2.1 Cell Structure Revision Notes

1. Overview

Cells are the fundamental building blocks of all living organisms. This topic explores the microscopic structures (organelles) found within animal, plant, and bacterial cells, explaining how their specific shapes and components allow them to perform the essential processes of life.

Key Definitions

• Cell: The basic structural and functional unit of all living organisms.
• Tissue: A group of cells with similar structures, working together to perform a shared function.
• Organ: A structure made up of a group of tissues, working together to perform specific functions.
• Organ System: A group of organs with related functions, working together to perform body functions.
• Organism: A living thing that has (or can develop) the ability to act or function independently.
• Organelle: A specialized structure within a cell that performs a specific function (e.g., mitochondria).

Core Content

Cell Structures and Functions

All living cells contain certain components, but plant, animal, and bacterial cells have distinct differences.

Structure	Description/Function	Found In
Cell Wall	Made of cellulose; provides structural support and protection.	Plant, Bacteria
Cell Membrane	Controls the movement of substances in and out of the cell.	All cells
Nucleus	Contains genetic material (DNA) and controls cell activities.	Plant, Animal
Cytoplasm	Jelly-like substance where most chemical reactions occur.	All cells
Chloroplasts	Contains chlorophyll; the site of photosynthesis.	Plant (green parts)
Ribosomes	Tiny structures responsible for protein synthesis.	All cells
Mitochondria	The site of aerobic respiration, releasing energy for the cell.	Plant, Animal
Vacuoles	Large/permanent in plants (contains cell sap); small/temporary in animals.	Plant, Animal

Bacterial Cells

Bacterial cells are much smaller than plant or animal cells and do not have a nucleus.

• Circular DNA: A single loop of genetic material that floats freely in the cytoplasm.
• Plasmids: Small, circular rings of extra DNA that can be transferred between bacteria.
• Cell Wall: Made of a different substance (murein/peptidoglycan) than plant cell walls.

Production of New Cells

• Principle: New cells are produced by the division of existing cells. This process (mitosis) allows for growth and the repair of damaged tissues.

Specialised Cells: Structure and Function

Cells are often adapted to carry out specific roles:

1. Ciliated cells: Have tiny hairs (cilia) to sweep mucus containing dust and bacteria upward in the trachea and bronchi.
2. Root hair cells: Have a long extension to increase surface area for the absorption of water and mineral ions.
3. Palisade mesophyll cells: Tall cells packed with chloroplasts for maximum photosynthesis.
4. Neurones: Long, thin cells that conduct electrical impulses over long distances in the body.
5. Red blood cells: Contain haemoglobin and have a biconcave shape (no nucleus) to maximize the transport of oxygen.
6. Sperm and egg cells (gametes): Specialized for reproduction; sperm have tails for swimming, and eggs contain nutrient stores for the embryo.

Levels of Organisation (Examples)

• Cell: Muscle cell
• Tissue: Muscle tissue
• Organ: The Heart
• Organ System: Circulatory system
• Organism: A Human

Extended Content (Extended Only)

(Note: There are no specific Supplement objectives listed for the 2.1 Cell Structure section of the syllabus provided; all requirements are covered in the Core section above.)

Key Equations

While not a chemical equation, the calculation of Magnification is frequently tested in this topic:

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

• Image size: The size of the object in a drawing/photo (measured with a ruler).
• Actual size: The real-sized object (usually given in mm or µm).
• Units: Always ensure both sizes are in the same units (usually mm) before dividing.
• Unit Conversion: $1\text{ mm} = 1000\text{ µm}$.

Common Mistakes to Avoid

• ❌ Wrong: Thinking all plant cells have chloroplasts.
  • ✓ Right: Only plant cells exposed to light (like leaves) have them; root hair cells do not have chloroplasts.
• ❌ Wrong: Confusing the cell wall and cell membrane.
  • ✓ Right: All cells have a membrane, but animal cells never have a cell wall. The wall is the outermost layer in plants/bacteria.
• ❌ Wrong: Saying the nucleus is the "brain" of the cell.
  • ✓ Right: Use biological terms: the nucleus controls cell activities and contains genetic information.

Exam Tips

• Command Word "State": This is the most common command word for this topic. Give a short, concise answer without lengthy explanations.
• Identification: Be prepared to label a diagram of a cell. Look for the cell wall first—if it's there, it’s a plant or bacterial cell. If there is a nucleus, it cannot be a bacteria.
• Structure to Function: If asked to "explain" how a cell is adapted, always link the physical feature to the job it does (e.g., "Root hair cells have a long extension to increase surface area for faster absorption").
• Calculations: You may be given a numerical value like 5.1 (as seen in past papers). Always show your working for magnification calculations to gain partial marks even if the final answer is wrong.