Cellular Evolution and Internal Dynamics

Photosynthesis: An Ancient Process

Evolutionary Origin: Photosynthesis initially evolved in bacteria.
Blue-Green Algae (Cyanobacteria): Photosynthesizing bacteria are often referred to as blue-green algae, a term derived from the visible color of their clumps or groups.
Pigment Similarity: These prokaryotic organisms possess the same pigments, primarily chlorophyll, that make chloroplasts in plants and eukaryotic algae appear green. This highlights a fundamental biochemical link between these diverse life forms.
Micrograph Reference: A micrograph of a chloroplast was shown to illustrate the internal structure containing these pigments.

Palaeo DNA Example: The discussion included an example of mitochondrial DNA (mtDNA) extracted from a mammoth, referred to as "paleo DNA."
Gene Representation: Marked sections on the DNA molecule represent genes.
Structure: Unlike the linear chromosomes found in the nuclear genome of eukaryotes, mitochondrial DNA is typically circular, resembling the DNA of prokaryotes.
Dependent Organelles: Mitochondria and chloroplasts cannot survive independently outside of a host cell.
Size: The mammoth mtDNA cited was 16,842 BP (base pairs).
- Base Pairs (BP): In genetics, BP denotes the number of adenine (A), thymine (T), guanine (G), and cytosine (C) nucleotide pairs present in a DNA molecule.
- Comparison to Eukaryotic Chromosomes: This size is minuscule when compared to eukaryotic chromosomes; for instance, the human chromosome number one is approximately 300,000,000 base pairs long, demonstrating a vast difference in genetic material quantity.

Prokaryotic Precursors: Prokaryotes initiated their evolution around 4 billion years ago.
Eukaryotic Emergence: A new form of life, the eukaryotic cell, emerged roughly 2 to 2.5 billion years ago.
Mechanism of Evolution: Eukaryotes evolved through the absorption and integration of one prokaryote by another, larger prokaryote.
Role of Archaea and Bacteria: It is believed that both Archaea and Bacteria were involved in this intricate process.
Two-Step Engulfment: The theory posits a sequential series of endosymbiotic events:
- Mitochondrial Origin: Initially, a larger prokaryotic cell engulfed another prokaryote, which subsequently evolved into the mitochondrion. This foundational event marked the beginning of the first eukaryotes.
- Chloroplast Origin (for Photosynthetic Eukaryotes): Later, in the evolutionary lineage leading to plants and eukaryotic algae, a second endosymbiotic event occurred. A photosynthesizing bacterium (blue-green algae) was engulfed and became integrated into the existing eukaryotic cell, evolving into the chloroplast.

Mitochondrial and Chloroplast Similarities to Bacteria: Compelling evidence supports the endosymbiotic theory:
- Own DNA: Both mitochondria and chloroplasts possess their own DNA, which is circular, much like the DNA found in bacteria.
- Ribosome Structure: Their ribosomes are structurally and functionally similar to bacterial ribosomes.
- Division Mechanism: They divide via a process that mirrors bacterial cell division, often referred to as binary fission. This process includes replicating their internal chromosome and then dividing the cytoplasm to form two daughter organelles. This cellular division mechanism is akin to how single-celled bacteria reproduce to create two identical daughter cells from an original cell.
Implication: These similarities strongly suggest that eukaryotic cells, like our own, are the result of multiple ancestral cellular components coming together and integrating over evolutionary time.

Definition: The cytoskeleton comprises a complex network of crisscrossing protein elements within the cytoplasm of eukaryotic cells.
Intracellular Movement: It enables the movement and streaming of materials inside the cell.
- Amoeba Example: An amoeba demonstrates pronounced cytoplasm streaming and the extension of pseudopods ("false legs") for movement, which is facilitated by its cytoskeleton.
Additional Functions: In some cells, the cytoskeleton also plays roles in enzyme synthesis (as a scaffold for protein production) and acts as an intracellular transport system, likened to a "railroad."

Neuron Overview: A generalized neuron consists of a cell body and an axon.
- Neurotransmitter Production: Neurotransmitters are synthesized in the cell body.
- Neurotransmitter Release: Upon an electrical signal (firing), neurotransmitters are released from the axon terminals into synapses.
Protein Filament Highway: The axon contains a specialized protein filament structure that serves as a "highway" for transporting materials.
Motor Proteins: Special motor proteins are responsible for this transport.
- Mechanism: These proteins attach to vesicles containing neurotransmitters. Utilizing ATP as energy, they undergo conformational changes, effectively "walking" along the protein filament. This precise movement delivers the vesicles from the cell body all the way down to the axon terminals, where their contents are released.

Microfilaments: These are one of the three primary types of protein filaments within the cytoskeleton.
- Structure: They are characterized by being long and slender.
- Composition: Primarily composed of the protein actin.
- Function 1: Muscle Contraction: Actin is fundamental to muscle cell contraction across all muscle types (cardiac, voluntary/skeletal, and smooth muscle). Muscle movement occurs when motor proteins interact with and pull on actin filaments, facilitating shape changes and force generation.
- Function 2: Cilia and Flagella Movement: The movement of cellular appendages like cilia and flagella is also based on the intricate interaction between motor proteins and these filaments.
- Versatility of Actin: Actin is a highly complex and versatile molecule, found in various cellular contexts where it performs diverse structural and dynamic functions, often through its interaction with different motor proteins.
- Motor Protein Visualization: The visual animation of motor proteins carrying cargo (like vesicles) along filaments often depicts them with small "feet," illustrating their ATP-driven