Cell Theory: Structure, Function, and Evolution of Cells
Cell Theory
All living organisms are composed of cells. The cell is the structural and physiological unit of living things, where metabolic processes occur. Cells contain hereditary information, passed from mother to daughter cells.
Comparison: Prokaryotes vs. Eukaryotes
Prokaryotes
- Typical of small cells (e.g., monera)
- No defined nucleus
- No cell envelope; may have a capsule
- Genetic material is circular DNA; no mitosis
- Lack cytoskeleton and most organelles
- Small ribosomes
- Cell wall usually present
Eukaryotes
- Typical of protists, fungi, plants, and animals
- True nucleus with nuclear membrane
- Genetic material consists of multiple linear DNA molecules
- Cytoskeleton and various organelles present
- Large ribosomes
Comparison: Animal vs. Plant Eukaryotic Cells
Plant cells are similar to animal cells but differ in several aspects:
- Plant cells are surrounded by a cell wall.
- Plant cells contain large vacuoles and various types of plastids.
- Plant cells lack centrioles.
Cellular Components
Golgi Apparatus
Composed of membranes; involved in protein transport, protein synthesis, polysaccharide synthesis, and lipid synthesis. In plant cells, it synthesizes polysaccharides for the cell wall.
Centriole
Cylindrical structure of microtubules and filaments; maintains cell shape and contributes to cell movements.
Lysosomes
Vesicles containing enzymes that digest food within cells.
Plasma Membrane
Encloses the cell; restricts the passage of material between the outside and inside of the cell.
Mitochondria
Organelle where chemical reactions occur to provide energy for cellular activities (metabolism).
Nucleus
Organelle surrounded by a double membrane with pores; contains the cell’s genetic information (chromatin, which condenses into chromosomes). The nucleolus, where ribosomes originate, is also present. In plant cells, the nucleus often occupies an eccentric position due to large vacuoles.
Cell Wall (Cellulose)
Surrounds the membrane; provides protection and rigidity to the cell.
Peroxisome
Vesicles containing oxidative enzymes.
Plastids
Several types exist; chloroplasts are where photosynthesis occurs.
Endoplasmic Reticulum (ER)
Network of membranes extending from the nuclear membrane throughout the cytoplasm. Smooth ER lacks ribosomes and acts in lipid synthesis. Rough ER has ribosomes and acts in protein processing and transport.
Ribosomes
Structures that perform protein synthesis.
Vacuole
In plant cells, stores pigments, reserve substances, or waste products. In animal cells, can be digestive or contractile.
Cell Origin and Evolution
It is believed that the first cell (progenote) arose when self-replicating material and biomolecules were isolated within a phospholipid membrane.
Cell Evolution
Progenotes were anaerobic and heterotrophic, obtaining energy directly from the environment. As organic matter became scarce, cells evolved to create their own metabolism. Over time, cells developed the ability to produce and control energy, leading to the emergence of photosynthesis.
Prokaryotic Cells
Three types of prokaryotic cells developed: archaea, eubacteria, and urkaryotes, which are thought to be the ancestors of eukaryotes.
Eukaryotic Cells
Eukaryotic cells emerged around 1500 million years ago. The currently accepted theory of their origin is the endosymbiotic theory proposed by Lynn Margulis.
Endosymbiotic Theory
- A primitive urkaryotic cell, anaerobic and heterotrophic, lost its cell wall and folded its membrane to increase absorptive surface area.
- Membrane folding formed vesicles, initiating intracellular digestion. Membrane fragments with circular DNA attached formed a precursor to the nucleus.
- Synthesis of fibers and microtubules enabled cell movements and endocytosis.
- Incorporation of spirochetes through endocytosis led to symbiosis, resulting in cilia, flagella, and centrioles, enhancing cell mobility.
- The membrane system and the nuclear precursor evolved into true organelles (ER, Golgi, nucleus), forming a primitive phagocyte.
- This phagocyte endocytosed various prokaryotic cells, establishing an endosymbiotic relationship. These cells became organelles (mitochondria, peroxisomes). In endosymbiosis, the phagocyte provided safety and nutrients, while the prokaryotes enhanced metabolism.