MCD: Cells Usama Asif C ELLS 1: Cells and Organelles Professor Michael Ferenczi ([email protected]) 1. Understand what constitutes a cell, and the scale of cells and molecules Definition: Cell the basic unit from which living organisms are made, consisting of an aqueous solution of organic molecules enclosed by a membrane. All cells arise from existing cells, usually, by a process of division. -­‐‑ The body is made up of organs and tissues that are made up of cells and extracellular fluid. Membranes separate them from one another -­‐‑ Some cells can live independently of one another e.g. protozoa -­‐‑ When cells come together, they ‘specialise’ to give the organism an advantage and form tissues. This can even be observed in single-­‐‑celled protozoa, which come together to form colonies when individual cells specialise e.g. slime moulds, dictyostelium -­‐‑ Some cells (stem cells) divide to form colonies or clones, which then specialise by turning on or turning off particular genes, triggered by signals from their immediate environment (developmental biology) -­‐‑ There are around 200 cells in the body -­‐‑ Genes code for proteins that can be enzymes that induce the formation of specialised cells. The cells differ because some genes are switched on in some cells and some are switched off in others -­‐‑ Cell polarity is established, one end of the cell looks different from the other end i.e. one side, the apical side and the basal side are different. The areas in the cell are different too -­‐‑ The cytoskeleton is formed from polymers with mechanical properties, which form the basis of it. Organelles, cell-­‐‑cell contacts, specialised secretion and absorption are all controlled by genes -­‐‑ Development is controlled by the body’s in-­‐‑built development programme and also depends on external factors that are responded to. The importance of epigenetic factors is being recognised -­‐‑ Even identical twins are not exactly the same because some phenotypic features are determined by the environment and even identical twins cannot share exactly the same environment -­‐‑ The nuclear pore has proteins which forms pores and this controls what goes in and out of the nucleus All cells: • have a cell membrane that separates the outside from the organised interior. • contain DNA as the genetic material (exceptions) • contain several varieties of RNA molecules and proteins, most of the latter are enzymes. • are composed of the same basic chemicals: carbohydrates, proteins, nucleic acids, minerals, fats and vitamins. • regulate the flow of nutrients and wastes that enter and leave the cell. • reproduce and are the result of reproduction. • require a supply of energy. • are affected and respond to the reactions that are occurring within them and many of the environmental conditions around them; this information is continually processed to make metabolic decisions. MCD: Cells Usama Asif Scales Dimension of a Cell = 10-­‐‑50 µμm Dimension of a virus = 10nm 2. Demonstrate the following on a suitable transmission electron micrograph: nucleus; nucleolus; nuclear envelope; mitochondrion; rough endoplasmic reticulum; smooth endoplasmic reticulum; ribosomes; Golgi apparatus; secretory granule; plasma membrane; cytoskeletal components 1 Nucleus
1a Euchromatin
1b Heterochromatin
2 Nucleolus
3 Nuclear membrane
4 Nucleopore
5 Ribosomes
6 Rough endoplasmic reticulum (with ribosomes)
7 Smooth endoplasmic reticulum
8a Longitudinal- and cross-sections of a mitochondrion showing aristae
8b Longitudinal- and cross-sections of a mitochondrion showing tubules
8c Longitudinal- and cross-sections of a mitochondrion of the prismatic type
8d Longitudinal- and cross-sections of a mitochondrion showing saccules
9 Longitudinal- and cross-sections of a centriole
10 Golgi Apparatusus
11 Golgi vesicles
12 Endopinosomes, endopinocytotic vesicles
13 primary lysosomes
14 secondary lysosomes, phagocytosomes
15 tertiary lysosomes
16, 17 Multivesicular bodies
18 peroxisomes, microbodies
19 Secretory granules
20 Microtubules
21 Actin filaments
22 Intermediate filaments linking to desmosomes
24 Glycogen granules
25 Fat droplets
26 Synapse
27 Synaptic body
28 Cell membrane with glycocalyx(polysaccharides and glycoproteins)
29 Intercellular space
30
31 Tight junction
32 Adherens junction
33 Fascia Adherens with actin filaments (cardiac)
34 Punctum Adherens
35 Macula Adherens (desmosome)
36 Hemidesmosome (joins cell to basal lamina)
37 Gap junction
38 Microvilli with glycocalyx
39 Cilia
40 Basal bodies of microcilia
41 Stereocilia
42 Basal lamina (basement membrane or basement lamina)
MCD: Cells Usama Asif 3. Describe the predominant types of molecules in a cell • Basement membrane: o selective barrier for macromolecules o type IV collagen network, laminins, type XV collagen • Extracellular fluid (ECF): o ions (Na+, Cl-­‐‑, PO42-­‐‑, CO32-­‐‑, Mg2+, Ca2+) o soluble proteins o soluble carbohydrates, sugars o vitamins o amino acids o hormones o nucleotides (ATP) o lipids o cholesterol • Lymph • Plasma • Saliva, urine, bile, sweat, milk etc. 4. Identify the essential characteristics of prokaryotic and eukaryotic cells. -­‐‑ Eukaryotic cells are ones that have membrane bound organelles. The word eukaryote means true nucleus -­‐‑ Prokaryotic cells have no membrane bound organelles. All of their cellular components lie in the cytoplasm -­‐‑ Eukaryotic cells seem to have evolved from aggregates of prokaryotic cells that became interdependent upon one another and eventually merged or fused to make one whole cell -­‐‑ Thus, eukaryotes have a larger degree of organisation than prokaryotes, because they contain organelles and structures that are separated from one another by membranes -­‐‑ The evolution of cells seems to have come from a relationship of endosymbiosis of prokaryotic cells within other prokaryotic cells. What this means is that the organelles in eukaryotic cells derive from the incorporation (or invasion by) prokaryotes. Mitochondria are a prime example; they have their own genome and have other characteristics that are shared with prokaryotes 5. Understand that movements of molecules and organelles in cells and that the movement of cells are essential processes. -­‐‑ Molecules move spontaneously, via diffusion (Brownian motion) -­‐‑ Other forms of movement require energy via the hydrolysis of ATP to cause: o Movement of molecules up against their concentration gradient o Movement of organelles o Movement of cell membranes (ruffling) o Growth and migration of cells o Cell division o Muscle contraction (via motor proteins) o Etc. MCD: Cells Usama Asif 6. Explain the relationship of individual cells to the organisation of the whole body. -­‐‑ Understanding modern medicine and modern research requires knowledge of cellular processes -­‐‑ Development and repair are based on programming the cell cycle and turning on differentiation mechanisms -­‐‑ Diseases are often problems with the cell: o Cancer is when cellular development programmes are failing o Infections occur when cellular defence mechanisms fail to prevent bacterial invasion o Viruses take over the chemical machinery of the cell -­‐‑ Cells come together to form tissues, which in turn come together to form organs. The organs come together to form the organism -­‐‑ If we understand the cell, we can understand the organism 7. Understand that cancer is a disorder of cell division -­‐‑ Cancer is a disorder of cell division -­‐‑ There are certain mutations that lead to cancer. These mutations can occur in any of the these regions in the genetic code to cause cancer, but it is usually an accumulation of mutations over time that causes cancer -­‐‑ Mutations that can lead to cancer: o Switch on of ‘divide’ signals (oncogenes) o Switch off of ‘don’t divide’ signals (tumour suppressor genes) o Loss of the correction mechanism of DNA replication o Loss of the escape mechanism from cell division (i.e. cannot get out of the cell cycle) o Loss of the limit on the number of times the cell can divide o Loss of control-­‐‑keeping mechanisms i.e. those that keep cells within the tissue boundary o Ability to evade body defence mechanisms o Ability to recruit blood vessels into the growing tumour (tumour angiogenesis) o Ability to migrate into the circulation (blood and lymph) o Ability to establish tumours in the wrong tissue MCD: Cells Usama Asif C ELLS 2: Infectious agents Dr Brian Robertson ([email protected]) 1. Name the main types of infectious agent causing disease in humans -­‐‑ A variety of pathogens can cause human infectious disease, though these organisms represent a fraction of the commensal bacteria making up the normal human flora, which is beneficial to health -­‐‑ The main types include: o Viruses are not free-­‐‑living organisms but obligate intracellular parasites e.g. SARS, HIV, Influenza. Intracellular replication followed by budding o Bacteria are prokaryotes that replicate by binary fission e.g. Neisseria meningitidis, Mycobacterium tuberculosis, Shigella spp. o Fungi are single celled eukaryotes that exist as spores or filaments. Systemic infections with these organisms (e.g. Candida albicans, Aspergillus fumigatus) often affect immunocompromised people o Protozoa are single-­‐‑celled eukaryotes. e.g. (Plasmodium spp.) malaria, Leishmania spp. (leishmaniasis) o Helminth parasites are multi-­‐‑cellular organisms that include tapeworms, flukes and roundworms 2. List the key differences between prokaryotes and eukaryotes -­‐‑ The following features distinguish prokaryotic cells from eukaryotic cells: -­‐‑ Cell walls of prokaryotes contain peptidoglycan, and the membranes of prokaryotes are limited to the cell membrane. There are no distinct organelles in prokaryotes while nuclei containing one or more (linear) chromosomes are found in viruses. DNA in prokaryotes is generally in the form of a single (circular) chromosome. Prokaryotes have a single copy of the chromosome, and are thus described as having the haploid number of chromosomes, whereas eukaryotes can be diploid or haploid (haploid means that mutations are always expressed) -­‐‑ Eukaryotes have a cytoskeleton, prokaryotes have too, but it is less well defined -­‐‑ Prokaryotes do not have internal membranes whereas eukaryotes do (organelles) -­‐‑ Prokaryotes are simpler and generally smaller than eukaroytes 3. Give examples of each type of infectious agent and the disease it causes 4. Name the distinguishing features of the different types of infectious agent and explain how they replicate Viruses -­‐‑ These are not cells in their own right. They are obligate intracellular parasites -­‐‑ They contain RNA or DNA genetic material and make use of host cell nuclear synthetic machinery to replicate -­‐‑ They do show host specificity but they infect almost all other life forms (including bacteria) -­‐‑ They divide by budding out of the host cell -­‐‑ They have various routes of infection MCD: Cells -­‐‑
Usama Asif One example of a virus is the human immunodeficiency virus (HIV). It uses the enzyme reverse transcriptase to convert its own RNA genome into a DNA template, which it integrates into the host genome. This makes the host cell manufacture the proteins that the virus needs to replicate Bacteria -­‐‑ They are fundamentally different from all other living things, because they are prokaryotes -­‐‑ They contain a chromosome of DNA but there is no defined nucleus. The nuclear area is where the DNA is contained (in nucleoids) -­‐‑ They are widely distributed in nature and some, but not all, are pathogenic (e.g. Neisseria and Shigella) -­‐‑ They replicate via binary fission and have various routes of infection -­‐‑ For example, the bacterium Shigella is an invasive pathogen. It is transmitted via the faecal-­‐‑oral route and has an infectious dose of only 10-­‐‑100 bacteria. It enters the cell via a vacuole and replicates intracellularly. Using host actin, it spreads from cell to cell -­‐‑ Septicaemic disease is characterised by rapid progression of a bacterial infection, septic shock and a severe inflammatory response. It is characterised by high levels of bacteraemia i.e. bacteria in the blood Fungi -­‐‑ They are eukaryotic organisms and occur as yeasts, filaments or both -­‐‑ The yeasts bud to divide. They have filaments called hyphae, which have cross walls or septa -­‐‑ They cause cutaneous, mucosal and/or systemic mycoses -­‐‑ For example, Candida, is a yeast infection and can infect the genitals and/or the oral tract Protozoa -­‐‑ These are unicellular eukaryotic organisms -­‐‑ They are parasites that can infect the intestines, the blood and other tissues -­‐‑ They replicate in the host via binary fission or by formation of trophozoites inside a cell -­‐‑ They have a complicated life cycle involving two hosts -­‐‑ Infection is acquired by ingestion or through a vector, such as an insect -­‐‑ Two examples are malaria and leishmaniasis Helminth parasites -­‐‑ These include roundworms, tapeworms and flatworms -­‐‑ They are multicellular eukaryotic organisms – metazoa with a eukaryotic cell structure -­‐‑ They are visible to the naked eye and can exist outside the host -­‐‑ They have variable life cycles -­‐‑ Examples include Ascaris and schistosomiasis MCD: Cells Usama Asif C ELLS 3: Cell membranes Professor Michael Ferenczi ([email protected]) 1. Explain the formation of phospholipid bilayers in an aqueous environment -­‐‑ A living system must be separated from the environment to maintain complex order -­‐‑ The cell is a bag containing substances that are relevant for its function -­‐‑ The bag is made up of the membrane -­‐‑ It forms the limit of the cell and surrounds the intracellular components. The membrane needs to be selectively permeable, impermeable to macromolecules, biochemical intermediates but needs to be permeable to nutrients (in) and waste products (out) -­‐‑ Phospholipids have a hydrophilic head (polar) and a hydrophobic tail – to avoid water, the tails pack together -­‐‑ Suspended in water, they form micelles or droplets. They can also arrange themselves into bilayers (two molecules thick) -­‐‑ Droplets in cells are called liposomes 2. Draw the structure of phosphatidylcholine and identify the component parts -­‐‑ The most abundant phospholipid class is the phosphatidylcholines. The diagram above shows the general structure of a phosphatidylcholine molecule -­‐‑ There are others, that have different lengths of fatty acid tails, different polar head groups etc., such as phosphatidylserine, phosphatidylethanolamine, sphingomyelin etc. -­‐‑ The fatty acid tails can be saturated or unsaturated. This decides on how well packed they are. The more unsaturated the fatty acid, the more sparsely packed it is MCD: Cells Usama Asif -­‐‑
Cholesterol is a steroid, it increases or decreases the membrane stiffness depending on the temperature and nature of the membrane. It also changes interactions with the cytoskeleton 3. Describe the permeability properties of a phospholipid bilayer with respect to macromolecules, ions, water and organic compounds (including drugs) 4. Distinguish simple diffusion, facilitated diffusion and active transport of ions and molecules across cell membranes -­‐‑ Phosphoipids can flip-­‐‑flop between layers of the membrane -­‐‑ Phospholipids can diffuse between each other in the same layer – i.e. with their neighbours – leading to a dynamic membrane -­‐‑ This means that membranes are much more fluid than calculations would predict -­‐‑ The membranes are soluble to small, neutral molecules and fat-­‐‑soluble molecules -­‐‑ They are impermeable to large, hydrophilic or charged molecules Phospholipid bilayers are permeable to: -­‐‑ Water molecules and a few other small, uncharged, molecules like oxygen and carbon dioxide, which diffuse freely in and out of the cell -­‐‑ Diffusion of water across membrane is called osmosis -­‐‑ Diffusion down the concentration gradient is possible -­‐‑ Diffusion against the concentration gradient requires energy or exchange and is called active transport. The energy source is via the hydrolysis of ATP. This occurs using a carrier protein -­‐‑ Facilitated diffusion is movement of hydrophilic (charged) molecules down their concentration gradient through protein pores that hide the ionic charges from the hydrophobic core of the lipid bilayer. Proteins (or protein assemblies) offer a water-­‐‑filled channel. The channel can be ‘gated’ Lipid bilayers are not permeable to: -­‐‑
Cations (K+, Na+, Ca2+ ) but some do leak through, down the concentration gradient -­‐‑
Anions (Cl-­‐‑, HCO3-­‐‑) -­‐‑
Small hydrophilic molecules like glucose -­‐‑
Macromolecules like proteins and RNA -­‐‑
There are proteins on the membrane that are expressed to fulfil this function. They carry and form channels to allow the passage of substances through MCD: Cells Usama Asif 5. Understand the picket and fence model of membrane proteins 6. Categorise the functions of membrane proteins -­‐‑
Membrane proteins diffuse freely within small compartments and can hop between the compartments -­‐‑
The proteins appear to be constrained by fences. Some are less mobile, and appear to be attached to the fence (pickets): -­‐‑ The fences are made up by a membrane skeleton, which form compartments that are where membrane proteins lie -­‐‑ The fences function by steric hindrance i.e. they physically prevent the movement of molecules beyond their boundary -­‐‑ They seem to be made up of components of the actin cytoskeleton -­‐‑ The cell membrane proteins are moved in the membrane along the actin filaments by myosin motors i.e. cadherin -­‐‑ Cell membranes and organelle membranes contain proteins. -­‐‑ These proteins increase the cell fluidity and they confer new properties to the membranes -­‐‑ Many membrane proteins are involved in transport and in the transmission of signals -­‐‑ The proportion of protein to lipids varies from cell type to cell type. The protein composition is different in the inner and outer leaflets of the lipid bilayer. The protein composition also depends on the organelles -­‐‑ Functions of membrane proteins: o Transport (Na+-­‐‑Glucose transporter) o Receptor-­‐‑ for hormones and growth factors o Cell recognition and adhesion o Electron carrier (cellular respiration and photosynthesis in mitochondria and chloroplasts) 7. Explain the movement of Na+ and K+ ions across the cell membrane against a concentration gradient and the consequences of failure of such a movement 8. Explain how the entry of glucose and amino acids into the cell against a concentration gradient is coupled to ATP dependent Na+ transport -­‐‑
The electrochemical gradient is a combination of the concentration and electrostatic gradients generated by ions -­‐‑
The chemical gradient tends to move particles down the gradient, spontaneously -­‐‑
The electrostatic gradient tends to move particles according to their charge MCD: Cells Usama Asif -­‐‑
-­‐‑
-­‐‑
-­‐‑
-­‐‑
Equilibrium can be reached so that the actual ratio of the intra-­‐‑ and extracellular concentration ultimately depends on the existing membrane potential The dissipation of the gradients is ultimately prevented by the action of sodium-­‐‑potassium pumps The high concentrations of fixed ions inside the cell means that water is drawn in by osmosis The sodium-­‐‑potassium pump maintains the osmotic balance and stabilises the cell volume by exporting sodium out of the cell. The sodium ion gradient is thus maintained This gradient is exploited by certain processes in certain cells to drive the transport of the sugars and amino acids, and also to generate electrical signals by deliberately causing the influx of sodium into the cell, disrupting the membrane potential 9. Explain how external chemical signals can be sensed at the interior of a cell -­‐‑ The Na/K pump is found on the membrane of all cells and consists of two polypeptide chains – α (1000 amino acids) and β (300 amino acids) -­‐‑ The α-­‐‑chain spans the membrane ten times, forming a hydrophilic pore -­‐‑ It works by pumping out three sodium ions for every two potassium ions pumped in -­‐‑ It works as such: o It is mediated by successive conformational transitions of the pump molecule o It is driven by phosphorylation of an aspartyl residue (red explosion) o This is followed by hydrolysis of the aspartylphosphate (blue explosion). The changing color and shape is meant to indicate the changing "ʺconformational energy state"ʺ of the pump molecule relative to its substrates -­‐‑ The membrane potential is due to a difference in electric charge on the two sides of the membrane -­‐‑ It results form the activity of electrogenic pumps such as the Na/K pump or the proton pump -­‐‑ It can also result from passive ionic diffusion -­‐‑ Potassium concentration inside the cell is high, to balance the fixed anions and is pumped into the cell by the Na/K pump. It can also travel through leak channels -­‐‑ Sodium concentration inside the cell is low, also due to the action of the Na/K pump -­‐‑ The Na/K pump exchanges 3 Na+ ions from inside the cell for two K+ ions on the outside. -­‐‑ There are two consequences: o Ionic gradients are created: less Na+ and more K+ inside the cell than outside o A charge gradient is created, as more positive charges are pushed out than are coming in. This results in the inside of the cell being at a more negative potential than the outside -­‐‑ This imbalance gives rise to the membrane potential 10. Be able to calculate the membrane potential from the Nernst equation -­‐‑ The Nernst equation is as such: 𝑅𝑇
𝐶!
E! =
⋅ ln 𝑍𝐹
𝐶!
R = the gas constant Z = the charge of the ion concerned F = Faraday’s number (charge/mol of ion) T = temperature Co = [ion] outside the cell Ci = [ion] inside the cell MCD: Cells Usama Asif 11. Understand the role of membranes in synaptic transmission, using the neuromuscular junction as an example -­‐‑
The action potential occurs when the resting membrane potential is deliberately disturbed -­‐‑
They occur in elongated cells i.e. nerves and muscle -­‐‑
The disturbance opens sodium channels, which cause an influx of sodium to enter the cell and cause depolarisation, from about -­‐‑70mV to +50mV -­‐‑
During depolarisation, the refractory period arises, where these channels are temporarily inactivated to prevent further depolarisation -­‐‑
The voltage-­‐‑gated potassium channels open to repolarise the cell -­‐‑
The Na/K pump maintains the membrane potential The neuromuscular junction -­‐‑ This is the highly complex structure located between a motor neurone and a muscle -­‐‑ Acetylcholine is the neurotransmitter used in this -­‐‑ Stimulation at the NMJ is what causes muscle contraction -­‐‑ Outlined below is how it works: o Depolarisation of the muscular postsynaptic membrane results in a propagated action potential o The wave of depolarisation extends into the t-­‐‑tubules (invaginations of the cell membrane) to transmit the activation signal into the core of each muscle cell in the motor unit o Close contact with the sarcoplasmic membrane via triadic junctions involving the dihydropyridine (t-­‐‑tubule membrane) and ryanodine receptors (sarcoplasmic reticulum membrane) results in calcium release from the sarcoplasmic reticulum o Calcium diffusion into the myofilament lattice and calcium binding onto the troponin on the thin filaments (actin) in skeletal and cardiac muscle result in activation of the contractile machinery and contraction Other transport mechanisms Glucose -­‐‑ Glucose is membrane-­‐‑impermeant -­‐‑ Glucose moves down the concentration gradient into the cell -­‐‑ Glucose binds to a specific glucose transporter which functions by a flip-­‐‑flop mechanism -­‐‑ The transport is ‘facilitated’ – sodium is transported in with it (symporter) -­‐‑ There are several different proteins. Some are insulin-­‐‑sensitive Pinocytosis: engulfment by the membrane of extracellular solute and small molecules which end up in small intracellular membrane-­‐‑bound vesicles Phagocytosis: engulfment by the membrane of extracellular objects such as bacteria, cell debris, other cells. Again these end up in intracellular membrane-­‐‑bound vesicles Exocytosis: Movement of proteins and other molecules (e.g. hormones, blood clotting factors) from intracellular vesicles towards the extracellular space by fusion with the cell membrane. MCD: Cells Usama Asif C ELLS 4: Blood and blood cells Dr Michael Emerson ([email protected]) 1. List the main functions of the blood -­‐‑ Blood fulfils many functions. o It acts as a connective tissue o It transports many substances to the cells by connecting every part of the body o It aids heat distribution throughout the body o It has a function in immunity o Haemostasis – thrombogenesis and thrombolysis o Support – i.e. with the penis o Maintains homeostasis -­‐‑ Blood serves cells as such: 2. List the major components of blood -­‐‑ Erythrocytes (red blood cells) -­‐‑ Leukocytes (white blood cells) -­‐‑ Platelets -­‐‑ Plasma 3. Describe the essential features of the erythrocyte and list its major functions -­‐‑ Blood cells are called erythrocytes -­‐‑ Their function is primarily with respiratory transport i.e. oxygen to tissues and carbon dioxide away from tissues. This is done by the protein haemoglobin -­‐‑ They are a biconcave disc, which maximises their surface area for diffusion. They are also flexible, so they can squeeze through the smallest vessels -­‐‑ Erythrocytes can be produced on demand MCD: Cells -­‐‑
-­‐‑
Usama Asif When there is a low arterial oxygen level, caused by altitude, lung or heart disease etc., there is a process induced The process is called erythropoiesis – the process of making new erythrocytes: o The low oxygen level is detected by the kidney o The kidney secretes a hormone called erythropoietin o Erythropoietin stimulates bone marrow stem cells (haematopoietic stem cells) to differentiate into erythrocytes – this is erythropoiesis o The levels of erythrocytes in the blood increases. This consequently increases the level of haemoglobin o This therefore increases the oxygen carrying capability of the blood, so blood oxygen levels increase o This is detected by the kidney and the levels of erythropoietin secreted by the kidney decreases (negative feedback) Erythrocyte life cycle -­‐‑ Red blood cells are produced in bone marrow from precursors (haematopoietic stem cells) which produce haemoglobin then lose organelles -­‐‑ Immature erythrocytes contain ribosomes. These are called reticulocytes -­‐‑ High circulating levels of reticulocytes is a diagnostic tool for conditions caused by anaemia, chemotherapy etc. -­‐‑ The reticulocytes are removed through the reticulo-­‐‑endothelial system, by phagocytic macrophages in the spleen -­‐‑ Their lifespan is about 120 days (short, no nuclei; 1% or 250 billion cells per day) -­‐‑ The life cycle is dependent on dietary iron. Iron deficiency causes anaemia 4. Explain the importance, basic structure and role of haemoglobin -­‐‑ Haemoglobin is important because it is the molecule that transports oxygen to tissues -­‐‑ It consists of four subunits, each is a polypeptide with a haem group. Thus haemoglobin has a quaternary structure -­‐‑ The haem group contains ferrous iron (Fe2+), each of which binds a single oxygen molecule -­‐‑ There is an oxidised form of haemoglobin, that contains ferric iron (Fe3+) – this is called methaemoglobin. This cannot carry oxygen – however, this only happens after exposure to poisons such as benzene etc. -­‐‑ There is a different form in the foetus – with a greater oxygen affinity -­‐‑ Males have higher levels of haemoglobin than females (15.8g/100ml blood in males vs. 13.7 in females) -­‐‑ It is broken down to form bilirubin as its major breakdown product (as well as other things) MCD: Cells -­‐‑
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Usama Asif Hb has a low affinity for oxygen due to the conformational shape of globin molecules When oxygen binds to the Hb initially, it breaks the conformation and opens up the structure This allows the second oxygen to bind more easily and so on This is cooperative binding and allows more oxygen to be carried 5. Define anaemia and list the major causes and subclasses 6. Relate the types of anaemia to red blood cell volume -­‐‑ Anaemia is defined as having a low blood haemoglobin concentration -­‐‑ There are three types of anaemia: o Microcytic anaemia – characterised by a small MCV § This is caused by a failure of haemoglobin synthesis, usually caused by an iron deficiency § Menstruation (a daily loss of blood), GI tract lesions, parasitic infections o Normocytic anaemia – characterised by a normal MCV § This is usually caused by acute blood loss o Macrocytic anaemia – characterised by a large MCV § This is caused by a failure of DNA synthesis and cell division, leading to a reduced division of progenitor cells, so there are fewer but larger (compensatory) erythrocytes § Folic acid deficiency (folic acid is required for thymine synthesis) – apparent in pregnancy § Vitamin B12 deficiency (required for folic acid actions) – autoimmune diseases can cause this by destroying B12 uptake in the gut. Also caused by pernicious anaemia, being a vegetarian or a vegan -­‐‑ Sickle cell anaemia is caused by a mutation in the gene that codes for haemoglobin leading to distorted erythrocytes. It is autosomal recessive – having both copies of the gene leads to the disease, having one leads to sickle cell trait, which = malaria resistance -­‐‑ Polycythaemia is a condition characterised by excess erythrocytes. This can be a physiological response to high altitudes. It can become pathological when the increased viscosity of the blood leads to heart problems MCD: Cells Usama Asif 7. Explain simply the major functions of leukocytes and platelets 8. Explain simply the concepts of immune responses and passive immunity Leukocytes -­‐‑ These are white blood cells and use the circulation for transport -­‐‑ They travel near the capillary wall and invade tissue space to fight infections -­‐‑ They are classified by structure and dye binding -­‐‑ There are many types of leukocyte: o Polymorphonuclear granulocytes – these have multilobed nuclei with many granules § Neutrophils (phagocytic and the most abundant leukocyte) § Eosinophil (allergic response) § Basophils (produce histamine) o Lymphocytes – these are the antibody producing cells of the immune system o Monocytes (phagocytic) Polymorphonuclear granulocytes -­‐‑
These have a segmented nucleus and are full of cytoplasmic granules -­‐‑
They are the first on the scene during an immune response -­‐‑
They adhere to blood vessels in infected area and migrate to tissue -­‐‑
They engulf, kill and digest microorganisms -­‐‑
They release inflammatory mediators: toxic oxygen products (such as peroxides), digestive enzymes, vasodilators, chemotaxins etc. B-­‐‑lymphocytes -­‐‑ These mature in bone marrow -­‐‑ They are responsible for humoral (antibody-­‐‑mediated) immunity -­‐‑ When they encounter a foreign antigen, they initiate immunoglobulin (antibody) production, such as: IgM; IgG; IgA; IgD; IgE -­‐‑ Antibody-­‐‑antigen reactions assist phagocytosis by precipitation; agglutination (clumping) or coating in antibody (opsonisation) or prevent attachment of the microorganism to tissues (neutralisation) -­‐‑ The primary immune response: first exposure, antibodies appear after latent period, peak then fall -­‐‑ The secondary response: greater, quicker, longer response due to memory cells (long lived B-­‐‑
lymphocytes) -­‐‑ Passive immunity is achieved by injecting immunoglobulins (vaccine) or across the placenta (colostrum in some species) T-­‐‑lymphocytes -­‐‑ These are thymus dependent (derived in bone marrow, migrate to thymus, acquire surface antigenic molecules and become immunologically competent in the thymus) They are responsible for cellular immunity (i.e. not antibodies) -­‐‑ They circulate in the blood. If they encounter a foreign antigen, they undergo blast transformation and their progeny have receptors complimentary to the antigen that was encountered -­‐‑ These are called activated T-­‐‑lymphocytes. They secrete chemotaxins (attract macrophages), lymphotoxin (kills cells), interferon (kills viruses) etc. There are many types: o Cytotoxic T cells (CD8+, “attack cells”) o Helper T cells (CD4+, secrete cytokines to help B and T cells, essential, HIV infects) o Suppressor cells (modify lymphocytes responses) MCD: Cells Usama Asif Monocytes -­‐‑ These have a large, single horse-­‐‑shoe nucleus -­‐‑ They appear after granulocytes -­‐‑ Once in the tissue, they become macrophages (“big eaters”) -­‐‑ They engulf microorganisms, tissue debris and dead polymorphs etc. -­‐‑ They also secrete inflammatory mediators and stimulate angiogenesis (vessel growth = repair) -­‐‑ They ingest and store antigens. They present modified antigen to lymphocytes to initiate the adaptive immune response Normal leukocyte count -­‐‑ Leukocytosis – raised count of leukocytes – caused by infections, cancer etc. -­‐‑ Leukopenia – low count of leukocytes – caused by chemotherapy, HIV etc. Platelets -­‐‑
These are derived from megakaryocytes and are 2 -­‐‑ 3 µμm in diameter (small) -­‐‑
The normal platelet count is 25 x 104/ml. They have a life span of 8 -­‐‑ 10 days -­‐‑
They have many granules, many organelles, but no nucleus -­‐‑
They express surface receptors for platelet activators (e.g. collagen in vessels or thrombin from coagulation cascade) -­‐‑
They adhere to exposed collagen in wound or atherosclerosis -­‐‑
The release of granules promotes platelet aggregation -­‐‑
Platelets produce thromboxane A2 from cyclooxygenase enzyme. Aspirin inhibits cyclooxygenase and is an anticlotting factor -­‐‑
They are involved in clot or thrombus formation -­‐‑
The vascular endothelium produces e.g. prostacyclin and nitric oxide which inhibit platelet activation 9. List the major functions of plasma -­‐‑
This is the fluid component of the blood and act as the carrier of the solids in the blood -­‐‑
It contains organic and inorganic substances dissolved in water -­‐‑
There are plasma proteins: o They exert osmotic pressure to maintain blood volume o Albumins Carrier molecules e.g. hormones, bile salts, water insoluble drugs o Globulins o Fibrinogen – the precursor to fibrin, which is involved in clotting and platelet aggregation -­‐‑ Serum is plasma with the proteins removed (due to clotting)