🔬 Biology B2: Cells – The Basic Building Blocks of Life

Hello future Biologists! Welcome to the microscopic world of Cells. Don't worry if this chapter seems tricky at first; cells are just like tiny, highly organised factories or cities, each with specialized jobs. Understanding cells is essential because they are the fundamental units that make up every living thing, from the smallest bacteria to the largest whale. Let's dive in!

Remember from B1: Cells carry out the seven characteristics of living organisms (Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition).

1. General Cell Structure and Comparison (B2.1 Core)

All cells share some basic structures. These are the components needed for survival, known as organelles.

1.1 Animal Cell Structure (Core)

Animal cells are typically irregular in shape and have these key parts:

  • Cell Membrane: The outer boundary, controlling what enters and leaves the cell. Think of it as the security gate.
  • Nucleus: The control centre of the cell. It contains the genetic material (DNA) and controls all cell activities.
  • Cytoplasm: A jelly-like substance filling the cell. Most chemical reactions (metabolism) happen here.
  • Mitochondria: Tiny structures where aerobic respiration takes place, releasing energy for the cell. (The power station!)
  • Ribosomes: Very small organelles responsible for making proteins (protein synthesis). (The assembly line!)
  • Vacuoles: Small, temporary storage spaces (sometimes present, but not always a prominent feature like in plants).

1.2 Plant Cell Structure (Core)

Plant cells contain all the components of an animal cell, PLUS three additional key features that give them structure and allow them to photosynthesise.

  • All animal cell components: Cell membrane, Nucleus, Cytoplasm, Mitochondria, Ribosomes.
  • Cell Wall: A rigid layer made of cellulose found outside the cell membrane. It provides strength and fixed shape to the cell. (The protective outer armour).
  • Chloroplasts: Organelles containing the green pigment chlorophyll. This is the site of photosynthesis (making food using light energy).
  • Permanent Vacuole: A large central sac filled with cell sap (water, salts, sugars). It pushes the cytoplasm against the cell wall, maintaining the cell's rigid shape (turgor).
Quick Comparison: Plant vs. Animal Cells

To compare them easily, remember these key differences:

Plant Cells Have:

  1. Cell Wall
  2. Chloroplasts
  3. Large Permanent Vacuole
Animal Cells Have:
  1. No Cell Wall
  2. No Chloroplasts
  3. Small or temporary vacuoles (if any)

1.3 Structure of a Bacterial Cell (B2.1 Core)

Bacteria are prokaryotic cells, meaning they are much simpler and lack a true nucleus or membrane-bound organelles (like mitochondria or chloroplasts).

Key structures (limited to syllabus content):

  • Cell Wall: Present, outside the membrane, but not made of cellulose.
  • Cell Membrane: Controls entry/exit.
  • Cytoplasm: Jelly-like substance where reactions occur.
  • Ribosomes: Present, used for protein synthesis.
  • Circular DNA: The main genetic material, which is a single, large loop of DNA, floating freely in the cytoplasm (not in a nucleus).
  • Plasmids: Small, extra loops of circular DNA that carry additional genes (like antibiotic resistance).

Did you know? Bacteria are typically about 100 times smaller than eukaryotic (plant or animal) cells!

2. The Jobs of the Organelles (B2.1 Core)

Every part of the cell has a specific function crucial for the cell’s survival and metabolism:

Functions Summary Table
  • Nucleus: Contains genetic material (DNA) and controls all cell activities (like growth and reproduction).
  • Cytoplasm: Site of most chemical reactions; holds the organelles in place.
  • Cell Membrane: Selectively controls the passage of substances into and out of the cell. (This is important for the next chapter, B3!)
  • Cell Wall (Plants/Bacteria): Provides mechanical strength and maintains the cell's rigid shape.
  • Mitochondria: Site of aerobic respiration to release energy (ATP) for metabolism.
  • Chloroplasts (Plants): Site of photosynthesis (converting light energy into chemical energy, i.e., glucose).
  • Ribosomes: Site of protein synthesis.
  • Vacuole (Plants): Stores cell sap, maintains turgor pressure (rigidity).
  • Circular DNA/Plasmids (Bacteria): Contain the genetic instructions.

Key Takeaway: Cells are organised into compartments called organelles, each performing a specific vital function. Plant cells have cell walls, large vacuoles, and chloroplasts, unlike animal cells. Bacteria lack a nucleus and true membrane-bound organelles.

3. Cell Specialisation and Organisation

3.1 Where Do Cells Come From? (B2.1 Core)

The simple rule of biology is stated clearly in the syllabus:

New cells are produced by division of existing cells.

This process allows organisms to grow, repair damage, and reproduce.

3.2 Specialised Cells (B2.1 Core)

Just like a city needs different workers (builders, doctors, drivers), your body needs different kinds of cells to perform specific tasks. Specialised cells have specific shapes and structures that make them highly adapted for one job.

Examples of Specialised Cells and Their Functions:
  1. Ciliated Cells:
    • Function: Movement of mucus in the trachea (windpipe) and bronchi (air tubes).
    • Adaptation: Have tiny hair-like structures called cilia that sweep rhythmically to move mucus and trapped dust/pathogens out of the lungs.
  2. Root Hair Cells:
    • Function: Absorption of water and mineral ions from the soil.
    • Adaptation: Possess a large protrusion (the 'hair') that significantly increases the surface area for absorption.
  3. Palisade Mesophyll Cells:
    • Function: Main site of photosynthesis in leaves.
    • Adaptation: Tightly packed and contain lots of chloroplasts (found close to the upper surface of the leaf to catch maximum light).
  4. Neurones (Nerve Cells):
    • Function: Conduction of electrical impulses.
    • Adaptation: They are long and thin, allowing electrical signals to travel quickly over long distances.
  5. Red Blood Cells:
    • Function: Transport of oxygen around the body.
    • Adaptation: Contain the pigment haemoglobin (which binds to oxygen) and lack a nucleus (to maximise space for haemoglobin). They are biconcave shaped to increase surface area.
  6. Sperm and Egg Cells (Gametes):
    • Function: Reproduction. They fuse during fertilisation.
    • Adaptation (Sperm): Have a flagellum (tail) for movement (motility) and contain many mitochondria to provide energy for swimming.
    • Adaptation (Egg): Large size, often non-motile, and contain large energy stores (yolk) for the developing embryo.

3.3 Levels of Organisation (B2.1 Core)

Living organisms are highly structured. The syllabus requires you to understand the hierarchy from the smallest functional unit (the cell) up to the entire organism.

C T O O O (A handy mnemonic!)

  1. Cell: The basic structural and functional unit of a living organism. (Example: a muscle cell).
  2. Tissue: A group of similar cells working together to perform a specific function. (Example: Muscle tissue or Xylem tissue in plants).
  3. Organ: A structure made up of different tissues working together to perform a specific function. (Example: The Heart, the Liver, or a Leaf).
  4. Organ System: A group of different organs working together to carry out a particular function. (Example: The Digestive System or the Circulatory System).
  5. Organism: A complete living thing, made up of several organ systems working together. (Example: You, a dog, a mango tree).

Key Takeaway: Cells are specialised for their job, which means their structure fits their function. They build up levels of complexity from cells to tissues, organs, organ systems, and finally, the complete organism.

4. Cell Maths: Magnification and Size (B2.2 Core & Supplement)

When we look at cells through a microscope, we need to know how much bigger the image is than the real cell. This is where magnification comes in.

4.1 The Magnification Formula (B2.2 Core)

Magnification is the ratio of the image size (what you see under the microscope) to the actual size (the real size of the object).

The formula is:

$$ \text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} $$

Tip for rearranging: Remember the I AM triangle. Image size (I) goes on top, Actual size (A) and Magnification (M) go on the bottom.

  • If you want the Actual size (A): \( \text{Actual size} = \frac{\text{Image size}}{\text{Magnification}} \)
  • If you want the Image size (I): \( \text{Image size} = \text{Actual size} \times \text{Magnification} \)

IMPORTANT: When using this formula, the units for Image size and Actual size must be the same so that the magnification value has no units! (Core requirement is to use millimetres, mm).

4.2 Unit Conversion (B2.2 Supplement)

Since cells are incredibly small, biologists often use units smaller than millimetres (mm), particularly the micrometre (μm).

You need to be able to convert between these units:

$$ 1 \text{ mm} = 1000 \mu\text{m} $$

How to Convert:

  • To go from mm to μm, multiply by 1000. (e.g., 0.5 mm = 500 μm)
  • To go from μm to mm, divide by 1000. (e.g., 20 μm = 0.02 mm)

Common Mistake: Students often forget to convert units before plugging numbers into the magnification formula. Always ensure the Image size and Actual size are in the same units (e.g., both in mm or both in μm).

Key Takeaway: Use the formula I/A x M to calculate size or magnification. Be careful with unit conversion, especially remembering that 1 mm = 1000 μm. You got this!