Cell Biology Overview and Cell Theory

Study Notes

Cell biology, formerly called cytology, is a branch of biology.
It studies cells, their physiological properties, structure, interactions with their environment, division, function, and death.
This study is conducted at both the microscopic and molecular levels.

Early studies described cells as consisting of protoplasm (essential substance) containing a nucleus and surrounded by a membrane (cell/plasma membrane).
The protoplasm surrounding the nucleus is called cytoplasm, and the nucleus's protoplasm is called nucleoplasm.
Electron microscopy (EM) revealed two fundamental cell types:
- Prokaryotes (before nucleus): simpler, without a nuclear envelope or compartmentalization.
- Eukaryotes (true nucleus): more complex, with a nuclear envelope and organelles.

Feature	Prokaryote	Eukaryote
Size of cell	Typically 0.2-2.0 µm in diameter	Typically 10-100 µm in diameter
Nucleus	No nuclear membrane or nucleolus	True nucleus with nuclear membrane and nucleolus
Organelles	Absent	Present; examples include lysosomes, Golgi complex, endoplasmic reticulum, mitochondria, and chloroplasts
Flagella	Consist of two protein building blocks	Complex; consists of multiple microtubules
Glycocalyx	Present as a capsule or slime layer	Present in some cells that lack a cell wall
Cell Wall	Usually present; chemically complex; examples include peptidoglycan in bacterial cell walls, or cellulose or chitin in plants	Usually present; chemically complex in plants, some protists, fungi
Plasma Membrane	No carbohydrates and generally lacks sterols	Sterols and carbohydrates that can serve as receptors
Cytoplasm	No cytoskeleton or cytoplasmic streaming	Cytoskeleton; cytoplasmic streaming
Ribosomes	Smaller size (70S)	Larger size (80S); smaller size (70S) in organelles
Chromosome (DNA)	Usually single circular chromosome; typically lacks histones	Multiple linear chromosomes with histones
Cell Division	Binary fission	Involves mitosis
Sexual Recombination	None; transfer of DNA only	Involves meiosis

Structure	Function
Nucleus	DNA synthesis, RNA synthesis, assembly of ribosomal subunits; contains chromosomes and nucleolus
Ribosomes	Protein synthesis
Rough ER	Synthesis of membrane lipids and proteins; packaging of proteins in vesicles for transport
Smooth ER	Lipid synthesis; detoxification in liver cells; calcium ion storage; modification and transport of macromolecules
Golgi apparatus	Modification and transport of macromolecules; formation of lysosomes and transport vesicles
Lysosomes	Digestion of ingested food, bacteria, damaged organelles, macromolecules; recycling materials
Vacuoles	Digestion; storage of chemicals; cell enlargement; water balance
Peroxisomes	Metabolic processes, breakdown of H2O2
Mitochondria	Conversion of chemical energy of food to chemical energy of ATP
Chloroplasts	Conversion of light energy to chemical energy of sugars

Cell support, movement, and communication*:
Cytoskeleton: maintains cell shape, anchors organelles, aids in movement, and transmits signals
Extracellular matrix: binding of cells in tissues, surface protection, and regulation of cellular activities
Cell junctions: cell communication and tissue integrity
Cell walls: maintenance of cell shape for structure and protection, and interaction between cells

Composed of a mix of proteins and lipids.
Lipids include phospholipids and cholesterol, forming a phospholipid bilayer.
Proteins include integral (penetrate the bilayer) and peripheral (on surfaces) proteins.
Carbohydrates (oligosaccharides) are attached to proteins (glycoproteins) or lipids (glycolipids), acting as receptors and recognition molecules.

The cell membrane regulates material exchange.
Passive Transport: Movement down a concentration gradient; includes diffusion (small molecules like CO2 and H2O), and facilitated diffusion (large molecules via transmembrane proteins).
Active Transport: Movement against a concentration gradient; requires energy and transmembrane proteins.
Bulk Transport: Movement of large particles; includes endocytosis (into cell—pinocytosis for fluids, phagocytosis for solids) and exocytosis (out of cell).