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IAL Biology – Unit 4 Topic 5 – Energy Flow, Ecosystems and The Environment Photosynthesis is used to capture the light energy using chlorophyll and use it to convert carbon dioxide and water into simple sugars The energy from light is used to break the strong H-O bonds in the water molecules. The hydrogen which is released is combined with carbon dioxide to form glucose (fuel for cells). Oxygen is released into the atmosphere as a waste product. Structure of ATP ATP is a nucleotide with 3 phosphate groups attached. Hydrolysis of ATP When energy is needed, the third phosphate bond can be broken by a hydrolysis reaction. This is catalysed by the enzyme ATPase. The result is adenosine diphosphate, a free inorganic phosphate group (Pi) and energy. About 34kJ of energy is released per mole of ATP hydrolyzed. Some energy is lost as heat and the rest is used for biological activity in the cell which requires energy. This is a reversible reaction. The energy needed to drive the synthesis of ATP usually comes from catabolic reactions or redox reactions. As a result, an ATP molecule provides an immediate supply of energy for your cells. FYI – phosphorylation is the process of adding a phosphate group to ADP to make ATP. Synthesis of ATP The main way in which ATP is synthesized is by the removal of hydrogen atoms from several of the intermediate compounds in a metabolic pathway. When 2 hydrogen atoms are removed from a compound, they are collected by a hydrogen carrier or acceptor. The acceptor is reduced. Electrons from the hydrogen atoms are then transferred along a series of carriers known as an electron transport chain. The components of the chain are reduced when they receive the electrons, and oxidized again when they transfer the electrons to the next part of the chain. These redox reactions release a small amount of energy which is used to drive the synthesis of a molecule of ATP. Light-dependent stage of photosynthesis This stage of photosynthesis occurs on the thylakoid membranes of the chloroplasts. Has 2 main functions: to break up water molecules in a photochemical reaction, providing hydrogen ions to reduce carbon dioxide and produce carbohydrates in the light-independent stage / to produce ATP, which supplies the energy to build carbohydrates. When a photon (smallest unit of light) of light hits a chlorophyll molecule, the energy is transferred to the electrons of the chlorophyll molecule. The
electrons are excited and are raised to higher energy levels. If an electron is raised to a sufficiently high energy level, it leaves the chlorophyll molecule completely. The excited electron is collected by a carrier molecule called an electron acceptor and this results in the synthesis of ATP by one of 2 processes: cyclic photophosphorylation and non-cyclic photophosphorylation. Both these processes occur at the same time and in both cases, ATP is formed as the excited electron is transferred along an electron transport chain. In non-cyclic photophosphorylation, reduced NADP is also produced. Cyclic Photophosphorylation Cyclic photophosphorylation involves only photosystem I (PSI) and drives the production of ATP. When light hits a chlorophyll molecule in PSI, a light-excited electron leaves the molecule. It is collected by an electron acceptor and transferred directly along an electron transport chain to produce ATP. When an electron returns to the chlorophyll molecule in PSI, it can then be excited in the same way then. Non-cyclic Photophosphorylation During non-cyclic photophosphorylation, water molecule are broken down, providing hydrogen ions to reduce NADP. ATP is also produced. This process involves both photosystem I and photosystem II. In the light, photons constantly hit chlorophyll molecules in both PSI and PSII. This excited the electrons to a higher level. They are, therefore, lost from the chlorophyll molecule and collected by electron acceptors. An excited electron from PSII is collected by an electron acceptor and transferred along an electron transport chain to PSI, driving the synthesis of one molecule of ATP. PSI receives an electron to replace one that was lost to the light-independent reactions. Now the chlorophyll molecule in PSII is missing one electron and so it is unstable. The original electron cannot be returned to the chlorophyll because it has continued on to PSI. Another electron is therefore needed to restore the chlorophyll to its original state and this comes from the breaking down of water molecules, a process known as photolysis (depends on light). Water molecules dissociate spontaneously into hydrogen and hydroxide ions. As a result, there are many hydrogen and hydroxide ions in every part of the cell, including in chloroplasts. These ions are used to replace the lost electrons from chlorophyll. Once the chlorophyll molecule in PSII has received an electron it is restored to its original state, ready to be excited again when hit by another photon of light. At the same time, electrons in PSI are also being excited by light and collected by an electron acceptor. Electrons are transferred along an electron transport chain and collected by the electron acceptor, nicotinamide adenine dinucleotide phosphate (NADP). The NADP also collects a hydrogen ion from the dissociated water to form reduced NADP. Reduced NADP Provides reducing power. ATP Provides energy (in light-independent reactions of photosynthesis). Many hydrogen ions are removed by NADP and many hydroxide ions remain. The hydroxide ions react together to form oxygen and water. Electrons are freed as a result of the reaction and are absorbed by chlorophyll. 4 chlorophyll molecules regain electrons in the production of one molecule of oxygen:
Products of photosynthesis GALP (glyceraldehyde 3-phosphate) is the primary end-product of photosynthesis. Some of the GALP is used directly in cellular respiration. Some of the GALP produced is used to produce glucose in a process called gluconeogenesis. This glucose may be converted into disaccharides such as sucrose for transport around the plant; into polysaccharides such as starch for energy storage; and into cellulose for structural support. The GALP that enters cellular respiration is used to provide energy in the form of ATP. Compounds from these pathways are also used as the building blocks of amino acids. The molecules combine with nitrates from the soil. GALP can also continue round the Calvin cycle and combine with phosphates from the soil to produce nucleic acids. Some of the GALP that enters the cellular respiration pathways is converted into a chemical called acetyl coenzyme A. This compound is then used to synthesize the fatty acids needed for the production of phospholipids for membranes, and lipids needed for storage. Structure of chloroplasts Each chloroplast is surrounded by an outer and an inner membrane with a space between the two, known as the chloroplast envelope. Inside the chloroplast is a system of membranes that are arranged in layers called grana. A single granum is made up of layers of membrane discs known as thylakoids. This is where the green pigment chlorophyll is found. Granal membranes are covered in particles. These particles are involved in ATP synthesis. The grana are joined together by lamellae, extensions of the thylakoid membranes. These lamellae connect two or more grana. They also act as a skeleton inside the chloroplast, maintaining a distance between the grana so that they receive the maximum light and function as efficiently as possible. The membrane layers are surrounded by a matrix called the stroma. The stroma contains all the enzymes needed to complete the process of photosynthesis and produce glucose. Chlorophyll is another major adaptation of the chloroplasts. It is a light- capturing, photosynthetic pigment.
It is a mixture of closely related pigments. These include chlorophyll a
(blue green), chlorophyll b (yellow-green), the chlorophyll carotenoids
(orange carotene and yellow xanthophyll) and also a grey pigment phaeophytin, which is a breakdown product of the others. Each of the pigments absorbs and captures light from particular areas of the light spectrum. As a result, much more of the energy from the light falling on the plant can be used than if only one pigment was involved.
Absorption spectra The absorption spectrum describes the different amounts of light of different wavelengths that a photosynthetic pigment absorbs. (Represented as a graph) Action spectra A graph demonstrating the rate of photosynthesis against the wavelength of light. Chromatography Chloroplast pigments can be separated using chromatography. The pigments travel up the solid medium at different speeds and are readily separated using a suitable solvent (using paper or silica gel). You can determine the Rf values and compare them to the Rf values of known pigments in the same solvent. Important to compare the Rf values using the same solvent because the pigments can have very different values with different solvents. Relationship between GPP, NPP and R Gross primary productivity (GPP) in plants is the rate at which plants make new material by photosynthesis. Measured as: Plants use at least 25% of the material they produce for their own metabolic needs. They respire, breaking down glucose to produce ATP. This is an exothermic process, so it increases the internal energy store of the surroundings. The rest of the material is stored as new tissue in plants. This energy store is known as the net primary productivity (NPP).
Abiotic Factors Light: The amount of light in a habitat has a direct effect on the numbers of organisms found there such as plants as they depend on light for photosynthesis. Plants growing in low light have adaptations – extra chlorophyll or a different ratio of photosynthetic pigments that are sensitive to lower light levels. Larger leaves to absorb more light. In temperate countries, some plants reproduce early to avoid the shade caused by larger plants. Animals are affected by light levels indirectly, as a result of the distribution of food plants. Temperature: Each organism has a range of temperature for growth and reproduction. Reproduction does not occur above or below that range. Temperature influences enzyme-controlled reactions in plants and affects ectothermic animals (reptiles). Endothermic animals maintain body temperature. Wind and water currents: Wind increases cooling and water loss from the body so adds to the environmental stress on an organism. Currents are most damaging to populations when the strength increases suddenly, such as when flooding occurs. Water availability: Affects survival through rainfall, rate of evaporation and rate of loss by drainage through the soil.
Low water → death or adaptations (e.g. camels, kangaroo rats, cacti).
High water → growth bursts, e.g. desert seeds germinate and flower after
rain. Temporary abundance provides food and leads to population explosions. Oxygen availability: Affects both water and soil organisms. Cold/stagnant water = low oxygen → organisms die. Soil air spaces provide oxygen for roots to respire. Waterlogged soil = roots cannot obtain oxygen → plants die. Example: Mangroves have aerial roots for oxygen in waterlogged soils. Edaphic Factors (soil structure and mineral content): Sandy soil - loose, shifting structure so very little to grow on it, but good drainage (water passes through them rapidly, carrying with it minerals that are needed by the plants). This leaching of minerals reduces the population density of plants that can grow in the soil. Clay soil - Retains water, hard to drain. Easily waterlogged. Populations it will support is limited. Loam = ideal soil, mixture of particle sizes, good drainage and mineral holding. Biotic Factors Predation: Prey population increases, more food for predators and so eventually predator population grows. Predators will increase to a point at which they are eating more prey than are replaced by reproduction so numbers of prey will fall. Reduces food supply of predators so they will not produce as many offspring, their numbers will fall, numbers of prey will increase again. Finding a mate: Reproduction drives population changes. Probability of finding a mate affects abundance. The abundance of species reproducing sexually depends on mate availability. Single individuals in new areas often cannot reproduce sexually. Territory: Many animals have territorial behaviour — areas defended against other organisms. Territories ensure breeding pairs have sufficient resources to raise young.
Parasitism and Disease: Parasitism and disease weaken hosts and reduce reproduction. Dense populations = faster disease spread because individuals closer together. In low biodiversity ecosystems, disease impact is greater since most individuals are of the same species. In diverse communities, some species remain unaffected and food sources remain available. Niche The role of an organism within the habitat in which it lives. Abundance = the relative representation of a species in a particular ecosystem. Abundance is affected by abiotic and biotic factors. Distribution = where a species of organism is found in the environment and how it is organized. Niche affecting abundance (according to the case studies) Abundance increased where extra food was available and predation was limited. Also increased when more nesting sites were available (abundance limited by availability of habitat). Goat populations grew rapidly and outcompeted tortoises for food, reducing tortoise abundance – intraspecific competition due to overlapping niches. Niche affecting distribution (according to case studies) Population of a species found where there is vegetation to feed on and protection from predators. Population expanded due to abundance of food due to crops. Population declines because of less food and drought. Density-independent factors – Factors affecting the number of organisms occupying a niche which are the same regardless of population size. Example: temperature extremes, natural disasters, climate change. Density-dependent factors – Factors affecting the number of organisms occupying a niche which are depending on the number of organisms in a specific area. Example: disease, parasitism, competition. Limit populations when density is high. Competition – Individual organisms have to compete for density- dependent factors and this can determine the size and density of a population. Competition occurs when 2 organisms compete for a resource that is in limited supply. Intraspecific competition: Competition between members of the same species within the same niche for a limited resource. Example: male peacocks competing for mates. As a result of this, some individuals may not survive or may not reproduce and so population growth slows. Interspecific competition: Occurs when different species within a community compete for the same resources. The niches of the species overlap. Competition will reduce the abundance of the competing species. If there is a greater density of one species, or it has a faster reproduction rate than its competitor, then one of the species may become extinct in that area. Ecosystem evolvement Primary succession: Starts with an empty inorganic surface, such as bare rock (observed usually after a volcanic eruption or landslide). The first stage of the succession is colonization (process by which new species spread to new areas), and the first organisms to appear are opportunists or pioneer species such as algae, mosses and fungi. These organisms can penetrate the rock surface by dissolving the rock with acids they secrete and penetrating tiny cracks with root hairs and hyphae.
Climate change describes a large-scale change in global or regional weather patterns that occurs over a period of many years. Dendrochronology The dating of past events using tree ring growth. Trees increase in width each year by cell division in a layer of their trunks. The contrast between small cells formed in poor conditions and large cells formed in good conditions gives the appearance of rings. Counting rings gives approximate tree age and provides a climate record. Factors affecting reliability - Growth depends on temperature, CO₂, rainfall, and light. Conditions vary within years → more than one ring can form. To check reliability, scientists: Compare results from different places. If patterns match, climate trends are consistent. Data can also be compared with coral reefs (isotope data) for confirmation as the sea temperatures change and this gives another valuable record of climate change. Peat Bog Records Peat bogs are made of partly decomposed plant material, mainly Sphagnum mosses. It is very acidic, cool and anaerobic which prevents bacteria from decomposing organic material. Peat preserves pollen grains, moss spores and plant tissues. Layers of peat represent different time periods, allowing reconstruction of past climates. Each plant species thrives under certain conditions: Cotton grass and some species of Sphagnum moss cool and wet conditions. Other species of Sphagnum moss and species of Polytrichum drier conditions. Pine/chestnut pollen warmer periods. Birch tree pollen cooler periods. By analysing pollen composition, scientists infer temperature and moisture trends over time. Undisturbed peat cores provide continuous climate records up to 10, years. Increasing data reliability Dendrochronology and peat data can be cross- checked using radiocarbon dating (calibration). Calibration allows dating of samples accurately and matching to historical events. Correlation A strong tendency for two sets of data to vary together. Causal relationship One event happens as a direct result of another, with a clear mechanism by which one factor causes a given change. Greenhouse gases trap heat (infrared re-radiation), warming the Earth (the greenhouse effect). Human activities (burning fossil fuels, deforestation) increase greenhouse gases — anthropogenic cause. The Carbon Cycle Biotic phase: Phase during which the inorganic ions are incorporated in the tissues of living things. Abiotic phase: Phase during which the inorganic ions are returned to the non-living part of the ecosystem. Carbon sinks: Carbon sinks are reservoirs where carbon is removed from the atmosphere and locked into organic and inorganic compounds. Carbon dioxide in the air and water is taken in by plants during photosynthesis to form carbon compounds in plants. Carbon compounds in plants are passed to animals by feeding. Plants and animals release carbon dioxide back into the air by respiration. When plants and animals die, their bodies become dead organic matter in the soil. Decomposers break down dead organic matter and release carbon compounds, and during respiration by decomposers , carbon dioxide
returns to the air. Some dead organic matter may not fully decompose and can form fossil fuels over long periods through fossilisation. When fossil fuels are burned during combustion , carbon dioxide is released back into the air. Some carbon becomes stored in the oceans and forms carbonate rocks through sedimentation. Weathering and volcanic activity can release carbon dioxide from carbonate rocks back into the atmosphere. Carbon cycle can be applied to methods to reduce atmospheric levels of carbon dioxide Use of carbon sinks- carbon removed from the atmosphere and is locked there. Use of carbon neutral material. Use of biofuels. Use of renewable energy resources. Reforestation Predicting future climate change Data on greenhouse gases can be extrapolated and used in models to make predictions about what will happen to temperature and other aspects of global climate in the future. These extrapolations can be used in other models to predict the long-term effects of increased temperature on the environment. Limitations of climate change models: Impossible to tell the exact impact of carbon dioxide on global warming and predict the impact of global warming on particular aspects of the world climate. In addition, extrapolations from past data cannot take into account known factors in the future, including how current trend in resource usage and technologies may change. Effect of temperature on the rate of enzyme activity The effect of temperature on the rate of any reaction can be expressed as the temperature coefficient (Q 10 ). A 10°C rise in temperature (within 0–40°C) approximately doubles the rate of enzyme-controlled reactions. Each species has an optimum temperature range suited to its habitat. Tropical species: adapted to narrow ranges; small temperature rises can be fatal. Temperate species: adapted to wider ranges; can tolerate greater variation. Evolution Evolution (changes in frequency of alleles) can result from gene mutation and natural selection. If the environment changes, a new selection pressure is exerted on a population of organisms. As a result of the change in conditions, certain alleles may become advantageous or disadvantageous. A new set of advantageous alleles is selected for and eventually a new species better suited to the new conditions may evolve. When populations become isolated in some way, the normal gene flow between the individuals is reduced because they cannot meet and mate. This can lead to the formation of two different 21 species as the populations respond to slightly different selection pressures. Through natural selection, the populations may become so different they can no longer breed successfully. Populations with greater genetic variation have higher survival chances. Climate change and isolation One of the main drivers of speciation is isolation. When populations become isolated in some way, the normal gene flow between the individuals is reduced because they cannot meet and mate. Allopatric speciation - Occurs when populations become physically or geographically isolated. Different selection pressures act on each group → different alleles become advantageous. Over time, populations become genetically distinct → cannot interbreed → new species form.
particular nutrient or for antibiotic resistance. This acts as a marker gene to show which microorganisms have been genetically modified. Growing a pure culture (techniques): The nutritional requirements of different microorganisms vary greatly. You can produce a medium that will favor the growth of the organism you want to culture and inhibit the growth of others. This allows you to identify the colony you want and then re- inoculate it to produce a single pure culture. May need to control the range of nutrients available, or antifungal chemicals that will reduce or prevent the growth of all microorganisms except the one you want. There are indicator media that cause certain types of bacteria to change color. Colonies with or without color change can be isolated and cultured. Usage: Important to be able to isolate disease-causing organisms from those of the normal body flora, so that a disease can be diagnosed and appropriate treatment planned. Measuring the growth of bacterial cultures (methods) Cell counts (haemocytometer): Single-celled fungi which are cultured in nutrient broth can be counted using a microscope and a haemocytometer. A haemocytometer is a specialized thick microscope slide with a rectangular indentation and engraved grid of lines that is used to count cells. You dilute the sample of nutrient broth by half with an equal volume of trypan blue (stains dead cells blue so you can identify and count only the living cells). Then you can view and count the cells using a microscope. Each corner of the haemocytometer grid has a square divided into 16 smaller squares. The number of cells in each of these 4 sets of 16 squares is usually counted and the mean is calculated. The haemocytometer is calibrated so that the number of bacterial or fungal cells in one set of 16 squares is equal to the number of cells Optical methods (Turbidity): Turbidimetry, a specialized form of colorimetry is another way of measuring the number of cells in a culture. As the numbers of bacterial cells in a culture increase, it becomes increasingly turbid (cloudy). As a solution becomes more turbid, it absorbs more light, so less light can pass through it. By measuring how much light passes through a sample, the amount of light absorbed can be identified which indicate how many microorganisms are present. Dilution Plating Another way of counting is dilution plating which is used to find the total viable cell count. Based on the idea that each of the colonies on an agar plate has grown from a single, viable microorganism on the plate. A solid mass of microbial growth is often present after culturing and it is not possible to identify the individual colonies. Solution diluting
the original culture in stages until you reach a point when you can count the colonies. Can calculate the total viable cell count for the original sample by multiplying the number of colonies by the dilution factor. Because 2 or more plates are often used to count individual colonies, you can obtain a mean, giving an almost accurate number of the cells in a particular sample. Area and Mass (fungi) When you culture fungi, you can assess growth by measuring the diameter of the individual areas of mycelium. Used to compare growth rates in different conditions. Inoculate identical Petri dishes containing identical growth medium with the same number of fungal spores. Culture the Petri dishes at different temperatures, with several identical dishes grown at each temperature. After a specific period of time, measure the diameter of each fungal colony and calculate the mean value for the diameter at each temperature. Optimum temperature for growth is close to the temperature at which you obtained the largest mean diameter. Same technique can be used for bacteria but the growth of the colony tends to be slower and therefore less easy to measure because the microorganisms themselves are so small. Bacterial growth curve (4 stages) Lag phase: When bacteria are adapting to their new environment and are not reproducing at their maximum rate. Log/exponential phase: When the rate of bacterial reproduction is close to or at its theoretical maximum, repeatedly doubling in a given time period. Stationary phase: When the total growth rate is 0 as the number of new cells formed by binary fission is equal to the number of cells dying. Death/decline phase: When reproduction has almost stopped and the death rate of cells is increasing. Graph is a sigmoid (S-shaped) curve. Exponential growth rate constants Calculate the number of bacteria in a population using the formula:
how viruses attach to the cells which they infect. The VAPs respond to particular molecules on the host cell surface. Some viruses have: A lipid envelope which is produced from the host cell membrane and which covers the genetic material and protein coat. The presence of the envelope makes it easier for the viruses to pass from cell to cell, but it does make them vulnerable to substances such as ether which will dissolve the lipid membrane. Classification of viruses DNA viruses – Have DNA as their genetic material. The viral DNA acts directly as a template for both new viral DNA and for the mRNAs needed to induce synthesis of viral proteins. Examples: λ (lambda) phage and smallpox virus. RNA viruses – Have RNA as their genetic material. Much more likely to mutate than DNA viruses. Contain a single strand of RNA. Examples: tobacco mosaic viruses, Ebola fever. Retrovirus – Have a protein capsid and a lipid envelope. The single strand of viral RNA controls the synthesis of a special enzyme called reverse transcriptase. This is responsible for making DNA molecules corresponding to the viral genome. This DNA is then incorporated into the host cell DNA and is used as a template for new viral proteins and ultimately a new viral RNA genome. Example: Human immunodeficiency virus (HIV). Virus life cycle Viruses reproduce only within the cells of their host. Bacteriophages inject their genome into the host bacterial cell, but the main part of the viral material remains outside the bacterium. The viral DNA forms a circle or plasmid within the bacterium. Some types of viruses enter the cell by endocytosis and the host cell then digests the capsid, releasing the viral genetic material. Commonly, the viral envelope combines with the host cell surface, releasing the rest of the virus inside the cell membrane. Plant viruses usually enter the plant cell using a vector to penetrate the cellulose cell wall. Once a virus is in the host cell, there are 2 different routes of infection: lysogenic pathway and lytic pathway. Lysogenic pathway: Most viruses insert their DNA into the host DNA so it is replicated every time the host cell divides. This inserted DNA is called a provirus. Messenger RNA is not produced from the viral DNA because one of the viral genes causes a repressor protein to be produced.
This makes it impossible to translate the rest of the viral genetic material. This virus is said to be latent (the state of the non-virulent virus within the host cell) during this period of lysogeny, when the virus is part of the reproducing host cells. Under certain conditions, for example when the host is damaged, viruses in this lysogenic state are activated. The amount of repressor protein decreases and the viruses enter the lytic pathway and become virulent. Lytic pathway: Sometimes, the viral genetic material is replicated independently of the host DNA immediately after it enters the host. Mature viruses are made and eventually the host cell bursts, releasing large numbers of new virus particles to invade other cells. The virus is now disease-causing and the process of replicating and killing cells is known as the lytic pathway. Retrovirus lifecycle Their genetic material is viral RNA. This cannot be used as mRNA but is translated into DNA by the specific enzyme reverse transcriptase in the cytoplasm of the host cell. This viral DNA enters the nucleus of the host cell where it is inserted into the host DNA. Host transcriptase enzymes then make viral mRNA and new viral genome RNA. New viral material is synthesized and the new viral particles leave the cell by exocytosis. Host cell continues to function as a virus-making factory and at the same time the new viruses continue to infect other cells. Viruses and Disease Viruses cause the symptoms of disease by the lysis of the host cells, by causing the host cells to release their own lysosomes and digest themselves from the inside or by the production of toxins that inhibit cell metabolism. Each virus is specific and this specificity is due to the presence or absence of cell markers on the surface of host cells. Each type of cell has its own recognition markers and different types of viruses can only bind to particular markers. Tuberculosis TB is caused by the bacterium Mycobacterium tuberculosis which is spread by droplet infection. The disease is spread by breathing, coughing and sneezing. People who are malnourished, ill or have problems with their immune systems (or even with HIV/AIDS) are more vulnerable to the disease and are much more likely to develop active TB. Other common source of infection is from the bacterium Mycobacterium bovis, which affects cattle and people in close contact with it. TB affects the respiratory system, damaging and destroying lung tissue. It also suppresses the immune system. Symptoms: weakness, coughing up blood (appear at the end of the disease process). Primary infection – In the primary infection, the bacteria which have been inhaled into the lungs multiply slowly, often causing no obvious symptoms. If you have a healthy immune system, there will be a localized inflammatory response forming a mass of tissue called a tubercule, which contains dead bacteria and macrophages. After about 8 weeks, the immune system controls the bacteria, the inflammation disappears and the lung tissue heals. The bacteria has an adaptation that allows it to avoid the immune system. This means some bacteria may survive the primary infection stage. The
Inoculation through infected blood; can also be infected by infected blood products. From a mother to her fetus in the early stages of pregnancy, during birth or through breastfeeding. HIV to AIDS: Symptoms of AIDS result from the effect of HIV on the immune system. HIV attaches to receptors on T helper cells and is then able to infect these cells. HIV is a retrovirus. This mean that once it enters the T helper cell, it controls the host DNA and replicates. The host T helper cell is destroyed when the new viruses leave it. At the same time, other cells of the immune system called T killer cells recognize and destroy some of the heavily infected T helper cells. This causes a large decrease in the number of T helper cells. As a result, the immune system cannot fight other pathogens. Therefore, individuals with HIV are vulnerable to secondary infections such as pneumonia. Course of HIV/AIDS infection: Stage 1 (Acute HIV syndrome) – First few weeks after infection symptoms include: fever, headaches, tiredness and swollen glands. Some have no symptoms. Between 3 and 12 weeks after infection, HIV antibodies appear in the blood, making the person test HIV positive. Stage 2 (The asymptomatic or chronic stage) – Once the infection is established all symptoms disappear. In fit, young people with access to effective anti-retroviral drugs this stage can last many years. For people with little food or medicine, this stage of the disease will be relatively short. During the asymptomatic stage, the virus replicates, infecting the T helper cells but is kept under control by the T killer cells. As this stage progresses, secondary infections develop because the immune system is unable to deal with the situation. Stage 3 (Symptomatic disease) – Eventually the number of viruses attacking the immune system becomes so great that the whole immune system starts to fail. The normal T helper cell count falls from 500 to 200 per mm^3 of blood. Patients suffer from symptoms such as weight loss, fatigue, diarrhea, night sweats and low-grade infections such as thrush. Stage 4 (Advanced AIDS) – As T helper cell numbers fall, severe symptoms begin to appear such as major weight loss, dementia as brain cells become infected, cancers (e.g. Kaposi’s sarcoma) and serious infections such as TB and cryptococcal meningitis. Final stage of advanced AIDS is always death. Treating AIDS: AIDS is an incurable disease but there are methods to limit the spread of it. Education programmes help people to understand the ways in which HIV is spread and how to prevent it. Celibacy / Only having one sexual partner / Using clean needles if injecting drugs / Using condoms to prevent the spread of the virus from one partner to another. Difficult to produce an effective vaccine for HIV because: the virus mutates rapidly so the antigens on the viral coat keep changing in the years after infection. This makes it harder for the immune system to recognize the virus and destroy it. The rate of change only slows down as the T cell count starts to fall to very low levels so selection pressure on the virus is reduced. Natural
selection also favors mutations that enable the virus to replicate particularly fast. This allows the virus to infect many cells very quickly. As a result, by the time a vaccine has gone through all its development and safety tests the virus has changed and the vaccine is not effective. In addition, AIDS is found mainly in humans. There are, therefore, not many animals that scientists can use as a model for their research. Combined drug therapies which are anti-retroviral and stimulate the immune system mean that AIDS is becoming a treatable, long-term disease. The drugs are quite expensive and so are mostly used in the developed world. Large quantities of this drug are supplied to developing countries for no/minimal cost however without comprehensive medical infrastructure and where there is great political instability, the drugs are often not available to those who need them. Methods of spread (routes pathogens take when entering the body) For any disease to be spread, the pathogen needs to enter the body of the new host. This happens through the eyes, nose, mouth, ears, anus or urinogenital openings. The alternative is for microorganisms to enter directly into the blood through the skin. Pathogens are transmitted in a variety of ways: Vectors: A living organism that transmits infection from one host to another. E.g. malaria, dengue, yellow fever. Inhalation: When you cough, sneeze or talk, millions of droplets are expelled from your respiratory tract. These droplets all contain pathogens. Part of the water in these droplets evaporates, leaving very small droplets which are full of pathogens and small enough to remain suspended in the air for a long time. When these droplets are inhaled by another individual, the pathogens enter into a new respiratory tract and another infection is established. E.g. flu, tuberculosis. Ingestion: Many of the pathogens that cause gut diseases are transmitted by faecally contaminated food or drink. Risk of infection is greatest from raw or undercooked food. Often only a small number of disease-causing organisms need to be absorbed to cause disease. E.g. Salmonella poisoning, hepatitis A. Fomites: Fomites are inanimate objects that carry pathogens from one host to another. Hospital towels and bedding can be a risk. E.g. Staphylococcus infections. Direct contact: Direct contact is often important in the spreading of skin diseases in small children. Many sexual diseases are also spread by direct contact of the genital organs and then the pathogens pass through mucous membranes. E.g. gonorrhea, impetigo, Ebola. Inoculation: A pathogen can be inoculated into the body directly through a break in the skin. This transmission might be through sexual contact if the skin is damaged, via an injury from contaminated medical instruments or shared needles in drug abuse. Infected animal may bite, lick or scratch you. E.g. rabies, HIV. Barriers to entry Pathogens have to cross several natural barriers before infection can occur. Epithelial defences (skin): Skin is an impenetrable layer strengthened by keratin, a fibrous structural protein. It forms a physical barrier between the many pathogens in the environment and the delicate, blood-rich tissues beneath the skin. An oily substance produced by the skin, called sebum,
