Biology AS Level, OCR
It states that the cell is the fundamental structure of all living matter, and that cells can only develop
from other existing cells. ANS: What is cell theory? (2 points)
All living organisms are composed of cells, and since most cells are too small to see without a
microscope, we cannot observe the functions of cells and their organelles without one. ANS: Outline the
importance of microscopes in the study of living organisms.
Before then, microscope magnification was not as powerful, and so scientists were unable to see and
identify single cells or cell components. ANS: Why was cell theory not fully developed before the mid-
19th century?
1. Place the stage micrometer on the stage and align it with the eyepiece graticule.
2. Remember that each division on the stage micrometer is 10 micrometres.
3. Each eyepiece division is calculated by dividing the number of eyepiece divisions by the number of
micrometres.
4. Replace the stage micrometer with a specimen to measure. ANS: Describe the process of calibrating a
microscope.
How many times larger the image is than the actual size of the object. ANS: Define magnification.
It has two lenses - the objective lens near the specimen, and the eyepiece lens, through which the
specimen is viewed. ANS: How does a compound light microscope work?
A mount used to observe solid specimens, where the specimen is sectioned and a cover slip is placed
over the top. ANS: What is a dry mount?
A mount used to observe specimens suspended in water or immersion oil. ANS: What is a wet mount?
,A mount used to observe living specimens where the sample is squashed gently between the slide and
the cover slip. ANS: What is a squash slide?
A mount used to observe liquid samples by smearing the sample across the slide with a cover slip,
creating a thin, even coating of the substance. ANS: What is a smear slide?
This is used to separate bacteria into gram-positive and gram-negative bacteria. Crystal violet is applied
first to a bacterial specimen, then iodine, which fixes the dye, and then the slide is washed with alcohol.
Gram-positive bacteria retain the dye while gram-negative lose it as they have thinner cell walls. ANS:
Describe the gram stain technique.
This is used to differentiate Mycobacterium from other bacteria. A lipid solvent carries dye into the cells,
which are then washed with a dilute acid-alcohol solution. Mycobacterium retain the stain. ANS:
Describe the acid-fast technique.
By providing contrast, it allows us to differentiate between cell organelles that would otherwise be hard
to identify. ANS: Why do we need to stain samples?
The shortest distance between two objects that can still be seen as separate entities. ANS: Define the
term 'resolution'.
A beam of electrons illuminates the specimen, showing a more detailed cell ultrastructure. ANS: How
does electron microscopy work?
Transmission Electron Microscopy - electrons are transmitted THROUGH a specimen and focused to
produce an image. ANS: What is TEM?
Scanning Electron Microscopy - electrons are sent across the SURFACE of a specimen, and the reflecting
electrons are collected, giving 3D images. ANS: What is SEM?
, Light = inexpensive to buy and operate
Electron = expensive ANS: Relative costs of light and electron microscopes?
Light = small and portable
Electron = large and must be installed ANS: Relative portability of light and electron microscopes?
Light = simple sample preparation that doesn't usually lead to distortion.
Electron = complex sample preparation that often leads to distortion. ANS: Relative difficulties of
sample preparation of light and electron microscopes?
Light = natural colour of the sample, or colour of a stain, is seen.
Electron = black and white images are produced but can be coloured digitally. ANS: Relative colours of
the samples produced from light and electron microscopes?
Light = up to x2000
Electron = over x500,000 ANS: Relative magnifications of light and electron microscopes?
Light = 200nm
Electron = TEM: 0.5nm and SEM: 3-10nm ANS: Relative resolving power of light and electron
microscopes?
A visible structural detail caused by processing the specimen, and is not actually a feature of the
specimen itself. ANS: What is an artefact?
This refers to the chemical reactions of both the synthesis and the breaking down of molecules. ANS:
Define the term 'metabolism'.
It states that the cell is the fundamental structure of all living matter, and that cells can only develop
from other existing cells. ANS: What is cell theory? (2 points)
All living organisms are composed of cells, and since most cells are too small to see without a
microscope, we cannot observe the functions of cells and their organelles without one. ANS: Outline the
importance of microscopes in the study of living organisms.
Before then, microscope magnification was not as powerful, and so scientists were unable to see and
identify single cells or cell components. ANS: Why was cell theory not fully developed before the mid-
19th century?
1. Place the stage micrometer on the stage and align it with the eyepiece graticule.
2. Remember that each division on the stage micrometer is 10 micrometres.
3. Each eyepiece division is calculated by dividing the number of eyepiece divisions by the number of
micrometres.
4. Replace the stage micrometer with a specimen to measure. ANS: Describe the process of calibrating a
microscope.
How many times larger the image is than the actual size of the object. ANS: Define magnification.
It has two lenses - the objective lens near the specimen, and the eyepiece lens, through which the
specimen is viewed. ANS: How does a compound light microscope work?
A mount used to observe solid specimens, where the specimen is sectioned and a cover slip is placed
over the top. ANS: What is a dry mount?
A mount used to observe specimens suspended in water or immersion oil. ANS: What is a wet mount?
,A mount used to observe living specimens where the sample is squashed gently between the slide and
the cover slip. ANS: What is a squash slide?
A mount used to observe liquid samples by smearing the sample across the slide with a cover slip,
creating a thin, even coating of the substance. ANS: What is a smear slide?
This is used to separate bacteria into gram-positive and gram-negative bacteria. Crystal violet is applied
first to a bacterial specimen, then iodine, which fixes the dye, and then the slide is washed with alcohol.
Gram-positive bacteria retain the dye while gram-negative lose it as they have thinner cell walls. ANS:
Describe the gram stain technique.
This is used to differentiate Mycobacterium from other bacteria. A lipid solvent carries dye into the cells,
which are then washed with a dilute acid-alcohol solution. Mycobacterium retain the stain. ANS:
Describe the acid-fast technique.
By providing contrast, it allows us to differentiate between cell organelles that would otherwise be hard
to identify. ANS: Why do we need to stain samples?
The shortest distance between two objects that can still be seen as separate entities. ANS: Define the
term 'resolution'.
A beam of electrons illuminates the specimen, showing a more detailed cell ultrastructure. ANS: How
does electron microscopy work?
Transmission Electron Microscopy - electrons are transmitted THROUGH a specimen and focused to
produce an image. ANS: What is TEM?
Scanning Electron Microscopy - electrons are sent across the SURFACE of a specimen, and the reflecting
electrons are collected, giving 3D images. ANS: What is SEM?
, Light = inexpensive to buy and operate
Electron = expensive ANS: Relative costs of light and electron microscopes?
Light = small and portable
Electron = large and must be installed ANS: Relative portability of light and electron microscopes?
Light = simple sample preparation that doesn't usually lead to distortion.
Electron = complex sample preparation that often leads to distortion. ANS: Relative difficulties of
sample preparation of light and electron microscopes?
Light = natural colour of the sample, or colour of a stain, is seen.
Electron = black and white images are produced but can be coloured digitally. ANS: Relative colours of
the samples produced from light and electron microscopes?
Light = up to x2000
Electron = over x500,000 ANS: Relative magnifications of light and electron microscopes?
Light = 200nm
Electron = TEM: 0.5nm and SEM: 3-10nm ANS: Relative resolving power of light and electron
microscopes?
A visible structural detail caused by processing the specimen, and is not actually a feature of the
specimen itself. ANS: What is an artefact?
This refers to the chemical reactions of both the synthesis and the breaking down of molecules. ANS:
Define the term 'metabolism'.
