Cell Membrane Functions

Plasma Membrane

• Proteins are responsible for most of the functions of the membrane and are mobile within it, describing the membrane as a fluid-mosaic model.
• Biochemical functions of the plasma membrane:
• Energy conversion processes such as oxidative phosphorylation
• Regulation of molecular and ionic composition of cell compartments through pumps and channels
• Control of genetic information flow between cells
• Production and reception of chemical signals
• Consists of important enzymes that catalyze reactions such as lipid biosynthesis and xenobiotic metabolism

Cytoplasm

• Important cellular compartment and central reaction space of the cell
• Many important pathways of metabolism take place in the cytoplasm, including:
• Glycolysis
• Pentose phosphate pathway
• Gluconeogenesis
• Fatty acid synthesis
• Protein biosynthesis (translation)
• Glycogenesis (glycogen synthesis) and glycogenolysis (glycogen degradation)
• Consists of a supportive network of proteinaceous fibers and filaments called the cytoskeleton
• Functions of the cytoskeleton:
• Maintaining each cell's shape
• Facilitating cell movements
• Guiding the movement of organelles within the cell
• Components of the cytoskeleton:
• Microtubules (found in axons and dendrites of nerve cells)
• Microfilaments (found in protein actin in muscle)
• Intermediate fibers (found in protein keratin in skin)

Mitochondria

• Sausage-shaped structure bounded by two membranes: outer membrane (smooth and permeable to most molecules) and inner membrane (impermeable to ions and other molecules)
• Functions:
• Fatty acid metabolism (degradation) occurs in the matrix
• Oxidative phosphorylation reactions occur, producing energy as ATP in the presence of O2
• Citric acid cycle (Krebs cycle) occurs in the mitochondria, oxidizing amino acids, fatty acids, and carbohydrates
• Mitochondria has its own protein synthesizing apparatus, including DNA (mtDNA), ribosomes, tRNA, and mRNA, and plays a role in its own replication (DNA synthesis)

Endoplasmic Reticulum (E-R)

• System of interconnected membranous tubules
• Two forms of E-R:
• Rough E-R: involved in the synthesis of proteins for membrane and exporting it from the cell, with ribosomes attached to it
• Smooth E-R: functions in lipid synthesis, calcium storage, and biotransformation (converting lipid-soluble molecules to water-soluble and excreting them)

Ribosomes

• Small organelles that function in the biosynthesis of proteins
• Complex structure containing two irregularly shaped subunits of unequal size
• Number and distribution of ribosomes depend on:
• Metabolic activity
• Type of protein synthesized
• Cells with high rates of protein synthesis have larger numbers of ribosomes

Golgi Apparatus

• Flattened sac-like membranous vesicles located near the nucleus and close to the E-R and plasma membrane
• Functions:
• Packaging and secretion of cell products such as newly synthesized proteins and lipids
• Modifying products of ER like glycoproteins and membrane phospholipids
• Manufacturing macromolecules such as polysaccharides

Lysosomes

• Small sac-like organelles produced in E-R and Golgi apparatus
• Functions:
• Breaking down cell components either micro or macromolecules by digestive enzymes
• Digesting cell organelles
• Autophagy: recycling the cell's own organic material

Peroxisomes

• Small spherical membranous organelles that contain oxidative enzymes
• Functions:
• Generating and breaking down toxic molecules such as reactive oxygen species (ROS)
• Removing H+ atoms from substrates to oxygen, producing H2O2 (hydrogen peroxide)