imagine the line at the centre of the graph is the optimum condition, the wiggly line shows temperature rising and falling, but negative feedback means the level always returns to normal.
in positive feedback, the level doesnt return to normal, but continues to travel away normal.
Homeostasis uses negative feedback.
Negative feedback detects a change then brings levels back to normal. So when internal temperature rises, homeostasis uses negative feedback to return levels to normal.
Positive feedback does the opposite. It amplifies levels above normal.
It’s used to rapidly activate something. A example of positive feedback in a useful way is to create a blood clot:
however, positive feedback can also be bad:
Positive feedback is NOT used in homeostasis.
been a long time since i’ve actually posted on here, but with january here and exams on the horizon, i’m revising for OCR F214 module. however im sure that the syllabuses overlap and hence some aspects will (hopefully be useful to others :))
starting off with homeostasis (hence the big cheesy picure ;)) but will switch between other subjects so if you’re lost just hit search, type a word and hopefully it should appear (eventually). if not then ask and i’ll see what i can do :)
Some youtube clips worth having a look at - for anyone studying biology, chemistry, maths or physics at AS level. Good stuff, fun to watch (except the torture of watching someone else eat a delicious cupcake when you sit at home wishing you had one ;)) and broken down so is easy to get - from the clips i’ve seen, not a great fan of maths so kinda avoided that one :p - so all in all worth checking out :)
http://www.youtube.com/user/smartledore?feature=results_main
As you should know, ecosystems are dynamic systems (they change), and the process by which they change over time is called succession.
There are two types of succession: Primary succession and Secondary succession. Primary succession starts on new areas of land with NO organic material or soil (eg. bare rock/sand), for example, new land exposed when sea level drops, or a new surface formed by a volcanic eruption. Secondary succession is when the land has been destroyed or damaged, so succession must start again; the area has been cleared of all plants and must re-develop (eg. plants growing after a forest fire).
The succession occurs in stages called seral stages. The stages for primary succession are:
1) PIONEER SPECIES COLONISE THE LAND. The abiotic conditions are harsh, there’s no soil to retain water, so these species are specialised to tolerate and survive in these conditions.
2) PIONEER SPECIES CHANGE ABIOTIC CONDITIONS. As they die, the humus (dead organic material) is decomposed by microorganisms and create a thin, basic soil.
3) NEW ORGANISMS CAN MOVE IN AND GROW. The soil means more water csn be retained and more nutrients are available for plants. As each organism arrives, dies and is decomposed, more soil is built up and larger plants can live there. Once enough and large enough plants live there, habitats emerge and animals can move in and survive.
As more species move in and live there, the ecosystem becomes more complex and diversity increases.
The final seral stage is called the complex community, this is when the ecosystem is at its largest and most complex and won’t change much more; it’s at a steady state.
Secondary succession occurs the same way, except there is already soil present so succession starts at a later seral stage.
At each trophic level of a food chain, energy is lost. This is because not all the energy created/taken in by that organism will be transferred onto the one that eats it. Reasons for this are:
The energy that can be transferred is the energy that is stored or used for growth (becomes biomass) and can be transferred to the next trophic level.
Energy is also lost at the start of a food chain by producers, the energy that IS taken up by the producers for photosynthesis is called the PRIMARY PRODUCTIVITY. If we start with 100% availale energy from the sun, 60% is lost because:
This means that only 40% of the original available energy is used by the plant. This 40% is called the GROSS PRODUCTIVITY, as most of it is used up during the organisms life processes - 30% is lost (called RESPIRATORY LOSS) and 10% is available to the next trophic level (called NET PRODUCTIVITY).
As we go up the food chain, less energy is available from each organism as at each trophic level more energy is lost (respiratory loss) because, generally speaking, the organisms get larger and prey gets harder to catch so more energy is needed. This means less energy is transferred.
So technically speaking, if we were to eat a lot of grasshoppers (they use less energy and are at the beginning of the food chain) it would be more efficient than eating cows (which use up a lot of energy due to their size). Or you could just eat fruit ;)
An ecosystem is all the living and non-living things occurring together and the interelationships between them; the biotic and abiotic factors of an environment.
BIOTIC FACTORS = living things (predators, food, humans)
ABIOTIC FACTORS = non-living things (temperature, water availability, oxygen levels)
Ecosystems are dynamic systems - they’re always changing. The components of an ecosystem are:
A niche is the role each species plays in an ecosystem and is almost impossible to define, it’s different for every species - it’s impossible for two different species to share the same niche. Things that define a species niche would include what it eats, how it eats, what it excretes etc. (great detail on everything an organism does that would somehow effect its ecosystem).
The main way energy enters the ecosystem is photosynthesis when a plant converts light energy into a form that can be used by other organisms (although in sea ecosystems, bacteria use chemicals from deep sea vents as an energy source). Plants are hence called producers; they produce energy for the ecosystem.
Energy is transferred through the living organisms of an ecosystem when one organism eats another. These organisms are the consumers; they consume other organisms. Depending on what trophic level (a trophic level is a stage of a food chain occupied by a particular group of organisms) of the food chain the organism is, it is either a primary, secondary, or tertiary consumer.
So it goes:
PRODUCER —> PRIMARY CONSUMER —> SECONDARY CONS. —> TERTIARY CONS.
for example:
LETTUCE —> SLUG —> HEDGEHOG —> FOX
The arrows of a food chain show the transfer of energy from one trophic level to the next. A food chain show simple lines of this energy transfer, food webs show how lots of food chains in an ecosystem overlap.
Energy ‘locked up’ in things that can’t be eaten (eg. bones, faeces) get recycled back into the ecosystem by decomposers which break down dead or undigested organic material. Examples of decomposers are fungi and bacteria.
Mutations are changes to the sequences of nucleotide bases in a gene. There are five different types of mutation:
Mutations can be neutral, harmful, or beneficial.
Neutral mutations have no overall effect on the organism. this can be because :
Beneficial mutations have an advantageous effect on an organism - it increases it’s chance of survival. For example, if a bacteria can produce enzymes that break down some antibiotics, a beneficial mutation on the gene that codes for these enzymes could make them work on a wider range of antibiotics, making them more resistant.
Beneficial mutations are passed onto the next generation by natural selection.
Harmful mutations have a disadvantageous effect on an organism - it decreases its chance of survival. For example, Cystitis Fibrosis can be caused by a deletion of 3 bases on the gene that codes for CFTR. This mutation leads to excess mucus production which effects the lungs and leaves you open for infection.
In the wild, or general before modern medicine and other helpful things that weren’t so predominant in the past, mutations like these would mean the organism either died, or couldn’t reproduce to pass on their genes to offspring.
Transcription is the first stage in protein synthesis. It creates a single strand of mRNA (messenger RNA) which uses the template strand of DNA to create a copy of the coding strand. One thing that makes it different to the DNA coding strand is there is no Thymine on mRNA, instead the base is replaced with Uracil (so it’ s A-U, not A-T when complementary base pairing with mRNA)
A copy of DNA has been transcribed onto the mRNA strand, and the mRNA moves to a ribosome for the next stage of protein synthesis, translation
DNA is a sequence of nucleotide bases. A double-helix winding up that has a sugar-phosphate backbone and and nitrogenous bases bound together by hydrogen bonds. There are four different bases which can vary, however the sugar-phosphate backbone remains the same (made up of a pentose sugar and a phosphate group). The four different bases are:
Adenine + Thymine (A-T) bond together, and Guanine + Cytosine (G-U) are always together; they are complementary bases and this is called complimentary base pairing.
Genes are a unit of heredity. They are a section of DNA that codes for polypeptides (proteins) which results in a characteristic, eg. a gene for eye colour will code for a protein that results in you having particular coloured eyes.
A few genes are found in the mitochondria, but the majority are in the nuclues on chromosomes, each gene has a fixed place (a locus) on the chromosome.
The genetic code is a sequence of nucleotide bases that make a gene. Every three base pairs on a gene codes for an amino acid (the building blocks of proteins), so as a whole gene codes for a polypeptide/protein, one genetic code (one triplet of bases) codes for one amino acid which - when bonded together with other amino acids - creates the protein.
There are some rules to the genetic code:
couldnt find many helpful sites so decided to make my own, mostly cos writing/creating things helps me revise, hopefully it'll help some others too :)
if you're looking for something in particular, click search and see if you can find it (type in a keyword for a particular topic eg. type DNA and genes, mutations, development and other relevant topics, definitions or diagrams should come up), otherwise message me - no guarantee i'll be able to find it, i am a student, not the oracle ;)
