Study Notes: Chapter 1 – Cell Structure (9700 AS Level Biology)

Hello future Biologists! Welcome to the fundamental chapter of AS Biology: Cell Structure. Everything in biology—from fighting diseases to understanding inheritance—comes back to the cell. Think of the cell as the smallest, most complex factory in the world. By the end of this chapter, you’ll be able to navigate the cell’s internal machinery and distinguish between the major types of life! Let’s dive in.

1.1 The Microscope in Cell Studies: Seeing the Unseen

Before we explore cells, we need the tools to see them. Microscopes allow us to study structures that are far too small for the naked eye. Mastering microscopy is a key practical skill!

Key Terms Defined

  • Magnification: How many times larger the image appears compared to the actual size of the specimen.
    Analogy: Magnification is like using the zoom on your phone camera—you make the object look bigger.
  • Resolution (Resolving Power): The minimum distance apart two objects must be for them to be seen as two separate items. It is essentially the clarity or sharpness of the image.
    Analogy: Resolution is like the quality of your TV screen—high resolution means you can clearly distinguish fine details without them blurring together.

Light Microscope vs. Electron Microscope

Different microscopes give us different views of the cell, mainly due to differences in their resolution.

Feature Light Microscope Electron Microscope (TEM/SEM)
Radiation Source Light (photons) Electron beam
Max Resolution About 200 nm (0.2 µm) About 0.1 nm (much higher resolution)
Max Magnification Up to x1500 Up to x500,000 or more
Specimen Can be living or dead Must be dead (vacuum required)
Colour Coloured (if stained) Black and white (electron micrographs)
Quick Review: Why use an Electron Microscope (EM)?

The EM has a much higher resolution because electrons have a much shorter wavelength than visible light. This allows scientists to observe the ultrastructure (the extremely fine details) of organelles, which is impossible with a light microscope.

Calculations: Magnification and Actual Size

You must be able to calculate magnification and the actual size of a specimen.

The core formula is:
$$ \text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} $$

Step-by-step Tip for Calculations:

  1. Convert Units: This is the biggest source of error! Always ensure Image size and Actual size are in the same unit before using the formula.
    • 1 millimetre (mm) = 1000 micrometres (\(\mu\)m)
    • 1 micrometre (\(\mu\)m) = 1000 nanometres (nm)
  2. Rearrange for Actual Size: If you are given the magnification and image size, you will rearrange the formula: $$ \text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} $$
Using the Eyepiece Graticule and Stage Micrometer

In practical work, you use a ruler placed in the eyepiece (the eyepiece graticule) and a tiny slide ruler (the stage micrometer) to measure real sizes.

  • The stage micrometer has fixed, known divisions (e.g., 1 mm or 100 \(\mu\)m).
  • The eyepiece graticule is an arbitrary scale that changes in size relative to the objective lens used.
  • Calibration: You must calibrate the eyepiece graticule against the stage micrometer at each magnification (objective lens setting) to find out the actual length of one graticule division.

Key Takeaway: Microscopy provides us with the detail (resolution) and size (magnification) necessary to study cells. Remember that electron microscopes trade the ability to view living things for vastly improved resolution.

1.2 Cells as the Basic Units of Living Organisms

All living organisms are made of cells, and these cells require energy (in the form of ATP, produced primarily through respiration) to carry out energy-requiring processes like movement or active transport.

Eukaryotic Cells: Animal vs. Plant

Eukaryotic cells (found in animals, plants, fungi, and protoctists) are complex cells defined by having a true nucleus and other organelles enclosed by double membranes.

While they share many organelles, plants and animals have key structural differences:

Feature Animal Cells Plant Cells
Cell Wall Absent Present (made of cellulose)
Chloroplasts Absent Present (site of photosynthesis)
Vacuole Small, temporary vesicles Large, permanent central vacuole (surrounded by the tonoplast)
Shape Irregular/flexible Fixed, regular shape (due to cell wall)
Centrioles Present (involved in cell division) Absent in flowering plants, present in lower plants

Detailed Structure and Function of Eukaryotic Organelles

We need to know the structure and function of the major eukaryotic organelles (the "mini-organs" of the cell).

1. The Nucleus (The Control Centre)
  • Structure: Largest organelle, surrounded by a nuclear envelope (a double membrane) containing nuclear pores to allow molecule exchange (like mRNA). Contains chromatin (DNA wrapped around histone proteins) and one or more dense regions called the nucleolus.
  • Function: Contains the cell's genetic material (DNA), controlling cell activities. The nucleolus is responsible for making ribosomal RNA (rRNA) and assembling ribosomes.
2. Ribosomes (The Protein Synthesizers)
  • Structure: Tiny organelles made of rRNA and protein. They are not membrane-bound.
    • In the cytoplasm of eukaryotic cells: 80S ribosomes (larger).
    • In mitochondria and chloroplasts: 70S ribosomes (smaller, like those in bacteria).
  • Function: Site of translation (protein synthesis), where amino acids are assembled into polypeptides.
3. Endoplasmic Reticulum (ER) (The Production Line)
  • A network of flattened sacs (cisternae) extending from the nuclear envelope.
  • Rough Endoplasmic Reticulum (RER):
    Structure: Covered with 80S ribosomes on its surface.
    Function: Folding and modifying proteins synthesized on its ribosomes, ready for transport.
  • Smooth Endoplasmic Reticulum (SER):
    Structure: Lacks ribosomes.
    Function: Synthesis of lipids (fatty acids and phospholipids) and steroids, and detoxification of drugs and poisons (especially in liver cells).
4. Golgi Body (Golgi Apparatus/Complex) (The Post Office)
  • Structure: Stacks of flattened, membrane-bound sacs called cisternae (not interconnected like the ER). Often accompanied by small vesicles.
  • Function: Modifying, sorting, and packaging proteins and lipids made in the ER into vesicles for secretion or delivery to other organelles. The final processing and shipping centre.
5. Mitochondria (The Power House)
  • Structure: Oval shape, enclosed by a double membrane. The inner membrane is highly folded into finger-like projections called cristae. The central space is the matrix. Contains 70S ribosomes and small circular DNA (evidence of endosymbiosis).
  • Function: Site of the final stages of aerobic respiration, producing large amounts of ATP (the energy currency).
6. Chloroplasts (Plant Only) (The Solar Panel)
  • Structure: Large, typically lens-shaped, enclosed by a double membrane. Internal stacks of flattened discs (thylakoids) are grouped into grana. The fluid surrounding the grana is the stroma. Contains 70S ribosomes and small circular DNA.
  • Function: Site of photosynthesis, converting light energy into chemical energy (glucose).
7. Lysosomes (The Recycling/Waste Disposal Unit)
  • Structure: Small, spherical vesicles containing powerful hydrolytic enzymes, enclosed by a single membrane.
  • Function: Digests old cell parts, foreign material (like bacteria engulfed by phagocytosis), and waste products.
8. Cytoskeleton and Motility Structures
  • Centrioles and Microtubules: Centrioles (found in animal cells) are involved in forming the spindle during cell division. Microtubules are protein threads forming part of the cytoskeleton, providing structural support and acting as pathways for motor proteins.
  • Cilia: Hair-like projections on the cell surface (e.g., in the trachea). Made of microtubules. Function in movement of substances or the cell itself.
  • Microvilli: Finger-like projections of the cell surface membrane (e.g., on epithelial cells in the small intestine). Function to massively increase the surface area for absorption.
9. Plant-Specific Boundary Features
  • Cell Wall: Outermost rigid layer in plants (mainly cellulose). Provides mechanical strength and maintains turgidity (shape).
  • Large Permanent Vacuole and Tonoplast: The large central vacuole stores water, ions, and maintains turgor pressure. The tonoplast is the single membrane surrounding the vacuole.
  • Plasmodesmata: Tiny channels or gaps that pass through the cellulose cell walls, connecting the cytoplasm of adjacent plant cells, allowing for communication and transport.

Key Takeaway: Eukaryotic cells are highly compartmentalized, with each organelle performing a specific job defined by its unique structure, ensuring efficiency. Memorizing the function requires linking it back to the organelle's shape (e.g., cristae increase surface area for respiration in mitochondria).

1.3 Prokaryotic Cells: Bacteria

Prokaryotic cells (like bacteria) are structurally simpler than eukaryotes. They are typically much smaller and lack the complex internal compartmentalization.

Key Structural Features of a Typical Bacterium

  1. Unicellular: They exist as single cells.
  2. Size: Generally small, about 1–5 \(\mu\)m in diameter.
  3. Cell Wall: Present, made of peptidoglycan (not cellulose, like plants).
  4. Genetic Material: Possesses a large, singular, circular molecule of circular DNA found naked in the cytoplasm (not enclosed by a nucleus). Small loops of extra DNA called plasmids may also be present.
  5. Ribosomes: Possesses 70S ribosomes (smaller than the 80S found in the eukaryotic cytoplasm).
  6. Membrane-bound Organelles: They have a critical absence of organelles surrounded by double membranes (no nucleus, no mitochondria, no RER/SER, etc.).

Did you know? The fact that prokaryotes have 70S ribosomes and eukaryotic mitochondria/chloroplasts also have 70S ribosomes is key evidence supporting the Endosymbiotic Theory!

Comparing Eukaryotic and Prokaryotic Cells

This comparison is vital for exams! Focus on the absence of features in prokaryotes.

Differences in Structure:
  • Genetic Organisation: Eukaryotes have linear DNA (chromatin) enclosed in a nucleus. Prokaryotes have circular, naked DNA floating in the cytoplasm (nucleoid region).
  • Organelles: Eukaryotes have extensive membrane-bound organelles (Mitochondria, Golgi, ER). Prokaryotes lack these (though they might have infoldings of the cell membrane for processes like respiration).
  • Ribosomes: Eukaryotes have large 80S ribosomes (70S in mitochondria/chloroplasts). Prokaryotes have small 70S ribosomes.
  • Cell Wall Composition: Eukaryotes (Plants) have cellulose walls. Prokaryotes have peptidoglycan walls.

Struggling Student Tip: The prefix "Pro-" means "before," and "Eu-" means "true." Prokaryotes existed *before* true compartmentalization evolved, hence their simple structure.

1.4 Viruses: The Non-Cellular Structures

Viruses are often studied alongside cells, but they are fundamentally different: they are non-cellular structures. This means they cannot be considered the basic unit of life because they cannot carry out metabolic processes or reproduce independently—they must hijack a host cell.

Key Viral Structure

All viruses share two key components:

  1. Nucleic Acid Core: This is the genetic material, which can be either DNA or RNA (but not both).
  2. Capsid: A protective outer shell made of protein.

Some viruses (like the influenza virus or HIV) also have a third feature:

3. Outer Envelope: A layer made of phospholipids, often derived from the host cell membrane when the virus leaves.

Key Takeaway: Viruses are parasites that consist only of genetic instructions (DNA/RNA) wrapped in a protein coat (capsid). Their classification as non-cellular is why they are so hard to treat with antibiotics, which target cell structures (see Topic 10).