Week 6 - Oct 6

Lipids, Membranes & Membrane Transport

Introduction to Biological Membranes

Biological membranes are essential structures composed primarily of lipids and proteins. They serve as selective barriers, controlling the movement of substances into and out of cells and organelles.

• Lipids form the basic structure of the membrane, creating a hydrophobic barrier.

• Proteins are embedded within or associated with the lipid bilayer, facilitating transport and communication.

Solutions and Tonicity

Types of Solutions

The movement of water and solutes across membranes is influenced by the relative concentrations of solutes in different solutions.

• Hypertonic Solution: Higher solute concentration compared to another solution. Water moves out of the cell, causing it to shrink.

• Hypotonic Solution: Lower solute concentration compared to another solution. Water moves into the cell, causing it to swell.

• Isotonic Solution: Equal solute concentration compared to another solution. No net movement of water.

Types of Membrane Transport

Overview of Transport Mechanisms

Substances cross biological membranes by several mechanisms, classified by energy requirement and the involvement of proteins.

• Simple Diffusion (Passive): Movement of small, nonpolar molecules directly through the lipid bilayer, down their concentration gradient.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion (Passive): Movement of substances down their concentration gradient with the help of membrane transport proteins.

• Active Transport: Movement of substances against their concentration gradient, requiring energy input.

Facilitated Diffusion

Mechanism and Proteins Involved

Facilitated diffusion allows polar or charged molecules to cross the membrane with the help of specific proteins, without energy input.

• Occurs along the concentration gradient (from high to low concentration).

• Two main types of transport proteins:

  • Channel Proteins

  • Carrier Proteins

Channel Proteins

• Create hydrophilic channels for specific ions or water molecules to pass through the membrane.

• Transport does not involve conformational changes during solute passage.

• Most are specific for a particular ion or water (e.g., aquaporins).

Aquaporins are specialized channel proteins that facilitate rapid water transport. Their discovery was awarded the 2003 Nobel Prize in Chemistry to Peter Agre.

Gated Channel Proteins

• Many channel proteins are gated, opening or closing in response to specific stimuli:

  • Voltage (electrical changes)

  • Ligand (molecule binding)

  • Mechanical pressure

• Conformational change opens or closes the channel, but once open, solute passage does not require further conformational change.

Example: The CFTR (Cystic Fibrosis Transmembrane Conductance Regulator) is a gated chloride channel regulated by ATP and phosphorylation.

Role in Nerve Impulse Transmission

• Nerve impulses rely on the opening and closing of gated ion channels (e.g., Na+, K+ channels) to propagate electrical signals along neurons.

Factors Affecting Diffusion Rate Through Channels

• The diffusion rate across the membrane depends on:

  1. Concentration gradient of the solute

  2. Number of channel proteins present

  3. Open or closed state of the channels

Carrier Proteins

• Undergo conformational changes to transport specific solutes across the membrane.

• Transport is still passive (down the concentration gradient).

• Example: GLUT-1 carrier protein transports glucose into cells.

1. Molecule attaches to transporter binding site.

2. Carrier protein changes conformation.

3. Transporter opens to other side and solute diffuses away.

Active Transport

Overview

Active transport moves substances against their concentration gradient, requiring energy input.

• Essential for maintaining concentration differences across membranes.

Types of Active Transporters

• Pumps: Use ATP or light as an energy source (e.g., sodium-potassium pump).

• Coupled Transporters: Use the energy stored in ion gradients (e.g., symporters and antiporters).

Mechanism of Pumps (e.g., Na+/K+ ATPase)

1. ATP phosphorylates the protein, putting it in a high-energy state.

2. Solute binds to the protein.

3. Protein undergoes a conformational change, opening the binding site to the other side of the membrane.

4. Solute is released; binding affinity decreases.

5. Protein reverts to its original shape.

Pumps and Membrane Potentials

• Pumps create a membrane potential—a difference in charge across the membrane.

• This is crucial for nerve impulse transmission and muscle contraction.

Electrochemical Gradients

• Electrochemical gradient is the sum of the chemical (concentration) gradient and the electrical (charge) gradient.

$\text{Electrochemical gradient} = \text{chemical (concentration) gradient} + \text{electrical (charge) gradient}$

The Sodium-Potassium Pump (Na+/K+ ATPase)

• Moves 3 Na+ ions out of the cell and 2 K+ ions into the cell per ATP hydrolyzed.

• Maintains high K+ and low Na+ inside the cell.

Stepwise Mechanism:

1. Na+ binds to the pump from inside the cell.

2. ATP phosphorylates the pump, causing a conformational change.

3. Na+ is released outside the cell.

4. K+ binds from outside the cell.

5. Phosphate group is released, pump returns to original conformation.

6. K+ is released inside the cell.

Membrane Potential or Potential Difference

• The difference in electrical charge between the inside and outside of the cell membrane.

• Measured in millivolts (mV).

Importance: Nerve Impulse Transmission

• Nerve impulses are electrical currents traveling along axons, generated by the movement of ions through pumps and gated channels.

• Pumps and gated channels work together to create and propagate action potentials.

Coupled Transport (Cotransport)

Mechanism

Coupled transport uses the energy stored in ion gradients to move other molecules against their concentration gradients.

• Symport: Both molecules move in the same direction across the membrane.

• Antiport: Molecules move in opposite directions.

Example: Na+/Glucose Cotransporter

• Uses the Na+ gradient (created by the Na+/K+ pump) to drive glucose uptake into cells against its concentration gradient.

• Binding of Na+ and glucose induces a conformational change, allowing both to be transported into the cell.

• Affinity for glucose decreases after transport, releasing glucose into the cytosol.

Transport Type	Direction of Movement	Energy Source	Example
Simple Diffusion	Down gradient	None	O2, CO2
Facilitated Diffusion (Channel)	Down gradient	None	Ion channels, Aquaporins
Facilitated Diffusion (Carrier)	Down gradient	None	GLUT-1
Active Transport (Pump)	Against gradient	ATP	Na+/K+ ATPase
Coupled Transport (Symport/Antiport)	Against gradient (for one molecule)	Ion gradient	Na+/Glucose Cotransporter

Additional info: The coordinated action of pumps, channels, and cotransporters is essential for physiological processes such as nutrient absorption, nerve signaling, and osmoregulation.