Chapter 8 Membrane Structure and Function The Plasma Membrane is the boundary that separate the living cell from it nonliving surroundings. This membrane: •1 is about 8 nm in diameter •2 surrounds the cell and controls chemical traffic into and out of the cell. •3 Is selectively permeable ( ‫ )نفاذ‬making some substances pass more easily than

others. •4 Has a unique structure which determines its function. Membrane models have evolved to fit new data: Membrane function is determined by its structure. Early models of the plasma membrane were deduced from indirect evidence: 1. Evidence: lipids and lipid soluble materials enter cells more rapidly than other substances. Conclusion: membranes are made of lipids (Overton 1895). 2. Evidence: Amphipathic phospholipids will form an artificial membrane on the surface of water with only the hydrophilic heads immersed in water. Conclusion: because of their molecular structure, phospholipids can form membranes (Langmuir 1917). 3. Evidence: Phospholipids contents isolated from red blood cells is just enough to cover cells with two layers (Gorter and Grendel 1925). Conclusion: cell membranes are phospholipid bilayers. 4. Evidence: Membranes isolated from red blood cells contain proteins and lipids. Conclusion: there is protein in biological membranes.

In 1935, Danielli and Davson proposed the sanswich model of plasma membrane. 28

•5 cell embrane is made of a phospholipid bilayers sandwiched between two layers of globular protein. •6 the hydrophilic heads of phospholipids are oriented towards the protein layer forming a hydrophilic zone. •7 the hydrophobic tails of phospholipids are oriented in between polar heads forming a hydrophobic zone. •8 the membrane is about 8 nm thick. There are problems with this model: 1. not all membranes are identical in structure. 2. a membrane with an outside layer of proteins would be an unstable structure. In 1970, Singer and Nicolson proposed the Fluid mosaic model in which proteins are individually embedded in the phospholipid bilayer rather than forming a solid coat on the surface. A membrane is a fluid mosaic of lipids, proteins and carbohydrates A. The fluid quality of membranes: •9 Membranes are held together by weak hydrophobic interactions. •10Most membrane lipids and proteins can move laterally within the membrane. •11Molecules rarely flip-flop transversely across the membrane. •12Phospholipids move quickly along membrane plane (2 um per second). •13Membrane proteins move slowly than lipids. Membranes must be fluid to work properly: •14unsaturated hydrocarbon tails enhance membrane fluidiy. •15membranes solidify if temperature decreases to critical point. •16Cholesterol in plasma membranes of eukaryotes maintain fluidity by: -1 less fluid at warmer temperature. -2 More fluid at lower temperature by preventing close packing of phospholipids.

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B. Membranes as mosaic of structure and function: A membrane is mosaic of different proteins embedded in the phospholipid bilayer. These proteins vary in structure and function. •17 Integral proteins: inserted into the membrane. The may be:

-3 unilateral: reaching only partway across the embrane. -4 transmembrane: exposed on both sides of the membrane. •18 Peripheral proteins: not embedded but attached to membrane’s surface.

-5 may be attached to integral proteins fro outside. -6 may be held by cytoskeleton filaments inside. C. Membranes carbohydrates and cell-cell recognition: Cell-cell recognition: The ability of a cell to determine if other cells encountered are alike or different from itself. Cell-cell recognition is very important in the functioning of an organism. It is the basis for: •19 sorting of an animal embryo’s (‫ ) جني‬cells into tissues and organs. •20 Rejection (‫ ) رفض‬of foreign cells by the immune system ( ‫)جهاز الناعة‬.

Cell-cell recognition is made possible by certain molecules on the external side of the plasma membrane: •21usually branched oligosaccharides ( 15 monomers) •22some covalently bonded to lipids: glycolipids. •23Most covalently bonded to proteins: glycoproteins. •24Vary from species to species and between individuals of the same species. Permeability of the plasma membrane: Plasma membrane is selectively permeable: allowing some substances to cross more easily than others. 1. Nonpolar hydrophobic molecules: •25dissolve in the membrane and cross easily (e.g. hydrocarbons and O2). •26Smaller molecules pass more easily. 30

2. Polar hydrophilic molecules: •27small molecules like H2O and CO2 can pass through the membrane. •28Larger polar uncharged molecules like glucose cannot pass easily. •29All ions have difficulty penetrating the hydrophobic layer of the membrane. Transport Proteins: Hydrophilic substances which can not pass through plasma membrane are transported by proteins: Transport proteins. •30may provide a hydrophilic tunnel through the membrane. •31 may bind to a substance and physically move it across the membrane.

•32are specific for the substance they transport. Passive Transport : Is the diffusion ( ‫ )أنتشار‬of substances down a concentration gradient ( ‫)تدرج التركيز‬. Diffusion: In the absence of other forces, substances diffuse from where it is more concentrated to where it is less concentrated. •33a substance diffuse down its concentration gradient. •34A substance diffuse down its own concentration gradient and is not affected by other substances. •35Diffusion does not require energy. Passive Transport: •36diffusion of substances across biological membranes. •37Passive process which does not require the cell to expend energy. •38Rate of diffusion is regulated by the permeability of the membrane, some molecules diffuse more easily than others. •39Water diffuse freely across most cell membranes.

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Osmosis is the passive transport of Water: Definitions: Hypertonic solution: A solution with a greater solute concentration than that inside the cell. Hypotonic solution:A solution with a lower solute conction than that inside the cell. Isotonic solution: A solution with an equal solute concentration on both side of the cell. Osmosis: Diffusion of water across selectively permeable membrane. Water diffuse down its concentration gradient. •40if two solutions of different concentration are separated by a selectively permeable membrane that is permeable to water but not to the solute, water will diffuse from solution of less concentration to solution of higher concentration. •41If two solutions of the same concentration are separated by such membrane, water molecules diffuse equally on both sides of the membrane. Osmotic Pressure: •42osmotic pressure of pure water is zero. •43 osmotic pressure of a solution is proportional ( ‫ )يتناسب‬to its osmotic

concentration, the greater the solute concentration; the greater the osmotic pressure. Cell survival depends on balancing water uptake and loss Water balance of animal cells: •44 animals cells do not have cell walls and they are not tolerant of

excessive osmotic uptake or loss of water. •45In an isotonic environment, volume of the cell will remain stable. •46 In a hypertonic environment, cell will lose water by osmosis and shrivel. •47 In a hypotonic environment cell will gain water by osmosis and swell (lyse).

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Water balance of cells with cell walls:

•48cells of prokaryotes, some protests, fungi and plants have cell walls outside the plasma membrane. •49 In

a hypertonic environment, cells will lose water by

osmosis and will plasmolyze and die. •50 In

a hypotonic environment, water moves by osmosis into

the cell, causing it to swell until internal pressure against the cell wall equals the osmotic pressure of the cytoplasm, the cell is turgid ; normal situation. •51 In

an isotonic environment, there will be no movement of water into

or out of the cell, it will become flaccid and may die. Facilitated diffusion: Is a diffusion of solutes across plasma membrane with the help of transport proteins. •52 is passive transport because solutes are transported down its

concentration gradients. •53 Help the diffusion of many polar molecules and ions that are impeded

( ‫ )تعاق‬by the phospholipid bilayer. •54 Transport proteins remain in place in the membrane and translocate

solutes by alternating ( ‫ )تتبادل‬between two conformations.

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Active Transport: Energy requiring process during which a transport protein pumps a molecule across membrane against its concentration gradient. •55is generally uphill (+∆G) and requires the cell to expend energy. •56 Helps cells maintain steep ionic gradients across the membrane (e.g.

Na, K, Mg, Ca and Cl ions.) •57Transport proteins involved in active transport get energy from ATP to pump molecules against their concentration gradients. An example is the sodium potassium pump (see Campbell figure 8.14). Voltage across membranes: Because anions ( ‫ )ايونات سالبة‬and cations ( ‫ )ايونات موجبة‬are unequally distributed across the plasma membrane, all cells have voltage across their plasma membrane. Membrane potential: Voltage across membranes. •58 The cell’s inside (cytoplasm) is negatively charge with respect to the outside.

•59Favors diffusion of oations into cell and unions out of the cell. Two forces drive passive transport of ions across membranes: -7 concentration gradient of the ion. -8 Effect of membrane potential on ions. Electrochemical gradient: Diffusion gradient resulting from the combined effects of membrane potential and concentration gradient. Electronic Pump: A transport protein that generates voltage across a membrane. •60Na / K ions ATPase is the major electronic pump in animal cells. •61 A proton pump is the major electronic pump in plants, bacteria and fungi.

•62Voltage created by electronic pump are sources of potential energy available to do cellular work.

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Cotransport: Is a process where a single ATP powered pump actively transport one solute and indirectly drives the transport of other solutes against their concentration gradient. •63ATP powered pump actively (active transport) transports one solute and creates potential energy in the gradient it forms. •64 Another transport protein couples the solute’s downhill diffusion as it leaks

back across the membrane with a second solute’s uphill transport against its concentration gradient.

Expcytosis and Endocytosis: Exocytosis -process of exportin ‫ ))إخراج‬macromolecules from a cell by fusion of vesicles with the plasma membrane -Vesicles usually formed from the ER or Golgi and move towards plasma membrane -Used by secretary cells to export products (e.g. insulin)

Endocytosis:

Endocytosis -process of importing ‫ ))إدخال‬macromolecules into a cell by forming vesicles derived from the plasma membrane -used by cells to import extracellular products.

- phagocytosis/ Pinocytosis & Receptor-mediated endocutosis

Phagocytpsis: endocytosis of solid particles ( e.g. food particles). Pinocytosis: endocytosis of fluid droplets. •65droplets of the extracellular fluid are taken into the cell by vesicles. •66The process in not specific. All solutes dissolve in the droplets. Receptor- mediated endocytosis: process of importing specific macromolecules into the cell by vesicles. -more specific than pinocytosis. -The molecule with its receptor enters the cell. -A molecule that binds to a specific receptor site of another molecule is called ligand.

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