The Fundamental Unit of Life

1. Discovery of the cell

The word cell was first used by Robert Hooke in 1665. Looking at a thin slice of cork (the dead bark of an oak tree) under a crude compound microscope he had built himself, Hooke saw small, box-like compartments that reminded him of the tiny rooms (cellulae) of a monastery — he named them cells. He published his findings in Micrographia. Cork cells are dead and empty; what Hooke actually observed were the rigid cellulose cell walls left behind after the living contents had dried out.

Anton van Leeuwenhoek (1674), a Dutch lens-grinder, improved the microscope to magnifications of over 200x and became the first person to observe living cells. He saw free-living organisms in pond water and described red blood cells and bacteria — organisms he called "animalcules."

Robert Brown (1831) discovered the nucleus while examining orchid root cells. Purkinje (1839) coined the term protoplasm for the living, jelly-like material inside cells. These discoveries set the stage for a unified theory.

2. Cell theory

By 1838 a large body of microscopic observations had accumulated. Two scientists synthesised them into a single theory:

• Matthias Schleiden (1838), a German botanist, concluded that all plants are made of cells.
• Theodor Schwann (1839), a German zoologist, extended this to animals: all animals too are made of cells, and the cell is the basic structural unit of life.
• Rudolf Virchow (1855) added the third, crucial pillar: Omnis cellula e cellula — new cells arise only from pre-existing cells, not from non-living matter.

Together, the modern cell theory states: (i) all living organisms are composed of cells and products of cells; (ii) the cell is the basic unit of structure and function; (iii) all cells arise from pre-existing cells.

Why it matters: the cell theory unified biology. It explained how life maintains continuity (cells divide to make new cells), how organisms grow (by adding more cells), and where diseases begin (in malfunctioning cells).

3. Prokaryotic vs eukaryotic cells

All cells can be placed into one of two categories based on the organisation of their genetic material:

Key point: prokaryotes are among the oldest life forms on Earth and include many important bacteria (both harmful and beneficial). Despite lacking a true nucleus, they carry out all essential life processes.

4. Plant cell vs animal cell

Both are eukaryotic, but they differ in several important ways:

Remember: both have plasma membrane, nucleus, mitochondria, ER, Golgi apparatus and ribosomes in common.

5. Cell membrane — the gatekeeper

The plasma membrane (cell membrane) is the outermost living boundary of all cells (both plant and animal). It is an extremely thin (about 7–8 nm) flexible sheet made of a phospholipid bilayer with proteins embedded in it — this is described as the fluid mosaic model (Singer and Nicolson, 1972). The "fluid" part means the lipids and proteins can move laterally; the "mosaic" part refers to the mosaic-like pattern of proteins in the lipid sea.

The plasma membrane is selectively permeable — it allows some substances to pass freely, restricts others, and blocks others completely. This lets the cell control its internal environment.

Diffusion: movement of a substance (like CO2 or O2) from a region of higher concentration to a region of lower concentration across the membrane. No energy is needed. Example: O2 diffuses into cells during respiration; CO2 diffuses out.

Osmosis: the movement of water molecules specifically, across a selectively permeable membrane, from a region of higher water concentration (dilute solution) to a region of lower water concentration (concentrated solution). Three situations:

• Hypotonic solution (less solute outside): water moves into the cell — cell swells. In plant cells this creates turgor pressure; the cell becomes turgid. In animal cells the cell may burst (cytolysis).
• Isotonic solution (equal solute concentration): no net movement of water — cell remains unchanged.
• Hypertonic solution (more solute outside): water moves out of the cell — cell shrinks. In plant cells the contents shrink away from the wall — this is called plasmolysis. In animal cells the cell crenates.

Endocytosis: the plasma membrane can engulf large particles by folding around them. Amoeba uses this to engulf food particles. It shows the flexible nature of the membrane.

6. Cell wall — the armour of plant cells

Found in plant cells, fungi, and bacteria (absent in animal cells). In plants the cell wall is made primarily of cellulose, a polysaccharide. It lies outside the plasma membrane.

Functions:

• Gives the cell a definite shape and structural rigidity.
• Provides mechanical strength and protection against mechanical damage and infection.
• Allows plant cells to withstand wide changes in the surrounding medium without bursting (because even when the cell becomes turgid, the wall resists further expansion).
• Permits cell-to-cell communication through plasmodesmata — tiny channels that pass through adjacent cell walls.

The cell wall is fully permeable — it does not select what passes through. It is also non-living. It is the plasma membrane just inside that acts as the selective barrier.

When a plant cell is placed in a hypertonic solution, the plasma membrane and cytoplasm pull away from the cell wall — this is plasmolysis. The space between the membrane and wall fills with the external solution.

7. Nucleus — the control centre

The nucleus is the most prominent organelle of a eukaryotic cell, usually spherical and located near the centre. It is bounded by the nuclear envelope — a double membrane with small openings called nuclear pores through which molecules move between the nucleus and cytoplasm.

Contents of the nucleus:

• Nucleoplasm: the fluid filling inside the nucleus.
• Nucleolus: a dense, rounded body inside the nucleus (there may be one or more). It is the site of ribosome synthesis and is rich in RNA. It disappears during cell division.
• Chromatin: long, thread-like strands of DNA wound around histone proteins. During cell division chromatin condenses into distinct rod-shaped chromosomes. Each chromosome carries many genes — specific segments of DNA that code for proteins and determine inherited traits.

Functions of nucleus:

• Acts as the control centre — directs all metabolic activities of the cell.
• Contains the hereditary information (DNA) that is passed from parent to offspring.
• Controls cell division and the synthesis of proteins that carry out all cell functions.

In prokaryotes, there is no nuclear membrane; the DNA lies in a region of the cytoplasm called the nucleoid. Prokaryotes may also have extra DNA loops called plasmids.

8. Cytoplasm and cytosol

Cytoplasm is the fluid-filled space inside the plasma membrane, excluding the nucleus. It is sometimes called the "ground substance" of the cell. The fluid part of the cytoplasm (the matrix in which organelles are suspended) is the cytosol — a gel-like mixture of water, salts, enzymes, and organic molecules.

Functions of cytoplasm:

• Serves as the site of many metabolic reactions (e.g., glycolysis — the first stage of respiration — occurs in the cytosol).
• Provides a medium for organelles to be suspended and move within the cell.
• Transports nutrients and waste products within the cell by cytoplasmic streaming (cyclosis).

All membrane-bound organelles are embedded in the cytoplasm. The cytoplasm together with the nucleus is called protoplasm — the "living matter" of the cell (a term coined by Purkinje, 1839).

9. Mitochondria — the powerhouse

Mitochondria (singular: mitochondrion) are rod-shaped or oval organelles found in the cytoplasm of all eukaryotic cells. They are called the powerhouse of the cell because they produce most of the cell's energy.

Structure: double-membrane organelle. The outer membrane is smooth; the inner membrane is folded into finger-like projections called cristae, which greatly increase the surface area for chemical reactions. The fluid inside the inner membrane is the matrix.

Function: Mitochondria carry out aerobic respiration — they break down glucose and other fuel molecules using oxygen to produce ATP (adenosine triphosphate), the universal energy currency of cells. ATP is used for every energy-requiring activity: muscle contraction, nerve impulse, biosynthesis, active transport.

Why they are special:

• They have their own circular DNA and their own ribosomes (70S, like bacterial ribosomes).
• They can make some of their own proteins and can self-replicate (divide independently of the cell).
• This is evidence for the endosymbiont theory — ancient free-living bacteria were engulfed by early cells and became mitochondria.

Cells with high energy demands (muscle cells, liver cells) have very large numbers of mitochondria.

10. Endoplasmic reticulum (ER)

The endoplasmic reticulum is an extensive network of interconnected membranous tubes and flat sacs (cisternae) that extends from the nuclear envelope to the plasma membrane — essentially the cell's internal highway system.

There are two types:

• Rough Endoplasmic Reticulum (RER): studded with ribosomes on its outer surface, giving it a "rough" appearance. RER is the main site of protein synthesis and processing. Proteins made here are often destined for export from the cell or for use in membranes.
• Smooth Endoplasmic Reticulum (SER): lacks ribosomes and has a smooth appearance. SER is the site of lipid (fat) synthesis — it makes cell membrane phospholipids, steroid hormones, and oils. It also helps detoxify drugs and poisons (especially in liver cells) and stores calcium ions in muscle cells.

Functions of ER:

• Serves as the intracellular transport system — carries proteins and lipids from their site of synthesis to the Golgi apparatus.
• Helps in the formation of the nuclear envelope during cell division.
• Forms vacuoles in some cells.

11. Golgi apparatus — the packaging and dispatch unit

Discovered by the Italian scientist Camillo Golgi (1898), the Golgi apparatus consists of a stack of flat, membrane-bound sacs called cisternae arranged in a parallel series, often near the nucleus.

Functions:

• Receives proteins and lipids from the ER (brought in small transport vesicles).
• Modifies and processes them — for example, adding sugar groups to proteins (glycosylation).
• Sorts and packages processed materials into vesicles (membrane-bound sacs).
• Dispatches the vesicles to their destinations: to the plasma membrane for secretion outside the cell, to other organelles, or to become lysosomes.
• Plays a key role in the formation of lysosomes.
• Synthesises the components of the cell wall in plant cells.

12. Lysosomes — the suicidal bags

Lysosomes are small, spherical, single-membrane organelles that contain a variety of powerful digestive (hydrolytic) enzymes capable of breaking down all types of biological macromolecules — proteins, lipids, carbohydrates and nucleic acids. They are formed by the Golgi apparatus.

Functions:

• Intracellular digestion: they fuse with food vacuoles (formed by endocytosis) and digest ingested food particles. This is how cells like Amoeba digest food.
• Autophagy: they digest and recycle worn-out, damaged or excess organelles and cellular debris, keeping the cell clean.
• Defence: in white blood cells, lysosomes destroy bacteria and other foreign particles that enter the body.

Why "suicidal bags"? When a cell is severely damaged or at the end of its life, lysosomes may burst open and release their enzymes into the cytoplasm, digesting the entire cell's contents and causing cell death. This "self-destruction" function is why lysosomes are famously called the suicide bags of the cell. This process (autolysis) is also important in controlled cell death during normal development (e.g., webbing between fingers disappears in a human foetus).

The enzymes in lysosomes work best at an acidic pH (about 4.8), which is maintained by pumping H+ ions into the lysosome. If the lysosome ruptures, the cytoplasm (pH ~7.2) inactivates most enzymes, limiting accidental damage.

13. Vacuoles — storage chambers

Vacuoles are membrane-bound spaces in the cytoplasm filled with fluid or solid contents. The membrane surrounding a vacuole is called the tonoplast.

In plant cells:

• There is typically one large central vacuole that can occupy 50–90% of the total cell volume in a mature plant cell.
• It is filled with cell sap — a solution of salts, sugars, amino acids, pigments and waste products.
• It helps maintain turgor pressure — the pressure that keeps plant cells firm and gives the plant its structural support (wilting occurs when vacuoles lose water).
• Stores metabolic wastes and toxic substances, keeping them away from the cytoplasm.
• May store pigments (e.g., the blue-purple colour in beetroot cells is pigment in the vacuole).

In animal cells:

• Vacuoles are small and temporary, formed during processes like endocytosis (food vacuoles) or storing water temporarily.
• There is no permanent large central vacuole.

In unicellular organisms like Amoeba, food vacuoles form when the cell engulfs food; contractile vacuoles in Paramecium remove excess water (osmoregulation).

14. Plastids — the colour-and-food makers (plant cells only)

Plastids are double-membrane organelles found only in plant cells and algae. They are absent in animal cells. Like mitochondria, plastids have their own DNA and ribosomes, and they can self-replicate.

There are three main types of plastids:

• Chloroplasts: the most important plastid. They are green because they contain the pigment chlorophyll (and other pigments like carotenoids). Chloroplasts are the sites of photosynthesis — the process by which plants convert light energy + CO2 + water into glucose and oxygen. Inside the chloroplast are stacks of membrane sacs called grana (singular: granum) connected by membranes called stroma lamellae; the fluid between them is the stroma. Light reactions occur in the grana; dark reactions (Calvin cycle) occur in the stroma.
• Chromoplasts: contain coloured pigments other than chlorophyll — carotenoids (yellow, orange, red). They give bright colours to flowers and fruits, attracting pollinators and seed dispersers. Chloroplasts can convert into chromoplasts as fruits ripen (tomatoes turn from green to red).
• Leucoplasts: colourless plastids found in parts of plants not exposed to light (roots, seeds, underground storage organs). They store food reserves: amyloplasts store starch, elaioplasts store oils and fats, aleuroplasts store proteins.

Plastids and mitochondria both having their own DNA supports the theory that they originated from ancient free-living prokaryotes that entered into a symbiotic relationship with early eukaryotic cells.

15. Centrosome — the division organiser (animal cells)

The centrosome is a small organelle found in animal cells (and in lower plants) but absent in most higher plant cells. It is located near the nucleus. Each centrosome contains two barrel-shaped structures called centrioles arranged at right angles to each other.

Function: during cell division (mitosis and meiosis), the centrosome organises the formation of the spindle fibres — the network of fibres that pulls chromosomes to opposite ends of the cell so that each daughter cell gets the correct number of chromosomes. Without centrioles, animal cells cannot divide normally.

Higher plant cells manage cell division without centrioles, using different mechanisms to organise the spindle, which is why centrosomes are absent in them.