Welcome to Cells and Cell Structure!

Hello future Biologists! Get ready to zoom in on the fundamental unit of life: the cell. This chapter, "Cells and Cell Structure," is the bedrock of Unit 1, setting the stage for understanding the variety and organization of all living organisms. We’ll be looking at the tiny, intricate components inside cells (organelles) and learning how simple cells (prokaryotes) differ from complex ones (eukaryotes).


Don't worry if this seems tricky at first—we will use clear analogies and break down the complex structures step-by-step. Let's start by learning how we even look at these microscopic wonders!

3.1.2.1 The Structure of Eukaryotic Cells

1. Tools for Seeing the Invisible: Microscopy

Since most cells are too small to be seen with the naked eye, we rely on microscopes. The choice of microscope depends on what you need to see.

Key Concepts in Microscopy
  • Magnification: How much larger the image appears compared to the real object size.
  • Resolution: The ability to distinguish between two separate points. High resolution means a sharper, clearer image. (Think of blurry vs. HD TV).
Microscope Types and Limitations

1. Optical (Light) Microscope

  • Principle: Uses light and glass lenses.
  • Advantage: Can view living samples; relatively cheap.
  • Limitation: Low resolution and maximum magnification (around x1500) because light has a relatively long wavelength.

2. Electron Microscopes (Use beams of electrons, which have a much shorter wavelength, giving higher resolution.)

a) Transmission Electron Microscope (TEM)

  • Principle: Electrons pass *through* a thin specimen.
  • Advantage: Highest resolution and magnification, showing ultrastructure (the details *inside* organelles).
  • Limitation: Must be viewed in a vacuum (specimen must be dead); produces a 2D image.

b) Scanning Electron Microscope (SEM)

  • Principle: Electrons scan the *surface* of a specimen.
  • Advantage: Provides stunning 3D images of the surface structure.
  • Limitation: Resolution is lower than TEM.
Calculating Magnification

You must be able to use and rearrange the magnification formula.

$$ \text{Magnification} = \frac{\text{size of image}}{\text{size of object}} $$

Remember to ensure that the image size and the actual object size are in the same units (e.g., both micrometres).

2. Preparing the Cell: Cell Fractionation

If you want to study just the mitochondria, how do you separate them from the rest of the cell? You use Cell Fractionation and Ultracentrifugation.

This process separates the cell components based on their size and density:

  1. Homogenisation: The tissue is broken up in a cold, buffered, isotonic solution. (Cold reduces enzyme activity; Buffered maintains pH; Isotonic prevents osmosis, keeping organelles intact).
  2. Filtration: The resulting mixture (homogenate) is filtered to remove large debris.
  3. Ultracentrifugation: The sample is spun in a centrifuge at increasing speeds. The heaviest organelles settle out first, forming a pellet.

Order of separation (from heaviest/first pellet to lightest/last pellet):

Nice Men Can Listen Really Well

Nucleus → Mitochondria & Chloroplasts → Lysosomes & ER → Ribosomes & Cell Wall fragments.

Quick Review: Microscopy & Fractionation

TEM: Highest resolution, 2D ultrastructure.
Magnification: Image size / Actual size.
Ultracentrifugation: Separates organelles by density, with the Nucleus pelleting first.

3. The Organisation of Eukaryotic Cells

Eukaryotic cells (like those in animals, plants, fungi, and protists) are complex, containing structures called membrane-bound organelles.

In complex organisms, cells work together: Cells form Tissues, Tissues form Organs, and Organs form Systems (like the digestive system).

Key Eukaryotic Organelles and Functions (You must know their appearance and ultrastructure)

1. Nucleus (The Control Centre)

  • Appearance: Largest organelle, surrounded by a double membrane (nuclear envelope) with pores. Contains chromatin (DNA associated with proteins called histones).
  • Function: Controls the cell's activities (by controlling transcription/protein synthesis) and stores the cell's genetic material (DNA) in linear chromosomes.

2. Mitochondria (The Powerhouse)

  • Appearance: Oval-shaped, surrounded by a double membrane. The inner membrane is folded into finger-like projections called cristae, increasing the surface area. The fluid interior is the matrix.
  • Function: Site of aerobic respiration, producing the bulk of the cell’s ATP (the immediate source of energy).

3. Chloroplasts (The Plant Food Factory)

  • Appearance: Large organelles (found in plant and algal cells), surrounded by a double membrane. Internal membranes stack up to form grana (piles of thylakoids). The fluid inside is the stroma.
  • Function: Site of photosynthesis, converting light energy into chemical energy (glucose).

4. Ribosomes (The Protein Builders)

  • Appearance: Very small, not membrane-bound. Made of protein and ribosomal RNA (rRNA). Eukaryotic ribosomes are 80S (S = Svedberg unit, relating to size).
  • Function: Site of protein synthesis (translation).

5. Endoplasmic Reticulum (ER) (The Internal Highway)

  • Appearance: A network of flattened sacs (cisternae) extending from the nuclear envelope.
  • Function:
    • Rough ER (RER): Covered in ribosomes. Folds and processes proteins made on the ribosomes.
    • Smooth ER (SER): Lacks ribosomes. Synthesises, stores, and transports lipids and carbohydrates.

6. Golgi Apparatus (The Post Office)

  • Appearance: A stack of flattened, membrane-bound sacs (cisternae) and vesicles.
  • Function: Modifies, sorts, and packages proteins and lipids received from the ER into vesicles for transport, secretion, or delivery to other organelles.

7. Lysosomes (The Waste Disposal Unit)

  • Appearance: Small, spherical sacs enclosed by a single membrane. Contain powerful digestive (hydrolytic) enzymes.
  • Function: Digests waste materials, worn-out organelles (autolysis), or engulfed foreign material (phagocytosis).

8. Plasma Membrane (Cell-Surface Membrane) (The Gatekeeper)

  • Appearance: A partially permeable barrier, described by the fluid-mosaic model (phospholipids, proteins, and carbohydrates).
  • Function: Controls the passage of substances (like ions and molecules) into and out of the cell. Some cells have infoldings called microvilli to increase the surface area for exchange/absorption.

9. Cell Wall (Structural Support - Plants/Algae/Fungi)

  • Appearance: Rigid outer layer (composed mainly of cellulose in plants).
  • Function: Provides mechanical strength and prevents the cell from bursting when water enters by osmosis.

10. Cell Vacuole (The Storage Tank - Plants)

  • Appearance: Large, central sac filled with cell sap (water, salts, glucose). Surrounded by a membrane called the tonoplast.
  • Function: Maintains turgor pressure (keeps the plant rigid) and stores chemicals.
Analogy Aid: The Organelle City

Imagine the cell is a factory city:
Nucleus: City Hall (Holds blueprints, controls operations).
RER/Ribosomes: Production Lines (Making and processing proteins).
SER: Manufacturing Bay (Making lipids and carbs).
Mitochondria: Power Plant (Generates energy/ATP).
Golgi: Post Office (Sorting and packaging goods for delivery).
Lysosomes: Waste Recycling Plant (Breaking down unwanted materials).

3.1.2.2 The Structure of Prokaryotic Cells (Bacteria)

Prokaryotic cells (like bacteria) are the simplest form of life. They are generally much smaller than eukaryotic cells and lack the complex compartmentalization.

Key Differences: Prokaryotes vs. Eukaryotes

1. Genetic Material

  • Eukaryotes: DNA is linear, found inside a nucleus, and is associated with histone proteins.
  • Prokaryotes: There is no nucleus. The DNA is a single, circular molecule, free in the cytoplasm, and not associated with proteins.

2. Internal Structure

  • Eukaryotes: Possess a cytoplasm filled with many membrane-bounded organelles (Mitochondria, Golgi, etc.).
  • Prokaryotes: Cytoplasm lacks membrane-bounded organelles.

3. Ribosomes

  • Eukaryotes: Larger ribosomes (80S).
  • Prokaryotes: Smaller ribosomes (70S).

4. Cell Wall Composition

  • Eukaryotes (Plants): Cell wall made of cellulose.
  • Prokaryotes: Cell wall made of the glycoprotein murein (also called peptidoglycan).

Additional Features of Prokaryotes

Many prokaryotic cells also possess structures not found in eukaryotes:

  • Plasmids: Small, circular loops of DNA that carry extra genes (like antibiotic resistance).
  • Capsule: A slimy layer surrounding the cell wall that protects the bacterium from being engulfed by immune cells (phagocytosis) and prevents dehydration.
  • Flagella (singular: flagellum): Long, tail-like appendages used for locomotion (movement).
Common Mistake Alert!

Students often confuse ribosomes with membrane-bound organelles. Remember: Ribosomes are found in ALL cells (Prokaryotic and Eukaryotic) and they are NOT membrane-bound. Prokaryotes only lack *membrane-bound* structures.

Key Takeaway: Cells and Cell Structure

Understanding cell structure is essential for explaining how different organisms function and how they are classified (the diversity of life). Eukaryotes rely on complex, specialized organelles to manage tasks, while prokaryotes achieve life processes using simpler structures and a highly efficient cell membrane and cytoplasm. The differences in organelles (or lack thereof) mean these two cell types carry out metabolic activities in fundamentally different locations.