Cell Biology Timeline and Eukaryotic Cell Structure
Timeline of Key Cell Biology Discoveries
- 1590 – Hans and Zacharias Janssen built the first microscope.
- 1665 – Robert Hooke named the cell.
- 1670 – Anton van Leeuwenhoek designed a simple microscope.
- 1674 – van Leeuwenhoek discovered animalcules (bacteria and protozoa).
- 1838 – Schleiden started developing cell theory in plants.
- 1839 – Theodor Schwann extended cell theory to all animals.
- 1858 – Rudolf Virchow proved that all cells come from the division of pre-existing cells.
- 1888 – Santiago Ramón y Cajal demonstrated that nervous tissue is also formed of cells.
1. Living Things
1.1 Characteristics of Living Things
Living things are complex, organized organisms that perform three vital functions:
- Nutrition: exchange of matter/energy, metabolism → growth/development.
- Interaction: detect and respond to stimuli → maintain homeostasis.
- Reproduction and inheritance: asexual or sexual; nucleic acids transmit genetic information to offspring.
Their hierarchical organization follows the principle of emergence: each higher level has new properties beyond the sum of the parts. There are also acellular structures (for example, viruses): they lack cells and metabolism, are not considered fully living, and reproduce only inside host cells using host enzymes and structures.
2. Microscopes and the Discovery of Cells
Cells are the basic units of life, discovered thanks to microscopes.
Cell Theory
- Cells are structural units: all living things are made of one or more cells.
- Cells are functional units: they carry out life processes.
- Cells are reproductive units: new cells come from the division of existing cells.
- Cells are genetic units: they contain hereditary information that controls functions and is passed to offspring.
Basic Components
Most cells contain a plasma membrane, cytoplasm, genetic material, and ribosomes.
Types of Cells
- Prokaryotic cells: no internal membranes or membrane-bound organelles, no nucleus; DNA located in a nucleoid. These cells are very small (typically 0.5–5 μm) and are always unicellular; they do not form true multicellular organisms.
- Eukaryotic cells: have a nucleus and membrane-bound organelles; larger; can be unicellular or multicellular; include animal and plant cells.
Animal cells are typically heterotrophic and often spherical (shape varies with function). Plant cells are typically autotrophic and often polyhedral.
3. Eukaryotic Cells
Eukaryotic cells contain a plasma membrane, nucleus, and cytoplasm. They may also include a cell wall, cilia, or flagella.
Plasma Membrane and Functions
The plasma membrane is a fluid phospholipid bilayer with embedded proteins. In animal cells it commonly contains cholesterol and oligosaccharides. Functions include protecting the cell, maintaining shape, controlling transport, sending and receiving signals, enabling cell recognition, and forming cell-to-cell connections.
Transport: small molecules pass through the lipid bilayer or protein channels; large particles are transported by endocytosis and exocytosis using vesicles.
3.2 Cell Coverings (Cell Walls)
Cell walls are rigid structures outside the plasma membrane, composed mainly of carbohydrates. Types include:
- Bacteria: peptidoglycan.
- Plants: cellulose.
- Fungi: chitin.
Fibrous components are embedded in an amorphous matrix. Functions: provide shape and rigidity and protect from toxins and pathogens.
3.3 Cytoplasm
The cytoplasm contains the cytosol, cytoskeleton, and organelles.
Cytosol
The cytosol is the aqueous matrix rich in bioelements; it is the site of most metabolic reactions.
Cytoskeleton
The cytoskeleton is a three-dimensional network of protein filaments that maintains cell shape, supports organelles, and enables movement of vesicles and other substances.
Filaments include:
- Microfilaments (actin): thin and flexible.
- Intermediate filaments: strong, typically found in animals.
- Microtubules (tubulin): thick; form the spindle, centrosomes, cilia, and flagella.
Organelles
- Endoplasmic reticulum (ER): Rough ER (RER) has ribosomes → protein synthesis. Smooth ER (SER) lacks ribosomes → lipid synthesis and detoxification.
- Golgi apparatus: stacks of membrane sacs (dictyosomes) with cis and trans faces; modify proteins and lipids, form secretion vesicles, and produce lysosomes.
- Lysosomes: single-membrane vesicles containing digestive enzymes; degrade external substances and old organelles.
- Vacuoles: single-membrane vesicles in animal and plant cells; store substances and help maintain plant turgidity.
- Mitochondria: double membrane; divide by binary fission; perform aerobic respiration (glucose + O₂ → CO₂ + H₂O + energy). Structure: outer and inner membrane (cristae), intermembrane space, and matrix (contains circular DNA and ribosomes).
- Chloroplasts (plants): divide by binary fission; perform photosynthesis (CO₂ + H₂O + energy → glucose + O₂). Light-dependent reactions occur in the thylakoid membrane (sunlight to chemical energy); light-independent reactions occur in the stroma (chemical energy to glucose). Structure: double membrane, stroma (circular DNA and ribosomes), and thylakoids stacked into grana.
Endosymbiotic Theory
The endosymbiotic theory explains the origin of mitochondria and chloroplasts:
- Primary endosymbiosis → mitochondria: an anaerobic cell engulfed an aerobic bacterium, which became the mitochondrion, leading to eukaryotic animal cells.
- Secondary endosymbiosis → chloroplasts: a primitive eukaryote (already with mitochondria) engulfed a photosynthetic bacterium, which became the chloroplast, leading to eukaryotic plant cells.
Evidence includes circular DNA, presence of ribosomes, and division by fission.
Ribosomes are compact, non-membrane structures made of rRNA and proteins; they consist of large and small subunits and perform protein synthesis. Ribosomes can be free in the cytosol, attached to RER, or found in mitochondria and chloroplasts.
Centrosome (animals): consists of two centrioles; organizes microtubules, forms the mitotic spindle, and is involved in the formation of cilia and flagella.
3.4 The Nucleus
The nucleus is a defining feature of eukaryotic cells and is visible during interphase. In animal cells the nucleus is typically spherical and centrally located; in plant cells it is often flattened and displaced by vacuoles.
Main Components of the Nucleus
- Nuclear membrane/envelope: two membranes (the outer membrane has ribosomes and is continuous with the RER; the inner membrane lacks ribosomes), with a perinuclear space and nuclear pores that mediate exchange between nucleus and cytoplasm.
- Nucleoplasm: internal fluid containing water, ions, nucleotides, RNA, and proteins.
- Nucleolus: a dense, spherical region that produces ribosomes; there may be one or multiple nucleoli per nucleus.
- Chromatin: DNA associated with histone proteins; histones compact and organize DNA.