Download Cells - the basic unit of living organisms

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Biology AS
Cells - the basic unit of living organisms
You all know what cells are (or if you don’t, please go to umm…Kindergarten!?) but of
course we do forget the very basics sometimes (I mean…come on…how many of you STILL
remember what 1 + 1 equals?!). So just to embarrass you all…
Cells…
(a)
(b)
(c)
(d)
…are building blocks of structures
…are derived from others by cell division
…contain information for growth, development and functioning
…are the place where all chemical reactions take place
The DNA of all cells of a species (organism) is identical. People do not understand why
cells are different. Quite amazing if you think about it really.
Prokaryotic Cells : primitive
(e.g. Escherichia Coli bacteria)
Eukaryotic Cells: complex
(e.g. palisade or liver cell)
0.5 µ
Up to 40 µ (1,000 – 10,000 times bigger)
Found in bacteria and green algae
Found in plants, animals, fungi and protist
No nuclear membrane: no nucleus.
Only one strand of DNA in the cytoplasm
Distinct nuclear membrane
– a bound nucleus
No chromosomes – only one strand of DNA
DNA on chromosomes
Few organelles in cytoplasm
Many organelles in cytoplasm
No membrane-bound organelles
(e.g. mitochondria / chloroplasts)
Organelles ARE membrane bound
No mitochondria
(mesonomes carry out respiration instead)
Mitochondria (for aerobic respiration)
Small ribosomes (free in the cytoplasm)
Larger ribosomes (attached to a membrane)
No mitosis / meiosis – binary fission instead
Mitosis and meiosis
Flagella lack 9, 2 arrangement
(needed to aid movement)
Flagella have 9, 2 fibril arrangement
(points in bold are key points)
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Biology AS
Cell diagrams
(yes, you do need to know all the bits…)
Prokaryotic Cell
There may also be lipid
or glycogen granules
within the cell, so look
out for those!
Eukaryotic Cell
Key
1
2
3
4
5
6
7
8
Plasma Membrane
Smooth Endoplasmic Reticulum
Vacuole
Golgi Body
Mitochondrion
Ribosomes
Centriole
Rough Endoplasmic Reticulum
Parts of the nucleus
9
Nuclear Envelope
10
Nucleolus
11
DNA / Chromosomes
12
Nuclear Pore
13
Nucleoplasm
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Biology AS
Though eukaryotic cells contain many organelles, it is important to know which are in plant
cells, which are in animal cells and what their functions are.
Organelle
1. Cell wall
Present in
plant cell
Present in
animal cell
Function
Yes
No
strength, resist pressure created
when water enters
2. Plasma membrane
Yes
Yes
selectively controls the
movement of substances into and
out of cells
3. Nucleus
Yes
Yes
contains DNA which holds the
genetic information
4. Mitochondria
Yes
Yes
produces large amounts of
A.T.P. by aerobic respiration
5. Chloroplast
Yes
No
photosynthesis
6. Rough endoplasmic
reticulum
Yes
Yes
protein synthesis
7. Smooth endoplasmic
reticulum
Yes
Yes
synthesis of lipids
8. Golgi apparatus
Yes
Yes
modification and packaging of
proteins
With a light microscope, you can only see (in good preparation):
cell wall
vesicles
nucleus
vacuole
chromosomes
chloroplasts
With an electron microscope, you can see everything. (And because of the amazing guy who
discovered it all (Mr. Microscope), you have to learn all the structures! Lucky you ☺)
Nucleus
-
spherical about 10-20 µ
surrounded by a double membrane (called a nuclear envelope)
the nuclear envelope has a structure like a cell membrane
it is pierced with pores to allow mRNA to get to the ribosomes in protein synthesis
Inside the nucleus there is a nucleolus, the function of which is to make RNA
the bulk of the nucleus is contained within the nucleoplasm (i.e. ground materials
containing fixed numbers of chromosomes)
- The chromosomes are made of nucleoproteins and control the biochemistry of the
cell
- with an electron microscope, it is possible to see the pores
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Biology AS
Cell Membrane
- highly selective region made of phospholipids
- regulates the uptake and release of materials from the cell
- is approximately 7.5nm in thickness
- the outside contains glycocalyx which has unique markings for cell recognition
- the advantage of this is that it has an immunological memory – so it can learn
which cells to kill – therefore there is efficient recognition.
- the disadvantage of this is that it means that there can be rejection in transplants
Cellulose cell wall
Plant cells only
- not part of the cell: extra cellular only
- provides rigidity
- fully permeable:
prevents cell from rupturing
o prevents the intake of excess water
- polysaccharide
Cytoplasm
- subdivided
- the protoplasm is the name for all of the materials inside the cell membrane
- the cytoplasm is the name for all the material inside the membrane but not the
nucleus
- i.e. all the ground materials including organelles are in the cytoplasm
- the cytoplasm forms a cytoskeleton
o 90% water and inorganic salts and organic molecules
o organelles are absent in prokaryotic cells
Mitochondria
- size: 1µ wide, 2.5 µ long (would not be seen under light microscope)
- numbers vary according to the type of cell (normally 1000 per cell)
- muscle (and sperm) cells have more
- site of ATP production
- electron microscope reveals the internal structure
- organelle bounded by a double membrane (which is highly folded)
- giving rise to CRISTAE, which project into the interior of the organelle
- cristae are involved in “oxidative phosphorylation” and electron transport
- many of the enzymes are embedded in the wall of the cristae
- interior
o consists of organic matrix containing numerous chemical compounds
o site of “Krebs Cycle”
- DNA is present in mitochondria so
that it can replicate itself
- in plant mitochondria, the cristae are
plate-like
- in animal mitochondria, the cristae
are finger-like
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Biology AS
Cell Wall
- consists of many cellulose fibres
- cemented together by a mixture of other organic substances
- cellulose:
polysaccharide (polymer of glucose)
o consists of long chains of glucose molecules
o bound by adjacent molecules
- in the cell wall, there are around 2000 parallel cellulose molecules, which are
packed to form “microfibrils”
- there, in turn, are bundled together to form “fibrils”
- the structure is like fibre glass – in the sense that the cell wall has great strength –
due to the many strong fibres and “glue” that holds them together
- All plant cells start by having a primary wall, which is flexible
- this grows with the cell
- the fibrils in this wall run in all directions
- Most plant cells develop a secondary wall
- this is thicker than the primary wall
- many additional layers are deposited outside the primary wall
- in each layer of the secondary wall, the fibrils run mainly in the same direction
- in older cells, more layers are laid down, but the fibrils run at different angles
- the consequence of this is that the overall structure has great strength and
prevents any further increase in size
Example: Xylem
- lignin is laid down
- this further strengthens the secondary wall
- this increases the strength of the supporting tissues (e.g. in trees and shrubs)
Conclusion (a)
(b)
(c)
cell wall has several functions:
rigidity and strength (i.e. resists expansion when the cells are turgid)
allows communication between cells (i.e. cytoplasmic connections –
plasmodesmata in cell wall)
forces cell to grow in a certain way (i.e. shape) (e.g. long tube – xylem)
Chloroplast
- before the electron microscope, this was only seen as a body with a series of layers
- with the electron microscope, lamellae are confirmed to be these layers
- also: grana:
stacks of densely packed membranes
o linked by lamellae
- stroma (cytoplasm of chloroplast)
- double membrane (responsible for “fixation” of CO2) (i.e. the site of enzymes,
which fix CO2)
- in eukaryotes only
- size: 3 – 10 µ in diameter
- just visible with a light microscope (but is undifferentiated)
- photosynthetic pigments are located on internal membranes (grana)
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