Learn & Review: GCSE Biology - Cell Types and Cell Structure (2026/27 exams)

Summary of Cell Structures and Types

This summary outlines the fundamental concepts of cells, their basic building blocks, and a comparison of different cell types, focusing on eukaryotic (animal and plant) and prokaryotic (bacterial) cells.

What are Cells?

• Basic Building Blocks of Life: Cells are the smallest units of life capable of independent replication.
• Replication: Cells can divide to create more cells.
  • In multicellular organisms (like animals and plants), cell division is primarily for growth or replacing dead cells.
  • In unicellular organisms (like bacteria), cell division results in the reproduction of a whole new organism.
• Multicellularity: Organisms like animals and plants are composed of numerous cells, with hundreds of different types in humans, and an estimated over 40 trillion cells in an adult.

Subcellular Structures (Organelles)

Subcellular structures, also known as organelles, are the components that make up a cell.

Common Structures in Animal and Plant Cells (Eukaryotic)

• Cell Membrane: Surrounds the cell, controlling the passage of substances in and out.
• Nucleus: Contains the cell's genetic material (DNA) and controls cellular activities.
• Cytoplasm: A gel-like substance filling the cell, where organelles are suspended and chemical reactions occur.
• Mitochondria: Responsible for providing energy to the cell through aerobic respiration, breaking down sugars like glucose.
• Ribosomes: Sites where proteins are synthesized (made).

Additional Structures in Plant Cells

• Cell Wall: A rigid outer layer made of cellulose, providing structural support and preventing the cell from bursting due to excess water.
• Permanent Vacuole: A large sac containing "cell sap" (sugars, salts, water), which the cell can use when needed.
• Chloroplasts: The site of photosynthesis, where plants use sunlight to create sugars. They contain chlorophyll, a green pigment that absorbs light energy.

Bacterial Cells (Prokaryotic)

Bacteria are prokaryotes and are unicellular organisms.

• Shared Structures with Eukaryotic Cells:
  • Cell Membrane
  • Cell Wall
  • Ribosomes
  • Cytoplasm
• Key Differences from Eukaryotic Cells:
  • No Nucleus: Genetic material (a single circular strand of DNA) floats freely in the cytoplasm. This DNA may be referred to as a circular chromosome or nucleoid.
  • No Mitochondria
  • No Chloroplasts
• Additional Structures in Some Bacteria:
  • Plasmids: Small, circular DNA molecules that can carry extra genes, such as antibiotic resistance.
  • Flagella: Thread-like structures that rotate to propel the bacterium, enabling movement.