قناة سنة ثانية بشري (دفعة 53)

12. Transport Across Membranes A) Transport of Small Molecules 1. Passive (Simple) Diffusion • Movement from high to low concentration • No energy needed • Depends on solubility • Example: ion channels 2. Facilitated Diffusion • Uses specific carrier proteins (e.g., glucose transporter) • No energy needed • Stops when carriers are saturated • Affected by: • Number of carriers • Affinity • Hormones (e.g., insulin ↑ glucose carriers) 3. Active Transport • Movement from low → high concentration • Requires energy (ATP) • Example: Sodium–Potassium pump (Na⁺/K⁺-ATPase) B) Transport of Large Molecules 1. Endocytosis • Uptake of large particles/molecules • Types: • Pinocytosis: fluid uptake; occurs in all cells • Phagocytosis: “cell eating”; occurs in macrophages & granulocytes 2. Exocytosis • Release of substances produced inside the cell • Example: release of insulin from pancreatic beta cells

7. Functions of Membrane Lipids A) Fluidity • Allows movement of lipids and proteins. • Needed for endocytosis and exocytosis. • Increased by higher temperature & more unsaturated fatty acids. Clinical Correlations • ↓ Fluidity affects receptors/ion channels → contributes to Alzheimer’s disease & hypertension. • PUFA (cis-double bonds) ↑ fluidity → ↑ insulin sensitivity → ↓ risk of diabetes. B) Selective Permeability • Membrane controls what enters/leaves the cell. 8. Membrane Proteins Two types: A) Peripheral Proteins • Weakly attached to membrane surface. • Removed by salt solutions. B) Integral (Transmembrane) Proteins • Embedded deeply in bilayer. • Removed only by detergents. • Function as receptors, enzymes, ion channels, and antigens. 9. Functions of Membrane Proteins 1. Transport of substances 2. Cell receptors 3. Immunoglobulins on lymphocytes 4. Energy production in mitochondria 5. Membrane-bound enzymes 10. Special Example: Erythrocyte (RBC) Membrane A) Ankyrin & Spectrin • Peripheral proteins • Maintain biconcave RBC shape • Spectrin gene mutation → Hereditary spherocytosis → hemolytic anemia B) Glycophorin • Integral membrane glycoprotein • Rich in sialic acid • Functions: • ABO blood group antigens • Gives RBCs a hydrophilic coat → prevent adhesion C) Anion Channel Protein • Important for CO₂ transport in blood. 11. Membrane Carbohydrates • Present as glycoproteins & glycolipids • Found only on the outer surface of the membrane Functions 1. Act as receptors 2. Determine blood group type 3. Help sperm recognize the ovum (fertilization)

⭐ Biological Cell Membrane 1. Definition The cell membrane (plasma membrane) is a thin barrier that surrounds the cell and separates the internal environment from the external environment. Membranes also surround organelles such as the nucleus and mitochondria. 2. Metabolic Functions of the Cell Membrane 1. Contains specific pumps and channels for substance transport. 2. Contains specific receptors for hormones, lipids, and other molecules. 3. Generates chemical and electrical signals. 4. Site of ATP production in the inner mitochondrial membrane. 3. Chemical Composition Cell membranes are mainly composed of: • Lipids • Proteins • Carbohydrates 4. Membrane Lipids A) Phospholipids 1. Glycerophospholipids: contain glycerol, fatty acids (palmitate C16, stearate C18), and bases (choline, serine, ethanolamine, inositol). 2. Sphingophospholipids: contain sphingosine; the main type is sphingomyelin, rich in the myelin sheath. B) Glycolipids • Lipids containing sugar. • Cerebrosides: contain one sugar (glucose/galactose). • Gangliosides: contain 3 or more sugars. C) Cholesterol • Abundant in plasma membrane. • Less in mitochondrial/nuclear membranes. 5. Lipid Bilayer • Lipids are amphipathic: hydrophilic head + hydrophobic tail. • Arranged in a bilayer with heads facing outward and inward, and tails facing each other. 6. Stability of the Lipid Bilayer Stabilized by: • Hydrophobic interactions (major force) • Van der Waals forces between tails • Hydrogen bonds & electrostatic interactions with water

مراجعه خت فت هيا cell membrane

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