Edexcel biology Unit 2

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  • Created on: 04-05-13 17:51
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The Voice of the Genome-Cell Ultra structure
The 3 basic ideas about cells are:
They are building blocks of structure in living things
Are the smallest units of life
Are derived from other pre-existing cells by division
Contains a blueprint for their growth, development and behaviour
Have sites of chemical reactions of life (metabolism)
Transmission electron microscopy
In the electron microscope (EM), a beam of electrons is used to produce a magnified image.
The electron beam is generated by an electron gun and is focused on electromagnets
In transmission electron microscopy of biological material, the electron beam is passed
through an extremely thin section of tissue.
Membranes and other structures present in cells are stained with heavy metal ions, making
them electron opaque so they stand out
In scanning electron microscopy, the surface of the whole specimen is scanned by a beam of
The 3D image is created from electrons reflected from the surface and also from electrons
generated their secondary electrons.
Larger specimens can be viewed by scanning electron microscopy than by transmission
electron microscopy, but the reaction is not as great
Eukaryotic cells have a nucleus and prokaotic cells don't
The ultrastructure of an animal cell
The fluid around the organelles in an eukaryotic cell is called the cytosol which is formed and
used in the chemical reactions of life (metabolism which involves the chemical metabolites).
They cytosol and organelles are contained within the plasma membrane, allowing the
metabolites to move within the organelles
Introducing organelles
Nucleus- is the largest organelle (10-20µm diameter). It's surrounded by a double membrane
with 100nm pores (diameter) and pores make up 1/3 of the nuclear membrane's surface
diameter. The nucleus contains chromosomes which appear dispersed as a diffuse network
(chromatin). The nuclei are present in nucleus (is tiny and rounded and is where ribosomes are
Mitochondria- are metabolically active cells and are 0.5-1.5µm wide and 3.0-10.0µm long. It
has a double membrane-its inner membrane is infolded to form cristae. The interior of the
mitochondria contains an aqueous solution of metabolites and enzymes (matrix) and
mitochondrion is the site of aerobic respiration
Ribosomes- are the site of protein synthesis. They are 25nm in diameter and are built of 2
subunits and don't have membranes. They are made of protein and nucleic acid (RNA)
Rough endoplasmic rectilium- have ribosomes attached to its outer surface. Vesicles are then
pinched off. Vesicles are small, spherical organelles bound by a single membrane used to
transport and store substances around the cell. Example- RER is the site of protein synthesis
and discharge from cells e.g. digestive enzymes

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Smooth endoplasmic rectilium- have no ribosomes and are the site of synthesis of substances
needed by cells. Example- SER is important in the manufacture
Endoplasmic rectilium- has a network of folded single membranes forming interconnected
sheets, tubes or sacs. The origin of endoplasmic rectilium is the outer membrane of nucleus
which remains attached
Golgi apparatus- contains a stack-like collection of flattened membranes sacs one side of
stack of membranes is formed by fusion of membranes of vesicles from endoplasmic
rectilium.…read more

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Some cells are totally dependent on other cells for example nerve cells are adapted for the
transport of nerve impulses but are dependent on blood cells for oxygen, and on heart
muscle cells to pump the blood
Specialised cells in mulitcellular organisms become organised into tissues and organs during
A tissue is a group of similar cells specialised to perform a particular function e.g. muscle
An organ is a collection of different tissue which perform a specialised function e.g. leaf in a
plant.…read more

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In animal cells, division is by in-tucking of plasma membrane at spindle equator and pinching
cytoplasm in half
In plant cells the Golgi apparatus forms vesicles of new cell wall materials which collect along
the line of equator spindle a.k.a. the cell plate.
The vesicles coalesce and form new plasma membrane and cell walls between the 2 cells
Observing chromosomes during mitosis experiment
1. The tip of a root (5mm) is cut off and retained
2. Tip is transferred to a watch glass
3.…read more

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Without the reductive nuclear division the chromosome number would double with each
Mammals reproduce sexually only but plants can reproduce both sexual and asexual
Self fertilisation is when the sperm and egg come from the same individual but cross
fertilisation comes from sperm and egg form different individuals.
Meiosis, the reduction of division
In meiosis, 4 daughter cells are produced, each with half the number of chromosomes of the
parent cell.…read more

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Crossing over segments of individual maternal and paternal chromosomes- results in new
combinations of genes on chromosomes of the haploid cells produced. It involves:
1. Homologous chromosomes paired in a bivalent
2. If chromatids break at corresponding points along their length, their rejoing may
cross over
3. The chromatids finally separate and move to haploid nuclei in meiosis 2, producing
new genetic combination.…read more

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Steps leading to fertilisation
In mammals fertilisation occurs in upper oviduct and during intercourse the PH of the vagina
is quite acidic but the alkaline secretion of prostate gland helps to neutralise the acidity and
provide a neutral environment for the sperm
Waves of contraction of the muscular walls of the uterus guides the semen to the site of
As sperm nucleus enters the oocyte, the completion of meiosis 1 is triggered and the male
and female haploid nuclei come together to form the…read more

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Make up the bulk of the embryo as it Occurs in growing and adult body, with most
commences development from the inner cell organs where they replace dead/damaged cells
mass of blastocyst e.g. in bone
A.k.a pluripotent stem cells- meaning having the a.k.a. multipotent stem cells- meaning capable of
potential to differentiate into `very many but not giving rise to a restricted range of cell types i.e.…read more

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Eggs are then mixed with sperm in a shallow dish and are checked using a microscope to
ensure fertilisation
Zygotes then incubated at body temperature for 2-3 days
Microscope examination to confirm that the embryos have reached the 4-8 cell stage
Up to 3 embryos are transferred into uterus in the expectation that will implant
successfully ­ embryo can be frozen for further use
2.…read more

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It is now possible to obtain ES cells from an It is an important overriding principal for some
embryo at the 8 cell stage without causing the people that humans should not tamper with
death of the embryo `nature' in a deliberate way
It is possible to regulate and control ES cell Stem cell research is a costly technology, mostly
research to serve: beneficial to the lives of a very limited number
Advances in understanding and treating of people of developed nations, whereas if
infertility…read more


Divya Sareen

You amazing person!! Thankyou so much !! very detailed.


thankyou! um look at my unit 1 notes and see if you like them as well


Thank you, What did you get for this exam? 


Um I don't know because this is the first time I'm taking A levels- I'll tell you as soon as I get my results- and your welcome!


omg life saver!!! thankyou ! :)

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