Biology - Foundations in Biology - Cell Structure

How do light microscopes work and what are they primarily used for?

Light microscopes use visible light and glass lenses to bend light and create a magnified image on the viewer’s eye. They are used for looking at live and stained specimens.

Describe an average image made by a light microscope.

• Only large organelles seen, such as the nucleus and the cell wall/cell membrane

• Colourful due to cell staining

• Lowest magnification and lowest resolution of all microscopes

How do you prepare a light microscope for viewing?

1. Set up the microscope for viewing by plugging it into a power source and turning it on.

2. Clip the slide onto the stage.

3. Use the stage adjuster to move the stage as desired.

4. Starting on the lowest magnification objective lens, use the coarse and fine focus to adjust the image.

5. Repeat step 4, using different magnifications.

What are the advantages and disadvantages of using a light microscope?

Advantages:

• Least expensive method

• Can see live organisms

• Simple set up and use

Disadvantages:

• Lowest magnification and resolution of any microscope

• Can’t see smaller/non-membrane bound organelles

How do TEMs work and what are they primarily used for?

TEMs (transmission electron microscopes) fire a beam of electrons through a specimen to create an image. They are primarily used for creating higher magnification and resolution images of smaller specimens and a wider range of organelles.

What are the advantages and disadvantages of a TEM?

Advantages:

• Higher magnification and higher resolution

• Can see smaller/non-membrane bound organelles such as ribosomes.

Disadvantages:

• Can only be used in a vacuum, so no live specimens can be viewed.

• Very expensive and complicated set up and use

• Can only view thinner tissues.

Why can you only view thinner tissues using a TEM?

Because the image is created by the specimen absorbing electrons. Denser tissues absorb more electrons and show darker in the image, and lighter tissues absorb less electrons and show lighter in the image. Thick tissues absorb most electrons and therefore do not create a reliable image.

How do SEMs work and what are they primarily used for?

SEMs (scanning electron microscopes) pass an electron beam over the specimen (or ‘scans’ it) which are then captured in a cathode ray tube, forming a computational image. They are primarily used for creating higher magnification and higher resolution, 3D images.

What are the advantages and disadvantages of SEMs?

Advantages:

• High magnification and high resolution.

• Can create 3D images

• Can accept larger, more bulky samples

Disadvantages:

• Expensive set up and complicated use

• Creating 3D images takes a long time

• Can only be used in a vacuum

How do you calibrate an eye piece graticule?

1. Check the size/scale of the graticule.

2. Align the eyepiece graticule with the stage micrometer.

3. Count how many divisions there is on the eyepiece graticule that correspond to the divisions on the micrometer (e.g. for every 10 divisions on the stage graticule, there is 21 divisions on the micrometer).

4. Calculate the length between each division on the graticule using the ratio (e.g. 1mm = 42 gu therefore 1 gu = 1/42 mm or 0.0238mm)

What is the difference between magnification and resolution?

Magnification is a scale of how many times larger the image size is compared to the actual size, and resolution is the minimum distance that two objects can be seen as distinct objects in the image.

What is the difference between primary structures and ultra structures?

Primary structures are structures that can be seen under a light microscope (cell membrane, nucleus etc) and ultrastructures are structures that can only be seen with an electron microscope.

List the ultrastructures in a eukaryotic cell.

• Nucleus

• Mitochondria

• Chloroplasts

• Lysosomes

• Rough Endoplasmic Reticulum (RER)

• Golgi apparatus

• Ribosomes

Describe the function and the size of the nucleus.

• It is the main site of DNA in eukaryotes

• Preservation, replication, and expression of genetic information

• Synthesises RNA for protein synthesis

• Copies DNA for cell division

• 10 micrometers in diameter

Describe the function and the size of the mitochondria.

• Internal membrane contains the enzyme needed to synthesis ATP (adenosine triphosphate)

• Site of aerobic respiration

• Metabolic pathways are divided by inner membranes

• 1 micrometer wide and 5 micrometers long

Describe the function and the size of chloroplasts.

• Photosynthesis

• Metabolic pathways are closely related to that of the mitochondria

• 5 to 10 micrometers in diameter

Describe the function and the size of the lysosomes

• Digestion of compounds taken in by endocytosis

• Recycling of material within the cell

Describe the function and the size of the Golgi apparatus.

• Processing and packaging

• Synthesising lysosomes to contain potentially dangerous catabolic enzymes

• Produces secretory vesicles

• Makes more phospholipids (for cell membrane)

• 1 to 3 micrometers in diameter with cistern are being 0.5 micrometers thick

What are catabolic enzymes and secretory vesicles?

Catabolic enzymes are enzymes that break down big molecules into smaller ones, and vesicles are small packets that sit on the cell membrane that enable exo and endocytosis

Describe the function and the size of the RER.

• It is difficult to study the RER as it is difficult to extract intact

• Starts biosythesis for proteins and lipids (this continues in the Golgi apparatus)

• Has ribosomes attached unlike the smooth endoplasmic reticulum

• Proteins made in the RER will secrete out of the cell

• 26 to 56 nanometers thick

Describe the function and the size of ribosomes.

• Protein synthesis

• NOT membrane bound (they are present in both prokaryotes and eukaryotes)

• Subunits of ribosome are synthesised in the nucleus of eukaryotes

• 20 nanometers in diameter

Describe the function and size of the nucleolus.

• Helps regulate aging and immune responses

• Site of ribosome synthesis in the nucleus

• Site of RNA transcription

• 0.5 to 5 micrometers in diameter

Describe the function and size of the plasma membrane.

• Protects the cell from external environments

• Has selective permeability and can let molecules in and out of the cell

• Helps the cell maintain its structure

• 7 to 10 nanometers thick

Describe the function and size of the flagella.

• Allows prokaryotic cells to be motile (they can move)

• Made up of three parts: the filament, hook, and basal body

• Can be monotrichous, lophotrichous, amphitrichous, and peritrichous

• 12 to 30 nanometers in diameter and 5 to 20 micrometers in length

Describe the function and size of the centrioles.

• They help chromosomes separate evenly during mitosis and meiosis

• Determines the location of the nucleus

• Maintains cell shape and polarity

• Anchors the flagella to prokaryotic cells

• About 250 nanometers in diameter and 150 to 500 nanometers in length

Describe the function and size of the cell wall.

• It gives the plant cell strength and structure

• Has selective permeability to allow molecules in and out of the cell

• Protects the cell from pathogens and its external environment

• 0.1 to several micrometers