unit 2 biology notes

all topics

HideShow resource information


Living organisms vary, and this variation is influenced by genetic and environmental factors: 

  • variation exists between members of a species ( a group of organisms which can breed and produce fertile offspring) 
  • it is important to take a random sample of a population because the sample is a representative of the population and so that there is no investigator bias.
  • variation can result from both environmental and genetic factors
  • dicontinuous variation is clear-cut distinctions in phenotype (physical characteristics e.g blood group) this is caused by allelles of one gene
  • continuous variation is unclear distinctions in phenotype e.g mass-caused by several genes 
  • homozygous recessive- a characteristic that only appears when two recessive alleles are present
1 of 17

Standard Deviation

Standard Deviation measures how far spread the results are from the mean e.g in height very few people are giants or dwarves.

On average 68% of all measurements fall into 1SD and 95% falls into 2SD

2 of 17

DNA - structure

DNA is an information carrying molecule. It's sequence of bases determines the structure of proteins, including enzymes. 

  • DNA has a double helix structure, meaning large amounts of information can be stored in small volumes
  • it can withstand hight temperatures 
  • it is also stable so information is not corrupted when getting passed to a generation
  • There are 4 Nitrogenous bases A T C G 
  • A pairs with T 
  • C pairs with G
  • Each DNA strand is made of two nucleotides in each 'step' two sugar - phosphate backbones and two nitrogenous bases joined by hydrogen bonds, they are on opposite strands and are anti parallel
  • (hydrogen bonds are used as they are strong and can be easily broken when needed for protein synthesis and cell division
  • Each nucleotide is held by a strong phosphodiester bond as a results of a CONDENSATION REACTION
3 of 17


  • A gene is a chemical code that contains the instructions for making a complete protein or polypeptide/ a section of DNA that contain coded information, as a specific sequence of bases, the code for polypeptides that determine the nature and development of organisms
  • different forms of genes are known as alleles 
  • (meaning the polypeptides they code for may also differ) 
  • a specific amino acid is coded for by a codon (3 bases) this also determines the sequence in a polypeptide
  • in prokaryotes the DNA molecules are circular and are not associated with proteins
  • in eukaryotes some of the DNA does not code for polypeptides,known as INTRONS
  • INTRONS= DNA that does not code for polypeptides
  • Genome is the term used to refer to all the different genes in a single individual
  • a karyotope is the full complement of human chromosomes arranged in numbered homologous chromosomes-females have ** males XY
  • a chromatin is the DNA and its associates proteins- form in which the DNA is in when the cell is not dividing - uncoiled DNA found in the nucleus 
  • meiosis happens in the productuon of gametes they are haploid so there is only one of each homologous chromosomes
4 of 17


chromosomes replicate
form two identical chromatids
join at the centromere
DNA is replicated also 

replicated chromsomes coil up and joined at the centromere 

nuclear membrane breaks down and chromosomes line up on the equator of the cell
spindle fibres form and attach to the centromeres

chromatids are pulled to oppisite poles of the cell as the spindle fibres shorten

nuclear membranes develop & chromosomes uncoil
two cells which form are exact copies of the original cell 

5 of 17


Meiosis involves two cell divisions
First division: the cell has 46 chromosomes (each consisisting of 2 chromatids) or homologous pairs knows as bivalents
After the first division there is 23 chromosomes (which still consist of a pair of chromatids) 

At the end of the second division each cell has 23 independent chromosomes so each gene is present once, there are no clones so one of each pair of chromosomes goes into each gamete.

Advantage of meiosis is that genetic variation is increased, as new combinations of alleles are produced, which can result in new adaptions, the successful gene combinations for which can be passed on to their offspring

  • Genetic Variation can come about as a result of independent assortment chromosomes can be reshuffled in any order producing gametes with a different combination of genes. 
    Random fertilisation means any female gamete can join with any male gamete

6 of 17

Similarities and differences in DNA result in gene

  • a reduction in genetic diversity can mean that the organism may not be abkle to evolve successfully to changing conditions-results in the reduction of the gene pool
  • battery hens can suffer from reduction in genetic diversity because theyre selectively bred
  • the intensive breeding often results in health problems such as arthritus and lack of calcium due to laying so many eggs
  • crops are also selectively bred to create crops that have desirable characteristics and a fewer not so desirable ones
  • GENETIC DIVERSITY IS the differences in the DNA of individuals of a species
  • the found effect is when a new colony is started by a few members of the original colony meaning genetic diversity is decreased because the select few are just breeding with each other.
  • the bottleneck event (e.g hunting) is where the population is reduced for at least one generation, resulting in there being less genetic diversity when the population recovers
7 of 17


HAEMOGLOBIN is a protein with a quaternary structure, and is made of 4 polypeptide chains (2 beta and 2 alpha)

  • each chain contains haem groups which can associate with oxygen  
  • it changes the shape of the haemoglobin molecule 
  • making it easier for the other molecules of oxygen to bind
  •  at low partial pressures of oxygen there is not much oxygen available so the organism has a higher affinity for oxygen at low partial pressures
  • at high partial pressures of oxygen the organism haemoglobin has a lower affinity for oxygen but the oxygen molecules are able to disassociate easier ( for use in aerobic respiration) 
  • the larger the mammal the higher the affinity for oxygen
  • in humans haemoglobin disassociates with oxygen more readily in the presence of carbon dioxide this is needed for respiring muscles
8 of 17

Plant Cells

  • cellulose is linked by glycosidic bonds, but these are strengthened by hydrogen bonds to form bundles called micro fibrils
  • when these link fibres form ( micro fibrils --> macro fibrils --> then fibres)
  • cellulose molecules are long and straight so they are perfect for forming structural fibrils/
  • Starch is a polysacharide made of alpha-glucose and are joined by glycosidic bonds it is made of amylase and amlopectin
  • starch and glycogen are used as storage molecules - they are compact and insoluble so osmosis is prevented
  • starch is found in plants and glycogen is found in animals 
  • a palisade cell has lots of chloroplasts around the cell membrane for photosynthesis to occur
  • they are long and thin to increase surface area
  • chloroplasts are bound by a double membrane
  • the thylakoid membrane is a phospholipid bilayer containing many chlorophyll molecules- this captures light energy which is then transferred to protein molecules GENERATING ATP
  • granum is the stacks of thylakoid that increase the efficiency of light dependant reactions
  • cellulose make the cell wall prevents cell bursting, keeps it turgid
  • stroma contains enzymes, ribosomes, lipid droplets and starch grains
9 of 17

During the cell cycle, genetic information is copi

  • DNA is replicated through a process called semi-conservative replication
  • when the process takes place the new molecule has one 'old' strand and one 'new' strand made up of the free nucleotides. 
  • evidence for this = research carried out, shows that if you start with a heavy nitrogen-14 labelled DNA molecule these molecules will not be has heavy when centrifuged
  • this is because half of the DNA molecule is now combined with a new lighter strand of DNA
10 of 17

DNA- replication

  • in DNA replication the DNA helicase 'UNZIPS' the DNA two strands, by breaking the hydrogen bonds between the polynucleotide strands
  • DNA polymerase forms the phosodiester bonds between the nucleotides ( it does not join the nucleotides to the DNA it forms the phosphodiester bond between the ribonucleotides to form the mRNA
  • instead of a thymine (T)  base mRNA is used for protein synthesis 
  • bases are attached to the coding strand of DNA using RNA polymerase (transcription)
  • mRNA then leaves the DNA as a single strand and DNA zips up again
  • the mRNA then leaves through pores of the nucleus in the cytoplasm
  • translation then begins, the start codon attaches to a ribosome which reads the sequence until the end of the codon
  • tRNA have complementary bases on one end and an amino acid chain on the other, which forms a primary then tertiary structure 
  • MITOSIS - is used for growth and repair
  • at the end of the DNA replication process two cells are produced
11 of 17

Meitosis- disadvantages

  • during the cell cycle proto-oncogenes stimulate cell division and also inhibit cell death
  • tumour suppressor genes prevent cell division or lead to cell death
  • however when proto-oncogenes mutate, oncogenes are formed which are not affected by tumour suppressor genes
  • this uncontrolled cell division results in the formation of a tumour and therefore cancer benign tumours stay where they are, whereas malignant tumours travel in the blood to the other parts of the body to become new tumours, (metastasis)
12 of 17

Specialised Cells

In complex multicelluar organisms, cells are organised into tissues --> then organ --> then systems


  • when cells become specialised for different functions it is called cell differentiation
    Unspecialised cell --> tissue --> organ
13 of 17

Gas Exchange 1

Factors such as metabolic rate and size affect the requirements of organism and this gives rise to adaptions such as specialised gas exchange surgaces and mass transport systems

  • the surface area to volume ration dictates how gases are exchanged in single celled organisms there is no need for a gas exchange system BUT there is for a larger organism
  • the larger the animal, the smaller its surface area to volume ration
  • the smaller the animal the harder it is for it to gain or lose heat
  • large mammals compensate by having adaptions to help them control heat loss and gain
  • in cold areas, mammals have a reduced surface area, so less heat is lost and vice versa for hot areas
  • in insects blood is not used to transport gases for gas exchange, gases enter and leave through spiracles
  • cuticle is impermeable so it can conserve water, but also its impermeable to to o2 and co2


14 of 17

Gas Exchange 2 (insects)

  • the advantage of spiracles is that oxygen reaches the mitochondria directly in the most active muscle (such as the flight muscle - the one that controls the wings)
  • larger insects such as grasshoppers use a pumping motion to ventilate the tissues
  • in order to get extra o2 into the muscles is while resting - a small amount of water leaks into the ends of the tracheoles respire (partially anaerobically) lactate is produced, which is soluble and reduces the water potential of the muscle cells and water passes from the tracheoles into the muscle cells
  • this reduces the diffusion distance for oxygen when it is needed the most
15 of 17

Gas Exchange in Fish

  • In fish the exchange surface is made up of lamellae which are on fillaments which make up the gill arch protected behind the bony operculum
  • the lamellae provide a short diffusion pathway distance as the capillaries run close to the surgace and the lamellae themselves are one cell thick
  • water is drawn in when the fish opens its mouth which then forced over the lamellae and out through the operculum
  • to maintain a steep concentration gradient, a counter current system is in place which facilitates gas exchange
  • amd eber more numerous lammellae have a rich blood suppy their thin membranes allow blood to come very close to the water.
  • the blood in the capillaries flows in the opposite direction to the way water flows over the gills hence the countercurrent system
  • in a countrercurrent system the blood always has a lower concentration of oxygen than the water
  • a diffusion gradient is maintained along the entire length of the filament - this ensures that oxygen can diffuse into the blood at every point along the filament and that blood leaving has a high conc of o2
16 of 17

Gas Exchange in plants

  • In plants the echange surface changes depending on the sort of climate they grow in
  • the cuticle prevents gas exchange the stomata provides an opening for gases to enter and escape, controlled by guard cells
  • guard cells close the stomata when they are not turgid (lost water) which prevents water loss and during times when co2 cannot be used for photosynthesis (at night times when suns not out)
  • the mesophyll layer has many air spaces to allow gases to circulate to and from cells by diffusion
  • as well as sllowing co2 to enter rapidly there is also the danger of waterloss from the stomata this overcome by closing during darkness or to take up large amounts of water
  • cacti which live in dry areas are known and xerophytes and have special adaptions to overcome the shortage of water
  • leaves are reduced to spines, roots run close to the soil surface to take advantage of the occasional showers and a 2nd set which penetrate the water table (therefore very long)
  • stem is also thick to allow storage of water and it has reduced number of stomata & thicker wall cuticle
  • leaves coil up,so that only the cuticle faces the outside the advantage of this is because
17 of 17


No comments have yet been made

Similar Biology resources:

See all Biology resources »