Cell Organisation (AQA A-Level Biology): Revision Notes
📚Revision Notes
Cell Organisation
Cell specialisation
In multicellular organisms, cells undergo specialisation to perform specific functions more efficiently. This process allows organisms to carry out complex life processes that would be impossible with just one type of cell.
All cells in an organism originate from mitotic divisions of a fertilised egg, meaning they contain identical genetic information. However, during development, each cell becomes adapted for a particular role through selective gene expression. While every cell contains all the genes needed to develop into any cell type, only certain genes are active (switched on) in each specialised cell, whilst others remain inactive (switched off).
Note
The key to cell specialisation lies in selective gene expression - it's not that different cell types have different genes, but rather that they express different combinations of the same genes at different times and levels.
Cellular differentiation provides several advantages:
- Each cell type can perform its specific function more effectively
- Different cell types can work together to carry out complex processes
- The organism functions more efficiently as a whole
Specialised cells not only differ in shape and structure, but also in the number and type of organelles they contain. This relationship between structure and function is fundamental to understanding how cells work.
Example
Worked Example: Organelle Distribution in Specialised Cells
Muscle cells: Contain many mitochondria to provide energy for contraction
- High energy demand = high mitochondria number
Sperm cells: Have numerous mitochondria concentrated in the middle piece
- Need energy for swimming motion
Bone cells: Contain very few mitochondria
- Lower energy requirements due to reduced metabolic activity
White blood cells: Rich in lysosomes for destroying pathogens
- Defence function requires enzymes for breaking down harmful substances
Comparison: Muscle cells have fewer lysosomes compared to white blood cells because their primary function is contraction, not pathogen destruction.
Tissues
A tissue consists of a collection of similar cells that work together to perform a specific function. Cells are organised into tissues to increase working efficiency in multicellular organisms.
Note
The organisation of cells into tissues represents the first level of cooperation in multicellular organisms, where similar cells coordinate their activities for greater efficiency than individual cells could achieve alone.
Examples of tissues
Epithelial tissues are found in animals and consist of sheets of cells that line organ surfaces. These tissues often have protective or secretory functions. Several types exist:
- Thin, flat epithelial cells facilitate diffusion in structures like lung alveoli
- Ciliated epithelium lines ducts such as the trachea, where cilia move mucus across the epithelial surface
- Xylem tissue occurs in plants and comprises several similar cell types. This tissue transports water and mineral ions throughout the plant whilst also providing mechanical support to the plant structure.
Organs
An organ represents a combination of different tissues that coordinate to perform various functions, though they typically have one predominant major function.
Animal organ example: the stomach
Example
Worked Example: The Stomach as an Organ
The stomach functions primarily in food digestion and demonstrates how different tissues work together:
Muscle tissue: Churns and mixes stomach contents
- Smooth muscle contracts rhythmically to break down food mechanically
Epithelial tissue: Protects the stomach wall and produces digestive secretions
- Secretes gastric juice containing enzymes and acid
Connective tissue: Holds the other tissues together
- Provides structural support and contains blood vessels
Plant organ example: the leaf
Example
Worked Example: The Leaf as an Organ
A leaf serves as an organ specialised for photosynthesis and gas exchange, containing several tissue types:
Palisade mesophyll: Made of leaf palisade cells that carry out photosynthesis
- Contains many chloroplasts for light absorption
Spongy mesophyll: Adapted for gaseous diffusion between air spaces
- Loosely packed cells allow gas movement
Epidermis: Protects the leaf whilst allowing gaseous diffusion
- Contains stomata for gas exchange
Phloem: Transports organic materials away from the leaf
- Carries sugars produced by photosynthesis
Xylem: Transports water and ions into the leaf
- Provides water needed for photosynthesis
Distinguishing organs from tissues
Important
Critical Distinction: Organs vs Tissues
Not all structures are easily classified as organs. Blood capillaries, for example, are not organs as they consist of just one tissue type (epithelium).
However, arteries and veins are organs because they contain multiple tissue types including epithelial, muscle and connective tissues, despite sharing the same major function of blood transport with capillaries.
Key rule: Organs must contain multiple tissue types working together, regardless of whether they share similar functions with simpler structures.
Organ systems
Organ systems consist of organs working together as a coordinated unit. These systems can be grouped to perform particular functions more efficiently.
Examples of human organ systems
The digestive system processes and digests food. Key organs include:
- Salivary glands, oesophagus, stomach, duodenum, ileum, pancreas and liver
The respiratory system enables breathing and gas exchange. Main organs include:
- Trachea, bronchi and lungs
The circulatory system pumps and circulates blood throughout the body. Primary organs include:
- Heart, arteries and veins
Each system demonstrates how organs with related functions work together to carry out complex physiological processes that maintain life in multicellular organisms.
Summary
Key Points to Remember:
- Cell specialisation occurs through selective gene expression, allowing cells to perform specific functions more efficiently
- The hierarchy of organisation proceeds: cells → tissues → organs → organ systems
- Tissues consist of similar cells performing the same function, while organs contain different tissue types working together
- Specialised cells vary not only in shape but also in organelle number and type according to their function
- Organ systems coordinate multiple organs to carry out complex life processes effectively