Membrane

Unit 2: Membranes & Transport

Main takeaway: Cell membranes are selectively permeable barriers that regulate exchange and communication. Substances cross them by passive processes, which do not require cellular energy, or by active transport, which uses energy directly or indirectly. Diffusion, osmosis, channels, and carriers are central mechanisms.

Lecture Outline and Membrane Functions

• Lecture topics: Plasma membrane; passive transport; channels and carriers (transporters); active transport; examples.

• Plasma membrane functions:

1. Physical barrier

2. Exchange

3. Communication

4. Attachment

• Membrane structure: The fluid mosaic model describes the plasma membrane.

• Phospholipids are amphipathic:

• Hydrophilic, polar heads

• Hydrophobic, nonpolar tails

• Membrane proteins:

• Peripheral proteins attach to the inner or outer membrane surface.

• Integral proteins extend partly or fully across the membrane and include channels and carriers.

• Membranes are selectively permeable: relatively few molecules, such as gases and small lipid-soluble compounds, cross readily.

• Relative sizes: Ion channels and transporters are distinct membrane structures; no specific size measurements were provided.

Passive and Active Transport

Passive Transport

Passive transport does not require energy from the cell. Materials move down their concentration gradients, from higher to lower concentration.

Includes:

1. Simple diffusion

2. Osmosis (water movement)

3. Facilitated diffusion

4. Bulk filtration

Active Transport

Active transport requires energy to move molecules against their concentration gradients, from lower to higher concentration.

• Primary active transport: Directly consumes ATP.

• Secondary active transport: Uses energy stored in a pre-existing concentration gradient.

Diffusion

Fick’s Law

In one dimension, the solute flux depends on the solute’s concentration gradient and diffusion coefficient. With the conventional sign indicating movement down the gradient:

$J_S = -D_S \frac{dC_S}{dx}$

where $J_S$ is the flux of solute $S$, $D_S$ is its diffusion coefficient, and $dC_S/dx$ is its concentration gradient.

Seven Properties of Simple Diffusion

1. It is a passive process.

2. Net movement is from high to low concentration.

3. It requires a concentration gradient.

4. It is rapid over short distances.

5. It is directly related to temperature.

6. It is inversely related to molecular size.

7. It occurs in an open system or across a partition.

Diffusion Over Time and Distance

• Molecules spread randomly over time; diffusion is nonlinear over distance and time.

• The average distance a diffusing particle travels from its starting point over time $t$ is described by the root mean square (RMS) distance:

$\text{RMS distance} = \sqrt{2D_S t}$

• Diffusional distance increases with the square root of time, unlike linear motion, where distance increases in direct proportion to time.

Factors Affecting Diffusion Across a Membrane

• Membrane surface area

• Lipid solubility

• Molecular size

• Membrane thickness

• Concentration gradient, determined by concentrations outside and inside the cell

• Composition of the lipid layer

Structure–Function Relationship

• Surface area is a factor in Fick’s equation.

• Structures that need to maximize diffusional flux tend to have expanded surface area and attenuated (reduced) linear dimensions.

Osmosis and Forces Acting on Cells

• Osmosis is the diffusion of water across a selectively permeable membrane.

• Osmolarity is the number of particles in solution.

• Adding solutes lowers the concentration of water.

• Tonicity refers to the outside environment relative to the cell, including whether it has more or fewer solutes.

• Osmotic force is created by diffusion of water.

• Hydrostatic force is the pressure required to block osmosis.

• Membranes can differ in permeability: some are freely permeable, while others are selectively permeable.

Channels and Carriers

Ion Channels

• Ion channels are selective for particular ions.

• Selectivity is associated with a conserved amino-acid sequence in the channel’s selectivity filter.

• In the potassium channel KcsA, conserved glycines are highlighted in the filter sequence; other chemically similar amino acids are also conserved.

• As the filter sequence becomes less conserved, channels may lose selectivity for $K^+$ over $Na^+$ and become more permeable to $Ca^{2+}$.

• Channelopathies are diseases or disorders resulting from ion channel dysfunction.

• Ion-channel gating mechanisms are a classification topic; specific mechanisms were not detailed.

• TTX was listed as an example, without further explanation.

Carrier/Transporter-Mediated Transport

• Carriers (transporters) transport solutes, including ions and molecules, across cell membranes.

• Carriers are not evolved for speed.

• In the alternating access model, a carrier cycles through conformations in which its binding site is accessible to only one side of the membrane at a time.

• A carrier never forms an open channel all the way through the membrane.

• In the occlusion state, the solute is accessible from neither the inside nor the outside.

• Carriers typically transport one to three molecules per conformational cycle. Their transport rate is slower than channel-mediated transport; a single channel opening can allow thousands of ions per second to pass.

Facilitated Diffusion

• Facilitated diffusion is passive but requires a carrier protein.

Active Transport Examples

Primary Active Transport: Sodium–Potassium Pump

• The sodium–potassium pump is a 3 Na⁺/2 K⁺ ATPase.

• In the cycle described:

1. Three $Na^+$ ions from the intracellular fluid (ICF) bind the protein.

2. ATP supplies a phosphate group, phosphorylating the ATPase.

3. The protein changes conformation and releases three $Na^+$ ions into the extracellular fluid (ECF).

4. Two $K^+$ ions from the ECF bind.

5. The protein changes conformation again, releasing two $K^+$ ions into the ICF.

Secondary Active Transport

• Typically, one or more solutes move down their concentration gradients, providing energy for other solutes to move up their gradients.

• SGLT (sodium–glucose cotransporter) illustrates this process in the intestine or kidney:

1. $Na^+$ binds to the carrier.

2. Sodium binding creates a site for glucose.

3. Glucose binds, changing the carrier’s conformation.

4. $Na^+$ is released into the cytosol, and glucose follows.

• The diagram shows high $Na^+$ and low glucose on one side, and low $Na^+$ and high glucose on the other.

Other Transport Mechanisms

• Bulk filtration: Moves solutes and/or water in the same direction. Unlike diffusion, it is not caused by the random motion of individual molecules.

• Transport between or through cells:

• Paracellular: Between cells.

• Transcellular: Through cells.

• Transcellular and paracellular transport were noted as topics that become more important later in the course.

Additional Course Context

• The material is from Unit 2: Membranes & Transport, presented by Dr. John Redden, Physiology and Neurobiology, University of Connecticut.