2.1.3 studying cells

Question 1

Optical microscope

Answer 1

Use light to form a 2D image

Visible light, longer wavelength so lower resolution

Low magnification x1500

Question 2

Optical adv

Answer 2

Can see living organisms

Question 3

Optical dis

Answer 3

2D image

Only used on thin specimens

Low resolution; can’t see internal structures of organelles

Low magnification

Question 4

SEM

Answer 4

Uses electrons to form a 3D image

Beams of electrons scan surface, knocking off electrons from the specimen

Electrons shorter wavelength so higher resolution

High magnification

Question 5

SEM adv

Answer 5

3D image

Higher resolution; can see internal structures of organelles

High magnification

Used on thick specimens

Question 6

SEM dis

Answer 6

Vacuum; can’t see living organisms

Lower resolution than TEM

Question 7

TEM

Answer 7

Uses electrons to form a 2D image

Electromagnets focus beam of electrons onto specimen

More dense= more absorbed = darker

Electrons shorter wavelength so higher resolution

High magnification

Question 8

TEM adv

Answer 8

High resolution; see internal structures of organelles

High magnification

Question 9

Magnification equation

Answer 9

Magnification = size of image / size of real object

Question 10

Magnification

Answer 10

How much bigger the image of a sample is compared to the real size

Question 11

Resolution

Answer 11

How well distinguished an image is between 2 points; shows amount of detail; limited by wavelength of radiation used e.g. light

Question 12

Measuring the size of an object viewed with an optical microscope

Answer 12

Line up eyepiece graticule with stage micrometer

Use stage micrometer to calculate the size of divisions on eyepiece graticule at a particular magnification

Take the micrometer away and use the graticule to measure how many divisions make up the object

Calculate the size of the object by multiplying the number of divisions by the size of division

Recalibrate eyepiece graticule at different magnifications

Question 13

Preparing a ‘temporary mount’ of a specimen on a slide

Answer 13

Place a thin section of specimen

Add a drop of a stain (e.g iodine in potassium iodide solution used to stain starch grains in plant cells)

Add a cover slip by carefully tilting and lowering it, trying not to get any air bubbles

Question 14

Cell fractionation

Answer 14

Homogenise tissue to break open cells

Place in a cold, isotonic, buffered solution as cold reduces enzyme activity so organelles aren’t broken down. Isotonic so water doesn’t move in/out of cell via osmosis so they don’t burst/shrivel. Buffered to keep pH constant so enzymes don’t denature

Filter homogenise to remove large, unwanted debris e.g. whole cells, connective tissue

Question 15

Ultracentrifugation

Answer 15

Centrifuge homogenate in a tube at low speed

Remove pellet of heaviest organelle and spin supernatant at higher speed

Repeat at higher and higher speed until organelles separated out

Separated in order of mass/density

Nuclei > chloroplasts > mitochondria > lysosomes > er > ribosomes

Question 16

Considerable period of time in which the scientific community distinguished between artefacts and cell organelles

Answer 16

Repeatedly prepared specimens in different ways

If an object could only be seen with one preparation technique, but not another, it was more likely to be an artefact rather than organelle