Learning Objectives. The Phospholipid Bilayer. Chapter 5: Plasma Membrane: Structure and Function 1. Lectures by Tariq Alalwan, Ph.D.

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1 Biology, 10e Sylvia S. Mader Lectures by Tariq Alalwan, Ph.D. Learning Objectives Describe the structure of the plasma membrane and the diverse functions of the embedded proteins. Describe what is meant by a semipermeable membrane. Predict the effect of osmotic conditions on animal versus plant cells. Compare and contrast the passive means of crossing a plasma membrane. Compare and contrast the active means of crossing a plasma membrane. The Phospholipid Bilayer The plasma membrane is a phospholipid bilayer with partially or wholly embedded proteins Phospholipids are amphipathic molecules that have both hydrophilic and hydrophobic regions Nonpolar tails (hydrophobic) are directed inward Polar heads (hydrophilic) are directed outward to face both extracellular and intracellular fluid Cholesterol a lipid found in animal plasma membranes that helps modify the fluidityof the membrane The proteins are scattered throughout the membrane forming a mosaicpattern Structure and Function 1

2 Plasma Membrane of an Animal Cell Plasma Membrane Structure The plasma membrane is asymmetrical, how? Membrane proteins may be integral (embedded) or peripheral Integral proteins are found in the membrane and are held in place by the cytoskeleton and the extracellular matrix (ECM) Peripheral proteins are found on the inner membrane surface ECM are only found in animals and their functions include supporting the plasma membrane and communicating between cells Structure and Function 2

3 Fluid Mosaic Model The fluid mosaic model describes the plasma membrane The fluid component refers to the phospholipids bilayer of the plasma membrane (PM) The mosaic component refers to the protein content in the PM Fluidity of the plasma membrane allows cells to be pliable (flexible) Protein movements are limited by interactions with the cytoskeleton and ECM Membrane Fluidity Four main factors contribute to membrane fluidity Temperature at body temperature, the phospholipid bilayer has the consistency of olive oil Membrane phospholipid h id tail length shorter hydrocarbon tails can move sideways (lateral) more easily; rarely flip flop, why? The degree of unsaturation of membrane phospholipid tails Amount of cholesterol keeps the hydrocarbon tails fluid at cold temperatures, and stabilizing them at high temperatures Membrane Fluidity (cont.) Phospholipid Movement With Cholesterol Unsaturated/Saturated Structure and Function 3

4 Carbohydrate Chains Membrane contain carbohydrate chains linked to phospholipds Glycolipids and proteins Glycoprotein on the extracellular surface Glycocalyx a sugar coat in animal cells that facilitates cellular adhesion, protection, signal reception and cell cell recognition Carbohydrate chains vary by number (from 15 to 100 s), sequence of sugars and whether the chain is branched (a fingerprint ) Carbohydrate chains are the basis for A, B and O blood groups in humans Functions of Membrane Proteins The manner in which a protein associates with a membrane depends on its structure and can be categorized as follows Channel proteins Carrier proteins Cell Recognition proteins Receptor proteins Enzymatic proteins Junction proteins Chanel Proteins Allows passage of molecules or ions freely through membrane They facilitate diffusion by forming hydrophilic transmembrane channels H + ions across mitochondrial inner membrane during ATP production Faulty Cl channel causing cystic fibrosis Channel proteins are only responsible transport for passive Structure and Function 4

5 Carrier Proteins Selectively interact with a specific molecule so that it can cross the plasma membrane to enter or exit the cell This process often requires energy (ATP) When ATP is involved with actively moving molecules through the membrane the process is called active transport Example: Na + K + pump of nerve cells Cell Recognition Protein Glycoproteins and some glycolipids serve as surface receptors for cell recognition and identification (cellular fingerprint) Important in that the immune system cells can distinguish between one s own cells and foreign cells The major histocompatibility complex (MHC) glycoprotiens are different in each individual MHC determines organ transplant acceptance or rejection Receptor Proteins Receptor proteins serve as binding or attachment sites Protein has a specific shape so that specific molecules can bind to them Binding of a molecule (e.g. insulin hormone) can influence the liver to store glucose Pygmies are short due to their faulty PM hormone receptors that cannot interact with growth hormone Structure and Function 5

6 Enzymatic Proteins Many enzymes are embedded in membranes, which attract reacting molecules to the membrane surface Catalyzes a specific reaction Adenylate cyclase is a membrane bound enzyme that is involved in ATP metabolism Cholera toxin activates the adenylate cyclase enzyme in the intestinal cells Results in the loss of H 2 O, Na + and K + from the intestinal cells (i.e. dehydration) Junction Proteins Form various types of junctions between animal cells Signaling molecules that pass through gap junctions allow the cilia of cells lining the respiratory tract to beat at the same time Tight junctions joining animal cells in order to form a specific function Example nervous system in animal embryos Permeability of the Plasma Membrane Plasma membrane is differentially (selectively) permeable Allows some material to pass freely Inhibits (blocks) passage of other materials Some materials enter or leave the cell only by the using cell energy By regulating chemical traffic across its plasma membrane, a cell controls its volume and its internal ionic and molecular composition Structure and Function 6

7 Types of Transport: Active vs. Passive Passive Transport No ATP requirement; includes diffusion and facilitated transport Molecules follow concentration gradient (i.e. from high to low concentration) Concentration the number of molecules of a substance in a given volume Gradient a physical difference between two regions so that molecules will tend to move from one of the regions toward the other (i.e. concentration, pressure & electrical charge) Active vs. Passive (cont.) When the distribution of molecules is not equal, and we have a gradient, there is a net movement of molecules along down the gradient Example: Cellular respiration Concentration of O 2 is lower inside a cell than outside Concentration of CO 2 is higher inside the cell than outside Active vs. Passive (cont.) Active Transport Requires carrier protein Molecules move through the membrane against the concentration gradient Requires energy in form of ATP Movement out of the cell involving changes of the membranes & formation of vesicles is exocytosis Movement of materials into the cell is endocytosis Structure and Function 7

8 Diffusion A solution consists of: A solvent (liquid) and a solute (dissolved solid) Diffusion the net movement of solute molecules down their own concentration gradient, from a region of higher concentration to one of lower concentration, until molecules are equally distributed In terms of cellular activity, diffusion: Requires no energy However, the cell has no control over diffusion, and the rate of diffusion is quite slow Diffusion (cont.) The rate of diffusion can be affected by: Temperature (higher temperature faster molecule movement) Molecule l size (smaller molecules l often move more easily) Concentration (Initial rate faster with higher concentration) Electrical & pressure gradients of the two regions (greater the gradient differential, the more rapid the diffusion) Membrane Transport Materials that may move through membranes freely by simple diffusion include: CO 2 O 2 Small lipid soluble molecules Passive transport (carrier proteins): H 2 O (aquaporin) Glucose Many small ions Some amino acids Structure and Function 8

9 Osmosis Focuses on solvent (water) movement rather than solute Osmosis diffusion of water across a differentially (selectively) permeable membrane Solute concentration on one side high, but water concentration is low Solute concentration on other side low, but water concentration is high Water diffuses both ways across membrane but solute can t Net movement of water is toward low water (high solute) concentration Osmosis Demonstration Osmotic pressure is the pressure that develops due to osmosis The more solute particles present, the higher the osmotic pressure Significance of Osmosis Absorption of water from the soil by plant roots Turgidity is developed by the process of osmosis which provides mechanical strength in plants Re absorption of water by the kidneys Absorption of water by the digestive tract (i.e. stomach, small intestine and the colon) Structure and Function 9

10 Types of solutions: Isotonic Isotonic Solution Solute and water concentrations are equal on both sides of membrane This results in no net movement of water into or out of cells the cell neither swells nor shrinks Osmotically balanced Physiological or normal saline consists of 0.9% NaCl in water, which is isotonic to red blood cells (RBCs) Types of solutions: Hypotonic Hypotonic Solution The solution surrounding the cell has a lower solute concentration (i.e. more water) than the cell This results in a net movement of water into the cells Cells placed in a hypotonic solution will swell May cause animal cells to burst lysis Hypotonic Environments Cells which typically exist in hypotonic solutions (fresh water), use various mechanisms such as The contractile vacuoles found in protists (e.g. paramecium) are used to expel excess water Well developed kidneys in freshwater fish to excrete large volume of diluted urine Plant cells use osmotic pressure to their advantage When plant cells immersed in water, the vacuole (containing the stored molecules) gain water which increases the turgor pressure This pressure forces the cytoplasm against the plasma membrane and cell wall, helping to keep the cell rigid Structure and Function 10

11 Types of solutions: Hypertonic Hypertonic Solution The surrounding solution has a higher solute concentration (i.e. less water) than the cell Cells placed in a hypertonic solution will shrink Plasmolysis Antibiological activities used in food preservation (i.e. meats, fruits and vegetables are pickled, salted, or mixed with concentrated sugar solutions to prevent bacterial & fungal growth) Hypertonic Environments Salt water is hypertonic to the cells of freshwater organisms Central vacuole in plants lose water and the plasma membrane pulls away from the cell wall Plasmolysis occurs in plants when the soil or water around them contains high concentrations of salts or fertilizers Marine animals cope in various ways Sharks increase/decrease urea in blood Fishes excrete salts across their gills Facilitated Transport: Carrier Proteins Facilitated Transport Small molecules (i.e. glucose & amino acids) Can t get through membrane lipids Combine with carrier proteins Follow concentration gradient (i.e. no ATP) Structure and Function 11

12 Active Transport Across a Membrane Active Transport Small molecules (i.e. glucose & amino acids) Move against concentration gradient Requires a direct expenditure of energy Requires two carrier protein active sites: one to recognize the substance to be carried one to release ATP to provide the energy for the protein carriers or "pumps The sodium potassium pump The Na + K + Pump Bulk Transport: Exocytosis Macromolecules are transported into or out of the cell inside vesicles Vesicle formation requires ATP Exocytosis vesicles formed from Golgi apparatus fuse with plasma membrane and secrete contents Hormones, neurotransmitters & digestive enzymes are secreted by exocytosis Example: insulin, made in pancreatic cells, are secreted by exocytosis Regulated secretion occurs when plasma membrane receives a signal (i.e. rise in blood sugar) Structure and Function 12

13 Exocytosis Bulk Transport: Endocytosis Endocytosis substances that enter the cell by vesicle formation There are three mechanisms of endocytosis: Phagocytosis large, solid particles into vesicle, il such as a bacterium Pinocytosis liquid or very small particles, such as macromolecules, go into the vesicle Receptor Mediated Endocytosis specific form of pinocytosis using a receptor protein Vesicle membrane is added to plasma membrane Phagocytosis Phagocytosis ( cell eating ) Cell ingests large solid particles such as food or bacteria Folds of plasma membrane enclose the cell or particle, forming a phagocytic vacuole Vacuole may fuse with lysosomes, which degrade the ingested material Examples amoeba & macrophage Structure and Function 13

14 Pinocytosis Pinocytosis ( cell drinking ) Cell takes in dissolved materials Droplets of fluid are trapped by folds in the plasma membrane, which h pinch off into the cytosol as vesicles Vesicles become smaller as liquid in the vesicles is transferred into the cytosol Examples Blood cells & plant root cells Receptor Mediated Endocytosis A form of pinocytosis, occurs when specific macromolecules bind to plasma membrane receptors The macromolecules are taken into the cell via coated vesicles that pinch from the plasma membrane Receptors for specific molecules are concentrated in coated pits (i.e. layer of fibrous protein) on the plasma membrane Coating detaches from vesicle, and uncoated vesicle fuses with a lysosome Receptor Mediated Endocytosis (cont.) Pits are associated with exchange of substances between cells (e.g. maternal and fetal blood) System is selective and more efficient than pinocytosis Defects in receptor mediated endocytosis are responsible for certain diseases such as hypercholesterolemia LDL receptors cannot bind to the coated pit, thus the cells are unable to take up cholesterol Access cholesterol accumulates in the circulatory system Will cause heart attacks & atherosclerosis Structure and Function 14

15 Receptor Mediated Endocytosis Structure and Function 15