reviewer for LT 3.1
Cell Membrane structures
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The cell membrane follows the “Fluid Mosaic Model”. This is the model used by the biologists to
describe the structure of the cell membrane.
Fluid – moves about freely
Mosaic – made up of different things that together make a pattern
It is semi – permeable meaning it only allow certain substances to pass through.
The cell membrane is composed of proteins embedded in a bilayer of phospholipids
Phospholipid bilayer – double layer of phospholipid molecules
Hydrophilic heads (water-loving) – can be seen
inside and outside the cell (intracellular and
extracellular)
Hydrophobic tails (water-fearing) – can be seen
sandwiched between the hydrophilic heads
Cell Transport
Cell Membrane
Intracellular
Extracellular
Solute and Solvent
Solute and Solvent
75% sodium
concentration
25% sodium
concentration
(water and sodium)
(water and sodium)
This is not balanced at all. The movement of concentration will be OUTSIDE of the cell or from an
AREA OF HIGH CONCENTRATION TO AN AREA OF LOW CONCENTRATION to be able to attain
equilibrium. (50% - 50%)
*if it already established equilibrium, the net movement of the substances is zero.
TERMS
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concentration - the measure of the amount of a sub-component in a solution
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solute - substance being dissolved
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solvent - substance that dissolves
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concentration gradient - the difference in concentration between two areas (The areas are
typically separated by a membrane.)
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tonicity – ability of a solution to cause a cell to gain or lose water
PASSIVE TRANSPORT
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movement of molecules/substances across a cell membrane that does not require energy.
It has three types: simple diffusion, facilitated diffusion and osmosis
The greater the difference in concentration (ex: 95% - 5%), the faster the movement of
molecules.
1. Simple diffusion
- The movement of molecules is from an area of HIGH CONCENTRATION to an area of LOW
CONCENTRATION (remember: to attain equilibrium) down/with the concentration gradient.
SUBSTANCES: small, non – polar molecules (the bind between molecules are WEAK) like fats,
oxygen, carbon dioxide.
2. Facilitated diffusion
- The movement of molecules is from an area of HIGH CONCENTRATION to an area of LOW
CONCENTRATION (remember: to attain equilibrium) down/with the concentration gradient via
SPECIFIC TRANSPORT PROTEINS.
Transport Proteins:
• Channel Protein – this is also called the “gateway”, allowing specific solutes to pass through.
(VERY VERY SPECIFIC TYPE OF TRANSPORT PROTEIN)
• Carrier Protein – changes shape to allow molecules to pass through.
Note: It doesn’t mean that since it can change shape, all molecules will pass through.
Remember that the cell membrane is semi – permeable in nature so it only allows certain
substances to pass through.
SUBSTANCES: large, polar molecules (the bind between the molecules are STRONG) like
glucose, amino acids, and chlorine ion.
3. Osmosis
- The diffusion of water molecules from an area of LOW CONCENTRATION to an area of HIGH
CONCENTRATION (so there will be a possibility of 100% - 0% or vice versa)
SUBSTANCES: water ONLY
TONICITY OF SOLUTIONS
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concentration of solute/solvent in a solution in comparison to the concentration of another
solution
influences the movement of water into or out of a cell via osmosis (if that’s the case, the
movement of water molecules is from an area of LOW CONCENTRATION to an area of HIGH
CONCENTRATION
it has three types: hypertonic, hypotonic and isotonic solution
*tonicity is more on extracellular rather than intracellular
1. Hypertonic
- ‘hyper’ means above; ‘tonic’ means strength
- the water concentration inside the cell is less concentrated so it will
move OUTSIDE the cell, to the area of high concentration.
PLANT CELL: shrinks (because of the cell wall)
ANIMAL CELL: shriveled
2. Hypotonic
- ‘hypo’ means below; ‘tonic’ means strength
- the water concentration outside the cell is less concentrated so it will
move INSIDE the cell, to the area of high concentration.
PLANT CELL: turgid (because of the cell wall)
ANIMAL CELL: burst
3. Isotonic
- ‘iso’ means same; ‘tonic’ means strength
- the water concentration outside and outside the cell is equal in solute
and solvent concentration, therefore, no net water movement.
PLANT CELL: normal
ANIMAL CELL: normal
High
concentration
Less
Less
concentrated
concentration
Low
concentration
High
concentration
Same
concentration
Same
concentration
ACTIVE TRANSPORT
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The movement of molecules/substances up/across the concentration gradient using the energy
from ATP (Adenosine triphosphate)
It has two kinds: molecular and bulk transport
1. Molecular Transport (keyword: molecules/ions, small particles)
- Movement of ions through the plasma membrane up/against the concentration gradient.
Example: Sodium – Potassium Pump (important for nerve impulses/nerve transmitters)
*Sodium-Potassium Pump (aka Protein Pump) - moves three Na+ ions OUT of the cell and two K+ ions
INTO the cell, using ATP
2. Bulk Transport (keyword: bulk, whole, large)
- movement of large amounts of substances (or whole cells) through the plasma membrane up/ against
the concentration gradient.
2 Types:
a. Endocytosis (keyword: engulf, take in, delivery inside; ‘endo’ means INSIDE)
- engulfing of large molecules (or even whole cells) occurs by infoldings or formation of pouches of
the cell membrane.
b. Exocytosis (keyword: release, delivery outside; ‘exo’ means OUTSIDE)
- expulsion of materials from a cell; occurs when a vesicle (or vacuole) fuses with the plasma
membrane