ESSAY HELP Paper 3 A level Biology AQAWatch
The transfer of energy within and between organisms
The transfer of energy is of pivotal importance in biology as energy is what allows life to be sustained. The main way in which energy is supplied is from the sun. Light energy from this source is converted into chemical energy by plants. When light hits a chlorophyll molecule, the light-dependent reaction takes place and photoionisation takes place, causing electrons to leave the chlorophyll molecule. Electrons are accepted and move along an electron transport chain in which energy is lost at each stage - this energy is used to produce ATP and reduced NADP. During respiration, the chemical energy is lost as it is used up for growth and movement or it binds to other biological molecules to make up the biomass of the plant. The energy in this biomass passes along the food chain.
However, in each trophic level, less energy is passed on. 90% of light energy from the sun is reflected back into space by dust and clouds. At each trophic level, only 10-20% of energy is passed on because some of the material in the biomass is indigestible or it is not consumed. This is why there are such few trophic levels, as there is not enough energy to support a large population of organisms. Another way in which the transfer of energy is limited in organisms is the limiting factors of photosynthesis and as a consequence, less light energy from the sun is converted to chemical energy.
One example of a photosynthetic limitation is low levels of carbon dioxide as it is required for the light independent reaction of photosynthesis, where it reacts with RuBP to form glycerate 3-phosphate. Lower light levels are also a limiting factor for the light-dependent stage of photosynthesis because fewer electrons become excited and photoionisation slows down, so electrons are not carried down the electron transport chain. The consequence of this is that less ATP is produced, and less energy is released.
> Responses to stimuli/nerve impulses
> Muscle contraction
> Mass transport
These are the only topics I can think of.
Best of luck
1. A wide range of ideas/reasons/factors, each of which will be assigned about 2-3 marks - so your having described p-synthesis alone will probs earn you a max of 3 marks out of the 20 odd assigned to this part, more like 2 marks because you have not even mentioned the key term light reaction OR transduction of light energy into chemical energy OR carbon fixation (see point (2) below).
2. A clear manifestation of your knowledge of biological terms together with their precise meaning and their use in the right context.
3. A succinct understanding of the association/links between different parts of the syllabus/different systems of the living organism/interplay between different aspects of the biosphere (the essay is called "synoptic" for a reason!)
4. It is an ESSAY, so ideas should have a logical flow and some degree of coherence.
5. 3 marks are assigned to the command of English and to point (4) in terms of syntax and grammar (punctuation, spelling, tense, figure of speech, singular/plural). Your English sounds very good considering the poor teaching of the subject these days and the generally low standard among students; you would probs get 2 out of the 3, simply because there just is not sufficient abundance of text there for the examiner to make an impartial appraisal.
You will probs get 4-5 out of 25 for his answer (sorry!)
ONE FINAL BUT CRUCIAL POINT I WOULD RAISE IS:
READ THE Q carefully TWICE and work out EXACTLY what is required. (Here you have completely ignored the word "within" in the Q - this has been addressed very well by GrandMedic above, but you could add reabsorption in proximal tubule cells, absorption after digestion of glucose + amino acids by active transport, the Na+, K+ ATPase in cardiac muscle and elsewhere, the role of NAD+/NADH, the biochemical processes [particularly in the liver] that general thermal energy, the sweat glands in loss of heat energy etc.) Finally, have a look at my attached article, and keep using the tips therein regularly as you do past papers.
M (specialist biology tutor)