Cell: Structure and Functions - Comprehensive Study Guide

Introduction to Cytology and Cell Discovery

• The cell is the smallest structural and functional unit capable of performing all essential life processes.

• Cytology is the formal study of cells, including their detailed structure and various functions.

• Robert Hooke (1665) coined the term "cell" after observing a honey-comb-like structure in a thin section of cork.

• Leeuwenhoek was the first to discover and observe a living cell.

Classification of Organisms by Cellular Structure

• Acellular Organisms:

  • Organisms that do not possess a true cellular structure, such as viruses.

  • Viruses exhibit both living and non-living characteristics.

  • Viruses remain inactive outside of a host and only reproduce inside living cells.

• Unicellular Organisms:

  • Organisms composed of a single cell that independently performs all vital functions, including respiration, digestion, and reproduction.

  • Examples include Bacteria and Amoeba (which lacks a definite shape).

• Multicellular Organisms:

  • Organisms composed of many cells that perform specialized functions.

  • Examples include Animals and Plants.

The Cell Theory and its Expansion

• Matthias Schleiden, a German Botanist, and Theodor Schwann, a German Zoologist, proposed the Cell Theory in 1838–39.

• Core tenets of the theory include:

  • All living organisms are composed of cells.

  • The cell is the basic structural and functional unit of life.

• In 1855, Rudolf Virchow proposed an important expansion to the theory with the statement "Omnis-cellula-e-cellula," which means "All living cells arise from pre-existing cells."

Diversity in Cell Shape and Size

Cells vary widely in their morphology and dimensions based on their specific functional requirements.

• Cell Shapes:

  • Red Blood Cells (RBCs): Biconcave.

  • Nerve Cells: Long and branched.

  • White Blood Cells (WBCs): Irregular.

• Size Records and Examples:

  • Largest known cell: Unfertilized ostrich egg.

  • Smallest cell: Pleuropneumonia-like organisms (PPLO), specifically Myplasma gallisepticum, measuring between $0.1 \times 10^{-6}\,\text{m}$ and $0.3 \times 10^{-6}\,\text{m}$ (or $0.1-0.3\,\mu m$).

  • Longest animal cell: Nerve cell, which can reach lengths of up to a few meters. Its length helps in feeling sensations, such as heat, and reacting quickly by moving a body part away from danger.

  • Largest unicellular plant: Acetabularia, measuring approximately $10\,cm$.

  • Largest unicellular animal: Amoeba, measuring approximately $1\,mm$.

  • Largest human cell: Female ovum, measuring approximately $1\,mm$.

  • Smallest human cell: Sperm.

Prokaryotic versus Eukaryotic Cells

Cells are classified into two types based on the presence or absence of a nucleus:

• Prokaryotic Cells:

  • They lack a true nucleus and membrane-bound organelles.

  • Genetic material exists as naked DNA in a specialized region called the nucleoid.

  • Examples include bacteria and archaea.

  • All bacterial cells are prokaryotic and can carry out special forms of sexual reproduction called conjugation and transformation.

• Eukaryotic Cells:

  • Complex cells possessing a true nucleus enclosed by a nuclear membrane.

  • They contain various membrane-bound organelles.

  • Examples include plant cells, animal cells, fungi, and protists.

Structural Organization of the Cell

Microscopic examination reveals three basic features common to most cells: the cell membrane, cytoplasm, and nucleus.

• Cell Wall:

  • Found only in bacterial, plant, and fungal cells.

  • It is an outer, non-living covering that surrounds the plasma membrane.

  • It protects the cell from mechanical injury, temperature variation, moisture conditions, and osmotic shock.

  • Chemical composition: Plant cell walls are primarily cellulose, hemicellulose, and pectin; fungal cell walls are made of chitin.

  • Animal cells do not possess a cell wall.

• Cell Membrane (Plasma Membrane):

  • The outermost living covering that encloses the cytoplasm and nucleus.

  • It is selectively permeable, allowing only certain substances to move in and out of the cell.

  • It facilitates the separation of cells from each other and the surrounding medium.

  • The Fluid Mosaic Model, proposed by Singer and Nicolson in 1972, describes the structure of the plasma membrane.

• Effects of Surrounding Solutions:

  • Hypotonic Solution: The surrounding solution has a lower solute concentration than the cell; water enters the cell, causing it to swell and potentially burst.

  • Hypertonic Solution: The surrounding solution has a higher solute concentration than the cell; water moves out, causing the cell to shrink.

  • Isotonic Solution: Solute concentration is equal inside and outside; there is no net movement of water, and the cell size remains normal.

The Living Protoplasm and Cytoplasm

• Protoplasm:

  • Described by Huxley in 1863 as the "Physical basis of life."

  • It is an elastic, viscous, colorless, colloidal system that constitutes the living part of a cell where all life activities occur.

  • Components include the cytoplasm, nucleus, and other cell organelles.

• Cytoplasm:

  • A jelly-like fluid located between the cell membrane and the nucleus.

  • It contains various cell organelles.

  • Functions include the intracellular distribution of nutrients, metabolites, and enzymes.

  • It is the site for many metabolic reactions.

The Nucleus and Genetic Material

• The nucleus was discovered by Robert Brown in 1831.

• It acts as the control center of the cell, regulating all metabolic and hereditary activities.

• Components of the Nucleus:

  • Nuclear membrane.

  • Nucleoplasm.

  • Nucleolus: Responsible for the synthesis of ribosomes.

  • Chromatin material: Composed of DNA and proteins.

• Chromatin, DNA, and Genes:

  • During cell division, chromatin condenses to form chromosomes.

  • A chromosome is a thread-like structure made of DNA and protein that carries genetic information.

  • The centromere is the constricted region of a chromosome that ensures proper separation during cell division.

  • A gene is a small segment of DNA on a chromosome that controls a specific trait.

  • DNA is a double helical structure made of two strands of nucleotides. Each nucleotide contains a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

• Anucleated Cells:

  • Mature mammalian RBCs lack a nucleus.

  • Sieve tube elements of mature phloem lack a nucleus.

  • The absence of a nucleus in these cells maximizes their specific functional efficiency.

Cytoplasmic Organelles: Energy and Synthesis

• Mitochondria:

  • Often known as the "powerhouse of the cell."

  • They are tiny rod-shaped structures that provide energy by oxidizing food through cellular respiration.

  • They contain their own circular DNA, which is similar to bacterial DNA.

• Ribosomes:

  • Smallest non-membranous organelles.

  • Appear as dense, spherical particles either attached to the Endoplasmic Reticulum or free in the cytoplasm.

  • They are the site of protein synthesis and are termed the "protein factory of the cell."

  • These are the only organelles found in both prokaryotic and eukaryotic cells.

• Endoplasmic Reticulum (ER):

  • A membrane-bound network of channels consisting of cisternae, vesicles, and tubules.

  • Serves as the "skeleton" or "transport system" of the cell, moving materials internally.

  • Smooth Endoplasmic Reticulum (SER): Lacks ribosomes; involved in lipid synthesis.

  • Rough Endoplasmic Reticulum (RER): Studded with ribosomes; involved in protein synthesis.

Specialized Cell Organelles

• Vacuoles:

  • Sac-like structures containing "cell sap" (a solution of water, mineral salts, and sugar).

  • In plant cells, they are large, permanent, and occupy most of the cell's volume, helping to maintain turgidity.

  • In animal cells, vacuoles are small and temporary, or may even be absent entirely.

• Lysosomes:

  • Commonly called "suicidal bags."

  • Single membrane-bound, spherical, tiny sac-like bodies.

  • They serve as a reservoir for approximately 50 types of hydrolytic enzymes.

  • They facilitate the digestion of materials taken in via endocytosis.

• Golgi Bodies (Golgi Complex):

  • Consist of a network of flattened tubules, small vesicles, and large Golgian vacuoles.

  • Functions include the formation of lysosomes, acrosome of sperms, yolk, and melanin granules.

  • They are involved in the storage of secretion products and the secretion of hormones like insulin.

• Centrosomes:

  • Found mainly in animal cells; they assist in the process of cell division.

Plastids and Plant Cell Specialization

Plastids are colored or colorless bodies scattered in the cytoplasm of plant cells but are absent in animal cells.

• Chromoplasts: Colored plastids (excluding green) that give fruits and flowers their specific colors.

• Leucoplasts: Colorless plastids used for storing food, specifically starch, protein, and oil.

• Chloroplasts: Contain chlorophyll for photosynthesis and possess their own DNA.

Comparison of Plant Cells and Animal Cells

Cellular Organization and Division of Labour

• Multicellular organisms exhibit a division of labour because specific functions are carried out by specialized groups of cells.

• Tissue: A group of cells with similar origin and structure that perform a specific function.

• Tissues are designed to provide the highest possible efficiency for their specific tasks.

• Structural Hierarchy: Certain tissues combine to form organs; groups of organs combine to form an organ-system.