Topic 4 - Student. Membrane structure and Function 2024.2025.pdf

BIOLOGY 1 ASB0204 Topic 4 Membrane Structure and Function Biology Unit Centre for Foundation Studies in Science of Universiti Putra Malaysia Outline 4.1 :Plasma Membrane Structure and Function - Plasma membrane components - Per...

BIOLOGY 1 ASB0204 Topic 4 Membrane Structure and Function Biology Unit Centre for Foundation Studies in Science of Universiti Putra Malaysia Outline 4.1 :Plasma Membrane Structure and Function - Plasma membrane components - Permeability of plasma membrane 4.2 :Passive and Active Transport 4.3 :Modification of Cell Surfaces Learning outcomes 1. Explain the structure and function of membrane. 2. Compare passive and active transport. 3. Explain the modification that occurs at cell surfaces. 4  Subtopic 4.1: Plasma Membrane Structure and Function Plasma membrane; ❑ Made up of phospholipid bilayer ❑ Separating a cell from its external environment: essential to origin of life ❑ Membrane proteins are critical in cell membrane activities: ❑ Proteins associated with the plasma membrane transport materials: transmit information; serve as enzymes ❑ Cell adhesion molecules connect cells to one another to form tissues ❑ Cylindrical shape allow them to associate with water most easily as bilayer ❑ Other amphipathic molecule with different shape tend to form spherical structure in water ❑ The electron microscope revealed a three-layered structure, suggesting plasma membrane was uniform and no more than 10 nm thick. ❑ J. Singer and G. Nicolson proposed the fluid mosaic model in 1972: embedded proteins loosely associate with the bilayer, like a dynamic mosaic Singer Nicholson Fluid mosaic model Fluid Mosaic Model of the Plasma Membrane Outside cell Plasma membrane carbohydrate extracellular chain Matrix (ECM) hydrophobichydrophilic glycoprotein tails heads phospholipid glycolipid filaments of cytoskeleton Inside cell peripheral protein integral protein cholesterol 7  How does membrane move? ❑ A membrane is held in together by weak hydrophobic interactions. ❑ Most membrane lipids and some proteins can drift laterally within the membrane ❑ Molecules rarely flip transversely (flip-flop) across the membrane, because hydrophilic parts would have to cross the membrane’s hydrophobic core.  Subtopic 4.1: Plasma Membrane Structure and Function Component of Plasma Membrane 1. Phospholipid bilayer ❑ Two parts: Hydrophobic region - made up from two fatty acid chains Hydrophilic region – negatively charged phosphate group ❑ Flexible: allowing cell membranes to change shape without breaking ❑ Self-sealing, and spontaneously round up to form closed vesicles ❑ Can fuse with other bilayers: allowing vesicles to transfer materials from one compartment to another, or secrete a product from the cell External surface lined with hydrophilic polar head Nonpolar, hydrophobic, fatty acid tails sandwiched in between Cytoplasmic surface lined with hydrophilic polar head Component of Plasma Membrane 2. Other lipids and cholesterol molecules: Affect the fluidity of plasma membrane When outside temperature is low, some organisms alter fatty acid content of their membrane lipids to increase relative proportions of unsaturated fatty acids Unsaturated hydrocarbon tails enhance membrane fluidity because kinks at the carbon-to-carbon double bonds hinder close packing of phospholipids. Component of Plasma Membrane 2. Other lipids and cholesterol molecules: Affect the fluidity of plasma membrane Cholesterol molecules in membranes act as “fluidity buffers,” keeping hydrocarbon chains fluid at low temperatures and stabilizing them at high temperatures At low At high temperature: temperature: Cholesterol Cholesterol prevent the stiffen the membrane from membrane from freezing by not becoming too allowing contact fluid. between certain phospholipids tails. Component of Plasma Membrane 3. Protein molecules: Three types: Integral membrane proteins: amphipathic proteins firmly bound to the membrane Transmembrane proteins: integral proteins that extend completely through the membrane Peripheral membrane proteins: located on inner or outer surface of plasma membrane, bound to exposed regions of integral proteins Peripheral protein Integral protein Transmembrane protein Integral protein Peripheral protein Membrane proteins and its function: 1. Channel Protein Allow passage of protein through membrane via channel in the protein 2. Carrier Protein Combine with the substance to be transported Assist passage of molecules through membrane 3. Cell recognition protein Glycoprotein/glycolipids on the extracellular surface (ID tags) Carbohydrate are short branched chains of less than 15 sugars Help the body recognize foreign substances Membrane proteins and its function: 4. Receptor Proteins Bind with specific molecules Allow a cell to respond to signals from other cells 5. Enzymatic Proteins Some are enzymes that carry out metabolic reactions directly. 6. Junction Proteins Forms various of junction in animal cells Attach adjacent cells  Examples of different types of membrane protein Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Channel Protein: Carrier Protein: Cell Recognition Allows a particular Selectively interacts Protein: molecule or ion to with a specific The MHC (major cross the plasma molecule or ion so histocompatibility membrane freely. that it can cross the complex) glycoproteins Cystic fibrosis, an plasma membrane. are different for each inherited disorder, The inability of some person, so organ is caused by a persons to use transplants are difficult faulty chloride (Cl–) energy for sodium- to achieve. Cells with channel; a thick potassium (Na+–K+) foreign MHC mucus collects in transport has been glycoproteins are airways and in suggested as the attacked by white blood pancreatic and cause of their obesity. cells responsible for liver ducts. immunity. a. b. c. Receptor Protein: Enzymatic Protein: Junction Proteins: Is shaped in such a Catalyzes a specific Tight junctions join way that a specific reaction. The membrane cells so that a tissue molecule can bind to protein, adenylate can fulfill a function, as it. Pygmies are short, cyclase, is involved in when a tissue pinches not because they do ATP metabolism. Cholera off the neural tube not produce enough bacteria release a toxin during development. growth hormone, but that interferes with the Without this because their plasma proper functioning of cooperation between membrane growth adenylate cyclase; cells, an animal hormone receptors sodium (Na+) and water embryo would have no are faulty and cannot leave intestinal cells, and nervous system. interact with growth the individual may die hormone. from severe diarrhea. d. e. f. Permeability of the Plasma Membrane Plasma membrane is selectively permeable; only allow some substance to move across the membrane Inhibits passage such as polar molecules Water moves across plasma Small, non-charged molecules membrane through specialized (CO2, O2, gylcerol, alcohol) protein named aquaporin – freely pass the membrane – speed up water transport follow concentration gradient across the membrane  Water moves across the plasma membrane through aquaporin ❑ Allow cell to equalize water pressure differences between interior and exterior environment ❑ Prevent cell membrane from burst Ions and polar molecules (glucose, amino acids) across the membrane is assisted by carrier proteins ❑ Carrier protein recognize particular shapes of molecules before changing its shape and transporting the molecules across the membrane Many ions and small molecules move through membranes by diffusion ❑ Random motion of particles resulting in net movement “down” their own concentration gradient  Some molecules must move against concentration gradient with the expenditure of energy ✔ Active transport Bulk transport is the way large particles enter or exit the cell. ✔ Exocytosis – fusion of vesicles with the membrane plasma membrane moves a particle to outside the membrane ✔ Endocytosis – vesicle formation moves a particle to inside the plasma membrane  Subtopic 4.2: Passive and Active Transport Passive Transport: Diffusion Diffusion: Movement of molecules down a concentration gradients Involved many ions and small molecules The rate of diffusion is determined by particles’ size and shape, their electric charges, and the temperature Two types: ▪ Simple diffusion ▪ Facilitated diffusion Simple diffusion: Osmosis ∙ Special case of diffusion ∙ Focuses on solvent (water) movement rather than solute ∙ Diffusion of water across a selectively permeable membrane ∙ Solute concentration on one side is high, but water concentration is low ∙ Solute concentration on other side is low, but water concentration is high ∙ Water can diffuse both ways across membrane but the solute cannot ∙ Net movement of water is toward low water (high solute) concentration ∙ Osmotic pressure is the pressure that develops due to osmosis. Tonicity: Relative concentrations of solutes in two fluids separated by a selectively permeable membrane can differ ▪ Isotonic Solutions ∙ Solute and water (solvent) concentrations are equal on both sides of cellular membrane. ∙ There is no net gain or loss of water by the cell. ▪ Hypotonic Solutions ∙ Concentration of solute in the solution is lower than inside the cell. ∙ Cells placed in a hypotonic solution will swell. ∙ Causes turgor pressure in plants ∙ May cause animal cells to lyse (rupture) ∙ Protozoans living in fresh water environments have contractile vacuoles to rid themselves of excess water. ▪ Hypertonic Solutions – Concentration of solute is higher in the solution than inside the cell. – Cells placed in a hypertonic solution will shrink. Crenation in animal cells – Example of red blood cells placed in a hypertonic solution (higher than 0.9% sodium chloride) Plasmolysis in plant cells – Examples are dead plants along a salted roadway and marine animals that are able to make their blood isotonic with the environment. Responses of Animal Cells to Osmotic Pressure Differences Facilitated Diffusion ▪ Occurs when molecule are passively transported across the membrane using transport protein Transport proteins are specific; can transport only certain types of molecule or ion Transport protein undergo transformational change in shape to allow the molecule to enter cell ▪ Examples of molecules: Glucose and amino acids  Subtopic 4.2: Passive and Active Transport Active Transport ▪ The movement of molecules against their concentration gradient ▪ Movement from low to high concentration ▪ Movement facilitated by specific carrier proteins ▪ Requires expenditure of energy in the form of ATP ▪ Example: Sodium-potassium pump Exocytosis Bulk transport Endocytosis ⮚ Transportation of macromolecules into and out of the cell by vesicles Active Transport: Sodium-Potassium Pump The sodium–potassium pump uses ATP to pump Na ions out of the cell and K ions into the cell ▪ 2 K ions are in for every 3 Na ions out ▪ Membrane becomes polarized ▪ Membrane potential is created because of the separation of charges ▪ Electrochemical gradients store energy used to drive other transport systems Active Transport: Exocytosis ▪ Intracellular vesicle fuses with the plasma membrane as secretion occurs: Hormones, neurotransmitter, digestive enzymes are secreted via this manner ▪ During exocytosis, membrane of the vesicles become part of the plasma membrane, because both are nonpolar ▪ The vesicles are produced by Golgi body that carry cell product to the membrane Active Transport: Endocytosis ▪ Endocytosis – Cells engulf substances into a pouch, which becomes a vesicle. ▪ Occurs in three ways: Phagocytosis – Large, solid material is taken in by endocytosis. – Example: human white blood cells can engulf debris or viruses Pinocytosis – Vesicles form around a liquid or very small particles (cell drinking). Receptor-Mediated Endocytosis – Specific form of pinocytosis using receptor proteins and a coated pit Lysosome fuse with vacuole and pour potent hydrolytic enzyme onto ingested material. Three ways of Endocytosis  Subtopic 4.3: Modification of Cell Surfaces Extracellular Matrix (ECM) Meshwork of proteins and polysaccharides in close connection with the cell that produced them. Component of ECM 1. Collagen 2. Elastin 3. Fibronectin 4. Integrin 5. Proteoglycans Extracellular Matrix (ECM) Component Functions Collagen resists stretching. Elastin provides resilience to the ECM. Fibronectin adhesive protein which binds to integrin. Integrin plays role in cell signaling. Proteoglycans attach to a long, centrally placed polysaccharide that resists compression of extracellular matrix. assist cell signaling by regulating passage of material through the ECM to the plasma membrane. Cell Junctions Allow cells to behave in coordinated manner. Adhesion Junctions Gap Junctions intercellular plasma filaments between membrane cells channels are desmosome joined internal allows cytoplasmic communication Tight Junctions plaques important in heart form impermeable hemidesmosome muscle and barriers intermediate smooth muscle e.g.: urine stay in filament kidney tubules Plant Cell Walls Plants have a freely permeable cell wall, with cellulose as the main component. Plasmodesmata penetrate the cell wall. Connecting cytoplasm between cells. Allow passage of water and small solutes between cells. Cells in woody plants have a secondary cell wall containing lignin and more cellulose fibers than the primary cell wall.  PLASMA MEMBRANE Structure & Transportation Modification of cell Function across membrane surfaces Structure ECM Phospholipid bilayer Meshwork of proteins and polysaccharides Other lipids and Cholesterols Components: Collagen Protein molecules: Elastin Channel Fibronectin Carrier Integrin Cell recognition Proteoglycans Receptor Enzymatic Junction Cell Junctions Adhesion Functions Tight Separating a cell from its external environment Gap Permeability: Transportation Metabolism Cells recognition References 1. Mader, SS & Windelspecht, M (2019). Biology (14th ed.). McGraw-Hill Education. Membrane Structure and Function. 2. Solomon, EP, Martin, CE, Martin, DW, Berg, LR (2019). Biology (11th Ed.). Cencage Learning. Biological Membranes. Page 106-130.  Chapter 5 Biological Membranes page 106-130 36 37