Cellular Organelles

Cellular Organelles and Signaling Overview

• Mechanical sensors transmit extracellular matrix changes into the cell, while plasma membrane receptors serve as binding sites for contact and chemical signaling, most of which are glycoproteins.
• Contact signaling is crucial for development and immunity, and some infectious agents use it to identify target tissues.
• Chemical signaling involves ligands binding to specific receptors, including neurotransmitters, hormones, and paracrine chemicals, with G proteins acting as middlemen between extracellular and intracellular messengers.
• The cytoplasm consists of cytosol, organelles, and inclusions, with organelles performing specific cell functions and inclusions varying by cell type.
• Mitochondria are the cell's power plants, producing most of its ATP supply, and their density reflects a cell's energy requirements.
• Mitochondria have an outer smooth membrane and an inner membrane with shelflike cristae, where enzymes break down food fuels in a multistep process called aerobic cellular respiration.
• Mitochondria contain their own DNA, RNA, and ribosomes, and can increase their number in response to increased ATP requirements.
• Ribosomes are sites of protein synthesis, with free ribosomes making soluble proteins and membrane-bound ribosomes synthesizing proteins for incorporation into cell membranes or export from the cell.
• Endoplasmic Reticulum (ER) is an extensive system of interconnected tubes and parallel sacs, with rough ER studded with ribosomes that manufacture secreted proteins and integral proteins for cellular membranes.
• Rough ER also functions as the cell's membrane factory and attaches sugar groups to proteins for eventual use.
• Smooth ER lacks ribosomes and is involved in lipid metabolism, detoxification, and calcium ion storage.
• The ER is continuous with the outer nuclear membrane and accounts for about half of the cell's membranes, playing important roles in protein synthesis and membrane production.

Cell Junctions and Passive Membrane Transport

• Desmosomes are abundant in high mechanical stress tissues like skin and heart muscle
• Gap junctions are communicating junctions between adjacent cells, allowing the passage of ions and small molecules
• Different types of gap junctions are composed of different transmembrane proteins, determining what can pass through them
• Passive membrane transport involves diffusion of molecules down their concentration gradient
• Simple diffusion allows small nonpolar lipid-soluble molecules to directly diffuse through the lipid bilayer
• Facilitated diffusion allows the passive transport of larger or polar molecules through carrier or channel proteins
• Carrier-mediated facilitated diffusion involves carrier proteins that change shape to transport specific molecules
• Channel-mediated facilitated diffusion involves transmembrane proteins that transport substances, usually ions or water, through aqueous channels
• Osmosis is the diffusion of a solvent, such as water, through a selectively permeable membrane
• Osmotic pressure and tonicity play a role in determining the distribution of water in different fluid-containing compartments of the body
• Osmotic imbalances can cause cells to swell or shrink until the solute concentration is balanced or the membrane breaks
• Tonicity refers to the ability of a solution to change the shape of cells by altering their internal water volume, with isotonic and hypertonic solutions having different effects on cell shape