AS unit 2 biology notes (missing biodiversity)

notes on unit 2 edexcel AS biology (missing biodiversity)

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Biology Unit 2 Edexcel AS notes, Liam Johnson.
Section 1, Cells:
Cells can be Eukaryotic or Prokaryotic.
Eukaryotic Prokaryotic
Large cells Small celss
Dna is linear Dna is circular
Nucelus present (dna inside) No nucleus (dna in cytoplasm)
No cell wall for animals, cellulose wall in plants Cell wall of polysaccharide (not chitin or
and chtin wall in fungi cellulose)
Many organelles, mitochondrion Few organelles, no mitochondrion
Big ribosomes Small Ribosomes
Example = skin cell Example = E coli bacterium
Organelles, structure + function:
Nucleus: Surrounded by nuclear envelope (double membrane) with a lot of pores. Contains chromatin
and a nucleolus.
Chromatin is made from protein + DNA. Pores let stuff such as RNA to move from nucleus to
cytoplasm.
Lysosome: Round organelle with a membrane.
Contains digestive enzymes, can be used to digest invading cells or worn out bits of the current cell.
Vesicle: Small sac in cytoplasm with a membrane.
Transports stuff in and out of cell. Formed at endoplasmic reticulum, golgi apparatus or cell surface.
Ribosome: Floats free in cell or is attached to rough ER.
Makes proteins from amino acids.
ER: Smooth ER or Rough Er. System of membranes in a fluid space, RER has ribosomes attached.
Smooth makes lipids, Rough makes complex proteins from the ribosomes.
Golgi Apparatus: Fluid filled flat sacs, vesicles are seen at the edge.
Packs proteins and lipids for transport. Makes lysosomes.
Centriole:
Hollow cylinders, containing a ring of microtubules (small protein cylinders)
Involved in separating chromosomes.
Mitochondrion:

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Biology Unit 2 Edexcel AS notes, Liam Johnson.
Oval shaped, double membrane. Inner one folded to make cristae. Matrix inside which has enzymes
for respiration. Site of aerobic respiration, ATP made here.
Protein Transport:
Protein made at ribosome (RER makes ones for transport, lone ones make protein to stay in
cytoplasm)
Protein made at RER are folded inside the RER.
Taken to the golgi apparatus in vesicles.
Golgi apparatus might process more.
Proteins enter more vesicles to move around the cell. Some are secreted.…read more

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Biology Unit 2 Edexcel AS notes, Liam Johnson.
Mitosis:
4 stages. Remember PMAT.
Prophase:
Chromosomes condense, centrioles start to move to opposite sides of the cells. Nuclear envelope
breaks down and the chromosomes lie free in the cytoplasm. Spindles form from the centriole.…read more

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Biology Unit 2 Edexcel AS notes, Liam Johnson.
Section 2: Reproduction
Production of gametes:
Gametes are sex cells that are found in an organism that produces sexually.
Form together to form a zygote.
Males = sperm Female = Egg
In plants male gametes inside pollen grains and females inside ovules.
Each gamete has half the number of usual chromosomes (haploid)
Sperm specialisation:
Acrosome containing enzymes to break down the zona pellucida, lots of mitochondrion inside the tail
and a flagella to move it.…read more

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Biology Unit 2 Edexcel AS notes, Liam Johnson.
Genetic differentiation:
Occurs in Meiosis.
Before first division, homologous pairs pair up.
Two of the chromatids in each pair twist around eachother. The twisted bits break off and rejoin the
other chromatid.
Chromatids still contain the same genes but have a different combination of alleles.
Four new cells after meiosis contain different alleles.
Independent assortment:
Four daughter cells have completely different combinations of chromosomes.…read more

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Biology Unit 2 Edexcel AS notes, Liam Johnson.
When tube reaches ovary it grows through the microphyte (hole in the ovule wall) and into the ovule
sac inside. Once inside the tube nucleus disintegrates and the two gametes are released.
One gamete fuses with the egg nucleus to form a zygote to become the seed. Second nucleus fuses
with two nuclei in the centre called polar nuclei. This forms a cell with a large nucleus which divides to
become a food store.…read more

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