Membrane Structure and Function: Study Notes for Cell Biology

Chapter 7: Membrane Structure and Function

Membrane Structure

The plasma membrane is a dynamic structure that separates the cell from its environment and regulates the movement of substances in and out of the cell. Its organization and properties are essential for cellular function.

• Phospholipid Bilayer: The fundamental structure of the membrane consists of a double layer of phospholipids, with hydrophilic (water-attracting) heads facing outward and hydrophobic (water-repelling) tails facing inward.

• Fluid Mosaic Model: Proposed by Singer and Nicholson in 1972, this model describes the membrane as a mosaic of proteins and other molecules embedded in a fluid lipid bilayer. Most lipids and some proteins can move laterally within the layer.

• Membrane Fluidity: Fluidity is influenced by the types of fatty acids in phospholipids and the presence of cholesterol. Unsaturated hydrocarbon tails (with kinks) increase fluidity, while saturated tails and cholesterol decrease it. Cholesterol acts as a 'fluidity buffer,' preventing the membrane from becoming too rigid or too fluid depending on temperature.

• Membrane Proteins: Proteins are either integral (embedded within the bilayer, sometimes spanning it as transmembrane proteins) or peripheral (attached to the surface or to other membrane components). Integral proteins have hydrophilic and hydrophobic regions, allowing them to interact with both the lipid bilayer and the aqueous environment.

• Membrane Carbohydrates: Carbohydrates are present as branched oligosaccharides covalently bound to lipids (glycolipids) or proteins (glycoproteins). They play a key role in cell recognition and signaling.

Membrane Components

• Lipids: Phospholipids, cholesterol, glycolipids

• Proteins: Integral and peripheral

• Carbohydrates: Glycoproteins and glycolipids

Table: Membrane Components and Their Functions

Component	Location	Main Function
Phospholipids	Bilayer	Barrier, fluidity
Cholesterol	Within bilayer	Regulates fluidity
Integral Proteins	Embedded in bilayer	Transport, signaling
Peripheral Proteins	Surface of bilayer	Cell shape, signaling
Carbohydrates	Extracellular surface	Cell recognition

Membrane Proteins

Membrane proteins are crucial for the diverse functions of the cell membrane.

• Integral Proteins: Located within the core of the lipid bilayer; some span the entire membrane (transmembrane proteins). They have hydrophilic and hydrophobic regions.

• Peripheral Proteins: Not embedded in the bilayer; bound to molecules within the bilayer or to integral proteins.

Six Major Functions of Membrane Proteins

• Transport: Move substances across the membrane (e.g., channels, pumps).

• Enzymatic Activity: Catalyze reactions at the membrane surface.

• Signal Transduction: Receive and transmit signals from the environment.

• Cell-Cell Recognition: Identify and interact with other cells.

• Intercellular Joining: Connect cells together.

• Attachment: Anchor the membrane to the cytoskeleton and extracellular matrix.

Membrane Carbohydrates

Carbohydrates on the cell surface are essential for cell-cell recognition and communication.

• Branched Oligosaccharides: Short chains of sugars attached to lipids (glycolipids) or proteins (glycoproteins).

• Role in Cell Recognition: Determine cell fate in the embryo and help the immune system recognize foreign cells. Example: ABO blood group antigens are carbohydrates on glycoproteins.

Table: ABO Blood System and Membrane Carbohydrates

Blood Type	Carbohydrate on RBC	Antibodies Produced
A	A antigen	Anti-B
B	B antigen	Anti-A
AB	A and B antigens	None
O	None	Anti-A and Anti-B

Traffic Across Membranes

The plasma membrane is selectively permeable, allowing some molecules to cross more readily than others. This property is essential for maintaining cellular homeostasis.

• Hydrophobic molecules cross easily; hydrophilic molecules require assistance.

• Some molecules can only cross in one direction, helping regulate concentrations.

Transport Proteins

Transport proteins assist ions and polar molecules in crossing the membrane. Transport is specific and can be passive (no energy required) or active (energy required).

• Hydrophilic channels: Allow passage of ions and polar molecules.

• Carrier proteins: Bind to molecules and change shape to shuttle them across.

Passive Transport: Diffusion, Facilitated Diffusion, and Osmosis

Diffusion

Diffusion is the movement of solute across a membrane from a region of higher concentration to lower concentration, driven by the concentration gradient.

• Equation: $J = -D \frac{dC}{dx}$ (Fick's Law; J = flux, D = diffusion coefficient, dC/dx = concentration gradient)

• Occurs as long as the membrane is permeable to the solute.

Facilitated Diffusion

Movement of small molecules down their concentration gradient, assisted by membrane proteins (channels or gated channels).

Osmosis

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

• Water moves from a hypotonic (lower solute concentration) to a hypertonic (higher solute concentration) solution.

• Aquaporins: Specialized channels for water transport.

Table: Tonicity and Cell Response

Solution Type	Animal Cell	Plant Cell
Hypotonic	Lysed (bursts)	Turgid (normal)
Isotonic	Normal	Flaccid
Hypertonic	Shriveled	Plasmolyzed

Active Transport

Active transport moves solutes against their concentration gradients, requiring energy (usually ATP).

• Pumps: Membrane proteins that use energy to transport ions or molecules.

• Example: Sodium-potassium pump maintains high K+ inside and high Na+ outside the cell.

• ATP (Adenosine Triphosphate): Provides energy by transferring a phosphate group (phosphorylation).

Sodium-Potassium Pump Cycle

1. Cytoplasmic Na+ binds to the pump.

2. Na+ binding stimulates phosphorylation by ATP.

3. Phosphorylation changes the protein's shape, expelling Na+ outside.

4. K+ binds on the extracellular side, triggering release of the phosphate group.

5. Loss of phosphate restores the protein's original shape.

6. K+ is released inside, and the cycle repeats.

Equation: ATP Hydrolysis

$\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Ion Pumps and Membrane Voltage

Some ion pumps generate voltage across membranes by separating charges, creating potential energy.

• Na+-K+ pump: Pumps Na+ out and K+ in.

• Proton pump: Transfers H+ ions out of the cell.

Co-Transport

Co-transport uses the energy from downhill diffusion of one solute to drive the uphill transport of another solute.

• Proton pump: Adds H+ to the extracellular environment.

• Co-transporter: Uses H+ gradient to allow diffusion of H+ and another molecule (e.g., sucrose) together.

Transport of Large Molecules: Exocytosis and Endocytosis

Large molecules are transported across the membrane via vesicles in processes called exocytosis and endocytosis.

• Exocytosis: Secretion of macromolecules by vesicles fusing with the plasma membrane.

• Endocytosis: Uptake of macromolecules by the cell, including:

  • Phagocytosis ('cell eating')

  • Pinocytosis ('cell drinking')

  • Receptor-mediated endocytosis (specific uptake via receptors)

Example Questions

• Cholesterol in Membranes: Arctic char, living in cold environments, are likely to have more cholesterol in their membranes to maintain fluidity.

• Sodium-Potassium Pump: Requires energy because it uses a membrane protein, transports ions, and moves solutes against their concentration gradients.

• Solute Transport: Polar solutes require transport proteins but do not require ATP for passive transport.

Key Terms

• Phospholipid bilayer

• Fluid mosaic model

• Integral and peripheral proteins

• Glycoproteins and glycolipids

• Diffusion, osmosis, facilitated diffusion

• Active transport, sodium-potassium pump, ATP

• Exocytosis, endocytosis, phagocytosis, pinocytosis

Additional info: These notes expand on the provided materials with definitions, examples, and tables for clarity and completeness, suitable for college-level cell biology exam preparation.