1.7 Membrane Transport Mechanism

Introduction

Cells need food materials, salts, water, and oxygen. They also need to get rid of substances such as carbon dioxide. The plasma membrane controls the movement of these substances into and out of the cell. This movement is called membrane transport.

Definition

Active Transport: The movement of molecules or ions across a membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, is called active transport.

In simple words: Active transport is the movement of substances against the concentration gradient, which requires energy.

Key Points

• Movement is usually in one direction only.
• It requires energy (ATP).
• It uses transport proteins (carrier proteins).
• It moves substances against the concentration gradient (from low to high concentration).

Comparison: Passive vs Active Transport

Core Concept

• Membrane transport is dependent upon:
• The permeability of the membrane
• The solute concentration
• The size of the solute
• The charge of the solute

• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport

• Simple diffusion, facilitated diffusion, and osmosis are passive transport mechanisms.
• Active transport is an active mechanism (requires energy).

Detailed Explanation

Types of Membrane Transport

1. Simple Diffusion

Intermediate Level

Concentration Gradient

• The concentration gradient is the difference in concentration between two regions.
• Substances naturally move from high concentration to low concentration (down the gradient).
• This is called passive transport because it does not require energy.
• Moving against the gradient (low to high) requires energy and is called active transport.

Why Do Molecules Move?

• Molecules are in constant random motion.
• This random motion causes them to spread out from areas of high concentration to areas of low concentration.
• This is called diffusion.
• When equilibrium is reached, the concentration is equal on both sides, but molecules continue to move randomly.

Why Do Some Molecules Need Help?

• The plasma membrane is a phospholipid bilayer.
• The interior of the bilayer is hydrophobic (water-fearing).
• Small, nonpolar molecules (O₂, CO₂) can dissolve in the hydrophobic interior and pass through easily.
• Large, polar molecules (glucose) and ions (Na⁺, K⁺) cannot pass through the hydrophobic interior.
• They need transport proteins to help them cross.

Transport Proteins

• Form a pore or channel through the membrane.
• Allow specific molecules or ions to pass through.
• Can be open (always open) or gated (open or closed in response to a signal).

• Bind to the molecule.
• Change shape to move the molecule across the membrane.
• Release the molecule on the other side.
• Are affected by temperature and saturation.

Osmosis and Water Potential

• Osmosis is the diffusion of water across a semipermeable membrane.
• Water moves from a region of higher water potential to a region of lower water potential.
• Water potential is a measure of the tendency of water to move.
• Pure water has the highest water potential.
• Adding solutes lowers the water potential.

Active Transport

• Moves substances against the concentration gradient.
• Requires energy (ATP).
• Uses carrier proteins (often called pumps).
• Example: Sodium-potassium pump – moves Na⁺ out of the cell and K⁺ into the cell, both against their concentration gradients.

Important Points

• 1. Membrane transport is a set of mechanisms that control the movement of solutes through biological membranes.
• 2. Membrane transport depends on:
• - Permeability
• - Solute concentration
• - Size of solute
• - Charge of solute
• 3. There are four types of transport mechanisms:
• - Simple diffusion
• - Facilitated diffusion
• - Osmosis
• - Active transport
• 4. Simple diffusion, facilitated diffusion, and osmosis are passive transport mechanisms.
• 5. Simple diffusion: Molecules move directly through the membrane from high to low concentration. No transport proteins needed.
• 6. Concentration gradient: The difference in concentration between two regions.
• 7. Facilitated diffusion: Molecules move from high to low concentration with the help of transport proteins (channel or carrier).
• 8. Channel proteins: Form pores; can be open or gated.
• 9. Carrier proteins: Bind to molecules, change shape, and release them on the other side.
• 10. Osmosis: Water moves from low solute concentration to high solute concentration through a semipermeable membrane.
• 11. Semipermeable membrane: Allows solvent molecules (water) to pass through but not larger solute molecules.
• 12. Osmotic pressure: The minimum pressure applied to a solution to stop the flow of solvent molecules through a semipermeable membrane.
• 13. Water potential: A measure of the tendency of water to move from one place to another.
• 14. Active transport: Movement of molecules or ions from low to high concentration, against the concentration gradient. Requires energy (ATP) and transport proteins.
• 15. Comparison of simple and facilitated diffusion:
• - Both are passive.
• - Both move substances from high to low concentration.
• - Both do not directly require chemical energy.
• - Simple diffusion does not require transport proteins; facilitated diffusion does.
• - Simple diffusion is generally slower but more straightforward; facilitated diffusion is faster but affected by factors such as temperature and membrane proteins.

Remember

• - Four types of transport:
• 1. Simple diffusion
• 2. Facilitated diffusion
• 3. Osmosis
• 4. Active transport
• - Passive: Simple diffusion, facilitated diffusion, osmosis
• - Active: Active transport (requires ATP)
• - Simple diffusion: Direct, no proteins, high to low
• - Facilitated diffusion: Needs proteins, high to low
• - Channel proteins: Pores (open or gated)
• - Carrier proteins: Bind, change shape, release
• - Osmosis: Water across semipermeable membrane, low solute to high solute
• - Active transport: Low to high, requires ATP and proteins
• - Concentration gradient: Difference in concentration
• - Osmotic pressure: Pressure to stop osmosis
• - Water potential: Tendency of water to move

Common Mistakes

• 1. Confusing simple diffusion and facilitated diffusion – Both are passive and move substances from high to low concentration. The difference is that facilitated diffusion requires transport proteins.
• 2. Thinking osmosis is the movement of solute – Osmosis is the movement of water (solvent), not solute.
• 3. Thinking active transport does not require energy – Active transport requires energy (ATP) because it moves substances against the concentration gradient.
• 4. Forgetting that channel proteins and carrier proteins are different – Channel proteins form pores; carrier proteins bind and change shape.
• 5. Thinking all molecules can pass through the membrane easily – Only small, nonpolar molecules can pass through easily. Large, polar molecules and ions need transport proteins.
• 6. Confusing concentration gradient with water potential – Concentration gradient is the difference in solute concentration; water potential is the tendency of water to move.
• 7. Thinking osmotic pressure and water potential are the same – Osmotic pressure is the pressure required to stop osmosis; water potential is a measure of the tendency of water to move.
• 8. Forgetting that osmosis is a passive process – Osmosis does not require energy.
• 9. Thinking active transport moves substances from high to low concentration – Active transport moves substances from low to high concentration (against the gradient).
• 10. Mixing up the examples of osmosis – Feeling thirsty after salty food, dialysis of kidney, swelling of resins and seeds.

Quick Revision

• Membrane transport is a set of transport mechanisms that control the movement of solutes such as ions and small molecules through biological membranes.
• Membrane transport depends on permeability, solute concentration, size, and charge of the solute.
• There are four types of transport mechanisms:
• Simple diffusion, facilitated diffusion, and osmosis are passive transport mechanisms.
• Concentration gradient: The difference in concentration between two regions.
• Semipermeable membrane: Allows solvent molecules (water) to pass through but not larger solute molecules.
• Osmotic pressure: The minimum pressure applied to a solution to stop the flow of solvent molecules through a semipermeable membrane.
• Water potential: A measure of the tendency of water to move from one place to another.
• Comparison of simple and facilitated diffusion:
• Both are passive.
• Both move substances from high to low concentration.
• Both do not directly require chemical energy.
• Simple diffusion does not require transport proteins; facilitated diffusion does.
• Simple diffusion is generally slower but more straightforward; facilitated diffusion is faster but affected by factors such as temperature and membrane proteins.

60-Second Revision

• Four transport types: Simple diffusion, facilitated diffusion, osmosis, active transport
• Passive: Simple diffusion, facilitated diffusion, osmosis
• Active: Active transport (ATP)
• Simple diffusion: Direct, no proteins, high to low
• Facilitated diffusion: Proteins needed, high to low
• Channel proteins: Pores (open/gated)
• Carrier proteins: Bind, change shape, release
• Osmosis: Water, semipermeable membrane, low solute to high solute
• Active transport: Low to high, ATP, proteins
• Concentration gradient: Difference in concentration
• Osmotic pressure: Pressure to stop osmosis
• Water potential: Tendency of water to move
• Warning: Osmosis is the movement of water, not solute. Active transport requires energy. Simple diffusion and facilitated diffusion are both passive but facilitated diffusion needs proteins.