What Is a Cell? | Structure, Organelles, and Functions (Full Guide)
A cell is the smallest unit of life capable of performing all vital functions. Every living organism—from tiny bacteria to humans—is made up of one or more cells.
Cells are alive, they carry out life processes independently, and they are responsible for essential activities such as respiration, excretion, growth, and energy production. Because cells are microscopic, they can only be observed using a microscope.
Main Parts of a Cell
A cell has three main parts.
From outside to inside, these are:
- Cell membrane
- Cytoplasm
- Nucleus
A. Cell Membrane
The cell membrane protects the cell from external influences and gives it shape. It has a flexible structure with tiny gateways called pores, which allow selective exchange of substances.
Key Features of the Cell Membrane
- Maintains the shape of the cell
- Protects the cell from external factors
- Controls the entry and exit of materials
- Prevents the cell from disintegrating
- Selectively permeable (not all substances can pass)
- Made of proteins, lipids, and carbohydrates
- It is a living structure
Cell Wall
The cell wall is a tough, protective layer surrounding the cell membrane.
It is found in plants, bacteria, and fungi, but not in animal cells.
Features of the Cell Wall
- Provides durability and structural support
- Protects the plant against physical impacts
- Mostly made of cellulose
- Fully permeable
- Non-living structure
B. Cytoplasm
The cytoplasm is a jelly-like fluid between the cell membrane and the nucleus. It is semi-transparent and mostly composed of water.
Functions of the Cytoplasm
Vital processes such as:
- Respiration
- Digestion
- Excretion
- Energy production
- Nutrient synthesis
take place in the cytoplasm.
The specialized structures that perform these tasks are called organelles.
The cytoplasm also contains enzymes, hormones, minerals, lipids, carbohydrates, and proteins.
Organelles in the Cell
1. Endoplasmic Reticulum (ER)
A network of channels that spreads throughout the cell.
- Transports substances within the cell
- Helps in the synthesis and storage of certain materials
- Functions like a transportation highway
2. Golgi Apparatus
- Produces and packages secretory substances
- Abundant in cells that produce secretions
- Salivary glands, sweat glands, and mammary glands contain many Golgi bodies
- In flowering plants, it helps produce nectar
3. Mitochondria
Known as the powerhouses of the cell.
- Produce energy by breaking down nutrients with oxygen
- Found in high numbers in energy-demanding tissues (muscles, nerves, liver)
4. Lysosomes
- Digest and break down large molecules
- Destroy harmful microorganisms
- Remove old or damaged cell parts
- Found in animal cells and simple plant cells
5. Ribosomes
- The smallest organelles
- Found in all cells except viruses
- Responsible for protein synthesis
- High numbers in growing cells, liver cells, and secretory cells
6. Centrosome
- Contains a pair of centrioles
- Essential for cell division
- Found in animal cells and primitive plant cells
7. Vacuole
- Stores nutrients, waste products, and water
- Large in plant cells, small in animal cells
8. Plastids (Plant Cells Only)
Plastids are found only in plant cells and are responsible for color, storage, and photosynthesis.
Types:
a. Chloroplast
- Green-colored plastid
- Performs photosynthesis
- Produces food and oxygen
- Contains high amounts of chlorophyll
- Found in green parts of plants
b. Chromoplast
- Gives plants red, orange, and yellow colors
- Found in fruits, flowers, roots, and seeds
c. Leucoplast
- Colorless
- Stores starch, lipids, and proteins
- Found in underground structures (roots, tubers, seeds)
C. Nucleus
The nucleus is the control center of the cell.
- Manages all vital activities
- Stores genetic material (DNA)
- Directs growth, division, and protein production
Exceptions
- Bacteria have no true nucleus; DNA floats freely in the cytoplasm
- Mature red blood cells have no nucleus
- Striated muscle cells contain multiple nuclei
Chromosomes
During cell division, DNA condenses and forms chromosomes.
The number of chromosomes is specific for each species.
Chromosome Number and Biological Complexity
Does Chromosome Number Show How Advanced an Organism Is?
The number of chromosomes in an organism does not indicate its level of complexity or development.
A species with more chromosomes is not necessarily more advanced than one with fewer chromosomes.
For example, some simple organisms may have a higher chromosome count than humans, while many advanced organisms have fewer chromosomes.
Does Having the Same Chromosome Number Mean Two Species Are Related?
Having the same number of chromosomes does not mean that two organisms are genetically similar or closely related.
Chromosome number is not a measure of evolutionary closeness.
Two completely different species may share the same chromosome count but have entirely different genetic structures.
DNA, Chromosomes, Genes and Cell Structure – Complete Guide
What Is DNA?
DNA (Deoxyribonucleic Acid) is the genetic control molecule of the cell.
It carries all the instructions necessary for a living organism to grow, develop and function.
- DNA has a double-stranded, twisted helix structure.
- Every trait of an organism is stored inside DNA.
What Is a Gene?
A gene is a functional segment located on DNA.
Genes determine many characteristics such as:
- Eye color
- Skin color
- Blood type
- Gender
- Some inherited traits
From Cell to Nucleotide — Biological Hierarchy
Living organisms are organized in the following order:
Cell → Chromosome → DNA → Gene → Nucleotide
Each level is smaller and contains more specific genetic information.
Differences Between Plant and Animal Cells
Major Structural Differences
| Feature | Plant Cell | Animal Cell |
|---|---|---|
| Chloroplast | Present | Absent |
| Cell Wall | Present | Absent |
| Centrosome | Absent in advanced plants | Present |
| Vacuole | Large and few | Small and many |
| Shape | Generally rectangular | Round or irregular |
| Lysosome | Rare in plants | Common in animals |
Historical Development of Cell Knowledge
Scientists and Their Contributions
- Zacharias Janssen – Invented the first microscope.
- Robert Hooke – First person to observe and name the “cell” while examining cork tissue.
- Anton van Leeuwenhoek – Observed tiny moving organisms in pond water.
- Robert Brown – Discovered the nucleus of plant cells.
- Purkinje, Schwann, Mohl – Named the living substance inside the cell “cytoplasm.”
- Rudolf Virchow – Completed the modern cell theory.
- Siemens – Developed the electron microscope, allowing cells to be viewed in great detail.
Cell Theory
Modern cell theory consists of three main principles:
- All living organisms are made of one or more cells.
- The cell is the basic unit of structure and function in living things.
- All cells arise from pre-existing cells through division.
Prokaryotic and Eukaryotic Cells
Eukaryotic Cells (Advanced Cells)
Cells with a true nucleus are called eukaryotic.
Examples:
- Plants
- Animals
- Humans
- Fungi
- Amoeba, Euglena, Paramecium
Prokaryotic Cells (Primitive Cells)
Cells without a nucleus are called prokaryotic.
Examples:
- Bacteria
Prokaryotic cells do not contain mitochondria, endoplasmic reticulum, Golgi apparatus or chloroplasts.
From Cell to Organism
In multicellular organisms:
- Similar cells group together to form tissues.
- Tissues form organs.
- Organs work together to form systems.
- All systems together create the organism (the living being).
Cell → Tissue → Organ → System → Organism