Cell Membrane Structure and Function

• Selective permeable membrane separates the intracellular space from the extracellular space.
• Observed under an electron microscope (EM), a tri-laminar membrane is visible, featuring two parallel dark lines separated by a light zone.
• The fluid mosaic model comprises a lipid bilayer, membrane proteins, and membrane carbohydrates.

Lipid Bilayer

• Composed mainly of amphipathic phospholipid molecules with polar, hydrophilic heads and non-polar, hydrophobic fatty acid tails.
• Cholesterol molecules maintain optimal fluidity by preventing solidification at low temperatures and restricting movement at high temperatures.
• Exhibits self-sealing properties, forming closed vesicles and fusing with other membranes during exocytosis or endocytosis.

Membrane Proteins

• Include integral and peripheral types, and are mostly amphipathic.

Integral Membrane Proteins

• Penetrate the hydrophobic interior of the lipid bilayer, with some extending partway through.
• Most are transmembrane proteins, firmly bound within the membrane.

Peripheral Membrane Proteins

• Not embedded in the lipid bilayer.
• Loosely associated with the membrane, located on its inner or outer surfaces.

Functions of Membrane Proteins

• Intercellular joining: Membrane proteins hook together in cell junctions.
• Transport: Channel proteins facilitate the passage of ions or specific molecules. Pumps or carriers perform active or passive transport.
• Enzymatic activity: Embedded enzymes catalyze chemical reactions.
• Signal transduction: Transmit signals to the interior of the cell usually through hormones.
• Cell-cell recognition: Glycoproteins serve as identification tags between cells.
• Cell-ECM adhesion: Integrins attach the ECM to the cell's cytoskeleton.

Membrane Carbohydrates

• Form the cell coat or glycocalyx, protecting the cell.
• Short, branched polysaccharide chains attach to membrane proteins, forming glycoproteins, or to the lipid bilayer, forming glycolipids.
• Act as markers distinguishing one cell from another.
• Human blood groups (A, B, AB, and O) reflect variations in the carbohydrate part of glycoproteins on red blood cell surfaces.

Mitochondria

• Known as the powerhouse of the cell.
• Self-replicating organelles capable of growth and division.
• Numbers increase in active cells, such as liver (800-2000) and muscle cells (hundreds to thousands), based on energy demands.
• Site of cellular respiration, using nutrients and oxygen to produce energy in the form of ATP.
• Cellular respiration includes glycolysis, the citric acid cycle (Krebs cycle), and oxidative phosphorylation.
• Contain ribosomes and circular DNA.
• Enclosed by a smooth outer membrane and a folded inner membrane with cristae, increasing surface area for cellular respiration.
• The inner membrane divides the mitochondrion into the inter-membrane space and the mitochondrial matrix.
• Enzymes that make ATP are integrated into the inner mitochondrial membrane.