Cell Membrane and Transport PDF

Cell Membrane and Transport Physiology Chapter 2 Cell membrane characteristics The cell membrane is the outer part of the cell and is a selectively permeable layer. It has a dynamic and flexible structure. It is accepted as the fluid-mosaic model. The cell membrane is 7.5-1...

Cell Membrane and Transport Physiology Chapter 2 Cell membrane characteristics The cell membrane is the outer part of the cell and is a selectively permeable layer. It has a dynamic and flexible structure. It is accepted as the fluid-mosaic model. The cell membrane is 7.5-10 nanometer thick. It contains 55% protein, 25% phospholipids, 13% cholesterol, 4% other lipids, and 3% carbohydrates. The main structure is a double-layered lipid bilayer. Membrane lipids The lipid layer is made of phospholipid molecules. The head-side of phospholipids contains phosphate and they are hydrophilic. The tail parts are made of fatty acids and are hydrophobic, so they are positioned toward the inside of the double-layer membrane. Cholesterol is also found in the membrane as dissolved form Cholesterol determines the fluidity and elasticity of the membrane. If cholesterol increases, the membrane becomes more rigid.  Membrane lipids Types of phospholipids: Phosphatidylcholine - serine (negatively charged) - ethanolamine - inositol (found in small amounts but plays a crucial role in cell signaling) - cardiolipin (in mitochondria) - sphingomyelin (in neuron membranes). Phosphatidylserine and ethanolamine are only in the inner leaflet, Phosphatidylcholine and sphingomyelin are only in the outer leaflet. There are an inner-outer layer asymmetry between the phospholipid layers Phospholipid movements: left- right- flexion - flip-flop.  Membrane proteins There are two types: integral proteins and peripheral proteins The general functions of membrane proteins include: Transport of molecules (Channel proteins and carrier proteins) Enzymatic activity Cell communication (Receptors) Intercellular connections, binding to the cytoskeleton and extracellular matrix (Adhesion molecules) Recognition of cells (MHC proteins).  Membrane carbonhydrates It is found only on the outer surface of the cell membrane. When carbohydrates attach to proteins on the outer surface of the membrane, they form glycoproteins. When carbohydrates attach to lipids on the outer surface of the membrane, they form glycolipids. Glycoprotein + glycolipid = Glycocalyx layer (cell coat). Functions of the glycocalyx layer: It gives a negative charge to the cell due to sialic acid, thus repelling negatively charged substances. It helps cells stick to each other. It determines the specificity of the cell and plays a role in recognizing foreign substances.  Membrane permeability Substances Can Pass? Reason Small, nonpolar, fat-soluble molecules can diffuse through the lipid Oxygen (O2) Yes bilayer. Small, nonpolar, fat-soluble molecules can diffuse through the lipid Carbon Dioxide (CO2) Yes bilayer. Vitamins A, D, E, K Yes Fat-soluble vitamins can easily pass through the lipid bilayer. Alcohol Yes Alcohol is fat-soluble and can pass through the lipid bilayer. Water can pass in limited amounts due to its small size, often Water (H2O) Limited through aquaporins. Charged particles (ions) cannot pass through the hydrophobic core Ions (Na+, K+, Cl-) No of the membrane. Large, polar molecules need transport proteins to pass through the Glucose No membrane. Large, polar molecules need transport proteins to pass through the Amino Acids No membrane.  Factors Affecting Membrane Permeability Effect on Factors Reason Permeability Higher temperature increases the fluidity of the membrane, making it Temperature Increases easier for substances to pass. More cholesterol makes the membrane more rigid, decreasing Cholesterol Content Decreases permeability. Saturation of Fatty Saturated fatty acids pack tightly, reducing permeability, while Varies Acids unsaturated fatty acids increase fluidity and permeability. Larger molecules cannot pass through the tightly packed lipids as easily Molecule Size Decreases as smaller molecules. Polarity of Polar molecules face resistance from the hydrophobic core of the Decreases Molecules membrane, while non-polar molecules pass more easily. Presence of Transport proteins allow specific molecules to pass through the Increases Transport Proteins membrane, increasing permeability for those substances. Lipid-soluble molecules dissolve in the membrane, making it easier for Lipid Solubility Increases them to pass, while water-soluble substances cannot pass as easily.  Membrane transporting The transport of substances depends on the size, concentration, and whether they dissolve in water or fat. Small substances - through active or passive transport Large substances - through endocytosis or exocytosis  Passive transport Passive diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration, without energy (ATP). It occurs naturally due to the concentration gradient, and there are three types of passive diffusion: 1. Simple diffusion 2. Facilitated diffusion 3. Osmosis  Simple diffusion Simple diffusion is the process by which small, nonpolar molecules (O2-CO2) pass directly through the lipid bilayer of the membrane without the help of transport proteins. Factors Affecting Simple Diffusion: Concentration Gradient: The larger the difference in concentration, the faster the rate of diffusion. Temperature: Higher temperatures increase the kinetic energy of molecules, speeding up diffusion. Molecule Size: Smaller molecules diffuse faster than larger ones. Membrane Surface Area: A larger membrane surface area increases the rate of diffusion. Membrane Thickness: A thinner membrane allows faster diffusion. Lipid Solubility: Nonpolar, lipid-soluble molecules pass more easily through the membrane  Facilitated idffusion Glucose, amino acids, or ions move across the cell membrane with the help of special proteins called carrier or channel proteins. These proteins allow the molecules to move from an area of high concentration to an area of low concentration, without using energy. Vmax is the maximum speed at which these carrier proteins can transport molecules. It happens when all the carrier proteins are working at full capacity, meaning they are fully "saturated" with molecules. At this point, even if there are more molecules outside the cell, the transport rate cannot increase because all the carrier proteins are already busy.  Key Points About Vmax: Carrier Saturation: When the concentration of molecules increases, the rate of transport also increases, but only up to a certain point. Once all the carrier proteins are busy, the rate reaches its Vmax, and it can't get faster. Limited Capacity: Unlike simple diffusion, facilitated diffusion has a limit due to the number of carrier proteins. Vmax shows the maximum transport speed, which depends on how many carriers are available. 1. Affinity (Km): Km is the concentration of the molecule needed to reach half of the Vmax. 2. A low Km means the protein binds to the molecule easily and can reach Vmax at lower concentrations. A high Km means more molecules are needed to reach the maximum speed. Example with Glucose: Glucose enters the cell through carrier proteins like GLUT. As blood glucose increases, the transport rate rises until all the GLUT proteins are fully used (saturated). At this point, the transport rate reaches Vmax, meaning it can't increase further, even with more glucose. In short, Vmax is the maximum speed at which transport proteins can move molecules. When all the proteins are busy, the transport speed stays constant.  Osmosis Osmosis is the diffusion of water molecules across a semipermeable membrane, from an area of low solute concentration to an area of high solute concentration. Osmosis is vital for maintaining water balance in cells. Continues until the concentration of water (or solute) is equal on both sides of the membrane, creating osmotic equilibrium.  Active transport It is the transportation of substances by combining carrier proteins. Substances pass here using the electrochemical gradient. Substances are transported from low concentration to high concentration, so energy is spent. There are 2 types: primary active transport and secondary active transport. Key Points: Energy is needed: active transport uses ATP Protein pumps: Special proteins, called pumps, help in this process. They "pump" the molecules across the membrane Sodium-potassium pump: It moves sodium (Na+) out of the cell and potassium (K+) into the cell, both against their concentration gradients, using energy from ATP. Primary active transport energy from ATP is directly used to move molecules against their concentration gradient (from low to high concentration). Types of Pumps: 1.Sodium-potassium (Na-K) pump 2.Calcium (Ca2+) pump 3.Proton (H+) pump  Na+/K+ Pump The Na-K pump moves 3 sodium ions (Na+) out of the cell and brings 2 potassium ions (K+) in. This pump works against the concentration gradient for both sodium and potassium. It uses energy from ATP to function. The importance of the Na-K pump: It provides the balance of sodium (Na+) and potassium (K+) inside the cell. It helps maintain cell volume and prevents excessive water intake. After stimulation of nerve cells, it re- establishes the ion balance inside and outside the cell and makes the cell excitable again.  Secondary active transport ATP is not used directly, but the ion gradient created by ATP is used indirectly. It transports another molecule by using the Na+ concentration gradient expelled during primary active transport as energy. Sodium-glucose transporter (SGLT): Na in, Glucose in Sodium-calcium exchanger (NCX): Na in, Ca out Sodium-hydrogen exchanger (NHE): Na in, H out  Differences Between Primary Active Transport and Secondary Active Transport Primary Active Transport Secondary Active Transport Energy comes directly from ATP. Energy comes from the ion gradient, not directly from ATP. Molecules are moved directly from low to An ion (like sodium) moves from high to low high concentration using ATP. concentration, while another molecule moves from low to high concentration. Examples: Na-K pump, Ca²⁺ pump. Examples: Sodium-glucose transporter (sodium gradient moves glucose).  The cell takes in large molecules or liquids from the outside by folding its membrane inward. The cell takes in these substances to digest or use them. Endocytosis There are 3 main type: Phagocytosis, Pinocytosis, receptor mediated endocytosis (klatrin and caveol)  Phagocytosis and Pinocytosis Phagocytosis (Cell engulfment): A cell takes in large particles (e.g. bacteria). A particle is surrounded by a membrane and taken into the cell. Macrophages, neutrophils are digest the bacterias by phagocytosis Pinocytosis (Cell drinking): A cell takes in small drops of liquid and solutes. A portion of the cell membrane folds inward to form a small vesicle. Generally, cells that aim to take fluid from the external environment with the cell membrane perform pinocytosis. Epithelial cells: In the absorption of nutrients in the intestine Endothelial cells: Provide material exchange between the blood and tissues. Immune cells: They use it to take antigens or foreign substances in small pieces. Difference: Phagocytosis takes in large solid particles, while Pinocytosis takes in small drops of liquid. Receptor mediated endocytosis Clatrin-mediated and caveolae- mediated endocytosis provide controlled and efficient work while taking in specific substances in cells. Phagocytosis Clathrin-mediated Caveolin-mediated Endocytosis Endocytosis Takes up large particles Takes up large specific Takes up small, lipid-soluble (bacteria) molecules (cholesterol) molecules Used for defense Used for nutrient uptake Used for signaling and transport of small molecules (by immune cells) and regulation Requires more energy Requires less energy Requires less energy (for lipid raft formation) Engulfs large particles by Forms clathrin-coated pits Forms small invaginations wrapping the membrane and called caveolae forms phagosomes  Exocytosis Exocytosis is the process of expelling substances from the cell. The cell takes the substances it wants to expel into a vesicle. This vesicle moves towards the cell membrane, fuses with the membrane and the substances inside are released. waste substances, neurotransmitters, hormones, proteins