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Mechanisms of Transport Through the Cell Membrane
Membrane Permeability
Cell membrane is made up of lipids
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Allows passage of only lipid-soluble substances e.g. CO2, O2, steroid hormones, etc.
Does not allow passage of water-soluble substances e.g. sodium ions, chloride ions, etc.
Transport proteins on the cell membrane make holes in the membrane
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Allow passage of water-soluble substances.
Types of Transport Mechanisms
Diffusion
Passive process.
Driven by electrochemical gradient i.e. substance moves down the electrochemical gradient.
e.g. glucose concentration is higher outside the cell than inside โ glucose diffuses from outside to inside down this concentration gradient.
Electronegativity inside the cell attracts positively charged ions and repels negatively charged ions.
Active Transport
Substance is moved against electrochemical gradient.
Energy is required.
Depending on source of energy there are two types of active transport: Primary and Secondary.
Primary Active Transport
Energy is obtained directly from ATP.
e.g. Na-K-ATPase pump
Moves Na from inside to outside the cell (against electrochemical gradient).
Moves K from outside to inside the cell (against electrochemical gradient).
Uses ATP for this.
Secondary Active Transport
Energy is obtained from downhill movement of another solute.
e.g. Na-Glucse Cotransporter
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Allows downhill movement of Na
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Uses energy from this movement of Na to move glucose uphill
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Here transport of glucose is secondary active transport.
Osmosis
Movement of water from low osmolarity to high osmolarity.
Passive process.