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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 interbreed
and produce fertile offspring
· It is important to take a random sample (each individual is chosen entirely by chance, and
has equal chance of being selected) of a population
· This is important so the sample is representative of the population, and so that there is no
investigator bias
· Standard deviation measures how far spread…read more

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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…read more

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· Crossing over takes place during first prophase division-two of the
chromatids on a homologous pair of chromosomes can cross over the
point at which they cross is known as the chiasma
· When the chromosomes separate during anaphase, parts of the
chromatids are swapped from one chromatid to another
· This means that alleles of genes that occur on the same chromosome
can be combined in new ways
2.…read more

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Cellulose allows plant cells to be turgid and stops them from bursting
· As cellulose molecules are long and straight, they are perfect
for forming structural fibres
· Starch is a polysaccharide made of -glucose, and are joined
by glycosidic bonds-it is made of amylose and amylopectin
· Starch and glycogen are used as a storage molecules, as they
are compact and insoluble, and so osmosis is prevented
· Although both starch and glycogen are branched, glycogen
has far more branches
· Starch is used…read more

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For protein synthesis, mRNA is used-uracil is used
instead of thymine as a base
· Bases are attached to the coding strand of DNA using
RNA polymerase (transcription)
· mRNA then leaves the DNA as a single strand, and
the DNA zips up again due to RNA polymerase
· The mRNA then leaves through pores in the nucleus,
into the cytoplasm
· Translation then begins-the start codon attaches to a
ribosome, which reads the sequence until the end
· tRNA have complementary bases on…read more

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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)
2.6- In complex multicellular organisms, cells are
organised into tissues, then organs, then systems
· when cells become specialised for different functions it is called
cell differentiation
· Unspecialised cells tissue organ system
2.…read more

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In plants, the exchange surface changes
depending on the sort of climate they grow
· The cuticle prevents gas exchange, the
stomata (mainly on the underside of the
leaf) provides an opening for gases to enter
and escape, controlled by guard cells
· Guard cells Xylem brings water into the leaves
close the
stoma when Phloem removes the product of
they are not photosynthesis
turgid (and have therefore lost water), which
prevents water loss and during times when
carbon dioxide cannot be used for…read more

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Arterioles are able to contract, and divert the blood, such as to the muscles rather than the
stomach during exercise for example
· Veins have the same layers as arteries, but are much thinner, as the blood has a lower
pressure compared to the arteries, and the lumen is therefore larger
· As the muscles contract and squeeze the veins, blood is helped back from the legs and arms
as the squeezing of the veins forces the blood to flow towards the heart-when the muscles…read more

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This is slow, as the cytoplasm and membrane provide a lot of resistance, and so as there is
less resistance in the cellulose fibres, water mostly passes via the apoplast route (through
the cell walls)
· Just before the xylem, there is an epidermis cell and the casparian strip (a waterproof
barrier), which forces water to move through the cytoplasm, rather than through the cell
· Water can move up the xylem by root pressure-when ions are absorbed via active transport
by the root…read more



Brilliant, Thank you so much!! This is amazing 

tilda mascall

Fantastic resorce, thanks :)

Miss Meera J

Humza Productions!!!!

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