AQA Biol5 Essay Marking Thread/Feedback

Have all of you been timing your essays? Are you giving yourself approximately 30 minutes in the exam?

Yeah, Ive been doing them in 35 minutes. I tend to write loads to 30minutes isnt really feasible for me :P

So, your Gibbs Free energy value must have been more than zero.





Sorry for the dry joke.

Exam conditions just aren't standard for me

Anyone up for all brainstorming ideas for possible essays? If we picked a general area and all make suggestions I reckon between us we could come up with some decent points!

Yeah sure Role of ATP maybe? Lots of stuff to write for that! Errr something about proteins? Anyone else got any ideas

Muscle contraction, action potential (Sodium-potassium pump), Krebs cycle(?).., erm...anyone else?

Calvin cycle, glycolysis (phosphorylation of glucose), oxidative phosphorylation, active transport (basically what you said about action potentials) role of ATP immediate energy source due to unstable phosphate bonds, quick hydrolysis.
U could start off by mentioning what ATP stands for Adenosine triphosphate and describing its structure. Also it's role in joining together tRNA and the amino acids for protein synthesis.
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Hi I thought the detail that you went into with the enzymes was good. Also the mention of photosynthesis, as that gets you brownie points for mentioning plants. Maybe go into a little more detail for the muscle contraction, actually talk about the process of muscle contraction? And then maybe add in a few more topics, just to make it a bit longer, could talk about:
DNA - enzymes used in DNA replication - DNA polymerase, RNA polymerase.
Transcription factors - Oestrogen - second messenger
Control of cell division - Growth factors bind to specific receptor proteins causes genes to be switched on.
Protein channels in unit 1 - specific protein channels for certain molecules
Sodium-potassium pump - only specific to sodium and potassium

Overall was a good essay though, I would say its on the 15/16 range.

Could somebody please please pleaseeee mark this essay? I would appreciate it so much! The title is 'suggest reasons for variation between organisms'.

Suggest Reasons for Variation Between Organisms

There are many reasons that organisms on Earth vary. This can be intraspecific variation (where individuals of the same species vary) or interspecific variation which describes the variation between species. These variations may be caused by either genetics or the environment, or even a combination of both.

Firstly, there are 2 main types of cell that have variation between them. Prokaryotes have no nucleus whereas eukaryotes do have a nucleus to store genetic variation. Prokaryotes store their DNA in circular rings called plasmids and have structures like flagellum which help them to move and is an adaption to their environment that eukaryotes do not have. These are the domains which are further divided into archea, bateria and eukaryotes and are the organisms that all multicellular organisms evolved from. This is an example of interspecific variation at the simplest level and is important to understand evolution.

Interspecific variation is explained by the differences in DNA between species. This can be explained by evolution. It is thought that the first living organism on earth was a single celled ameba which has sinced evolved in different ways to produce the variety of different organisms that are now present on earth. Evolution of the eukaryotic cell is responsible for the human species today. This evolution is thought to have arisen from symbiosis between the eukaryotic cell and other bacterial cells which ‘worked together’ to both benefit each other. This was the start of multicellular organisms. Variation between the genetic material in eukaryotic cells, caused by mutation, meant that some of the cells were better suited to environments than others. This caused those without the mutation to be selected against, leaving them to die out and only adapted eukaryotes remained. This natural selection saw the evolution of many different species which adapted differently depending on their environments which has lead to the variation we have today.

Different species have varied anatomies which have resulted from evolution as they have adapted to their environments. This is illustrated by the different gaseous exchange systems between bony fish and human species. Humans have a gaseous exchange system adapted to exchange oxygen and carbon dioxide between air and so air enters through the mouth or nose, into the pharynx, through the larynx, through the trachea, bronchus and bronchioles and finally into the alveoli where gaseous exchange takes place. However, bony fish have a different gas exchange system and have gills so that they can extract oxygen from water. The gaseous exchange system of fish also allows them to use the counter-current principle in which water flows in the opposite direction to blood to optimise diffusion. THis variation is shown further as small organisms such as some insects have no gas exchange system at all and rely solely on diffusion and plants use diffusion of CO2 into their stomata for photosynthesis. This is an example of inter species variation, as there is variation between organisms in order for their chances of survival to be increased.

Intraspecific variation can also be caused by natural selection. Sometimes the members of the same species can be geographically or temporally isolated from each other which can lead to the creation of a new species, as the former members can no longer breed together to produce fertile offspring. Geographical isolation happens when a species is split up by a physical barrier - for example floods or melting ice. The 2 groups then experience different selective pressures so there will eventually be a difference in allele frequencies as different alleles will be advantageous in the different groups. Changing allele frequencies will lead to differences accumulating in the gene pools of the 2 populations leading to changes in the phenotype which can cause the 2 populations to become so varied from each other that they become reproductively isolated and become different species with variation between them

Mutations are a reason for intraspecific variation. Mutations can be additions of a nucleotide base, deletions of a nucleotide base or substitutions of a nucleotide base. Additions or deletions will always cause variation of some type as they will cause a frameshift in the DNA which will change the triplet coding so different amino acids are coded for which will therefore change the phenotype of the individual. Different mutations in different individuals leads to different changes in the phenotype so this is a reason for variation between organisms. A substitute mutation may also cause such variation however this is not always the case due to the degenerate nature of DNA. Mutagenic agents are factors that increase the liklihood of mutation such as some chemicals and some types of radiation, such as ultraviolet. These can therefore be said to be a reason for variation between organisms.

Intraspecific variation is caused by processes such as crossing over and independent assortment during meiosis. In meiosis crossing over happens in prophase 1 when the homologous chromosomes are synapsed. The chromatids intertwine and exchange segments so that the mixture of genetic material is changed called ‘genetic recombination’. This means the chromatids have new combinations of alleles than their parents had, so is a reason for variation. Independent assortment refers to the random separation and assortment of non-homologous chromosomes during meiosis and produces different combinations of gamete so is an important source of variation between individuals. When this is combined with recombination and the random fusion of gametes, the potential for variation is enormous.

Finally, another reason for intraspecific variation between organisms is due to the environment. This is when variation is not caused by genetic causes such as mutation or evolution, but it caused by external factors. For example, human beings may use hair dye or have tattoos which change their appearance but will not be passed on to the next generation.

To conclude, there are many reasons for variation between organisms including interspecific reasons such as evolution of new species and differences in anatomy. Intraspecific variation can also be caused by natural selection, mutations, meiosis and there are also environmental influences. When all of these factors are combined, the opportunity for variation on Earth is massive.

Hey, well done on the essay, this is really good!! I thought this was quite a difficult title to be able to get a high mark for breadth of knowledge, but you've covered lots of different topics from the whole spec, and included some extra information, outside of the spec. Maybe the only other thing examiners would look for is the mention of plants, as you've done a lot on bacteria (Which is good) and animals. But I would still say that this is definitely in the 20-23 range.

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Would really appreciate it if someone would take a look at my essay please! It's just posted above.. Thank you!