Lipid Bilayer Composition

• Water is a polar molecule that can bond with other molecules, facilitating dissolution and is termed hydrophilic.
• Hydrophobic molecules cannot form hydrogen bonds.
• These bonds are fundamental for cell structure.
• Membranes that surround cells and organelles act as barriers that control exchange.
• Membranes comprise lipids (phospholipids, glycolipids, sterols), proteins, and small amounts of carbohydrates.

Lipid Bilayer Composition

• The three major classes of lipids include phospholipids, glycolipids, and sterols like cholesterol.
• Phospholipids are the most abundant lipids.
• A phospholipid molecule consists of a glycerol backbone.
• Attached to the backbone is composed of a phosphate molecule liked to choline or serine forming a hydrophilic head group.
• Also attached are two nonpolar fatty acid chains that form hydrophobic tails.
• Tails of fatty acids can be saturated or unsaturated.
• Phospholipids organize with hydrophobic tails away from water, forming two lipid bilayers.

Lipid Bilayers as Barriers

• They occur in almost all biological membranes, regulating the movement of substances.
• Molecules such as nonpolar, oxygen, nitrum (N2), and CO2 are permeable.
• Water and glycerol are mostly permeable.
• Glucose and sucrose are mostly impermeable.
• Molecules such as Na+, K+, H+, and Cl- are impermeable.
• Membrane permeability to molecules depends on their size and hydrophilic/hydrophobic nature.

Membrane Proteins

• Membrane proteins are classified as integral or peripheral.
• Integral proteins are difficult to isolate and interact with the hydrophobic interior of the lipid bilayer, embedding them in the bilayer.
• Peripheral proteins are hydrophilic and located on the membrane surface, interacting with hydrophilic regions of integral proteins.

Functions of Membrane Proteins

• Transport proteins facilitate the movement of molecules in and out of the cell, including channel proteins.
• Recognition proteins aid in cell-cell recognition, essential for the immune system.
• Receptors receive chemical signals.
• Enzymes catalyze enzymatic activities and reactions.

Selectivity in Membrane Permeability

• The movement of molecules by simple diffusion depends on their size and hydrophilic nature, determining membrane permeability.
• Facilitated diffusion and active transport use proteins to move molecules across the membrane.
• These transport proteins selectively recognize specific molecules or classes of molecules.
• Selective permeability results from facilitated diffusion and active transport.

Transport Across Membranes: Active Transport

• Active transport moves molecules from lower to higher concentration regions, requiring energy (ATP).
• Pump proteins move ions or molecules against their concentration gradient.
• It helps in the uptake of essential nutrients.
• It helps in the removal of secretory or waste materials from the cell.
• It helps maintaining correct ion concentrations in the cells.

Types of Active Transport

• Direct active transport uses energy from exergonic reactions like ATP hydrolysis to transport molecules.
• Vesicles transport large amounts of material.
• Endocytosis moves particles into the cell where the cell membrane engulfs particles.
• Phagocytosis ingests large solids.
• Pinocytosis ingests liquids.
• Phagocytosis involves the membrane surrounding, engulfing, and transporting particles with a phagosome to the lysosome.
• Pinocytosis involves the cell taking in small amounts of extracellular fluid.
• Exocytosis is the reverse; it secretes bulk material.
• The to-be-removed material is enclosed in vesicles that fuse with the plasma membrane.

Passive Transport: Simple Diffusion

• Simple diffusion moves molecules down a concentration gradient (high concentration to low concentration).
• It's a spontaneous process (can lead to equilibrium) that doesn't require energy.
• Gas exchange: red blood cells transport oxygen from the lungs to body cells.
• Oxygen diffuses in the alveoli down the concentration gradient to blood in capillaries.
• Oxygen is carried to tissues, where it diffuses from high to low concentration areas.
• The same process occurs with carbon dioxide.

Osmosis

• It occurs with diffusion of water across a selectively permeable membrane from lower to higher solute concentrations.
• Solute concentration is higher inside the cell, leading to net water movement into the cell.
• Permeability depends on the size and polarity of molecules.

Facilitated Diffusion

• It involves the movement of solutes down their concentration gradient, mediated by carrier or channel proteins.
• The movement is facilitated by transport proteins.

Channel Proteins

• They are transmembrane proteins that form channels for polar molecules like ions.
• They are highly selective, with different channels needed for different ions.
• Ion channel selectivity is determined by the binding sites of hydrophilic amino acids and the size of the pore.
• Channels open and close in response to stimuli like voltage changes, binding of small molecules, or mechanical forces.
• When open, ions pass through the pore down the concentration gradient. When closed, the pore is plugged.
• Carrier proteins bind to solute molecules, undergo conformational changes, and transfer molecules across the membrane.