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Diapositiva 1 PLASMA MEMBRANE BY DR. NAGLAA KHEDR Functions of the Plasma Membrane .Separates the cell from the external aqueous environment. Transports nutrients into and metabolic wastes out of the cell. Maintains the proper ionic composition, pH (≈7.2), and osmotic pressure of the cytosol. It is a semi-permeable and selective barrier that restricts the entry and exit of compounds. Membrane lipids, proteins, glycolipids, and glycoproteins serve several functions as will be mentioned later. Basic Structure of Cell Membrane Structure of Plasma Membrane Lipid bilayer (two sheets of phospholipids), embedded with proteins and strengthened with cholesterol molecules. Many of the proteins and lipids on the external sheet contain covalently bound carbohydrate chains (glycoproteins and glycolipids). This layer of carbohydrate on the outer surface of the cell is called the glycocalyx. The membrane is referred to as a fluid mosaic because it consists of a mosaic of proteins and lipid molecules that can move laterally in the plane of the membrane. Phospholipid Bilayer Membrane Lipids Phospholipids consist of two long-chain, nonpolar fatty acyl groups esterified to two of the three OH groups in glycerol. The third OH group of glycerol is esterified to phosphate. The phosphate group is also esterified to a hydroxyl group on another hydrophilic compound. The principal phospholipids in the membrane are the glycerol lipids phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine and the sphingolipid sphingomyelin. Phospholipid Structure: e.g. Phosphatidylcholine The phospholipids in each layer of the plasma membrane are arranged with their hydrophilic head groups (the phosphate and its associated esterified group) facing the aqueous medium and their fatty acyl tails forming a hydrophobic membrane core. The close packing of the nonpolar tails is stabilized by the hydrophobic effect and van der Waals interactions between them. Ionic and hydrogen bonds stabilize the interaction of the phospholipid polar head groups with one another and with water. •Because phospholipids possess both hydrophilic and hydrophobic properties, they are said to have amphipathic nature. •Because of their hydrophobic core, bilayers are virtually impermeable to salts, sugars, and most other small hydrophilic molecules. •Membrane fluidity: the plasma membrane has the consistency of olive oil at body temperature due to unsaturated phospholipids. Cholesterol is another important lipid component of cell membranes that is embedded in the hydrophobic areas of the inner core. Most bacterial cell membranes do not contain cholesterol. Cholesterol affects fluidity: At body temperature it lessens fluidity by restraining the movement of phospholipids. At colder temperatures it adds fluidity by not allowing phospholipids to pack close together. Membrane Proteins Proteins of the Plasma Membrane Provide 6 Membrane Functions: 1) Transport proteins 2) Receptor proteins 3) Enzymatic proteins 4) Cell recognition proteins 5) Attachment proteins 6) Intercellular junction proteins Integral Membrane Proteins Membrane proteins are either integral proteins (intrinsic), which span the cell membrane, or peripheral proteins (extrinsic), which are attached to the membrane surface through electrostatic bonds. Hydrophilic regions of the proteins protrude into the aqueous medium on both sides of the membrane, while hydrophobic regions are embedded in the membrane core. Membrane Carbohydrates: Carbohydrates constitute 2-10% of the weight of plasma membranes. The hydrophilic carbohydrate layer, called the glycocalyx, protects the cell against digestion and restricts the uptake of hydrophobic compounds. The variable carbohydrate components of the glycolipids on the cell surface function as cell recognition markers. For example, the A, B, or O blood groups are determined by the carbohydrate composition of the glycolipids. Cell surface glycolipids may also serve as binding sites for viruses and bacteria toxins before penetrating the cell. For example, the cholera AB toxin binds to GM1-gangliosides on the surface of the intestinal epithelial cells. Transport of Molecules across the Plasma Membrane: 1. Passive transport: ATP energy is not needed to move the molecules: 2. Diffusion, Facilitated Diffusion, Osmosis 3. Active transport: ATP energy is required to move the molecules. 4. Vesicular transport. Active and Passive Transport DIFFUSION 1) Diffusion describes movement of particles directly through the phospholipids of the plasma membrane from an area of high concentration to an area of low concentration until equally distributed. 2) Molecules that pass across cell membrane by diffusion include: I. Gases (oxygen, carbon dioxide) II. Water molecules (rate slow due to polarity) III. Lipids (steroid hormones) IV. Lipid soluble molecules (hydrocarbons, alcohols, some vitamins) V. Small uncharged molecules (NH3) Diffusion is important for cells and human biological processes: 1. Cell respiration 2. Alveoli of lungs 3. Capillaries 4. Red Blood Cells 5. Medications: time-release capsules FACILITATED DIFFUSION Facilitated diffusion is the net movement of molecules through the plasma membrane from a high concentration to a low concentration with the aid of transport proteins (channel or carrier proteins). Molecules that pass across cell membrane by facilitated diffusion: 1. Ions (Na+, K+, Cl-) 2. Sugars (Glucose) 3. Amino Acids 4. Small water soluble molecules 5. Water (faster rate) Channel proteins allow ions, small solutes, and water to pass. Carrier proteins move glucose and amino acids. When the molecule binds to the transporter protein, the protein then undergoes a conformational change that allows the transported molecule to be released on the other side of the membrane. Facilitated diffusion is important for cells and human biological processes: oCells obtain food for cell respiration oNeurons communicatation oSmall intestine cells transport food to bloodstream oMuscle cells contract Specific types of facilitated diffusion 1. Counter Transport – the transport of two substances at the same time in opposite directions. Protein carriers are called antiports. 2. Co-transport – the transport of two substances at the same time in the same direction. Protein carriers are called symports. 3. Gated Channels – receptors combined with channel proteins. When a chemical messenger binds to a receptor, a gate opens to allow ions to flow through the channel. OSMOSIS AND TONICITY oOsmosis means diffusion of water through the semi- permeable plasma membrane. oWater moves from side with high amount of water to side with lower amount of water. Movement stops when osmotic pressure equals hydrostatic pressure. oOsmosis is important for cells and human biological processes: oCells remove water produced by cell respiration. oLarge intestine cells transport water to bloodstream. oKidney cells form urine. Tonicity oTonicity refers to the total solute concentration of the solution outside the cell. oIsotonic solutions (iso-osmotic) have the same concentration of solutes as the suspended cell. oExample: blood plasma has high concentration of albumin molecules to make it isotonic to tissues. oWhat will happen to a cell placed in an isotonic solution? The cell will have no net movement of water and will stay the same size. oHypotonic solutions have a lower solute concentration than the suspended cell. oWhat will happen to a cell placed in a hypotonic solution? The cell will gain water and swell. If the cell bursts, then we call this lysis (rupture of red blood cells = hemolysis). oHypertonic solutions have a higher solute concentration than a suspended cell. What will happen to a cell placed in a hypertonic solution? The cell will lose water and shrink (shrinkage of red blood cells = crenation). Effect of Tonicity on Red Blood Cells ACTIVE TRANSPORT Molecules move from areas of low concentration to areas of high concentration with the aid of ATP energy. Requires protein carriers called pumps. Importance of the active transport 1. Bring in essential molecules: ions, amino acids, glucose, nucleotides. 2. Rid cells of unwanted molecules (e.g. sodium from urine in kidneys). 3. Regulate the volume of cells by controlling osmotic potential. 4. Control cellular pH. 5. Re-establish concentration gradients to run facilitated diffusion (E.g. sodium-potassium pump and proton pumps). Sodium-potassium Pump (Na+,K+-ATPase) Three (3) sodium ions move out of the cell and then two (2) potassium ions move into the cell. Driven by the splitting of ATP to provide energy and conformational change to proteins. Used to establish an electrochemical gradient across neuron cell membranes. Sodium-potassium PumpSodium-Potassium Pump (Na+,K+- ATPase) 1. Three sodium ions bind to the transporter protein on the cytoplasmic side of the membrane. 2. When ATP is hydrolyzed to ADP, the carrier protein is phosphorylated and undergoes a change in conformation that causes the sodium ions to be released into the extracellular fluid. 3. Two potassium ions then bind on the extracellular side. Dephosphorylation of the carrier protein produces another conformational change, and the potassium ions are released on the inside of the cell membrane. 4. The transporter protein then resumes its original conformation, ready to bind more sodium ions. VESICULAR TRANSPORT Vesicular transport occurs when a membrane extends to surround a particle and encloses it into a vesicle. Exocytosis: Movement of large molecules bound in vesicles out of the cell with the aid of ATP energy. Endocytosis:
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