2.1 Cell Structure

This section focuses on the different types of cells found in plants, animals, and bacteria, and their key internal structures. Understanding these structures is fundamental to understanding how these organisms function.

Animal Cells

Animal cells are eukaryotic cells, meaning they have a nucleus and other membrane-bound organelles. The main structures include:

• Cell Membrane: The outer boundary of the cell, controlling what enters and exits.
• Cytoplasm: The jelly-like substance filling the cell, where many chemical reactions occur.
• Nucleus: Contains the genetic material (DNA) and controls the cell's activities.
• Mitochondria: The 'powerhouses' of the cell, producing energy (ATP) through respiration.
• Ribosomes: Involved in protein synthesis.
• Lysosomes: Contain enzymes to break down waste materials and cellular debris.
• Cytoskeleton: A network of protein filaments that provides structural support and helps with cell movement.

Plant Cells

Plant cells are also eukaryotic but have some unique structures not found in animal cells. These adaptations support the plant's structure and function.

• Cell Membrane: Similar to animal cells, controlling entry and exit.
• Cytoplasm: The cell's internal fluid.
• Nucleus: Contains the genetic material.
• Mitochondria: For energy production.
• Ribosomes: For protein synthesis.
• Cell Wall: A rigid outer layer providing support and protection. Made of cellulose.
• Chloroplasts: Contain chlorophyll and are the site of photosynthesis.
• Vacuole: A large central vacuole that stores water, nutrients, and waste. Helps maintain cell turgor pressure.

Bacterial Cells

Bacteria are prokaryotic cells, meaning they lack a nucleus and other membrane-bound organelles. Their structure is simpler than eukaryotic cells.

• Cell Wall: A rigid outer layer providing shape and protection.
• Cell Membrane: Controls entry and exit.
• Cytoplasm: The internal fluid.
• Nucleoid: Contains the bacterial DNA, but it is not enclosed in a nucleus.
• Ribosomes: For protein synthesis.
• Capsule: An outer layer providing protection and aiding in adherence to surfaces.
• Flagella: Whip-like structures used for movement.
• Pili (Fimbriae): Hair-like structures used for attachment.

Comparison of Cell Structures

The following table summarizes the key differences between plant, animal, and bacterial cells.

Structure	Animal Cell	Plant Cell	Bacterial Cell
Cell Wall	Absent	Present (cellulose)	Present (peptidoglycan)
Chloroplasts	Absent	Present	Absent
Large Central Vacuole	Small, if present	Large, prominent	Absent
Nucleus	Present	Present	Absent (Nucleoid)
Mitochondria	Present	Present	Absent
Ribosomes	Present	Present	Present
Cell Membrane	Present	Present	Present