Cell - The Unit of Life

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 8.

Why is the cell the smallest thing that counts as alive?

A mitochondrion can make energy. A chloroplast can trap light. A nucleus holds the instructions. But take any one of them out of the cell and it cannot live on its own. Only a complete cell can do everything life needs: take in food, release energy, build its own parts, respond and reproduce. That is the central idea of this chapter, and the rest is a tour of what a cell is made of.

How we found the cell

Antonie von Leeuwenhoek was the first to see and describe a live cell, and Robert Brown later discovered the nucleus. The theory that ties it all together came from three people.

The modern cell theory therefore says two things: all living organisms are made of cells and their products, and all cells arise from pre-existing cells. Notice how Virchow's step mattered. Without it, people could still believe cells formed spontaneously from non-living matter.

How big, and what shape?

Cells differ hugely in size. Mycoplasma are the smallest cells, about 0.3 µm. Bacteria are 3 to 5 µm, a human red blood cell is about 7 µm across, the ostrich egg is the largest isolated single cell, and nerve cells can be among the longest. Shapes follow function: disc-like, polygonal, columnar, cuboidal, thread-like or irregular.

Prokaryotic cells: complete, but without a nucleus

Prokaryotes are bacteria, blue-green algae (cyanobacteria), mycoplasma and PPLO (pleuro pneumonia like organisms). Bacteria come in four shapes: bacillus (rod), coccus (sphere), vibrio (comma) and spirillum (spiral). A prokaryotic cell has a cell wall around the membrane (except mycoplasma), a cytoplasm, and no membrane-bound nucleus or organelles except ribosomes.

The genetic material is naked: a single circular DNA lying in the cytoplasm, not enveloped by a nuclear membrane. Many bacteria also carry small extra circular DNA pieces called plasmids, which give them extra traits such as antibiotic resistance.

The cell envelope. Most bacteria are wrapped in three layers that act as one protective unit: the outermost glycocalyx, then the cell wall, then the plasma membrane. The glycocalyx is a loose slime layer in some bacteria and a thick, tough capsule in others. The cell wall gives the cell its shape and strength and stops it bursting or collapsing. Bacteria that take up the Gram stain are Gram positive, and those that do not are Gram negative.

Beyond the textbook. For the exam: Gram positive bacteria retain the stain, Gram negative do not, and the difference lies in the cell envelope. In reality: Gram positive bacteria have a thick wall of peptidoglycan, while Gram negative bacteria have a thin peptidoglycan layer plus an extra outer membrane. Why it matters: it is why some antibiotics work well on one group and poorly on the other.

Mesosomes and chromatophores. A mesosome is a folding of the plasma membrane into vesicles, tubules or lamellae. It helps in cell wall formation, DNA replication and distribution to daughter cells, respiration and secretion, and it increases surface area and enzyme content. Chromatophores are pigment-containing membranous extensions in cyanobacteria.

Beyond the textbook. For the exam: mesosomes are real structures with the jobs listed above. In reality: many microbiologists now think mesosomes are artefacts, folds produced when cells are chemically fixed for electron microscopy, rather than structures present in living cells. Why it matters: the functions in your textbook belong to the membrane folds as the textbook describes them, so answer with those functions in the exam and expect to meet this debate in later study.

Moving and sticking. A bacterial flagellum has three parts: the basal body (anchored in the envelope), the hook, and the filament, the longest part that extends outward. Pili are elongated tubular protein structures, and fimbriae are small bristle-like fibres. Neither helps movement. Fimbriae help bacteria stick to rocks in streams and to host tissues.

Inside the cytoplasm. Prokaryotic ribosomes are 70S (50S plus 30S subunits), about 15 nm by 20 nm, and are where proteins are made. Several ribosomes on one mRNA form a polyribosome (polysome). Inclusion bodies are reserve materials that lie free in the cytoplasm without a membrane: phosphate granules, cyanophycean granules and glycogen granules. Gas vacuoles occur in blue-green, purple and green photosynthetic bacteria.

Eukaryotic cells: compartments everywhere

Eukaryotic cells have a membrane-bound nucleus and membrane-bound organelles, which divide the cytoplasm into compartments. A compartment lets the cell run incompatible reactions side by side, such as digestion in one place and synthesis in another. The two cell types compare like this.

Plant and animal cells differ in a few classic ways.

The plasma membrane: a fluid mosaic

The membrane is made of phospholipids arranged in a bilayer, with the polar (water-loving) heads facing outwards and the non-polar (water-fearing) tails pointing inwards, plus cholesterol. Proteins are embedded in it: integral proteins are partly or totally buried, and peripheral proteins sit on the surface. In 1972, Singer and Nicolson proposed the fluid mosaic model. The lipid bilayer is quasi-fluid, so proteins can move sideways, and that fluidity matters for cell growth, secretion, endocytosis and cell division. In the human red blood cell, the membrane is about 52% protein and 40% lipid.

Things cross the membrane in three ways. Passive transport needs no energy: small neutral molecules move by simple diffusion, water moves by osmosis, and polar molecules need a carrier protein because they cannot pass through the non-polar lipid core. Active transport uses ATP to move ions or molecules against a concentration gradient, as the Na⁺/K⁺ pump does.

The cell wall (plants, algae, fungi)

The wall is a non-living, rigid layer that gives the cell shape, protects it and acts as a barrier to unwanted large molecules. In algae it contains cellulose, galactans, mannans and minerals such as calcium carbonate. In other plants it is cellulose, hemicellulose, pectins and proteins. In fungi it is made of chitin. A young cell has a growing primary wall; as the cell matures, a secondary wall is laid down on the inside. The middle lamella is a layer of calcium pectate that glues neighbouring cells together. Plasmodesmata pass through the wall and middle lamella and connect the cytoplasm of adjacent cells.

The endomembrane system: one coordinated factory

The endomembrane system includes the endoplasmic reticulum (ER), the Golgi apparatus, lysosomes and vacuoles. These organelles work together, passing material by vesicles. Mitochondria, chloroplasts and peroxisomes are not part of it, because their functions are not coordinated with the others.

The ER is a network of tubular structures dividing the cell into a luminal and an extra-luminal compartment. Rough ER (RER) has ribosomes on its outer surface, makes proteins for secretion, and is continuous with the outer nuclear membrane. Smooth ER (SER) has none, and it is the major site of lipid synthesis; in animal cells it makes steroid hormones.

Pause and predict: a cell that secretes large amounts of a steroid hormone and another that secretes digestive enzymes. Which has more smooth ER, and which has more rough ER?

Check your answer: the steroid-secreting cell is rich in smooth ER, because steroids are lipids. The enzyme-secreting cell is rich in rough ER, because enzymes are proteins made on ribosomes.

The Golgi apparatus, discovered by Camillo Golgi in 1898, is a stack of flat, disc-shaped cisternae, 0.5 to 1.0 µm across, arranged near the nucleus. It has two faces: the convex cis (forming) face, which receives vesicles from the ER, and the concave trans (maturing) face, which sends them out. The Golgi's job is packaging, and it is also where glycoproteins and glycolipids are formed.

Lysosomes are membrane-bound vesicles made by the Golgi. They are full of hydrolytic enzymes (lipases, proteases and carbohydrases) that work best at acidic pH and digest carbohydrates, proteins, lipids and nucleic acids.

Vacuoles are membrane-bound spaces holding water, sap, waste and other useless materials. A single membrane, the tonoplast, bounds the vacuole, and in plants it pumps ions in against a gradient. Amoeba has a contractile vacuole for excretion and osmoregulation, and many protists form food vacuoles by engulfing food.

Mitochondria and plastids: the energy organelles

Mitochondria are usually sausage shaped, 0.2 to 1.0 µm in diameter (average 0.5 µm) and 1.0 to 4.1 µm long. Each is bound by two membranes. The outer membrane is a smooth limiting boundary, and the inner membrane folds inwards into cristae that face the matrix and greatly increase the surface area. The mitochondrion is the site of aerobic respiration, making ATP, so it is called the power house of the cell. The matrix holds a single circular DNA molecule, some RNA, and 70S ribosomes, and mitochondria divide by fission.

Plastids are found in plant cells and euglenoids. They are classified by pigment.

A chloroplast is 5 to 10 µm long and 2 to 4 µm wide. Chlamydomonas has a single large chloroplast, while a mesophyll cell has 20 to 40. The chloroplast is bound by two membranes, and the space inside the inner membrane is the stroma, which contains enzymes for making carbohydrates and proteins, a small circular DNA and 70S ribosomes. Embedded in the stroma are flat membranous sacs called thylakoids, stacked like coins into grana and joined by stroma lamellae. Chlorophyll sits in the thylakoids.

Beyond the textbook. For the exam: learn the table above, including double membrane, circular DNA and 70S ribosomes. In reality: those three features are the main clues that mitochondria and chloroplasts descend from free-living bacteria that were engulfed by an ancestral cell, an idea called endosymbiosis. Why it matters: it turns a list of facts into one story, and it explains why 70S ribosomes appear in these two organelles of an 80S cell.

Pause and predict: a researcher finds a cell whose organelle has a double membrane, circular DNA and 70S ribosomes, but no thylakoids. Is it a chloroplast or a mitochondrion?

Check your answer: a mitochondrion. Thylakoids are exclusive to chloroplasts, so their absence points to a mitochondrion.

Here is the classic match-the-column set.

Ribosomes, cytoskeleton, cilia and centrioles

Ribosomes, first seen by George Palade in 1953, are made of RNA and protein and have no membrane. The eukaryotic ribosome is 80S (60S + 40S subunits) and the prokaryotic ribosome is 70S (50S + 30S). The "S" stands for Svedberg unit, a measure of sedimentation that reflects density and size. S values do not add up: 50 + 30 does not equal 70, because the unit depends on shape as well as mass. Ribosomes in mitochondria and chloroplasts are 70S, smaller than the 80S ribosomes in the cytoplasm.

The cytoskeleton is a network of microtubules, microfilaments and intermediate filaments. It gives mechanical support, allows movement and maintains cell shape.

Cilia and flagella are hair-like outgrowths of the cell membrane. Cilia are short and work like oars, moving the cell or the fluid around it. Flagella are longer and move the cell. Eukaryotic flagella are structurally different from bacterial ones. Their core, the axoneme, has nine doublets of microtubules around the edge and two single microtubules at the centre, the 9+2 array. The central pair is enclosed in a central sheath, which connects to one tubule of each peripheral doublet by a radial spoke, giving nine spokes in all. The doublets are also linked to each other by interdoublet linkers. Both structures arise from basal bodies, which resemble centrioles.

The centrosome contains two centrioles lying at right angles to each other, surrounded by pericentriolar material. Each centriole is a cartwheel of nine evenly spaced peripheral fibrils, each fibril a triplet of tubulin, with a central protein hub joined to the triplets by radial spokes. Centrioles form the basal bodies of cilia and flagella and the spindle apparatus during cell division in animal cells.

The nucleus and chromosomes

The nucleus was described by Robert Brown. Its envelope is two parallel membranes with a gap of 10 to 50 nm called the perinuclear space. The outer membrane is continuous with the ER and carries ribosomes. At several places the two membranes fuse to form nuclear pores, through which RNA and proteins move in both directions.

Inside, the nucleoplasm contains the nucleolus, a spherical body with no membrane, which is the site of ribosomal RNA synthesis, and it is larger and more numerous in cells that make a lot of protein. It also contains chromatin, made of DNA, histones, non-histone proteins and RNA. A human cell has about two metres of DNA packed into 46 chromosomes (23 pairs). A few cells have no nucleus at all: mammalian red blood cells and the sieve tube cells of vascular plants.

Every chromosome has a primary constriction, the centromere, with disc-shaped kinetochores on its sides. The position of the centromere decides the type.

Some chromosomes also have a non-staining secondary constriction at a fixed place, which makes a small fragment called the satellite.

Microbodies are minute membrane-bound vesicles that contain various enzymes. They occur in both plant and animal cells.

Exam traps

60-second revision

Where this chapter connects

• Previous: Structural Organisation in Animals built animal organs out of tissues; this chapter goes down to the cells that make those tissues.
• Next: Biomolecules explains the proteins, lipids and nucleic acids that make up the membranes and organelles you just studied.
• Related: Biological Classification uses the prokaryote and eukaryote split, Cell Cycle and Cell Division uses centrioles and chromosomes, and Photosynthesis in Higher Plants and Respiration in Plants happen inside the chloroplast and mitochondrion. Biotechnology: Principles and Processes puts bacterial plasmids to work as vectors.