Essential Biological Processes: Metabolism, Transport, and Transcription

Metabolic Pathways

Metabolic pathways consist of chains and cycles of enzyme-catalyzed reactions. Chemical reactions occur in a sequence of small steps. Some form a chain, while others form a cycle where the end product is the reactant that starts the pathway. These are categorized as:

• Anabolic pathways: Build up molecules.
• Catabolic pathways: Break down molecules.

Functions of Membrane Proteins

• Hormone binding sites: For example, insulin.
• Immobilized enzymes: With the active site on the outside (e.g., small intestine).
• Cell adhesion: To form tight junctions between groups of cells in tissues and organs.
• Cell-to-cell communication.
• Channels for passive transport: Allow hydrophilic particles across via facilitated diffusion.
• Pumps: Proteins for active transport that use ATP to move particles across the membrane.

Active Transport

The movement of a substance against the concentration gradient.

Endosymbiotic and Exocytosis Theories

Endosymbiotic Theory

Suggests that mitochondria were originally prokaryotic cells that performed aerobic respiration. A large prokaryotic cell took in this small cell by endocytosis, and they began to live in a mutualistic association.

Exocytosis

A secretory vesicle fuses with the plasma membrane, releasing the vesicle's contents to the cell exterior. The vesicle membrane becomes part of the plasma membrane.

Transcription

The synthesis of RNA using DNA as a template. DNA strands are unzipped by RNA polymerase, and free nucleotides in the nucleus fit into place, where G pairs with C and A pairs with U. RNA polymerase gathers the nucleotides in the correct place, forming covalent bonds.

Initiation

Transcription begins near an area at the end of a gene called the promoter. Thanks to transcriptional factors, RNA polymerase is able to bind to the promoter. This allows RNA polymerase to untwist and break the hydrogen bonds between the bases, identifying the antisense strand (template) and the sense strand (DNA strand with the same sequence as the mRNA).

Elongation

RNA polymerase adds complementary nucleotides to the antisense strand. Because RNA does not have thymine, uracil (U) complements adenine (A). Two phosphates are removed from the nucleoside to provide the energy required to create covalent bonds between the nucleotides.

Termination

RNA polymerase finds a sequence that signals the end of transcription. The mRNA is released, and the DNA strands rejoin, forming hydrogen bonds and twisting back into a helix.

Post-Termination

This involves the modification of mRNA before it leaves the nucleus. Introns (non-coding sequences) are removed, and exons (coding sequences) are spliced together to produce mature RNA. In prokaryotic cells, there is no post-transcriptional modification because they lack introns.