Download 3.1 AS Unit: Cells, Exchange and Transport Module 1: Cells 1.1.1

AS Biology: OCR Syllabus
Module 1.1.1
3.1 AS Unit: Cells, Exchange and Transport
Module 1: Cells
1.1.1 Cell Structure
Candidates should be able to:
(a)
state the resolution and magnification that can be achieved by a light microscope, a transmission
electron microscope and a scanning electron microscope;
Light Microscope
Transmission Electron
Microscope
Scanning Electron
Microscope
Resolution
0.2 μ (200nm)
0.2nm
0.2nm
Magnification
≈ ×1500 / 2000
Over 500 000
250 000
(b)
explain the difference between magnification and resolution;
Resolution
“the ability of an optical system to distinguish between two adjacent
objects”
Magnification
increases the apparent size of an object”
Resolving power
“the degree of detail that can be seen with a microscope”
The resolving power is inversely proportional to the wavelength of the
radiation used (i.e. the shorter the wavelength, the greater the
resolution).
(c)
Stains:
(d)
explain the need for staining samples for use in light microscopy and electron microscopy;
-
most biological structures are transparent
the stain gives a contrast between different structures
the stain combines with certain chemicals in the structure
- Iodine solution:
Starch
→
blue-black
- Eosin solution:
cytoplasm →
pink
- Feulgens agent
DNA →
dark red / purple
- Aceto-orcein agent
calculate the linear magnification of an image (HSW3);
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(e)
Key
1
2
3
4
5
6
7
8
Module 1.1.1
describe and interpret drawings and photographs of eukaryotic cells as seen under an electron
microscope and be able to recognise the following structures: nucleus, nucleolus, nuclear envelope,
rough and smooth endoplasmic reticulum (ER), Golgi apparatus, ribosomes, mitochondria,
lysosomes, chloroplasts, plasma (cell surface) membrane, centrioles, flagella and cilia;
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
(f)
outline the functions of the structures listed in (e);
Organelle
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
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Module 1.1.1
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
9.
Centriole
Yes
Yes
organises spindle during mitosis
10. Flagella
Yes
Yes
locomotion
11. Cilia
Yes
Yes
chemical sensation, signal
transduction
(g)
outline the interrelationship between the organelles involved in the production and secretion of
proteins (no detail of protein synthesis is required);
Proteins are created via a process called protein synthesis. This beings in the nucleus and
then moves to the Golgi apparatus where it is modified and packaged. This means that
certain chemical groups may be added to it. This protein can then be expelled if needed via
a process called exocytosis or can be used within the cell.
(h)
explain the importance of the cytoskeleton in providing mechanical strength to cells, aiding
transport within cells and enabling cell movement;
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained within the cytoplasm.
The cytoskeleton is present in all cells; it was once thought this structure was unique to
eukaryotes, but recent research has identified the prokaryotic cytoskeleton. It is a dynamic
structure that maintains cell shape, protects the cell, enables cellular motion (using
structures such as flagella, and cilia), and plays important roles in both intracellular
transport (the movement of organelles, for example) and cellular division.
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AS Biology: OCR Syllabus
(i)
Module 1.1.1
compare and contrast, with the aid of diagrams and electron micrographs, the structure of
prokaryotic cells and eukaryotic cells;
Prokaryotic Cell
There may also be lipid or
glycogen granules within
the cell, so look out for
those!
(for Eukaryotic cell, see part (e))
(j)
compare and contrast, with the aid of diagrams and electron micrographs, the structure and
ultrastructure of plant cells and animal cells
See part (f) for the difference between plant and animal cells.
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
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Module 1.1.1
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
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
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-
Module 1.1.1
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)
(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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Module 1.1.1
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.
- 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)
packages from the
endoplasmic reticulum
starts from
- protein molecules are transferred between cisterneae
- secretory vesicles pass through the cytosol and fuse with adjacent cisterna
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
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Module 1.1.1
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
- often several ribosomes collectively (polysome)
- 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:
ƒ to release enzymes outside the cell (exocytosis)
ƒ to fuse with a particle taken in by exocytosis and digest it internally
(residual bits may be expelled by exocytosis)
ƒ 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
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