Biology Unit 3: Living and Growing

Stem cells are undifferentiated cells. This means that they have no set role yet and therefore can be made into different types of cells and therefore be used to treat medical conditions. Stem cells can be found in both adult cells and embryonic cells. Embryonic cells are more effective because they can be made into more things and are easier to find but there are ethical debates against such use because taking the stem cells kills the embryo.  

Humans have a double circulatory system. This means that blood travels in two circuits: from the heart to the lungs and back, and from the heart to the body and back. This involves all four chambers of the heart:

• the right atrium receives the deoxygenated blood from the body through the vena cava 
• the tricuspid valve allows blood to be pumped into the...
• right ventricle and out of the...
• pulmonary vein and to the lungs

Then:

• the left artium receives the oxygenated blood from the lungs through the pulmonary artery 
• the bicuspid valve allows the blood to be pumped into the...
• left ventricle and out of the...
• aorta to the body

This can be remembered using LORD: Left Oxgenated, Right Deoxygenated

Remember: the left side of the heart that you look at is the actually the right hand side!

There are three types of blood vessel that carry blood to and from your heart. Each vessel has different features and roles, these are:

• Arteries: go away from the heart! They have thick, muscular and elastic walls in order to resist the high pressure at which blood flows through them in order to reach the whole body. 
• Veins: go in to the heart! As they flow against the force of gravity, they require help to carry the blood back to the body. For example, lumen which encourages blood flow and valves which prevent back flow. 
• Capillaries: they link arteries and veins and allow the exchange of nutrients through their permeable walls. 

There are two rapid phases of growth, more commonly called growth spurts, in human growth; one just after birth and one at puberty. Different parts of the body also grow at different rates because the different parts of the organism may be needed at different times. However, growth can be measured in many different ways, each with their advantages and disadvantages:

• height/length: easy but only measures growth in one direction
• wet mass: easier than dry mass and more accurate than length but the water content of organisms can vary easily
• dry mass: most accurate but requires the organism to be dead and dried 

Dolly the sheep was made through a process called nuclear transfer, which is a way of cloning animals. Cloning could help to produce animals with desired characteristics very quickly in the future, and even to produce human products like insulin. Dolly the sheep was made by:

• using a donated egg cell with the nucleus extracted
• inserting a nucleus from a body (udder) cell into the egg cell
• shocking the egg with electricity in order to make the cell divide
• the embryo was then inserted into the surrogate mother
• the sheep produced was an identical clone to the sheep the nucleus came from

It is easier to clone plants than animals because plants retain the ability to differentiate. By cloning plants, it makes things easier for farmers who can produce many plants at oncce, all identical, but it reduces the genetic variety of plants and makes them more susceptible to illness

Plants can also be cloned by a process called tissue culture:

• plant is selected for it's certain characteristics
• cuttings are taken from the plant and placed into a test tube/dish for growth
• cuttings are grown in a growth medium using an aseptic technique (stops bacteria getting in)

Genetic engineering is altering the genetic base code of an organism in order to produce an organism with a desired characteristic very quickly. However, some people argue that genetic engineering could produce unknown side effects and therefore don't agree with its use. This is done using: (the brackets is knowledge that you will need to know in OCR Biology Unit 6)

• The desired gene is discovered, usually from another organism, and isolated
• The gene is then extracted 
• The other organism's DNA is then cut open (using restriction enzymes) 
• The gene is inserted into their DNA (using ligase enzymes)
• The organism is then allowed to reproduce, replicating the genetic modification in their offspring

Some examples of genetic engineering are:

• producing resistance in plants - they won't die when exposed to a herbicide (herbicides kill plants: think 'herbs' whereas pesticides kill insects which are 'pests')
• Extracting the beta-carotene gene from carrots and inserting it into rice in order to produce a large amount of vitamin A in the rice. This solves the lack of vitamin A in 3rd world countries
• getting bacteria to produce human insulin for those with diabetes (this is explained in detail in unit 6!)

Red blood cells are well adapted to their oxygen carrying role in the body. This is because they have:

• a large surface area to volume ratio - basically small but with a lot of space for blood to react with the cell
• a biconcave shape - to easily pass through small blood vessels
• no nucleus - to be able to carry more oxygen
• haemoglobin - to react with oxygen in the lungs to form oxyhaemoglobin. This allows the blood cell to carry the oxygen until it reaches the muscles and other cells where are reverse reaction occurs forming oxygen at the cell and haemoglobin in the blood.

Plasma is the liquid part of your blood and is responsible for carrying:

• glucose
• hormones
• carbon dioxide
• waste products (like urea - excess amino acids)
• plasma proteins like antibodies

In the body, enzymes catalyse reactions like respiration and photosynthesis, making them faster. However, there is only one enzyme for each product that needs to be broken down because enzymes have a high specificity. This product is called a substrate. The lock and key theory defines enzymes as having one 'key' or substrate per 'lock' or active site. The active site of an enzyme is where the substrate fits in.

However, enzymes are  effected by pH and temperature because each enzyme has an optimum pH and temperature. The optimum is the best possible conditions for the most efficient work. At low temperatures, particles are moving slowly so little collisions occur and the reaction is slow, yet at too high a temperature the enzyme, as a protein molecule, will denature. This changes the shape of the active site completely and therefore the original substrate can no longer 'fit'. 

You can see how enzymes are effected by temperature by calculating the temperature coefficient or the Q10. The equation for this is:

Q10 = rate at HIGH temp / rate at LOW temp

Similar to mitosis, meiosis is a type of cell division. This time, the cells produced are NOT genetically identical and are used to make gametes (sex cells) or other haploid cells (cells containing one chromosome only). The process of meiosis is like going through mitiosis twice and can be summarised as: 

• DNA double strands that make up the chromosome 'unzip' to form two single strands
• Each individual strand is copied to produce 4 strands
• 4 strands pair up by realigning with the base pairs like a template to form 2 X-shaped chromosomes
• Chromosomes pairs move to opposite poles of the cell (1st division like in mitosis)
• The pairs are pulled apart as they move to the four opposite corners of the cell (2nd division)
• 4 genetically different haploid cells are produced

However, DNA is too large to travel out of the nucleus to the site of protein synthesis, called the ribosomes, and therefore a copy is required to travel there. Thus, an enzyme reads the DNA and makes a copy called messenger RNA or mRNA. The mRNA copy then travels to the ribosomes where it is read by an organelle, which is able to organise the amino acids into the triplet base code stated by the mRNA.

Different proteins are made using a different order of the bases in the mRNA molecule. The different types of proteins are: 

• Enzyme
• Hormones (e.g. insulin)
• Haemoglobin
• Structural proteins (e.g. collagen – used for skin)

Aerobic respiration occurs in plenty of oxygen and releases energy from a food molecule called ATP. The equation is: C6H12O6 + 6O2 → 6CO2 + 6H2O 

When the muscles don't receive enough oxygen, for example in strenuous exercise, anaerobic respiration occurs. However, because of this lack of oxygen, glucose is unable to break down completely, causing a build up of lactic acid. It is this acid that causes pain and fatigue in muscles and this form of respiration also releases less energy. The equation is: glucose → lactic acid

After exercise, breathing and heart rate must remain high in order to rectify the production of lactic acid. This works by pumping lactic acid to the liver where the excess oxygen is used to help the liver break it down. 

The production of carbon dioxide and use of oxygen can be used to measure the sum of all the reactions in the body, also called the metabolic rate because the more oxygen required, the more is occuring. The equation for this is called the respiratory quotient or RQ. RQ = carbon dioxide produced / oxygen used

Mitosis is the process of cell division that causes cells to replicate to produce identical copies. The stages are as follows: (please note that a chromosome is a section of DNA)

• DNA double strands that make up the chromosome 'unzip' to form two single strands
• Each individual strand is copied to produce 4 strands
• 4 strands pair up by realigning with the base pairs like a template to form 2 X-shaped chromosomes
• A spindle forms and the chromosomes are arranged across the equator of the cell
• Chromosomes move to opposite poles of the cell
• 2 genetically identical copies are produced

Mitosis is used to make body cells.

Selective breeding is often used when there are little left of a species in order to preserve the healthiest and most advanced animals. However, this process refins the mating groups and often involves inbreeding (breeding between the same family: e.g. sister mates with brother). By mating in the same group it reduces the variety of alleles available and causes the accumulation of a number of recessive characteristics. This can be seen in some species of dog. The lack of different alleles also means that there is less variation between the species and therefore the species cannot adapt as easily to change.

Bacteria cells are very different to our own cells because:

• they lack a true nucleus
• they store their DNA in a coiled molecule in the cytoplasm called a plasmid
• they have a tail for movement called a flagellum or flagella (plural)

Similarly, plant growth is also very different to our own growth because:

• plants can carry on growing
• plants cells elongate and enlarge rather than divide
• plant cells only divide in areas called meristems
• plants retain the ability to differentiate

Two scientists, Watson and Crick, exposed the structure of DNA in the 1960s. They x-rayed DNA to show that it formed a double helix, held together by the 4 bases.

DNA is a chemical that contains your genetic code and is used for everything from making gametes (sex cells) to your average protein. It is formed by two strands, coiled together into a double helix and held together by cross links between the 4 base pairs: G and C, A and T. DNA can be changed when it is exposed to radiation, this is called mutation and causes the rearrangement of amino acids and therefore a different DNA structure.

Being multicellular allows an organism to become larger and more complex because each cell has a different role/job. This is called cell differentiation. However, being multicellular also requires:

• communication between all cells
• ability to supply the cells with enough nutrients
• ability to control heat and gas exchange