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Summary Summarised AS level Biology notes
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AS Bio notes Raida 12DCell structure:
Cell: the basic unit of all living organisms
Surrounded by a cell surface membrane and contains genetic material (DNA) and
cytoplasm, containing organelles
Organelles: a functionally and structurally distinct part of a cell (ex. A Ribosome or
Mitochondria)
Eukaryote: an organism whose cells contain a nucleus and other membrane-bound
organelles
Prokaryote: an organism whose cells do not contain a nucleus or any other membrane-
bound organelles
Nucleus: a relatively large organelle found in eukaryotic cells but absent from prokaryotic
cells; the nucleus contains the cells DNA, therefore controlling the activities of the cell
Surrounded by 2 membranes which together form the nuclear envelope
1.2 Cell biology and microscopy
There are 2 different types of microscope:
1. Light microscope: light as a source of radiation
2. Electron microscope: electrons as a source of radiation
1.3 Plant and animal cells as seen with a light microscope
UNITS OF MEASUREMENT:
FRACTION OF METRE UNITS SYMBOL
One thousandth = 0.001 = 1/1000 = 10^-3 Milimetre mm
One millionth = 0.000001 = 1/1000000 = 10^-6 Micrometre μm
One thousand millionth = 0.000000001 = 1/1000000000 = 10^- Nanometre nm
9
A photomicrograph is a photograph of a specimen as seen with a light microscope
(staining)
Many of the cell contents are colourless and transparent, so they need to be stained with
coloured dyes to be seen
The chromatin in the nuclei is particularly heavily stained
Plant cells have not been stained because the chloroplasts contain the green pigment
chlorophyll and are easily visible without staining
> Features that animal and plant cells have in common:
Cell surface membrane:
All cells, both prokaryotes and eukaryotes, are surrounded by a very thin cell surface
membrane
Sometimes referred to as Plasma membrane
Partially permeable
Controls the exchange of materials between the cell and its environment
, Nucleus:
All eukaryotic cells contain a nucleus
Relatively large structure
Stains intensely and is therefore very easy to see with the microscope
The deeply staining material in the nucleus is called Chromatin
Chromatin is a mass of coiled threads. The threads are seen to collect together to form
chromosomes during nuclear division
Chromatin contains DNA, the molecule containing the instructions (genes) that control
the activities of the cell
Inside the nucleus, an even more deeply staining area is visible, the Nucleolus
Made of loops of DNA from several chromosomes
Number of nucleoli is variable, 1-5 being common in mammals
One of the main functions of nucleoli is to make ribosomes
Cytoplasm:
All living material inside the cell is called Protoplasm
All living material outside the nucleus is called Cytoplasm
[cytoplasm + nucleus = protoplasm]
Cytoplasm is an aqueous material, with a fluid to a jelly-like consistency
Using a light microscope, many small structures can be seen within it known as
organelles
An organelle can be defined as a functionally and structurally distinct part of a cell
Organelles are often, but not always, surrounded by one or two membranes so that their
activities can be separated from the surrounding cytoplasm
Organising cell activities in separate compartments is essential for a structure as
complex as an animal or plant cell to work efficiently
Mitochondria:
Only just visible using light microscope, they’re also the most numerous organelles seen
with the light microscope
They can move about, change shape and divide
They’re specialised to carry out aerobic respiration
Golgi apparatus:
Collects and processes molecules within the cell, particularly proteins
> Differences between plant and animal cells:
One of the structures commonly found in animal cells which is absent from plant cells is
Centrioles. Plant cells also differ from animal cells in terms of cell walls, large permanent
vacuoles and chloroplasts
Centrioles:
Appears as a small structure close to the nucleus
Made of microtubules in circular pattern
, Cell walls
An extra structure outside the cell surface membrane
The wall is relatively rigid because it contains fibres of cellulose (a polysaccharide which
strengthens the wall)
Gives the cell a definite shape
Prevents the cell from bursting when water enters by osmosis, allowing large pressures
to develop inside the cell
Cell walls may be reinforced with extra cellulose or with a hard material called Lignin for
extra strength
They are freely permeable, allowing free movement of molecules and ions through to the
cell surface membrane
Plasmodesmata:
Plant cells are linked to neighbouring cells by means of pores containing fine strands of
cytoplasm, these structures are called plasmodesmata
Lined with cell surface membrane
Movement through the pores is thought to be controlled by the structure of the pores
Vacuoles:
Sac-like structures surrounded by a single membrane called Tonoplast which controls
the exchanges between the vacuole and cytoplasm
Although animal cells may possess small vacuoles which are temporary structures,
mature plant cells often possess a large, permanent, central vacuole
In plants, vacuoles help to regulate the osmotic properties of cells (the flow of water
inwards and outwards)
The fluid in the vacuole is a solution of pigments, enzymes, sugars, and other organic
compounds (including some waste products), mineral salts, oxygen, and carbon dioxide
For example, the pigments which colour the petals of certain flowers and the parts of
some vegetables, may be found in vacuoles
Chloroplasts:
Organelles specialised for the process of photosynthesis
Found in the green parts of the plant, mainly in the leaves
Relatively large so are easily seen with a light microscope
It is even possible to see tiny ‘grains’/grana (granum) inside the chloroplasts
Parts of the chloroplast contain chlorophyll, the green pigment which absorbs light during
photosynthesis
1.4 Measuring size and calculating magnification
Magnification is the number of times larger an image of an object is than its real size
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