Biology B2

• Osmosis is a special type of diffusion involving the movement of water through a selectively permeable membrane. The water will move from where it is in higher concentration to where water is in lower concentration.

Osmosis in Plant Cells

• Water can move in and out of plant cells depending on the concentration of the solution surrounding the cells.
• When water moves into a plant cell, the vacuole increases in size, pushing the cell membrane against the cell wall.
• The cell wall prevents too much water entering and stops the cell from bursting unlike animal cells.
• The force of the extra water pushing against the cell wall makes it Turgid. This gives the plant structure and is essential in keeping plants upright. This is why wilting occurs when there is a lack of water. Cells that are not turgid are Flaccid.
• When too much water leaves plant cells plasmolysis occurs. During plasmolysis so much water leaves the cell that the cell contents shrink, pulling the cell membrane away from the cell wall.

Transpiration- is the loss of water from the leaves of the plant by evaporation. Evaporation takes place through the stomata of the leaf. A constant stream of water through the plant is for 3 reasons-

• Supply of water for photosynthesis
• Transport of minerals through the xylem system and into leaves
• Water enters cells by osmosis to provide support
• To conserve water the stomata can close stopping water loss and gas exchange.

Measuring water uptake using a Potometer-  measures the water uptake of a leafy shoot. As water is evaporates from the leaves of the cut shoot, the shoot sucks water up through the potometer. The distance the air bubble moves in a period of time can be used to calculate the rate of water uptake

• The reservoir allows the apparatus to be reset so that replicate results can be recorded or the water uptake can be measured in different environmental conditions. It is important that the apparatus is sealed properly to prevent unwanted air bubbles entering the equipment

Factors Increasing Water Uptake- Wind Speed/Temperature/Humidity/Higher temperature, higher wind speed and low humidity will INCREASE rate of evaporation./Lower temperature, lower wind speed and high humidity will DECREASE rate of evaporation.

The Circulatory System has three main components- the blood, blood vessels and the heart. It has two main functions- Transport of Blood Cells, absorbed food, hormones, urea etc. Also the Protection Against Disease

The blood is composed of many different materials and acts as a solute for the transport of substances around the body. It is essential for gas exchange and respiration as it carries food particles, oxygen and carbon dioxide.

Composition of Blood-

• Red Blood Cells- the function of these cells is to carry oxygen around the body. It is the haemoglobin which enables them to do this. It has a large surface area due to the biconcave shape and does not contain a nucleus.
• White Blood Cells- the blood contains two types of white blood cell- Lymphocytes and Phagocytes. Lymphocytes produce antibodies while phagocytes engulf bacteria.
• Platelets- these are important in blood clotting and the formation of scabs.
• Plasma- this is the liquid part of the blood. The plasma is responsible for the transport of the blood cells, absorbed food molecules, carbon dioxide, hormones and urea.

The heart pumps the blood to the lungs and around the body. This is why the heart has two sides- the right hand side pumps the blood to the lungs while the left hand side pumps the blood to the body. This is why the left hand side is thicker, as more pressure is needed to pump the blood further.

• The right atrium receives deoxygenated blood from the body. This passes into the right ventricle where it is pumped out in the pulmonary artery to the lungs.

• In the lungs the blood becomes oxygenated and returns to the left atrium through the pulmonary vein. The oxygen-rich blood passes into the left ventricle and is pumped into the aorta around the body.

•  Humans have a double circulatory system which means that the blood travels through the heart twice in one circulation.

• The heart itself receives blood from the coronary arteries, which branch from the aorta immediately after leaving the heart. These are fine vessels which run over the surface− 

Arteries – carry blood under pressure away from the heart (usually oxygenated blood) A=AWAY

Veins– carry blood back to the heart and have valves which prevent the backflow of blood (usually deoxygenated blood) veIN=In

Capillaries – allow the exchange of material with tissues through permeable walls;

Heart Disease-  is caused by cholesterol being present in high levels in the arteries. Over time this leads to narrowing of the arteries, making it difficult for blood to flow through them. This is particularly likely to happen in the coronary arteries, hence Coronary Heart Disease.

Eventually the blood may be totally blocked stopping cells from receiving oxygen and glucose. This will lead to death of heart muscles and for the heart to stop beating; this is a heart attack.

Strokes- If a blockage occurs in the brain, a stroke may result. Again, cells are deprived of oxygen and glucose so die and this stops the brain from functioning correctly. This may lead to paralysis.

Prevention

• Regular exercise helps the body in many ways-
• Helps reduce heart disease or strokes by burning fat which may clog up arteries.
• Heart muscle will be strengthened allowing an increased output of blood.
• Burns fat which may otherwise lead to obesity.
• When we exercise our body needs more energy and the heart has to pump more blood to our muscles so that they have enough oxygen for respiration.

Pastuer- Previous to Pasteur’s work it was assumed that microorganisms appeared from nowhere, this was known as Spontaneous Generation. Pasteur proved that microorganisms were not spontaneously created. Instead they were always present in the air. This is proved as juice open to the air would be contaminated while juice that was not directly open to the air would not be contaminated.

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Defence Mechanisms of the Body- The human body is well adapted to protect us against infection. The body is successful in preventing most microorganisms from gaining entry and it has effective defences if microorganisms do enter.

• Skin- The skin is an excellent barrier to microorganisms. Any openings in the skin such as the nose and respiratory system have a mucous membrane which traps microorganisms.
• Clotting- This stops blood from escaping but also stops microorganisms from entering through the cut in the skin.

Antigens and Antibodies

• Invading microorganisms have chemicals on their surface that the body can recognise to be foreign. These are antigens and they cause lymphocytes to produce antibodies.
• Antibodies have a shape that is complimentary to the antigens. The antibodies join with the microorganisms and cause them to clump together. Once this happens, they are easily destroyed by phagocytes in a process known as phagocytosis.

Phagocytosis-

• White Blood Cells can surround microorganisms and engulf them. Chemicals inside the phagocyte can then digest the microorganism.

Immunity

• Natural- Innate (From Birth antibodies passing from mother to baby across the placenta)  and Acquired (develops during life- catching the disease and recovering)
• Artificial- Active (having a vaccination- this causes a body to produce its own antibodies) and  Passive (antibodies injected into the body)

• Vaccinations involve the use of dead or modified pathogens that are injected into the body. The pathogens still have the antigens on their surfaces that cause the body to make antibodies at a high enough level to stop the individual becoming ill later.
• The process is exactly the same as if you actually caught the disease except you don’t get sick. Sometimes boosters are needed to prolong the effect over a human lifetime.

Edward Jenner and the First Vaccination-

• Jenner noticed that milkmaids who had had the minor illness cowpox did not catch the serious illness smallpox. He concluded that having cowpox protected them from smallpox.
• He tested this by deliberately infecting a young boy with cowpox. After a period of time, he infected him with smallpox. The boy did not catch smallpox. As the cowpox was so similar to the smallpox microorganism, the boy had built up immunity to both.

Vaccination and Travel-

• People who travel to some foreign countries are required to be vaccinated against certain diseases. This is because some countries have pathogens that cause serious illnesses that do not exist in the British Isles. We will not have any antibody defences against these pathogens as we have not been in contact with their antigens before.

Antibiotics e.g. Penicillan- These are chemicals which can damage and kill living microbes. They can be used against bacterial infections. Antibiotics kill bacteria or stop their growth. Antibiotics combat a range of bacteria and they act in a different manner to antibodies.

Overuse fo Antibiotics

• Bacterial resistance to antibiotics is becoming a major problem and is making many antibiotics ineffective against various bacteria. The overuse of antibiotics is largely responsible and it is important that antibiotics are only used when necessary.
• Some bacteria have developed resistance to the extent that they are now referred to as Superbugs. These bugs are resistant to most types of antibiotic and can be a very serious problem in hospitals.
• The spread of Superbugs can be reduced by good hygiene and cleanliness. A greater care in administering antibiotics and only when necessary can also reduce the effect of Superbugs.

Alcohol-Many people drink alcohol when socialising and are unlikely to suffer any serious harm if drank in moderation. However, many people, including many teenagers, drink too much and cause harm to themselves.

Why do teenagers drink too much- Peer Pressure-Experimentation -Escape from problems]

Harm caused by alcohol-Violence- Absence from college or work-Family breakup-Breakdown in relationships-Drink-driving-Binge Drinking is a particular problem. 

Reducing the harm caused by Alcohol- Drink less each time, drink low-alcohol drinks or drink more slowly-Drink on fewer occasions-Education on the effect of alcohol-Never drink or drive-Do not drink until the legal age limit of 18

Smoking- can seriously damage health and has been banned in many countries in certain situations.

Tar- Causes bronchitis, emphysema and lung cancer

Nicotine- Addictive and affects heart rate

Carbon Monoxide- Combines with red blood cells to reduce the oxygen carrying capacity of blood

Illegal Drugs-

Cannabis- It is widely used throughout the UK, due to its availability and low cost. When taking cannabis, people feel relaxed or “chilled out”. Cannabis can lead to mental health problems and to taking more damaging drugs.

Cocaine- Cocaine can give users a “high” and is very addictive. It is short lived so users normally increase their dose. Overdose can result in death.

Both of these drugs are illegal. Cocaine is a Class A drug which is the most dangerous category while Cannabis is a Class B drug. 

Chromosome- Most living cells contain a nucleus. The nucleus is the control centre because it contains chromosomes that are subdivided into smaller sections called genes. There are hundreds of genes in each chromosome. Chromosomes occur in functional pairs except in sex cells.

Genes- It is the genes in our body that control characteristics such as eye and hair colour. Inside genes and chromosomes there is a very important molecule that gives them their properties

DNA- is deoxyribonucleic acid. Genes are short lengths of DNA that code for a particular protein or characteristic.

• DNA consists of two phosphate and sugar strands held together by bases linked by hydrogen bonds. This unit is repeated along the length of the DNA molecule.
• The four bases can combine only in the order
• Adenine—Thymine
• Guanine—Cytosine
• These are referred to as Base Pairings.
• One unit of sugar, phosphate and base is called a nucleotide.
• These bases form two separate strands which then join together with hydrogen bonds and twist into the double helix shape.

Chromosome- Most living cells contain a nucleus. The nucleus is the control centre because it contains chromosomes that are subdivided into smaller sections called genes. There are hundreds of genes in each chromosome. Chromosomes occur in functional pairs except in sex cells.

Genes- It is the genes in our body that control characteristics such as eye and hair colour. Inside genes and chromosomes there is a very important molecule that gives them their properties

DNA- is deoxyribonucleic acid. Genes are short lengths of DNA that code for a particular protein or characteristic.

• DNA consists of two phosphate and sugar strands held together by bases linked by hydrogen bonds. This unit is repeated along the length of the DNA molecule.
• The four bases can combine only in the order
• Adenine—Thymine
• Guanine—Cytosine
• These are referred to as Base Pairings.
• One unit of sugar, phosphate and base is called a nucleotide.
• These bases form two separate strands which then join together with hydrogen bonds and twist into the double helix shape.

• Erwin Chargaff- In 1950 he discovered that although the arrangement of bases in DNA varied, there was always an equal amount of adenine and thymine. Similarly there was always an equal amount of guanine and cytosine.

• Rosalind Franklin & Maurice Wilkins- They used x-ray diffraction in which x-rays are fired into molecules of DNA and the ways in which the DNA scatters the x-rays provides information about its three-dimensional structure.

• James Watson & Francis Crick- They built on the work of the previous two scientists to deduce how the bases were arranged and also to conclude that the DNA molecule is arranged as a double helix.

Mitosis- allows organisms to grow, to replace worn out cells and to repair damaged tissue;

• Most living organisms grow by increasing their cell number. Cells double in number by splitting in half. It is important that the two new cells created are the exact same as the original cell- they are clones.
• This means every cell in the body has the same number and kind of genes and chromosomes.

Asexual reproduction- produces genetically identical offspring. This is because gametes are not involved and the cells from the adult simply reproduce identical copies of themselves by mitosis to form a new individual

• Because new plants are identical, they are referred to as clones. The big advantage is that only one parent is needed.
• Advantages include the fast production of new plants and the fact that desirable qualities are passed down.
• However, because there is no variation, all of the plants are equally susceptible to a particular disease.

Gregor Mendel discovered the basic rules of genetics by carrying out experiments with pea plants. He crossed tall plants with dwarf plants and allowed the seeds to develop. These seeds were then planted and new plants grew up. Plants are crossed by taking pollen from the anther of one plant to the stigma of the other plant.

• Each parent plant contains two genes for height, one on each of the pair of chromosomes.
• Tall = TT
• Dwarf = tt
• During Meiosis the pair of chromosomes separate and one goes into each gamete. So the gamete contains either one T gene or a t gene.

• During fertilisation the T and t genes are brought together, so the F1 offspring contain one T and one
• t gene. We call this Tt.

• The F1 (First generation) are all tall. This is because the T is Dominant and suppresses the t gene which is called Recessive.
• The F1 offspring are inter-crossed and the resulting seeds formed are allowed to grow. This generation is known as the F2 Offspring.
• The F1 offspring are crossed with each other to obtain the F2 offspring.
• The F2 offspring are a mixture of tall and dward plants in the ratio of 3:1.
• The TT plants are tall,
• The Tt plants also are tall because T allele is dominant to the recessive t allele.
• The tt plants are dwarf.
• If an individual has two genes coding for the same allele e.g. flower colour. They are said to be homozygous.
• TT – Homozygous (Tall)
• tt – Homozygous (Dwarf)
• If they have two genes coding for different alleles, they are said to be heterozygous.
• Tt – Heterozygous

Alleles: These are different forms of the same gene, found at the same locus on a pair of homologous chromosomes E.g. the gene for height can have the alleles T or t.

Homozygous: An organism is homozygous for a trait if it has 2 identical alleles, found at the same locus on a pair of homologous chromosomes. E.g. Tall (TT) and dwarf (tt).

Heterozygous: An organism is heterozygous for a trait if it has 2 different alleles, found at the same locus on a pair of homologous chromosomes. E.g. Tt (Tall)

Dominant: A gene is dominant if it produces the same trait whether it is present in the homozygous or heterozygous condition. E.g. The genes for tall can be TT or Tt.

Recessive: A recessive gene is only expressed when it is present in the homozygous condition. E.g. a gene for dwarf is tt.

Genotype: This is the genes the organism has for a particular trait. E.g. the trait for tall has the genotype TT or Tt.

Phenotype: This is the physical expression of the genotype. E.g. the genotype TT gives a tall individual

• Humans have 22 pairs of normal chromosomes and one pair of sex chromosomes. The male sex chromosomes are XY and females have two ** chromosomes. 

• As the sex chromosomes act in the same way as in other genetic crosses, we can see that equal amount of males and females are produced when using a punnet’s square. 

The Sex Cells- Sperm and Egg cells are gametes. They are haploid- meaning they only have half the number of chromosomes than normal body cells.

Fertilisation

• If a sperm and egg meet and fuse in the oviduct, fertilisation will result. Fertilisation involves the haploid nuclei of the sperm and egg fusing and restoring the diploid number of chromosomes.
• The fertilised cell becomes the first cell of the new individual, the zygote. This cell then divides by mitosis and grows into a bunch of cells as it travels down the oviduct.
• The bunch of cells then becomes an embryo and is implanted on the wall of the uterus. At this point the uterus has developed a thick lining that holds and nourishes the embryo.
• At the point where the embryo begins to develop in the uterus lining, the placenta and umbilical cord form. A protective membrane, the amnion, develops around the embryo. The cushions the embryo, which soon develops into a foetus which has features more recognisable as a baby.
• During pregnancy useful materials including oxygen and glucose pass from the mother to the foetus through the placenta

Testosterone (produced by the testes) and oestrogen (produced by the ovaries) are sex hormones and recall the secondary sexual characteristics they cause to develop;

Males-Body hair and pubic hair develops/ Sexual organs enlarge/Body becomes more muscular/ voice deepens/ Sexual awareness and drive increases

Females-Hair grows in pubic regions and in the armpits/Sexual organs enlarge and breasts develop  Pelvis and Hips widen/ Menstruation begins/ Sexual awareness and drive increases 

The menstrual cycle- occurs in females from puberty until the end of reproductive life. The purpose of the menstrual cycle is to prepare the reproductive system for pregnancy by controlling the monthly release of an egg and renewing and replacing the uterine lining.

Reasons for fertility problems include-Failure of ovary to produce eggs-The oviducts may be blocked or twisted, possibly due to infections-Complications of some sexually transmitted infections-The lining of the uterus does not develop properly to enable implantation to occur-The vagina may be hostile to sperm entering e.g. lining may be too thick or acidic-Males may not produce enough sperm or the sperm may not be healthy-Impotence

Fertility Drugs-These are given to the woman to increase production of eggs. This may solve the problem if low egg production is the issue but if there are other problems such as blocked ovaries, in vitro fertilisation may be needed.

In Vitro Fertilisation-The woman is given fertility drugs so that several eggs are produced. These are collected from the ovaries surgically-Sperm is donated and the sperm and eggs are mixed in the laboratory--Successful embryos are placed in the mother’s uterus (making sure the lining is thick enough). If the process is successful an embryo will implant in the uterus lining--Usually only a small number of embryos are placed in the mother’s uterus to give balance between ensuring a successful pregnancy and avoiding multiple births.

Contraception

Mechanical – the condom as a barrier to prevent the passage of sperm and also prevent the spread of sexually transmitted infections, some of which can lead to infertility if left untreated (gonorrhoea, chlamydia and HIV leading to AIDS);

Chemical – the contraceptive pill that changes hormone levels and stops the development of the ovum;

Surgical – male and female sterilisation to prevent the passage of sperm and ova respectively; and − an awareness that contraception can raise ethical issues for some people.

Cystic Fibrosis- is caused by a recessive allele. Individuals with cystic fibrosis must have both alleles- cc. If two parents are carriers (Cc) there is a 25% probability that a child will be homozygous and have cystic fibrosis.

Mutations are random changes in the number of chromosomes (Down Syndrome); or structure of genes and can be triggered by environmental factors.

• This condition is not caused by a recessive allele but by an error in the formation of the parental gametes.
• In this condition a malformed gamete of 24 chromosomes combines with a normal gamete of 23 chromosomes. The affected individual has 47 chromosomes in all cells in their body and this causes Down Syndrome.

Genetic screening -Genetic screening may be used to reduce the incidence of diseases or conditions caused by problems with our chromosomes or genes. It involves testing people for the presence of a particular allele or genetic condition.  Cells are taken from the amniotic fluid surrounding the baby in the womb and they are then allowed to multiply in laboratory conditions. 

Ethical Issues- It is possible that if Down Syndrome is detected that the mother may choose to have an abortion.

The basic techniques used to produce human insulin (for the treatment of diabetes) is the transfer of a human insulin gene into a plasmid of a bacterial cell to form a genetically modified bacterium which then multiplies and produces human insulin

• In genetic engineering a piece of DNA can be taken from one organism and incorporated into another organism. Typically a gene that makes a useful product is incorporated into bacteria and the bacterium becomes a factory that makes the desired product.

Continuous -Gradual change in a characteristic across a population with no distinct categories; most individuals will be average with fewer at both extremes

• Height, weight, reaction time

Discontinuous-Individuals can be grouped into distinct groups with no overlap

• Tongue Rolling, blood group

Causes of Variation

Genetic- this describes variation due to gentic make up e.g. eye colour

Environmental- this describes varation due to the environmen, for example plants look very different if they are watered or receive alot of light- even if they are genetically similiary

• If the members of a species vary then some will be better adapted to survive than the other. This means that the best adapted or fittest survive which is the basis for evolution. Small changes over time can mean that organisms change as particular characteristics are favoured.

• Sometimes entire species may not be well enough adapted to survive in a changing world and they no longer survive. This leads to extinction of a species.

Charles Darwin and Natural Selection-

• Charles Darwin concluded that natural selection was the driving force for evolution. His main points were There is variation among the individuals in a population-If there is a competition for resources there will be a struggle for existence-The better adapted individuals survive this struggle which leads to survival of the fittest and these individuals are more likely to pass their genes onto the next generation.

Below are two examples of natural selection-

• Antibiotic Resistance in Bacteria- When treated when an antibiotic some bacteria may be resistance. These bacteria survive as the rest are killed. The resistant bacteria survive to breed and soon become the dominant type in the population.
• Peppered Moth- The peppered moth exists in two forms- light coloured and dark. In non-polluted areas the light form is well camouflaged on the bark of trees whereas the black form is easily spotted and eaten. In these areas the light form is common. In industrial areas where the trees are black with pollution the black forms are better adapted and survive better and increase in number.