Cell Theory, Prokaryotic Cells and Cell Membrane

The cell is the basic structural and functional unit of life. This page explains cell theory, from Schleiden and Schwann to Virchow, and how cells differ in size and shape. It then covers prokaryotic cells: the cell envelope, mesosome, flagella, pili, ribosomes and inclusion bodies. It ends with the eukaryotic cell plan, the fluid mosaic model of the cell membrane and the cell wall, in the order of the NCERT Class 11 chapter Cell: The Unit of Life. NEET often pairs cell theory scientists with their work in match-the-list questions.

1. Unity of Life and the Cell

1.1 Unity underlying diversity

• Biology is the study of living organisms.
• Describing their form and appearance only showed how diverse they are.
• Cell theory showed the unity beneath this diversity: all life forms have a cellular organisation.
• Cell theory also raised a puzzle: living phenomena are seen only while the cellular organisation remains intact (integrity of the cell).
• Living phenomena here mean physiological and behavioural processes.
• The study of cells covers two themes: cell structure and cell growth by division.

1.2 Reductionist biology

• Life processes can be studied by a physico-chemical approach using cell-free systems.
• This approach describes the processes in molecular terms.
• First, living tissues are analysed for elements and compounds, showing which organic compounds are present.
• Next, we ask what these compounds do in a cell and how they carry out processes such as digestion, excretion, memory, defence and recognition.
• This gives the molecular basis of physiological processes and explains abnormal processes in disease.
• ★ Exam imp Reductionist biology: this physico-chemical approach, which applies the concepts and techniques of physics and chemistry to biology.

1.3 G.N. Ramachandran (1922-2001)

• An outstanding figure in the field of protein structure.
• Founder of the 'Madras school' of conformational analysis of biopolymers.
• Discovered the triple helical structure of collagen, published in Nature in 1954.
• Analysed the allowed conformations of proteins using the Ramachandran plot.
• These two contributions rank among the most outstanding in structural biology.
• Born on 8 October 1922 in a small town near Cochin, on the south-western coast of India.
• His father, a professor of mathematics at a local college, shaped his interest in mathematics.
• Graduated in 1942 as the top-ranking student of B.Sc. (Honours) Physics, University of Madras; Ph.D. from Cambridge University in 1949.
• At Cambridge he met Linus Pauling, whose models of the $\alpha$-helix and $\beta$-sheet turned his attention to collagen.
• He died on 7 April 2001, at the age of 78.

1.4 What is a cell?

• The presence of the cell, the basic unit of life, is what makes an organism living.
• All organisms are composed of cells.
• Unicellular organisms: made of a single cell. Multicellular organisms: made of many cells, as in humans.
• Unicellular organisms can (i) exist independently and (ii) perform all the essential functions of life.
• Anything less than a complete cell cannot ensure independent living.
• ★ Exam imp Hence the cell is the fundamental structural and functional unit of all living organisms.
• Antonie von Leeuwenhoek first saw and described a live cell.
• Robert Brown later discovered the nucleus.
• The microscope, and its improvement into the electron microscope, revealed all the structural details of the cell.

2. Cell Theory

Cell theory took shape in this order of time:

1. 1838, Matthias Schleiden (German botanist): studied many plants and found that all plants are made of different kinds of cells, which form the plant's tissues.
2. 1839, Theodore Schwann (German zoologist): studied animal cells and found a thin outer layer, now called the plasma membrane.
3. From plant tissues, Schwann concluded that the cell wall is a unique character of plant cells.
4. Schwann's hypothesis: the bodies of animals and plants are made of cells and products of cells.
5. Schleiden and Schwann together formulated the cell theory. It did not explain how new cells are formed.
6. 1855, Rudolf Virchow: cells divide, and new cells form from pre-existing cells (Omnis cellula-e cellula).
7. Virchow modified the hypothesis of Schleiden and Schwann and gave cell theory its final shape.

3. An Overview of the Cell

3.1 Plant cell and animal cell

• Onion peel cell (a typical plant cell): a distinct cell wall forms the outer boundary, with the cell membrane just inside it.
• Human cheek cell: an outer membrane is the delimiting structure.
• Each cell has a dense, membrane-bound nucleus. It contains the chromosomes, which carry the genetic material, DNA.
• ★ Exam imp Eukaryotic cells: have membrane-bound nuclei. Prokaryotic cells: lack a membrane-bound nucleus.
• Cytoplasm: the semi-fluid matrix that fills the cell in both prokaryotes and eukaryotes.
• The cytoplasm is the main arena of cellular activities; its chemical reactions keep the cell in the 'living state'.
• A typical eukaryotic cell has a cell membrane, nucleus and cytoplasm; plant cells also have a cell wall outside the cell membrane.

3.2 Organelles

• Besides the nucleus, eukaryotic cells have membrane-bound organelles: endoplasmic reticulum (ER), Golgi complex, lysosomes, mitochondria, microbodies and vacuoles.
• Prokaryotic cells lack such membrane-bound organelles.
• ★ Exam imp Ribosomes are non-membrane bound organelles found in all cells, prokaryotic and eukaryotic.
• Inside a cell, ribosomes occur in the cytoplasm, within chloroplasts (plants) and mitochondria, and on the rough ER.
• Centrosome: another non-membrane bound organelle of animal cells; it helps in cell division.

3.3 Size and shape of cells

• Cells differ greatly in size, shape and activities.
• ★ Exam imp Mycoplasmas are the smallest cells, only 0.3 $\mu$m long.
• Bacteria could be 3 to 5 $\mu$m long.
• The largest isolated single cell is the egg of an ostrich.
• In multicellular organisms, human red blood cells are about 7.0 $\mu$m in diameter.
• Nerve cells are some of the longest cells.
• Shapes: disc-like, polygonal, columnar, cuboid, thread-like or even irregular.
• The shape of a cell may vary with the function it performs.

4. Prokaryotic Cells

4.1 Members, size and shape

• ★ Exam imp Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
• They are generally smaller and multiply more rapidly than eukaryotic cells.
• They vary greatly in shape and size.

Numbers to Remember

• Typical eukaryotic cell: 10-20 $\mu$m.
• Typical bacteria: 1-2 $\mu$m.
• PPLO: about 0.1 $\mu$m.
• Viruses: 0.02-0.2 $\mu$m.

4.2 Basic organisation

• Prokaryotes show many shapes and functions, yet their cell organisation is fundamentally similar.
• ★ Exam imp All prokaryotes have a cell wall around the cell membrane, except mycoplasma.
• The cytoplasm is a semi-fluid matrix that fills the cell.
• There is no well-defined nucleus. The genetic material is basically naked, not enclosed by a nuclear membrane.
• Genomic DNA: a single chromosome of circular DNA.
• Plasmids: small circular DNA molecules outside the genomic DNA, present in many bacteria.
• Plasmids give bacteria unique phenotypic characters, such as resistance to antibiotics.
• Plasmid DNA is used to monitor bacterial transformation with foreign DNA.
• A nuclear membrane is found only in eukaryotes.
• Prokaryotes have no organelles like those of eukaryotes, except ribosomes.
• Prokaryotes have something unique: inclusions.
• Mesosome: a specialised, differentiated form of the cell membrane, characteristic of prokaryotes. It is an infolding of the cell membrane.

4.3 Cell envelope and its modifications

• Most prokaryotic cells, especially bacteria, have a chemically complex cell envelope.
• It is a tightly bound, three-layered structure.

1. Glycocalyx: the outermost layer.
2. Cell wall: the middle layer.
3. Plasma membrane: the innermost layer.

• Each layer has a distinct function, but together they act as a single protective unit.
• Bacteria fall into two groups by differences in the cell envelope and by their response to Gram staining.
• Gram positive: take up the Gram stain. Gram negative: do not take it up.
• Glycocalyx differs in composition and thickness among bacteria.

• Cell wall: determines the shape of the cell and gives strong structural support, so the bacterium does not burst or collapse.
• Plasma membrane: selectively permeable; it interacts with the outside world. Its structure is similar to that of eukaryotes.

4.4 Mesosome and chromatophores

• The mesosome forms by extensions of the plasma membrane into the cell.
• These extensions take the form of vesicles, tubules and lamellae.

Functions of the mesosome

• Help in cell wall formation.
• Help in DNA replication and distribution to daughter cells.
• Help in respiration.
• Help in secretion processes.
• Increase the surface area of the plasma membrane and its enzymatic content.

• Chromatophores: other membranous extensions into the cytoplasm that contain pigments, found in some prokaryotes such as cyanobacteria.

4.5 Flagella, pili and fimbriae

• Bacterial cells may be motile or non-motile.
• Motile bacteria have flagella: thin filamentous extensions from the cell wall.
• Bacteria show a range in the number and arrangement of flagella.
• ★ Exam imp A bacterial flagellum has three parts: filament, hook and basal body.
• The filament is the longest part; it extends from the cell surface to the outside.
• Pili and fimbriae are also surface structures, but they play no role in motility.

4.6 Ribosomes and inclusion bodies

• In prokaryotes, ribosomes are associated with the plasma membrane.
• They measure about 15 nm by 20 nm.
• ★ Exam imp They have two subunits, 50S and 30S, which together form the 70S prokaryotic ribosome.
• Ribosomes are the site of protein synthesis.
• Polyribosome (polysome): a chain of several ribosomes attached to a single mRNA; its ribosomes translate the mRNA into proteins.
• Inclusion bodies: reserve material stored in the cytoplasm. They are not bound by any membrane and lie free.
• Examples: phosphate granules, cyanophycean granules and glycogen granules.
• Gas vacuoles are found in blue-green bacteria and in purple and green photosynthetic bacteria.

5. Eukaryotic Cells

• Eukaryotes include all the protists, plants, animals and fungi.
• Membrane-bound organelles give an extensive compartmentalisation of the cytoplasm.
• They have an organised nucleus with a nuclear envelope.
• They have a variety of complex locomotory and cytoskeletal structures.
• Their genetic material is organised into chromosomes.
• All eukaryotic cells are not identical; plant and animal cells differ.

6. Cell Membrane

6.1 Chemical composition

• The detailed structure of the membrane was studied only after the electron microscope arrived in the 1950s.
• Before that, chemical studies, especially on human red blood cells (RBCs), helped scientists deduce its possible structure.
• The cell membrane is mainly made of lipids and proteins.
• ★ Exam imp The major lipids are phospholipids, arranged in a bilayer.
• The polar heads face the outer sides; the hydrophobic tails face the inner part.
• This keeps the nonpolar tails of saturated hydrocarbons away from the watery (aqueous) environment.
• The membrane also contains cholesterol.
• Later biochemical work showed that membranes also have protein and carbohydrate.
• The ratio of protein to lipid varies greatly between cell types.
• ★ Exam imp The human erythrocyte membrane has about 52 per cent protein and 40 per cent lipids.

6.2 Membrane proteins

Membrane proteins are classified by how easily they can be extracted:

6.3 Fluid mosaic model

• ★ Exam imp Singer and Nicolson (1972) proposed an improved model, the fluid mosaic model, which is widely accepted.
• The quasi-fluid nature of the lipid lets proteins move sideways (laterally) within the bilayer.
• This ability to move within the membrane is measured as its fluidity.

Why fluidity matters

• Cell growth.
• Formation of intercellular junctions.
• Secretion.
• Endocytosis.
• Cell division.

6.4 Transport across the membrane

• One of the most important functions of the plasma membrane is the transport of molecules across it.
• The membrane is selectively permeable to some molecules present on either side.
• Passive transport: movement across the membrane without using energy.
• Simple diffusion: neutral solutes move along the concentration gradient, from higher to lower concentration.
• Water also moves from higher to lower concentration. Osmosis: the movement of water by diffusion.
• Polar molecules cannot pass through the nonpolar lipid bilayer. A carrier protein of the membrane helps them across.
• Active transport: a few ions or molecules move against the concentration gradient, from lower to higher concentration.
• Active transport is energy-dependent and uses ATP, for example the $\mathrm{N}{\mathrm{a}}^{+}/{\mathrm{K}}^{+}$ pump.

7. Cell Wall

• ★ Exam imp The cell wall is a non-living, rigid structure that covers the plasma membrane of fungi and plants.

Functions of the cell wall

• Gives shape to the cell.
• Protects the cell from mechanical damage and infection.
• Helps in cell-to-cell interaction.
• Acts as a barrier to undesirable macromolecules.

Layers, from between two cells to the inside of one cell:

1. Middle lamella: a layer mainly of calcium pectate that holds, or glues, neighbouring cells together.
2. Primary wall: the wall of a young plant cell. It can grow, and this capacity falls as the cell matures.
3. Secondary wall: formed later on the inner side of the primary wall, towards the membrane.

• Plasmodesmata pass through the cell wall and middle lamella and connect the cytoplasm of neighbouring cells.

8. Exam Essentials

Pairs to Match

Numbers to Remember

• 1838 Schleiden; 1839 Schwann; 1855 Virchow; 1972 Singer and Nicolson; 1950s electron microscope studies of membranes.
• Mycoplasma 0.3 $\mu$m; bacteria 3 to 5 $\mu$m; human RBC about 7.0 $\mu$m.
• Eukaryotic cell 10-20 $\mu$m; typical bacteria 1-2 $\mu$m.
• PPLO about 0.1 $\mu$m; viruses 0.02-0.2 $\mu$m.
• Prokaryotic ribosome: about 15 nm by 20 nm; 70S = 50S + 30S subunits.
• Human erythrocyte membrane: about 52% protein and 40% lipids.
• 3 layers in the bacterial envelope; 3 parts of a bacterial flagellum; 4 basic bacterial shapes.
• G.N. Ramachandran: born 1922; collagen triple helix 1954; died 2001.

Examples to Remember

9. Quick Revision

• The cell is the structural and functional unit of life; unicellular organisms live independently.
• Leeuwenhoek first saw and described a live cell; Robert Brown discovered the nucleus.
• Schleiden (1838) plants, Schwann (1839) animals; Virchow (1855): cells from pre-existing cells.
• Reductionist biology: a physico-chemical approach using cell-free systems, applying physics and chemistry to biology.
• Eukaryotic cells have a membrane-bound nucleus and organelles; prokaryotic cells do not.
• Ribosomes occur in all cells; the centrosome occurs in animal cells.
• Smallest cells: Mycoplasma (0.3 $\mu$m); largest isolated cell: ostrich egg; among the longest: nerve cells.
• Prokaryotes: bacteria, blue-green algae, mycoplasma, PPLO; cell wall in all except mycoplasma.
• Bacterial DNA is naked and circular; plasmids can carry antibiotic resistance.
• Envelope: glycocalyx (slime layer or capsule), cell wall, plasma membrane.
• Mesosome: membrane infolding for wall formation, DNA replication, respiration and secretion.
• Flagellum: filament, hook, basal body; pili and fimbriae do not help in motility.
• Prokaryotic ribosomes are 70S (50S + 30S); inclusion bodies lie free in the cytoplasm.
• Fluid mosaic model (1972): proteins move laterally in a quasi-fluid lipid bilayer.
• Cell wall: non-living; middle lamella of calcium pectate; plasmodesmata link cells.

10. Solved Examples

11. Practice Questions

Common Mistakes to Avoid