Science - Combined (0653) | Cells

Hello IGCSE Scientists! Welcome to the World of Cells!

The cell is the most fundamental unit of life. Everything alive, from the smallest bacterium to the largest whale, is made of cells. Understanding cells is like learning the alphabet of biology – once you know the basic components and how they work, you can understand everything else!

This chapter (B2) will explore the structures inside different types of cells, how cells fit together to make organisms, and how we calculate their tiny sizes. Don't worry if the names sound complicated – we will break them down piece by piece!

Section 1: Levels of Organisation in Living Organisms (B2.1 Core 5)

Before diving into the cell itself, let's understand how billions of cells work together to form a whole living organism. This hierarchy helps us understand complexity.

Key Hierarchy of Organisation

• Cell: The basic structural and functional unit of all living organisms. (Example: A single nerve cell or a palisade cell.)
• Tissue: A group of similar cells working together to perform a specific function. (Example: Muscle tissue in your heart, or xylem tissue in a plant.)
• Organ: A structure made up of different tissues working together to perform a main function. (Example: The stomach, the heart, or a leaf.)
• Organ System: A group of organs working together to carry out a major function. (Example: The digestive system or the circulatory system.)
• Organism: An individual living thing, made up of one or more organ systems. (Example: A human, a tree, or a bacterium.)

Section 2: The Structure and Function of Animal and Plant Cells (B2.1 Core 1, 4)

Although they look very different on the outside, animals and plants share many similar cell structures. These are called eukaryotic cells.

Core Cell Structures (Shared by Animals and Plants)

• Cell Membrane:
  Function: Controls which substances can enter or leave the cell (it is partially permeable). Think of it as the security guard for the cell.
• Nucleus:
  Function: Contains the genetic material (DNA) and controls the activities of the cell. It's the "control centre" or "brain" of the cell.
• Cytoplasm:
  Function: A jelly-like substance where most of the cell's chemical reactions take place. It fills the cell and holds the organelles.
• Mitochondria:
  Function: Site of aerobic respiration, releasing energy for the cell. These are the cell's power stations.
• Ribosomes:
  Function: Where protein synthesis occurs (where proteins are made).

Structures Unique to Plant Cells

Plant cells have three extra components that make them special, allowing them to carry out photosynthesis and provide support.

• Cell Wall:
  Function: Provides structural support and a fixed shape to the cell. It is made mostly of cellulose and is fully permeable.
• Chloroplasts:
  Function: Contain the green pigment chlorophyll and are the site of photosynthesis (making food using light energy).
• Vacuole (Large Central):
  Function: Stores water, nutrients, and waste. When full of water (cell is turgid), it pushes the cytoplasm against the cell wall, providing support.

Section 3: Bacterial Cell Structure (B2.1 Core 2)

Bacteria are much simpler organisms. They are prokaryotic (meaning they existed before the nucleus). You need to know their structure, limited to these specific components:

Structure of a Bacterial Cell

• Cell Wall: Offers protection and structural shape (Note: it is chemically different from a plant cell wall).
• Cell Membrane: Controls substances entering and leaving.
• Cytoplasm: Site of chemical reactions.
• Ribosomes: Used for protein synthesis.
• Circular DNA: The main genetic material, which is not enclosed within a nucleus (it floats freely in the cytoplasm).
• Plasmids: Small, extra rings of DNA that often carry genes for special functions, such as antibiotic resistance.

Common Mistake Alert: Do not say a bacterium has a nucleus or mitochondria – it does not! Its DNA is circular and floats in the cytoplasm.

Section 4: Specialised Cells (B2.1 Supplement 6)

In multicellular organisms (like humans and plants), cells become specialised – they develop specific features (adaptations) to perform a particular job efficiently.

A. Root Hair Cells (Absorption)

• Function: Absorbs water and mineral ions from the soil.
• Key Adaptation: They have a long, thin extension (the root hair) that significantly increases the surface area for absorption.

B. Palisade Mesophyll Cells (Photosynthesis)

• Function: Primary site of photosynthesis in leaves.
• Key Adaptation: They are packed with numerous chloroplasts (the site of photosynthesis) and are located near the upper surface of the leaf to catch maximum sunlight.

C. Red Blood Cells (Transport of Oxygen)

• Function: Transport oxygen from the lungs to the body tissues.
• Key Adaptations:
  1. They contain the protein haemoglobin which binds to oxygen.
  2. They lack a nucleus (when mature), allowing more space for haemoglobin.
  3. They have a biconcave shape, which increases the surface area for rapid oxygen uptake and release.

Section 5: Calculating Size and Magnification (B2.2 Core 1, 2 & Supplement 3)

When we look at biological specimens under a microscope, we need to know how much bigger the image is than the real object. This is calculated using the magnification formula.

The Magnification Formula (B2.2 Core 1)

Magnification is the ratio of the image size to the actual (real) size of the specimen.

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

Memory Aid: Think of a triangle – Image (I) is on top, Magnification (M) and Actual (A) are on the bottom. If you want M, divide I by A. If you want I, multiply M by A. If you want A, divide I by M.

Units and Conversions (B2.2 Core 2 & Supplement 3)

When performing calculations, it is crucial that the image size and the actual size are in the same units. We often deal with very small units:

• Millimetres (mm): Used to measure visible objects or larger drawings.
• Micrometres (\(\mu\text{m}\)): Used to measure the actual size of cells and organelles.

You must be able to convert between these units:

1 millimetre (mm) = 1000 micrometres (\(\mu\text{m}\))

Tip for conversion:

• To go from mm to \(\mu\text{m}\): Multiply by 1000.
• To go from \(\mu\text{m}\) to mm: Divide by 1000.

Calculation Example (B2.2 Core 2)

A student draws a palisade cell. The drawing (Image size) measures 30 mm. The actual size of the cell is known to be 20 \(\mu\text{m}\). What is the magnification of the drawing?

Step 1: Ensure units are the same. Convert the actual size to mm or the image size to \(\mu\text{m}\). Let's convert the image size to \(\mu\text{m}\):
Image size = 30 mm
\(30 \text{ mm} \times 1000 = 30\,000 \mu\text{m}\)

Step 2: Apply the formula.
$$ \text{Magnification} = \frac{\text{Image size}}{\text{Actual size}} $$
$$ \text{Magnification} = \frac{30\,000 \mu\text{m}}{20 \mu\text{m}} $$
$$ \text{Magnification} = 1500 $$

The magnification is \(1500\times\). (Magnification has no unit, as it is a ratio).

Did you know?

The biggest single cell in the world is the yolk of an ostrich egg! However, most cells are much smaller, needing microscopes to be seen clearly. The size of cells is limited because they rely on the movement of substances (like oxygen and waste) across their surface area – if a cell gets too big, its volume increases faster than its surface area, meaning transport becomes inefficient.

You've mastered the building blocks of life! Now, you are ready to explore how these cells move materials in and out in the next chapter. Keep up the great work!