Membrane Structure and Function: Study Notes

Chapter 7: Membrane Structure and Function

Concept 7.1: Cellular Membranes are Fluid Mosaics of Lipids and Proteins

Cellular membranes are dynamic structures composed primarily of lipids and proteins, with carbohydrates playing important roles in cell recognition. The fluid mosaic model describes the arrangement and movement of these molecules within the membrane.

• Phospholipid Structure: Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails. When placed in water, they spontaneously form bilayers with hydrophobic tails facing inward and hydrophilic heads facing outward.

• Fluid Mosaic Model: Membranes are described as 'fluid mosaics' because proteins float in or on the fluid lipid bilayer like boats on a pond.

• Peripheral vs. Integral Proteins: Integral proteins span the membrane, while peripheral proteins are attached to the membrane surface.

• Carbohydrates in Membranes: Carbohydrates are found on the extracellular surface of the membrane, often attached to proteins (glycoproteins) or lipids (glycolipids), and play a key role in cell-cell recognition.

• Diagram: A typical membrane diagram should label integral and peripheral proteins, carbohydrate components, and lipid components.

Example: The ABO blood group antigens are determined by carbohydrate structures on the surface of red blood cells.

Concept 7.2: Membrane Structure Results in Selective Permeability

Membranes regulate the passage of substances into and out of cells, allowing some molecules to cross more easily than others. This property is known as selective permeability.

• Hydrophobic vs. Hydrophilic Molecules: Hydrophobic (nonpolar) molecules, such as O2 and CO2, can dissolve in the lipid bilayer and pass through the membrane rapidly. Hydrophilic (polar) molecules and ions, such as glucose and Na+, cannot pass as easily.

• Transport Proteins: Channel proteins and carrier proteins facilitate the movement of specific molecules across the membrane.

• Diffusion Rates: The rate at which a molecule crosses the membrane depends on its size, polarity, and the presence of transport proteins.

• Distinguishing Proteins: Channel proteins form hydrophilic channels, while carrier proteins undergo conformational changes to transport substances.

Example: Aquaporins are channel proteins that facilitate the rapid transport of water across the cell membrane.

Concept 7.3: Passive Transport is Diffusion of a Substance Across a Membrane with No Energy Investment

Passive transport involves the movement of substances across the membrane without the use of cellular energy (ATP). This includes diffusion, osmosis, and facilitated diffusion.

• Diffusion: The movement of molecules from an area of higher concentration to an area of lower concentration.

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Transport proteins help specific molecules cross the membrane down their concentration gradient.

• Concentration Gradient: The difference in concentration of a substance across a space represents potential energy for diffusion.

• Solution Types:

  • Isotonic: Solute concentration is equal inside and outside the cell; no net water movement.

  • Hypertonic: Higher solute concentration outside the cell; cell loses water.

  • Hypotonic: Lower solute concentration outside the cell; cell gains water.

• Direction of Water Movement: Water moves from areas of low solute concentration to high solute concentration.

Example: Red blood cells placed in a hypotonic solution will swell and may burst due to water influx.

Equation:

$\text{Net movement} = \text{[High concentration]} \rightarrow \text{[Low concentration]}$

Concept 7.4: Active Transport Uses Energy to Move Solutes Against Their Gradients

Active transport requires energy, usually in the form of ATP, to move substances against their concentration gradients via specific transport proteins.

• Active vs. Passive Transport: Active transport moves substances from low to high concentration (against the gradient), while passive transport moves substances down their gradient.

• Transport Proteins: Channels, carriers, and pumps are involved in active transport. Pumps, such as the sodium-potassium pump, are especially important in maintaining cellular ion gradients.

• Types of Active Transport:

  • Primary Active Transport: Direct use of ATP (e.g., Na+/K+ pump).

  • Secondary Active Transport (Cotransport): Uses the energy from the movement of one substance down its gradient to drive the movement of another substance against its gradient.

• Bulk Transport: Large molecules are transported via endocytosis (phagocytosis, pinocytosis, receptor-mediated endocytosis) and exocytosis.

• Pinocytosis vs. Receptor-Mediated Endocytosis: Pinocytosis is the non-specific uptake of extracellular fluid, while receptor-mediated endocytosis is highly specific, involving receptor proteins.

Example: The sodium-potassium pump maintains high K+ and low Na+ concentrations inside animal cells.

Equation:

$\text{Na}^+/\text{K}^+ \text{ pump:} \ 3\ \text{Na}^+ \text{ out},\ 2\ \text{K}^+ \text{ in},\ \text{per ATP hydrolyzed}$

Summary Table: Types of Membrane Transport

Type	Energy Required?	Direction (relative to gradient)	Example
Simple Diffusion	No	Down	O2 crossing membrane
Facilitated Diffusion	No	Down	Glucose via carrier protein
Osmosis	No	Down (water potential)	Water via aquaporin
Active Transport	Yes (ATP)	Against	Na+/K+ pump
Cotransport	Indirect (uses gradient)	Against (for one solute)	Sucrose-H+ cotransport in plants
Bulk Transport	Yes (ATP)	Varies	Endocytosis, exocytosis