Cells - Structure, Organelles, and Functions

Cells - Cells are the basic structural and functional units of life. Every living organism — from the smallest bacteria to the largest tree — is made up of cells. Some organisms, like amoeba, consist of only a single cell, while others, like humans, contain trillions of cells that work together to maintain life.

This article provides a detailed, student-friendly explanation of cell structure, organelles, types of cells, and their functions, making it easier to understand one of biology’s most fundamental topics.

What Is a Cell?

A cell is the smallest unit of life that can perform all life processes — such as growth, metabolism, and reproduction. The term “cell” was first coined by Robert Hooke in 1665, when he observed cork cells under a microscope. Later, Schleiden and Schwann proposed the Cell Theory, which states:

1. All living organisms are made up of one or more cells.
2. The cell is the basic structural and functional unit of life.
3. All cells arise from pre-existing cells (added by Rudolf Virchow).

Types of Cells

There are two main types of cells based on structural organization:

1. Prokaryotic Cells

• Simple cells without a true nucleus.
• Genetic material lies freely in the cytoplasm (nucleoid region).
• Found in bacteria and cyanobacteria.
• Example: E. coli.

2. Eukaryotic Cells

• Complex cells with a well-defined nucleus and membrane-bound organelles.
• Found in plants, animals, fungi, and protists.
• Example: Human cell, Onion cell.

Plant Cells vs. Animal Cells

Feature	Plant Cell	Animal Cell
Cell wall	Present	Absent
Vacuole	One large central vacuole	Many small vacuoles
Chloroplasts	Present (for photosynthesis)	Absent
Centrosome	Absent	Present
Shape	Usually rectangular	Usually round or irregular
Stored Food	Starch	Glycogen

Structure of a Cell

Each cell has three main parts:

1. Cell wall (in plants only)
2. Cell membrane (plasma membrane)
3. Cytoplasm (containing organelles)

Let’s understand each component in detail.

1. Cell Wall

• Found in plants, algae, fungi, and some bacteria.
• Made primarily of cellulose fibers.
• Provides rigidity, protection, and support to the cell.
• The wall is pitted and allows water and dissolved substances to pass through easily.

2. Cell Membrane (Plasma Membrane)

• A thin, flexible membrane that surrounds the cell.
• Separates the cell’s contents from the external environment.
• Semi-permeable: controls what enters and leaves the cell.
• Gives shape to the cell and maintains internal balance (homeostasis).

3. Cytoplasm

• The jelly-like fluid between the nucleus and the cell membrane.
• Made up of water, salts, and organic molecules.
• Contains all cell organelles suspended within it.
• Helps maintain cell shape through its network of microtubules and filaments.

Major Cell Organelles and Their Functions

a. Nucleus

• The control center of the cell.
• Contains DNA, which carries genetic information.
• Surrounded by a double membrane (nuclear envelope) with pores for material exchange.
• Contains the nucleolus, which produces ribosomes.
• Regulates cell growth, metabolism, and reproduction.

b. Ribosomes

• Small round organelles responsible for protein synthesis.
• Found free in cytoplasm or attached to the endoplasmic reticulum (Rough ER).
• Proteins made here are used for cell repair, enzyme production, and growth.

c. Endoplasmic Reticulum (ER)

A network of membranous channels extending throughout the cytoplasm.

• Rough ER: Has ribosomes; synthesizes and transports proteins.
• Smooth ER: Lacks ribosomes; synthesizes lipids and detoxifies harmful substances.
• Acts as an internal transport system between the nucleus and cytoplasm.

d. Golgi Apparatus (Golgi Bodies)

• Stack of flattened membrane sacs near the nucleus.
• Modifies, sorts, and packages proteins and lipids received from ER.
• Forms secretory vesicles and lysosomes.
• Often called the “post office” of the cell for distributing molecules.

e. Lysosomes

• Small membrane-bound vesicles formed by Golgi bodies.
• Contain digestive enzymes that break down waste materials and worn-out cell parts.
• Sometimes called “suicidal bags” because they can digest the entire cell if damaged.
• Play a role in defense by destroying harmful microbes.

f. Mitochondria

• Known as the “powerhouse of the cell.”
• Double membrane structure with inner folds called cristae that increase surface area.
• Perform cellular respiration to produce ATP (Adenosine Triphosphate) — the energy currency of the cell.
• Contain their own DNA, enabling them to reproduce independently.

g. Vacuoles

• Membrane-bound sacs filled with fluid.
• Store water, nutrients, and waste materials.
• In plants: one large central vacuole helps maintain turgor pressure.
• In animals: several small vacuoles for temporary storage.

h. Centrosome

• Found only in animal cells near the nucleus.
• Contains two centrioles made of microtubules arranged in 9 triplets.
• Helps in cell division by forming the spindle fibers that separate chromosomes.
• Also aids in the formation of cilia and flagella for movement.

i. Plastids (in Plant Cells Only)

There are three main types of plastids:

1. Chloroplasts

• Contain chlorophyll, the green pigment responsible for photosynthesis.
• Convert solar energy into chemical energy (glucose).
• Have a double membrane and stacks of thylakoids called grana.
• Found in leaves and green stems.

2. Chromoplasts

• Contain carotenoid pigments (red, yellow, orange).
• Give color to fruits, flowers, and some roots like carrots.

3. Leucoplasts

• Colorless plastids used for storage of starch, fats, and proteins.
• Found in roots and storage organs like potatoes.

Nucleolus

• Found inside the nucleus.
• Main function: produces ribosomes that later migrate to the cytoplasm.

Functions of the Cell

Cells carry out numerous life-sustaining functions, including:

1. Energy Production: through respiration in mitochondria.

2. Protein Synthesis: by ribosomes and ER.

3. Storage and Secretion: by vacuoles and Golgi apparatus.

4. Reproduction: through cell division (mitosis and meiosis).

5. Defense: by lysosomes and white blood cells.

6. Photosynthesis: by chloroplasts in plant cells.

Interesting Facts About Cells

• The average human body has around 37 trillion cells.
• The largest cell is the ostrich egg, while the smallest are mycoplasma bacteria.
• Nerve cells can be over a meter long, transmitting signals across the body.
• Cells communicate using chemical messengers called hormones.

Cell Organelles and Their Functions

Organelle	Function
Cell Wall	Provides shape and protection (plants only)
Cell Membrane	Controls entry and exit of substances
Cytoplasm	Site of cellular activities
Nucleus	Controls all cell functions
Ribosomes	Synthesizes proteins
ER	Transports and processes materials
Golgi Apparatus	Packages and distributes substances
Lysosomes	Digests waste materials
Mitochondria	Produces energy (ATP)
Vacuoles	Stores nutrients and wastes
Centrosome	Helps in cell division
Chloroplasts	Performs photosynthesis
Nucleolus	Produces ribosomes

FAQs About Cells

Q1. Who discovered the cell?

Robert Hooke discovered the cell in 1665 while observing cork under a microscope.

Q2. What is the main difference between plant and animal cells?

Plant cells have a cell wall and chloroplasts; animal cells do not.

Q3. What is the function of mitochondria?

Mitochondria generate energy through respiration, producing ATP.

Q4. Which organelle is called the “suicidal bag”?

Lysosomes — because they digest old or damaged cell parts.

Q5. What is the role of chloroplasts?

Chloroplasts help plants make food using sunlight through photosynthesis.

Q6. Why is the nucleus called the control center of the cell?

Because it contains DNA that regulates all cellular activities and reproduction.