Lecture 2 Membrane Structure and Transport PDF

Membranes structure, function and transport 1 Membranes structure, function and transport. Be able to; 1. Draw and label a diagram of the cell membrane. 2. Describe the components and function(s) of eukaryotic cell membranes. 3. Explain why cell membranes are ‘fluid...

Membranes structure, function and transport 1 Membranes structure, function and transport. Be able to; 1. Draw and label a diagram of the cell membrane. 2. Describe the components and function(s) of eukaryotic cell membranes. 3. Explain why cell membranes are ‘fluid mosaics of lipids and proteins’? 4. Explain the reason for the fluidity of membranes. 5. Explain why selective permeability is related to the membrane structure. 6. Describe passive transport across cell membranes (the movement of a substance across a membrane by the process of diffusion). 7. Describe active transport across cell membranes. 8. Describe bulk transport across cell membranes. 9. Distinguish between endocytosis and exocytosis with the aid of diagrams and explain how endocytosis can be specific 2  Diseases associated with dysfunction of cell membranes 1. Cystic fibrosis (CF) is caused by a mutation in a membrane protein which normally helps move water and salt across the cell membrane. In CF the cells do not secrete enough water, the mucus in the lungs becomes extremely thick. 2.A membrane protein contributes to Alzheimer’s disease that lead to the accumulation of amyloid proteins. Amyloid protein fibres are associated with a great variety of human diseases including Alzheimer’s disease and the prion conditions. https://www.alz.org/alzheimers-dementia/what-is-alzheimers/brain_tour_part_2 Cell Membrane Characteristics Functions and characteristics of cell membranes: 1. Membranes exhibit selective permeability (Allowing some substances to pass across it more easily than others) 2. Cell membranes separate the internal (intracellular) cell environment from the outside (extracellular environment) maintaining a different concentration of molecules and ions inside and outside of the cell 3. Help organisms maintain homeostasis (balance or equilibrium) by controlling what substances may enter or leave cells 4. Selective permeability of cell membranes is related to the membrane structure. (Cell membrane is a phospholipid bilayer with proteins embedded or attached) Membrane composed of a phospholipid bilayer. Phospholipid Bilayer  Composed of an inner leaflet (layer 1) an outer leaflet (layer 2) 4  Draw and label diagrams Phospholipid Bilayer Micelle Membrane composed of a phospholipid bilayer. Phospholipid Bilayer  Define hydrophobic Composed of an inner leaflet (layer 1) and hydrophilic?? an outer leaflet (layer 2) 5 Permeability of cell membranes Permeability refers to the ease with which molecules cross biological membranes. Because of the chemical and structural nature of the phospholipid bilayer (hydrophobic core) only hydrophobic lipid soluble molecules and some small molecules can diffuse across. Whether a molecule can pass through the membrane depends on its size and its electrical nature Cell membranes are impermeable to: i. Small polar ions (charged) like K+, Na+, Ca2+, Cl-, HCO3 - ii. Polar hydrophilic (water soluble) molecules: glucose, polar amino acids, nucleotides. Why are they repelled? Repelled by the hydrophobic fatty acid components of the interior region of the plasma membrane. Macromolecules like proteins & RNA. Why repelled ? Large size. The electrons spend more time with one atom giving it an overall negative charge 6 ii. Cell membranes are permeable to: 1) Gases: CO2, O2, N2. 2) All small Nonpolar (uncharged) molecules. 3) Nonpolar (hydrophobic – fat soluble) uncharged molecules: e.g: certain dyes, some carcinogens, anticancer agents. 4)Small polar molecules: Ethanol, ether, H2O, steroid hormones, small lipophilic (fat soluble) molecules. Non polar covalent bonds Atoms share electrons equally Entry and exit of molecules and ions is carefully regulated. To enter or leave a cells most molecules and all ions must be transported across the membrane by special transport proteins embedded in the membrane. 7 Structure and chemical composition of cell membranes Structure: Cell membranes are mainly composed of a phospholipid bilayer with proteins. Phospholipids contribute to the overall structure of the membrane and form the bilayer. Note the two fatty acid (FA) components that make up the ‘lipid tails’ of a phospholipid molecule can be the same or different.  Note the charge on the phosphate head group of the phospholipid molecule and the uncharged nature of the fatty acid tails. 8 Phospholipid molecule - draw and label simple schematic Structure and chemical composition of cell membrane Chemical Composition: Phospholipids Phospholipids are amphipathic molecule meaning that they have; 1. Nonpolar hydrophobic (water insoluble/ “water hating”) fatty acid tail, 2. Polar hydrophilic (water soluble/ “water loving”) phosphate and glycerol head. Diagrams below information: know the components listed in the diagram 10 Structure and chemical composition of cell membrane Phospholipid The phosphate group is negatively charged, making the head polar and hydrophilic, or “water loving.” The phosphate heads are therefore attracted to the water molecules in their environment. The lipid tails of fatty acids are uncharged, nonpolar and hydrophobic, or “water hating” A hydrophobic molecule repels and is repelled by water. Phospholipids spontaneously form a lipid bilayer in aqueous environments. Lipid tails can consist of saturated fatty acids and some contain unsaturated fatty acids. 11 Diagrams for illustration Structure and chemical composition of cell membrane. Other lipid components Glycolipids (short chains of monosaccharides attached to lipid) are amphipathic molecules, involved in membrane stability, cell-cell recognition, adhesion, chemical signalling. Glycoproteins involved in chemical signalling, lubricant (mucins), cell-cell recognition. Cell Receptors on membrane surfaces are usually Glycoproteins or Glycolipids. Cholesterol is an amphipathic molecules which contributes to membrane fluidity and stability. The hydrophilic region bonds to the hydrophilic heads and the hydrophobic regions fit in between the tails of the membrane layer contributing to fluidity and stability. Diagram for 13 illustration purposes  14 Draw and label this diagram of a general plasma membrane Structure and chemical composition of cell membrane. Proteins Proteins are located in or on the lipid bilayer. Proteins determine most of the membrane’s specific functions. Types of membrane proteins: Peripheral proteins Loosely bound to the internal or external surface of the membrane, but not embedded in the bilayer. Integral proteins Are embedded and penetrate the hydrophobic fatty acid core. Many are transmembrane proteins, spanning the entire membrane. Other integral proteins only extend partway into the hydrophobic core. Some transmembrane proteins have a hydrophilic channel through which hydrophilic molecules can be transported into the cytoplasm. 13 Some functions of membrane proteins To adapt to changing conditions, cells must sense their external environments and respond accordingly. 14 Eukaryotic Cell Membrane Draw a clearly labelled pencil drawing of the plasma membrane and describe the role of it’s components 18 Fluidity of cell membranes ‘Fluid Mosaic’ model is used to describe the structural appearance of cellular membranes. Mosaic - proteins like pebbles scattered throughout the phospholipid bilayer. Cellular membranes are ‘fluid mosaics’ of lipids and proteins https://www.youtube.com/watch?v=Qqsf_UJcfBc (membrane fluidity). Fluidity: Membranes are not static sheets of phospholipid molecules stacked tightly together. 19 Diagrams for illustration purposes Fluidity of cell membranes 1. Unsaturation FA’s in membranes, introduce ‘kinks’ into the molecule that results in poor stacking and permits greater motion of the lipid molecules and increased fluidity. Saturated FA’s without ‘kinks’ stack more tightly together and decreased fluidity. Diagram for illustration purposes Organisms can increase or decrease the number of saturated or unsaturated FA’s in their membrane phospholipids. At high temperatures cells can introduce more phospholipids with saturated fatty acids which permits tighter stacking preventing the membrane from becoming too fluid. The reverse may occurs at low temperatures. 2. Cholesterol in animal cell membranes (not in prokaryotes) contributes to control membrane fluidity. At 37°C it stabilises membranes by restraining phospholipid movement, preventing membranes from becoming too fluid at this temperature and higher temperatures. At low temperatures it prevents phospholipids from stacking too tightly thus keeping the membrane more fluid so it behaves as a ‘temperature buffer’. 20 Fully saturated fatty acids in the extended form pack into nearly crystalline arrays, stabilized by many hydrophobic interactions. The presence of one or more cis double bonds interferes with this tight packing, and results in less stable aggregates. https://www.youtube.com/watch?v=hpECyJbtQ38 21 Fluidity of cell membranes Importance of the fluidity and selective permeability of plasma membranes. 1. Cells can change shape in response to the environment and permits exocytosis and endocytosis 2. Allows free diffusion of molecules such as oxygen, carbon dioxide, and small hydrocarbons 3. Permits the cell to maintain the composition of the cytoplasm independent of the external environment. 19 Membrane structure animation Transport of ions and molecules across cell membranes Cells must exchange material with the extracellular environment. A steady traffi c of molecules and ions move across cell membranes in both directions. There are three mechanisms by which substances can cross cell membranes A. Passive transport. No energy required. B. Active transport Energy required. C. Bulk transport Energy required. 20 A. Passive Transport Movement that doesn’t require the cell to use energy. At dynamic equilibrium in cells {3rd region of diagram} as many molecules cross the membrane in one direction as in the other. There are 3 types of passive transport across cell membranes. 1. Simple Diffusion: Movement of molecules or ions down a concentration gradient from a high concentration  low concentration. Molecules that cross the plasma membrane in this way are: CO2, O2. urea - small nonpolar molecules that dissolve and move through the hydrophobic membrane interior 21 A. Passive Transport https://www.youtube.com/watch?v=1636lRv2uGE 2018 11min Passive V Active transport 2. Facilitated diffusion: when polar molecules and ions cross the membrane aided by transport proteins. TWO types of transporter proteins (i) Channel proteins (ii) Carrier proteins (i). Channel proteins: provide a protein lined passageway through which specific molecules or ions move down their concentration gradient. E.g.: Aquaporins: Carry water molecules (1993 N prize). Ion channels: Channel proteins that carry ions like Na, K+, Cl-and open or close in response to stimuli (Very fast :108/sec) (ii). Carrier proteins: Carrier proteins typically have a “binding site” which will only bind to the substance they are supposed to carry. Bind specifically. Change shape in order to carry the molecule across. 25 https://www.youtube.com/watch?v=-aWL84eW7z8 Channel and carrier proteins (A) Channel proteins form open pores through which molecules of the appropriate size (e.g., ions) can cross the membrane. (B)Carrier proteins selectively bind the small molecule to be transported and then undergo a conformational change to release the molecule on the other side of the membrane. 26 Diagram for information A cartoon model of carrier-mediated transport The solute acts as a ligand that binds to the transporter protein…. … and then a subsequent shape change in the protein releases the solute on the other side of the membrane. https://www.ibiblio.org/virtualcell/textbook/chapter3/cmf3.htm  Diagram for illustration purposes A. Passive Transport 3. Osmosis: The diffusion of water across a selectively permeable membrane. Water molecules diffuse across membranes from a region with a low solute concentration  a region with a higher solute concentration until the solute concentration is equal on both sides of the membrane. A dynamic equilibrium is reached when the concentration of solute is equal on both sides of the membrane. Osmosis (movement of water) is a very important process in cell biology If the total concentration of all dissolved solutes is not equal on both sides of a membrane, Effects of Osmosis on the balance of water between a cell and it’s environment Definition: Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water by Osmosis. A solution's tonicity is related to its osmolarity, which is the total concentration of all solutes in the solution. When solutions of different osmolarities are separated by a membrane permeable to water, but not to solute, water will move from the side with lower osmolarity to the side with higher osmolarity. Most cells are in some kind solution. Three terms that may be used to describe the tonicity of a solution 1. Isotonic solution: Solute concentration outside = Solute conc. inside the cell. No net movement of water molecules by osmosis in either direction 2. Hypertonic solution: Solute conc. outside > Solute inside the cell. Cell lose water by osmosis undergoing crenation (animal cells) and plasmolysis (plant cells) 3. Hypotonic solution: Solute conc. outside < Solute inside the cell. Water enters cells by osmosis. Hypotonic / Hypertonic solutions in nature can cause osmotic problems for organisms. Organisms must have mechanisms of Osmoregulatio2n9  Crenation (blood cells) 30 Draw, label and describe the processes for animal  Diagram for illustration purposes B. Active Transport http://www.uic.edu/classes/bios/bios100/lectures/diffusion.htm Active transport Moves substances across membranes against/up their concentration gradient. From low concentration  high concentration. http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles%20of%20Life2e/Animated%20Tutorials/pol2e_at_0503_Active_Transport/ o2e_at_0503_Active_Transport.html video ATPs role in active Transport. Active transport requires energy, in the form of ATP. It occurs through specific proteins embedded in the membrane. Active transport permits cells to maintain high internal concentrations of certain ions relative to the conc. of these ions outside the cell. E.g.: Cells maintain High K+ inside cell and low Na+ inside cell through active transport. 31  https://www.youtube.com/watch?v=b3sYO-IDkxY C. BULK TRANSPORT (Uses energy) Endocytosis & exocytosis 1. Endocytosis: The general process of taking bulky material into a cell. i)Phagocytosis - Solid particles taken in across the plasma membrane. White Blood Cells (WBCs) carry out phagocytosis of bacteria and viruses. Phagosome formed, phagosome fuses with a lysosome  Phagolysosome. Material hydrolysed by hydrolytic enzymes. Nutrients released into cells cytoplasm and waste in the residual body released by exocytosis (10 RBCs/day). ii)Pinocytosis or “cell drinking” used by cells to move fluids containing dissolved particles like salts and soluble proteins across the plasma membrane. Most cells carry out pinocytosis. Non selective. Cells lining the intestine absorb fat droplets by pinocytosis. All eukaryotic cells are continually ingesting fluid and molecules by pinocytosis, only specialised phagocytic cells ingest large particles. iii)Receptor – mediated endocytosis. Involves very specific receptor proteins on the cell membrane outer surface that binds to and transports specific molecules from the extracellular environment into the cell. Example the LDL cholesterol receptor. (Receptor mediated endocytosis is a special form selective pinocytosis) https://www.ncbi.nlm.nih.gov/books/NBK9831/ https://www.youtube.com/watch?v=lNKKg6hIBcM https://www.youtube.com/watch?v=CUXQsAdxuYw Summary Endocytosis http://highered.mheducation.com/sites/0072495855/student_view0/chapter2/animationphagocytosis.html 33 https://www.youtube.com/watch?v=K7yku3sa4Y8&feature=related Phagocytosis - describe the process with the aid of a labelled diagram 34  2. Exocytosis is the reverse of endocytosis. It is the process by which materials packaged in secretory vesicles are secreted/exited from a cell when the vesicle membrane fuses/unite with the plasma membrane. It occurs in all cells but is most important in secretory cells (e.g. cells that produce digestive enzymes) and nerve cells (neurotransmitters). Cell changes shape – requires energy E.g.: Hormones or wastes released from cell 35 Diagram for illustration purposes https://www.thoughtco.com/what-is-exocytosis-4 Summary definitions 36