Structure of Cell: Components and its Function

STRUCTURE OF CELL

INTRODUCTION

Cells are the smallest functional units of life. Every living organism—small or large—is made up of one or many cells. Organisms like bacteria have only one cell (unicellular), while humans and animals are composed of millions of cells (multicellular).

The study of cells, including their structure and functions, is called Cytology.

Cells are often called the building blocks of life because tissues, organs, and entire organ systems are formed by groups of cells working together.

Levels of structural organization:

Cell → Tissue → Organ → Organ System → Body

HISTORY

The understanding of cells developed gradually through scientific discoveries:

• 1665 – Robert Hooke
  First observed dead cork cells using a microscope. He named them “cells” meaning small rooms.
• Anton van Leeuwenhoek
  Observed living cells with better magnification and noted their movement (motility).
• 1831 – Robert Brown
  Identified and described the nucleus inside plant cells.

These discoveries formed the foundation of modern cell biology.

TYPES OF CELLS

Living organisms have two major categories of cells:

1. Prokaryotic Cells

(Greek: pro = before, karyon = nucleus)

• Do not have a true nucleus.
• Genetic material present as a nucleoid, not enclosed by a membrane.
• Do not contain membrane-bound organelles.
• Examples: Bacteria, Blue-green algae.

2. Eukaryotic Cells

(Greek: eu = true, karyon = nucleus)

• Have a well-defined nucleus surrounded by a nuclear membrane.
• Found in animals, plants, fungi, and protozoa.
• Contain several membrane-bound organelles such as mitochondria, lysosomes, Golgi bodies, etc.
• Much larger and more complex than prokaryotic cells.

Difference Between Prokaryotic and Eukaryotic Cells

PLASMA MEMBRANE

The plasma membrane (cell membrane) forms the outer boundary of the cell and controls what enters and leaves the cell.

The most accepted model describing its structure is the Fluid Mosaic Model proposed by Singer and Nicolson.

Structure of Plasma Membrane

• Made of a double layer of phospholipids.
• Contains proteins, cholesterol, and carbohydrates embedded within it.
• Phospholipids have a:
• Hydrophilic (water-loving) head
• Hydrophobic (water-hating) tail

Functions of Plasma Membrane

• Acts as a protective barrier.
• Maintains the internal environment of the cell.
• Controls movement of substances into and out of the cell.
• Helps in cell recognition and communication.
• Plays a role in intercellular signaling.

TRANSPORT OF MATERIAL ACROSS THE MEMBRANE

Materials move across the membrane by passive or active transport mechanisms.

1. PASSIVE TRANSPORT

• Does not require energy.
• Movement occurs from high concentration → low concentration.

Types of Passive Transport

a) Diffusion
Movement of solute molecules from an area of high concentration to low concentration.

Two types:

• Simple Diffusion – substances pass directly through the membrane.
• Facilitated Diffusion – substances pass through transport proteins (carrier or channel proteins).

b) Osmosis
Movement of water molecules from high water concentration to low water concentration across a selectively permeable membrane.

2. ACTIVE TRANSPORT

• Requires energy (ATP).
• Movement occurs from low concentration → high concentration (against gradient).

Examples:

• Sodium-Potassium Pump
  Maintains ion balance inside nerve and muscle cells.

Bulk Transport

Used for movement of large particles or liquids.

a) Endocytosis ― Transport into the cell

• Phagocytosis – “cell eating”; uptake of solid particles.
• Pinocytosis – “cell drinking”; uptake of fluids.
• Receptor-mediated endocytosis – very specific uptake using receptors.

b) Exocytosis ― Transport out of the cell

• Used to release hormones, enzymes, waste, etc.

CELL ORGANELLES

Organelles are specialized structures inside the cell that perform specific functions—just like organs in the human body. Most organelles are membrane-bound and are suspended in the cytoplasm.

Major organelles include:
Nucleus, Mitochondria, Ribosomes, Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Peroxisomes, Centrosome, and components of the Cytoskeleton.

1. NUCLEUS

The nucleus was discovered by Robert Brown (1831).

It is the largest and most important organelle in eukaryotic cells and serves as the control center of the cell.

Structure

• Surrounded by a double membrane called the nuclear envelope.
• Contains:
• Nucleolus – site of ribosome formation
• Chromatin – DNA + proteins
• Nuclear pores – allow exchange of materials
• Nucleoplasm – fluid inside the nucleus

Functions of Nucleus

1. Stores genetic material (DNA).
2. Controls protein synthesis.
3. Regulates cell growth and cell division.
4. Transfers hereditary information to the next generation.
5. Essential for cell reproduction—cells without a nucleus cannot divide.

2. MITOCHONDRIA

Discovered by Albert von Kolliker.

Known as the “Powerhouse of the Cell” because they produce ATP (energy).

Structure

• Double membrane organelle.
• Inner membrane folded into cristae.
• Contains its own DNA, ribosomes, and enzymes.
• Found abundantly in energy-demanding tissues like muscle and liver.

Functions

1. Produces ATP through cellular respiration.
2. Helps regulate metabolism.
3. Plays an important role in cell death (apoptosis).
4. Detoxifies ammonia in liver cells.
5. Essential for hormone synthesis (testosterone, estrogen).
6. Helps in formation of certain blood components.

3. RIBOSOMES

Discovered by George E. Palade (1955).

Ribosomes are non-membrane organelles made of RNA and proteins.

Types

• 70S ribosomes – found in prokaryotes
• 80S ribosomes – found in eukaryotes

Functions

• Site of protein synthesis.
• Convert genetic code (mRNA) into amino acid chains using tRNA.
• Essential for formation of enzymes, hormones, and structural proteins.

4. ENDOPLASMIC RETICULUM (ER)

The ER is a network of membranes forming channels throughout the cell.
It provides transportation, storage, and manufacturing functions.

There are two types:

i. Rough Endoplasmic Reticulum (RER)

• Has ribosomes on its surface (appears rough).
• Involved in protein synthesis.
• Helps in protein folding and sorting.

ii. Smooth Endoplasmic Reticulum (SER)

• No ribosomes (appears smooth).
• Involved in:
1. Lipid, phospholipid, and cholesterol synthesis
2. Production of steroid hormones
3. Carbohydrate metabolism
4. Detoxification of drugs in liver cells
5. Storage and release of calcium ions in muscle cells

5. GOLGI BODIES (GOLGI APPARATUS)

Discovered by Camillo Golgi (1898).

Appears as a stack of flattened membranes called cisternae.

Two Faces

• Cis-face: Receiving side
• Trans-face: Shipping side (releases vesicles)

Functions

1. Packages and modifies proteins and lipids.
2. Forms lysosomes.
3. Produces glycoproteins and glycolipids.
4. Helps form complex carbohydrates.
5. Important in forming plant cell walls.

6. LYSOSOMES

Discovered by Christian de Duve (1955).

Known as “Suicide bags of the cell” because they contain powerful digestive enzymes.

Functions

1. Break down waste materials (autophagy).
2. Digest materials taken in from outside (heterophagy).
3. Destroy harmful substances.
4. Release enzymes outside the cell (exocytosis) when needed.

Lysosomes protect the cell by ensuring harmful substances are broken down safely.

7. CYTOSOL AND CYTOSKELETON

Cytosol

The fluid portion of the cytoplasm where chemical reactions occur.

Functions:

• Site of metabolic reactions.
• Helps transport molecules.
• Supports signal transduction pathways.

Cytoskeleton

A network of protein fibres providing structural support.

Types of Cytoskeleton Fibers

8. PEROXISOMES

Discovered by Christian de Duve.

Small, spherical membrane-bound organelles containing the enzyme catalase.

Functions

1. Break down hydrogen peroxide (H₂O₂) into water + oxygen.
2. Protect cells from oxidative damage.
3. Perform β-oxidation of fatty acids.
4. Important in detoxification reactions.

9. CENTROSOME AND CENTRIOLES

The centrosome is located near the nucleus and contains a pair of centrioles.

Functions

1. Organizes microtubules.
2. Forms the mitotic spindle during cell division.
3. Helps with chromosome movement during mitosis.

Centrioles are essential for maintaining the internal organization and support of the cell.

10. CELL EXTENSIONS

Some cells have structures that extend from the plasma membrane for movement or increasing surface area.

Types of Cell Extensions