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Cell Structure
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.
Present in
plant cell
Present in
animal cell
Function
1. Cell wall
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
(Ribosomes)
Yes
Yes
protein synthesis
7. Smooth endoplasmic
reticulum
Yes
Yes
synthesis of lipids
8. Golgi apparatus
Yes
Yes
modification and packaging of
proteins
Organelle
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With a light microscope, you can only see (in good preparation):
cell wall
nucleus
chromosomes
vesicles
vacuole
chloroplasts
With an electron microscope, you can see everything.
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
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
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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 Cylce”
- 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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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 thank 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)
cell wall has several functions:
rigidity and strength (i.e. resists expansion when the cells are turgid)
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(b)
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allows communication between cells (i.e. cytoplasmic connections –
plasmodesmata in cell wall)
(c)
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)
(a)
Membrane system
- site of light reactions (photosynthesis)
- chlorpophyll pigments
- enzymes
- electron carriers
- flattened, fluid-filled sacs (called “thylakoids” – which are stacked to form grana)
- lamellae between grana
(b)
Stroma
- site of dark reactions in photosynthesis
- photosynthetic gel containing enzymes associated with “calvin cycle” and sugars
and organic acids
(c)
Starch grains
- excess sugars
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Golgi apparatus (a.k.a. Golgi body)
- series of complex tubules
- flattened cisternae
- present in cells which actively produce enzymes
- modify and package proteins produced by ribosomes
- e.g. carbohydrates are added to proteins in the golgi body – to become
glycoproteins, which are released into the cytoplasm and are fused with the surface
membrane to be released outside.
- released as hormones or enzymes
- the golgi body is also involved with the transportation of lipids within the cells and
the formation of lysosomes (which contain digestive enzymes)
- lysosomes fuse with and digest unwanted substances in the cells or old worn-out
organelles
golgi vesicle
released from
cisternae
cytasol (between cysternae)
starts from
packages from the
endoplasmic reticulum
- protein molecules are transferred between cisterneae
- secretory vesicles pass through the cytosol and fuse with adjacent cisterna
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Endoplasmic Reticulum
- membrane-bound flattened sacs or tubules – cisternae
- form a network – continuous with outer membrane of nuclear envelope
- same structure as other membranes in cell
- increases the surface area within the cell
o sites of metabolic activity
o channels for movement of substances
o separate different activities of the cell proceeding simultaneously
2 types
1.
Rough endoplasmic reticulum
- the bulk of the endoplasmic reticulum
- encrusted with ribosomes
- isolates and transports proteins synthesised by the ribosomes
- many of these proteins are not required by the cell, but are secreted by the cell (e.g.
digestive enzymes, hormones etc.)
- rough endoplasmic reticulum acts as a transport system for these products.
2.
Smooth endoplasmic reticulum
- not encrusted with ribosomes
- involved in synthesis and transport of lipids and steroids
Summary
Endoplasmic reticulum - cytoskeleton - surface for chemical reactions and pathway for
transport of products
ribosomal
Ribosomes
- composed of RNA (rRNA)
- each consists of 3 sub-units – one slightly larger than the other (“cottage loaf”
shape)
- assembles amino acids into proteins
- operates in conjunction with mRNA
- bound to endoplasmic reticulum
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- often several ribosomes collectively (polysome)
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- N.B. not all ribosomes are associated with the endoplasmic reticulum. In growing
cells or cells that are making proteins for internal use, large numbers of ribosomes
are found free in the cytoplasm.
Lysosomes
- formed by golgi body
- they are vesicles containing enzymes
- carry out intracellular digestion
- 0.2µ – 0.5µ wide
- enzymes include nuclease, protease, lipase, acid phosphotase (i.e. hydrolases)
o Function:
to release enzymes outside the cell (exocytosis)
to fuse with a particle taken in by exocytosis and digest it internally
(residual bits may be expelled by exocytosis)
Autophagy: digest “old worn-out” parts of cell / organelle
Centrioles
- not seen in plant cells
- act as anchors for spindle fibres when they pull chromosomes apart
Microtubules
- fine tubular organelles making up the cytoskeleton in the cytoplasm (i.e. maintain
shape)
- straight and hollow of various lengths
- made of protein sub-units called “tubulin”
- used to make centrioles and spindles for cell division