The cell is the basic structural and functional unit of the human body. All tissues and organs are made up of cells, and normal body function depends on coordinated cellular activities.
A typical human cell contains a plasma membrane, cytoplasm, nucleus and specialized organelles. Each organelle performs specific functions while working together to maintain cellular homeostasis.
Interactive 3D Model of a Cell
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Basic Composition of the Cell
The cell contains water, electrolytes, proteins, lipids and carbohydrates. These substances provide structural components, energy and the molecular machinery required for cellular function.
- Water: The major component of the cell and an important medium for biochemical reactions.
- Electrolytes: Include sodium, potassium, calcium, magnesium, chloride and phosphate.
- Proteins: Perform structural, enzymatic, transport and regulatory functions.
- Lipids: Important components of cell membranes and energy storage.
- Carbohydrates: Provide energy and participate in cell recognition and structural functions.
Cellular Organelles
Endoplasmic Reticulum
The endoplasmic reticulum is a membrane-bound network within the cytoplasm. It is divided into rough and smooth endoplasmic reticulum.
- Rough ER: Contains ribosomes and is involved mainly in protein synthesis and processing.
- Smooth ER: Participates in lipid synthesis, detoxification and calcium storage.
Ribosomes
Ribosomes are the sites of protein synthesis. They may occur freely in the cytoplasm or remain attached to the rough endoplasmic reticulum.
Golgi Apparatus
The Golgi apparatus modifies, sorts and packages proteins and lipids received from the endoplasmic reticulum. It also contributes to the formation of secretory vesicles and lysosomes.
Lysosomes
Lysosomes contain hydrolytic digestive enzymes that break down cellular waste, damaged organelles and other materials taken into the cell.
Peroxisomes
Peroxisomes contain oxidative enzymes and are involved in fatty-acid oxidation and detoxification reactions.
Secretory Vesicles
Secretory vesicles transport substances to the plasma membrane for release outside the cell by exocytosis.
Mitochondria
Mitochondria are major sites of aerobic energy production. They generate ATP, which provides energy for many cellular processes.
Cytoskeleton
The cytoskeleton is formed by protein filaments that help maintain cell shape and participate in intracellular transport and cellular movement.
Nucleus and Nucleolus
The nucleus contains most of the cell's genetic material in the form of DNA. It regulates gene expression and controls many cellular activities.
The nucleolus is a specialized region within the nucleus where ribosomal RNA is synthesized and ribosomal subunits are assembled.
Plasma Membrane
The plasma membrane forms the outer boundary of the cell. It is selectively permeable and controls movement of substances between the intracellular and extracellular environments.
The membrane also participates in cell signaling, cell recognition and communication with neighboring cells.
Transport Across the Cell Membrane
- Simple diffusion: Movement of substances down their concentration gradient without energy expenditure.
- Facilitated diffusion: Passive movement through membrane proteins such as channels or carriers.
- Active transport: Movement of substances against a concentration or electrochemical gradient using energy.
- Endocytosis: Uptake of material into the cell through formation of membrane vesicles.
- Exocytosis: Release of substances from the cell through fusion of vesicles with the plasma membrane.
Cellular Energy and ATP
ATP is the major immediately usable energy source of the cell. It is required for active transport, protein synthesis, muscle contraction, cellular movement and many other metabolic processes.
Cellular Movement
Cells and cellular structures can move through mechanisms involving the cytoskeleton and motor proteins. Examples include movement of cilia, flagella, intracellular transport and migration of certain cells.
High-Yield MBBS Revision
| Structure | Main Function |
|---|---|
| Plasma membrane | Selective barrier; transport, signaling and cell recognition. |
| Nucleus | Contains DNA and regulates genetic activity. |
| Nucleolus | rRNA synthesis and ribosome assembly. |
| Ribosomes | Protein synthesis. |
| Rough ER | Protein synthesis and processing. |
| Smooth ER | Lipid synthesis, detoxification and calcium storage. |
| Golgi apparatus | Modification, sorting and packaging of proteins and lipids. |
| Lysosomes | Intracellular digestion and breakdown of cellular waste. |
| Peroxisomes | Fatty-acid oxidation and detoxification reactions. |
| Mitochondria | ATP production through aerobic metabolism. |
| Cytoskeleton | Cell shape, intracellular transport and movement. |
Quick MBBS Takeaway
The cell is the basic structural and functional unit of the body. The plasma membrane regulates exchange with the extracellular environment, the nucleus controls genetic activity, ribosomes synthesize proteins, the endoplasmic reticulum and Golgi apparatus process cellular products, lysosomes digest cellular material, peroxisomes perform oxidative reactions, and mitochondria produce ATP. Together, these structures maintain cellular function and homeostasis.
Key Points for Revision
- The cell is the basic structural and functional unit of the human body.
- The plasma membrane is selectively permeable.
- The nucleus contains most of the cellular DNA.
- Ribosomes are responsible for protein synthesis.
- Rough ER is mainly involved in protein synthesis.
- Golgi apparatus modifies, sorts and packages cellular products.
- Lysosomes contain hydrolytic digestive enzymes.
- Peroxisomes participate in oxidation and detoxification.
- Mitochondria are major sites of aerobic ATP production.
- ATP supplies energy for many cellular processes.
- Active transport requires energy, whereas simple diffusion does not.