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Transcript
Membrane. Mechanism of transport charge and non charge partial throw membrane structure of cell. 1 Plan • Biological membrane • Transport Across Cell Membranes – Passive Transport – Active Transport – Endocytosis/Exocytosis 2 All living cells and many of the tiny organelles internal to cells are bounded by thin membranes. These membranes are composed primarily of phospholipids and proteins and are typically described as phospholipid bi-layers. 3 Phospholipids self assemble into different structures because their hydrophobic and hydrophilic ends repel each other 4 In this sketch, the spheres represent the phosphate end, which is polar and water soluble (hydrophilic). The twin extensions represent the fatty acid components which are not water soluble (hydrophobic). 5 Cell membranes also contain cholesterol in the phospholipid bilayer. In some membranes there are only a few cholesterol molecules. Cholesterol makes the bilayer stronger, more flexible but less fluid, and less permeable to 6 water-soluble substances such as ions and monosaccharides. • A biological membrane or biomembrane is an enclosing or separating membrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content. The cellular membranes should not be confused with isolating tissues formed by layers of cells, such as mucous and basement membranes. 7 8 Transport Across Cell Membranes Essential and continuous parts of the life of a cell are the taking in of nutrients and the expelling of wastes. All of these must pass through the cell membrane. Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vescicle attaches to the cell membrane from the inside and then opens to form a pocket, expelling its contents to the outside. This may be called exocytosis. The cell membrane may also envelope something on the outside and surround it, taking it into the cell. This may be called endocytosis or phagocytosis. There are also examples where molecules move across a membrane from a region of low concentration to an region of high concentration, and this requires a source of energy to "pump" the molecules uphill in concentration. Such processes are called active 9 transport. Transport Across Cell Membranes • 3 Types • 1. Passive Transport • 2. Active Transport • 3. Endocytosis/Exocytosis 10 Transport Across Cell Membranes • 1. Passive Transport 11 Brownian Motion and Concentration Gradients Brownian Motion: matter is made up of tiny particles that are in constant motion 12 Molecules always move 1) randomly 2) from areas of high concentration to areas of low concentration This is called DIFFUSION 13 Concentration Gradient The difference in concentration between the high and low concentration areas. The greater the difference in concentration – the faster the particles move Eventually equilibrium is reached 14 Why does this happen? • At high concentrations the molecules ricochet off of each other and move towards the area of low concentration 15 Equilibrium • Remember: at equilibrium the particles DO NOT stop moving, they continue to move back and forth across the concentration gradient. 16 Passive Transport NO ENERGY REQUIRED • Small molecules move by diffusion (water, oxygen, carbon dioxide) • When WATER molecules move by diffusion across a membrane we call it OSMOSIS 17 Diffusion Diffusion refers to the process by which molecules intermingle as a result of their kinetic energy of random motion. Consider two containers of gas A and B separated by a partition. The molecules of both gases are in constant motion and make numerous collisions with the partition. If the partition is removed as in the lower illustration, the gases will mix because of the random velocities of their molecules. In time a uniform mixture of A and B molecules will be produced in the container. 18 Steady-State Diffusion Flux proportional to concentration gradient = dC dx Fick’s first law of diffusion C1 C1 C2 x1 x C2 dC J D dx x2 dC C C2 C1 if linear dx x x2 x1 D diffusion coefficient 19 19 Facilitated Transport (also called facilitated diffusion) • Another form of passive transport • Used for molecules that are too large to cross the membrane by diffusion (i.e. glucose), and for charged molecules 20 Facilitated Diffusion • Carrier proteins bind to larger molecules, and change their shape so molecules can diffuse through. • Channel proteins provide water filled pores for charged ions to pass through 21 Osmosis If two solutions of different concentration are separated by a semi-permeable membrane which is permeable to to the smaller solvent molecules but not to the larger solute molecules, then the solvent will tend to diffuse across the membrane from the less concentrated to the more concentrated solution. This process is called osmosis. Osmosis is of great importance in biological processes where the solvent is water. The transport of water and other molecules across biological membranes is essential to many processes in living organisms. The energy which drives the process is usually discussed in terms of osmotic pressure. 22 3. Selective Filter – SEMIPERMEABLE – only allows certain substances in or out • Regulated by particle size and by selective transport by membrane proteins 23 Osmosis 24 Active Transport Across Cell Membranes There are numerous situations in living organisms when molecules move across cell membranes from an area of lower concentration toward an area of higher concentration. This is counter to what would be expected and is labeled "active transport". There is a very strong tendency for molecules to move from higher concentration to low, just based on thermal energy. Molecules at normal temperatures have very high speeds and random motions. For example, water molecules at 20°C have an effective or rms speed of over 600 m/s or over 1400 miles/hr! This motion from areas of high concentration to low is called diffusion. There are times when membranes are impermeable to some molecules because of their size, polarity, etc. and only the smaller solvent molecules like water molecules will move across the membrane. This is called osmosis, and the tendency to transport the solvent molecules is 25 quantified in terms of osmotic pressure. Active Transport • Molecules move against the concentration gradient (low to high) • Energy must be provided (even when we are resting, 40% of our energy is spent on active transport!) 26 Active Transport • Uses specialized transport proteins and protein pumps 27 Why spend so much energy on active transport? Maintains internal cell environments (i.e. cell’s electrical gradient, roots pull in minerals from soil, filtering blood in your kidneys) 28 If a molecule is to be transported from an area of low concentration to an area of high concentration, work must be done to overcome the influences of diffusion and osmosis. Since in the normal state of a cell, large concentration differences in K+, Na+ and Ca2+ are maintained, it is evident that active transport mechanisms are at work. 29 Many crucial processes in the life of cells depend upon active transport. Included in the illustration above is the sodium-potassium pump which is a vital cell process. Active transport mechanisms may draw their enegy from the hydrolysis of ATP, the absorbance of light, the transport of electrons, or coupling with other processes that are moving particles down their concentration gradients. 30 The Sodium-Potassium Pump The process of moving sodium and potassium ions across the cell membrance is an active transport process involving the hydrolysis of ATP to provide the necessary energy. It involves an enzyme referred to as Na+/K+ATPase. This process is responsible for maintaining the large excessof Na+ outside the cell and the large excess of K+ ions on the inside. A cycle of the transport process is sketched below. It accomplishes the transport of three Na+ to the outside of the cell and the transport of two K+ ions to the inside. This unbalanced charge transfer contributes to the separation of charge across the membrane. The sodium-potassium pump is an important contributer to action potential produced by nerve cells. This pump is called a P-type ion pump because the ATP interactions phosphorylates the transport protein and causes a change in its conformation. 31 The Na-K pump 32 The Na-K pump cycle 33 The sodiumpotassium pump moves toward an equilibrium state with the relative concentrations of Na+ and K+ shown at left. 34 Functions of the Cell Membrane 1. Barrier – keeps “baddies out, cell organelles in Bacteria pollution resistant 35 Functions of the Cell Membrane 2. Organization – membranes surround and package materials in vessicles, lysosomes reticulum endoplasmic 36 2. Organization – membranes organize complex reactions like photosynthesis and cellular respiration 37 Two components of an electrochemical gradient 38