Cell structure

Welcome to the World of Cells!

Welcome to the first part of your Foundations in Biology module! In this chapter, we are zooming in on the basic unit of life: the cell. Whether you are looking at a giant redwood tree or a tiny bacterium, everything starts with cell structure. We will explore how we see these tiny structures using microscopes and what all those bits inside the cell (organelles) actually do. Don’t worry if some of the long names seem scary at first—we’ll break them down together!

1. Seeing the Unseen: Microscopy

Cells are far too small to see with the naked eye. To study them, we use microscopes. There are three main types you need to know for your OCR A Level:

Types of Microscopes

1. Light Microscope: Uses light to see images. It’s what you likely use in class. It’s great for looking at living cells and whole tissues, but it has low magnification and resolution.
2. Transmission Electron Microscope (TEM): Fires a beam of electrons through a thin sample. It gives very high resolution and lets us see the ultrastructure (the tiny details inside) of organelles. The images are 2D.
3. Scanning Electron Microscope (SEM): Bounces electrons off the surface of a sample. This creates amazing 3D images of the outside of cells or structures.

Did you know? Even though electron microscopes are powerful, they can only view dead samples because the process happens in a vacuum!

Magnification vs. Resolution

These two terms are often confused, but they are very different:
- Magnification: How many times bigger the image is compared to the real object.
- Resolution: The ability to distinguish between two separate points. A higher resolution means a clearer, sharper image. Think of it like a camera: magnification is the "zoom," but resolution is the "megapixels."

The Magnification Formula

You will need to calculate magnification in your exam. Use the "I AM" triangle to remember it:
I = Image Size (what you measure with a ruler)
A = Actual Size (the real size of the cell)
M = Magnification

The formula is: \( magnification = \frac{size\ of\ image}{size\ of\ real\ object} \)

Quick Tip: Always make sure your units are the same before calculating! Usually, you'll need to convert millimeters (mm) to micrometers (\(\mu m\)) by multiplying by 1,000.

Staining and Slide Preparation

Most cells are transparent. To see them under a light microscope, we use stains.
- Differential Staining: This is when we use specific stains to bind to specific cell structures. For example, Acetic orcein binds to DNA and stains chromosomes dark red, while Methylene blue acts as an all-purpose stain.

Key Takeaway: Electron microscopes (TEM/SEM) have much higher resolution than light microscopes, allowing us to see individual organelles in detail.

2. Eukaryotic Cell Ultrastructure

Eukaryotic cells (like those in plants, animals, and fungi) contain membrane-bound organelles. Think of a cell like a factory; each organelle has a specific job to keep the factory running.

The "Control Center" Organelles

- Nucleus: The largest organelle. It contains the cell's genetic material (DNA) in the form of chromatin. It controls the cell's activities.
- Nucleolus: A dark spot inside the nucleus that produces ribosomes.
- Nuclear Envelope: A double membrane with pores that allow molecules (like mRNA) to move in and out of the nucleus.

The "Production and Transport" Organelles

- Rough Endoplasmic Reticulum (RER): A system of membranes covered in ribosomes. Its job is to fold and process proteins.
- Smooth Endoplasmic Reticulum (SER): Similar to RER but has no ribosomes. It makes lipids (fats) and steroids.
- Golgi Apparatus: A stack of flattened sacs. It acts like a "post office," modifying proteins and packaging them into vesicles for transport.
- Ribosomes: Tiny dots where protein synthesis happens. They can be free in the cytoplasm or attached to the RER.

The "Energy and Cleanup" Organelles

- Mitochondria: The site of aerobic respiration, where ATP (energy) is produced. They have a folded inner membrane called cristae.
- Lysosomes: Small bags of digestive enzymes. They break down waste material and old organelles.
- Chloroplasts: Found in plants. They capture light for photosynthesis.

The Cell Surface and Support

- Plasma Membrane: The "skin" of the cell that regulates what enters and leaves.
- Cell Wall: Found in plants (made of cellulose) and fungi. It provides strength and support.
- Centrioles: Small hollow cylinders involved in cell division (spindle formation).
- Flagella and Cilia: Hair-like extensions used for movement or moving substances across the cell surface.

Quick Review Box:
- Protein maker? Ribosomes.
- Protein packager? Golgi.
- Energy maker? Mitochondria.
- Waste disposal? Lysosomes.

Key Takeaway: Each organelle has a specific structure that suits its function (e.g., the folded membranes in mitochondria increase the surface area for energy production).

3. The Protein Production Line

One of the most important processes in the cell is how it makes and secretes proteins. This shows how organelles work together (interrelationship). Don't worry if this seems like a lot; just follow the steps!

Step-by-Step: Protein Secretion
1. The DNA in the nucleus is copied into mRNA.
2. The mRNA leaves through a nuclear pore and goes to a ribosome on the RER.
3. The ribosome makes the protein.
4. The protein is folded inside the RER and put into a transport vesicle.
5. The vesicle moves to the Golgi apparatus, where the protein is modified (e.g., adding a sugar chain).
6. The modified protein is packaged into a secretory vesicle.
7. The vesicle moves to and fuses with the plasma membrane, releasing the protein outside the cell (this is called exocytosis).

Key Takeaway: Organelles do not work in isolation; they are part of a coordinated system to produce vital molecules like hormones and enzymes.

4. The Cytoskeleton

The cytoskeleton is a network of protein fibers running through the cytoplasm. Think of it like the scaffolding of a building or the tracks of a railway.

It has three main functions:
1. Mechanical Strength: It keeps the cell's shape stable and stops it from collapsing.
2. Transport: It acts like "tracks" that motor proteins use to move organelles and vesicles around the cell.
3. Movement: It enables the whole cell to move (like an amoeba) or moves parts of the cell (like flagella).

Key Takeaway: The cytoskeleton provides the internal structure and transport system needed for a cell to function and divide.

5. Prokaryotic vs. Eukaryotic Cells

Finally, we need to compare the cells we've been talking about (Eukaryotes) with simpler cells like bacteria (Prokaryotes).

Common Mistakes to Avoid:

- Mistake: Thinking prokaryotes have no DNA.
Correction: They do have DNA, but it is circular and "naked" (not inside a nucleus).
- Mistake: Thinking both have the same size ribosomes.
Correction: Eukaryotes have larger 80S ribosomes; Prokaryotes have smaller 70S ribosomes.

Key Differences Table

Prokaryotes:
- Much smaller (1-5 \(\mu m\))
- No nucleus (DNA is in a loop in the cytoplasm)
- No membrane-bound organelles (No mitochondria, Golgi, etc.)
- Cell wall made of peptidoglycan (not cellulose)
- May have extra loops of DNA called plasmids

Eukaryotes:
- Larger (10-100 \(\mu m\))
- Nucleus present
- Many membrane-bound organelles
- Cell wall (if present) made of cellulose or chitin

Key Takeaway: Prokaryotes are much simpler and smaller than eukaryotes. They lack a nucleus and other specialized "rooms" (organelles) that eukaryotes have.

Final Summary Review

1. Microscopes let us see cells; Electron microscopes have the best resolution.
2. Organelles are specialized parts of a cell with specific jobs (e.g., RER for proteins, Mitochondria for energy).
3. Protein synthesis involves a team effort between the nucleus, RER, Golgi, and vesicles.
4. The cytoskeleton is the cell's scaffolding and transport track.
5. Prokaryotes are simple, small, and have no nucleus, unlike the complex Eukaryotes.