Cell: The Unit of Life

2.1 WHAT IS A CELL?

Biology is the study of living organisms, which range from microscopic entities like Amoeba to enormous creatures like blue whales.
All organisms share a fundamental characteristic: they are composed of cells, which serve as the building blocks of life.

Definition of a Cell

A cell is the fundamental structural and functional unit of all living beings.
- It is the smallest part of an organism capable of independent existence and performing essential life functions.
Every organ in the body (e.g., skin, brain, muscle, bone) is made up of hundreds of thousands of cells.
Every part of a plant (e.g., leaf, flower, root, wood) contains a large number of cells.
Each cell has its own life cycle, with old and weak cells continually being replaced by new ones.
- For example, red blood cells die after about 120 days.

2.2 Parts of a Cell

Part of Cell	Main Characteristics	Chief Function(s)
1. Cell wall (Only in plant cells)	- Outermost layer in plant cells - Rigid and non-living - Freely permeable - Composed of cellulose	- Provides rigidity and shape - Protection - Allows substances in solution to enter the cell
2. Plasma membrane	- Outermost layer in animal cells - Thin and flexible living membrane - Contains fine pores - Semi-permeable	- Regulates entry and exit of substances - Maintains cell shape
3. Cytoplasm	- Contents within the plasma membrane, excluding the nucleus - Mixture of water, soluble inorganic and organic compounds	- Medium for metabolic activities - Initial steps of respiration (anaerobic)
4. Endoplasmic reticulum (ER)	- Irregular network of tubules continuous with plasma membrane and nuclear membrane - Can be smooth or rough	- Synthesis and transport of proteins, involved in cell metabolism
5. Mitochondria	- Sausage-shaped, double-walled organelles with folds called cristae - Have their own DNA	- Energy production (ATP) through aerobic respiration
6. Golgi apparatus / Dictyosome	- Stacks of flattened membrane sacs - In animal cells as Golgi apparatus; in plant cells as Dictyosome	- Synthesis and secretion of enzymes and hormones - Packaging and transport of materials
7. Ribosomes	- Scattered in cytoplasm or attached to ER - Composed mainly of RNA	- Protein synthesis
8. Lysosomes	- Membranous sacs containing various enzymes	- Intracellular digestion, destroy unwanted materials - Recycle damaged organelles
9. Centrosome (Animal cells only)	- Region surrounding centrioles near the nucleus	- Spindle fibre formation during cell division
10. Plastids (Plant cells only)	- Variety includes chloroplasts, chromoplasts, leucoplasts	- Photosynthesis, color formation, food storage
11. Nucleus	- Largest organelle, spherical, dense, contains the primary genetic material	- Regulates cell functions - Cell division - Houses DNA
12. Nucleolus	- Substructure within the nucleus	- Produces ribosomes and aids in protein synthesis
13. Chromatin fibres	- Network of thread-like structures that condense into chromosomes during cell division	- Contains genetic information
14. Vacuoles	- Clear spaces containing water and substances in solution - Larger in plant cells, smaller in animal cells	- Store nutrients and waste products - Provide turgidity in plant cells
15. Granules	- Small particles containing stored nutrients like starch or glycogen	- Food storage for cells

2.3 Cell Theory

Proposed by Virchow in 1858: all cells arise from pre-existing cells.
Major points of Cell Theory:
  1. The cell is the smallest structural unit in all living things.
  2. The cell is the functional unit of all living things.
  3. All cells arise from pre-existing cells.

Examples of Cell Theory

Structural Unit: In both a mango tree and a frog, examining any part reveals a cellular structure.
Functional Unit: Functions in frogs, such as movement, metabolism, digestion, and absorption, depend on cellular activities. In plants, cells perform photosynthesis, nutrient absorption, and other vital functions.
Cells and Replacement: The organisms consist of millions of cells that constantly die and regenerate through cell division.
Life Origins: Both frogs and mango trees begin life as single cells (fertilized egg for frogs, embryo for mango).

2.4 Differences between Plant and Animal Cells

Feature	Plant Cells	Animal Cells
1. Cell wall	Present (cellulose)	Absent
2. Centrosome	Absent	Present
3. Vacuoles	Prominent and large	Small and temporary
4. Plastids	Present (chloroplasts, chromoplasts, leucoplasts)	Absent
5. Size	Usually larger	Usually smaller
6. Cytoplasm	Less dense, fewer intercellular spaces	Denser and fills nearly entire cell
7. Arrangement of Cytoplasm	Pushed to periphery due to large vacuoles	More even distribution

2.5 Protoplasm

Definition: Protoplasm is the living substance found within cells, which includes both the cytoplasm and the nucleus.
It is semi-liquid, somewhat colorless, and its chemical composition includes essential elements such as carbon, hydrogen, oxygen, nitrogen, sulfur, iron, and phosphorus.
The protoplasm functions as the site for metabolic activities vital for the cell's survival.

2.6 Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells:
  - Lacks a defined nucleus (nucleoid).
  - Contains a single strand of DNA.
  - Typically small ribosomes.
  - Example: Bacteria.
Eukaryotic Cells:
  - Well-defined nucleus with a nuclear membrane.
  - Several strands of DNA (chromosomes).
  - Larger ribosomes.
  - Includes all plants and animals.

2.7 Cellular Activity

Every activity of living organisms relies on cellular activities:
  1. Growth is driven by cell proliferation and increase in size.
  2. Healing and repair of injuries result from cell division.
  3. Movement is facilitated by the contractility of cells or cellular components.

2.8 Structure of a Generalized Cell

A generalized cell consists of three essential parts: 1. Plasma membrane, 2. Nucleus, and 3. Cytoplasm.
Cell Boundaries:
- The cell membrane is living and porously regulates substance movement, while the cell wall in plants offers rigidity and shape.
Cytoplasm:
- Semi-liquid medium where organelles reside and where numerous biochemical reactions occur.

2.9 Specialized Functions of Cell Structures

Mitochondria: Energy production site storing energy as ATP.
Golgi Apparatus: Modifies, packages, and transports proteins and lipids.
Lysosomes: Digestive vesicles that break down waste and excess cellular components, often termed "suicide bags".

2.10 Unique Features of Cells

Vacuoles: Serve different functions; large in plant cells for storage and turgor pressure, smaller in animal cells.
Plastids: Important in photosynthesis and storage of pigments and starches in plants.

Progress Checks

Review questions on cell organelles and their functions.

2.11 Understanding Cell Functions Through Examples

Cells exhibit diverse functions depending on their structure and location; for example, nerve cells transmit impulses reliably, muscle cells contract for movement, and guard cells in leaves control gas exchange.

2.12 Summary

Cells are integral to life, serving as the basic unit of structure and function across organisms.
Through the interactions of various organelles, cells perform the essential life functions necessary for growth, repair, and homeostasis.