Chapter 2 Cells and cell structure

Question 1

2.1 What is a microscope?

Answer 1

Microscopes refer to instruments that magnify the image of an object.

Question 2

2.1 What is the object?

Answer 2

The object refers to the material put under a microscope.

Question 3

2.1 What is the image?

Answer 3

The image refers to the appearance of the object viewed under a microscope.

Question 4

2.1 What is magnification?

Answer 4

The magnification of an object refers to how many times bigger the image is compared to the object.

Question 5

2.1 What is the formula triangle of magnification? (include the important note regarding units)

Answer 5

Actual size of object = size of image/ magnification
- It is important to remember that when calculating magnification, units of length need to be equal for both actual size of object and size of image

https://www.savemyexams.co.uk/igcse/biology/cie/22/revision-notes/2-organisation-of-the-organism/2-4-size-of-specimens/2-4-1-magnification/

Question 6

2.1 How do you convert between units of measurements?

Answer 6

m to cm = x100 cm to mm= x10 (also open link for table)

https://www.elevise.co.uk/gab1c.html

Question 7

2.1 What is resolution?

Answer 7

Resolution of a microscope refers to the minimum distance apart two objects can be for them to appear as two seperate items.
In other words greater resolution= image produced is more clear and precise

Question 8

2.1 What does resolving power depend on?

Answer 8

Whatever the type of micrsocope, the resolving power depends on the wavelength/ type of radiation used

Question 9

2.1 What is the difference between magnification and resolution?

Answer 9

High magnification= bigger image size
High resolution= increase in clarity of image

Question 10

2.1 Does higher magnification automatically refer to higher resolution?

Answer 10

Every microscope has a limit of resolution, up to this limit higher maginification will increase resolution. However beyond this limit, higher magnification won’t do this (object whilst appearing larger will just be more blurred).

Question 11

2.1 What is cell fractionation? And why is it important?

Answer 11

• Cell fractionation refers to the process by which cells are broken up and the different components they contain including organelles, are sepearted out. It occurs in two stages; homogenation and ultracentrifugation.
• Cell fractionation allows various organelles’ structures and functions to be studied as it leads to the isolation of a large number of organelles.

Question 12

2.1 What needs to be done before cell fractionation? (incude breakdown)

Answer 12

Before cell fractionation can take place, the tissue needs to be placed in a cold, isotonic buffered solution;
Cold - in order to reduce enzyme activity that might break down the organelles
Isotonic- in order to prevent organelles from bursting/ shrinking due to osmotic gain/loss (isontonic solution has the same water potential as the og. tissue)
Buffered- in order to maintain a constant pH level

Question 13

2.1 Cell fractionation stage 1: What is the process of homogenation?

Answer 13

• The cells are broken up by a homogeniser (blender) which releases the organelles from the cells. And then the resultant fluid (homogenate) is filtered to remove any complete cells and large pieces of debris.

Question 14

2.1 Cell fractionation stage 2: What is ultracentrifugation? And describe the process.

Answer 14

• Ultracentrifugation- process by which the fragments in the filtered homogenate are seperated in an ultracentrifuge (spins tubes of homogenate at very high speeds in order to create a centrifugal force)
  The process occurs as follows;
  1. Tube of filtrate (homogenate) is placed in ultracentrifuge and is spun at a slow speed
  2. The heaviest organelle (nuclei) is forced to the bottom of the tube, forming a sediment/pellet
  3. Fluid at the top of the tube (supernatant) is removed, leaving just the sediment of the nuclei
  4. The supernatant is transferred to another tube and spun in the ultracentrifuge at a faster speed than before
  5. The next heaviest organelles (mitochondria), are forced to the bottom of the tube
  6. Process is continued in this way so that, at each increase in speed, the next heaviest organelle is sedimented and seperated out

Question 15

2.1 What is the order by which the organelles are seperated out?

Answer 15

Nuclei
Mitochondria
Lysosome
Ribosome

Question 16

2.2 What are the characteristics and limitations of optical/ light microscopes?

Answer 16

• Use visible light to magnify images

Limitations:
- Low resolution/resolving power (200nm/0.2μm) - because of relatively long wavelength of light
- Photomicrograph/image may contain artefacts (things that result from the way the specimen is prepared) = makes it difficult to distinguish whether the artefact is part of the tissue or not

Question 17

2.2 What are that characteristics of electron microscopes?

Answer 17

• Electron microscopes use a beam of electrons rather than light
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  The main two advanatages of this is;
• An electron beam has a very short wavelength = higher resolution/ resolving power
• Electrons are negatively charged so the beam can be focussed using electromagnets
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• Since electrons are absrobed by molecules in air, a near-vaccum has to be created within the chamber of an electron microscope in order for it to work effectively (if air present, it would stop electrons from reaching the specimen)

Question 18

2.2 What are the 2 types of electron microscopes?

Answer 18

• Transmission electron microscope (TEM)
• Scanning electron microscope (SEM)

Question 19

2.2 What are the charactersitics of the transmission electron micrsocope (TEM)?

Answer 19

• Consists of an electron gun that produces a beam of electrons focused onto the specimen by a condenser electromagnet
• Beam penetrates a thin section of the specimen
• Parts of this specimen absorbs electrons= appears dark
• Other parts of this specimen allow electrons to pass through= appears bright
• Image (called photomicrograph) produced on a screen as 2D images
• Resolving power/resolution (0.1nm) and maginification power higher than SEM

Question 20

2.2 What are the limits of the transmission electron microscope (TEM)?

Answer 20

• Living specimens can’t be observed= whole system must be in a vacuum
• Complex ‘staining’ process required and even then the image is in black and white only (colour can be added digitally)
• Photomicrograph/image may contain artefacts (things that result from the way the specimen is prepared) = makes it difficult to distinguish whether the artefact is part of the tissue or not
• Specimens must be extreemly thin to allow the electrons to penetrate the specimen= results in flat 2D image (can partly get over this by taking series of sections through a specimen which can then be built up to produce a 3D image by looking at a series of photomicrographs produced - however it is a slow and complicated procedure) = development of SEM has overcome this problem

Question 21

2.2 What are the characteristics of the scanning electron microscope (SEM)?

Answer 21

• Consists of an electron gun that produces a beam of electrons focused onto the specimen by a condenser electromagnet
• Directs beam of electrons onto surface of specimen from above rather than penetrating it from below (like TEM)
• Beam is passed back and fourth across proportion of the specimen in a regular pattern
• Reflected scattering of electrons depends on contours of specimen surface
• Can build up/produce a 3D image by computer analysis of the pattern of electron scattering and secondary electrons produced
• Resolving power/resolution (around 20 nm) and magnification power is lower than TEM

Question 22

2.2 What are the limits of the scanning electron microscope (SEM)?

Answer 22

• Living specimens can’t be observed= whole system must be in a vacuum
• Complex ‘staining’ process required and even then the image is in black and white only (colour can be added digitally)
• Photomicrograph/image may contain artefacts (things that result from the way the specimen is prepared) = makes it difficult to distinguish whether the artefact is part of the tissue or not