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BUILDING BLOCKS Building blocks of life: organic compounds • Why organic? • Because they all contain CARBON • The main organic compounds are: 1) Carbohydrates –sugars, starches, glycogen 2) Lipids (fats) 3) Proteins 1) Carbohydrates: • Main function is to provide a quick source of energy for all the functions cells have to perform ie metabolic energy • Metabolism = sum of all the chemical reactions in your body • Energy in a cell is ‘stored’ in the form of ATP • What do the initials ATP stand for? • A few carbohydrates have a structural function 2) Lipids (fats) • Most lipids are hydrophobic which means they are insoluble in water • Fatty acids =smaller molecules and can travel in blood • Larger fatty molecules need help to travel around the body; so they are attached to protein – called lipoproteins Functions of lipids: • form part of cell membrane; • steroids (eg cholesterol and the hormones cortisol and aldosterone); • vitamins A,D,K & E; • sex hormones • And can be converted in to ENERGY to drive metabolism 3) Proteins: • More complex in structure than carbohydrates and lipids • Comprised of amino acids • Some have a structural role – Collagen and keratin • Others a physiological role eg – enzymes (…….ase, eg salivary amylase) – muscle contraction (actin & myosin) – antibodies – some hormones eg insulin – haemoglobin Other important organic compounds: • Nucleic acids: • DNA & RNA – Comprised of nucleotides – Found in the cell nucleus • ATP – Stores and transfers energy eg in muscle contraction, moving substances across the cell membrane, synthesising larger molecules from smaller ones Inorganic substances are also very important: • Other elements including trace elements eg Iron, calcium, potassium, iodine; Zinc, Manganese….. • Acids: H+ plus an anion (ie a -ve charge) • Bases: OH- plus a cation (ie a +ve charge) Atoms and Ions What ions are we interested in? • • • • Na + Cl – K+ Ca 2+ NB If you do not know what these symbols mean, then find out! Question: What do the + and – attached to the atom names above mean? • These particles and their movement across the cell membrane will be very important in our understanding of how neurons function, and how nerves ‘fire’ muscles into action. So…now test yourself: • Amino acids are the building blocks of……… • Lipids are soluble/insoluble in water • True or false: – carbohydrates have primarily a structural function. – Glucose has 5 carbon atoms – Lipids are found in the cell membrane • ATP stands for…………………….and its role in the cell is to………………………………. A typical animal cell: • A typical cell: The cell membrane: a phospholipid bilayer The cell membrane is bilaminar ie 2 layered FLUID MOSAIC MODEL • Mosaic=pattern of small pieces fitted together • Membrane likened to “a mosaic of proteins floating like icebergs in a sea of lipids” ( T & G, 1996, p55) Functions of membrane proteins: • Integral proteins: • Some proteins act as channels; they have a pore through which certain substances can pass into or out of the cell • Others act as transporters or carriers to move a substance from one side of the membrane to the other • Others as receptors or recognition sites: attach to a nutrient, hormone or neurotransmitter • Other functions: enzymes, cytoskeleton anchors, cell identity markers How molecules and ions get into and out of cells: diffusion, osmosis and active transport • The composition of the cell membrane is crucial to the transport of substances into and out of the cell • The plasma membrane that surrounds the cell is impermeable to MOST dissolved substances ie these cannot get in or out easily • BUT the cell membrane is permeable to some substances Selective permeability: what can and cannot pass into the cell • Substances that can dissolve in lipids CAN pass • Most large molecules CANNOT pass • Molecules and ions which carry a charge MAY pass but only through special channels or by being carried by a transporter molecule (see later) • So, the presence of channels and transporters in the membrane determine what can pass; they are usually very selective How substances move into and out of the cell: by both PASSIVE PROCESSES and ACTIVE PROCESSES Movement across the membrane: • Passive processes: diffusion (simple and facilitated), osmosis, bulk flow • Active processes: active transport (primary and secondary), vesicular transport (endoand exo-cytosis) • Let us first look at some of the PASSIVE processes Diffusion • Substances will move from an area of high concentration to an area of low concentration –down (or with) their concentration gradient • Two of these substances are oxygen and carbon dioxide, which move into and out of cells by a process called diffusion. • For example, in the alveolus of the lung, oxygen is in a higher concentration in the alveolar sac than in the capillary supplying the alveolus, and so oxygen will move into the blood stream; carbon dioxide is in a higher concentration in the blood stream and so will diffuse out into the sac. Gas exchange in the alveolar sac: See also Fig 14.3 Page 216 in A & McH (2006) Total width of air-blood exchange region is 0.2-0.6 µm Simple diffusion: watch the animation and answer the quick quiz questions at the end • http://highered.mheducation.com/sites/0072 943696/student_view0/chapter3/animation__ how_diffusion_works.html • What is meant by a ‘concentration gradient’? This is an important concept in understand movement of substances across the cell membrane Facilitated diffusion: watch the animation and answer the quick quiz questions at the end • http://highered.mheducation.com/sites/0072 943696/student_view0/chapter3/animation__ how_facilitated_diffusion_works.html Osmosis • Diffusion of a solvent (in this case, water) through a selectively permeable membrane; ie the cell membrane) • Water moves from an area of high concentration to an area of low concentration • What determines the concentration of water molecules is the inclusion of other substances called the solute Demonstration of osmosis: watch the animation and answer the quick quiz questions at the end • http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch apter2/animation__how_osmosis_works.html Remember: The cell membrane is a phospholipid bilayer • Substances that are soluble in lipid will be able to pass through • the cell membrane is permeable to oxygen, carbon dioxide, steroids, fat soluble vitamins and many other substances including water • Water is also able to easily diffuse through the cell membrane through pores in the channels • Ions (Na+, Cl-, K+, Ca2+ , HCO3-, and urea) diffuse in and out through protein channel pores down their concentration gradients Now test yourself: • What is meant by polarity? • True or false: – Osmosis requires energy in the form of ATP – Diffusion is a special case of osmosis – Substances will move by diffusion up their concentration gradient – Facilitated diffusion requires a special carrier protein in the cell membrane How substances move into and out of the cell: PASSIVE PROCESSES ACTIVE PROCESSES The active processes: 1) Primary active transport 2) Secondary active transport 3) Vesicular processes (endocytosis and exocytosis) The structure of the cell membrane: • The cell membrane is composed of a …………………… bilayer • Cells are surrounded by fluid…and have fluid inside them • The concentration of substances inside and outside the cell is usually different • To enter a cell, a substance may have to move AGAINST its concentration gradient Cell membrane/plasma membrane potential or resting potential • A tiny electrochemical potential difference exists between the inside and outside of the cell • This membrane potential is a means of storing potential energy (like a battery) and is important in excitable cells (such as neurons and muscle fibres –see next lecture) • Have a look at the following slide: what is the charge just outside the cell: positive or negative? And inside the cell, it is……………..? Cell membrane potential Humanbiologylab.pbworks.com Another factor that contributes to the resting potential: • Membrane permeability : • In a resting nerve or muscle cell, the permeability of the plasma membrane is 50100 GREATER to K + than to Na+ • For more detail, see Tortora & Grabowski p343 (8th Ed) Active processes: • Active transport: • Used when substances have to get into cells against the concentration gradient • ENERGY is required for this process, in the form of ATP • Integral membrane proteins act as ‘pumps’ to push specific ions and molecules across the membrane Actively transported substances include: • • • • • Na + Cl – K+ Ca 2+ Plus others eg H+, I- • NB if you are unfamiliar with these symbols, then ensure you understand what they mean 1) Primary active transport: • The cell uses energy from ATP to change the shape of transport proteins in the plasma membrane • This change of shape is accompanied by movement of substances across the cell membrane • The sodium pump is an example of primary active transport. Exports Na+ from the cell and imports K+ • Hundreds of sodium pumps exist in the cell membrane per square micrometer (µm3) NB1µm = diameter of a smoke particle Sodium Pump in action: • http://highered.mheducation.com/sites/0072495855/stude nt_view0/chapter3/animation__sodiumpotassium_exchange_pump__quiz_1_.html • sodium pump animation • NB There are 3 animations in this series, the first 2 seem similar • Don’t forget the short quiz to test yourself after watching the animation 2) Secondary active transport: • secondary active transport animation • The sodium pump maintains a large concentration gradient of Na+ across the plasma membrane; this electrochemical gradient can be used as a source of energy. • A sodium ion and another substance (eg glucose) bind to a symporter protein which then changes its shape and allows both substances to cross the membrane; one with its gradient (Na+) and one against its gradient (glucose) • The Na+ is then pumped back out using the sodium pump; ATP is required for this 3) Vesicular transport: Endocytosis and Exocytosis • Molecules which are too large to be moved by primary and secondary active transport are moved by mechanisms which involve being engulfed in folds of the plasma membrane • vesicular transport animation Sample exam questions: 1) Describe 3 functions for proteins in the human body. 2) What is meant by the fluid mosaic model in relation to the cell membrane? 3) What is the function of the sodium pump? 4) Explain how the resting potential of a cell membrane is achieved. Quiz: Match the type of cell transport, with its description Type of transport description Osmosis Cell uses energy from ATP to change the shape of transport proteins in the plasma membrane Primary active transport A sodium ion and another substance cross the membrane with the help of a symporter membrane protein Endocytosis Large molecules are engulfed in folds of the plasma membrane Secondary active transport Movement of water through a selectively permeable membrane down its concentration gradient Next week: We will look at these processes in relation to specialised cells: neurons and muscle cells WHAT TO DO NEXT: • Read: • Chapters 3 (pp 40-46 esp) and 6 (pp69-78) in Atkinson & McHanwell (2002) • Or • Chapter 11 in Seikel, King & Drumwright (2005) • Other reading: • Chapter 4 in Love & Webb (mainly nerve physiology) • Chapter 2 in Tortora et al (the chemical level of organisation)