Cells

• Ribosomes are responsible for protein synthesis in the cell.
• Cells are the smallest living units and the structural and functional unit of life
• The entire organism's function depends on the individual and combined activities of all its cells
• Biochemical functions of cells are dictated by the shape of the cell and its specific composition
• Continuity of life has a cellular basis: cells can arise only from other preexisting cells
• Over 250 different types of human cells exist, differing in size, shape, and subcellular components, leading to functional differences
• All cells have common structures: plasma membrane, cytoplasm, and nucleus
• The plasma membrane is a flexible barrier that separates intracellular fluid from extracellular fluid
• The plasma membrane is made of lipids, proteins, and carbohydrates
• The lipid bilayer of the plasma membrane consists of phospholipids, glycolipids, and cholesterol
• Membranes are fluid structures, with lipids and proteins moving easily in the bilayer
• Plasma membranes are selectively permeable, allowing the passage of small, nonpolar molecules
• Integral proteins are firmly inserted into the membrane and function as transport proteins, enzymes, or receptors
• Peripheral proteins are loosely attached to integral proteins and have various functions like enzymatic activity and cell-to-cell connections
• The glycocalyx consists of sugars sticking out of the cell surface and functions as biological markers for cell recognition
• Cell junctions like tight junctions, desmosomes, and gap junctions play roles in cell adhesion and communication
• Tight junctions prevent movement of molecules between cells, desmosomes provide physical strength, and gap junctions allow rapid communication
• Gap junctions are tunnels formed by transmembrane proteins that allow small molecules to pass between cells
• Gradients across the plasma membrane, like concentration and electrical gradients, influence cellular functionality
• Transport processes across the plasma membrane can be passive (simple diffusion, facilitated diffusion, osmosis) or active (primary and secondary transport, vesicular transport)
• In facilitated diffusion, transmembrane proteins help move solutes that are too polar to pass through the lipid bilayer
• Carrier-mediated facilitated diffusion involves carriers that transport specific polar molecules by changing shape
• Channel-mediated facilitated diffusion uses channels formed by transmembrane proteins to transport molecules down their concentration gradient
• Osmosis is the net movement of a solvent through a selectively permeable membrane from an area of high concentration to low concentration
• Diffusion of solutes and osmosis of water occur across the membrane until equilibrium of solutes and water is reached
• Equilibrium is when there is the same concentration of solutes and water molecules on both sides, with equal volume on both sides
• Osmolarity measures the concentration of the total number of solute particles in a solvent
• Water concentration varies with the number of solute particles because solute particles displace water molecules
• When solute concentration increases, water concentration decreases, and vice versa
• Osmolarity is equal to molarity times the number of ions (particles)
• Example: NaCl (1 particle) ionizes to Na+ and Cl− (2 particles), so a 1 M solution of NaCl equals a 2 Osm solution
• Osmolarity is expressed in osmoles/liter (osmol/L)
• Water moves by osmosis from areas of low solute (high water) concentration to areas of high solute (low water) concentration
• Tonicity is the ability of a solution to change the shape or tone of cells by altering the cells’ internal water volume
• Isotonic solution has the same osmolarity as inside the cell, so the volume remains unchanged
• Hypertonic solution has a higher osmolarity than inside the cell, so water flows out of the cell, resulting in cell shrinking (crenation)
• Hypotonic solution has a lower osmolarity than inside the cell, so water flows into the cell, resulting in cell swelling, which can lead to cell bursting (lysing)
• If the plasma membrane is severely damaged, substances can diffuse freely into and out of the cell, compromising concentration gradients
• If you drink too much water too quickly, it can lead to water toxicity because the extracellular fluid becomes hypotonic, causing cells to swell
• Active membrane transport includes active transport, primary active transport, secondary active transport, and vesicular transport