The urinary system
Carbon dioxide is removed by our lungs. Our urinary system is responsible for producing, storing and removing urine from our bodies. Our urine contains urea and any excess water.
The kidneys are part of the urinary system, together with the ureter, urethra and bladder. Humans have two kidneys. They are bean-shaped organs - approximately 11.5 cm long – which are found just below our ribcage, one on either side of our spine.
The renal arteries take blood with waste products to the kidneys to be filtered. Renal veins then return the filtered blood to be circulated around the body.
Blood vessels take the blood though the kidneys where the waste products are removed into convoluted tubules. These tubules join together to form the ureter, which transports urine to the bladder where it is stored. Urine is then passed from the bladder to the urethra to be released.
Since the kidney is responsible for the removal of waste from the blood, any damage (either from accidents or disease) can lead to a build-up of poisonous waste products in the body. We can survive without one kidney very well, but total kidney failure would be fatal if not treated. Treatment can take the form of dialysis on a kidney machine or a kidney transplant.
Kidney dialysis machine
A dialysis machine can keep a patient alive until a transplant becomes available. The patient’s blood is passed through the machine and cleaned of any waste products the kidneys would have otherwise removed. However, dialysis machines have several disadvantages:
They are expensive.
The patient must have his or her blood connected to the machine for several hours each week.
Patients must follow a very strict diet to avoid complications.
They only work for a limited time for a patient .
A kidney transplant can save the patient's life, and after a transplant the patient can live a relatively normal life. This is clearly a better option than a machine, but kidney transplants also have several disadvantages:
Any major surgery carries some risk.
The kidney may be rejected by the body of the patient and so drugs (immuno-suppressant drugs) are used constantly to help prevent rejection.
A precise match of tissue type is needed. About half the donated kidneys come from family members (this is known as a ‘living donor’).
There is a severe shortage of donors.
Urine is produced in microscopic structures in the kidney called nephrons. There are approximately 1 million nephrons in each kidney.
The glomerulus filters blood and produces glomerular filtrate. This filtrate contains water, glucose, salts and urea. Large molecules such as protein are too large to fit through the blood capillary walls. The Bowman's capsule collects the filtrate and it enters the tubules. All glucose is reabsorbed immediately into the blood capillaries.
As the rest of the filtrate travels through the tubules, water and salts needed by the body are reabsorbed into the blood capillaries. The loop of Henlé helps maintain the correct water balance in the body by filtering out salts. The waste, consisting of excess water, excess salts and urea, is urine. The collecting duct collects the urine, which is then transported in the ureter to the bladder. The bladder stores urine until the body is ready to expel it through the urethra.
This process can be summarised in three important steps:
Selective reabsorption – the useful substances (ions and sugars) are reabsorbed back into the blood from the tubules. The amount of water in the blood is regulated here to maintain it at a constant rate. This is known as ‘osmoregulation’.
Excretion of waste - urea and excess water and ions travel to the bladder as urine, to be released from the body.
ADH and regulating water content
The kidneys are the main organs a mammal uses to adjust its water content. This is done by controlling the volume of urine produced. The kidneys are controlled by a hormone called ADH (anti-diuretic hormone) which is produced by the pituitary gland.
The brain detects any changes in the water concentration of the blood.
If the water concentration falls, then more ADH is produced. In response to this, the kidney reabsorbs more water. This means that a small volume of very concentrated urine is produced. This reduces water loss.
If the water concentration rises, then less ADH is produced. In response to this, the kidney reabsorbs less water. This means that a large volume of very dilute urine is produced. This increases water loss.
ADH production is an example of a negative feedback mechanism. This is when a substance is produced in our bodies to return a system to normal. Here ADH is produced in response to low blood water levels (high salt concentration). Negative feedback is also seen in the menstrual cycle.
The menstrual cycle
A woman is fertile, on average, between the ages of 12 to 50. During these fertile years, a recurring process - known as the menstrual cycle - takes place each month. This involves the lining of the uterus preparing for pregnancy. If pregnancy does not occur, the lining breaks down and the woman has a period. This is also known as menstruation.
The menstrual cycle usually lasts approximately 28 days. The first day of the cycle is the first day of a woman’s period (menstruation). The period – which usually lasts for 3-7 days - is made up of blood and the uterus lining. It passes out of the body through the vagina.
After menstruation, the lining of the uterus builds up again (thickens) in preparation for a fertilised egg. Around day 14 of the cycle, an egg is released from a follicle in the ovaries - this is ovulation.
If this egg is fertilised and embeds itself in the thickened lining of the uterus, the lining is maintained and the woman becomes pregnant. If a fertilised egg does not embed itself, the lining breaks down and menstruation occurs - and so the cycle repeats itself.
The mensrual cycle
The menstrual cycle
The role hormones play
The menstrual cycle is controlled by the hormones oestrogen and progesterone.
Oestrogen is produced by the ovaries and makes the lining of the uterus repair itself and grow again after menstruation.
Progesterone is produced by the empty follicle in the ovary after the egg has been released. This hormone maintains the lining of the uterus during the second half of the menstrual cycle.
If a woman becomes pregnant the follicle continues to produce progesterone and a placenta is formed. If pregnancy does not occur, then both hormone levels drop towards the end of the menstrual cycle, the lining breaks down and menstruation occurs.
Hormones and the menstrual cycle
You need to know the roles of oestrogen and progesterone and also two other hormones: follicle stimulating hormone (FSH) and luteinising hormone (LH).
FSH and LH are both produced by the pituitary gland in the brain and are transported in the blood.
Low progesterone levels allow FSH levels to stimulate an egg (in a follicle in the ovary) to be matured. This encourages the production of oestrogen which repairs the uterus wall and stimulates a surge of LH. This triggers ovulation.
After the egg is released from the follicle, it develops into the corpus luteum. This produces progesterone which maintains the lining of the uterus and so stops menstruation. Progesterone inhibits FSH and LH and so remains high during pregnancy.
The menstrual cycle is an example of a negative feedback mechanism. This is when a substance is produced in our bodies to return a system to normal.
Negative feedback is also seen in the formation of ADH (anti-diuretic hormone) in the kidneys. In this case, ADH is produced when the brain detects too little water and so the kidney reabsorbs more water. The opposite happens when the brain detects too much water.
Eggs and sperm
Eggs (ova) and sperm have half the number of chromosomes of normal body cells. They are called haploid cells rather than diploid cells. A single egg cell is called an ovum. Collectively, eggs and sperm are called gametes or sex cells. When fertilisation occurs, an egg (ovum) and a sperm fuse to form a zygote.
An egg cell (ovum) is one of the largest cells in the human body and can just be seen without using a microscope.
Each egg cell has a haploid nucleus - containing only half the number of chromosomes of a normal cell nucleus. It has a large cytoplasm which contains the nutrients and mitochondria needed for mitosis (cell division) after fertilisation. And each egg has a special cell membrane which only allows one sperm to fertilise it.
Each sperm cell also has a haploid nucleus. It has a tail (for motility) which propels it through the cervix, uterus and fallopian tube towards the egg. And each one has many mitochondria (where respiration occurs) to release the energy needed for its journey.
Sperm cells also have special enzymes, called acrosomes, which allow them to break through the cell membrane of the egg.
Some women donate a small number (usually about ten) of their eggs (ova). Women are given hormones to help their eggs mature before they are removed during an operation. The medical risk is small. Donated eggs are normally then fertilised in vitro (see below).
In vitro fertilisation (IVF)
If a man is unable to produce enough sperm of suitable quality, IVF can be used. An egg (ovum) is removed from the woman (or a donor egg is used), and sperm is introduced to it outside of the body. The fertilised egg is then returned to the woman’s uterus. Babies born from this process are sometimes called ‘test tube babies’. This is an expensive procedure with a low medical risk.
Some people are worried about the ethical implications of IVF, and are concerned that couples may only want fertilised eggs with ‘desirable’ qualities. For example, they might want a girl if there are lots of boys in the family, or they may wish to avoid producing a baby with an inherited defect.
Surrogate mothers agree to become pregnant for another couple who are infertile. There are different laws that relate to this process in different countries. Surrogate mothers can become pregnant from IVF treatment (see above).
Use of hormones
Follicle stimulating hormone (FSH) can be given to women to help them mature and then release their own eggs. It can also be used to encourage the production of several mature eggs before they are removed for IVF treatment.
The genetics of fertilisation
Fertilisation occurs when a haploid egg (ovum) cell is fertilised by a haploid sperm cell to form a diploid zygote cell. This then grows into an adult organism. Genetic material (DNA) from both parents is combined in the zygote. Offspring are therefore similar to their parents, but not identical. This is called sexual reproduction and leads to genetic variety in offspring.
There are 23 pairs of chromosomes in humans (23 single chromosomes from each of their parents). The 23rd pair are called the sex chromosomes, and the combination of these determines whether offspring are male or female.
In women, both sex chromosomes are the same and are therefore called XX. In males they are different and called XY. When sperm and eggs (ova) are made during meiosis they separate to form different gametes.
As women have XX, their eggs always carry the X chromosomes. As males have XY, approximately half their sperm carry the X chromosome while the remainder carry the Y chromosome.
The gender of any baby is determined by whether an egg is fertilised by an X or Y sperm.
Gender-linked genetic disorders
There are a number of genetic disorders that are linked to the sex chromosomes, and so are inherited in different frequencies by men and women. The probabilities of inheriting these can be calculated in a similar way to the way you would work out the chances of inheriting other monohybrid inheritance disorders.
People with haemophilia have problems forming blood clots. This disorder is caused by a recessive gene on the X chromosome. Men only have one X chromosome, so they are more likely to have this disorder. Women with only one copy of this disorder do not have the symptoms - but are able to pass it to their children. Queen Victoria was a ‘carrier’ of this disorder and passed it to her son Leopold.
There are varying degrees of colour blindness - the inability to see colour or colours. A common example is red-green colour blindness, where people would not be able to see the number in the picture below. Colour blindness is also caused by a recessive allele on the X chromosome, so men are more likely to have this disorder too.
Microorganisms and immunity
There are three main types of microorganism: bacteria, viruses and fungi. Individually, they are too small to see without using a microscope, although groups (or colonies) of them can often be seen with the naked eye.
Bacteria grow by a process of replicating themselves. This is called binary fission. When bacteria possess the nutrients and appropriate conditions for growth, they are able to replicate very quickly indeed. This increase is called exponential growth and can lead to the rapid development of an infection. Exponential growth is shown in the graph below.
Bacteria grow using a medium, such as agar gel or milk, from which they obtain nutrients.
You need to know how to investigate the growth of bacteria in milk. One way of doing this is to use a dye called resazurin. This can be added to milk kept at different temperatures to see the relative growth of bacteria.
Resazurin changes from blue to pink to white as the presence of bacteria increases. A quicker change in colour indicates the presence of more bacteria.
A weakened or genetically-modified version of a pathogen is found or made.
This is given to the person to be vaccinated (normally by injection).
The person’s immune system responds - their body notices antigens(foreign proteins) on the pathogens.
White blood cells (lymphocytes) then produce antibodies which destroy the pathogens.
The first time we are infected with a pathogen it takes some time to produce the antibodies to fight it off.
However, our lymphocytes remember pathogens - and if we are exposed to the pathogen again, the lymphocytes respond by making antibodies much more quickly. This means that we don’t usually catch the disease a second time, and are now immune to it.
Active immunisation is most common. This can be artificial - as a result of a vaccination (see previous page). It can also be natural. Here the process is the same, but results from a natural exposure to a pathogen. Catching a common cold each winter is an example of this. We don’t catch the same cold each winter. There are over 200 known viruses which cause the same symptoms. After we have caught each one, we are immune to it.
Passive immunisation is less common and involves someone being given antibodies. This can also be natural (from a mother’s milk to her baby) or artificial, in the form of an injection.
Advantages and disadvantages of vaccinations
The advantages of vaccinations are obvious - they stop individuals becoming ill. If enough people are vaccinated, vaccinations can also stop pathogens infecting whole populations. This is called herd immunity.
There are also risks to having vaccinations. For instance, some people suffer from a mild reaction to the vaccine.
In recent years there has been much controversy surrounding the MMR vaccine. Some people used to think the vaccine - which is a combined treatment against measles, mumps and rubella - could cause autism in children. They decided not to risk letting their child have the vaccine and just hoped they would not catch measles, mumps or rubella.
But this meant that, as fewer and fewer children were vaccinated, the three diseases began to spread more easily and the number of cases began to increase. More recent studies by the World Health Organisation have shown that there is no link between the MMR vaccine and autism.
Vaccinations can never be completely safe because side-effect levels vary. So, when making a decision, these are some of the factors that need to be considered:
when fewer people are vaccinated, the number of cases of the disease increases
the chance of falling seriously ill or dying from the disease may be far greater than the chance of experiencing a serious side-effect
using a vaccine may be much cheaper than treating a very ill person
Memory lymphocytes and making antibodies
There are different types of lymphocytes but you only need to know about B lymphocytes which are produced and matured in bone marrow.
They produce the plasma cells that form the antibodies. They also produce memory cells that stay in the body for a long time - sometimes forever - and can trigger the production of antibodies if reinfection occurs.
Monoclonal antibodies are identical copies of antibodies that have been made in laboratories. They have a number of different uses. They need to be made in large numbers to work properly
Monoclonal antibodies are used in a wide variety of ways. They are used in pregnancy test kits to identify the small levels of a hormone called human chorionic gonadotrophin, which is present in the urine of pregnant women. They can also be used to locate blood clots as they bind to clots.
They can also be used to diagnose and then treat some cancers. They can bind to the cancerous cells and help the person’s immune system attack them.
This form of cancer treatment has a number of advantages over radiotherapy and chemotherapy, both of which can have unpleasant side effects including hair loss, suppression of immune response (meaning you are susceptible to infection by pathogens) and reduction in the ability to clot blood (meaning wounds take longer to heal and bruises are more extensive).
Louis Pasteur and aseptic technique
Louis Pasteur was a French scientist who lived in the 19th century. He discovered the first vaccine for rabies - but is more famously remembered for aseptic technique and pasteurisation (which is, of course, named after him).
Pasteur is famous for three experiments involving microorganisms.
He proved that microorganisms turned liquids sour.
He proved that microorganisms couldn’t grow if the liquids were boiled (this is pasteurisation).
He proved that microorganisms did not generate out of thin air but transferred from one medium to another (in the 19th century, people thought microorganisms just appeared).
Plant defences against microorganisms
Plants, like humans, are also attacked by pathogens. Two well-known examples of plant infections are Dutch elm disease and tobacco mosaic virus.
Dutch elm disease is a fungal disease that first appeared in the UK in the 1920s. It is spread by the elm bark beetle. The disease was able to kill many elm trees that had not managed to evolve a resistance to it.
The disease is thought to have originated in Asia, before it made its way to Europe and North America. A particularly aggressive strain took hold in the 1960s. It had a devastating impact - both in the UK and across Europe. By the end of the 20th century very few mature elm trees remained.
Tobacco mosaic virus was the first plant virus to be discovered (in 1930). However, the effects of this virus on the tobacco plant were well known for decades before its discovery. It attacks the leaves making them mottled or discoloured.
Plants do not have white blood cells and cannot produce antibodies to fight infection. However, they can produce some substances to defend themselves. These include several substances which humans have identified and we now use for our own purposes:
quinine is a substance produced by cinchona trees – it has been used to treat malaria
aspirin is produced by willow trees - it reduces pain and fever
The cost of plant infections
Infections of plants can have a devastating impact on the food supply.
Potato blight is a fungus-like organism that attacks potatoes and some tomato plants. Its spores live on the parts of plants that survive underground during winter (these parts are called ‘tubers’).
The disease can destroy entire crops. The leaves of plants become blotchy before mould appears on them. Then all parts of the plants become covered in the infection and rot away.
Potato blight was one of the factors that caused over one million Irish people to starve to death between 1845 and 1857 during the Irish Potato Famine.
A fungus is responsible for rice blast – which is found in over 80 countries. Lesions appear on the leaves and shoots of rice plants, flowering heads rot, and fewer seeds are produced. Each year, the disease causes massive waste. It is thought to destroy a quantity of rice equal to the amount that could have fed over 60 million people.
Photoperiodicity and circadian rhythms
Many plants detect and respond to changes in the length of daylight to control their growth and reproduction. This is called ‘photoperiodicity’. This process helps plants to flower and reproduce, germinate, grow roots and shoots, and drop their leaves at the appropriate time.
Animals and plants also live by circadian rhythms. These are almost 24-hour cycles.
Plants use these cycles to close their flowers at night and rotate their leaves towards the Sun during the day.
Animals, including humans, use circadian rhythms to regulate sleeping and feeding patterns, as well as hibernation and migration (eg the Eastern North American monarch butterfly). Jet lag is the result of circadian rhythm disruption.
Animal and plant behaviour
Animals and birds use different ways to attract a mate. Many male animals and birds use courtship behaviour to attract a female. This is seen in the spectacular way male frigate birds inflate their large red throat sacks, in the colourful display of feathers in male peacocks, and in way that male squirrels prove their fitness to potential mates.
Many animals and birds don’t mate for life. They have several different sexual partners in their lifetime or during one breeding season. Often there is one dominant ‘alpha male’ that mates with all of the sexually-mature females in his group. The alpha male is usually the largest or strongest male. This behaviour is seen in lions and also sea-lions.
Some animals are monogamous - they mate with one partner for life. This behaviour is seen in puffins and albatrosses. It is very unusual in mammals.
Many animals and birds look after their young in a variety of ways. These behaviours are called parental care. This gives their young the best possible chance of survival to ensure that the genes of the parents are passed on.
Female mammals carry their young in their uterus before they are born. An animal that does this – gives birth to living young rather than laying eggs - is said to be a viviparous animal. Once born, mammals care for their young by producing milk. The mother’s milk provides the baby with all the nutrients it needs. Suckling from their mother is also a relatively safe place to feed.
Birds also look after their young. Parents incubate eggs until the chick is ready to be born.
An extreme example of this can be seen in the behaviour of the male Emperor penguins in Antarctica. The male bird stands and incubates the egg for approximately two months in freezing cold winds – without eating any food - until his partner returns. The males can lose up to half their body weight in the process.
All newborn chicks are fed by one or both of their parents until they are old enough to leave their parents and live on their own.
The killdeer bird displays an unusual type of parental care behaviour. It nests on the ground and when predators try to take its eggs or chicks, it lures them away by pretending it has a broken wing. It’s a risky strategy for the parent bird but helps to give their young a good chance of survival.
Types of behaviour
Behaviour is defined as the response of an animal to a stimulus. Some responses are innate. They are not learned – they are instinctive and happen automatically. A newborn pup sucking milk from its mother is an example of an innate behaviour. Other behaviours are learned. These are called conditioned behaviours, and there are four types: operant, habituation, imprinting and classical.
This type of learned behaviour occurs by rewarding or punishing an animal. Teaching a dog to jump through a hoop by giving it treats is operant conditioning. This type of conditioning can be used to train:
sniffer dogs to find illegal drugs or bombs
police horses to remain calm in crowds or riots
dolphins (at sea life centres) to jump through hoops
Habituation is where an animal becomes steadily used to a stimulus or situation. It is sometimes known as a simple learning or desensitisation process. An example of habituation would be the action of prairie dogs which have lived alongside humans for some time. They have become familiar with the scents of humans in their territory and no longer make alarm calls when a scent is found.
Types of behaviour
Imprinting is the tendency of young animals to follow the first moving object they see. This is usually the mother. Imprinting usually occurs during a short, but critical, period of a young animal’s life.
This type of learned behaviour occurs without rewarding or punishing. Many dogs will run towards the door to begin their walk when their owner shakes their lead. This is classical conditioning.
A Russian scientist called Ivan Pavlov completed a famous experiment into classic conditioning. He observed that his dog produced lots of saliva when he showed it food. Every time he fed his dog, he rang a bell for a short while afterwards. Eventually, just ringing the bell alone was enough to make his dog salivate. It had been conditioned into salivating when it heard the bell - and not just when it saw food.
Communication in plants and animals
Animals are able to communicate with each other in many different ways. Some of the most important ones are given below.
There are many examples of this behaviour. Snakes hiss to warn off an approaching threat. Rabbits and hares thump their feet on the ground to alert each other when predators are nearby. And one or more meerkats will act as lookout and make loud noises to warn the rest of their group of danger.
Whales and dolphins are capable of making sounds under water to communicate with each other. Whale song can be heard over hundreds of kilometres.
A common example of a visual display that mammals exhibit is baring their teeth. In dogs and cats this is clearly a warning display.
Another interesting example is seen in honey bees returning to their hive after finding a nectar source. They complete a dance to show the other bees the location of the nectar. It is known as the waggle dance.
Communication in animals and plants
Many animals use chemicals to communicate. Dogs and cats mark out their territories by urinating on the boundaries. Skunks produce an unpleasant chemical smell to ward off predators. Pheromones are scent chemicals produced by some insects and some vertebrates. Female dogs produce pheromones from scent glands and in urine to provide their newborn pups with a feeling of comfort. Some species - including ants and bees - produce chemicals when attacked to warn others away from their colony or hive. Ants also mark their paths with chemicals.
Humans have evolved extremely complex ways in which we communicate through language. There are over 5,000 different languages spoken around the world - including sign language, which uses hand patterns and facial gestures in place of words. The most widely-spoken language is Mandarin Chinese.
Communication in plants
Plants also communicate with each other - and with other animals (particularly insects).
some plants release chemicals to warn nearby plants of attack
others have brightly-coloured flowers or flowers with bold patterns to attract insects for pollination
other plants attract pollinating insects with enticing scents
An ethologist is a scientist who studies animal behaviour. There are four famous ethologists you need to know about.
Jane Goodall (1934- ) is a British ethologist who has spent her life studying chimpanzees in Africa. She famously observed that they have distinct personalities and are capable of behaviour like hugging and tickling each other. Perhaps her most famous observation was that they are capable of using tools.
Dian Fossey (1932-1985) was an American ethologist who studied mountain gorillas in Africa. She lived very closely with them and they became habituated to her. Fossey famously became the first person to be recorded making peaceful contact with a wild gorilla. A photograph (taken in 1969) shows a young male named Peanuts touching her hand. She spent her later years working to prevent poaching.
Konrad Lorenz (1903-1989) was a German ethologist who won the Nobel Prize with Nickolaas Tinbergen (see below). He is most famously remembered for his work on imprinting. This is when young animals - often birds - copy their parents. If newly-hatched chicks first see another animal they can imprint on them instead of their own parents.
Nikolaas Tinbergen (1907-1988) was a Dutch ethologist who also won the Nobel Prize. He studied gulls and showed that their chicks instinctively knew to peck at red spots on their parents’ beaks to encourage them to regurgitate food.
Choice chambers are small boxes that have areas with different conditions. Animals, often woodlice, are put inside and their ‘choice’ for the different conditions is recorded by counting the numbers in each area after a short period of time. Typical experiments involving woodlice have combinations of light, dark, dry and damp areas. We would expect to see more woodlice in the dark and damp sections of the choice chamber. This is most like the conditions they like in real life - we tend to find woodlice under rocks and rotting wood.
Evidence for human evolution
Charles Darwin’s theory states that all organisms alive today evolved from more simple life forms. Two fossils named Ardi and Lucy provide evidence for human evolution. Both were found in Africa.
Ardi is a female human-like fossilised skeleton that dates from 4.4 million years ago. The bones that make up Ardi’s feet suggest that humans and chimpanzees evolved separately.
Lucy is also a female human-like fossilised skeleton, and dates from 3.2 million years ago. Lucy’s bones suggest that she walked in an upright position, like a human, but possessed a relatively small ape-like skull.
Many human-like fossils that provide additional evidence for human evolution were found by the archaeologists Mary and Louis Leakey. Mary was British and her husband, Louis, was Kenyan. The couple discovered fossils which date from 1.6 million years ago.
Tools also provide evidence for human evolution. Primitive tools (flint hand axes) have been found in remains from the Palaeolithic Age (10,000 to 2.5 million years ago). More advanced tools (arrowheads) have been found from the Mesolithic Age (6,000 to 10,000 years ago), and even more advanced tools have been found from the Neolithic Age (4,000 to 6,000 years ago). These dates are only approximate, and the tools have been dated from the environments they were found in.
Genetic evidence for human evolution
Our genetic code - DNA - is found within chromosomes in the nuclei of the majority of our cells. DNA is also present in our mitochondria. These are cell organelles found in the cytoplasm of our cells, and are where respiration occurs.
Scientists have now made a ‘mitochondrial DNA family tree’ by looking at mutations in this type of DNA. It is passed only from mothers to their children and so scientists have traced this family tree back to a common female human ancestor called African Eve. She is likely to have lived in Africa about 200,000 years ago.
Mitochondrial DNA is a better choice for looking at our evolution compared with nuclear DNA because it:
has a high mutation rate
is less likely to have degraded over time
is more abundant (common)
Darwin's theory of evolution states that all organisms alive today evolved from more simple life forms. Occasionally some animals and plants evolve together in a process called co-evolution.
There are two examples of co-evolution that you need to know:
- Hummingbirds and the flowers that they feed on have co-evolved. The flower is pollinated when the hummingbird drinks its nectar. The flower attracts the hummingbird whilst the bird’s beak is curved to allow it to reach the nectar.
- The caterpillar of the Old World Swallowtail butterfly has evolved to be resistant to the chemical defences of the fringed rue plant. This means that the caterpillar can feed on the plant without being poisoned.
Climate change and human behaviour
The end of the last ice age is thought to have been at the end of Palaeolithic Age. After this time, humans are thought to have spread quickly across Europe from Africa and might have crossed the Bering Strait Bridge (a stretch of land that joined Alaska - and so America - to Russia).
During the ice ages, human behaviour changed to suit the conditions. Larger animals were hunted, people probably lived in larger groups and travelled shorter distances. Approximately 100,000 years ago, all human ancestors had dark skin – but a lighter skin colour developed when the humans moved to colder climates with much less sun.
Fermentation reactions occur when microorganisms take in food and convert it into substances which are useful to them. The microorganisms also release waste substances such as carbon dioxide. The most common example of fermentation is when yeast (Saccharomyces cerevisiae) – a single-celled fungus - converts sugar (glucose) into alcohol. Here are the word and balanced formulae equations for this process: glucose → ethanol + carbon dioxide
The following conditions are maintained to maximise growth rates:
The fermenter is kept aseptic so only the desired microorganism grows.
Nutrients are provided to ensure that the microorganisms always have enough food to grow.
The optimum temperature and pH is maintained to ensure maximum growth.
There is an oxygen supply because most fermentation reactions are aerobic.
Agitation (stirring) takes place to ensure that the microorganisms, nutrients and temperature are evenly distributed.
Microorganisms and food production
We use microorganisms to make a large number of our food and drink products – these include bread, yoghurt, cheese and alcohol. Microorganisms are useful because:
they grow rapidly
they have DNA which is easy to manipulate
they can be grown in fermenters in almost any location (the local weather doesn’t normally affect their growth)
they can be grown using the waste products from other industrial processes
Mycoprotein - This is a general name for all the protein that is grown from fungi. It is commonly made in fermenters and grown from the fungus Fusarium. This protein is used instead of meat in a large number of vegetarian foods. Protein is important for growth and repair. Mycoprotein has the added advantage of being low in fat.
Yoghurt - Making yoghurt also uses microorganisms. Here bacteria ferment the milk and change it into yoghurt. Lactose is the main sugar in milk and the bacteria convert this into lactic acid. This increased acidity sours the milk, giving yoghurt its sharp taste. The lactic acid also helps to thicken yoghurt. Bacterial fermentation can be summarised by the following equation: Lactose →(bacteria) → Lactic acid
Enzymes are biological catalysts. They increase the rate of chemical reactions without being used up.
The enzyme invertase (sometimes called sucrase) is used by confectioners to make toffees, chocolates, mints and other soft centres. It is often produced by yeast and breaks down the sugar sucrose into two other sugars called glucose and fructose.
Many ‘biological’ washing powders now have enzymes in them to help break down and remove stains. The two types of enzymes used are:
proteases - which break down proteins into amino acids
carbohydrases - which break down carbohydrates into sugars
For many years, cheese was only made using the enzyme chymosin. This used only to be obtained from the stomachs of calves and so strict vegetarians were not able to eat cheese made using chymosin. Now the chymosin is produced by microorganisms after they have been genetically modified. This means that cheese made using chymosin from microorganisms no longer contains animal products - and so vegetarians are able to eat it.
Growth of Yeast : To investigate the growth of yeast, add a standardised small amount of it to a known volume of sugar solution in a series of test tubes. Incubate them at different temperatures and measure the height of the froth produced. A higher level of froth indicates more fermentation. Notice that more froth is produced up to an optimum temperature after which the amount of froth produced reduces.
Yoghurt making : To investigate yoghurt making, heat some milk in a beaker at 40°C for a few minutes. Add a starter culture of bacteria (Lactobacillus), and cover and incubate until it sets. You could check whether differences in temperature or pH affect the time it takes for the yoghurt to set.
Production of lactose-free milk : Lactose is the main sugar in milk that is broken down into galactose and glucose by the enzyme lactase. Measure out two identical volumes of milk. Add some lactase to one and keep both volumes in the same conditions overnight. In the morning use Benedict’s solution to test for glucose - which should only be present in the milk to which you added the enzyme.
Enzymes in food production : Pectin is a sugar which is found in the cell walls of plants. It is broken down by the enzyme pectinase. Cut up two identical portions of fruit. Place one in a beaker of water (the control) and the other in a beaker of water containing pectinase. After five minutes filter both solutions and record the volume of liquid produced. The portion of fruit exposed to the pectinase should produce more liquid.
Recombinant DNA technology
This process involves moving the gene (or genes) responsible for making a protein into a different organism. It commonly involves moving genes into bacteria which can then be grown in huge numbers in fermenters. These will make large quantities of the protein which can be collected. All insulin used by people with diabetes is now made in this way. In the past, it was made from the pancreases of pigs and oxen.
The process for making insulin using recombinant DNA technology is as follows:
The human gene for insulin production is identified and removed using enzymes called restriction enzymes.
The same restriction enzymes are used to cut open a plasmid (a small, circular section of DNA).
A ligase enzyme is then used to seal the human gene into the plasmid.
The plasmid is then inserted into a bacterium - which is grown into huge numbers of bacteria that all produce the human insulin.
Restriction enzymes : Restriction enzymes do not cut directly across the double strand of DNA because this would involve cutting any section of DNA into many different pieces and it would not be easy to remove an entire gene. Instead they cut across the double strands at two different places. The place where they cut across the DNA is called a sticky end. Restriction enzymes can be used to cut out specific genes, and also cut open places in the plasmid DNA where the genes will fit exactly.
Biotechnology and the world food supply
This rise in population is partly responsible for the lack of food that exists in certain countries. Other reasons for the shortage of food include:
poor quality soils and water shortages
poverty - meaning people can’t afford to buy seed or equipment to grow crops
To meet the demands of an increasing population, we must increase the amount of food we produce. This can be done by conventional plant-breeding programmes and pest management strategies. It can also be done by genetic modification. This is when a section of DNA is moved from one organism into another.
There are several common examples of genetic modification:
Vitamin A deficiency - which is common in some African and south-east Asian countries - often leads to blindness in children. To combat a deficiency in this particular vitamin, scientists have produced a sweet potato with increased levels of vitamin A.
Biotechnology and the world food supply
Tomatoes have now been genetically modified to have an antioxidant pigment called anthocyanin (a type of flavonoid). This is thought to have anti-cancer properties. It is present in high levels in purple-coloured fruits like blackberries. When tomatoes were genetically modified they turned purple.
This is a bacterium which is used as a vector when scientists create some genetically-modified plants.
A common example is how scientists used Agrobacterium tumefaciens to make herbicide-resistant crops. The process is as follows:
The DNA is inserted into Agrobacterium tumefaciens which is then inserted into the embryos of the crop plant.
These plants then grow into adult plants which are resistant to the herbicide - so that when the fields are sprayed, only weeds are killed.
This bacteria naturally produces a toxin which is poisonous to many insects. The gene for producing this poison has been inserted into crop plants which are now resistant to these insect pests. However, there are advantages and disadvantages to doing this.
The advantages of using this type of bacteria are:
Less insecticide has to be used
Crop yield is higher
The disadvantages of using this type of bacteria are:
The toxin could kill other, harmless insects
The Bacillus thuringiensis gene could be transferred into other wild plants
Some insect species have already evolved resistance to the toxin
The ethics of genetically modifying crops - Not all people agree with genetic modification. Some people think that there might be long-term problems that scientists don’t know about yet, or that genes from genetically-modified crops might spread to other plants and make ‘superweeds’.
A biofuel is a renewable fuel made from sustainable sources such as animal or food waste, wood and alcohol. Biofuels are a ‘green’ alternative to fossil fuels. Common examples include using vegetable oil or alcohol in cars, sometimes mixed with petrol.
A biodiesel refining plant in Motherwell
35,000 tonnes of old cooking oil and animal fat is filtered each year and chemically converted into diesel fuel
Biofuels are carbon neutral. This means that they only release as much carbon dioxide when they are burnt as was used to make them originally by photosynthesis. In this way, they don’t increase the amount of carbon dioxide in our atmosphere.
Some people worry that biofuels also have their drawbacks – for instance, large areas of land are used to grow some biofuels. This area of land might have been forest before, but now cannot act as a ‘carbon dioxide sink’ (a process whereby carbon dioxide is removed from the atmosphere). Also, the land now cannot be used by local people to produce their food.