The Fundamental Unit of Life

In 1665, Robert Hooke examined a thin slice of cork under a self-designed microscope and saw small box-like compartments, which he named cells (Latin for 'a small room'). What he actually saw were the dead cell walls of cork tissue. A few years later, in 1674, Anton van Leeuwenhoek used a better microscope and became the first person to see and describe a live cell — he observed free-living cells in pond water.

Later scientists added important pieces to this picture. Robert Brown discovered the nucleus inside cells in 1831. Purkinje named the fluid content of the cell protoplasm. Based on all this work, Schleiden and Schwann together proposed the Cell Theory, which states that all living organisms are made of cells and that the cell is the basic structural unit of life. Rudolf Virchow later expanded this theory by explaining that new cells arise from pre-existing cells through cell division.

Every function needed for life — obtaining nutrition, respiration, excretion, and reproduction — is carried out within cells or by cells working together. For example, a single-celled organism like Amoeba performs all life processes within one cell, while in a multicellular organism such as a human, trillions of cells group together to form tissues, which form organs, which form organ systems.

All cells can be sorted into two broad groups based on their internal organisation. In a prokaryotic cell, the nuclear material is not enclosed by a nuclear membrane; it lies directly in the cytoplasm as a poorly-defined region called the nucleoid. Prokaryotic cells also lack most membrane-bound cell organelles. Bacteria and blue-green algae (cyanobacteria) are common examples of prokaryotes, and they are generally very small, usually between 1 and 10 micrometres in size.

In a eukaryotic cell, the nuclear material is enclosed by a well-defined nuclear membrane, forming a true nucleus. Eukaryotic cells also possess membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus. Plants, animals, fungi, and protists (like Amoeba and Paramecium) are all made of eukaryotic cells, which are generally larger, ranging from about 5 to 100 micrometres.

For example, if a student looks at a stained smear of bacteria and a stained onion peel under a microscope, the bacterial cell will show no distinct nucleus, while the onion cell will clearly show a rounded, membrane-bound nucleus.

Every cell is enclosed by a thin covering called the plasma membrane (or cell membrane), made mainly of lipids and proteins. It is selectively permeable, meaning it allows the movement of some substances into and out of the cell while restricting others. This movement happens mainly by two processes: diffusion, where particles of a substance, such as a gas or liquid, move from a region of higher concentration to a region of lower concentration; and osmosis, the diffusion of water molecules through a selectively permeable membrane.

A simple example of osmosis is placing dried raisins in plain water — the raisins swell up as water enters them by osmosis. If the same raisins are instead placed in a concentrated sugar or salt solution, water moves out of the raisin, and it shrinks. When a plant cell is placed in a strong (hypertonic) salt or sugar solution, water leaves the cell by osmosis, and the cytoplasm shrinks away from the cell wall; this is called plasmolysis.

Besides the plasma membrane, plant cells, fungal cells, and bacterial cells also have a cell wall lying just outside the plasma membrane. In plant cells, the cell wall is mainly made of cellulose; in fungal cells it is made of chitin, and in bacterial cells it is made of a different chemical substance (peptidoglycan). The cell wall gives the cell a fixed shape and rigidity, and it protects the cell from bursting even when it absorbs a lot of water and becomes turgid, since the cell wall itself is a freely permeable layer that does not stop the passage of substances.

The nucleus is usually a dense, spherical structure found in the centre of a eukaryotic cell. It is separated from the cytoplasm by a double-layered, porous nuclear membrane, which allows the transfer of material between the nucleoplasm (fluid inside the nucleus) and the cytoplasm. Inside the nucleus lies a small, dense, spherical structure called the nucleolus, which is rich in RNA and protein and is involved in the formation of ribosomes, along with a network of thread-like structures known as chromatin.

Chromatin material is made up of DNA and protein. Just before a cell divides, this chromatin condenses to form rod-shaped structures called chromosomes. Chromosomes carry hereditary information from parents to offspring in the form of genes, which are functional segments of DNA. In this way, the nucleus not only controls the cell's activities but also plays a central role in inheritance.

A few cells lack a nucleus entirely — for example, the mature red blood cells of mammals have no nucleus. On the other hand, in organisms like Amoeba, the single nucleus directs and controls every cellular activity, showing how central the nucleus is to a cell's life.

The endoplasmic reticulum (ER) is a large network of membrane-bound tubes and sheets in the cytoplasm. Rough ER (RER) has ribosomes attached to its surface and helps in protein synthesis, while smooth ER (SER) has no ribosomes and helps in the synthesis of lipids (fats). The ER also helps transport materials within the cell and provides a surface for various chemical reactions.

The Golgi apparatus consists of a stack of membrane-bound, fluid-filled sacs. It receives materials from the endoplasmic reticulum, further processes and packages them, and then dispatches them to their proper destinations inside or outside the cell. It also forms lysosomes.

Mitochondria are rod-shaped or oval organelles enclosed by two membranes, with the inner membrane folded into structures called cristae. They are the site of aerobic respiration, where energy stored in food is released in a usable form as ATP, which is why mitochondria are called the powerhouse of the cell. Mitochondria have their own DNA and ribosomes, allowing them to make some of their own proteins.

Plastids are found only in plant cells and are of three types: chloroplasts, which are green because they contain chlorophyll and are the site of photosynthesis; chromoplasts, which give flowers and fruits their colours; and leucoplasts, which are colourless and store food such as starch, oil, or protein.

Vacuoles are storage sacs bound by a single membrane. Plant cells usually have one large central vacuole, which may occupy up to 90% of the cell's volume and is filled with cell sap; this vacuole helps maintain turgidity and rigidity of the cell. Animal cells have many small vacuoles instead. In Amoeba, a special contractile vacuole helps expel excess water and waste products from the cell, while a food vacuole helps digest food.

Lysosomes are membrane-bound sacs filled with digestive enzymes, formed by the Golgi apparatus. They break down worn-out cell parts, foreign material, and even bacteria. If a cell is damaged, lysosomes may burst and digest the cell's own components, which is why lysosomes are called the suicide bags of the cell.