Biological Membranes and Transport

Cell Membranes Composition
- Cell membranes are made of phospholipids.
- Phospholipids are amphipathic molecules (having both hydrophilic and hydrophobic parts) and they form a bilayer when mixed with water.
- The phospholipid bilayer is characterized as a flexible, fluid-like structure.
- The fluidity of a bilayer is influenced by its composition, including the types of lipids and proteins present.
- The phospholipids that make up bilayer membranes are synthesized in the smooth endoplasmic reticulum.
- Various types of proteins with distinct functions are associated with the bilayer membrane.

Definition: The plasma membrane serves as the barrier between the external environment and the interior of the cell.
Function in Eukaryotic Cells: Eukaryotic cells also possess numerous internal membranes that create distinct compartments known as organelles.

Phospholipids
- Phospholipids consist of a glycerol backbone, two fatty acid tails, and a phosphate group.
- The phosphate group is attached to the third carbon of glycerol, providing a charged component.
- This structure contributes to the amphipathic nature of phospholipids.

Formation of Glycerol: Glycerol has three carbons, and each carbon has hydroxyl groups (-OH) attached.
Phosphate Addition: A phosphate group is added to the third carbon of the glycerol.
Fatty Acid Attachment: Two free fatty acid tails are added through dehydration reactions to two of the glycerol carbons.

Fatty acid tails are composed of hydrocarbon chains.
Types of Fatty Acids:
- Saturated Fatty Acids: Contain all single bonds between carbon atoms.
- Unsaturated Fatty Acids: Contain one or more double bonds between carbon atoms, which introduces kinks in their structure.

Location: Cholesterol is embedded between the hydrophobic tails of phospholipids.
Functions of Cholesterol:
1. Reduces the fluidity of the membrane by plugging gaps, providing stability.
2. Prevents excessive tangling of hydrophobic tails, thus facilitating membrane integrity.

Types of Membrane Proteins:
- Transmembrane Proteins (Integral Proteins): Embedded within the membrane, extending across the bilayer.
- Peripheral Proteins: Loosely attached to the surface of the membrane.
Functionality: Proteins serve diverse roles including transporters, anchors, signal receptors, and enzymes.

Molecular Motion: Phospholipids exhibit constant lateral motion, rarely flipping to the opposite side of the bilayer.
Fluidity Dependence: The fluidity of the membrane is affected by the nature of the phospholipid tails (saturated vs. unsaturated) and the presence of cholesterol.
- Unsaturated tails provide increased fluidity due to kinks that prevent close packing.
- Shorter tails contribute to increased permeability.
Temperature Effect:
- High Temperature: Increases fluidity and permeability.
- Low Temperature: Decreases fluidity and permeability.

The fluid mosaic model describes biological membranes as structures with diverse components that are dynamic and can move laterally within the layer.
Adaptations for Different Environments: Organisms in hot climates often adjust their membrane's lipid composition and cholesterol levels to maintain appropriate fluidity and functionality.

High Permeability Substances: Oxygen (O2), Carbon dioxide (CO2), Nitrogen (N2) are small, nonpolar molecules and diffuse easily.
Moderate Permeability Substances: Water (H2O) and glycerol have low permeability.
Low Permeability Substances: Larger uncharged polar molecules (e.g., glucose, sucrose) and ions (e.g., Cl-, Na+)

Definition: Solutes (ions or molecules) diffuse across the lipid bilayer from areas of higher concentration to areas of lower concentration.
Nature of the Process: This is a passive process that does not require energy.
Equilibrium: Solutes will diffuse until equal concentrations are achieved across both sides of the membrane.

Separation of Solutes: Different solutes start on opposite sides and can diffuse across the bilayer.
Diffusion Process: Each solute undergoes a net movement along its own concentration gradient.
Equilibrium State: At equilibrium, solutes continue to move across the membrane at equal rates.

Transport Mechanism: Substances that cannot freely cross the lipid bilayer (e.g., large polar molecules) can utilize transport proteins for movement through the membrane.
Types of Transport Proteins:
- Channel Proteins: Facilitate the diffusion of small polar molecules or ions (e.g., K+ ions) through specific channels that may open or close.
- Carrier Proteins: Transport larger polar molecules (e.g., glucose) by binding to the substrate, inducing a conformational change that allows passage.
Mechanism of Action: Facilitated diffusion is also a passive process that relies on the concentration gradient without expending energy.

Definition: Osmosis is the diffusion of water across a selectively permeable membrane. The membrane allows only water molecules to pass while blocking solutes.
Example: A scenario with a 10% salt solution (hypertonic) on one side of the bilayer leads to the movement of water from a 5% salt solution (hypotonic) to the hypertonic side, resulting in volume change.
Directionality: Osmosis generally occurs from the side with more water (hypotonic) to the side with less water (hypertonic).

Hypotonic Solutions: Can lead to water influx and cell swelling.
Isotonic Solutions: Maintain equal osmotic movement of water resulting in stable cell shape.
Hypertonic Solutions: Lead to water efflux causing cell shrinkage.

Definition: Cells can move substances against their concentration gradient using active transport, which requires energy (e.g., ATP).
Process Characteristics:
- Direction of movement is from low concentration to high concentration.
- Utilizes pump proteins in the membrane, which are highly specific for the substances they transport.

Active transport involves the expenditure of energy to move molecules against a concentration gradient, vital for maintaining essential concentrations of ions and metabolites within the cell.