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AS level biology
Difference between Prokaryotes and Eukaryotes
The organelles found inside a Eukaryotic cell are:
Organelle Description Function
Nucleus A large organelle surrounded by Chromatins is made from
a nuclear envelope (double proteins and DNA. The pores
membrane) which contains allow substances to move
many pores. The nucleus between the nucleus and the
contains chromatins and are cytoplasm. The nucleolus
often a structure called makes ribosomes
Lysosome A round organelle surrounded Contains digestive enzymes,
by a membrane with no clear these are kept separate from
internal structure the cytoplasm by a surrounding
membrane but can be used to
digest invading cells or to break
down worn out components of
the cell.
Vesicles A small fluid-filled sac, which is Transport substances in and
in the cytoplasm, it is out of the cell and between the
surrounded by a membrane organelle.
Ribosomes A very small organelle that The site where proteins are
floats free in the cytoplasm or is made
attached to the rough
endoplasmic reticulum
Endoplasmic There are two types of The smooth ER synthesises
Reticulum endoplasmic reticulum these and processes lipids
are smooth ER (due to no The Rough ER folds and
ribosome's) and the rough ER, processes proteins that have
which is covered in ribosomes been made at the ribosomes
Golgi A group of fluid filled flattened It processes and packages new
Apparatus sacs. Vesicles are often seen at lipids and proteins. It also
the edge of the sacs makes lysosomes
Centrioles Hollow cylinders, containing a Involves with the separation of
ring of microtubules chromosomes during cell
Mitochondri Are double bound membrane, The site of aerobic respiration,
a the inner one is folded to form and is where ATP is produced.
cristae and inside that is the They are very active and
matrix. It contains enzymes require a lot of energy
which are used for respiration

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There are many differences between Eukaryotes and prokaryotes,:
Eukaryotes Prokaryotes
Larger cells (2-200um) Extremely small cells less and 2.0um
DNA is linear DNA is circular
Nucleus present-(DNA is inside) No nucleus-DNA is free in cytoplasm
No cell wall in animal cells, cellulose Cell wall made of polysaccharides but
cell wall in plants and chitin cell wall in not cellulose or chitin
Many organelles, mitochondria present Few organelles, no mitochondria
Large ribosomes Smaller ribosomes
Example Human liver cell Example E.Coli bacterium
Protein Transport
1.…read more

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The master gene produces mRNA which is translated into signal
proteins. These proteins switch on the genes responsible for producing the
proteins needed for the specialisation of the cell in each segment.
The Cell cycle and Mitosis
The cell cycle consists of a period of cell growth
and DNA replication called interphase and a
period of cell division called mitosis.
Interphase is sub divided into three separate
growth stages These are called G,S and G2. Mitosis
is used for growth, repair and asexual
reproduction.…read more

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Production of a gametes
In animals the male gamete is sperm, and in female it is an egg cell (ova). In plants
the male gamete is pollen grains, and the female gametes are ovules.
Gametes contain only half the normal amount of chromosomes, so in one gamete
in humans it will be 23.
1. The DNA replicates so there are
two identical copies of each
chromosomes called
2. The DNA condense to form
double armed chromosomes, made
from two sister chromatids
3.…read more

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Genetic variation:
Crossing over of chromatids is when before the first division of meiosis the
homologous pairs of chromosomes come together and pair up, two of the
chromatids in each pair twist around each other. The twisted bit breaks off
their original chromatid and rejoins onto the other chromatid recombining
their genetically material. The point where the chromatids break is called
the chiasmata.
Independent assortment of chromosomes is when the four daughter cells
formed from meiosis have completely different combinations of
chromosomes.…read more

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When the tube reaches the ovary, it grows through the micropyle, and into
the embryo sac within the ovule.
4. In the embryo sac, the tube nucleus disintegrates and the tip of the pollen
tube bursts releasing two male nuclei
5. One male nucleus fuses with the egg nucleus to make a zygote. This divides
by mitosis.
6. The other male nucleus fuses with two other nuclei called polar nuclei,
which is at the centre of the embryo sac.…read more

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Stem cells in Medicine
Some stem cell treatment already exists, such as treatment for leukaemia, where
bone marrow transplants are given. Scientists are researching treatments for
injuries such as: spinal cord injuries, and heart disease. The treatments can save
many lives, and also improve the quality of lives for many people.
There are two methods from where stem cells can come from, these are Adult stem
cells and embryonic stem cells.…read more

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There are many for and against arguments.
To help these arguments many authorities have been set up. These include looking
at the proposals of research to see if it should be allowed. This ensures that any
research involving embryos is carried out for a good reason. The licensing and
monitoring centres involved in embryonic stem cell research. There are many
guidelines which are involved. They also provide evidence and advice to
governmental officials.…read more

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Since 1850 the human population has on average got 8cm taller. This can be due to
many reasons such as:
Taller men have more children, resulting in a gradual change
Greater movements of people have lead to less inbreeding
Improved health, especially in protein
The end of child labour has allowed more energy to be put into growth.
Hair colour
In animals they make melanin using the enzyme Tyrosinase. This is the first step
along a chemical pathway.…read more


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