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OCR A A-level Biology Module 2 Notes Summary PART 1
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Concise notes with diagrams and key words highlighted, sticking to the OCR A Biology specification PART 1 - CELLS
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A Level BiologyMICROSCOPY OCR A Module 2
Optical (light) microscopes
• Resolution much lower than electron microscopes
• Magnification → overall magnification calculated by multiplying the eyepiece lens magnification by the
objective lens magnification
• Use light to create colour images
• Used to view whole cells and tissues (most organelles not visible)
• Can be used to view live specimens
• The eyepiece graticule must be calibrated against a stage micrometer (see next page)
Scanning electron microscopes (SEM) Transmission electron microscopes (TEM)
• Resolution and magnification → lower than TEM • Highest resolution
but much higher than optical microscopes • Highest magnification
• Use electrons to scan the surface of cells • Use an electron beam to view internal structure
(cannot see internal structures) of cells and organelles
• Create a 3D image in black and white • Create a 2D image in black and white
• Can only view dead specimens
You need to be able to recognise
• Preparation is more complex than for optical
cellular components of eukaryotic microscopes
cells from microscope images. • Specimens must be very thin
Differential staining in light microscopy
• Allows identification of different cell types and different organelles
• Allows white cells to be visualised and counted, and gives contrast to images
• Many different stains available → Sudan red stains cell membranes
→ Nile blue, methylene blue and haematoxylin all stain nuclei (DNA)
→ eosin stains proteins pink (including the cytoskeleton proteins)
→ iodine in potassium iodide solution stains starch blue-black
Magnification and resolution
• Resolution → the ability to separate two objects
→ a high resolution allows the microscope to distinguish between structures that are very
close
• Magnification → how many times larger the image is than the actual object
image size
10
x x1000 1000
x
magnification
>
-
Cm mi Um, nm
actual size :10 1000
: 1000
:
• To calculate the volume of a cell, use the volume of a sphere formula: V =
ar
Page 1 of 2 © Dr Zoë Huggett
, A Level Biology
MICROSCOPY OCR A Module 2
Calibrating the eyepiece graticule on an optical microscope
• Eyepiece graticule → fixed onto the eyepiece, has divisions with no scale
• Stage micrometer → placed onto the stage, has a scale
• To measure something with the eyepiece graticule
→ calibrate the eyepiece graticule with the stage -
eyepiece graticule
micrometer to find the length of one division on
-
stage micrometer
the graticule 0 1 2 3 5 4
6
(mmdivisions)
7
→ measure the object with the graticule (take
repeat measurements and calculate a mean) one division on the
→ convert the number of graticule divisions into a eyepiece graticule is 0.7mm
length e.g. in millimetres or micrometres
Preparing a microscope slide
• Add a drop of water to a glass microscope slide
• Put a thin section of your sample onto the water drop When taking measurements
using tweezers using a microscope, take
• Pipette a drop of stain at the edge of the sample multiple measurements from
• Carefully lower the cover slip at an angle using a different fields of view and
mounted needle calculate a mean.
• Use blotting paper to remove excess stain
Representing cell structure using annotated diagrams
• Use continuous lines with no shading or cross hatching
• Add a scale bar and title to the diagram Amitochondrion
outer mitochondrial
• Draw label lines using a ruler membrane
matrix
→ do not cross the lines
→ do not add arrowheads to the line
• Use two lines to represent a cell wall
• Annotate colours where appropriate
intermembrane
space we cristal
Make sure to follow these inner mitochondrial
membrane
rules rather than copying my
usual style of diagram! 0.5Mm
Page 2 of 2 © Dr Zoë Huggett
, A Level Biology
EUKARYOTIC CELLS OCR A Module 2
Animal cell structure
• Surrounded by the plasma (cell-surface) membrane
• Contain membrane-bound organelles
nucleus
·in
The cytoplasm contains
:my g
mitochondria smooth
a....
-·
endoplasmic many intracellular enzymes
reticulum and the cytoskeleton.
&
:ii.
-
ribosome ..........
I
o
a
8 ·
1000
·
·
·
·
plasma
membrane
B
·
1
......
·esssssss
·
Algal and fungal cells
- are also eukaryotic.
·
↑
·
-centricle
-
·
cytoplasm ·
·
·
lysosome Golgi apparatus
rough endoplasmic
reticulum
Plant cell structure
• Surrounded by the plasma (cell-surface) membrane and a cellulose cell wall
• Contain membrane-bound organelles
movigrice.
-
cellwall . .......
7
7 smooth
8g
·
-
:iii.
·
-
·
·
⑳ j.ii:
: endoplasmic
.
mR
-
:
I
ooo
- reticulum
Amyloplasts are starch
·
-
&
-
nucleus
grains surrounded by a double ·0- 8 -
·
O
·
membrane. They synthesise plasmodesma-
8 - ......
and store starch.
E· (
s
-
rough endoplasmic
reticulum
·
·
ribosome
-
Plasmodesmata are gaps ·
--
solgi apparatus
&
in the cell wall which allow
~plasma
⑧
·
exchange between cells. S
⑧ 0
membrane
vacuole
↑ -
cytoplasm amyloplast
Page 1 of 4 © Dr Zoë Huggett
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