TOPIC #4; Chapter 7 (phospholipids)

Chapter 7: Membrane Structure and Function

Definition of Plasma Membrane: The boundary that separates living cells from their surroundings, exhibits selective permeability, facilitating the movement of certain substances while restricting others.
Phospholipids: Most abundant lipid in membranes, amphipathic molecules consisting of:
- Hydrophobic Tails: Water-fearing regions that avoid water.
- Hydrophilic Heads: Water-loving regions that interact with water.
Phospholipid Bilayer: Formed by the alignment of phospholipids, where hydrophilic heads face outward towards the aqueous environments and hydrophobic tails point inward.

Definition: Describes the plasma membrane as a mosaic of protein molecules drifting in a fluid phospholipid bilayer.
Membrane Fluidity: Maintained by hydrophobic interactions, which are weaker than covalent bonds, allowing for the movement of lipids and proteins within the membrane.
- Types of Movement:
  - Lateral Movement: Lipids and some proteins can drift laterally at a rapid pace.
  - Transverse Flip-Flop: Rare movement of lipids across the membrane from one leaflet to another.

Protein Movement: Larger than lipids, so they are slower to move. Some are stationary while others are directed along the cytoskeleton by motor proteins.
Fluidity and Temperature: Membrane function relies on membrane fluidity, impacting permeability and enzyme activity.
Saturated vs. Unsaturated Fatty Acids:
- Saturated Fatty Acids: Less fluid due to tighter packing.
- Unsaturated Fatty Acids: More fluid due to kinks created by double carbon bonds.
Cholesterol: Modulates membrane fluidity; restrains phospholipid mobility at warm temperatures and prevents tight packing in cooler temperatures.

Types of Membrane Proteins:
- Integral Proteins: Span the lipid bilayer and are often transmembrane proteins; have hydrophobic regions embedded in the membrane and hydrophilic regions exposed to the aqueous environment.
- Peripheral Proteins: Loosely bound to the membrane surface, often interacting with integral proteins.
Six Major Functions of Membrane Proteins:
1. Transport
2. Enzymatic Activity
3. Signal Transduction
4. Cell-Cell Recognition
5. Intercellular Joining
6. Attachment to Cytoskeleton and Extracellular Matrix (ECM)

Glycolipids: Membrane carbohydrates attached to lipids.
Glycoproteins: Carbohydrates covalently bonded to proteins.
Function: These carbohydrates serve as markers to differentiate between cells.

Hydrophobic Molecules: Such as hydrocarbons and gases (CO2, O2) pass through the lipid bilayer easily.
Hydrophilic Molecules: Polar molecules like sugars struggle to cross the membrane unaided.

Transport Proteins: Facilitate the movement of polar molecules unable to permeate the lipid bilayer directly.
- Channel Proteins: Provide passageways for specific molecules or ions (e.g., aquaporins for water).
- Carrier Proteins: Bind to molecules and change shape to shuttle them across the membrane.

Diffusion: Movement of molecules from higher to lower concentration, achieving equilibrium.
- Dynamic Equilibrium: When concentrations are equal on both sides of a membrane; substances continue to move but with no net movement.
Osmosis: The diffusion of water across a selectively permeable membrane, influenced by solute concentrations.

Tonicity: Describes how surrounding solutions affect water balance in cells:
- Isotonic: Equal solute concentration; no net water movement.
- Hypertonic: Greater solute concentration outside the cell; cell loses water.
- Hypotonic: Lower solute concentration outside the cell; cell gains water, potentially causing it to burst.

Turgor Pressure: Caused by water entering plant cells, leading to rigidity.
Flaccid: Cell becomes limp in isotonic surroundings.
Plasmolysis: Occurs in hypertonic conditions, where water exits, causing the cell to shrink away from the cell wall.

Definition: Movement of solutes against their concentration gradient requiring energy (ATP).
Sodium-Potassium Pump: Example of active transport accounting for the exchange of Na+ and K+ across the plasma membrane.
Cotransport: Coupled transport mechanism where one solute moves down its gradient while another moves against it.

Exocytosis: Process where cells secrete molecules by fusing vesicles with the plasma membrane.
Endocytosis: Cell takes in molecules by forming new vesicles; includes:
- Phagocytosis: Engulfing particles into food vacuoles.
- Pinocytosis: Taking in extracellular fluid.
- Receptor-Mediated Endocytosis: Molecule binding triggers vesicle formation.