A2.2. Cell Structure

Cell StructurePresentation

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Animal, plant and fungal cells

Processes of life in unicellular organisms

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Prokaryotic and eukaryotic cells

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Summary

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Developments in microscopy

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Interactive 2

Using microscopes

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Drawing cells

Magnification calculations

Observing cells

Cell Theory

The Big Picture

Index

What does resolution refer to with respect to optical devices?What is the actual limit to the resolving power of the human eye? How does this compare to a bird of prey like an eagle? How large are cells? Organelles? Membranes? What is the resolving power of the different type of microscopes like light and electron microscopes? A scanning electron microscope was used to prepare the image shown in this figure which is an embryo on the head of a pin. What is the value of a SEM over a transmission electron microscope?

The Figure shows a hot spring extremophile community. This community thrives in 75°C water in the hills of New Mexico. The community in the picture is made up of sulfur bacteria (purple), algae and protozoa, all one celled organisms.

• How does the cell theory take into account the diversity of cell structure?
• }What features of cells are universal?
• What are some examples of features that are unique to certain cells?
• What are the implications of the cell theory?
• What are the limits to what the cell theory predicts or explains?

Guiding questions

What are the features common to all cells and the features that differ?How is microscopy used to investigate cell structure?

The soil and the cell structure

How the soil and cell structure are related?

How do use the soil?

• Build houses
• Walk on it
• Plant food in it
• Search for resources
• Raise animals
• Play sports

It's also easily taken for granted. Soils are incredibly diverse.

Soil is like a museum of cells. Some of the oldest and smallest cells on Earth can be found in soil!BacteriaEarthworms

Royal Society of London, 1664

• What are these techniques, and what have scientists discovered about cells?

• What have they found inside cells, and how has this knowledge impacted our understanding of the natural world?

These are just a few of the many questions that continue to drive research in the field of biology.

The Big Picture

Microscopes and cells

In 1665, Robert Hooke, an English scientist and inventor, made a ground-breaking discovery while examining a piece of cork through a primitive microscope. He observed that the cork was made up of small compartments, which he referred to as ‘cells’, because they reminded him of the small rooms or ‘cellula’ in a monastery. This discovery provided the first evidence that living organisms are made up of small, discrete units.Most cells are too small to be seen with the naked eye, so we rely on microscopes to view them.

Cells are the smallest units of self-sustaining life, and they come in a wide range of shapes, sizes and functions.

The Big Picture

Most cells are too small to be seen with the naked eye, so we rely on microscopes to view them.

As technology has advanced, scientists have developed various techniques to produce detailed images of cells

Cell Theory

Before microscopes

People used to believe that living organisms could spontaneously appear from non-living matter

Cells everywhere

Cell theory states that all living things are made of individual cells

Cells as individual beings

Cell theory states that cells are the basic units of life

The cell and its doughter cells

Cell theory states that all cells arise from other cells.

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Microscopy

Using microscopes

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Microscopes are scientific instruments used to magnify objects or images that are too small to be seen with the naked eye.

Since the invention of microscopes, technological advancements of methods and tools have continued to allow scientists to see the structures that make cells in increasing detail.When we are viewing an object using a microscope, we call the object being viewed a specimen . You may also make temporary mounts of specimens such as onion tissue and cheek cells.

Wet mount of onion tissue

Observing , drawing a photographing cells

Preparing a temporary (wet) mount of onion tissue.

Wet mount of Elodea (aquatic plant)

Wet mount of human cheek cells

Preparing a temporary (wet) mount of human cheek cells.

Experimental techniques – Safety

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Whenever you carry out a practical activity you should take the time to consider potential hazards , and the precautions you will take to reduce the risks posed by these hazards.

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The cell measures 20 μm. To convert from millimetres (mm) to micrometres (μm), multiply your value by 1000.0.02 mm x 1,000 = 20 μm

A plasma cell measures 0.02 mm in diameter. Find the diameter of the cell in micrometres (μm).

How many millimetres are found in 76 000 μm?

There are 76 mm in 76 000 μm. To convert from micrometres (μm) to millimetres (mm), divide your value by 1000.76,000 μm / 1000 = 76 mm

How many millimetres are found in 76 000 μm?

Measuring sizes and using an eyepiece graticulate

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Eyepiece graticulate

You can measure the actual sizes of structures visible through a microscope by using a scale inside the eyepiece, called a graticule.

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Calibrate the eyepiece graticulate

Stage micrometres are small, calibrated rulers that are mounted onto the stage of the microscope.

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Converting between units

Cells and cell structures are usually measured using micrometers (μm) as a unit of measurement.

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Magnification calculations

Magnification calculations

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Magnification

We use the following equation to calculate how much an image has been magnified:

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Image size

If you know magnification and actual size, you can rearrange this equation to calculate the size of the image:

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Actual size

If you know the image size and the magnification, you can rearrange this equation to calculate the actual size:

Scale bars may be put on micrographs or drawings, or shown alongside them. A scale bar is a straight line, labelled with the actual size that the bar represents. For example, a 10mm long scale bar on a micrograph with a magnification of ×10,000 would be labelled 1 µm

The question tells you the image size of the amoeba (90 mm) and the actual size (100 µm). This means you will use the equation: magnification = image size / actual size. Before you carry out the magnification calculation, you need to convert the measurements into the same units.There are 1000 µm in 1 mm, so to convert from µm to mm we divide by 1000: 100 00 µm / 1000= 0.1 mmNow that your measurements are in the same units, substitute them into the equation: magnification = 90 / 0.1magnification= x900

To calculate the size of the image, use the equation:image size = magnification x actual sizeSubstitute your values into the equation:image size = 400 x 11 µm = 4400 µmThe question has asked us to give our answer in millimetres. To convert from micrometres to millimetres, divide the value by 1000.4400 µm / 1000= 4.4 mm

To calculate actual size, use the equation:actual size = image size / magnification Substitute your values into the equation:actual size = 70 mm / 50000 = 0.0014 mmThe units for the actual size will be the same as the units for image size. This means that the actual size is 0.0014 mm.The question has asked us to give our answer in micrometres. To convert from millimetres to micrometres, multiply the value by 1000.0.0014 x 1000= 1.4 µm

Pretty easy, huh?

Worked examples

A student observes and draws an amoeba. The diameter of the amoeba in the drawing is 90 mm. The actual diameter of the amoeba is 100 µm. What is the magnification of the drawing?

A white blood cell is viewed under a microscope using a magnification of ×400. The graticule measures the actual size of the cell to be 11 μm. Calculate the size of the image produced in mm.

A type of bacterium called Escherichia coli is viewed under the microscope using a magnification of ×50 000. The size of the image produced is 70 mm.Calculate the actual size of this cell. Give your answer in micrometres.

Practical skills: drawing cells

Just like writing, drawing is a way in which we can communicate our observations to others. Follow these conventions when drawing your observations:

• Draw your image as large as possible in the space provided.
• Always use a pencil. This means that if you make a mistake, you can correct it.
• When drawing the plasma membrane, or the membrane of organelles, make sure you use continuous lines.
• Only draw the structures that you can actually see. Do not add in things that you think should be there, but cannot see.
• Label important structures such as organelles and the plasma membrane using straight lines, drawn with a ruler and pencil. Make sure your line touches the structure that you are identifying and that label lines do not cross over one another.
• Indicate the magnification used to view the specimen.
• You may also want to add a scale bar. See the video for a detailed guide for constructing scale bars.

The next interactive shows an example and a non-example of a drawing of an onion cell.

Compare image A with image B. Image A follows scientific conventions of how to record observations from a microscope, whereas image B contains many errors. What has the illustrator done incorrectly in image B?

This interactive shows an example and a non-example of a drawing of an onion cell.

Interactive Image

1. Interactive 2a is a magnified image of kidney tissue, viewed using an eyepiece graticule. Use the draggable ruler to measure the image size of the graticule and then calculate the magnification used to view this tissue. Make sure your image and actual size are in the same units before you carry out the calculation.

2. Interactive 2b is a magnification of a type of bacterium called E. coli, taken using an electron microscope. Use the draggable ruler to calculate the magnification and then measure the length of the bacteria in the image, and then calculate the actual size of the bacterium.

3. Interactive 2c shows a type of plant cell from the epidermis of an onion, viewed using an eyepiece graticule. (a) Move the ruler to measure the image size of the graticule and then calculate the magnification used to view this tissue. Make sure your image and actual size are in the same units before you carry out the calculation. (b) This image was viewed using a stain called iodine. Write a sentence explaining what a stain is and why iodine is a suitable stain to use for viewing plant cells, but not animal cells.

Because electrons have a much shorter wavelength than light, electron microscopes have a much higher resolution than light microscopes (200 nm vs 0.1 nm).

Commonly used to view cells and cellular structures. Rather than passing light through a specimen, electron microscopes pass a beam of electrons through a specimen. Electrons will be absorbed by the denser parts of the sample, and scattered or able to pass through less dense areas, after which they are picked up by an electron detector and used to form an image.

Electron microscopes

Microscopes that pass light through a specimen and then use lenses to magnify the image produced. But how can we visualize the smaller components of a cell that cannot be seen with a light microscope?

Light microscopes

Developments in microscopy

25,460x

400x

Comparison between light and electron microscopes

Developments in microscopy

Techniques commonly used in microscopy

Freeze fracture microscopy

It is a particularly useful technique for being able to visualise structures that are not normally visible, such as the internal plasma membrane.

Cryogenic electron microscopy

Freezing the sample improves the resolution of the image formed and reduces damage that may occur from the electron beam.

Immunofluorescence

Immunofluorescence is a technique used in light microscopy to better visualise certain structures.

Fluorescent dyes

As in immunofluorescence, the labelled areas will appear as brightly coloured spots, allowing visualisation of the target molecule throughout the specimen.

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Techniques commonly used in microscopy

Cell structure

All known cells share the following features:

Cell membrane, which controlls who goes in and who goes out the cell

Cytoplasm, the jelly fluid within cells

Ribosomes, responsible of making proteins

DNA as a genetic material.

Prokaryotic and Eukaryotic cells

A typical cell

Bacteria and Archaeae are both types of prokaryotes. Within prokaryotes, there is enormous variation and number of species. They are found everywhere, including the most inhospitable places; from boiling wells to deep mine shafts, to the ocean floor.

Prokaryotes were the first organisms to evolve on Earth (originated around 3.5 billions years ago) and they still have the simplest cell structure. They are mostly small in size and are found almost everywhere (in soil, in water, on our skin, in our intestines and even in pools of hot water in volcanic areas). Prokaryotes are unicellular organisms that do not contain membrane-bound organelles. Like eukaryotes, prokaryotes contain ribosomes, the site of protein synthesis. However, prokaryotic ribosomes (70s) are smaller than eukaryotic ribosomes (80s), where the unit "S" refers to Svedberg unit. All prokaryotic ribosomes are free in the cytoplasm.Prokaryotic cells usually range in diameter between 0.1 and 5.0 μm, whereas eukaryotic cells typically between 10 and 100 μm.

Prokaryote cell structure

The first and everywhere cell

Prokaryote cell structure

Explore the 3D animated model of a bacterial cell by rotating and zooming in and out. Click on the annotations to learn about the different structures.

Prokaryote cell structure

A prokaryotic cell viewed using a light microscope (left) vs a prokaryotic cell viewed using an electron microscope (right). Note the difference in resolution and how this allows you to see the cell structures more clearly in the electron micrograph.

You need to be able to identify cells in light or electron micrographs as prokaryote, plant or animal. In electron micrographs, you should be able to identify the following structures in prokaryotic cells: nucleoid region, cell wall, ribosomes and plasma membrane.

Practical skills: Experimental techniques

Eukaryotes, like all other living organisms, have a basic cell structure with cytoplasm inside a plasma membrane. In some eukaryotes, there is also a cell wall outside the membrane. Whereas the cytoplasm of a prokaryotic cell is one undivided space, eukaryotic cells are compartmentalized. Areas are separated from the rest of the cytoplasm by single or double membranes.Unlike prokaryotes, which are all unicellular, some eukaryotes are multicellular , meaning that the body of the organism consists of more than one cell.There is huge diversity within the eukaryotes.

Eukaryote cell structure

The diverse and complex cells

Eukaryotic cells contain membrane-bound cytoplasmic organelles, such as mitochondria and chloroplasts. This compartmentalization allows for the interior of the organelles to have separate conditions to the cytoplasm of the cell. The advantages of compartmentalisation include:

• The ability to create higher concentrations of certain substances within organelles.
• The ability to separate toxins and potentially damaging substances from the rest of the cell. For example, hydrolytic enzymes can be stored in structures called lysosomes , away from the cell cytoplasm.
• Control over conditions inside organelles (such as pH) to maintain the optimal conditions for the enzymes that function in those parts of the cell

Eukaryote cell structure

Compartimentalization

Eularyotic cell structure

8 Nutrition

7 Excretion

6 Reproduction

5 Growth

4 Movement

3 Homeostasis

2 Response to stimuli

1 Metabolism

All prokaryotic cells, and some eukaryotic cells are unicellular. Unicellular organisms have a body composed of only one cell. A single cell can be classed as an organism if it can carry out the life processes. What are the life processes common to all living things?The single cell that makes up the unicellular organism will be capable of carrying out all of the eight life processes:

Processes of life in unicellular organisms

Paramecium is a genus (group) of unicellular protozoa. Paramecia are usually less than 0.25 mm in size and are widespread in aquatic environments, particularly in stagnant ponds. They are heterotrophs, feeding on food particles they encounter in their environment. They can move in all directions using their cilia, small hair-like structures that cover the whole body and beat rhythmically to propel the cell in a given direction.

Unicellular organisms carrying out all processes of life

Chlamydomonas is a genus of unicellular green algae (Chlorophyta) distributed all over the world, in soil, fresh water, oceans and even in snow on mountain tops. The algae in this genus range in size from 10 to 30 μm in diameter and have a cell wall, a chloroplast, an ‘eye’ that detects light, as well as two flagella, which they use to swim using a breaststroke-type motion.Chlamydomonas are autotrophs; they can manufacture their own food using their large chloroplast to photosynthesize.

Unicellular organisms carrying out all processes of life

Centrioles are two cylindrical organelles that help to establish and organize the microtubules, playing an important role in cell division. Lysosomes are membrane-bound bags of hydrolytic enzymes that break down and destroy biological molecules and old cellular organelles. Lysosomes are found in high concentrations in phagocytic white blood cells where they will fuse with and destroy ingested pathogens .Some animal cells contain vacuoles. These tend to be much smaller than vacuoles found in plant cells. Animal vacuoles store water, nutrients and waste products.Some animal cells contain cilia, hair-like structures made of microtubules, important for the movement of substances past the cell. E.g. There are many cilia on the epithelial cells of the bronchi , which beat in unison to move microbes and debris up and out of the respiratory tract.

Animal cell structure

Animals, plants and fungi are all eukaryotic. They are similar in some ways but what distinguishes each cell type from the other?

Animal, plant and fungal cells

Animal cells viewed using a light microscope and an animal cell viewed using an electron microscope.

Animal cell structure

Animal, plant and fungal cells

Explore the model to see the typical features of an animal cell, rotating and zooming in and out. Click on the annotations to learn about the different structures.

Animal cell structure

Animal, plant and fungal cells

Plant cells also contain a cell wall made of a polysaccharide called cellulose. It protects the cell and resists osmotic pressure, maintaining the shape of the cell. The vacuole in plant cells is much larger than the vacuoles found in animal cells, and they have an important role in regulating the osmotic potential of the cell.Chloroplasts (double-membrane bound organelles) convert light energy into chemical energy in the process of photosynthesis. The chloroplast is one of many types of plastid (a small organelle responsible for manufacturing and storing chemical energy).

Plant cell structure

Animals, plants and fungi are all eukaryotic. They are similar in some ways but what distinguishes each cell type from the other?

Animal, plant and fungal cells

Plant cells viewed using a light microscope and (b) a plant cell viewed using an electron microscope.

Plant cell structure

Animal, plant and fungal cells

Explore the model to see the typical features of a plant cell, rotating and zooming in and out. Click on the annotations to learn about the different structures.

Plant cell structure

Animal, plant and fungal cells

Like plant cells, fungal cells have a cell wall, but their cell wall is made of a polysaccharide called chitin . Fungal cells contain large vacuoles, which degrade (break down) molecules in the cell, as well as acting as a storage site for small molecules such as ions.Like animal cells, some fungal male gametes may also contain centrioles for producing and organizing the cytoskeleton and playing a key role in cell division. The main function of centrioles is to produce cilia during interphase and the aster and the spindle during cell division.Some fungi are unicellular, such as Saccharomyces cerevisiae, and some are multicellular such as mushrooms and toadstools.

Fungal cell structure

Animals, plants and fungi are all eukaryotic. They are similar in some ways but what distinguishes each cell type from the other?

Animal, plant and fungal cells

Fungal cells viewed using a light microscope and an electron microscope.

Fungal cell structure

Animal, plant and fungal cells

Fungi reproduce by a process called budding. The budding scar is a crater-like ring of tissue that forms when a daughter cell buds from a parent cell. The number of budding scars on a fungal cell is indicative of how many times the cell has divided.

Fungal cell structure

Animal, plant and fungal cells

Animal, plant and fungal cells

Sieve tube elements

Aseptate hyphae in fungi

Mature red blood cells

Skeletal muscle

Anucleate and multinucleate eurkaryotic cells

There are some atypical eukaryotic cells. This means they do not contain, or contain abnormal numbers of the cell structures and organelles that are found in most other eukaryotic cells.

Atypical cell structures in eukaryotes

A Contains a nucleusB Cell wall made of chitinC Cell wall made of celluloseD CentriolesE ChloroplatsF Contain plastids such as chloroplastsG Can contain vacuolesH Contain mitochondriaI Contain 70S ribosomes in the cytoplasmJ Plasma membraneK LysosomesL Cilia

Organize the statements on the Venn diagram to compare and contrast between these three kingdoms of eukaryotic organisms.

Interactive 3

Outline how there may be differences in the cell structure of animal, plant and fungal cells.

Give examples of eukaryotic cells with atypical cell structure.

Name the eight processes that all living things carry out.

Identify how different unicellular organisms carry out each of the eight life processes.

Describe the structure of a typical eukaryotic cell and the function of eukaryotic cell structures and components.

Learning Outcomes

Now you are able to:

Outline cell theory and describe the structure and components of a typical cell.

Summarize how to make and stain temporary mounts of cells and tissues.

Describe how to use an eyepiece graticule and stage micrometre to measure sizes of a specimen.

Perform calculations involving actual size, image size and magnification.

Outline the applications of electron microscopy.

Describe the application of techniques that are commonly used in microscopy.

Outline the structures that are common to cells in all living organisms.

Describe the structure of a typical prokaryotic cell and the function of prokaryotic cell structures and components.

Pixologic Studio. (2014). Nerve cell. [Clip art]. GettyImages. https://www.gettyimages.com.mx/detail/ilustración/nerve-cell-artwork-ilustraciones-libres-de-derechos/478188147NOAA. (2019) Pelagibacter ubique. [Clip art]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Pelagibacter.jpg. Public domain.Royal Society of London. (1664). Robert Hooke's microscope. [Clip art]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Robert_Hooke%27s_microscope.png

And add a topping: give it interactivity and animation.

Improved light microscopes allowed the discovery of bacteria and other unicellular organisms.

Robert Hooke and Antoni van Leeuwenhoek, were independently credited with discovering microorganisms using light microscopes

Proposed solution / Timeline

17th century

19th century

19th century

20XX

20XX

20XX

Telling stories with order and hierarchy is fundamental.

The important thing is that everything fits the theme.

Often, the eyepiece lens of light microscopes will be fitted with an eyepiece graticule . Eyepiece graticules contain a scale or grid. When we look through the eyepiece lens this scale will be superimposed on the image of the specimen.

Water is the main component of cytoplasm and there are many substances dissolved or suspended in this water. Enzymes in the cytoplasm catalyse hundreds or even thousands of different chemical reactions. These reactions are the metabolism of the cell. Metabolism provides a cell with energy and produces all the proteins and other substances that make up the structure of a cell.

Immunofluorescence

A fluorescent tag, called a fluorophore, is attached to antibodies specific for antigens on a structure or cell being viewed. When the antibody binds to the antigen, the structure is then ‘tagged’ with immunofluorescence.When a certain wavelength of light is shone onto the fluorescence tag, the tag will emit light of a different wavelength that can then appear as brightly coloured spots, allowing the visualisation of the location of these target molecules.

Theory of Knowledge

Ada Yonath is a biochemist and crystallographer who is best known for her work in producing the first high resolution X-ray crystal structure of ribosomes. In 2009, she was jointly awarded the Nobel Prize in Chemistry. Ada Yonath is the first woman from the Middle East to be awarded a Nobel Prize in science, and the first woman in 45 years to be awarded the Nobel Prize in Chemistry.To what extent is scientific research becoming more representative of the global population? What are the reasons behind this? What factors continue to limit proportional representation of gender and culture in scientific research?

Robert Hooke’s drawing of cork cells

Cheek wet mount

1. Place a very small drop of the sodium chloride solution on the slide.
2. Use a toothpick to collect your specimen running the toothpick gently along the inside of your cheek.
3. Place your sample on the slide swirling the end of the toothpick around in the drop of sodium chloride solution.
4. Dispose adequately your toothpick (biohazard risk).
5. Put a drop of methylene blue over the sample.
6. Put the coverslip holding it at a right angle to the slide and then let it drop.
7. If the liquid seep out the sides use a paper towel to absorb the excessive liquid.

You also need to be able to draw and annotate diagrams of organelles (nucleus, mitochondria, chloroplasts, sap vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum and chromosomes) as well as other cell structures (cell wall, plasma membrane, secretory vesicles and microvilli) shown in electron micrographs. When annotating these diagrams, you should include the function of the labelled structures.

Practical skills

After studying this subtopic you need to be able to identify cells in light or electron micrographs as prokaryote, plant or animal.In electron micrographs of eukaryotic cells, you should be able to identify these structures: nucleus, mitochondrion, chloroplast, vacuole, Golgi apparatus, rough and smooth endoplasmic reticulum, chromosomes, ribosomes, cell wall, plasma membrane and microvilli.

Genes, made of DNA, contain the information needed for a cell to carry out all its functions. Many genes hold the instructions for making a protein. Some proteins are structural so are needed for growth and repair. Others act as enzymes, without which a cell cannot control chemical reactions and does not have a functioning metabolism.

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El contenido visual es un lenguaje transversal, universal, como la música. Somos capaces de entender imágenes de hace millones de años, incluso de otras culturas.

Freeze fracture microscopy involves freezing a sample and then using a specialised tool to break the sample into small pieces. These small pieces are then observed using an electron microscope to see the internal structure.

Freeeze fracture microscopy

• At this point your specimen may still be slightly blurry, so use the fine focus knob to make smaller adjustments in the distance between the objective lens and the specimen to bring your object into focus.
• Now that you have a clear image you can adjust the magnification by rotating the nosepiece to use a different objective lens.

When using a microscope:

• Start with the lowest magnification possible and the stage at the highest position.
• To get a clear image of the specimen you will need to adjust the focus of the microscope using the coarse focus and the fine focus.
• Look through the eyepiece and use the coarse focus knob to adjust the distance between the specimen and the objective lens until the object comes into focus.
• Turn the knob anticlockwise to move the specimen further from the lens, and if you need to move it back up, turn the knob clockwise.

At this point your specimen may still be slightly blurry, so use the fine focus knob/wheel to make smaller adjustments in the distance between the objective lens and the specimen to bring your object into focus. Now that you have a clear image you can adjust the magnification by rotating the nosepiece to use a different objective lens.

We can see on the figure hat the cell overlaps 4 eyepiece graticules, which means that the diameter of the cell is 4 × 5 µm = 20 µm.

Calibrating the eyepiece graticulate

The stage micrometre shows the actual size of the image using divisions that are each 100 µm (0.1 mm) apart.Each 100 µm division of the stage micrometre is equivalent to 20 eyepiece graticule divisions, which means that one graticule division is equal to 5 µm.

Nature of science: theories

Deductive reasoning can be used to generate predictions from theories. Based on cell theory, we can predict that a newly discovered organism will consist of one or more cells.

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This is the outer boundary of the cell and encloses all of its contents. The plasma membrane controls the entry and exit of substances. It is also very effective at preventing entry of unwanted or even toxic substances. It allows a cell to maintain concentrations of substances that are very different from those in the surrounding environment. The permeability of the plasma membrane relies on a structure based on lipids.

You will also come across the unit nanometres in other parts of this course as measurements involving proteins, viruses and wavelengths of light are often given in nanometres. One micrometre is equal to 1000 nanometres. Similarly, we can think of this in two ways:There are 1000 nanometres in one micrometre.There are 0.001 micrometres in one nanometre.

Microscopes magnify small objects and organisms that we are unable to see with the unaided eye. Cells and cell structures are usually too small to be measured using millimetres , so instead we usually use micrometres (μm) as a unit of measurement. One millimetre is equal to 1000 micrometres. We can think of this in two ways:There are 1000 micrometres in one millimetre.There are 0.001 millimetres in one micrometre.

Onion wet mount

1. Using a sharp scalpel, cut a small square of onion.
2. Using tweezers, peel off a thin inside layer of the onion.
3. Transfer the thin layer of onion onto a glass slide.
4. Using a pipette, add a small drop of iodine onto your specimen.
5. Starting with the cover slip at a 90° angle, gently lower the cover slip over the specimen to avoid bubbles.
6. If bubbles do occur, gently press the cover slip with the eraser end of a pencil to push out the bubble.

As cells and their structures are usually transparent, it can be hard to distinguish different parts using a light microscope. To help visualize certain structures, we use stains, which bind preferentially to particular structures or areas on a cell, making that structure easier to see. We use different stains to view different cell types. Iodine is a stain used to prepare slides of plant cells because it binds to the starch present in plant cells.

In this practical you should consider what you will do to minimize the risk posed by:

• A sharp scalpel
• Iodine solution, which can be harmful and a potential hazard
• Broken glass.

CLEAPSS provides resources and information sheets that you can use to understand hazards and risks posed by common laboratory chemicals and processes, as well as guidelines for ethical and safe disposal of used materials.

Addressing safety of self, others and the environment

Prokaryotic cells typically range between 0.1 and 5.0 μm in diameter, whereas eukaryotic cells typically have diameters ranging between 10 and 100 μm.

NOAA Ocean Exploration, 2019 Pixologic Studio, 2014

Cells can range from the tiny crescent-shaped bacteria such as Pelagibacter ubique, to long and thin animal nerve cells.

Hooke and Leeuwenhoek's discovery put the theory of spontaneous generation into question. Robert Hooke’s observations led to the development of a new theory, cell theory (Schwann, 1839)

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Theory of Knowledge

Many different scientists have contributed to our understanding of cell theory:

• In 1665, Robert Hooke discovered cork cells (non-living) using his microscope.
• In 1674, Antoni van Leeuwenhoek first observed living cells under the microscope.
• In 1838, Matthias Schleiden and Theodor Schwann compared their observations of plant and animal cells.
• In 1858, Rudolf Virchow stated ‘omnis cellula e cellula’ (all cells come from cells).
• In 1839, Theodor Schwann first proposed cell theory.

To what extent does scientific collaboration need to occur in the same time frame or location?