Cell — Study Notes

1. Introduction

The cell is the fundamental structural and functional unit of all living organisms. Robert Hooke first discovered cells in 1665 while examining a thin slice of cork with a self-made microscope; he observed honeycomb-like compartments and called them "cells." A few decades later, Antonie van Leeuwenhoek observed living cells and named them "animalcules." These discoveries laid the foundation of the cell theory, which was later formalised by Schleiden (1838) and Schwann (1839). Schwann, in 1839, proposed that the cells have a thin outer layer which is now known as the plasma membrane, and he was the first to recognise that plants and animals are composed of cells. Rudolf Virchow later added the crucial principle that all cells arise from pre-existing cells, completing the modern cell theory.

The study of cells gained momentum with the discovery of the electron microscope, which revealed the elaborate internal structure of cells. The cell shows tremendous diversity in size, shape and number. The smallest cell is Mycoplasma (about 0.3 micrometres), while the largest cells are the egg of an ostrich (about 15 cm) and the nerve cell of a giraffe. The shape of a cell is related to its function: amoeboid cells change shape, nerve cells are elongated for conduction, and muscle cells are cylindrical for contraction. This chapter presents a detailed account of the cell, its components, and the differences between prokaryotic and eukaryotic cells, including the structure and function of various cell organelles.

2. Cell Theory

The cell theory was proposed by Schleiden, Schwann and Virchow. Its main postulates are: 1. All living organisms are composed of cells and products of cells. 2. All cells arise from pre-existing cells (Rudolf Virchow).

Modern cell theory adds that the cell is the basic structural and functional unit of all living organisms, and that all biochemical activities of life are performed within cells. However, there are exceptions: viruses are not composed of cells, and mycoplasma are the smallest cells that can survive without oxygen.

3. Prokaryotic and Eukaryotic Cells

3.1 Prokaryotic Cells

Prokaryotes (bacteria, cyanobacteria, mycoplasma) have an organised nuclear material but no nuclear membrane, so the genetic material is not enclosed in a distinct nucleus. A single circular chromosome lies free in the cytoplasm, called the genophore or nucleoid. Membrane-bound cell organelles are absent. The ribosomes are of the 70S type. The prokaryotic cell has a cell wall, and in addition, some bacteria have a capsule (slime layer). The surface may bear flagella and pili. The flagellum of a prokaryote is composed of a protein called flagellin and differs fundamentally from the eukaryotic flagellum.

Structure of a Prokaryotic Cell

• Cell envelope: The outermost cell wall, the middle plasma membrane and the innermost glycocalyx (capsule or slime layer). The cell wall protects the cell and gives it shape, and it is made of peptidoglycan (murein) in most bacteria.
• Plasma membrane: A selectively permeable membrane made of lipids and proteins, which regulates the movement of substances in and out of the cell.
• Cytoplasm: Contains ribosomes (70S) and many inclusion bodies such as gas vacuoles, phosphate granules and cyanophycean granules. Mesosomes are invaginations of the plasma membrane into the cytoplasm, which help in cell wall formation, DNA replication and respiration, and increase the surface area of the membrane.
• Nucleoid: The region containing the circular DNA, which is not bound by a membrane.
• Plasmids: Extra-chromosomal circular DNA molecules that replicate independently of the chromosomal DNA, often carrying genes for special functions like antibiotic resistance.
• Flagella and pili: Flagella are filamentous, proteinaceous locomotory organs; pili (fimbriae) are small bristle-like fibres that help in attachment and conjugation.

3.2 Eukaryotic Cells

Eukaryotes (plants, animals, fungi, protists) have a well-defined nucleus enclosed by a double nuclear membrane, and membrane-bound organelles such as mitochondria, chloroplasts, Golgi apparatus and endoplasmic reticulum. The ribosomes are of the 80S type. The cytoplasm contains the cytoskeleton (microtubules, microfilaments and intermediate filaments) that maintains the shape of the cell. Eukaryotic cells are generally larger than prokaryotic cells.

4. The Eukaryotic Cell: Structure and Organelles

4.1 Cell Membrane

The cell membrane, or plasma membrane, is the outermost covering of animal cells and the inner limiting membrane of plant cells (which also have a cell wall). It is composed of lipids (mainly phospholipids) arranged in a bilayer, with proteins embedded in it, according to the fluid mosaic model proposed by Singer and Nicolson (1972). The membrane is selectively permeable, allowing only specific molecules to pass through. The main functions of the cell membrane include: protecting the cell, allowing transport of materials, providing recognition sites, and receiving chemical messages.

4.2 Cell Wall

The cell wall is a non-living rigid structure outside the plasma membrane, present in plant cells, bacteria and fungi. In plants, it is made of cellulose, hemicellulose and pectin; in fungi it contains chitin; in bacteria it is made of peptidoglycan (murein). The middle lamella, composed of calcium pectate, cements the cell walls of adjacent cells. The cell wall provides shape to the cell, protects it from mechanical damage and infection, and prevents excess water uptake.

4.3 Endomembrane System

The endomembrane system includes the endoplasmic reticulum, Golgi complex, lysosomes and vacuoles, which function as a coordinated unit. The endomembrane system does not include mitochondria, chloroplasts or peroxisomes.

4.4 Endoplasmic Reticulum (ER)

The ER is a network of membrane-bound tubes and sheets extending from the nuclear envelope into the cytoplasm. It is of two types: - Rough ER (RER): Studded with ribosomes on its surface, involved in the synthesis and transport of proteins. - Smooth ER (SER): Lacks ribosomes, involved in the synthesis of lipids, and in detoxification (especially in liver cells).

4.5 Golgi Apparatus

The Golgi apparatus consists of a system of stacked, flattened, membrane-bound sacs called cisternae, along with vesicles. It was first observed by Camillo Golgi. The Golgi complex is involved in the packaging, modification and transport of materials, and in the formation of lysosomes, glycoproteins and glycolipids. It is the "post office" of the cell, directing proteins to their destinations.

4.6 Lysosomes

Lysosomes are membrane-bound vesicular structures formed by the Golgi apparatus. They contain a variety of hydrolytic enzymes (hydrolases) that digest macromolecules. Lysosomes are called "suicide bags" because when a cell is damaged, lysosomes burst and the enzymes digest the entire cell. They help in the digestion of foreign materials and worn-out cell organelles.

4.7 Vacuoles

Vacuoles are membrane-bound sacs, the largest being the central vacuole of plant cells, which occupies up to 90% of the cell volume. They are enclosed by a membrane called the tonoplast. Vacuoles store water, food, waste products and various solutes, and in plant cells they maintain turgor pressure and cell shape. In Amoeba, the contractile vacuole is important for osmoregulation and excretion.

4.8 Mitochondria

Mitochondria are double-membrane-bound organelles, called the powerhouses of the cell. The outer membrane is smooth, and the inner membrane is folded into finger-like projections called cristae. The space inside the inner membrane is the matrix, which contains enzymes of the Krebs cycle, DNA, and ribosomes (70S). Mitochondria are the sites of aerobic respiration and ATP synthesis. They have their own DNA and ribosomes and can self-duplicate, so they are called semiautonomous organelles.

4.9 Plastids

Plastids are present only in plant cells, containing their own DNA and ribosomes. They are of three types: - Chloroplasts: Contain chlorophyll, the green pigment, and are the sites of photosynthesis. - Chromoplasts: Contain coloured pigments like carotenoids (yellow, orange, red). - Leucoplasts: Colourless plastids that store starch, oil and proteins; the amyloplasts store starch.

The chloroplast is bounded by two membranes. The inner membrane forms flattened sacs called thylakoids, which are stacked to form grana. The chlorophyll is present in the thylakoid membranes. The fluid ground substance of the chloroplast, called the stroma, contains enzymes for the dark reaction (Calvin cycle), DNA and ribosomes.

4.10 Ribosomes

Ribosomes are the sites of protein synthesis, composed of RNA and proteins. They are not membrane-bound. Prokaryotic ribosomes are of the 70S type (50S and 30S subunits), while eukaryotic ribosomes are of the 80S type (60S and 40S subunits). Ribosomes may be free in the cytoplasm or bound to the ER.

4.11 Cytoskeleton

The cytoskeleton is a network of protein filaments that provides shape, support and a framework for cell movement, present in the cytoplasm. It includes microfilaments, microtubules and intermediate filaments.

4.12 Centrosome and Centrioles

The centrosome is an organelle usually containing two cylindrical structures called centrioles, made of tubulin protein, arranged perpendicular to each other. Centrioles help in the formation of spindle fibres during cell division, and they give rise to basal bodies, cilia and flagella.

4.13 Cilia and Flagella

Cilia and flagella are hair-like projections that help in locomotion, consisting of a core called the axoneme. The axoneme has a central pair of tubules (9+2 arrangement) surrounded by a ring of nine pairs of microtubules. The central tubules are connected by bridges and are enclosed by a central sheath. The flagella of bacteria (prokaryotes) are different, being made of flagellin.

4.14 Nucleus

The nucleus is the most important cell organelle, usually spherical, bound by a double membrane called the nuclear envelope, which has pores that allow the exchange of materials. The nucleus contains the nucleolus (rich in RNA and proteins) and the chromatin (DNA and histone proteins). When the cell divides, chromatin condenses into chromosomes. The nucleus controls all the metabolic activities of the cell and carries the hereditary information.

4.15 Microbodies

Microbodies are membrane-bound minute vesicles that contain various enzymes. Peroxisomes contain catalase and oxidases and are involved in the breakdown of hydrogen peroxide and fatty acids, and they are present in both plant and animal cells. Glyoxysomes are specialised peroxisomes found in plant cells that help in the conversion of fats into carbohydrates.

Quick Revision Tables

Table 1: Prokaryotic vs Eukaryotic Cells

Table 2: Cell Organelles and their Functions

Table 3: Cell Discovery Timeline

Mind Map

Important Diagrams (SVG)

Common Mistakes

1. Students state that the cell theory was proposed only by Schleiden and Schwann, forgetting Virchow's contribution (omnis cellula e cellula).
2. The ribosome size is confused: prokaryotes have 70S, eukaryotes have 80S ribosomes.
3. Mesosomes are described as permanent organelles; they are dynamic invaginations of the plasma membrane.
4. The bacterial flagellum is said to be like the eukaryotic flagellum; it is made of flagellin and has no 9+2 arrangement.
5. The endomembrane system is wrongly said to include mitochondria and chloroplasts; these organelles are not part of it.
6. Lysosomes are called "suicide bags" because their enzymes digest the cell when it is damaged, not because they are harmful normally.
7. The plant cell wall is said to be made only of cellulose; it also contains hemicellulose and pectin, and the middle lamella is calcium pectate.
8. Plastids are said to be absent in animals; however, some animals do not contain plastids, but that is a plant-cell characteristic, so it is correctly stated, but students wrongly say leucoplasts are green - they are colourless.

Exam Tips

1. Remember cell discovery and theory timeline: Hooke (1665) to Virchow (cell from pre-existing cell) to Singer and Nicolson (fluid mosaic, 1972).
2. For every organelle, remember its structure-function link: cristae-surface area-ATP, thylakoids-chlorophyll-photosynthesis, cisternae-Golgi.
3. The difference table of prokaryotic vs eukaryotic cells is a sure-shot two-mark question.
4. Learn the 9+2 arrangement of cilia/flagella and the absence of membrane-bound organelles in prokaryotes.
5. Semiautonomous organelles (mitochondria and chloroplasts) with their own DNA and 70S ribosomes are a favourite assertion-reason topic.
6. Function of SER (lipid synthesis, detoxification) vs RER (protein synthesis) is frequently tested.
7. Note the sizes: smallest cell Mycoplasma, largest single cell ostrich egg; giant nerve cells of giraffe.

Conclusion

The cell is truly the unit of life, both structurally and functionally. The cell theory unifies all living organisms, while the comparison of prokaryotic and eukaryotic cells highlights the evolutionary gap between simpler and more complex forms of life. Every organelle in the eukaryotic cell performs a specialised function: the nucleus governs heredity and metabolism, mitochondria produce energy, chloroplasts capture light, ribosomes build proteins, and the endomembrane system packages and transports these products. The fluid mosaic model of the plasma membrane explains how cells interact with their environment. Together, these components enable the cell to maintain its internal environment, respond to stimuli and reproduce, laying the complete foundation for the study of biomolecules and cell division in the following chapters.