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AS Biology CIE
1. Cell Structure
CONTENTS
1.1 The Microscope in Cell Studies
1.1.1 The Microscope in Cell Studies
1.1.2 Magnification Calculations
1.1.3 Eyepiece Graticules & Stage Micrometers
1.1.4 Resolution & Magnification
1.1.5 Calculating Actual Size
1.2 Cells as the Basic Units of Living Organisms
1.2.1 Eukaryotic Cell Structures & Functions
1.2.2 Animal & Plant Cells
1.2.3 The Vital Role of ATP
1.2.4 Prokaryotic v Eukaryotic Cells
1.2.5 Viruses
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1.1 The Microscope in Cell Studies
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1.1.1 The Microscope in Cell Studies
Microscope Slide Preparation
In order to observe cellular material in more detail, specimens can be prepared for viewing
under a light microscope
Samples need to be thin enough to allow light to pass through
The type of preparation that is appropriate is dependent on the cellular material that needs
to be viewed
Slide preparation methods table
Samples sometimes need to be stained, as the cytosol and other cell structures may be
transparent or difficult to distinguish
To stain a slide the sample needs to be first air-dried and then heated by passing it
through a Bunsen burner flame – this will allow the sample to be fixed to the slide and to
take up the stain
As with the type of preparation required, the type of stain used is dependent on what type
of specimen is being used
Common microscope stains & uses table
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Drawing Cells
To record the observations seen under the microscope (or from photomicrographs taken)
a labelled biological drawing is often made
Biological drawings are line pictures which show specific features that have been
observed when the specimen was viewed
There are a number of rules/conventions that are followed when making a biological
drawing
The conventions are:
The drawing must have a title
The magnification under which the observations shown by the drawing are made must
be recorded
A sharp HB pencil should be used (and a good eraser!)
Drawings should be on plain white paper
Lines should be clear, single lines (no thick shading)
No shading
The drawing should take up as much of the space on the page as possible
Well-defined structures should be drawn
The drawing should be made with proper proportions
Label lines should not cross or have arrowheads and should connect directly to the
part of the drawing being labelled
Label lines should be kept to one side of the drawing (in parallel to the top of the page)
and drawn with a ruler
Drawings of cells are typically made when visualizing cells at a higher magnification power,
whereas plan drawings are typically made of tissues viewed under lower magnifications
(individual cells are never drawn in a plan diagram)
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Exam Tip
When producing a biological drawing, it is vital that you only ever draw what you see
and not what you think you see.To accurately reflect the size and proportions of
structures you see under the microscope, you should get used to using the
eyepiece graticule.
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1.1.2 Magnification Calculations
Magnification Calculations
Magnification is how many times bigger the image of a specimen observed is in
comparison to the actual (real-life) size of the specimen
The magnification (M) of an object can be calculated if both the size of the image (I), and
the actual size of the specimen (A), is known
An equation triangle for calculating magnification
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Worked Example
An image of an animal cell is 30 mm in size and it has been magnified by a factor of X
3000.
What is the actual size of the cell?
To find the actual size of the cell:
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The size of cells is typically measured using the micrometre (μm) scale, with cellular
structures measured in either micrometers (μm) or nanometers (nm)
When doing calculations all measurements must be in the same units. It is best to use the
smallest unit of measurement shown in the question
To convert units, multiply or divide depending if the units are increasing or decreasing
Magnification does not have units
Converting units of measurement
There are 1000 nanometers (nm) in a micrometre (µm)
There are 1000 micrometres (µm) in a millimetre (mm)
There are 1000 millimetres (mm) in a metre (m)
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Worked Example
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Step 1: Check that units in magnification questions are the same
Remember that 1mm = 1000µm
2000 / 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm
Step 2: Calculate Magnification
Magnification = image size / actual size = 50 / 2 = 25
So the magnification is x 25
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1.1.3 Eyepiece Graticules & Stage Micrometers
Eyepiece Graticules & Stage Micrometers
An eyepiece graticule and stage micrometer are used to measure the size of the object
when viewed under a microscope
The type of microscope and magnification used can vary signficantly so the eyepiece
graticule needs to be calibrated each time when measuring objects
The calibration is done using a stage micrometer, this is a slide with a very accurate known
scale in micrometres (µm)
The eyepiece graticule is a disc placed in the eyepiece with 100 divisions, this has no scale
To know what the graticule divisions equal at each magnification the eyepiece graticule is
calibrated to the stage micrometer at each magnification
Using stage micrometer & eyepiece graticule
A stage micrometer alongside an eyepiece graticule.
In the diagram, the stage micrometer has three lines each 100 µm (0.1 mm) apart
Each 100 µm division has 40 eyepiece graticule divisions
40 graticule divisions = 100 µm
1 graticule division = number of micrometres ÷ number of graticule division
1 graticule division = 100 ÷ 40 = 2.5 µm this is the magnification factor
The calibrated eyepiece graticule can be used to measure the length of the object
The number of graticule divisions can then be multiplied by the magnification factor:
graticule divisions x magnification factor = measurement (µm)
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Exam Tip
The calculations involving stage micrometers and eyepiece graticules are often
seen in exam questions, so make sure that you are comfortable with how to calibrate
the graticule and calculate the length of an object on the slide.
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1.1.4 Resolution & Magnification
Magnification
Resolution & Magnification
Magnification is how many times bigger the image of a specimen observed is in compared
to the actual (real-life) size of the specimen
A light microscope has two types of lens:
An eyepiece lens, which often has a magnification of x10
A series of (usually 3) objective lenses, each with a different magnification
To calculate the total magnification the magnification of the eyepiece lens and the
objective lens are multiplied together:
eyepiece lens magnification x objective lens magnification = total magnification
Resolution
Resolution is the ability to distinguish between two separate points
If two separate points cannot be resolved, they will be observed as one point
The resolution of a light microscope is limited by the wavelength of light
As light passes through the specimen, it will be diffracted
The longer the wavelength of light, the more it is diffracted and the more that this
diffraction will overlap as the points get closer together
Electron microscopes have a much higher resolution and magnification than a light
microscope as electrons have a much smaller wavelength than visible light
This means that they can be much closer before the diffracted beams overlap
The concept of resolution is why the phospholipid bilayer structure of the cell membrane
cannot be observed under a light microscope
The width of the phospholipid bilayer is about 10nm
The maximum resolution of a light microscope is 200nm (half the smallest wavelength
of visible light, 400nm)
Any points that are separated by a distance less than 200nm (such as the 10nm
phospholipid bilayer) cannot be resolved by a light microscope and therefore will not
be distinguishable as “separate”
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The resolving power of an electron microscope is much greater than that of the light
microscope, as structures much smaller than the wavelength of light will interfere with a
beam of electrons
Comparison of the electron microscope & light microscope
Light microscopes are used for specimens above 200 nm
Light microscopes shine light through the specimen, this light is then passed through
an objective lens (which can be changed) and an eyepiece lens (x10) which magnify
the specimen to give an image that can be seen by the naked eye
The specimens can be living (and therefore can be moving), or dead
Light microscopes are useful for looking at whole cells, small plant and animal
organisms, tissues within organs such as in leaves or skin
Electron microscopes, both scanning and transmission, are used for specimens above
0.5 nm
Electron microscopes fire a beam of electrons at the specimen either a broad static
beam (transmission) or a small beam that moves across the specimen (scanning)
The electrons are picked up by an electromagnetic lens which then shows the image
Due to the higher frequency of electron waves (a much shorter wavelength)
compared to visible light, the magnification and resolution of an electron microscope
is much better than a light microscope
Electron microscopes are useful for looking at organelles, viruses and DNA as well as
looking at whole cells in more detail
Electron microscopy requires the specimen to be dead however this can provide a
snapshot in time of what is occurring in a cell eg. DNA can be seen replicating and
chromosome position within the stages of mitosis are visible
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Light v Electron Microscope Table
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1.1.5 Calculating Actual Size
Calculating Actual Size
When investigating the size of organisms and biological structures you will use a
microscope of a specific magnification to produce an image
Photomicrographs are images obtained from a light microscope, these are used for
specimens above 200 nm (a bacteria cell is about 1000 nm)
Electron micrographs are images obtained from electron microscopes, both scanning
and transmission, these are used for specimens above 0.5 nm
Electron microscopes are useful for looking at organelles and biological molecules, eg.
DNA can be seen replicating
To better understand the images we produce using microscopes we need to know the
actual size of the specimen
Worked example: Calculating the actual size of a specimen
A scientist looks at a sample of red blood cells under a light microscope.
The eyepiece lens of the microscope has a magnification of x10 and an objective lens of
x40 was used to view the blood cells. The scientist takes a photomicrograph of the blood
cells, in which the average size of each cell is 3 mm.
What is the average size of the red blood cells in the sample? Give your answer in
micrometres.
Known values:
Eyepiece lens magnification: x10
Objective lens magnification: x40
Image size: 3 mm
Step 1: Calculate the total magnification of the specimen
eyepiece lens magnification x objective lens magnification
= total magnification
x10 x x40 = x400
Step 2: Calculate the image size in the units asked for (micrometres)
1 mm = 1000 μm
3 mm = 3000 μm
Step 3: Calculate the actual size of the red blood cell
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Therefore, the average size of a red blood cell in this sample is 7.5 micrometres
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1.2 Cells as the Basic Units of Living Organisms
1.2.1 Eukaryotic Cell Structures & Functions
Eukaryotic Cell Structures & Functions
Cell surface membrane
The structure of the cell surface membrane – although the structure looks static the
phospholipids and proteins forming the bilayer are constantly in motion
All cells are surrounded by a cell surface membrane which controls the exchange of
materials between the internal cell environment and the external environment
The membrane is described as being ‘partially permeable’
The cell membrane is formed from a phospholipid bilayer of phospholipids spanning a
diameter of around 10 nm
Cell wall
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The cell wall is freely permeable to most substances (unlike the plasma membrane)
Cell walls are formed outside of the cell membrane and offer structural support to cell
Structural support is provided by the polysaccharide cellulose in plants, and peptidoglycan
in most bacterial cells
Narrow threads of cytoplasm (surrounded by a cell membrane) called plasmodesmata
connect the cytoplasm of neighbouring plant cells
Nucleus
The nucleus of a cell contains chromatin (a complex of DNA and histone proteins) which is
the genetic material of the cell
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Present in all eukaryotic cells, the nucleus is relatively large and separated from the
cytoplasm by a double membrane (the nuclear envelope) which has many pores
Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the
nucleus, as well as allowing enzymes (eg. DNA polymerases) and signalling molecules to
travel in
The nucleus contains chromatin (the material from which chromosomes are made)
Usually, at least one or more darkly stained regions can be observed – these regions are
individually termed ‘nucleolus’ and are the sites of ribosome production
Mitochondria
A single mitochondrion is shown – the inner membrane has protein complexes vital for the
later stages of aerobic respiration embedded within it
The site of aerobic respiration within eukaryotic cells, mitochondria are just visible with a
light microscope
Surrounded by double-membrane with the inner membrane folded to form cristae
The matrix formed by the cristae contains enzymes needed for aerobic respiration,
producing ATP
Small circular pieces of DNA (mitochondrial DNA) and ribosomes are also found in the
matrix (needed for replication)
Chloroplast
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Chloroplasts are found in the green parts of a plant – the green colour a result of the
photosynthetic pigment chlorophyll
Larger than mitochondria, also surrounded by a double-membrane
Membrane-bound compartments called thylakoids containing chlorophyll stack to form
structures called grana
Grana are joined together by lamellae (thin and flat thylakoid membranes)
Chloroplasts are the site of photosynthesis:
The light-dependent stage takes place in the thylakoids
The light-independent stage (Calvin Cycle) takes place in the stroma
Also contain small circular pieces of DNA and ribosomes used to synthesise proteins
needed in chloroplast replication and photosynthesis
Ribosome
Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA
and protein
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Found freely in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in
eukaryotic cells
Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
70S (composed of 50S and 30S subunits) ribosomes in prokaryotes, mitochondria and
chloroplasts
Site of translation (protein synthesis)
Endoplasmic reticulum
The RER and ER are visible under the electron microscope - the presence or absence of
ribosomes helps to distinguish between them
Rough Endoplasmic Reticulum (RER)
Surface covered in ribosomes
Formed from continuous folds of membrane continuous with the nuclear envelope
Processes proteins made by the ribosomes
Smooth Endoplasmic Reticulum (ER)
Does not have ribosomes on the surface, its function is distinct to the RER
Involved in the production, processing and storage of lipids, carbohydrates and steroids
Golgi apparatus (golgi complex)
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The structure of the Golgi apparatus
Flattened sacs of membrane similar to the smooth endoplasmic reticulum
Modifies proteins and packages them into vesicles or lysosomes
Large permanent vacuole
The structure of the vacuole
Sac in plant cells surrounded by the tonoplast, selectively permeable membrane
Vacuoles in animal cells are not permanent and small
Vesicle
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The structure of the vesicle
Membrane-bound sac for transport and storage
Lysosome
The structure of the lysosome
Specialist forms of vesicles which contain hydrolytic enzymes (enzymes that break
biological molecules down)
Break down waste materials such as worn-out organelles, used extensively by cells of the
immune system and in apoptosis (programmed cell death)
Centriole
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The structure of the centriole
Hollow fibres made of microtubules, two centrioles at right angles to each other form a
centrosome, which organises the spindle fibres during cell division
Not found in flowering plants and fungi
Microtubules
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The structure of the microtubule
Makes up the cytoskeleton of the cell about 25 nm in diameter
Made of α and β tubulin combined to form dimers, the dimers are then joined into
protofilaments. Thirteen protofilaments in a cylinder make a microtubule
The cytoskeleton is used to provide support and movement of the cell
Microvilli
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The structure of the microvilli
Cell membrane projections that increase the surface area for absorption
Cilia
The structure of the cilia
Hair-like projections made from microtubules
Allows the movement of substances over the cell surface
Flagella
The structure of the flagella
Similar in structure to cilia, made of longer microtubules
Contract to provide cell movement for example in sperm cells
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1.2.2 Animal & Plant Cells
Electron Micrographs: Animal Cells
TEM electron micrograph of an animal cell showing key features

Exam Tip
You should be able to describe and interpret photomicrographs, electron
micrographs and drawings of typical animal cells.
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Electron Micrographs: Plant Cells
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TEM electron micrograph of a plant cell showing key features
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Structure of Animal & Plant Cells
The only structures found in animal cells but not plant cells are the centrioles and microvilli
Plant cells also have additional structures: the cellulose cell wall, large permanent vacuoles
and chloroplasts
The ultrastructure of an animal cell shows a densely packed cell – the ER and RER and
ribosomes form extensive networks throughout the cell in reality
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Plant cells have a larger, more regular structure in comparison to animal cells
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1.2.3 The Vital Role of ATP
The Vital Role of ATP
All organisms require a constant supply of energy to maintain their cells and stay alive
This energy is required:
In anabolic reactions – building larger molecules from smaller molecules
To move substances across the cell membrane (active transport) or to move
substances within the cell
In animals, energy is required:
For muscle contraction – to coordinate movement at the whole-organism level
In the conduction of nerve impulses, as well as many other cellular processes
In all known forms of life, ATP from respiration is used to transfer energy in all energyrequiring processes in cells
This is why ATP is known as the universal energy currency
Adenosine Triphosphate (ATP) is a nucleotide
The monomers of DNA and RNA are also nucleotides
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1.2.4 Prokaryotic v Eukaryotic Cells
Structural Features of Typical Prokaryotic Cells
Animal and plant cells are types of eukaryotic cells, whereas bacteria are a type of
prokaryote
Prokaryotes have a cellular structure distinct from eukaryotes:
Their genetic material is not packaged within a membrane-bound nucleus and is
usually circular (eukaryotic genetic material is packaged as linear chromosomes)
Prokaryotes lack membrane-bound organelles
They are many (100s/1000s) of times smaller than eukaryotic cells
Their ribosomes are structurally smaller (70 S) in comparison to those found in
eukaryotic cells (80 S)
Prokaryotic cells are often described as being ‘simpler’ than eukaryotic cells, and they are
believed to have emerged as the first living organisms on Earth
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Prokaryotic v Eukaryotic Cell Structures
Prokaryotic & Eukaryotic Cells Comparison Table
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1.2.5 Viruses
Key Features of Viruses
Viruses are non-cellular infectious particles that straddle the boundary between ‘living’
and ‘non-living’
They are relatively simple in structure; much smaller than prokaryotic cells (with diameters
between 20 and 300 nm)
Structurally they have:
A nucleic acid core (their genomes are either DNA or RNA, and can be single or doublestranded)
A protein coat called a ‘capsid’
Some viruses have an outer layer called an envelope formed usually from the membranephospholipids of a cell they were made in
All viruses are parasitic in that they can only reproduce by infecting living cells and using
their protein-building machinery (ribosomes) to produce new viral particles
Viruses are not cellular like prokaryotes and eukaryotes – this is just one example of a virus
structure
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