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Cambridge O Level Biology
# 1.1 Cell Structure & Function
# Contents
Animal & Plant Cells
Animal & Plant Cells Under the Microscope
Bacterial Cells
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Animal & Plant Cells
# Animal & Plant Cells
Animals
The main features of animals:
They are multicellular
Their cells contain a nucleus with a distinct membrane
Their cells do not have cellulose cell walls
Their cells do not contain chloroplasts (so they are unable to carry out photosynthesis )
They feed on organic substances made by other living things
They often store carbohydrates as glycogen
They usually have nervous coordination
They are able to move from place to place
A typical animal cell
Plants
The main features of plants:
They are multicellular
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Their cells contain a nucleus with a distinct membrane
Their cells have cell walls made out of cellulose
Their cells contain chloroplasts (so they can carry out photosynthesis )
They feed by photosynthesis
They store carbohydrates as starch or sucrose
They do not have nervous coordination
A typical plant cell
Cell Structures Found in Both Animal and Plant Cells Table
Structure Function
Nucleus Contains genetic material (DNA) which
controls the activities of the cell
Cytoplasm
A gel like substance composed of water and
cell solutes. It supports the internal cell
structures and is the site for many chemical
reactions
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Cell membrane
Holds the cell together, separating the inside
of the cell from the outside. Controls which
substances leave and enter the cell
Ribosomes Found in the cytoplasms, these are the site of
protein synthesis
Mitochondria
The site of aerobic respiration where energy is
released to the cell. Large numbers of
mitochondria are found in cells that are very
metabolically active, such as muscle cells
Cell Structures Found Only in Plant Cells Table
Structure Function
Cell wall Made of cellulose, gives extra support to the
cell
Chloroplasts Contain green chlorophyll pigments which
absorb light energy during photosynthesis
Permanent vacuole
Contains cell sap which is a solution of
dissolved sugars and ions. It is used for
storage and support of the cell structure
Animal and Plant Cell Diagrams
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An animal and plant cell as seen under a light microscope; only larger structures are visible under a light
microscope so smaller structures such as ribosomes will not be visible
Identifying Cell Structures and Function
Within the cytoplasm, the following organelles are visible in almost all cells except prokaryotes when
looking at higher magni cation (i.e. using an electron microscope):
Mitochondria (singular: mitochondrion) are organelles found throughout the cytoplasm
Ribosomes are tiny structures that can be free within the cytoplasm or attached to a system of
membranes within the cell known as Endoplasmic Reticulum
Endoplasmic reticulum studded with ribosomes looks rough under the microscope; this gives
rise to its name of Rough Endoplasmic Reticulum (often shortened to R.E.R. )
Vesicles can also be seen using a higher magni cation - these are small circular structures found
moving throughout the cytoplasm
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Structures in an animal cell visible under a light microscope and an electron microscope
Structures in a plant cell visible under a light microscope and an electron microscope
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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 speci c 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
Guidelines for microscope drawings
The conventions are:
The drawing must have a title
The magni cation 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 shading
The drawing should take up as much of the space on the page as possible
Well-de ned 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 visualising cells at a higher magni cation power, whereas
plan drawings are typically made of tissues viewed under lower magni cations (individual cells are
never drawn in a plan diagram)
Biological Drawing of an Animal Cell
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An example of a tissue plan drawn from an electron micrograph of an animal cell
Biological Drawing of a Plant Cell
An example of a tissue plan drawn from an electron micrograph of a plant cell
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Examiner 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 re ect the size and proportions of structures you see under the
microscope, you should get used to using the eyepiece graticule. You should be able to describe and
interpret photomicrographs, electron micrographs and drawings of typical animal cells.
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# Animal & Plant Cells Under the Microscope
# Examining Cells Under the Microscope
Viewing cells under the microscope
Many biological structures are too small to be seen by the naked eye
Optical microscopes are an invaluable tool for scientists as they allow for tissues, cells and larger
organelles to be seen and studied
Light is directed through a thin layer of biological material that is supported on a glass slide
This light is focused through several lenses so that an image is visible through the eyepiece
Getting a visible image requires a very thin sample of biological tissue because light has to pass
through the sample and into the lenses of the microscope
The most common specimens to observe under a light microscope are cheek cells (animal cells) and
onion cells (plant cells)
A stain is often used to ensure cell structures are clearly visible under the microscope
General method
Specimens must be prepared on a microscope slide to be observed under a light microscope
This must be done carefully to avoid damaging the biological specimen and the structures within it
Preparing a slide using a liquid specimen :
Add a few drops of the sample to the slide using a pipette
Cover the liquid/smear with a coverslip and gently press down to remove air bubbles
Wear gloves to ensure there is no cross-contamination of foreign cells
Preparing a slide using a solid specimen :
Use scissors to cut a small sample of the tissue
Peel away or cut a very thin layer of cells from the tissue sample to be placed on the slide (using a
scalpel or forceps)
Some tissue samples need to be treated with chemicals to kill/make the tissue rigid
Gently place a coverslip on top and press down to remove any air bubbles
A stain may be required to make the structures visible depending on the type of tissue being
examined
Commonly used stains include methylene blue to stain cheek cells and iodine to stain onion
cells
Take care when using sharp objects and wear gloves to prevent the stain from dying your skin
When using an optical microscope always start with the lowest power objective lens :
It is easier to nd what you are looking for in the eld of view
This helps to prevent damage to the lens or coverslip in case the stage has been raised too high
Preventing the dehydration of tissue:
The thin layers of material placed on slides can dry up rapidly
Adding a drop of water to the specimen (beneath the coverslip) can prevent the cells from being
damaged by dehydration
Unclear or blurry images:
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Switch to the lower power objective lens and try using the coarse focus to get a clearer image
Consider whether the specimen sample is thin enough for light to pass through to see the
structures clearly
There could be cross-contamination with foreign cells or bodies
Using a microscope diagram
Light microscopes have a lens in the eyepiece which is xed and two or three objective lenses of
di erent powers
Viewing plant tissue
An ideal tissue is the onion epidermis (found between the layers of onions) because it forms a layer just
one cell thick
Being a non-photosynthetic tissue, onion epidermis is not green as it does not contain any
chloroplasts
Apparatus
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The key components of an optical microscope you will need to use are:
The eyepiece lens
The objective lenses
The stage
The light source
The coarse and ne focus
Other apparatus used:
Forceps
Scissors
Scalpel
Coverslip
Slides
Pipette
Iodine solution
Viewing onion cells under the microscope diagram
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Care must be taken to avoid smudging the glass slide or trapping air bubbles under the coverslip
Viewing animal tissue
Human cheek cells are a good choice for examination under the light microscope because they are:
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Plentiful
Easy to obtain safely
Can be obtained without an overly intrusive process
Relatively undi erentiated and so will display the main cell structures
Safety considerations
Do not perform the sampling on a person who has a cold , cough , throat infection etc.
To avoid spreading the infection to others
Concentrated methylene blue is toxic if ingested
Wear gloves and do NOT allow children to handle methylene blue solution or have access to the
bottle of solution
Apparatus
Glass microscope slides
Cover slips
Paper towels or tissue
Staining solution
Methylene blue solution
0.5% to 1%
Dilute according to concentration of the stock solution
Plastic pipette or dropper
Sterile, individually packed cotton wool buds or swabs
Method
Brush teeth thoroughly with normal toothbrush and toothpaste
This removes bacteria from teeth so they don't obscure the view of the cheek cell
Take a clean, sterile cotton swab and gently scrape the inside cheek surface of the mouth for 5 10
seconds
Smear the cotton swab on the centre of the microscope slide for 2 to 3 seconds
Add a drop of methylene blue solution
Place a coverslip on top
Lay the coverslip down at one edge and then tilt it down at
This reduces bubble formation under the coverslip
Absorb any excess solution by allowing a paper towel to touch one side of the coverslip.
Place the slide on the microscope, with 4 x or 10 objective in position and nd a cell
Then view at higher magni cation to reveal more detail
Methylene blue stains negatively charged molecules in the cell, including DNA and RNA
This causes the nucleus and mitochondria appear darker than their surroundings
The cells seen are squamous epithelial cells from the outer epithelial layer of the mouth
Viewing cheek cells under the microscope diagram
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Parts of the cell that can be seen with a light microscope
Nucleus
Mitochondria
Cell membrane
Cytoplasm
Parts of the cell that cannot be seen with a light microscope
Ribosomes*
Endoplasmic reticulum*
Golgi*
Details of the nucleus, mitochondria and cell membrane
* these are parts of the cell that you don't need to know the names of, but they are included here as
examples of very small structures within the cell
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Examiner Tip
Remember that a cell is always a 3 dimensional object. If you search around your microscope's eld of
view you may nd a cell squashed up against the coverslip and will be able to see its 3 D structure in
the background.
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Bacterial Cells
# Bacteria Cells
Bacteria, which have a wide variety of shapes and sizes, all share the following biological
characteristics:
They are microscopic single-celled organisms
Possess a cell wall (made of peptidoglycan , not cellulose), cell membrane , cytoplasm and
ribosomes
Lack a nucleus but contain a circular chromosome of DNA that oats in the cytoplasm
Plasmids are sometimes present - these are small rings of DNA (also oating in the cytoplasm) that
contain extra genes to those found in the chromosomal DNA
They lack mitochondria, chloroplasts and other membrane-bound organelles found in animal
and plant cells
Some bacteria also have a agellum (singular) or several agella (plural). These are long, thin, whip-
like tails attached to bacteria that allow them to move
Examples of bacteria include:
Lactobacillus (a rod-shaped bacterium used in the production of yoghurt from milk)
Pneumococcus (a spherical bacterium that acts as the pathogen causing pneumonia)
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A typical bacterial cell
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Cambridge O Level Biology
# 1.2 Specialised Cells, Tissues & Organs
# Contents
Organisation of Cells
Magni cation Formula
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Organisation of Cells
# Specialised Cells
Specialised cells in animals
Specialised cells are those which have developed certain characteristics in order to perform speci c
functions
These di erences are controlled by genes in the nucleus
Cells specialise by undergoing di erentiation : this is a process by which cells develop the structure
and characteristics needed to be able to carry out their functions
Specialised Cells in Animals Table
Cell Function Adaptation
Ciliated cell
Movement of mucus in the
trachea and bronchi of the
chest cavity
Hair-like structures form on the
surface of the cell called cilia
These beat to move mucus and
trapped particles up and out of the
trachea
Nerve cell Conduction of impules
Long so that nerves can run to and
from di erent parts of the body to
the central nervous system
The cell has extensions and
branches for communication. The
axon is insulated for speed of
transmission of the nerve impulse
Red blood cell Transport of oxygen
Biconcave disc shape increases
surface area for more e cient
di usion of oxygen
Contains haemoglobin for to carry
oxygen
No nucleus to increase space for
carrying oxygen
Sperm cell Reproduction The head contains the genetic
material for fertilisation in a haploid
nucleus (containing half the normal
number of chromosomes)
The acrosome in the head contains
digestive enzymes so the sperm cell
can penetrate the egg cell
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The mid-piece has many
mitochondria to provide energy for
tail movement
Egg cell Reproduction
Contains a lot of cytoplasm which
has nutrients for the growth of the
early embryo
Haploid nucleus contains genetic
material for fertilisation
Cell membrane changes after
fertilisation by a single sperm cell so
no more sperm can enter
.
Diagrams of specialised cells in animals
Ciliated cell
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Nerve cell
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Red blood cells
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Sperm cell
Egg cell
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Specialised Cells in Plants Table
Cell Function Adaptation
Root hair cell Absorption of water and
minerals from soil
Root hair increases the surface area to
ensure maximum absorption of water
and minerals
Walls are thin to ensure water moves
through quickly
No chloroplast present as no light
underground
Xylem vessel
Conduction of water
through the plant;
provides support for the
plant
No top or bottom cell walls for
continuous ow of water
Cells are dead so no organelles to
ensure a smooth stream of water can
ow
Thickened cell walls with lignin to
provide structural support
Palisade mesophyll
cell Photosynthesis
Column shaped to maximise
absorption of sunlight
Contains many chloroplasts for
maximum photosynthesis
Diagrams of specialised cells in plants
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Root hair cell
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Xylem structure
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Palisade mesophyll cell
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Levels of Organisation in an Organism
Level Description
Cells Basic functional and structural units in a living organism
Tissues GRoups of cells of similar structure working together to perform the same
function
Organs Made from di erent tissues working together to perform a speci c function
Organ
systems
Groups of organs with related functions working together to perform body
functions
Levels of organisation diagram
An example of the di erent levels of organisation
Levels of Organisation Examples Table
Organ system Organ Tissue
Shoot system Leaf, stem, ower, fruit
Epidermis mesophyll
Xylem
Phloem
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Root system Root, tuber
Xylem
Phloem
Ground tissue
Digestive system Oesophagus, stomach,
small and large intestines
Muscle
Connective
Nerve
Epithelial
Circulatory system Heart, arteries, veins
Muscle
Connective
Nerve
Epithelial
Immune system Thymus, spleen Bone marrow
Respiratory system Trachea, bronchi, lungs
Muscle
Connective
Nerve
Epithelial
Excretory system Liver, kidney, skin, lungs
Muscle
Connective
Nerve
Epithelial
Nervous system Brain , spinal cord Nerve
Reproductive system Ovary, uterus, vagina, penis,
testes
Muscle
Connective
Nerve
Epithelial
Examiner Tip
Most incorrect answers here come from not being able to identify a tissue, so its worth making sure
you understand and remember that tissues are always made up of only one type of cell .
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# Magni cation Formula
# Magni cation Formula
Calculating magni cation and specimen size using millimetres as units
Magni cation is calculated using the following equation:
Magni cation = Image size Actual size
A better way to remember the equation is using an equation triangle:
Magni cation equation
Rearranging the equation to nd things other than the magni cation becomes easy when you
remember the triangle - whatever you are trying to nd, place your nger over it and whatever is left is
what you do, so:
Magni cation = image size / actual size
Actual size = image size / magni cation
Image size = magni cation x actual size
Remember magni cation does not have any units and is just written as x 10 or x 5000
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2015 2025 Save My Exams, Ltd. Revision Notes, Topic Questions, Past Papers Worked example
An image of an animal cell is 30 mm in size and it has been magni ed by a factor of x 3000. What
is the actual size of the cell?
To nd the actual size of the cell:
Worked example using the magni cation equation
Examiner Tip
To ensure you do not lose marks when answering exam questions on this skill:
. Always look at the units that have been given in the question - if you are asked to measure
something, most often you will be expected to measure it in millimetres NOT in centimetres -
double check the question to see! Remember to bring a ruler to the exam!
. Learn the equation triangle for magni cation and write it on the page straight away
. Dont forget that magni cation has NO UNITS - students often lose a mark because they put one
in
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