Membrane Structure and Function

Chapter 7: Membrane Structure and Function

Introduction to the Plasma Membrane

• Overview of topics covered this week:

  • Structure of the plasma membrane

  • Fluid properties of the membrane

  • Selective permeability of membranes

  • Osmosis

  • Transport across the membrane, including:

  • Facilitated diffusion

  • Active transport

  • Bulk transport

Structure of the Plasma Membrane

• The plasma membrane's primary role is to control the movement of substances into and out of the cell.

• Composition:

  • The plasma membrane consists of two lipid leaflets, making it a lipid bilayer.

  • Hydrophilic heads face outward, while hydrophobic tails face inward, forming a hydrophobic barrier to most water-soluble molecules.

Properties of the Plasma Membrane

• Passive transport and active transport:

  • Small molecules utilize passive transport without energy, sometimes with the help of proteins.

  • Active transport requires energy input and transport proteins.

  • Large molecules cross via bulk transport mechanisms: exocytosis or endocytosis.

Tonicity and Ion Concentration

• Essential for cells to maintain a different concentration of ions inside and outside. Specific concentrations for a mammalian cell:

  • Sodium [$ ext{Na}^+$] concentration inside << outside

  • Potassium [$ ext{K}^+$] concentration inside >> outside

  • Calcium [$ ext{Ca}^{2+}$] concentration inside << outside

  • Chloride [$ ext{Cl}^-$] concentration inside << outside

• Tonicity: overall solute particle concentration inside and outside the cell must be approximately the same for animal cells.

Membrane Components and Structure

• Main components: lipids, proteins, and carbohydrates

  • Major composition consists of phospholipids which are amphipathic: having hydrophobic and hydrophilic regions.

• Cellular membranes are referred to as fluid mosaics of lipids and proteins.

Fluid Mosaic Model

• Describes the membrane as a mosaic of protein molecules drifting in a fluid bilayer of phospholipids.

• Membrane proteins are not randomly distributed; they cluster for specific functions.

• Membrane stability comes from weak hydrophobic interactions that allow lipids and proteins to move laterally, but flipping from one layer to another is rare (occurs approximately once per month).

• Temperature affects membrane fluidity:

  • Membrane switches from fluid to solid state as temperature decreases.

  • Membranes rich in unsaturated fatty acids remain fluid at lower temperatures.

Cholesterol in Membranes

• Cholesterol has a variable effect on membrane fluidity; it restrains movement at warm temperatures and helps maintain fluidity at cooler temperatures.

• Adaptive mechanisms: some organisms adjust lipid composition based on environmental conditions.

  • Example: In winter wheat, the percentage of unsaturated phospholipids increases in autumn to prevent solidification.

Membrane Proteins and Their Functions

• Types of membrane proteins:

  • Peripheral proteins: Bound to the surface of membranes.

  • Integral proteins: Penetrate the hydrophobic core, many are transmembrane proteins that span the membrane.

• Functions of membrane proteins:

  • Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton and extracellular matrix (ECM).

• Cell surface proteins: Important in medicine (e.g., HIV's interaction with CD4 and CCR5).

Membrane Properties: Sidedness

• Membranes exhibit asymmetrical compositions, with carbohydrates mainly on the extracellular side.

• Importance for cell recognition: surface molecules attach to branched carbohydrate chains, forming glycolipids and glycoproteins.

Selective Permeability of Membranes

• Membranes regulate molecular traffic and have selective permeability; hydrophobic molecules pass easily while hydrophilic molecules require assistance.

• Transport proteins are crucial for the movement of hydrophilic substances.

  • Main classes:

  • Channel proteins form pores for specific molecules.

  • Carrier proteins bind and shuttle substances across membranes.

• Diffusion and facilitated diffusion mechanisms are employed for transport:

  • Diffusion is the movement of particles down their concentration gradient, requiring no energy.

Osmosis and Tonicity

• Osmosis is the diffusion of free water across a selectively permeable membrane and occurs from lower to higher solute concentration until balance is achieved.

• Types of solutions:

  • Isotonic: no net movement of water;

  • Hypertonic: net water loss leading to cell shrinkage;

  • Hypotonic: net water gain, leading to cell swelling and potential lysis.

Transport Mechanisms

• Facilitated Diffusion: Uses transport proteins to speed up passive movement across membranes, including channel and carrier proteins.

• Active Transport: Moves solutes against their gradients using energy, maintaining concentration differences. Example: sodium-potassium pump.

Mechanisms of Bulk Transport

• Bulk transport methods include:

  • Exocytosis: Vesicles release contents outside the cell.

  • Endocytosis: Cell engulfs material using vesicular transport, including three types:

  • Phagocytosis: cellular eating;

  • Pinocytosis: cellular drinking;

  • Receptor-mediated endocytosis: specific binding of substances triggers vesicle formation.

• Importance in cells: e.g., human cells use receptor-mediated endocytosis for cholesterol uptake via LDLs.

Conclusion

• Membrane structure and function are complex and essential for cellular operations, affecting everything from molecular transport to cellular interaction and signaling.

• The health of membranes underlies many biological processes and medical implications, emphasizing the importance of continued study in membrane biology.