Visualizing Cells (lecture 2)

Question 1

What is the minimum resolvable by unaided eye?

Answer 1

0.2 mm (200 micrometers)

Question 2

Minimum resolvable by light microscope?

Answer 2

200 nm ( This means that it can resolve down to cells and organelles but usually not beyond)

Question 3

Mimimum resolvable by the electron microscope?

Answer 3

0.2 nm (This means that it can resolve down from organelles, molecules, very, very little atoms)

Question 4

Robert Hooke

Answer 4

Built one of the first useful compound microscope

• observed the structure of a cork
• coined the term “cell”

Question 5

Chichona calisaya

Answer 5

Plant that provided early anti-malarial drugs. The compound quinine

Question 6

Briefly describe the main components of the light microscope

Answer 6

Detection and Analysis
Magnification-Ocular piece, objective lense
Light Gathering-objective
Illumination

Question 7

Electromagnetic Theory of Light Propagation

Answer 7

The concept that under some conditions, light is an electromagnetic wave composed of electric and magnetic components that oscillate perpendicular to one another

Question 8

What is diffraction?

Answer 8

A property of light. Phenomena that occurs when a wave encounters an obstacle or a slit. An interference of a wave.

Question 9

What is refraction?

Answer 9

When waves pass a boundary between two substances with different densities, such as air or water. This causes it to change direction and creates an angle of incidence and an angle of refraction. The angle of refraction is always less than the angle of incidence.

Question 10

Phase Contrast Microscopy

Answer 10

A type of light microscope. Ideal for observing thin, colorless/unstained organisms. Made it possible to observe living organisms that are transparent. Takes advantage of direct and diffracted light to give quality and defined images of transparent organisms.

Question 11

DIC Microscopy

Answer 11

DIC has strong advantages in uses involving live and unstained biological samples, such as a smear from a tissue culture or individual water borne single-celled organisms. Its resolution and clarity in conditions such as this are unrivaled among standard optical microscopy techniques.

Question 12

Describe the light path in DIC microscopy

Answer 12

1) Unpolarised light enters the microscope and it is polarized.
2) Polarized light is split into two rays .
3) Two rays are focused in the condenser for passage through the sample (the light passes the sample at different degrees of polarization)
4)
5
6) The second prism combines the two rays into one. The combination of the rays leads to interference, brightening or lightening the image at that point according to the optical path difference.

Question 13

What ultimately limits the resolution in Optical microscopy?

Answer 13

Resolution is ultimately limited by the wavelength of the illuminating light. Limit of Resolution approaches ~ ½ of the wavelength

Question 14

What does the symbol of Lambda mean?

Answer 14

Wavelength of light

Question 15

SEM vs TEM

Answer 15

SEM is based on scattered electrons while TEM is based on transmitted electrons
SEM focuses on the samples surface and composition while TEM provides details about the internal composition. SEM therefore only shows morphology.
The samples for TEM have to be cut thinner whereas there is no need for that in SEM

Question 16

Scanning Electron Microscope

Answer 16

SEM= Scanning electron microscope, an electron beam(~20kV) will scan the surface of the thick sample and signals that arise out of electron-beam specimen interaction are used for getting the desired information in your sample. ex: (i)secondary electron ( high resolution imaging of surface features, fractography); (ii) Back scattered electrons - composition contrast image, Orientation imaging (iii) X-rays ( chemical composition, EDS, WDS )

Question 17

Transmission electron Microscope

Answer 17

TEM =Transmission electron microscope, high KV(~200kV) electron beam is passed through electron transparent thick sample and the signals that are collected below the sample used to understand the sample character. ex: (i) Bright field image and dark field imaging = microstructural features like grains, dislocations, precipitates etc., (ii) SADP =selected area diffraction (crystal structure information of your sample)