GCSE BIOLOGY -> CELL BIOLOGY
CELL STRUCTURE
MICROSCOPES
LIGHT MICROSCOPES
1590s : Dutch spectacle makers Janssen made the first compound microscope
1650 : British scientist Hooke observed and drew cells using a compound
microscope
1800s : optical quality of lenses increased and microscopes similar to ones we use
today
The maximum magnification with a light microscope is around x1500. This means
that the resolution (The ability to distinguish between two points in an image) of light
microscope is very low. Subcellular structures cannot be identified.
ELECTRON MICROSCOPES
Electron microscopes use a beam of electrons rather than light rays. There are two
types;
Scanning electron microscope (SEM) has a large field of view so can be used
to examine the surface structure of specimens. SEMs are often used at lower
magnifications.
The transmission electron microscope (TEM) is used to examine thin slices or
sections of cells or tissues. These have a magnification of around x1000000.
Their limit of resolution is now less than 1nm. The TEM has revealed
subcellular structures within cells not visible with the light microscope.
Microscopes can be used to magnify the image of a specimen to make it appear
larger. To calculate the magnification of a microscope, use the equation;
Magnification of microscope = magnification of eyepiece x magnification of objective
lens
The formula to calculate the magnification is;
size of image
Magnification = ________________
real size of image
PRACTICAL – Preparing a slide
Animal cells
1. Place a small drop of water on a slide
2. Swap the inside of a cheek with a clean cotton bud
3. Gently rub cotton bud on slide and you can see the cells with the naked eye
Plant cells
1. Place a small drop of water on a slide
2. Peel some onion skin from the inside of a leaf of an onion bulb
, 3. Use forceps to transfer to the drop of water. Ensure it is flat and that there are
no air bubbles
4. Stain the sample with iodine
When viewing a slide with a microscope, a small square of thin glass called a
coverslip is placed over the specimen. It protects the microscope and prevents the
slide from drying out when it’s being examined. The coverslip is gently lowered onto
the slide and a mounted needle is used to hold the specimen in place.
Since most cells are colourless, a stain is used to add contrast.
RISKS
Care must be taken when looking down the microscope if the illumination is
too tight
Care must be taken when using stains
Care must be taken when handling coverslips, microscope slides, and
mounted needles.
PRACTICAL – Using a microscope
1. Rotate the objective lens so the lowest power lens is inline with the stage
2. Turn the coarse focus knob so the stage is as close to the lens as possible
3. Place the slide on the stage and line it up so that the specimen is at the centre
of the stage
4. Focus the slide using both the coarse and fine focus adjustments
5. Draw a low power image of what you see and then rotate to a higher power
objective lens
DRAWING IMAGES
Low Power Diagram
A plan to show the arrangement of distinct regions
Shows the outline of cells making up a tissue
High Power Diagram
Detailed image of part of a slide
Usually shows a single cell
CELL SIZE
Most cells are measures using the basic unit μm – this is a micrometer and is equal
to 1x10-6 m
Subcellular structures and viruses are even smaller than cells. This means the
smaller unit of a nanometer is needed to measure them and this is equal to 1x10-9 m
As you can see, these numbers are written in standard form. Standard form numbers
are written as
- X x 10n
- X is a number >1 but <10
- n is the index of the power, in a power of 10
CELLS
ANIMAL CELLS
CELL STRUCTURE
MICROSCOPES
LIGHT MICROSCOPES
1590s : Dutch spectacle makers Janssen made the first compound microscope
1650 : British scientist Hooke observed and drew cells using a compound
microscope
1800s : optical quality of lenses increased and microscopes similar to ones we use
today
The maximum magnification with a light microscope is around x1500. This means
that the resolution (The ability to distinguish between two points in an image) of light
microscope is very low. Subcellular structures cannot be identified.
ELECTRON MICROSCOPES
Electron microscopes use a beam of electrons rather than light rays. There are two
types;
Scanning electron microscope (SEM) has a large field of view so can be used
to examine the surface structure of specimens. SEMs are often used at lower
magnifications.
The transmission electron microscope (TEM) is used to examine thin slices or
sections of cells or tissues. These have a magnification of around x1000000.
Their limit of resolution is now less than 1nm. The TEM has revealed
subcellular structures within cells not visible with the light microscope.
Microscopes can be used to magnify the image of a specimen to make it appear
larger. To calculate the magnification of a microscope, use the equation;
Magnification of microscope = magnification of eyepiece x magnification of objective
lens
The formula to calculate the magnification is;
size of image
Magnification = ________________
real size of image
PRACTICAL – Preparing a slide
Animal cells
1. Place a small drop of water on a slide
2. Swap the inside of a cheek with a clean cotton bud
3. Gently rub cotton bud on slide and you can see the cells with the naked eye
Plant cells
1. Place a small drop of water on a slide
2. Peel some onion skin from the inside of a leaf of an onion bulb
, 3. Use forceps to transfer to the drop of water. Ensure it is flat and that there are
no air bubbles
4. Stain the sample with iodine
When viewing a slide with a microscope, a small square of thin glass called a
coverslip is placed over the specimen. It protects the microscope and prevents the
slide from drying out when it’s being examined. The coverslip is gently lowered onto
the slide and a mounted needle is used to hold the specimen in place.
Since most cells are colourless, a stain is used to add contrast.
RISKS
Care must be taken when looking down the microscope if the illumination is
too tight
Care must be taken when using stains
Care must be taken when handling coverslips, microscope slides, and
mounted needles.
PRACTICAL – Using a microscope
1. Rotate the objective lens so the lowest power lens is inline with the stage
2. Turn the coarse focus knob so the stage is as close to the lens as possible
3. Place the slide on the stage and line it up so that the specimen is at the centre
of the stage
4. Focus the slide using both the coarse and fine focus adjustments
5. Draw a low power image of what you see and then rotate to a higher power
objective lens
DRAWING IMAGES
Low Power Diagram
A plan to show the arrangement of distinct regions
Shows the outline of cells making up a tissue
High Power Diagram
Detailed image of part of a slide
Usually shows a single cell
CELL SIZE
Most cells are measures using the basic unit μm – this is a micrometer and is equal
to 1x10-6 m
Subcellular structures and viruses are even smaller than cells. This means the
smaller unit of a nanometer is needed to measure them and this is equal to 1x10-9 m
As you can see, these numbers are written in standard form. Standard form numbers
are written as
- X x 10n
- X is a number >1 but <10
- n is the index of the power, in a power of 10
CELLS
ANIMAL CELLS
