CIE AS/A Level Biology -1.1 The microscope in cell studies- Study Notes- New Syllabus

Ace A level Biology Exam with CIE AS/A Level Biology -1.1 The microscope in cell studies- Study Notes- New Syllabus

Key Concepts:

1 make temporary preparations of cellular material suitable for viewing with a light microscope
2 draw cells from microscope slides and photomicrographs
3 calculate magnifications of images and actual sizes of specimens from drawings, photomicrographs and electron micrographs (scanning and transmission)
4 use an eyepiece graticule and stage micrometer scale to make measurements and use the appropriate units, millimeter (mm), micrometre (μm) and nanometre (nm)
5 define resolution and magnification and explain the differences between these terms, with reference to light microscopy and electron microscopy

CIE AS/A Level Biology 9700-Study Notes- All Topics

Making Temporary Preparations of Cellular Material for Light Microscopy

🌱 Purpose of Temporary Preparations

To observe cells or tissues under a light microscope for study of structure and function.
Temporary slides allow quick examination without permanent mounting.

🧩 Materials Needed

Microscope slides and cover slips
Dropper or pipette
Staining solutions (e.g., iodine, methylene blue)
Distilled water
Forceps and scalpel or razor blade
Paper towels or blotting paper

🧬 Step-by-Step Procedure

Sample Collection

Select fresh cellular material (e.g., onion epidermis, cheek cells, leaf peel).

Preparation of the Sample

For solid samples (onion epidermis, leaf peel): Use forceps to peel a thin layer or scrape a small section.
For liquid samples (blood, pond water): Use a dropper to place a small drop on the slide.

Mounting the Sample

Place the sample flat on a clean glass slide.
Add a drop of distilled water or stain (if required) onto the sample to increase visibility.

Staining (Optional but Recommended)

Add a drop of stain like iodine (for plant cells) or methylene blue (for animal cells) to highlight structures such as the nucleus or cell wall.
Wait 1-2 minutes for stain to act.

Applying the Cover Slip

Hold the cover slip at an angle (about 45°) next to the sample.
Slowly lower it to avoid air bubbles.

Removing Excess Liquid

Use blotting paper or tissue to gently absorb excess liquid around the edges.

🧪 Tips for Clear Viewing

Avoid thick samples; thin layers allow light to pass through.
Minimise air bubbles under the cover slip.
Use appropriate stains to improve contrast.
Start viewing under low power, then switch to high power for details.

📊 Summary Table of Steps

Step	Description	Notes
Sample Collection	Choose fresh tissue or cells	Onion epidermis, cheek cells, etc.
Sample Prep	Thin layer or drop on slide	Use forceps or pipette
Mounting	Place sample + drop of water/stain	Ensures visibility
Staining	Add stain for contrast	Iodine for plants, methylene blue for animals
Cover Slip	Lower gently to avoid bubbles	45° angle recommended
Excess Liquid	Blot around edges	Prevents slide slipping

🧠 Key Points to Remember
Temporary slides are easy and fast but not durable for long-term storage.
Staining is crucial to see internal structures clearly.
Handle slides carefully to avoid breakage or contamination.

How to Draw Cells from Microscope Slides and Photomicrographs

🌱 Purpose

To create clear, accurate scientific drawings of cells observed under a microscope or from photomicrographs (microscope photographs).
Helps in studying and documenting cell structure.

🧬 Materials Needed

Microscope slide or photomicrograph
Pencil and sharpener
Eraser
Ruler (for scale lines)
White drawing paper or notebook
Colored pencils (optional for shading)

✍️ Step-by-Step Guide to Drawing Cells

Observe Carefully

Look closely at the microscope slide or photomicrograph.
Identify key structures like the cell membrane, nucleus, cytoplasm, organelles, etc.

Outline the Cell Shape

Draw the overall shape of the cell lightly using a pencil.
Keep proportions similar to what you see.

Add Internal Structures

Sketch important organelles (nucleus, vacuole, chloroplasts, etc.) inside the cell outline.
Use simple shapes and maintain relative size and position.

Include Details

Add details like cell wall (if plant cell), granules, or cilia as visible.
Avoid overcrowding; keep the drawing neat and clear.

Label Key Parts

Use straight lines or arrows to point from the structure to its name.
Write labels clearly outside the drawing.

Draw a Scale Bar

If scale is given (e.g., 10 µm), draw a scale bar to indicate actual size.
Use a ruler to make the scale bar proportional.

Use Appropriate Magnification

Mention the magnification used (e.g., ×400) near the drawing.

📊 Checklist for Scientific Cell Drawing

Step	Action	Important Notes
Observation	Identify visible features	Focus on clearly seen organelles
Outline	Light sketch of cell shape	Maintain correct proportions
Internal Details	Draw organelles and structures	Keep shapes simple and accurate
Labeling	Add labels with straight lines	Write clearly outside drawing
Scale Bar	Draw scale bar with ruler	Indicate actual cell size
Magnification	Write magnification used	Helps understand drawing scale

🧠 Key Takeaways
Accuracy and clarity are more important than artistic skill.
Scientific drawings are a communication tool, so neatness and correct labeling matter most.
Practice observing and sketching improves your understanding of cell structures.

Calculating Magnification and Actual Size of Specimens

🌟 Key Concepts

Magnification (M): How many times larger the image is compared to the actual specimen.
Actual size (A): The real size of the specimen or structure.
Image size (I): The size of the specimen in the drawing or micrograph.

📏 Basic Formula

\[
\text{Magnification} \ (M) = \frac{\text{Image size} \ (I)}{\text{Actual size} \ (A)}
\]

or rearranged for actual size:

\[
\text{Actual size} \ (A) = \frac{\text{Image size} \ (I)}{\text{Magnification} \ (M)}
\]

🧮 Step-by-Step Calculation

Calculating Magnification
Measure the size of the image (I) using a ruler (e.g., in mm or cm).
Obtain the actual size (A) from given data or scale bar (in µm or nm).
Apply formula:
\[
M = I \div A
\]
Calculating Actual Size
Measure the size of the image (I) on the drawing or micrograph.
Know the magnification (M) used to capture or print the image.
Calculate actual size (A) using:
\[
A = I \div M
\]

🔍 Example Calculation

Image size (I) of a cell in a drawing = 12 cm = 120 mm
Magnification (M) = 400×
Actual size (A) = ?
Convert image size to same unit (e.g., µm):
Since 1 cm = 10,000 µm, 12 cm = 120,000 µm
\[
A = \frac{I}{M} = \frac{120{,}000 \ \mu m}{400} = 300 \ \mu m
\]

🧪 Applying to Different Micrographs

Micrograph Type	Typical Magnification Range	Units of Actual Size
Light microscope	×40 to ×1500	micrometers (µm)
Electron microscope (TEM)	×10,000 to ×1,000,000	nanometers (nm)
Electron microscope (SEM)	×20 to ×100,000	nanometers to micrometers

📝 Tips for Accurate Calculations

Always convert units to be consistent (e.g., all in µm or all in mm).
Use the scale bar on photomicrographs when provided to measure actual distances.
Note the magnification given for electron micrographs carefully (TEM shows internal structure, SEM shows surface details).

📊 Summary Table

What to Calculate	Formula	What You Need
Magnification (M)	M = Image size ÷ Actual size	Image size, Actual size
Actual size (A)	A = Image size ÷ Magnification	Image size, Magnification

🧠 Key Takeaway
Magnification compares image size to real size; actual size is found by dividing image size by magnification.
Consistency in units and careful measurement is crucial.
Understanding these calculations helps interpret drawings, photomicrographs, and electron micrographs correctly.

Using an Eyepiece Graticule and Stage Micrometer to Make Measurements

🌱 What Are They?

Eyepiece Graticule: A small transparent scale fitted inside the microscope eyepiece. It has arbitrary divisions (usually 100 units) but no fixed measurement until calibrated.
Stage Micrometer: A slide with an accurately known scale (usually 1 mm divided into 100 divisions, so each division = 0.01 mm or 10 µm). Used to calibrate the eyepiece graticule.

🧬 Why Calibration Is Needed?

The eyepiece graticule divisions do not have a fixed size.
Calibration links the eyepiece scale units to actual lengths (µm or mm) for a specific magnification.

🔍 Step-by-Step Calibration & Measurement

Place the Stage Micrometer on the Microscope Stage
Focus on the scale marked on the micrometer slide.
Align the Eyepiece Graticule with the Stage Micrometer Scale
Look through the eyepiece and observe both scales superimposed.
Count How Many Eyepiece Divisions Match a Known Length on the Stage Micrometer
For example, 50 eyepiece units might equal 0.5 mm on the stage micrometer.
Calculate the Value of One Eyepiece Division (EPU)
Size of 1 EPU = Length on stage micrometer ÷ Number of eyepiece divisions
Using the example:
Size of 1 EPU = 0.5 mm ÷ 50 = 0.01 mm = 10 µm
Use the Calibrated EPU to Measure Specimens
Count how many eyepiece units span the specimen.
Multiply by the size of one EPU to get the specimen size.

🧮 Example: Measuring a Cell

Suppose a cell spans 30 eyepiece units.
From calibration, 1 EPU = 10 µm.
Actual cell size = 30 × 10 µm = 300 µm.

🧪 Unit Conversions

Unit	Equivalent	Use for
1 millimeter (mm)	1,000 micrometers (µm)	Visible cell size or tissue
1 micrometer (µm)	1,000 nanometers (nm)	Bacteria, organelles
1 nanometer (nm)	0.001 micrometer (µm)	Viruses, molecules

📊 Summary Table

Step	Action	Notes
Place stage micrometer	Focus on known scale	Slide with precise divisions
Align eyepiece graticule	View both scales superimposed	Both scales must be visible
Count divisions	Measure eyepiece units vs stage scale	Example: 50 EPU = 0.5 mm
Calculate size of 1 EPU	Stage length ÷ number of EPUs	Gives length per eyepiece unit
Measure specimen	Count specimen EPUs × size of 1 EPU	Gives actual specimen size

🧠 Key Points to Remember
Calibration must be done for each objective lens (magnification).
Always keep units consistent when calculating or reporting measurements.
Eyepiece graticule alone does not give measurements — it must be calibrated with a stage micrometer first.

Resolution and Magnification in Microscopy

🌟 Definitions

Magnification

The process of enlarging the appearance of an object so it looks bigger than its actual size.
It is the ratio of the image size to the actual size of the specimen.

Resolution

The ability of a microscope to distinguish two points as separate and distinct.
It determines the level of detail and clarity visible in the image.

🔍 Differences Between Magnification and Resolution

Feature	Magnification	Resolution
Meaning	Enlarging the image size	Ability to distinguish close objects
What it affects	How big the specimen looks	How clear and detailed the image is
Measured in	Times (e.g., ×400, ×1000)	Distance (usually in micrometers or nanometers)
Limitation	Unlimited in theory, but practical limits exist due to resolution	Limited by wavelength of radiation used
Result of poor…	Image too small or details hard to see	Blurry image; close points appear merged

🧪 Magnification and Resolution in Light Microscopy

Magnification: Typically, up to ×1500. Achieved using objective and eyepiece lenses combined.
Resolution: Limited by the wavelength of visible light (~400–700 nm). Maximum resolution ~200 nm (0.2 µm). Limits the ability to see structures smaller than this.

⚡ Magnification and Resolution in Electron Microscopy

Magnification: Much higher than light microscopes, ranging from ×10,000 to over ×1,000,000.
Resolution: Uses electron beams with much shorter wavelength than light, increasing resolution dramatically.
TEM resolution: ~0.1 nm.
SEM resolution: ~1–20 nm.

📊 Comparison Table: Light vs Electron Microscopy

Feature	Light Microscope	Electron Microscope
Magnification	Up to ×1500	Up to ×1,000,000+
Resolution	~200 nm (0.2 µm)	TEM: ~0.1 nm, SEM: 1–20 nm
Radiation Used	Visible light	Electron beam
Detail Visible	Large cell structures	Ultrastructure (organelles, molecules)
Sample Preparation	Simple	Complex, requires vacuum

🧠 Summary
Magnification makes the image bigger, resolution makes the image clearer and more detailed.
High magnification without good resolution produces a large but blurry image.
Electron microscopes outperform light microscopes in both magnification and resolution, allowing us to see much smaller details.

CIE AS/A Level Biology -1.1 The microscope in cell studies- Study Notes

CIE AS/A Level Biology -1.1 The microscope in cell studies- Study Notes- New Syllabus