Biology 2

Major Problems in the World

  • The major problems in the world result from the discrepancy between how nature operates and human thought processes.

Relevant Previous Knowledge

Principles of Cellular Life

Definition of a Cell

  • The term "cell" originates from the Latin word "cella", meaning "small room".

  • Initially coined by a microscopist observing cork structure.

  • A cell represents the basic structural and functional unit of all known living organisms.

  • All organisms are composed of one or more cells.

  • The advancement of the microscope significantly contributed to the discovery of cells.

  • Cells are often termed "the building blocks of life".

Characteristics of Cells

  • Organisms such as bacteria, amoebae, and yeasts can consist of a single cell.

  • The typical human body contains approximately one trillion cells.

Foundational Events in Cell Discovery and Cell Theory Development

Robert Hooke

  • Robert Hooke (18 July 1635 – 3 March 1703) introduced the term "cell".

  • An English polymath, acting as a scientist, natural philosopher, and architect.

  • First to observe microorganisms in 1665 using a self-built compound microscope.

  • Improved the design of the existing microscope to enable more detailed observations.

Hooke’s Microscope

  • Utilized three lenses and a stage light for illumination and magnification of specimens.

  • Notable observation of cork resulted in the conclusion that it was comprised of tiny pores, which he named "cells" after monastic cells.

  • Recorded his observations in the book "Micrographia".

Anton van Leeuwenhoek

  • Shortly after Hooke, Dutch scientist Anton van Leeuwenhoek discovered other minuscule organisms, such as bacteria and protozoa.

  • Renowned for mastering microscope construction and enhancing the design of simple microscopes.

  • Single lens microscopes could magnify objects by 200 to 300 times.

  • Coined the term "animalcules" for the small living organisms observed.

  • First to observe and describe spermatozoa in 1677 and examined dental plaque under the microscope.

  • Remarked about observing tiny animalcules with great wonder in a letter to the Royal Society.

Robert Brown

  • Robert Brown (21 December 1773 – 10 June 1858) was a Scottish botanist who significantly contributed to botany through pioneering use of the microscope.

  • In 1827, studied pollen grains of the plant Clarkia pulchella suspended in water, observing jittery motion of particles now known as amyloplasts (starch organelles) and spherosomes (lipid organelles).

  • This motion observed in both organic and inorganic particles led to identifying the phenomenon known as Brownian motion (pedesis), characterized as the random motion of particles suspended in a medium.

  • Discovered the cell nucleus.

Matthias Schleiden and Theodor Schwann

  • Matthias Schleiden (5 April 1804 – 23 June 1881) and Theodor Schwann (7 December 1810 – 11 January 1882) were German botanist and zoologist, respectively, and co-founders of cell theory.

  • They delineated differences between plant and animal cells, proposing that cells are fundamental units of both.

  • Misconceptions included Schleiden's view of cells being "seeded" by nuclei and Schwann's belief that animal cells "crystallized" from intercellular material.

Rudolf Virchow

  • Rudolf Virchow (13 October 1821 – 5 September 1902) was a German physician and anthropologist who identified a crucial component of cell theory in 1855.

  • Conducted research on cancer cells, concluding "Omnis cellula-e-cellula" (All cells come from cells), asserting that all cells arise from pre-existing cells.

The Cell Theory

  • Cell theory posits that all living organisms consist of cells as the smallest functional unit.

  • Key tenets of modern cell theory include:

    • All known living organisms are made of one or more cells (Schleiden & Schwann).

    • Cells are the fundamental unit of structure and function in all living organisms (Schleiden & Schwann).

    • Living cells arise from pre-existing cells by division (Virchow).

  • Expanded cell theory includes:

    • Cells transmit genetic material to daughter cells during cellular division.

    • Cells share similar chemical compositions among organisms of similar species.

    • Metabolic processes and energy flow occur within cells.

Unicellular vs. Multicellular Organisms

  • Unicellular organisms consist of only one cell fulfilling all life functions, while multicellular organisms comprise multiple cells with diverse functions.

  • Examples of unicellular organisms: bacteria, protists, yeast.

  • Examples of multicellular organisms: humans, trees, dogs, cows, chicken, etc.

Comparison of Unicellular and Multicellular Organisms

  • Unicellular organisms:

    • Composed of a single cell.

    • Simple body organization.

    • Life processes carried out by one cell.

    • The total cell body exposed to the environment.

    • Division of labor occurs at the organelle level.

    • Includes both eukaryotes and prokaryotes.

    • Usually exhibit short lifespans.

  • Multicellular organisms:

    • Composed of multiple cells.

    • Complex body organization.

    • Various cells perform different functions.

    • Only outer cells exposed to the environment.

    • Division of labor occurs at cellular, tissue, organ, and organ system levels.

    • Includes only eukaryotes.

    • Characteristically longer lifespans.

Cell Types

  • There are two primary types of cells:

    • Prokaryotic cells

    • Eukaryotic cells

  • Prokaryotic cells include bacteria and archaea.

  • Eukaryotic cells encompass plants and animals and are categorized based on their complexity, referred to as "Cellular Diversity".

Prokaryotic Cells

  • Prokaryotic cells are generally simpler in structure.

  • They lack an organized nucleus and most membrane-bound organelles (e.g., mitochondria, chloroplasts).

  • Nuclear materials (genomes/nucleoid) are dispersed in the cytoplasm without a nuclear membrane.

  • The endosymbiotic theory posits that eukaryotic cells may have originated from several prokaryotic ancestors.

  • Some prokaryotes survive in extreme conditions, including extreme temperature, pH, and radiation.

Archaea

  • Archaea are a subgroup of prokaryotes that exist under the most extreme conditions.

  • Early classification grouped them as archaebacteria, but this term is outdated.

  • They lack nuclei and thrive in inhospitable environments with high salinity, acidity, or anaerobic conditions.

  • Types of archaea include:

    • Thermophiles: Thrive in extreme temperatures.

    • Halophiles: Survive in high salt concentrations.

    • Methanogens: Produce methane during metabolism.

    • Acidophiles: Flourish under highly acidic conditions.

  • Collectively known as extremophiles.

Structure of Prokaryotic Cell - Bacterial

  • Features of bacterial cell structure include:

    • Capsule

    • Pilus

    • Cell wall

    • Plasma membrane

    • Nucleoid (DNA)

    • Cytoplasm

    • Ribosomes

    • Flagellum

Cell Envelope

  • The cell envelope typically consists of a cell wall and a plasma membrane, though some bacteria may possess an additional capsule.

    • Functions of the cell envelope:

    • Provides rigidity to the cell.

    • Separates internal from external environments, acting as a protective filter.

  • Not all prokaryotes possess a cell wall (e.g., Mycoplasma, Thermoplasm).

Cell Wall

  • The cell wall is composed of peptidoglycan, a composite structure of carbohydrates and amino acids.

  • Functions of the cell wall:

    • Acts as a barrier against external forces.

    • Prevents cytolysis due to osmotic pressure in hypotonic environments.

Plasma Membrane

  • Composed of a phospholipid bilayer rich in proteins.

  • Functions include:

    • Isolates cytoplasmic contents from the external environment.

    • Facilitates communication between cells.

    • Regulates the exchange of substances between the cytoplasm and the environment.

Nucleoid

  • The nucleoid is a non-membrane-bound region containing a compacted circular chromosome.

  • Functions of the nucleoid:

    • Critical for cell control and reproduction processes.

    • Sites of transcription and DNA replication.

Pilus

  • A hair-like structure facilitating bacterial adhesion, aiding colonization and infection.

Flagella

  • Filamentous organelles that promote bacterial locomotion.

    • Enable swimming and swarming behavior to navigate towards favorable conditions.

Ribosomes

  • Ribosomes are microscopic structures engaged in protein synthesis, present in all cells including bacteria.

Cytoplasm

  • Gelatinous liquid composing the internal environment of a cell, consisting of water, salts, and organic molecules essential for growth, metabolism, and replication.

Eukaryotic Cells

  • Eukaryotic cells are defined as those containing membrane-bound nuclei and organelles.

  • Included organisms: protozoa, fungi, plants, and animals, categorized within the kingdom Eukaryota.

  • Capable of maintaining diverse environments within a single cell to facilitate various metabolic reactions, enabling larger sizes compared to prokaryotic cells.

Characteristics of Eukaryotic Cells

  • Enclosed nucleus within a nuclear membrane.

  • Contains mitochondria.

  • Locomotory structures include flagella and cilia.

  • The outermost layer is a cell wall in plant cells but absent in animal cells.

  • Mitosis is the process of cellular division.

  • Comprises a cytoskeletal architecture.

  • Carries single, linear DNA encapsulating genetic information.

Structure of Eukaryotic Cell

Plasma Membrane

  • Separates the internal cell environment from the external surroundings.

  • Comprises specific embedded proteins essential for substance exchange.

Cell Wall

  • A rigid structure outside plant cells, absent in animal cells.

  • Provides cell shape and facilitates cell-to-cell interactions.

  • Acts as a protective layer against injuries and pathogens, consisting of cellulose, hemicellulose, pectins, and proteins.

Cytoskeleton

  • Present within cytoplasm, comprising microfilaments, microtubules, and fibers.

  • Functions to maintain cell shape, anchor organelles, and facilitate movement.

Endoplasmic Reticulum

  • A network of small tubular structures dividing the cell surface into luminal and extraluminal sections.

  • Types of Endoplasmic Reticulum:

    • Rough ER: Contains ribosomes.

    • Smooth ER: Lacks ribosomes, appearing smooth.

Nucleus

  • Enclosed nucleoplasm contains DNA and associated proteins.

  • The nuclear envelope comprises two layers (outer and inner membrane), both being permeable to ions, molecules, and RNA materials.

Golgi Apparatus

  • Constructed of flat disc-shaped structures known as cisternae, arranged near the nucleus.

  • Functions related to glycoprotein and glycolipid formation.

  • Absent in red blood cells of humans and sieve cells of plants.

Ribosomes

  • Main sites for protein synthesis composed of proteins and ribonucleic acids.

Mitochondria

  • Often regarded as the cell's "powerhouse", responsible for energy production.

  • Composed of an outer membrane and a folded inner membrane termed cristae, important for metabolic regulation.

Lysosomes

  • Referred to as "suicidal bags", they contain hydrolytic enzymes for digesting proteins, lipids, carbohydrates, and nucleic acids.

Plastids

  • Double-membraned structures exclusive to plant cells.

  • Types of plastids:

    • Chloroplasts: Contain chlorophyll and facilitate photosynthesis.

    • Chromoplasts: House carotene pigments providing red, yellow, or orange hues.

    • Leucoplasts: Colorless plastids that store oils, fats, carbohydrates, or proteins.

Cell Membrane - Structure and Function

  • The cell membrane (plasma membrane) is present in all cells; it separates cell interior from its external environment.

  • Composed of a lipid bilayer, it is semipermeable and regulates material transport into and out of the cell.

  • Provides structural protection and maintains a stable internal environment.

  • Transport functions include nutrient absorption and expulsion of toxic substances.

  • Membrane proteins may be glycoproteins or lipid proteins, facilitating cellular interactions.

  • The structural integrity of the plasma membrane varies across different cell types, containing significant cholesterol content.

  • Other organisms (e.g., plants and bacteria) often possess tougher cell walls, providing additional structural protection beyond the plasma membrane.