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Topic 2: CELLS
Chapter 3: CELL STRUCTURE
3.1 Methods of studying microscopes
Microscopy
● Instruments that produce a magnifier image of an object
● Optical microscopes use light to form an image. The relatively long wavelength of
light rays means that a light microscope can only distinguish between two objects if
they are 0.2nanometers
● The maximum useful magnification of an optical microscope is about x1500
Magnification
● Magnification is how much bigger the image is than the specimen
● Formula = magnification = size of image/ size of the real object
Resolution
● Resolution is how detailed the image is. More specifically it’s how well the
microscope distinguishes between two points that are close together
● If a microscope lens can’t separate two objects then increasing the magnification
won’t help
Cell fractionation
To look at organelles under an electron microscope first, they need to be separated from the
rest of the cell. Three steps:
1. Homogenisation: breaking up the cells. Cells are broken by a homogeniser (blender).
This releases organelles from the cell. The solution must be kept cold, to reduce
enzyme activity. It also should be isotonic so the water potential is the same. A buffer
solution should be added to maintain pH.
2. Filtration: removing big bits. The solution is filtered to remove any complete cells
3. Ultracentrifugation: Separating the organelles. Ultracentrifugation is the process by
which the fragments in the filtered homogenise are separated in a machine. For
animal cells, the following process happens:
~ A tube of filtrate is placed in the centrifuge and spied. At a slow speed
~ Heaviest organelles e.g. nuclei get going to the bottom. They form thick sediment at the
bottom. The rest of the organelles stay suspended in the fluid above the sediment-
supernatant
~ The supernatants are transferred to another tube and spin in the centrifuge at a high
speed.
~ Heaviest organelles are flung to the bottom and the process is repeated until all the
organelles are separated.
~ The organelle is speared in order of mass 1. Nucleus 2. Mitochondria 3. Lysosome 4.
Endoplasmic reticulum and finally 5. Ribosomes
3.2 Electron microscope
, ● They use electrons to form an image
● They have a higher resolution than optical microscopes so give a more detailed
image
● The maximum resolution is about 0.002 nanometers.
● The maximum useful magnification of an image left Ron microscope is about
x1500000
Types of electron microscopes
1. Transmission electron microscope (TEM)
TEMS use electromagnets to focus a beam of electrons and high is then transmitted through
the specimen
Denser parts of the specimen absorb more electrons which makes them look darker
Advantages: Give hog resolutions image so shows small objects
Disadvantage: can only be used in dead specimens
Can only be used in non-living specimens
2. Scanning electron microscope (SEM)
SEMs scan a beam of electrons across the specimen. This knocks off electrons from the
specimen which are gathered in a cathode ray tube to form an image.
The images you end up with will show the surface of the specimen and they appear in 3D.
Advantages: can be used on thick specimens. Can be 3D
Disadvantage: gives a lower resolution image than TEM
Can only be used on non-living Specimens
3.3 Microscope measurements and calculations
Measuring cells
● when using a light microscope we can measure the size of objects using an eyepiece
graticule
● It is placed in the eyepiece
● A scale is etched on it. Usually 10mm long and divided into subdivisions
● The scale on the eyepiece graticule cannot be used directly to measure the side of
objects under a microscope bid tube orbs because ur has a different magnification
● For each objective orbs the testicular can remain in position for future use provided
the same objective lens is used
Calibrating the eyepiece graticule
● To calibrate an eyepiece graticule you need to use a microscope slide called a stage
micrometre.
● When the eyepiece graticule scale and the stage micrometre scales are lined up, it is
possible to calculate the length of the divisions on the eyepiece graticule
● E.g. 10 units on the micrometre scale are equivalent to 39 units on the graticule
scale. There 1 unit on the micrometre scale equals 4 units on the graticule scale. As
each unit in the micrometre scale equals 10 manometer each unit in the graticule
equals 10:4 = 2.5 nanometers.
3.4 Eukaryotic cell structure
, Ultrastructure of eukaryotic cells
● Nucleus: A double membrane called the envelope containing around 3000 nuclear
pores that enable molecules to enter and leave. It also contains a chromatid and a
nucleolus which is the site of ribosome production. A granular jelly-like material
called nucleoplasm makes up the bulk of the nucleus. Function: control centre of the
cell and retains genetic material.
● Rough endoplasmic reticulum: a series of flattened sacs cells enclosed by a
membrane with ribosomes on the surface. RER folds and processes proteins made on
the ribosomes.
● Smooth endoplasmic reticulum: a system of membrane-bound sacs. SER produces
and processes lipids
● Golgi apparatus: a series of fluid-filled flattened and curved sacs with vesicles
surrounding the edges. Golgi apparatus processes proteins and lipids. It produces
lysosomes.
● Mitochondria: oval-shaped and bound by a double membrane called the envelope.
The inner membrane is folded to form projections called cristae with a matrix on the
inside containing all the enzymes needed for respirations
● Centrioles: Hollow cylinders containing a ring of microtubules arranged at right
angles to each other. Centrioles are involved in producing spindle fibres for cell
division.
● Ribosomes: are composed of two subunits and are the site of protein products.
● Lysosomes: vesicles containing digestive enzymes bound by a single membrane.
● Chloroplasts: carry photosynthesis. The main features include the chloroplast
envelope that surrounds the organelle, the grana are stacks of up to 100 disc-like
structures called thylakoids which contain pigment chlorophyll. Also, stroma where
the second stage of photosynthesis takes place.
● Cell wall: contains microfibrils of the polysaccharide cellulose embedded in a matrix.
Cellulose microfibrils have considerable strength and so contribute to the overall
strength of the cell wall which provides mechanical strength to prevent the cell from
bursting under pressure created by osmotic entry into the water.
● Vacuoles: are fluid-filled sacs bounded by a single membrane. It contains minerals,
amino acids and waste. They support herbaceous plants and are temporary food
stores.
3.5 Cell Specialisation and Organisation
Cell specialisation
● All cells perform certain basic functions. Cells are specialised in different ways to
perform a particular role.
● The first grip of cells in an embryo is identical. As it matures each cell becomes
specialised to be able to carry out a particular function
Tissues
● a collection of similar cells taut perform a particular function is called tissues
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