Lesson 5 Phospholipid Bilayer and Membrane PDF

Phospholipid Bilayer and Membrane Prepared By: Sheryl M. Mallari  Membrane Structure The fluid mosaic model of membrane structure contends that membranes consist of: ￭ phospholipids arranged in a bilayer ￭ globular proteins inserted in the lipid bilayer Cellular...

Phospholipid Bilayer and Membrane Prepared By: Sheryl M. Mallari  Membrane Structure The fluid mosaic model of membrane structure contends that membranes consist of: ￭ phospholipids arranged in a bilayer ￭ globular proteins inserted in the lipid bilayer Cellular membranes have four components: ￭ phospholipid bilayer ￭ transmembrane proteins ￭ interior protein network ￭ cell surface markers  CELL MEMBRANE ALL cells have a cell membrane made of proteins and lipids. SOME cells have cell membranes and cell walls ex: plants, fungi and bacteria ￭ Plant cells have a cell wall made of cellulose ￭ Bacteria and fungi also have cell walls, but they do not contain cellulose ￭ Cell membranes and cell walls are porous, allowing water, carbon dioxide, oxygen and nutrients to pass through easily.  Membrane Structure The membrane can be seen using Electron Microscope ￭ Transmission electron microscopes (TEM) can show the two layers of a membrane. ￭ Freeze-fracturing techniques separate the layers and reveal membrane proteins.  Phospholipid Bilayer The phospholipid bilayer (commonly written as "phospholipid bilayer") is a fundamental structure in biological membranes, including plant cells. It is composed of two layers of phospholipids arranged tail-to- tail, forming a flexible yet stable barrier. Phospholipid Components: Head: The polar (hydrophilic) "head" Tail: The non-polar (hydrophobic) "tails"  Phospholipid Bilayer Phospholipid structure consists of: ￭ Glycerol – a 3-carbon polyalcohol acting as a backbone for the phospholipid. ￭ Two fatty acids attached to the glycerol ￭ Phosphate group attached to the glycerol Fatty acids are chains of carbon and hydrogen that don’t mix with water because they are nonpolar. This makes them hydrophobic ("water-fearing"). The phosphate group is polar, meaning it mixes well with water. This makes it hydrophilic ("water-loving").  Membrane Proteins Membrane proteins are proteins embedded within or attached to the phospholipid bilayer of the cell membrane. Membrane proteins have various functions: 1. Transporters 2.Enzymes 3.Cell surface receptors 4.Cell surface identity markers 5.Cell-to-cell adhesion proteins 6.Attachments to the cytoskeleton  Membrane proteins have various functions: TRANSPORTER Membrane proteins that help transport materials (ions, nutrients, waste) across the cell membrane. ￭ Channel proteins: Form pores in the membrane for molecules (like ions or water) to pass through. ￭ Carrier proteins: Bind to specific molecules and help move them across the membrane, either by facilitated diffusion or active transport.  Membrane proteins have various functions: ENZYMES Some membrane proteins function as enzymes that catalyze chemical reactions at the membrane surface. ￭ ATPases: Involved in energy transport across the membrane. ￭ Digestive enzymes: Break down molecules outside the cell, like in the digestive tract.  Membrane proteins have various functions: CELL SURFACE RECEPTOR Membrane proteins that receive and respond to chemical signals from outside the cell. ￭ Hormone receptors: Bind to hormones (e.g., insulin receptors). ￭ Neurotransmitter receptors: Bind to neurotransmitters and trigger cellular responses, such as in nerve cells.  Membrane proteins have various functions: CELL SURFACE IDENTITY MARKER Membrane proteins (often glycoproteins or glycolipids) that identify cells and distinguish them from one another. This is important for immune responses. ￭ MHC proteins: Help the immune system recognize self from non-self ￭ Blood group antigens: Determine blood types (e.g., A, B, O) based on cell surface markers.  Membrane proteins have various functions: CELL TO CELL ADHESION PROTEIN These proteins help cells stick together, allowing tissue formation and communication between cells. ￭ Cadherins: Mediate cell-cell adhesion in tissues. ￭ Integrins: Attach cells to the extracellular matrix and also play a role in cell signaling.  Membrane proteins have various functions: ATTACHMENT TO THE CYTOSKELETON These proteins help cells stick together, allowing tissue formation and communication between cells. ￭ Spectrin: A protein that helps link the plasma membrane to the cytoskeleton, maintaining cell shape. ￭ Anchor proteins: Help anchor the cytoskeleton to the membrane.  TYPES OF MEMBRANE PROTEIN Peripheral Membrane Proteins It is a proteins that are loosely associated with the outer or inner surface of the cell membrane, but do not span the lipid bilayer Integral Membrane Proteins These proteins are embedded within the lipid bilayer, with some spanning the entire membrane (called transmembrane proteins). TYPES OF MEMBRANE PROTEIN  Different type of Transport In biological membranes, transport refers to the movement of molecules across the cell membrane. There are two main types of transport: passive and active transport. PASSIVE TRANSPORT Molecules move from an area of higher VESICULAR TRANSPORT (BULK TRANSPORT) concentration to an area of lower Large molecules or particles are concentration transported in vesicles, which are small ￭ Diffusion membrane-bound sacs that bud off from ￭ Facilitated Diffusion the cell membrane. ￭ Osmosis ￭ Endocytosis ￭ Exocytosis ACTIVE TRANSPORT Molecules move against their concentration gradient (from low to high concentration). ￭ Primary Active Transport ￭ Secondary Active Transport  PASSIVE TRANSPORT DIFFUSION The movement of molecules directly through the lipid bilayer from high to low concentration. ￭ Example: Oxygen (O₂) and carbon dioxide (CO₂) moving across the cell membrane  PASSIVE TRANSPORT FACILITATED DIFFUSION Transport of larger or polar molecules that cannot pass directly through the lipid bilayer. It requires integral proteins to help them cross ￭ Example: Glucose moving into cells via the glucose transporter.  PASSIVE OSMOSIS TRANSPORT A specific type of facilitated diffusion for water molecules through the membrane, often via aquaporins ￭ Example: Water moving into a cell from a dilute solution to a more concentrated solution. When two solutions have different osmotic concentrations ￭ the hypertonic solution has a higher solute concentration ￭ the hypotonic solution has a lower solute concentration  PASSIVE TRANSPORT OSMOSIS Organisms can maintain osmotic balance in different ways. Some cells use extrusion in which water is ejected through contractile vacuoles. Isosmotic regulation involves keeping cells isotonic with their environment. Plant cells use turgor pressure to push the cell membrane against the cell wall and keep the cell rigid.  PASSIVE TRANSPORT OSMOSIS How Organisms Deal with Osmotic Pressure Bacteria and plants have cell walls that prevent them from over- expanding. A protist like paramecium has contractile vacuoles that collect water flowing in and pump it out to prevent them from over- expanding. Saltwater fish pump salt out of their specialized gills so they do not dehydrate. Animal cells are bathed in blood. Kidneys keep the blood isotonic by remove excess salt and water  PASSIVE TRANSPORT OSMOSIS Hypertonic Solutions contain a high concentration of solute relative to another solution Hypotonic Solutions contain a low concentration of solute relative to another solution (e.g. the cell's cytoplasm). Isotonic Solutions contain the same concentration of solute as another solution  ACTIVE TRANSPORT PRIMARY ACTIVE TRANSPORT Direct use of energy (usually ATP) to transport molecules via a pump protein across the membrane against their concentration gradient. ￭ Example: Sodium-potassium pump (Na⁺/K⁺ pump), which moves sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell against their respective concentration gradients.  ACTIVE TRANSPORT SECONDARY ACTIVE TRANSPORT Uses the energy from the ion gradients created by primary active transport. This allows other molecules to be transported without direct use of ATP, but rather by the movement of ions down their concentration gradient. ￭ Example: Sodium-glucose symporter: Glucose is transported into the cell along with sodium ions, using the gradient created by the sodium-potassium pump.  VESICULAR TRANSPORT ENDOCYTOSIS The process of taking in material from outside the cell by engulfing it with the cell membrane to form a vesicle. ￭ Phagocytosis: The cell engulfs large particles like debris or pathogens (e.g., white blood cells eating bacteria). ￭ Pinocytosis: The cell engulfs extracellular fluid and dissolved substances.  VESICULAR TRANSPORT EXOCYTOSIS The process of moving large molecules or waste from the inside of the cell to the outside by vesicle fusion with the plasma membrane. ￭ Secretion of hormones (e.g., insulin from pancreatic cells) ￭ Secretion of nbeurotransmitters (e.g., synaptic vesicles releasing neurotransmitters at synapses).  TERMS TO KNOW Concentration – the amount of solute in a solution Solute – the dissolved substance in a solution. Solution – a mixture in which two or more substances are mixed evenly. Concentration gradient - the gradual difference in the concentration of solutes in a solution between two regions  IMPORTANCE OF PHOSPHOLIPIDS BILAYER AND MEMBRANE IN PLANTS Structural Integrity and Barrier Function ￭ It forms the basic structure of the cell membrane. Selective Permeability ￭ it regulates the passage of substances like water, ions, and small molecules in and out of the cell. Signal Reception and Communication ￭ The membrane proteins embedded in the bilayer are involved in signal transduction. Cell-Cell Communication and Interaction ￭ In plant cells, the thylakoid membranes inside chloroplasts contain proteins and pigments crucial for photosynthesis.  IMPORTANCE OF PHOSPHOLIPIDS BILAYER AND MEMBRANE IN PLANTS Role in Cell Growth and Division ￭ During cell division, the plant cell membrane plays an essential role in the formation of the new cell wall. Protection Against Pathogens ￭ The membrane acts as the first line of defense against pathogens and environmental stress