biology advanced level notes

A LEVEL BIOLOGY NOTES




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alone. Remember the learners who shall use these notes. I pray for their success; bless them according to
your riches in mercy and grace. In Jesus Christ’s name . AMEN.


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, FORM 5 TERM 1
TOPIC 1: CELL STRUCTURE AND FUNCTIONS
Patience and perseverance have a magical effect before which difficulties
disappear and obstacles vanish
KEY CONCEPT OBJECTIVES CONTENT SUGGESTED SUGGESTED
Learners should be (ATTITUDES, SKILLS LEARNING RESOURCES
able to: AND KNOWLEDGE) ACTIVITIES AND
NOTES
8.1.1 Microscopy  Calibrate eyepiece - calibration and  Calibrating  Relevant reference
graticule measurement eyepiece graticule. materials
 ICT tools
 Draw and - units of measurement  Observing cells  Braille
determine linear (millimetre, micrometre using light software/Jaws
dimensions of and nanometre) microscope.  Light Microscope
specimens  Measuring linear (X4, X10, X40
dimensions of objective lenses)
specimens.  Hand lenses
-magnification and  Graticules
 distinguish between resolution (refer to light  Discussing the  Stage micrometers
magnification and and electron concepts  Stains
resolution microscopes) magnification and  Prepared slides
resolution.
- wet mounts  Mounting
 prepare temporary - staining temporary slides.
slides  Staining wet
mounts with
appropriate stains.



MICROMETRY

Micrometry: The measurement of microscopic objects.

─ A micrometer is used.
─ In biology we often need to measure very small objects.
─ When measuring cells or parts of cells, the most useful unit is the micrometer (µm).
─ One micrometer is 1000 of a millimeter.
─ 1µm =1/1000mm.
─ Even smaller structures such as the organelles with such small sizes are measured using
even smaller units.
─ The Nanometers are used (nm).
─ 1nm=1/1000µm.

NB* Note that centimeters are not units in Biology, nm, µm, & mm are used.

Microscopy

─ The use of microscopes in Biology is called microscopy.
─ Microscopes used are light microscope and electron microscope.

Distinguish between resolution and magnification.


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, ─ Resolution is the ability of a microscope to distinguish two objects close together rather
than to see them as one object.
─ Magnification is the number of times an object is enlarged.

𝒊𝒎𝒂𝒈𝒆
𝑴𝒂𝒈𝒏𝒊𝒇𝒊𝒄𝒂𝒕𝒊𝒐𝒏 = 𝒔𝒊𝒛𝒆𝒐𝒇 𝒔𝒊𝒛𝒆𝒐𝒇𝒐𝒃𝒋𝒆𝒄𝒕.
𝒓𝒆𝒂𝒍


─ E.g. A person makes a drawing of an incisor tooth and the width of the actual tooth is 5mm
while the tooth drawing is 12mm .Calculate the magnification of the drawing?

Magnification = 12/5 =x2, 4

Calculation of magnification and the conversion of units.

─ Let’s say, the real diagram of a red blood cell is 7nm and asked to calculate the
magnification.

Step 1– measure the diameter of the cell in the diagram. You find that it is 30mm.

Step2-we have been given its real size so we need to convert 30mm to µm .there are
1000µm in a mm , so 30mm =30x1000µm

Step 3-we can now put the numbers in the equation.

Calculating magnification from a scale bar

─ When given a scale bar, there is no need to measure the leukocyte .We can simply use the
scale bar.
─ All you need to do is to measure the length of the scale bar and then substitute it’s measured
length and the length that it represents on the scale bar.
─ Remember to convert the measurements to µm.

Step 1– measure the scale bar.Here it is 24mm

Step2-substutute into the equation.

𝒊𝒎𝒂𝒈𝒆
𝑴𝒂𝒈𝒏𝒊𝒇𝒊𝒄𝒂𝒕𝒊𝒐𝒏 = 𝒔𝒊𝒛𝒆𝒐𝒇 𝒔𝒊𝒛𝒆𝒐𝒇𝒐𝒃𝒋𝒆𝒄𝒕.
𝒓𝒆𝒂𝒍

= length of scale Bar
Length of scale bar represents

Light microscope

─ A light microscope (compound microscope) uses the magnifying powers of the convex lens
to produce a magnified image of a small object.
─ Therefore the magnification of the light microscope I equal to the objective lens & the
magnifying lens.


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, Common stains used in light microscopy

Stain Use Colours produced
METHYLENE Staining living cells Dark blue nucleus , light cytoplasm 9in bacteria the
BLUE whole cell takes up the stain)
Iodine Staining living plant cells Very dark blue starch grains
Acidified Staining lignin (the Bright red
phloroglucinal substance in the cell walls
of xylem vessels)
Acetin orcein Nuclei and chromosomes Red
Light green Staining plant cell walls Green


Electron microscope.

─ Is like an up-side-down light microscope.
─ The radiation enters @ the top and the specimen at the bottom is viewed.
─ The principle is the same as on the light microscope in that, a beam of radiation electrons is
focused by condenser lenses and the image is magnified by further lenses.
─ A photograph taken by the electron microscope is called an electron micrograph
─ The major advantage of an electron microscope is its high resolving power i.e., 0, 5 nm in
practice.


Comparison of a light microscope and an electron microscope

Electron microscope Light microscope
Radiation source Electrons light
Wavelength About 0,005nm 400-700nm
Max resolution 0,5NM 200nm
Max magnification X250000 X1500
Lenses Electromagnets glass

Qn.Describe what is meant by the term resolution. [2]

Qn. State the maximum magnification that can be achieved by a light microscope and a
transmission electron microscope.Select your answers from the list below.

10x 40x 100x 400x 1500x 25 000x 50 000x 500 000x

light microscope ................................... x

transmission electron microscope ................................... x [Total 2 marks]
Qn. Explain why it would not be possible to see the same detail of a cell ,as can be seen using a
electron microscope on a light microscope. [3marks]

 Electron microscope has high resolution
 Because of shorter wave length
 More detail can be seen/ much clearer
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