Biomembranes: Structure and Function - PDF

MODULE 3: BIOLOGICAL MEMBRANE Biological Membranes -​ Also known as the cell membrane or plasma membrane -​ Selective permeable - allows some substances to pass while blocking others -​ It has a fluid-like consistency, similar to oil, to encourage ions and molecules to pass thr...

MODULE 3: BIOLOGICAL MEMBRANE Biological Membranes -​ Also known as the cell membrane or plasma membrane -​ Selective permeable - allows some substances to pass while blocking others -​ It has a fluid-like consistency, similar to oil, to encourage ions and molecules to pass through -​ Rough Endoplasmic reticulum (rough ER) is located closer to the nucleus -​ Composed of lipids, proteins, and carbohydrates The Fluid Mosaic Model -​ Describes the structure of biological membranes, explaining their dynamic and semi-permeable nature. -​ The current model for the plasma membrane structure and other cell membranes is that protein molecules “float” in a fluid phospholipid bilayer. ​ It is called a mosaic because of the different molecules embedded in it ​ The current model of plasma membrane structure, where protein molecules "float" in a fluid phospholipid bilayer. ​ Amphipathic since it is both hydrophilic and hydrophobic -​ Protects inside of the cell from outside ​ Extracellular side of the membrane -​ Glycolipid -​ Carbohydrate -​ Glycoprotein ​ The cytoplasmic side of the membrane -​ Sterol -​ Microfilaments of cytoskeleton -​ Peripheral protein -​ Integral protein Phospholipid Bilayer -​ Composed of phospholipids arranged in a bilayer with: ​ (one) Hydrophilic heads facing outward (interacting with water) -​ Polar and is located on two surfaces of the double-layered membrane  ​ (two) Hydrophobic tails facing inward (shielded from water) -​ Nonpolar, hydrophobic fatty acid chains of phospholipids project into the interior of the double-layered membrane -​ One tail is straight while the other has a kink due to cis double bond -​ Kink allows it to move -​ The bilayer provides fluidity, allowing the movement of proteins and lipids. Membrane Proteins -​ They move through the phospholipids -​ Integral Proteins - embedded within the membrane (hydrophobic core), often spanning the bilayer ​ Transmembrane proteins span across both layers. ​ It is composed of hydrophobic amino acid stretches, often forming alpha helices. ​ Embedded in the bilayer with hydrophobic regions interacting with lipid tails. -​ Peripheral proteins - loosely attached to the surface, interacting with other proteins and lipids -​ Functions ​ Transport Proteins -​ Transports energy -​ It tends to create energy as a byproduct -​ Produces energy or ATP ​ Signal Transduction -​ Detects something from outside and does something inside -​ The signaling molecule attaches to the receptor and signal transduction happens ​ Enzymatic Activity -​ Enzymes speed up chemical reactions; membranes are part of this reaction -​ If one ion tends to attach to one enzyme, it produces another ion -​ Proteins embedded tend to help with enzymatic activities ​ Cell-Cell Recognition -​ Recognize nearby cells ​ Intercellular Joining  -​ It happens especially if cells are similar ​ Attachment to the cytoskeleton and extracellular matrix (ECM) -​ ECM is a technical term for the outside environment -​ It needs to be attached to the microfibril and ECM to maintain the rigor of the plasma membrane Carbohydrates & Cholesterol -​ Carbohydrates (glycoproteins & glycolipids) - Involved in cell recognition and signaling -​ Cholesterol - maintains membrane stability and fluidity, preventing it from becoming too rigid or too fluid Movement Within the Membrane -​ Lateral diffusion - phospholipids move side-to-side within a single layer -​ Transverse diffusion (flip-flop) - phospholipid move between layers (is rare) -​ Phospholipid rotates within the layer -​ All membrane lipids are produced in the smooth ER (SER) ​ SER makes lipids contribute because it does not have ribosomes ​ Those lipids are the bilayer ​ Why smooth ER is located near the cell membrane ​ SER just makes and makes lipids -​ Enzymes in the SER join fatty acids, glycerol, phosphate, and head groups to make phospholipids ​ SER contains all of the components needed to create the head and tail -​ The phospholipid is inserted into one of the monolayers ​ Cells know where to go after -​ Flippases are enzymes that transfer specific lipids to the other bilayer, creating asymmetry ​ So the whole membrane will grow ​ They only transfer specific lipids ​ All lipids do one layer first -​ Integral proteins contribute to membrane asymmetry Membrane Fluidity -​ Fluidity allows for: ​ Rapid movement of membrane proteins within the bilayer  ​ Diffusion of lipids and proteins to their correct locations laterally, where they are inserted into a bilayer after their synthesis ​ Rarely, a lipid may flip-flop transversely across the membrane ​ Fusion of membranes and mix their molecules(e.g., during cell reproduction) ​ Equal distribution of membrane molecules in daughter cells after division ​ If membranes are too rigid, the transport of substances is hindered Membrane Protein Mobility and Functions -​ Membrane proteins can move within the bilayer. -​ It is a collage of different proteins, often grouped together, embedded in the fluid matrix of the lipid bilayer -​ Proteins determine most of the membrane’s specific functions -​ Larry Fry & Michael Edidin (Johns Hopkins University) Experiment: ​ Fused a mouse and human cell, observing protein mixing after 1 hour. ​ Demonstrated that membrane proteins move freely. Selective Permeability of the Membrane -​ Plasma membranes regulate what enters and exits the cell. ​ The cell must change materials with its surroundings ​ Plasma membranes are selectively permeable, regulating the cell’s molecular traffic -​ Hydrophobic (nonpolar) molecules (e.g., CO₂, O₂, hydrocarbons) diffuse rapidly through the lipid bilayer. ​ To make it fast, the cell needs to make a membrane protein specific to those ​ CO2 needed for photosynthesis ​ O2 is a product of photosynthesis ​ They don’t need to transport proteins to get out of the cell because they can easily dissolve in the lipid bilayer ​ Hydrocarbons can pass through lipid bilayer but slowly since their molecules are bigger than oxygen and carbon dioxide -​ Hydrophilic (polar) molecules (e.g., ions, sugars) do not cross easily and require transport proteins. ​ Transmembrane proteins are needed to make them move faster Transport Proteins -​ Allows passage of hydrophilic substances across the membrane -​ Channel Proteins: ​ Provide a hydrophilic tunnel for molecules or ions. ​ Example: Aquaporins facilitate water transport. ​ Passive transport – does not require energy. ​ If it fits, then it's in -​ Carrier Proteins: ​ Bind to molecules and change shape to transport them across the membrane. ​ Highly specific to the substances they move and need. ​ If the ion’s shape doesn’t conform to a carrier protein, the ion will not go in ​ Active transport - requires energy for confirmation Types of Transport -​ Passive Transport (No energy required) ​ Diffusion: -​ Diffusion of a population of molecules may be directional ​ Plasma Membrane in the middle and regulating the molecules’ movement -​ At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other -​ Substances diffuse down their concentration gradient, the region along which the density of a chemical substance increases or decreases  -​ No work must be done to move substances down the concentration gradient ​ Solutes move down bc its easier -​ The diffusion of a substance across a biological membrane is passive transport because no energy is expended by the cell to make it happen ​ Cells like passive transport because they don't need water -​ Molecules spread evenly into available space until equilibrium is reached. -​ Moves from high to low concentration. -​ It continues until equilibrium is reached. -​ If there are two molecules present on both sides of the membrane ​ One solute will go to the other side and vice versa ​ Until equilibrium is reached ​ One or two different solutes can pass through as long as the plasma membrane allows for it ​ Osmosis: -​ Diffusion of water across a selectively permeable membrane. -​ Water moves from a region of lower solute concentration to a higher solute concentration. ​ Facilitated Diffusion -​ Larger or charged molecules use transport proteins to cross. -​ Transport proteins speed up the passive movement of molecules across the plasma membrane. -​ It is still passive because solutes move down their concentration gradients, requiring no energy -​ Types of Transport Proteins ​ Channel proteins: Provide corridors for specific molecules or ions -​ Aquaporins facilitate the diffusion of water -​ Ions channels facilitate the diffusion of ions ​ In some ion channels, called gated channels, open or close in response to a stimulus  ​ Carrier proteins: Undergo subtle changes in shaper to move solutes across the membrane Effects of Osmosis on Water Balance -​ Tonicity (Effect of surrounding solution on a cell) ​ Cell walls play an important role ​ Isotonic Solution: -​ Solute concentration is the same inside and outside the cell. -​ No net water movement. -​ Plant cell: Flaccid. -​ Animal cell: Normal. ​ Hypertonic Solution: -​ Higher solute concentration outside the cell. -​ Water leaves the cell → shriveling (plasmolysis). -​ Plant cell: Plasmolyzed. -​ Animal cell: Shriveled. ​ Hypotonic Solution: -​ Lower solute concentration outside the cell. -​ Water enters the cell → swelling. -​ Plant cell: Turgid (normal for plants). -​ Animal cell: Lysed (bursts). Tonicity and Food Preservation -​ Drawing Moisture out of food is important so it doesn't spoil, so that's why it's being used for preservation -​ If we draw the moisture out of the food and replace it with solute -​ Air interacts with food, and without air, the contamination would be lessened -​ Salt & sugar are used in food preservation: ​ They create a hypertonic environment, drawing water out of food. ​ Reduces bacterial growth, preventing spoilage. Plasmolysis -​ In a hypertonic solution, water leaves the vacuole, causing the cytoplasm to shrink. -​ The cell wall remains intact, preventing the plant from collapsing entirely. Water Balance of Cells with Cell Walls -​ Cell walls help maintain water balance. -​ A plant cell in a hypotonic solution swells until the wall opposes further uptake; at this point, the cell is turgid (firm). -​ If a plant cell and its surroundings are isotonic, the cell becomes flaccid (limp). -​ In a hypertonic environment, plant cells lose water, causing the plant to wilt. ​ This loss of water can lead to a lethal effect called plasmolysis. Active Transport -​ Some molecules require energy (ATP) to move across the membrane. -​ Necessary for molecules moving against their concentration gradient. -​ Uses energy to move solutes against their concentration gradients. -​ Some transport proteins move solutes against their concentration gradients. -​ Performed by specific proteins embedded in the membrane. -​ Special membrane proteins are required for molecule transport. Membrane Potential -​ The voltage difference across a membrane. -​ Voltage is created by differences in the distribution of positive and negative ions across the membrane. Electrochemical Gradient -​ Two combined forces drive the diffusion of ions across a membrane: ​ Chemical force – the ion’s concentration gradient. ​ Electrical force – the effect of the membrane potential on ion movement. Electrogenic Pumps -​ Transport proteins that generate voltage across a membrane. -​ Sodium-potassium pump: The major electrogenic pump in animal cells. -​ Proton pump: The main electrogenic pump in plants, fungi, and bacteria. -​ Electrogenic pumps help store energy for cellular work. Cotransport  -​ Coupled transport by a membrane protein. -​ Occurs when the active transport of one solute indirectly drives the transport of another substance. -​ Works with proton pumps. ​ When a proton pump releases H⁺ ions, sucrose enters the cell through cotransport. ​ The process uses less energy because, for every H⁺ ion pumped out, sucrose is transported inside. -​ Plants commonly use the H⁺ gradient created by proton pumps to drive the active transport of nutrients into the cell. Bulk Transport Across the Plasma Membrane -​ Cells transport large quantities of substances via exocytosis and endocytosis. -​ Small molecules and water pass through the lipid bilayer or transport proteins. -​ Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles. -​ Bulk transport requires energy. -​ Exocytosis ​ ​ Transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell. ​ Used by many secretory cells to export products (e.g., removing waste). -​ Endocytosis ​ The cell takes in macromolecules by forming vesicles from the plasma membrane. ​ A reversal of exocytosis involves different proteins. -​ Phagocytosis (“cell eating”) – Example: Amoeba engulfing food. -​ Pinocytosis (“cell drinking”) – Used when cells need water in bulk. -​ Receptor-mediated endocytosis – A highly selective process ​ Specialized receptors in the plasma membrane help in cell-to-cell recognition. ​ The receptor binds with specific ions or molecules needed by the cell. -​ If the ion conforms to the receptor, the membrane forms a pit, pinches off, and transports the substance into the cell.