Physiology Textbook Chapter #3.pdf

Tuesday, September 17, 2024 Physiology Textbook Chapter #3 Objectives a) List the functions of the plasma membrane and the structural features that enable it to perform those functions. b) Describe the organelles of a typical cell, and indicate the speci c functions of each. c) Explain the func...

Tuesday, September 17, 2024 Physiology Textbook Chapter #3 Objectives a) List the functions of the plasma membrane and the structural features that enable it to perform those functions. b) Describe the organelles of a typical cell, and indicate the speci c functions of each. c) Explain the functions of the cell nucleus, and discuss the nature and importance of the genetic code. d) Summarize the role of DNA in protein synthesis, cell structure, and cell function. e) Describe the processes of cellular diffusion and osmosis, and explain their role in physiological systems. f) Describe carrier-mediated transport and vesicular transport mechanisms used by cells to facilitate the absorption or removal of speci c substances. Functions of the Plasma Membrane (4) a) physical isolation - plasma membrane is a physical barrier that separates the inside of the cell from the ECF b) regulation of exchange with environment - controls entry of ions and nutrients c) sensitivity to environment - contains receptors that allow the cell to recognize and respond to speci c molecules in its environment d) structural support - gives tissues stability Membrane Lipids - phospholipid bilayer: consists of two layers of phospholipid molecules (hydrophilic and hydrophobic) - contains cholesterol and steroids 1 fi fi fi  - the bilayer acts as a barrier, isolating the cell's interior from the external environment. - it controls the entry and exit of substances, maintaining the cell's internal conditions. Membrane Proteins - integral proteins: part of plasma membrane structure and cannot be easily separated from it without damaging or destroying the membrane - peripheral proteins: bound to inner or outer surface of membrane and are easily separated from it - anchoring proteins: attach plasma membrane to other structures and stabilize its position - recognition proteins: cells of immune system recognize other cells as normal or abnormal based on if there is recognition proteins - help cells identify each other and interact appropriately - essential for the immune response, allowing the body to distinguish between its own cells and foreign cells - enzymes: catalyze reactions - receptor proteins: sensitive to presence of speci c ions or molecules called ligands - carrier proteins: bind solutes and transport them across the plasma membrane - channels: some integral proteins contain a channel that forms a passageway through the plasma membrane - permit water and small solutes to move across the plasma membrane Membrane Carbohydrates - form layer called the glycocalyx (proteoglycans, glycoproteins and glycolipids) - functions: - lubrication and protection (glycoproteins and glycol lipids form thick layer that lubricates and protects plasma membrane) - helps anchor cells in place - glycoproteins and glycolipids can function as receptors - recognition — the immune system uses glycoproteins and glycolipids to distinguish between "self" and "foreign" cells 2 fi  - prevents the immune system from attacking the body's own cells and helps identify pathogens organelles: internal structures of cells that perform most of the tasks that keep a cell alive and functioning normally - has speci c functions - non-membranous organelles: not completely enclosed by membranes and all their components are in DIRECT contact with cytosol (e.g., cytoskeleton, centrosome with centrioles, ribosomes, and proteasome) - e.g., cytoskeleton, centrosome with centrioles, ribosomes and proteasomes - membranous organelles: isolated from cytosol by phospholipid membranes - e.g., ER, golgi apparatus, lysosomes, peroxisomes and mitochondria cytoskeleton: cell’s skeleton - gives the cytosol strength and exibility - made up of micro laments, intermediate laments and microtubules which affect cell shape and function - supports organelles and keeps them in their positions - determines where in cytoplasm enzymatic reactions take place and where speci c proteins are synthesized micro laments: smallest cytoskeletal structures; rod shaped - made up of actin (in skeletal muscle cells, actin causes contraction) - functions: - anchor cytoskeleton to integral proteins of the plasma membrane and give cell strength intermediate laments: - functions: - they strengthen the cell and help maintain its shape 3 fi fi fi fi fl fi fi  - stabilize organelle positions - anchor the cell to its surroundings. microtubules: hollow tubes built from protein called tubulin; largest components of cytoskeleton - functions: - provide strength and shape to the cell - transport vesicles and organelles within the cell via motor proteins - form structural components of organelles such as centrioles and cilia microvilli: small projections of plasma membrane on surface that have micro laments - increase the surface area to facilitate absorption of extracellular materials cilia: long extensions of plasma membrane that have microtubules - primary cilium - sensor - motor cilia move materials over cell surfaces agellum: whip-like extension of plasma membrane (found in sperm) centrosome: region of cytoplasm next to nucleus in cell; surround centrioles centrioles: made up of microtubules - important for chromosome movement during cell division ribosomes: organelles responsible for protein synthesis - free ribosomes: scattered throughout the cytoplasm - xed ribosomes: enter ER and are modi ed and packaged for us within cell or are secreted from the cell proteasomes: organelles that have protein-digesting enzymes (AKA: proteases) - functions: removes proteins (damaged or abnormal) from the cytoplasm 4 fi fl fi fi  Endoplasmic Reticulum (ER): network of intracellular membranes - functions: - synthesizes proteins, carbs and lipids - stores molecules or materials absorbed from cytosol - transports materials - detoxi es drugs and toxins - forms cisternae (hollow tubes) - 2 types: smooth ER (no ribosomes on outer surface) & rough ER (ribosomes on outer surface) Smooth ER: (functions) - synthesizes phospholipids and cholesterol needed for growth of plasma membrane - synthesizes steroid hormones (e.g., androgens and estrogens) - synthesizes and stores glycerides - synthesizes and stores glycogen Rough ER: (functions) - contains xed ribosomes, which are the sites where proteins are synthesized - modi es proteins, and packages and transports them to the golgi apparatus Golgi apparatus: - has discs called cisternae - functions: - modi es and packages secretions (e.g., hormones) - adds or removes carbs to or from proteins - packages enzymes within vesicles called lysosomes to be used in the cytoplasm 5 fi fi fi fi Lysosomes: vesicles that provide isolated environment for dangerous chemical reactions - produced by Golgi apparatus and have digestive enzymes that break down polymers - functions - remove damages organelles - primary lysosomes: have inactive enzymes; when fused with membranes of damaged organelles the enzymes become activated and secondary lysosomes are formed - secondary lysosomes: break down contents - destroy bacteria Peroxisomes: vesicles that have enzymes that break down hydrogen peroxide - functions: - absorb and break down fatty acids - because they break down hydrogen peroxide, they protect the cell from their potentially damaging effects Mitochondria: produces ATP - inner membrane: has folds called cristae (increase surface area to allow more proteins and enzymes to attach) that surround the matrix of the mitochondria - most energy-releasing reactions occur here - ATP is generated in mitochondria after glycolysis breaks down glucose into pyruvate in the cytosol - pyruvate enters the citric acid cycle in the mitochondrial matrix, releasing carbon dioxide and transferring hydrogen atoms to produce ATP via aerobic metabolism nucleus: central part of atom; control centre - determines the structure of the cell and what functions it can perform by controlling which proteins are synthesized, under what circumstances, and in what amounts - is surrounded by a membranous nuclear envelope, which encloses its contents, including DNA - in non-dividing cells, DNA is loosely coiled into chromatin 6  - during cell division, it condenses into visible chromosomes - contains nucleoli which synthesize ribosomal RNA - made up of RNA, enzymes and histones (help determine which information is available to the cell at any moment) Functions of the Nucleus: - stores genetic info - controls cellular activities (controls which proteins are synthesized and when and how much) - nucleoli synthesize ribosomal RNA which is essential for protein synthesis - surrounded by nuclear envelope which has pores that allow for communication and transport of materials between the nucleus and cytoplasm - cell repair (without a nucleus the cell cannot repair itself) genetic code: chemical ”language” the cell uses; set of instructions within a cell’s DNA that dictates how proteins are synthesized Nature of Genetic Code: - stored in DNA strands within the nucleus - DNA consists of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G) - the code is read in groups of three bases, known as triplets or codons - each triplet codes for a speci c amino acid (multiple triplets can code for the same amino acid) Importance of Genetic Code: - protein synthesis: synthesizes a ton of different proteins - determines structure and function of cells - allows for inheritance of traits by passing on the instructions for protein synthesis from one generation to the next - changes in genetic code (mutations) can affect protein structure and function which can lead to disorders and diseases 7 fi Role of DNA in Protein Synthesis - protein synthesis: assembling of functional polypeptides in cytoplasm - DNA strands: - a) coding strand (has triplets that specify sequence of amino acids in polypeptide), and - b) template strand (contains complementary triplets that are used as a template for mRNA production) Transcription: synthesis of RNA from a DNA template (“copy”) mRNA: carries info needed to synthesize proteins Process of Transcription - RNA polymerase is an enzyme that helps create messenger RNA (mRNA) from a DNA template. It starts at a speci c signal in a region called the promoter. As it moves along the DNA, it separates the strands and pairs the DNA bases with complementary RNA nucleotides. Instead of thymine (found in DNA), it uses uracil in RNA. For example, if the DNA base is adenine (A), RNA polymerase adds uracil (U) to the mRNA. The sequence of bases in the DNA determines the sequence of bases in the mRNA, with groups of three DNA bases (called triplets) matching up to three RNA bases (called codons). For instance, a DNA triplet of TCG corresponds to the mRNA codon AGC. This process ensures that the mRNA closely matches the coding strand of the gene, just replacing thymine with uracil. At “stop” signal, the RNA polymerase and mRNA detach from DNA strand and transcription ends. Translation: formation of chain of amino acids using info from an mRNA strand Process of Translation - tRNA binds and delivers speci c amino acids to the ribosome - Each tRNA has two important parts: - A "tail" that binds a speci c amino acid. 8 fi fi fi  - A loop containing three nitrogenous bases known as an anticodon (interacts with with a complementary codon on the mRNA strand during the translation process) - the mRNA is read in sequences of three bases (codons), and each codon matches with a tRNA's anticodon. e.g., if the mRNA sequence is AUG–CCG–AGC, it will pair with tRNAs that have the anticodons UAC, GGC, and UCG, respectively —> results in a speci c sequence of amino acids - phases of translation (3): - Initiation: The ribosome assembles around the mRNA - Elongation: tRNAs bring amino acids in sequence, extending the growing protein chain - Termination: The process ends when a stop codon is reached, completing the protein synthesis Cell Structure & Function & DNA - the DNA of the nucleus controls cell structure and function through the synthesis of speci c proteins - protein synthesis is the assembling of functional polypeptides in the cytoplasm, and the major events of protein synthesis are transcription and translation - By controlling protein synthesis, virtually every aspect of cell structure and function can be regulated, and changes in the extracellular environment may result in ions or molecules entering the cell directly through membrane channels or by binding to membrane receptors that then alter the intracellular (cytoplasmic) environment Permeability: property of plasma membrane that determines which substances can enter and leave cytoplasm - impermeable: nothing can pass through - freely permeable: anything can pass through - selectively permeable: lets in some things and not others Passage Across the Membrane - Passive Processes: move ions or molecules across plasma membrane without energy - Active Processes: require energy 9 fi fi  Passive Processes (Diffusion & Osmosis) - Diffusion: movement of a substance from area of higher concentration to an area of lower concentration - concentration gradient: different between high and low concentrations of a substance - factors that in uence diffusion: - a) distance: shorter distances speed up diffusion. - b) molecule size: smaller molecules diffuse more rapidly. - c) temp: higher temp = faster diffusion - d) concentration gradient: greater gradient = faster diffusion - diffusion across the plasma membrane depends on the substance’s size and lipid solubility - Simple Diffusion: diffuse across the phospholipid bilayer - Channel Mediated Diffusion: ions and compounds that are NOT lipid soluble cannot cross the plasma membrane so they need to go through a membrane channel in order to make it across - Leak channels: allow ions to pass across the plasma membrane - Osmosis: diffusion of water across a membrane - water molecules tend to ow across a selectively permeable membrane toward the solution with the higher solute concentration - osmotic pressure: the pressure required to stop the osmotic ow of water (driven by solute concentration differences) - e.g., Imagine two solutions separated by a membrane. Solution A has fewer solutes (more water), and Solution B has more solutes (less water). Water moves from A to B to balance solute concentrations. The pressure needed to stop this water movement is the osmotic pressure - hydrostatic pressure: pressure exerted by a uid due to its weight or force (driven by the weight or force of the uid) - e.g., In a column of water in a tube, the pressure at the bottom of the tube due to the weight of the water above is the hydrostatic pressure osmolarity: the solute concentration of the solution tonicity: a description of how the solution affects the shape of a cell 10 fl fl fl fl fl  Isotonic solution: a solution that has the same solute concentration as the cell - no net movement of water; the cell remains the same size and shape Hypotonic solution: a solution with a lower solute concentration compared to the cell - water ows into the cell, causing it to swell and potentially burst (hemolysis) Hypertonic solution: a solution with a higher solute concentration compared to the cell - water moves out of the cell, causing it to shrivel (crenation) Carrier-Mediated Transport: usage of specialized integral membrane proteins to move substances - can be active or passive - integral proteins bind to speci c ions and carry them across the plasma membrane - characteristics: - speci c: each carrier protein binds and transports only certain substances - limits: each carrier protein can only handle a certain amount of substrate at a time; when all available carrier proteins are occupied and working at their full capacity, they are considered saturated - once saturation occurs, the rate of transport cannot increase any further, even if there’s a larger concentration gradient (more substrate outside the cell than inside). - no matter how much more substrate is available, if all carriers are busy, transport remains at a maximum rate (think of an elevator) - regulation: hormones can help regulate activity of carrier proteins - symport/cotransport: carrier protein transports two different molecules in same direction - antitport/countertransport: carrier protein transports two different molecules in opposite directions - a) facilitated diffusion: a type of passive transport where essential nutrients, such as glucose and amino acids, are transported across the cell membrane with the help of carrier proteins - molecule binds to a receptor site on the carrier protein, which allows the molecule to diffuse across the plasma membrane - No ATP required - b) active transport: uses energy, typically from ATP, to move ions or molecules across the cell membrane (can go against their concentration gradient) 11 fi fl fi  - does not depend on the concentration gradient. This means cells can import or export substances regardless of their concentrations inside or outside the cell - 2 types: - i) primary active transport: direct use of ATP to transport molecules against a concentration gradient (e.g., Na+, K+ pump) - ii) secondary active transport: uses the energy from the concentration gradient of one molecule to drive the transport of another molecule (basically: the concentration gradient for one substance provides the driving force needed by the carrier protein, and the second substance gets a ”free ride.”) vesicular transport: moving materials into or out of the cell using small membranous sacs called vesicles - 2 types: - i) endocytosis: process of taking substances into the cell - 3 types: - a) receptor-mediated: targets speci c molecules (ligands) that bind to receptors on the cell surface. - process: - ligands bind to special proteins on the cell’s surface (receptors) - after binding, ligand-receptor pairs move to areas of membrane covered with a protein called clathrin which forms a pocket - the pocket pinches off, creating a clathrin coated vesicle - clathrin protein is release back into membrane for use, and the vesicle merges with primary lysosomes to create a secondary lysosome - inside secondary lysosome, ligands are processed, removed and absorbed into cytoplasm - after ligands are absorbed, the membranes of lysosome and vesicle separate - the vesicles fuse back with the plasma membrane becoming available again to bind more ligands - b) pinocytosis: process by which cells take in small amounts of extracellular uid 12 fi fl  - c) phagocytosis: a process where cells engulf solid particles, creating structures known as phagosomes - process: - cell forms extensions of its cytoplasm called pseudopodia (meaning "false feet”) which surround the solid object, enclosing it completely. - membranes of the pseudopodia fuse together, creating a vesicle called a phagosome that contains the engulfed material - phagosome merges with lysosomes (cellular compartments that contain digestive enzymes) which break down the contents of the phagosome, digesting the engulfed material - ii) exocytosis: process of expelling substances from the cell - vesicles fuse with the plasma membrane, allowing substances like hormones or waste products to be released into the extracellular environment - requires energy 13