Cells and microscopy

Welcome to the World of Cells and Microscopy!

Ever wondered how we know what’s happening inside our bodies at a level so small we can’t even see it? It all started with the invention of the microscope. In this chapter, we’re going to explore the "Cell Theory"—the idea that cells are the building blocks of all life—and look at the incredible tools and techniques scientists use to peer into this microscopic world. Don't worry if some of the names of organelles sound like a different language at first; we'll break them down together!

1. The Tools of the Trade: Microscopy

Before microscopes, people had no idea that cells existed. Microscopy is the reason we understand how diseases work and how our bodies grow. There are four main types of microscopes you need to know for your OCR Biology B course:

Types of Microscopes

• Light Microscope: The kind you likely use in class. It uses light and lenses.
  Pros: You can see living things and the images are in color.
  Cons: Low resolution (you can't see tiny details like ribosomes).
• Transmission Electron Microscope (TEM): Fires a beam of electrons through a thin slice of a sample.
  Pros: Incredible detail (high resolution) of the inside of cells.
  Cons: Samples must be dead, sliced very thin, and the image is black and white.
• Scanning Electron Microscope (SEM): Bounces electrons off the surface of a sample.
  Pros: Creates amazing 3D images of the surface.
  Cons: Samples must be dead; lower resolution than TEM.
• Confocal Scanning Microscope: Uses a laser to look at specific depths of a sample.
  Pros: Very sharp images and can look at "layers" of living tissue.

Quick Review: Think of a TEM like an X-ray (seeing inside) and an SEM like a 3D photograph (seeing the outside shape).

2. The "Cell Theory"

The Cell Theory is a "unifying concept" in biology. It simply states that:
1. All living things are made of cells.
2. The cell is the basic unit of life.
3. New cells are created from existing cells.

3. Measuring Cells: The Math Bit

Since cells are too small to measure with a normal ruler, we use a specific formula. You might want to memorize the "I AM" triangle to help you!

The Formula:
\(\text{magnification} = \frac{\text{image size}}{\text{actual size}}\)

Memory Aid: I = A \(\times\) M (Image = Actual \(\times\) Magnification). If you are looking for the Actual size, just cover the 'A' and you get Image divided by Magnification!

Using a Graticule and Stage Micrometer

How do we actually measure something under a lens? We use two "rulers":
1. Eyepiece Graticule: A glass disc with a scale (0-100) that stays in the eyepiece. The units are arbitrary (they don't mean anything yet).
2. Stage Micrometer: A slide with an accurate scale (usually in millimeters) placed on the stage.

The Process (Calibration): You line up the graticule with the micrometer to see how many "micrometer units" fit into one "graticule unit." This tells you the value of one division on your eyepiece scale at that specific magnification.

4. A Closer Look at Blood

In Biology B, we use blood to study different cell types. You need to know how to prepare a blood smear (or film).
Step-by-step: Place a drop of blood on a slide, use another slide (the spreader) at a 45-degree angle to pull the blood across into a thin layer, air dry it, and then use Leishman’s stain.

Why Stain? (Differential Staining)

Most cells are see-through! Staining adds color so we can see the structures. Leishman’s stain is special because it colors different parts of white blood cells differently, helping us identify them.

The "Blood Team" (Specialised Cells)

• Erythrocytes (Red Blood Cells): No nucleus, biconcave shape. Their job is to carry oxygen.
• Neutrophils: White blood cells with a multi-lobed nucleus. They squeeze through capillary walls to eat bacteria.
• Lymphocytes: White blood cells with a very large, round nucleus that takes up most of the cell. They produce antibodies.
• Monocytes: The biggest white blood cells with a bean-shaped nucleus.
• Platelets: Tiny fragments of cells that help your blood clot.

Key Takeaway: Structure = Function. A red blood cell has no nucleus to make more room for hemoglobin!

5. Eukaryotic Cell Ultrastructure

Eukaryotic cells (like ours and plants) have a nucleus and "membrane-bound organelles." Think of the cell as a busy factory:

• Nucleus & Nucleolus: The "Boss's Office." It contains DNA (the blueprints) and makes ribosomes.
• Plasma Membrane: The "Security Gate." Controls what enters and leaves.
• Mitochondria: The "Power Plant." Where aerobic respiration happens to produce ATP (energy).
• Ribosomes: The "Workers." They build proteins.
• Rough Endoplasmic Reticulum (RER): The "Factory Floor." Covered in ribosomes; it folds and processes proteins.
• Smooth Endoplasmic Reticulum (SER): Makes lipids (fats).
• Golgi Apparatus: The "Post Office." It modifies and packages proteins into vesicles to be sent where they are needed.
• Lysosomes: The "Recycling Bin." Contain enzymes to break down waste.
• Centrioles: Help with cell division.
• Cytoskeleton: The "Scaffolding." A network of fibers that keeps the cell’s shape and moves organelles around using motor proteins.

6. Plant Cells vs. Animal Cells

Plants have everything animal cells have, PLUS:

• Cell Wall: Made of cellulose for strength.
• Chloroplasts: For photosynthesis.
• Large Vacuole & Tonoplast: The vacuole stores sap, and the tonoplast is the membrane surrounding it. It keeps the cell firm (turgid).

7. Prokaryotic Cells (Bacteria)

These are much simpler and smaller.
Memory Aid: Pro rhymes with No (No nucleus). Eu rhymes with Do (Do have a nucleus).

Features of Prokaryotes:
- Circular DNA: No nucleus; the DNA just floats in the cytoplasm.
- Plasmids: Extra tiny loops of DNA.
- Flagella: Tail-like structures for swimming.
- Pili: Hair-like structures for sticking to things.
- Mesosome: In-foldings of the membrane (though some scientists debate their function!).

8. The Protein "Assembly Line"

One of the most important concepts is how organelles work together to make and secrete a protein. Here is the flow:

1. Nucleus: DNA is used to make a template for a protein.
2. Ribosomes (on RER): Use the template to build the protein.
3. RER: Folds the protein into its shape.
4. Vesicles: Transport the protein from the RER to the Golgi.
5. Golgi Apparatus: Modifies the protein (e.g., adding a sugar group).
6. Secretory Vesicles: Carry the finished protein to the plasma membrane.
7. Plasma Membrane: Fuses with the vesicle to release the protein outside (exocytosis).

The Cytoskeleton and Motor Proteins act like railway tracks and engines to move these vesicles between the organelles!

9. Modern Techniques: Flow Cytometry

Scientists now use Flow Cytometry to analyze blood quickly.
- Cells are labeled with fluorescent tags.
- They are passed through a laser beam one by one.
- The machine counts the cells and detects the tags to identify exactly what types of cells are in a sample. This is much faster than counting by hand with a microscope!

Quick Review Box:
- Magnification: How much bigger the image is.
- Resolution: How much detail you can see (the ability to see two points as separate).
- Organelles: Specialised "mini-organs" inside cells.
- Haemocytometer: A special slide used to count the number of cells in a specific volume of liquid.

Congratulations! You've just covered the essentials of Cells and Microscopy. Don't worry if the names of the organelles feel like a lot to take in—just remember the "Factory Analogy" and you'll be a pro in no time!