BIO; Methods of Studying Cells

what are the 2 key methods of studying cells

microscopes

cell fractionation + ultracentrifugation

what are the 3 key types of microscopes

optical (light) microscopes

transmission electron microscopes (TEM)

scanning electron microscopes (SEM)

define magnification

magnification of a microscope refers to;

how many times larger the image is compared to the actual size of the object

define resolution

resolution of a microscope refers to;

the minimum distance between two objects in which they can still be viewed as separate

how’s resolution determined in optical microscopes

by wavelength of light

how resolution determined in an electron microscope

by the wavelength of the beam of electrons

describe the optical (light) microscope

beam of light is condensed to create the image

poorer resolution - due to longer wavelength of light

small organelles in a cell are not visible

lower magnification

produces coloured images

can view living samples

describe the electron microscope

beam of electrons is condensed to create the image

higher resolution - electrons have a short wavelength

higher magnification

produces black and white images

only view non living samples - bc sample must be in a vacuum (no air)

how come optical microscopes have a poorer resolution than electron microscopes

light has a longer wavelength

and electrons have a shorter wavelength

why do samples have to be in a vacuum in electron microscopes

electrons are absorbed by air so sample must be in a vacuum (where there’s no air) to produce an image

how do transmission electron microscopes(TEM) work

using thin specimens, stained and placed in a vacuum.

an electron gun produces a beam of electrons that pass through the specimen

parts of the specimen that absorb the electrons - appear DARK

produces a 2D, black and white image of detailed internal structures of cell

how does a scanning electron microscope (SEM) work

specimen doesn’t need to be thin as electrons are not passing through the specimen;

electrons are beamed onto the surface and scattered in different ways depending on the 3D contours of the cell structures

produces 3D black and white image

not detailed of the internal structure of cells as electrons bounce bak

formula for calculating magnification/imagesize/actual size

magnfication=\overline{}\frac{imagesize}{actualsize}

how to use an eyepiece graticule

1. line up the stage micrometer and eyepiece graticule whilst looking through the eyepeice on the microscope

2. count how many divisions on the eyepiece graticule fit into one division on the micrometer scale

3. each division has a magnification of micrometer is 10um - used to calculate what one division on the eyepiece graticule is at that current magnification

what are the two steps of cell fractionation

homogenisation

ultracentrifugation

whats the first step of cell fractionation

homogenisation;

cells are broken open (homogenised) using a blender

cells are blended in a cold, isotonic and buffered solution

why must cells be prepared in a cold, buffered and isotonic solution

cold - to reduce enzyme activity as enzymes are released when breaking open the cell which damage the organelles

isotonic - same water potential to prevent osmosis as this could cause the organelles to shrivel/burst

buffered - pH buffer to prevent damage to organelles

whats the second step of cell fractionation

ultracentrifugation;

the filtered solution is spun at increasing speeds in a centrifuge;

the centrifuge forces causes pellets of the most dense organelles to form at the bottom first at the lower speeds e.g;

nuclei (most dense - lowest speed)

1st spin - chloropasts

2nd spin - mitochondria

3rd spin - lysosomes

4th spin - endoplsmic reticulum

5ht spin - ribosomes (least dense - highest speed)

PRACTICAL context; Students could use iodine in potassium iodide solution to identify starch grains in plant cells.

MS 1.8

involves creating thin samples (e.g., of potato or leaf), mounting them on slides using light microscopy techniques,

applying iodine staining to see starch amyloplasts (storage organelles).

Iodine in potassium iodide solution is added to the plant tissue, turns blue-black in presence of starch

how to prepare an optical microscope

1. Cut a very thin section of the tissue using scalpel

2. Place the sample onto the center of a clean microscope slide.

3. Add a drop of water or appropriate stain (e.g., iodine for onion cells, methylene blue for root tips).

4. Carefully lower a coverslip onto the specimen at an angle (e.g., using a mounted needle) to minimize air bubbles, which can obstruct the view.