Chapter 3 Plasma Membrane Transport 2024/2025 BIO091 PDF

CHAPTER 3 PLASMA MEMBRANE TRANSPORT BIO091 Semester 1 2024/2025 1 SUBTOPICS 3.1 : Membrane components and their 3.5 : Passive transport organization 3.5.1: Simple diffusion...

CHAPTER 3 PLASMA MEMBRANE TRANSPORT BIO091 Semester 1 2024/2025 1 SUBTOPICS 3.1 : Membrane components and their 3.5 : Passive transport organization 3.5.1: Simple diffusion 3.5.2: Osmosis 3.2 : Structure of plasma membrane – 3.5.3: Facilitated diffusion fluid mosaic model 3.6 : Active transport 3.3 : The role of membrane proteins 3.6.1: Electrogenic pump 3.6.2: Cotransport 3.4 : Membrane structure results in 3.6.3: Bulk transport selective permeability 2  LEARNING OUTCOMES List the components of plasma membrane and their function. 1 Describe the structure of the plasma membrane and the functions of each of its components. 2. Explain the role of membrane protein. 3 Explain how membrane structure results in selective permeability. 4 Explain the various transport mechanisms 5 across the membrane. 3 3.1 MEMBRANE COMPONENTS AND ITS ORGANIZATION ▪ The plasma membrane: ▪ separates living cell from its surroundings. ▪ controls traffic into & out of the cell it surrounds. ▪ exhibits selective permeability. ▪ Phospholipids ▪ the most abundant lipid in the plasma membrane. ▪ are amphipathic molecules, containing hydrophobic and hydrophilic regions. 4 5 3.2 STRUCTURE OF PLASMA MEMBRANE FLUID MOSAIC MODEL ▪ 1972 – S. Jonathan Singer and Garth Nicolson proposed the Fluid Mosaic Model ▪ It states that the membrane is a mosaic of proteins embedded in a fluid bilayer of phospholipids. 6 7 8 3.2 STRUCTURE OF PLASMA MEMBRANE THE FLUIDITY OF MEMBRANES ▪ Membranes are not static. ▪ Factors affecting fluidity : 9 1. Movement of phospholipid molecule ▪ held together by hydrophobic interactions. ▪ phospholipid molecules carry out flip-flop movement across the 2 layers of phospholipid ▪ lateral movement of phospholipids within the same layer of phospholipid ▪ some proteins move laterally 10 THE FLUIDITY OF MEMBRANES 11 2. Presence of unsaturated fatty acids ▪ As temperatures cool, membranes switch from a fluid state to a solid state. ▪ The temperature at which a membrane solidifies depends on the types of lipids it is made of. 2. Presence of unsaturated fatty acids The membrane rich in phospholipids with unsaturated hydrocarbon tails (unsaturated fatty acids), remains fluid at lower temperatures. Unsaturated hydrocarbon tails cannot pack together, as closely as saturated hydrocarbon tails. due to the presence of double bonds that form kinks in the tails. therefore, the membranes are more fluid. 2. Presence of unsaturated fatty acids The lipid composition of cell membranes can change as an adjustment to changing temperature. ▪ Example: In plants that tolerate extreme cold temperature (e.g., winter wheat), the percentage of unsaturated phospholipids increases in autumn, which keeps the membranes from solidifying during winter. 3. Presence of cholesterol ▪ Membrane must be fluid to work properly. When a membrane solidifies, its permeability changes & enzymatic proteins in the membrane may become inactive. ▪ The CHOLESTEROL has different effects on membrane fluidity at different temperatures. The cholesterol is wedged between phospholipids in the plasma membrane of animal cells. 3. Presence of cholesterol Cholesterol acts as a fluidity buffer for the membrane, it resists changes in membrane fluidity caused by changes in temperature. i. At warm temperatures (such as 37°C, body temperature), ✔ cholesterol restrains movement of phospholipids, membrane less fluid. ii. At cool temperatures, ✔ make membrane more fluid by preventing tight packing, & slow down solidification of membrane. 3.3 MEMBRANE PROTEINS AND THEIR FUNCTIONS Membrane proteins determine most of the Peripheral membrane protein membrane’s functions. 1. Peripheral membrane proteins ✓ Not embedded in the lipid bilayer. ✓ located on the inner or outer surface of the plasma membrane, usually bound to exposed regions of integral proteins by noncovalent interaction. 17 2. Integral proteins ✓ penetrate the hydrophobic core. ✓ Amphipathic proteins firmly bound to the membrane i. Hydrophilic segments – non-helical in contact with different aqueous solutions at the extracellular and cytoplasmic sides of the membrane. ii. Hydrophobic parts – has α-helical secondary structure or rolled-up β-pleated sheets. Transmembrane protein are Hydrophobic regions consist of one or more Integral proteins that span the stretches of nonpolar amino acids. membrane. 18 Membrane Proteins and Their Functions ▪ In many cases, a single membrane protein performs multiple tasks. ▪ Six major functions of membrane proteins: 19 20 Membrane Proteins and Their Functions a. As transport protein Plasma/cell membranes are very selective, allowing only certain solutes to enter or leave the cell, either through channels or carriers composed of membrane proteins. 21 Membrane Proteins and Their Functions Channel / Carrier protein Pore protein Transport a substance Provide a hydrophilic from one side to the channel across the other by changing its membrane, that is shape. selective for a particular solute. 22 Membrane Proteins and Their Functions b. Involve in enzymatic activity ▪ A membrane protein built into the membrane acts as enzyme. ▪ Active site exposed to substances in the adjacent solution. ▪ Several enzymes are organized to carry out sequential steps of a metabolic pathway. 23 Membrane Proteins and Their Functions c. Involve in signal transduction process ▪ Membrane protein acts as a receptor. ▪ Has a binding site with a specific shape that fits the shape of a chemical messenger. ▪ The signaling molecule causes the membrane protein to change shape. ▪ Allow message to relay to the inside of the cell. 24 Membrane Proteins and Their Functions d. Involve in cell-cell recognition ▪ Cell-cell recognition – a cell’s ability to distinguish one type of neighboring cell from another ▪ Membrane carbohydrates may be covalently bonded to lipids (glycolipids) or, more commonly, to proteins (glycoproteins). ▪ Some glycoproteins serve as identification tags 25 Membrane Proteins and Their Functions ▪ Cells recognize each other by binding to surface molecules, often carbohydrates or proteins, on the plasma membrane. ▪ Importance : Sorting of cells into tissues and organs in an animal embryo Basis for the rejection of foreign cells by the immune system in vertebrate animals Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual. ▪ Example: Human blood types (A, B, AB & O). 26 Membrane Proteins and Their Functions e. As Intercellular joining molecule Membrane proteins of adjacent cells may hook together in various kinds of junctions, e.g. ✔ Gap junctions : provide cytoplasmic channels between adjacent cells. ✔ Tight junctions : preventing leakage of extracellular fluid. 27  Membrane Proteins and Their Functions f. Attach to the cytoskeleton and extracellular matrix (ECM) Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, helps maintain cell shape stabilizes the location of certain membrane proteins. 28 3.4 MEMBRANE STRUCTURE RESULTS IN SELECTIVE PERMEABILITY Membrane proteins that act as transport protein play key roles in regulating transport ▪ Small hydrophobic molecules ▪ Hydrophilic molecules ▪ Nonpolar molecules ▪ Large uncharged polar molecules (e.g., ▪ Hydrocarbons, gases (such as CO2, glucose), charged molecules of any size, and and O2), and small polar but small ions (such as H+, Na+ and K+) do not uncharged molecules (such as H2O cross the membrane easily because of the and ethanol) can dissolve in the lipid hydrophobic core of the membrane. bilayer and pass through the ▪ Require transport protein to cross the membrane. membrane. 29 3.4 Membrane Structure Results In Selective Permeability 30 3.4 Membrane Structure Results In Selective Permeability Transport Proteins ▪ Transport proteins allow passage of hydrophilic substances across the membrane. ▪ Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel. ▪ Channel proteins called aquaporins facilitate the passage of water. 31 3.4 Membrane Structure Results In Selective Permeability ▪ Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane. ▪ A transport protein is specific for the substance it moves. 32  Two types of transport across plasma membrane. 33 3.5 PASSIVE TRANSPORT ▪ Types of passive transport: i. Simple Diffusion ii. Osmosis iii. Facilitated Diffusion 34 3.5.1 SIMPLE DIFFUSION A simple rule of diffusion: ▪ In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. a substance will diffuse down concentration gradient (the region along which the density of a chemical substance decreases). No energy is needed and a spontaneous process ▪ Molecules have a tendency to spread out evenly into the available space. ▪ Each molecule moves randomly, but diffusion of a population of molecules may be directional. 35 3.5.1 SIMPLE DIFFUSION 36  DIFFUSION OF ONE SOLUTE ▪ The molecules diffuse from where it is more concentrated to where they are less concentrated (diffusing down their concentration gradient). ▪ This leads to “dynamic equilibrium,” in which the solute molecules continue to cross the membrane but at equal rates in both directions.  DIFFUSION OF TWO SOLUTES ▪ Each type of molecule, diffuses down its concentration gradient. ▪ There will be a net diffusion of the purple molecules towards the left, even though the total solute concentration was initially greater on the left side. 3.5.2 OSMOSIS The diffusion water across a selectively permeable membrane from the region of lower solute concentration to a region of higher solute concentration until the solute concentration on both sides of the membrane is equal. Example: A U-shaped glass tube with a selectively permeable membrane separating 2 Water moves from an area of sugar solutions. Synthetic membrane has higher to lower free water pores. concentration (lower to higher solute concentration) 41 Tonicity is the ability of a solution surrounding a cell to cause that cell to gain or lose water. (– Campbell 12th edition) i. Isotonic solution: ▪ Its solute concentration is the same as that inside the cell; ▪ no net water movement across the plasma membrane. ii. Hypertonic solution: ▪ Its solute concentration is greater than that inside the cell; ▪ cell loses water. iii. Hypotonic solution: ▪ Its solute concentration is less than that inside the cell; ▪ cell gains water. 42 When placing the cell in a If an animal cell) is immersed When the cell is solution that is hypotonic to in an environment that is transferred to hypertonic the cell, isotonic to the cell, solution, water will enter the cell no net movement of cell will lose water to its faster that it leaves the water across the plasma environment. cell. membrane. Water flows across the Cell become shrivel & membrane at the same The cell swell and lyse probably die. rate in both directions. (burst). Example : effect of The volume of an animal cell is stable (iso = same). increasing salinity of a lake to animal 43 A plant cell is placed in a In isotonic solutions, In hypertonic solution, hypotonic solution, No net movement of There is a net outflow of Water enters the cell by water molecules. water by osmosis from osmosis. No change in the cell’s the cell. The plant cell swells but volume. The cell vacuole shrinks, do not burst. Cells become flaccid, the plasma membrane The cell become turgid plant wilts pulls away from cell wall (very firm). & the cell shrivels. Important for This phenomenon, herbaceous plants called plasmolysis, causes the plant cell to 44 wilt. 3.5 PASSIVE TRANSPORT 3.5.3 FACILITATED DIFFUSION ▪ Movement of solutes across a membrane, with the help of transport proteins (transmembrane proteins) ▪ Follows concentration gradient. ▪ Increase the rate of diffusion across cell membranes. ▪ The transport proteins are very specific on which chemicals they allow to pass through. ▪ Two types of transport proteins: i. Channel protein ii. Carrier protein 45  Two types of transport proteins: (a) Channel protein (b) Carrier protein 46  a) CHANNEL PROTEIN ▪ Provide passageways that allow specific molecules or ions to cross the membrane. ▪ Have hydrophilic passageways that allow water molecules or small ions to flow very quickly from one side of the membrane to the other. ▪ Example : Aquaporins, Ion channel 47 a) Channel Protein Aquaporins ▪ are the water channel proteins. ▪ Facilitate the massive amounts of diffusion of water molecules in plant & animal cells. ▪ Without aquaporins, the rate of water movement across the phospholipid bilayer is slow. 48 a) Channel Protein Ion channels i. Non-gated or leak channel: Always open and responsible for the permeability of specific types of ions. simply allow ions to pass through the channel without any impedance. 49 a) Channel Protein Ion channels i. Gated ion channels: Can open or close in response to a stimulus. The stimulus can be a: i. chemical (ligand), ii. electrical (change in voltage) or iii. mechanical. 50 i. Ligand-gated: open or close in response to ligand binding to receptor as neurotransmitter (Acetylcholine) ii. Voltage-gated: open or close in response to small voltage changes for example voltage- gated sodium channel iii. Mechanically-gated: open in response to physical deformation of the receptor, as in sensory receptors of touch and pressure (such as in skin, ear). 51  DISEASE RELATED TO CHANNEL PROTEINS : 1. CYSTIC FIBROSIS ▪ Genetic disease due to a mutated gene ▪ Causing channel proteins for chloride to be defective or absent ▪ Important to regulate the flow of water and salt across membranes of epithelial cells ▪ Build up of mucus in trachea, lungs , pancreas, liver, gallbladder, reproductive organs, sweat glands and other organs 52 1. CYSTIC FIBROSIS ▪ In the lungs, the chloride channel transports Cl- from the inside of cells to the outside of cells. ▪ Extracellular Cl- is required to maintain healthy mucus as it attracts water ▪ High concentration of Cl- inside cells also accumulates Na+ ▪ Causes mucus outside cells to become thicker and stickier than normal ▪ Thick mucus cannot be swept up by cilia ▪ Leads to persistent lung infection, chronic bronchitis, pneumonia 53 54 CYSTIC FIBROSIS 55  b) CARRIER PROTEIN ▪ Some small hydrophilic organic molecules, e.g., glucose and amino acids, use carrier protein to pass through the cell membrane by means of facilitated diffusion. ▪ Change in shape can be triggered by the binding of solute to the binding site of the carrier protein and release the transported molecule with no energy input required. ▪ Example: The glucose transporter, ✔ Glucose binds to the binding site of specific carrier protein ✔ Protein change shape ✔ Glucose molecule released into the cell 56 DISEASE RELATED TO CARRIER PROTEIN : 2. CYSTINURIA ▪ An inherited disease. ▪ Caused by too much amino acid cystine (also Lys, Arg) in the urine. ▪ Normally, after entering the kidneys, cystine dissolves and goes back into the bloodstream. ▪ In people with cystinuria, the carrier proteins that transport cystine across the membrane of kidney cells are defective or missing. ▪ As a result, cystine accumulates in the urine and forms crystals or stones, which may block the kidneys, ureters or bladder. 57 58  3.6 ACTIVE TRANSPORT ▪ Is the movement of ions or molecules across the cell membrane, aided by transport protein, against their concentration gradient. ▪ Uses carrier proteins ▪ Require energy expenditure provided by ATP. ▪ Cells that carry out active transport have: high respiratory rate. large number of mitochondria. ▪ Enables a cell to maintain an internal concentration of small solutes that differ from concentrations in its environment. 59 3.6 Active Transport 3.6.1 ELECTROGENIC PUMP ▪ Voltage is created by differences in the distribution of positive and negative ions across a membrane. ▪ Membrane potential is the voltage across a membrane. ▪ An electrogenic pump is a transport protein that generates a voltage across a membrane. ▪ Example: i. The sodium-potassium pump (animal cell) ii. The proton pump (plant cell, fungi, bacteria) 60 i) The sodium-potassium pump: ▪ Exchange Na+ for K+ across the plasma membrane. ▪ Pumps 3 Na+ out of the cell for every 2 K+ into the cell. i) The sodium-potassium pump: ▪ Exchange Na+ for K+ across the plasma membrane. ▪ Pumps 3 Na+ out of the cell for every 2 K+ into the cell. ii) Proton pump A pump that translocates positive charge in the form of H+. Actively transport H+ (protons) out of the cell. The voltage & [H+] gradient (electrochemical gradient) represent a dual energy source that can drive other processes, e.g., the uptake of nutrients. 63 3.6 Active Transport 3.6.2 COTRANSPORT A cotransport system, moves solutes across a membrane by indirect active transport. Two different solutes are transported at the same time. The movement of one solute (e.g., H+ or Na+) down its concentration gradient provides energy for the transport of another solute (nutrients such as sucrose or glucose) up its concentration gradient. However, an energy source such as ATP is required to power the pump (proton pump or Na+/K+ pump) that produces the concentration gradient. / high concentration of certain ion outside the cell 64 i) H+ - sucrose cotransporter ▪ Function of cotransport in plant: to load sucrose produced by photosynthesis into cells in the veins of leaves The process: i. Proton pump uses energy from ATP to pump H+ against its concentration gradient from inside the cell to the outside of the cell. ii. H+ accumulates outside of the cell. iii. H+ - sucrose cotransporter transports sucrose ▪ Plant cell uses H+ gradient generated by proton ATP-powered proton pump to drive the active (against its concentration gradient) along with the transport of amino acids, sugars, and several diffusion of H+ (down its electrochemical gradient) nutrients into the cell into the cell, 65 ii) Na+ - glucose Cotransporter To treat dehydration due to diarrhea or vomiting, patients are given a rehydration salt solution to drink. ▪ The solution contains a high concentration of salt and glucose. The process involves several transport proteins: i) Na+-glucose cotransporter present in the plasma membrane of epithelial cells of the intestine (brush border)(apical surface). ii) Na+-K+ pumps and glucose transport protein (GLUT-2 transport protein) located on the basolateral surface of the epithelium. iii) Aquaporins 66 The process: 1. Na+- K+ pump, pumps three Na+ out (and later enters blood capillary) and two K+ into the epithelial cell of the small intestine, causing low concentration of Na+ within the epithelial cells of the small intestine. 2. Drinking rehydration salt solution, causes a high concentration of Na+ in the lumen of the small intestine. 67 3. Due to the high concentration of Na+ in the Na+ couples up lumen of the small intestine, and low with glucose to diffuse in concentration of Na+ within the epithelial cells of the small intestine, Na+ diffuses into the epithelial cell (down the concentration gradient) through Na+ - glucose cotransporter. The Na+ - glucose cotransporter transports two Na+ and one glucose across the membrane, into epithelial cells of the small intestine. 68 4. Glucose molecules in the epithelial cell are transported by glucose transport proteins into blood capillaries by facilitated diffusion. 5. Due to Na+ entering the blood (via Na+/K+ pump (refer step 1)), the blood (in the lumen of the blood capillary) has a higher Na+ concentration compared to the epithelial cells of the small intestine. 69 6. Both Na+ and glucose molecules that enter the blood cause the blood to become hypertonic. 7. Water molecules move from the intestinal lumen across epithelial cells and into blood capillaries via aquaporins. 8. Patients slowly become rehydrated. 70 3.6 ACTIVE TRANSPORT 3.6.3 BULK TRANSPORT ▪ Small molecules and water enter or leave the cell through the lipid bilayer or via transport protein ▪ Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles ▪ Bulk transport requires energy 71 A. EXOCYTOSIS ▪ In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents outside the cell ▪ Many secretory cells use exocytosis to export their products ▪ Example : cells in the pancreas, nerve cells, plant cells 72 B. ENDOCYTOSIS ▪ In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane ▪ Endocytosis is a reversal of exocytosis, involving different proteins ▪ There are three types of endocytosis: 1. Phagocytosis 2. Pinocytosis (Cellular Drinking) 3. Receptor-mediated endocytosis 73  1. PHAGOCYTOSIS Phagocytosis EXTRACELLULAR Pseudopodium FLUID Solutes of amoeba ▪ Cell engulfs a particle by extending Pseudopodium pseudopodia around it. Bacterium 1 μm ▪ Pack it within a food vacuole. Food vacuole “Food” ▪ The vacuole fuses with a lysosome to or digest the particle. An amoeba engulfing a other bacterium via phago- particle cytosis (TEM) ▪ Examples in: Unicellular organism Amoeba Food Leukocytes, neutrophils and vacuole monocytes CYTOPLASM 74  2. PINOCYTOSIS (CELLULAR DRINKING) The process is similar to phagocytosis, except: ▪ used for the intake of dissolved materials rather than solids. ▪ Tiny droplets of fluids are trapped by the folds, in the plasma membrane pinocytic vesicle (fluid & dissolved solutes) ▪ Liquid content in the vesicle slowly transferred into cytosol ▪ Pinocytic vesicles are smaller than phagocytic vacuoles 75  3. RECEPTOR-MEDIATED ENDOCYTOSIS ▪ Specialized type of pinocytosis that enables cells to acquire bulk quantities of specific substances even if not very concentrated in extracellular fluid. ▪ Receptor proteins with specific receptor sites are embedded in the membrane and exposed to the extracellular fluid. ▪ Ligands are extracellular substances that bind specifically to a receptor site. ▪ Coated pits are the regions of the membrane containing a large number of receptor proteins. ▪ Each coated pit forms a vesicle containing the bound molecules ▪ Ingested material released from the vesicle ▪ Emptied receptors are recycled to the plasma membrane 76 77  3. RECEPTOR-MEDIATED ENDOCYTOSIS ▪ The intake of cholesterol molecules via receptor-mediated endocytosis by human cells for: synthesis of membranes precursor for the synthesis of other steroids ▪ Cholesterol travels in the blood in particles called low-density lipoproteins (LDL). ▪ Disease related to defective or missing receptor protein : hypercholesterolemia 78 ▪ Hypercholesterolemia - characterized by a very high level of cholesterol in the blood. ▪ LDL receptor proteins are defective - LDL particles cannot enter the cell. ▪ Cholesterol accumulates in the blood - contributes to early atherosclerosis. 79 OTHER SIGNIFICANCE OF ENDOCYTOSIS AND EXOCYTOSIS ▪ provides a mechanisms for rejuvenating or remodeling a cell membrane. ▪ occurs continuously and yet the area of the cell membrane remains fairly constant. ▪ the addition of membrane by one process (exocytosis) offsets the loss of membrane by the other (endocytosis). 80 Passive Transport Vs. Active Transport ATP 81 THA NK Y OU 82