Biology 25IB Cell Communication and Virology PDF

Biology 25IB Unit B: Cell Communication and Virology Structure and function of cell membranes https://www.youtube.com/watch?v=Pfu1DE9PK2w Inner life of the cell https://www.youtube.com/watch?v=wJyUtbn0O5Y Guiding Questions How do molecules of lipid and protei...

Biology 25IB Unit B: Cell Communication and Virology Structure and function of cell membranes https://www.youtube.com/watch?v=Pfu1DE9PK2w Inner life of the cell https://www.youtube.com/watch?v=wJyUtbn0O5Y Guiding Questions How do molecules of lipid and protein assemble into biological membranes? How do cells distinguish between the many different signals that they receive? What interactions occur inside animal cells in response to chemical signals? B2.1.2—Lipid bilayers as barriers The hydrophobic hydrocarbon chains that form the core of a membrane have low permeability to large molecules and hydrophilic particles, including ions and polar molecules, so membranes function as effective barriers between aqueous solutions. B2.1.10—Fluid mosaic model of membrane structure Fluid means that the molecules can move past one another. https://www.youtube.com/watch?v=Qqsf_UJcfBc B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity phosphate A phospholipid is like a unsaturated { triglyceride but has fatty acid 1 glycerol 2 fatty acids 1 phosphate. glycerol saturated fatty acid B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity No double bonds between carbon atoms. At least 1 double bond between carbon atoms. The double bond creates a bend in the fatty acid molecule B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity Effect of fatty acid type on membrane fluidity Saturated phospholipids Unsaturated phospholipids pack tightly reducing fluidity pack loosely increasing fluidity low permeability by simple diffusion higher permeability by simple diffusion stiffer more flexible higher melting point lower melting point B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity Cell membranes are specialized to have specific proportions of saturated and unsaturated phospholipids adapted for their function. Proportions of saturated and unsaturated phospholipids also depend on temperature the cell experiences. B2.1.11—Relationships between fatty acid composition of lipid bilayers and their fluidity Fish in Antarctic waters have a higher percentage of unsaturated fatty acids in their membranes than fish in warmer waters. Seedlings raised in warm greenhouses should be put outside for brief periods to give them time to change their membrane composition. Crocodile Icefish P 221 B2.1.12—Cholesterol and membrane fluidity in animal cells Cholesterol is lipid-soluble embedded in the hydrophobic core of the phospholipid bilayer involved in the fluidity/stiffness of the membrane B2.1.12—Cholesterol and membrane fluidity in animal cells fluid rigid At high temperatures, cholesterol At low temperatures, cholesterol decreases membrane fluidity and increases membrane fluidity and increases its rigidness. decreases its rigidness. Multicellularity created new requirements in organisms. Communication between cells became essential. Formation of tissues and organs required attachment between cells. Membrane proteins are used for these needs B2.1.4—Integral and peripheral proteins in membranes Membrane proteins have diverse structures, locations and functions. Hydrophobic sections embed in the lipid layer Hydrophilic sections are outside the membrane Hydrophobic sections may anchor peripheral proteins B2.1.4—Integral and peripheral proteins in membranes https://www.youtube.com/watch?v=Ym3mTa5WEOY B2.1.4—Integral and peripheral proteins in membranes Membrane proteins have diverse structures, locations and functions. Functions of Membrane Proteins JETRAT Junctions – connect and join cells together Enzymes – fixed in membranes localises metabolic pathways Transport – facilitated diffusion and active transport Recognition – may function as markers for cellular identification Anchorage – attachment points for cytoskeleton and extracellular matrix Transduction – receptors for peptide hormones B.2.1.9—Structure and function of glycoproteins and glycolipids Glycoproteins and glycolipids have carbohydrate chains extending into the extracellular environment which play a role in cell recognition. Glycolipids are lipids with Glycoproteins are simple carbohydrate conjugated proteins chains attached. (contain a non-protein part). B.2.1.9—Structure and function of glycoproteins and glycolipids A carbohydrate-rich environment surrounds the cell. It is called the glycocalyx. glycocalyx glycocalyx Glycocalyx of adjacent cells can fuse, bonding the cells together in tissues and organs. B1.1.7—Role of glycoproteins in cell–cell recognition Glycoproteins allow other things to recognize them. A cell-cell recognition B toxin recognition C virus recognition D antibody recognition E bacteria recognition This helps tissues to form by binding cells together. B1.1.7—Role of glycoproteins in cell–cell recognition Glycoproteins may act as antigens in a foreign body. eg. ABO blood groups in red blood cells. These glycoproteins have no known function in erythrocytes. B2.1.17—Adhesion of cells to form tissues (CAMs) Cell adhesion molecules (CAMs) are used for different types of cell–cell junction. They are integral proteins anchored in the cell membrane and protruding into the extracellular fluid. Most are glycoproteins. Detailed knowledge of different CAMs is not required B2.1.17—Adhesion of cells to form tissues (CAMs) Roles of cell-cell adhesion Maintain architecture of tissues and organs Control movement of extracellular fluids Immunity against foreign antigens T-cell activation The same type of CAMs are present in cells of the same type forming tissues. C2.1.1—Receptors as proteins with binding sites for specific signalling chemicals Signaling molecules transmit messages from one cell to another as a form of communication. Examples: hormones (eg. estrogen, insulin) neurotransmitters (eg. dopamine, serotonin, oxytocin) immune chemicals (eg. histamine, cytokines) autoinducers in quorum sensing C2.1.1—Receptors as proteins with binding sites for specific signalling chemicals Chemical signals Chemical signals produced by a cell bind to receptor proteins on another cell (receptors are integral glycoproteins). Binding causes changes in the receptor protein causing changes in the target cell. Target cell C2.1.1—Receptors as proteins with binding sites for specific signalling chemicals Signalling molecules that bind selectively to a specific site on a receptor are called ligands. It is similar to enzyme-substrate binding occurs at a specific site binding is specific - no other ligand can bind here receptors are not permanently changed by the binding But, the ligand can remain bound for longer. C2.1.1—Receptors as proteins with binding sites for specific signalling chemicals The receptor-ligand complex causes a conformational change in the glycoprotein receptor. This initiates a cascade of reactions in the cytoplasm of the target cell that causes a change in the function of the cell (often a change in gene expression). C.2.1.2—Cell signalling by bacteria in quorum sensing Quorum sensing: ability to detect when the population is large enough for the group to function cooperatively. Bacteria can communicate via signalling molecules called auto-inducers because they induce change in themselves. All bacterial cells secrete signaling molecules bacterium at a low rate which bind to receptors on the surface of the bacteria. This is an example of cell signalling! C.2.1.2—Cell signalling by bacteria in quorum sensing Quorum sensing: ability to detect when the population is large enough for the group to function as a group. As the number of bacteria increase, the concentration of signalling molecules will increase. When bacteria have a specific density of bacterium signal molecules bound, gene expression changes altering behavior of the bacteria. Bacteria behave differently as a group! C.2.1.2—Cell signalling by bacteria in quorum sensing At low concentration At high concentration of of autoinducer, active autoinducer, genes are genes benefit activated that benefit a individual cells. bacterium community of cells. C.2.1.2—Cell signalling by bacteria in quorum sensing Example 1: Symbiosis in bobtail squid and bacteria Vibrio fischeri have a symbiotic relationship. V. fischeri live in the light organ of the squid and get nutrients from the squid. C.2.1.2—Cell signalling by bacteria in quorum sensing The concentration of bacteria in the light organ is high enough to meet quorum. A quorum of V. fischeri in the squid causes emission of light energy (greenish-blue). This camouflages the squid in the moonlight. This is called mutualism because both species benefit! https://www.youtube.com/watch?v=mQ43fuJJW7M (0 - 2:00) Read page 415 of Oxford Biology to learn more. C.2.1.2—Cell signalling by bacteria in quorum sensing Biofilms: dental plaque, in water pipes - a quorum changes the behavior of the bacteria allowing them to create a film that sticks to surfaces. Single bacteria alone do not stick to surfaces. They require a cooperative group. C.2.1.2—Cell signalling by bacteria in quorum sensing Pathogenicity: Quorum sensing increases virulence of some bacterial diseases such as cholera (caused by Vibrio cholerae). mutant V. cholerae without quorum sensing are not pathogenic in mice. Interference of quorum sensing may act as an antibiotic by shutting down biofilm formation and toxin production. https://www.youtube.com/watch?v=q2nWNZ-gixI&t=3s C.2.1.2—Cell signalling by bacteria in quorum sensing Quorum sensing: cell signaling chemicals are used for bacteria to detect when population density is great enough to behave as a cooperative group. C2.1.6—Differences between transmembrane receptors in a plasma membrane and intracellular receptors in the cytoplasm or nucleus There are also intracellular receptor proteins in the cytoplasm of cells that respond to signalling molecules. These receptor proteins are hydrophilic so remain dissolved in cytoplasm or nucleus. The signaling molecules are hydrophobic so can pass through the membrane. C2.1.6—Differences between transmembrane receptors in a plasma membrane and intracellular receptors in the cytoplasm or nucleus Binding of the ligand to the receptor will cause changes in the cell. Example: Testosterone is a steroid hormone that passes through the membrane of target cells. It causes changes in gene expression resulting in the following secondary sex characteristics growth of facial hair growth of larynx to create a deeper voice and an adam’s apple increased muscle mass production of sperm in the testes etc C2.1.6—Differences between transmembrane receptors in a plasma membrane and intracellular receptors in the cytoplasm or nucleus Transmembrane receptors Transmembrane receptors and intracellular receptors are similar in that they both bind a signalling ligand and cause changes in target cell Intracellular receptors function. C2.1.6—Differences between transmembrane receptors in a plasma membrane and intracellular receptors in the cytoplasm or nucleus Transmembrane receptors Intracellular receptors Receptor protein is hydrophobic Receptor protein is hydrophilic to dissolve to embed in the membrane in the cytoplasm Signal molecule is hydrophilic so cannot pass through the Signal molecule is small and hydrophobic membrane to pass through the membrane Eg. protein hormones Eg. steroid hormones C2.1.7—Initiation of signal transduction pathways by receptors Signal Transduction Pathway After a signalling ligand binds to a receptor protein, it causes a series of interactions in the cell. Often it starts a cascade of reactions in the cytoplasm. C2.1.7—Initiation of signal transduction pathways by receptors Cascade of reactions The ligand is the 1st messenger. Ligand signals synthesis of a 2nd messenger (often cAMP). This amplifies the signal. C2.1.7—Initiation of signal transduction pathways by receptors Signal Transduction Pathway These reactions result in a cellular response often changing gene expression C2.1.7—Initiation of signal transduction pathways by receptors Signal Transduction Pathway These reactions result in a cellular response often changing gene expression amplify the signal C2.1.7—Initiation of signal transduction pathways by receptors Signal Transduction Pathway These reactions result in a cellular response often changing gene expression amplify the signal crosstalk can occur between more than one pathway to improve integration and efficiency of metabolism C2.1.9—Transmembrane receptors that activate G proteins An example of signal transduction in cells: G-protein-coupled receptors (GPCRs) are a family of transmembrane receptors that act like molecular switches turning on specific functions in cells. Many different stimuli from outside a cell can transmit a signal into the cell by binding to GPCRs. C2.1.9—Transmembrane receptors that activate G proteins GPCRs are associated with G proteins which are GPCR bound to the inner surface of the membrane. G proteins consist of 3 subunits (𝛂, 𝛃 and 𝞬). GDP is attached to the 𝛂 subunit when the complex is inactive. GDP is similar to ADP Inactive form (GDP attached) C2.1.9—Transmembrane receptors that activate G proteins Specific ligands (signalling molecules) can bind to the extracellular part of GPCR. ligand C2.1.9—Transmembrane receptors that activate G proteins Ligand binding will activate G protein by the replacement of the low energy GDP with high energy GTP. Active form (GTP attached)  C2.1.9—Transmembrane receptors that activate G proteins Activated G protein causes G protein to separate into its subunits. Activated subunits trigger a cascade of events using GTP energy resulting in a change in cell function. https://www.youtube.com/watch?v=Glu_T6DQuLU 3:10 https://www.youtube.com/watch?v=NL_YbPigDzg 3:23 C2.1.9—Transmembrane receptors that activate G proteins G protein activation changes activities in the cell. Activation of an ion channel Activation of an enzyme Change in cascade gene expression C2.1.9—Transmembrane receptors that activate G proteins G protein activation is widely used to communicate important signals between cells in multicellular organisms. Examples: - T cell activation - Nervous transmission (ion channel activation) - Endocrine system (hormone action) - Brain functions (learning, memory) - Embryonic development - Plant immunity - Plant growth and development C2.1.9—Transmembrane receptors that activate G proteins G protein activation is widely used to communicate important signals between cells in multicellular organisms. We use knowledge of these systems to produce therapeutic drugs! Linking Questions What are the roles of cell membranes in the interaction of a cell with its environment? What patterns exist in communication in biological systems? https://www.youtube.com/watch?v=-dbRterutHY (more depth than necessary)