Welcome to Cell Structure!
Hi there! Ready to dive into the amazing world of cells? This chapter, Cells as the basic units of living organisms, is the foundation of all AS and A Level Biology. Think of cells as the Lego bricks of life—understanding how they are built and how they work is essential for everything else we study, from genetics to physiology.
Don't worry if some terms seem new; we'll break down the complex structures and processes into easy, manageable steps!
1.1 The Microscope in Cell Studies: Seeing the Invisible
To study cells, we need microscopes. You must be able to describe how they work, handle the associated calculations, and understand their limitations.
Magnification and Resolution: What's the Difference?
These two terms are often confused, but they mean very different things:
- Magnification: This is how much larger the image appears compared to the actual specimen. If a cell is magnified 400 times, it looks 400 times bigger!
- Resolution (or Resolving Power): This is the ability to distinguish between two separate points that are close together. High resolution means a clearer, sharper image, not just a bigger one.
Analogy: Imagine a blurry photo. Increasing the magnification just makes the blur bigger. Increasing the resolution makes the picture sharp and allows you to see fine details.
Light Microscopy vs. Electron Microscopy
The type of microscopy dictates how much detail we can see.
| Feature | Light Microscope (LM) | Electron Microscope (EM) |
|---|---|---|
| Source | Light waves | Beam of electrons |
| Lenses | Glass lenses | Electromagnets |
| Resolution | Relatively low (max ~200 nm). Limited by wavelength of light. | Very high (down to ~0.1 nm). Enables visualization of ultrastructure. |
| Magnification | Lower (up to ~x1500) | Very high (up to x500,000) |
| Live Specimens | Can be used to view living cells (e.g., movement). | Cannot view living cells (must be in a vacuum). |
| Types | Standard compound LM | TEM (Transmission) and SEM (Scanning) |
Quick Tip: Electron microscopes (EMs) reveal the ultrastructure (the fine internal details of organelles), while light microscopes (LMs) are good for observing whole cells and tissues.
Calculating Magnification and Actual Size
You need to be able to calculate magnification and the real size of structures using the following formula. Remember to handle units correctly (mm, µm, nm).
- Millimetre (mm)
- Micrometre (\(\mu\text{m}\)): \(1 \text{ mm} = 1000 \ \mu\text{m}\)
- Nanometre (nm): \(1 \ \mu\text{m} = 1000 \text{ nm}\)
The Magnification Formula:
$$
\text{Magnification} = \frac{\text{Size of image}}{\text{Actual size of specimen}}
$$
Memory Aid (IMA Triangle): Cover the variable you want to find.
Image Size (I) / Magnification (M) x Actual Size (A)
Crucial Step: Unit Conversion!
Before doing any calculation, make sure the image size and the actual size are in the same unit (e.g., both in µm). For example, convert image size (often in mm) to actual size units (often in µm or nm).
Key Takeaway for 1.1: Magnification makes things bigger; resolution makes them clearer. Electron microscopes offer much higher resolution and magnification than light microscopes, allowing us to see the detailed internal structure (ultrastructure).
1.2 Cells as the Basic Units of Living Organisms
Living organisms are divided into three domains: Archaea, Bacteria (Prokaryotes), and Eukarya (Eukaryotes). You must be able to distinguish between these cell types and detail the structures within eukaryotic cells.
A. Eukaryotic Cell Structure (Plants and Animals)
Eukaryotic cells (cells with a true nucleus) are large and complex, containing many membrane-bound compartments called organelles. Each organelle has a specific function.
Key Organelles and their Functions
1. The Nucleus (The Control Centre)
- Structure: Usually the largest organelle, enclosed by a nuclear envelope (a double membrane) containing pores. Contains chromatin (DNA wound around histone proteins) and one or more dense regions called the nucleolus.
- Function: Contains the genetic material (DNA) and controls all cell activity. The nucleolus is responsible for synthesizing ribosomal RNA (rRNA) and assembling ribosomes.
2. Mitochondria (The Powerhouse)
- Structure: Oval-shaped, surrounded by a double membrane. The inner membrane is highly folded into cristae, increasing the surface area for respiration. The internal fluid is the matrix.
- Function: Site of aerobic respiration, where ATP (the universal energy currency) is generated.
Did you know? Mitochondria possess their own small, circular DNA and 70S ribosomes, suggesting they were once independent bacteria!
3. Ribosomes (Protein Factories)
- Structure: Small organelles made of rRNA and protein. They are not membrane-bound.
- Function: Site of protein synthesis (translation).
- Types: Eukaryotes generally have 80S ribosomes in the cytoplasm. Mitochondria and Chloroplasts have smaller 70S ribosomes.
4. Endoplasmic Reticulum (The Transport Network)
- Rough Endoplasmic Reticulum (RER): Covered in ribosomes. Involved in the synthesis and folding of proteins that are destined for secretion or membranes.
- Smooth Endoplasmic Reticulum (SER): Lacks ribosomes. Involved in synthesis of lipids (like phospholipids and steroids), detoxification of drugs and poisons, and calcium ion storage.
5. Golgi Body (Apparatus/Complex) (Packaging and Shipping)
- Structure: Stacks of flattened, membrane-bound sacs called cisternae.
- Function: Modifies, sorts, and packages proteins and lipids (from the ER) into vesicles for secretion or delivery to other organelles.
6. Lysosomes (Recycling Centres)
- Structure: Small, spherical vesicles containing powerful digestive (hydrolytic) enzymes.
- Function: Break down waste materials, worn-out organelles, and pathogens ingested by the cell (e.g., in phagocytes).
7. Cytoskeleton components (Support and Movement)
- Microtubules: Hollow tubes involved in cell shape, movement of organelles, and forming the core of cilia, flagella, and centrioles.
- Centrioles: A pair of structures made of microtubules, found in animal cells (and some lower plants). Involved in organizing the spindle fibres during cell division.
8. Cell Surface Membrane
- Structure: A fluid mosaic model (detailed in Topic 4) composed mainly of a phospholipid bilayer and embedded proteins.
- Function: Controls the passage of substances into and out of the cell; involved in cell signalling and recognition.
Structures Specific to Animal Cells
- Centrioles (involved in cell division).
- Often have microvilli: small, finger-like folds of the cell surface membrane, typically seen in absorptive cells (like those lining the small intestine), increasing surface area.
Structures Specific to Plant Cells
- Cell Wall: A strong, rigid outer layer outside the cell membrane, primarily made of cellulose. Provides shape, support, and protection against osmotic lysis (bursting).
- Chloroplasts: Site of photosynthesis. Double membrane structure, containing stacks of thylakoids (called grana) in the fluid stroma. Like mitochondria, they have small circular DNA and 70S ribosomes.
- Large Permanent Vacuole: A large sac in the centre, filled with cell sap and enclosed by a membrane called the tonoplast. Maintains turgor pressure (essential for plant support) and stores ions and waste.
- Plasmodesmata: Tiny channels or pores through the cell walls, allowing direct transport and communication between adjacent plant cells.
Key Takeaway for Eukaryotes: Eukaryotic cells are highly compartmentalized by membranes, allowing specialized functions (like respiration in mitochondria and photosynthesis in chloroplasts) to occur efficiently. Plant and animal cells differ mainly in their presence of a cell wall, large vacuole, and chloroplasts.
B. Prokaryotic Cells (Bacteria)
Prokaryotic cells (like typical bacteria) are much simpler and smaller than eukaryotic cells (typically 1–5 µm diameter). They are always unicellular.
Key Structural Features of Prokaryotes (What makes them different?)
Prokaryotes are defined primarily by what they lack, but also by their unique components:
- No Nucleus: The DNA is found in a region of the cytoplasm called the nucleoid.
- DNA Structure: Possess a single, large loop of circular DNA (not associated with histone proteins). They may also have smaller DNA rings called plasmids.
- Ribosomes: Only possess small 70S ribosomes.
- Cell Wall: Present and rigid, but chemically different from plant cell walls. It is made of peptidoglycan (also called murein).
- Organelles: Crucially, they have an absence of organelles surrounded by double membranes (i.e., no mitochondria, no chloroplasts, no RER/SER, no Golgi).
Comparison: Eukaryotic vs. Prokaryotic Cells
| Feature | Eukaryotes (Plants, Animals, Fungi, Protoctists) | Prokaryotes (Bacteria) |
|---|---|---|
| Size | Large (10–100 µm) | Small (1–5 µm) |
| Genetic Material | Linear DNA in chromosomes, contained within a nucleus. | Circular DNA, free in the cytoplasm (nucleoid). |
| Ribosomes | 80S (cytoplasm), 70S (mitochondria/chloroplasts) | 70S (cytoplasm) |
| Membrane Organelles | Present (Nucleus, ER, Golgi, Mitochondria, etc.) | Absent (No double-membrane organelles) |
| Cell Wall Composition | Cellulose (Plants) or Chitin (Fungi) | Peptidoglycan (Murein) |
Common Mistake Alert! Always remember that prokaryotes DO have ribosomes and a cell membrane, but they lack internal membrane-bound organelles.
C. Viruses: The Non-Cellular Life
Viruses are often studied alongside cells, but they are definitively non-cellular structures. They cannot carry out life processes (like respiration or reproduction) on their own; they must hijack a host cell.
Viral Structure Outline
- Nucleic Acid Core: The genetic material, which can be either DNA or RNA (but never both). This core contains the instructions for making new viruses.
- Capsid: A protective outer shell made of protein.
- Envelope (sometimes present): Some viruses (like HIV) have an outer layer derived from the host cell's membrane, which is made of phospholipids. This envelope helps the virus enter new host cells.
Did you know? Viruses are completely reliant on the host cell's machinery (ribosomes, ATP, enzymes) to replicate, which is why antibiotics designed to kill bacteria (which are complex cells) have absolutely no effect on viruses.
Key Takeaway for 1.2: Cells require ATP from respiration for energy. Eukaryotes are complex with membrane organelles; prokaryotes are simple, lacking a nucleus and double-membrane organelles. Viruses are non-living parasites made only of nucleic acid and protein (and sometimes an envelope).