The Fundamental Unit of Life

Robert Hooke discovered cells in 1665.

He observed thin slices of cork (the outer layer of the bark of an oak tree) under a self-designed compound microscope. He noticed that the cork was made up of tiny, box-like compartments that reminded him of small rooms in a monastery. He called these compartments "cells" (from the Latin word cellula, meaning "small room").

He published his observations in his famous book Micrographia (1665).

It is important to note that what Hooke saw were actually dead cell walls of the cork cells (no living contents were visible at that time). The living contents (cytoplasm, nucleus, etc.) were later studied by others.

The Cell Theory was later formally proposed by Schleiden (1838) and Schwann (1839), and further extended by Virchow (1855) who stated that all cells arise from pre-existing cells.

The cell is called the structural and functional unit of life for the following reasons:

Structural unit: The cell is the basic building block of all living organisms. All living things — whether unicellular (like bacteria, amoeba) or multicellular (like plants, animals, humans) — are made of cells. The body structure of every organism is organised at the cellular level. Even the most complex organisms are built from cells.

Functional unit: All life processes (functions) such as:

• Respiration — energy production occurs in cells (mitochondria)

• Digestion — enzymes are produced in cells

• Reproduction — occurs at the cellular level (cell division)

• Protein synthesis — occurs in ribosomes within cells

• Excretion — waste is removed at the cellular level

are all carried out within cells. Even a single cell (like a bacterium or amoeba) can perform all the functions of life independently. This is why the cell is the fundamental unit of both structure and function in living organisms.

Substances move in and out of the cell through the plasma membrane by the following processes:

(i) CO₂ — by Diffusion:

CO₂ is produced during cellular respiration inside the cell. The concentration of CO₂ inside the cell becomes higher than outside. CO₂ moves from the region of higher concentration (inside the cell) to the region of lower concentration (outside) across the plasma membrane by simple diffusion.

Similarly, O₂ (needed for respiration) diffuses into the cell from the surrounding fluid where its concentration is higher.

(ii) Water — by Osmosis:

Osmosis is the movement of water (solvent) molecules through a selectively permeable membrane from a region of higher water concentration (lower solute concentration, i.e., hypotonic solution) to a region of lower water concentration (higher solute concentration, i.e., hypertonic solution), until equilibrium is reached.

The plasma membrane is selectively permeable — it allows water to pass freely but controls the movement of solutes. The net direction of water movement depends on the relative concentrations of solutes inside and outside the cell.

The plasma membrane is called a selectively permeable membrane because it does not allow all substances to pass through freely — it selects which molecules can enter or leave the cell.

The plasma membrane allows:

Free passage to: Water (H₂O), oxygen (O₂), carbon dioxide (CO₂), and small uncharged molecules can pass through easily by simple diffusion or osmosis.

Controlled or restricted passage to: Ions (Na⁺, K⁺, Ca²⁺), glucose, amino acids, and large molecules — these need carrier proteins or energy-dependent processes (active transport).

No passage to: Large molecules like starch and proteins generally cannot cross the plasma membrane directly.

This selective permeability is essential for:

1. Maintaining the internal environment of the cell (homeostasis).

2. Regulating concentrations of ions and nutrients.

3. Removing waste products.

4. Allowing communication and signaling between cells.

The selective permeability is primarily determined by the lipid bilayer structure of the membrane and the specific protein channels and carriers embedded in it.

The two organelles that contain their own genetic material (DNA) and ribosomes are:

(i) Mitochondria:

Mitochondria contain their own circular DNA (mtDNA) and 70S ribosomes. They can synthesize some of their own proteins independently. This supports the endosymbiotic theory, which suggests that mitochondria evolved from ancient free-living bacteria that were engulfed by larger cells.

(ii) Chloroplasts (Plastids):

Chloroplasts (found in plant cells and algae) also contain their own circular DNA and 70S ribosomes. They too can synthesize some proteins on their own. Like mitochondria, they are also thought to have originated from ancient photosynthetic bacteria (cyanobacteria) through endosymbiosis.

Both organelles are capable of self-replication (dividing by binary fission, similar to bacteria) independently of the cell cycle.

If the organisation of a cell is destroyed due to some physical or chemical influence, the following will happen:

1. Lysosome rupture (Autolysis): When the cell's organisation is disrupted, the membranes of lysosomes (organelles containing powerful digestive enzymes) break down. The enzymes — including hydrolases, lipases, proteases, and nucleases — are released into the cytoplasm.

2. Self-digestion: These released enzymes begin to digest all the components of the cell — proteins, lipids, carbohydrates, and nucleic acids. This process is called autolysis (self-digestion).

3. Cell death: The cell loses its ability to carry out any life functions. All metabolic activities cease — no respiration, no protein synthesis, no transport. The cell ultimately dies.

This is why lysosomes are called the "suicide bags" of the cell — when triggered, they destroy the very cell they belong to.

In a broader context, this process (programmed cell death or apoptosis) is also used by the body intentionally — for example, to remove damaged cells, shape organs during development, or eliminate infected cells.

Lysosomes are known as "suicide bags" of the cell because they contain powerful hydrolytic (digestive) enzymes that can break down all types of biological molecules — proteins, carbohydrates, lipids, and nucleic acids.

Under normal conditions, these enzymes are safely enclosed within the lysosome's membrane. However, when a cell is:

• Damaged or injured

• Old and no longer needed

• Infected by a virus or bacteria

• Exposed to harmful chemicals or physical damage

the lysosome membrane breaks down and the digestive enzymes are released into the cytoplasm. These enzymes then digest all the cellular contents, leading to the complete breakdown and death of the cell.

This self-destructive process is called autolysis. Because the lysosome can destroy its own cell, it is fittingly called the "suicide bag."

Lysosomes also play important constructive roles: they digest worn-out organelles (autophagy), destroy foreign particles and bacteria (during phagocytosis by white blood cells), and are involved in bone remodeling.

Proteins are synthesised in ribosomes. Ribosomes are the sites of protein synthesis in all cells (both prokaryotic and eukaryotic).

In eukaryotic cells, ribosomes are found in two locations:

(i) Free ribosomes: Ribosomes that float freely in the cytoplasm. They synthesize proteins that are used within the cell (e.g., enzymes for metabolic reactions, structural proteins).

(ii) Ribosomes attached to the Rough Endoplasmic Reticulum (RER): These ribosomes synthesize proteins that are destined for:

• Secretion outside the cell (e.g., digestive enzymes, hormones like insulin)

• Incorporation into cell membranes

• Packaging in lysosomes

The RER transports these newly synthesized proteins to the Golgi apparatus for further processing and packaging.

Ribosomes are composed of two subunits (ribosomal RNA + proteins) and are the smallest organelles in the cell. In prokaryotes and in mitochondria/chloroplasts, the ribosomes are 70S type; in the cytoplasm of eukaryotes, they are 80S type.

Plant Cells have (Animal Cells lack):

1. Cell Wall: Plant cells have a rigid cell wall made of cellulose outside the plasma membrane. Animal cells have no cell wall (only plasma membrane).

2. Plastids (including Chloroplasts): Plant cells contain plastids. Chloroplasts (green plastids) contain chlorophyll and are the sites of photosynthesis. Animal cells lack plastids entirely.

3. Large Central Vacuole: Plant cells typically have one large central vacuole that can occupy up to 90% of the cell volume. It stores cell sap and maintains turgidity. Animal cells have small, temporary vacuoles if any.

4. Absence of Centrioles: Most plant cells lack centrioles. Animal cells have centrioles, which are important in cell division (forming the mitotic spindle).

Animal Cells have (Plant Cells usually lack):

1. Centrioles: Present in animal cells; absent in most plant cells.

2. Lysosomes: More prominent in animal cells. Plant cells have vacuoles that may perform similar functions.

3. Irregular shape: Animal cells tend to be round/irregular in shape. Plant cells are more regular/rectangular due to the rigid cell wall.

Both plant and animal cells have: Nucleus, mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, plasma membrane, and cytoplasm.

Amoeba obtains its food by a process called endocytosis, specifically phagocytosis ("cell eating").

The process occurs as follows:

1. Detection: When Amoeba detects a food particle (like bacteria, algae, or other microorganisms) nearby, it responds to it.

2. Pseudopod formation: The flexible plasma membrane of Amoeba extends outward forming finger-like projections called pseudopodia (meaning "false feet") around the food particle.

3. Engulfment: The pseudopodia surround and engulf the food particle completely, enclosing it within a membrane-bound vesicle called a food vacuole.

4. Digestion: Lysosomes fuse with the food vacuole and release digestive enzymes that break down the food into simpler substances (nutrients).

5. Absorption: The digested nutrients are absorbed into the cytoplasm and used for energy and growth.

6. Egestion: Undigested material is expelled out of the cell when the food vacuole fuses with the plasma membrane (exocytosis).

This ability to engulf food particles is possible because Amoeba has a very flexible plasma membrane (no cell wall).

Osmosis is the movement of water (solvent) molecules from a region of higher water concentration (lower solute concentration — hypotonic solution) to a region of lower water concentration (higher solute concentration — hypertonic solution) through a selectively permeable membrane, until equilibrium is established.

The driving force for osmosis is the difference in water potential (osmotic potential) across the membrane. Water always moves "down" its concentration gradient.

Example: When a raisin (dried grape) is placed in water, it swells up and becomes plump. The raisin contains a concentrated sugar solution inside its cells. The surrounding water is relatively less concentrated (more water molecules). Water moves by osmosis from outside (higher water concentration) into the raisin cells (lower water concentration) through the selectively permeable cell membrane.

Types of osmosis in cells:

Endosmosis: When a cell is placed in a hypotonic solution (more dilute than cell contents), water enters the cell. The cell swells up. In plants, this creates turgor pressure. Plant cells may become turgid.

Exosmosis: When a cell is placed in a hypertonic solution (more concentrated than cell contents), water leaves the cell. The cell shrinks. In plant cells, the cytoplasm shrinks away from the cell wall — this is called plasmolysis.

Observations after two hours:

Cup B and Cup C (raw potato + sugar):

Sugar creates a concentrated (hypertonic) solution inside the potato cups. The surrounding tray water is hypotonic (more dilute). Water moves by osmosis from the tray (high water concentration) into the potato cups (low water concentration). The water level inside the cups rises. The potato cups may also become slightly firmer (turgid).

Cup D (raw potato + water added):

Water is added to the cup. The concentration inside and outside the potato is approximately equal (both have relatively low solute concentration). There is little or no net osmosis. The water level remains approximately the same or changes very little.

Cup A (boiled potato + sugar):

The boiled potato's cells have been killed by heat. The plasma membranes are denatured and damaged — they are no longer selectively permeable. Since the membrane required for osmosis is not functioning, no osmosis occurs. The water level inside cup A does not rise significantly, even though sugar is present. This demonstrates that a living, functional selectively permeable membrane is essential for osmosis.

Conclusion: Osmosis requires a living selectively permeable membrane. The experiment demonstrates that water moves from regions of low solute concentration to high solute concentration across such membranes.

The Golgi apparatus (also called Golgi body or Golgi complex) was discovered by Camillo Golgi in 1898. It consists of a stack of flattened membrane-bound sacs (cisternae) and associated vesicles.

Functions of the Golgi apparatus:

1. Processing and modification of proteins and lipids: Proteins synthesized by ribosomes on the RER arrive at the Golgi apparatus, where they are modified by adding sugar chains (glycosylation), phosphate groups, or other chemical modifications.

2. Packaging and sorting: The Golgi packages the processed molecules into membrane-bound vesicles and sorts them, directing them to their correct destinations — the plasma membrane (for secretion), lysosomes, or other compartments.

3. Secretion: It is the major site from which secretory vesicles bud off and move to the cell surface, releasing their contents outside the cell (exocytosis). Example: digestive enzymes, hormones, antibodies.

4. Formation of lysosomes: Lysosomes are formed by budding from the Golgi apparatus. The Golgi packages digestive enzymes into lysosomes.

5. Synthesis of complex carbohydrates: The Golgi synthesizes polysaccharides like pectin and hemicellulose (used in plant cell walls).

The Golgi apparatus is often described as the "post office" or "packaging and dispatch centre" of the cell.

Mitochondria (singular: mitochondrion) is known as the "powerhouse of the cell".

Reason: Mitochondria are the sites of cellular respiration, where food (primarily glucose) is oxidised to produce ATP (adenosine triphosphate) — the energy currency of the cell.

The overall process:

ATP stores chemical energy in its high-energy phosphate bonds. When the cell needs energy for any activity (movement, transport, synthesis, etc.), it breaks down ATP to release the energy:

Mitochondria produce the bulk of ATP (through oxidative phosphorylation in the inner membrane) that powers virtually all cellular activities.

Structure: Mitochondria have a double membrane — an outer smooth membrane and a highly folded inner membrane (cristae). The folds (cristae) increase the surface area for ATP synthesis. The matrix (inner fluid) contains enzymes for the Krebs cycle.

Since they generate most of the cell's energy supply, mitochondria are aptly called the powerhouse of the cell.

Prokaryotic Cells:

1. Nucleus: No membrane-bound nucleus. The genetic material (DNA) is present in an irregular region called the nucleoid (not enclosed by a nuclear membrane).

2. Size: Generally smaller — 1–10 μm in diameter.

3. Membrane-bound organelles: Absent (no mitochondria, ER, Golgi, etc.).

4. Ribosomes: Smaller 70S ribosomes present free in cytoplasm.

5. Cell division: By binary fission (simple splitting).

6. Cell wall: Present (made of peptidoglycan in bacteria).

7. DNA: Single, circular DNA molecule. No histones.

8. Examples: Bacteria (e.g., E. coli, Staphylococcus) and Blue-green algae (Cyanobacteria).

Eukaryotic Cells:

1. Nucleus: True membrane-bound nucleus present. DNA is enclosed within a nuclear envelope (double membrane).

2. Size: Generally larger — 10–100 μm in diameter.

3. Membrane-bound organelles: Present — mitochondria, ER, Golgi apparatus, lysosomes, vacuoles, etc.

4. Ribosomes: Larger 80S ribosomes (in cytoplasm); 70S ribosomes in mitochondria and chloroplasts.

5. Cell division: By mitosis or meiosis.

6. Cell wall: Present in plant cells (cellulose) and fungi (chitin); absent in animal cells.

7. DNA: Multiple linear chromosomes with histone proteins.

8. Examples: Plant cells, animal cells, fungi, protists (e.g., Amoeba, Paramecium).