Assemblies of Molecules Cells and Groups

Assemblies of molecules, cells, and groups of cells within single cellular and multicellular organisms Plasma Membrane (BIO, BC) • General function in cell containment • Composition of membranes o Lipid components (BIO, BC, OC) ▪ Phospholipids • Lipid made of glycerol, two fatty acid tails, and a phosphate-linked head group • Biological membranes usually involve two layers of phospholipids with their tails pointing inward, an arrangement called a phospholipid bilayer ▪ Steroids • Maintains membrane fluidity and stability (cholesterol) • Can bind to steroid hormone receptors ▪ Waxes • Contributes to stability • Forms barrier against water o Protein components ▪ Integral membrane proteins = integrated into membrane • At least on hydrophobic region that anchors them to the hydrophobic core of the phospholipid bilayer • Those that span entire membrane are called transmembrane proteins ▪ Peripheral membrane proteins = outside and inside surfaces of membranes • Attached either to integral proteins or to phospholipids • Do not stick into the hydrophobic core of the membrane (more loosely attached) o Fluid Mosaic Model ▪ Describes the membrane as protein boats floating in a sea of lipids o Membrane dynamics ▪ Everything in the cell moves around ▪ Uncatalyzed movement (trans-bilayer diffusion) • Phospholipid on the outer leaflet (extracellular part of membrane) move into the inner leaflet (intracellular part of membrane) or vice versa • Very slow movement = no catalyst ▪ Lateral Diffusion • Movement of phospholipid from side to side • Fast type of movement ▪ Catalyzed Movement • Need a catalyst (protein) • Uses ATP, protein is called flippase • Floppase also uses ATP to bring phospholipid from inner leaflet to the outer leaflet ▪ *Many ways to move phospholipids around the membrane o Solute transport across membranes ▪ Thermodynamic considerations • Diffusion: Thermodynamically spontaneous process o Movement of solutes from higher concentration to lower concentration create a negative free energy change (Delta G) o As a result, energy will be produced as solutes move across the membrane o Increases the entropy of a system and decreases the free energy o Chemical concentration gradient and electrical gradient dictate direction of diffusion ▪ Osmosis • Colligative properties; osmotic pressure (GC) o Colligative properties: properties based on NUMBER of particles present, rather than type o Osmosis: water diffuses freely across the membrane, but not ions (so osmosis occurs readily) o Spontaneous net movement of solute molecules through a semi-permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides o Osmotic Pressure: “Pulling” pressure generated by concentration gradient encouraging osmosis ▪ Force that moves water down the concentration gradient ▪ Greater the difference in tonicity across the semi-permeable membrane, the greater the osmotic pressure ▪ In capillaries: osmotic pressure encourages flow of blood from tissues into capillaries. Hydrostatic pressure (created by slow movement of blood) counteracts osmotic pressure and encourages flow from capillaries into tissue ▪ Passive Transport • Any thermodynamically favorable movement of a solute across a membrane (down the gradient) • No energy is required due to the concentration gradient driving the movement of the solute • Simple diffusion: solute diffuses through the membrane without any help from a protein • e.g. Steroid Hormones ▪ Active Transport • Sodium/Potassium Pump o Secondary active transport that maintains negative potential across lipid bilayer o Pump 3 Sodium (Na+) out, 2 Potassium (K+) in o Cell maintains negative resting potential • Requires energy input • Always involves a protein • Primary active transport: ATP hydrolysis is coupled to transport molecules • Secondary active transport: ATP is first used to create a gradient, then potential energy is used to transport the molecules (indirectly use ATP) o Membrane channels ▪ To help ions cross the membrane, there are ion channels o Membrane potential ▪ The resting potential of the cell membrane is negative because of the sodium potassium pump o Membrane receptors ▪ Transmit signals from the extracellular space into the cytoplasm ▪ Ligand: hormones and neurotransmitters that require membrane receptors to cross the plasma membrane ▪ The binding of membrane receptors triggers production of second messengers ▪ Hormone types: • Small, hydrophobic: cross membrane, act on transcription usually, cause the increase of membrane/target proteins in a cell • Large, lipophobic: binds to receptor on cell membrane, could cause activation of second messenger which kicks off chemical cascade ▪ The same signaling molecule can have different effects depending on cell location o Exocytosis and Endocytosis ▪ Endocytosis types: • Phagocytosis: ligands signal for membrane to engulf particles • Pinocytosis: engulfs extracellular fluid, takes up solute • Receptor-mediated endocytosis: takes up ligands ▪ Exocytosis • Same as endocytosis but backwards o Intercellular junctions (BIO) ▪ Gap junctions • Small tunnels connecting cells • Action potentials, small molecules, ions can propagate through them • Present in cardiac and smooth muscle ▪ Tight junctions • Forms watertight seal from cell to cell – prevents fluids from moving past • Can act as barrier to prevent fluid from seeping past cells • Present in endothelial cells ▪ Desmosomes • Join two cells at a single point • Present in cells that experience stress due to sliding (skin, intestinal epithelium) Membrane-Bound Organelles and Defining Characteristics of Eukaryotic Cells (BIO) • Defining characteristics of eukaryotic cells: membrane bound nucleus, presence of organelles, mitotic division • Nucleus o Compartmentalization, storage of genetic information ▪ Nuclear membrane/nuclear envelope surrounds the nucleus ▪ Genetic information is stored inside the nucleus as DNA o Nucleolus: location and function ▪ Non-membrane bound area in nucleus ▪ Transcribes rRNA and assembles ribosomal subunits o Nuclear envelope, nuclear pores ▪ Nuclear envelope: double phospholipid bilayer wrapping around the nucleus ▪ Nuclear pores: large holes perforating envelope • Let’s RNA out but not DNA • Mitochondria o Site of ATP production o Inner and outer membrane structure (BIO, BC) ▪ Inner membrane invaginates to form cristae ▪ Electron transport chain takes place along inner membrane ▪ Intermembrane space separates outer and inner membranes ▪ Outer membrane permeable to small molecules (inner membrane is almost impermeable to these → need to maintain chemiosmotic gradient) o Self-replication ▪ Mitochondria have circular DNA that self-replicates (inherited by the mother) • Lysosomes: membrane-bound vesicles containing hydrolytic enzymes • • • o Digests things like food and viral/bacterial particles o Things that you want to digest gets into a vacuole by endocytosis or phagocytosis, and then the vacuole fuses with the lysosome (anything inside gets digested by the hydrolytic enzymes) o Rupture during apoptosis (lysosome count increases before apoptosis) ▪ Cells also shrinks and caspases (protease enzymes) are activated o Hydrolytic enzymes are activated by low pH ▪ pH lowered by lysosome pumping protons into its interior after binding to a food vesicle Endoplasmic reticulum o Rough and smooth components ▪ RER: synthesizes almost all proteins to be secreted out of cell ▪ SER: involved in gluconeogenesis and fat storage (in liver), detoxification of alcohol, cholesterol formation. Function depends on cell type o Rough endoplasmic reticulum sites of ribosomes ▪ Ribosomes on cytosolic side of RER ▪ Ribosomes attach to the outside of the RER and synthesis of protein into the lumen o Double membrane structure ▪ Double membrane separates ER lumen from cytosol o Role of membrane biosynthesis ▪ Supplies membrane with proteins and lipids ▪ Can facilitate membrane expansion o Role in biosynthesis of secreted proteins ▪ Enzymes essential for biosynthesis could be secreted Golgi apparatus: general structure and role in packaging and secretion o Flattened, membrane bound sacs o Organizes and concentrates proteins into vesicles full of proteins o Can secrete proteins out of cell or back into mitochondria or ER Peroxisomes: organelles that collect peroxides o Vesicles in cytosol involved production and breakdown of hydrogen peroxide o Also involved in lipid and protein storage, detoxification, and regulation of oxygen concentration o Essential for lipid breakdown, detoxification of drugs and chemicals in liver o Prevent damage from oxygen radicals Cytoskeleton (BIO) • General function in cell support and movement • Microfilaments: compositions and role in cleavage and contractility o Composition: actin filaments, globular subunits o Roles ▪ Cleave: pinches cytoplasm during cytokinesis ▪ Contractility: interact with myosin to cause muscle contractions • Microtubules: composition and role in support and transport o Composition: made up of 13 tubulin filaments arranged in a hollow tube o Roles ▪ Support: support shape of the cell ▪ Transport: serves as platform for vesicle (dynein and kinesin) o Also moves chromosomes around the cell (spindle apparatus) • Intermediate filaments: role in support o Not as flexible as the other two o Primarily used for structural rigidity o Keratin = intermediate filament • Composition and function of cilia and flagella • o Composition: 9 pairs of microtubules form a circle around 2 lone microtubules (9 + 2 arrangement) o Function: move fluid to cause cell or nearby substances to move Centrioles, microtubule organizing centers (MTOC) o MTCO: attaches “-“ end of microtubule o Centriole: production of flagella and cilia, form the centrosome (major MTOC in animal cells) o Microtubules radiate out of these barrel shaped structures, which are made of microtubules themselves Tissues Formed From Eukaryotic Cells (BIO) • Epithelial cells o Lines inside and outside of organs o 2 types ▪ Single layer: found in alveoli, capillaries ▪ Stratified: multiple layers, found in stomach, esophagus • Connective tissue cells o Two components of connective tissue ▪ 1) Cells ▪ 2) Matrix • Matrix can be made up of ground substance and fibers (need at least one) o Extracellular matrix: molecular network that holds tissue cells in place ▪ Fibroblasts secrete fibrous proteins that make the matrix o Consistency, rigidity of matrix varies from cell to cell o 3 Classes of molecules making up animal cell matrices ▪ 1) Glycosaminoglycans and proteoglycans: provides pliability to matrix ▪ 2) Structural proteins: provide strength to matrix ▪ 3) Adhesive proteins: help individual cells within tissues stick together