BIOG 1440 Textbook Notes, Lecutres 18-26
BIOG 1440 Textbook Notes, Lecutres 18-26 BIOG 1440
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Lecture 18: Chapter 50 pp. 11201126 Vertebrate Skeletal Muscle Skeletal muscle moves bones and body, has a hierarchy of smaller and smaller units Within a typical skeletal muscle is a bundle of long fibers Each fiber is a single cell with multiple nuclei Inside a muscle cell is a bundle of myofibrils (containing thick and thin filaments) Sarcomeres: basic contractile units of muscle, repeating sections Borders of sarcomeres line up in adjacent myofibrils forming striations Skeletal muscle = striated muscle Thin filaments attach at Z lines, thick filaments attach at M lines In resting myofibril, thick and thin filaments partially overlap Edge of sarcomere = thin filaments, center = thick filaments Muscle → bundle of muscle fibers → single muscle fiber → myofibril → sarcomere The SlidingFilament Model of Muscle Contraction Contracting muscle shortens, but filaments stay the same length Filaments slide past each other like a telescope Slidingfilament model: thick and thin filaments ratchet past each other, powered by myosin molecules Each myosin has a tail and head region Tail adheres to tails of other myosin molecules Head can bind ATP → hydrolysis of bound ATP converts myosin to a high energy form that binds to actin forming a cross bridge Myosin returns to its low energy form as it pulls the thin filament toward the center of the sarcomere Cross bridge is broken when a new ATP molecule binds to the head Contraction requires repeated cycles of binding and release Thick filament contains 350 heads Each of which forms and reforms about 5 bridges per second, driving the thick and thin past each other Powering repetitive contractions requires creatine phosphate and glycogen Transferring a P group from creatine phosphate to ADP synthesizes ATP Resting stores of creatine phosphate can sustain contractions for 15 seconds Breaking glycogen down into glucose can also replenish ATP stores During light/moderate activity, glucose is metabolized by aerobic respiration Yields enough power to sustain contractions for an hour During intense activity, ATP is generated by lactic acid fermentation Very rapid, but but generates much less ATP per glucose molecule, sustaining contractions for only about 1 minute The Role of Calcium and Regulatory Proteins Tropomyosin (regulatory protein) and troponin complex (additional regulatory proteins) are bound to the actin strands of thin filaments At rest, tropomyosin covers the myosinbinding sites along the thin filament (preventing actin and myosin from interacting) Ca2+ accumulation in the cytosol binds to the troponin complex, causing tropomyosin bound along the actin strands to shift position and expose the myosin binding sites on the thin filament Therefore when Ca2+ concentration rises, thick & thin filaments slide past each other and muscle fibers contract When Ca2+ concentration falls, the binding sites are covered and contraction stops Motor neurons trigger the release of Ca2+ into the cytosol of muscle cells → causing contraction This regulation of Ca2+ is a multistep process Arrival of an action potential at the synaptic terminal of a motor neuron triggers release of acetylcholine Binding of acetylcholine to receptors on the muscle fiber leads to a depolarization, triggering an action potential Action potential follows foldings of the plasma membrane called transverse (T) tubules These make close contact with the sarcoplasmic reticulum (SR) Specialized endoplasmic reticulum Action potential spreading along the T tubules triggers changes in the SR, opening Ca2+ channels, allowing Ca2+ to flow into the cytosol and bind to the troponin complex, initiating muscle fiber contraction Relaxation begins as transport proteins in the SR pump Ca2+ in from the cytosol When Ca2+ concentration in cytosol drops to a low level, regulatory proteins bound to the thin filament shift back to their starting position, blocking myosin binding sites as Ca2+ pumped from the cytosol accumulates in the SR Disease can cause paralysis by interfering with the excitation of skeletal muscle fibers by motor neurons In ALS motor neurons in spinal cord & brainstem degenerate and muscle fibers atrophy In Myasthenia gravis the body produces antibodies to the acetylcholine receptors of skeletal muscle and the transmission between motor neurons and muscle fibers decreases Nervous Control of Muscle Tension Contraction of a whole muscle, such as the bicep, is graded Contraction of a single skeletal muscle is a allornone twitch Nervous system produced graded contractions by either Varying the number of muscle fibers that contract Varying the rate at which muscle fibers are stimulated Each branched motor neuron may synapse with many muscle fibers (although each fiber is controlled by only one motor neuron) Motor unit consists of a single motor neuron and all the muscle fibers it controls Motor neuron produces action potential → all muscle fibers in motor unit contract Recruitment: as more and more motor neurons controlling the muscle are activated, tension developed by a muscle increases Some muscles ie. ones that control posture are always partially contracted Prolonged contraction can result in muscle fatigue due to depletion of ATP and dissipation of ion gradients required for normal electrical signaling Action potentials can add together, resulting in multiple twitches that add together, creating greater tension When rate is so high that muscle fiber cannot relax between stimuli, twitches fuse into one smooth sustained contraction called tetanus Skeletal Muscle Fibers Several distinct types of skeletal muscle fibers, each for a particular set of functions Classified by source of ATP and by contraction speed Oxidative and Glycolytic Fibers Oxidative fibers rely mostly on aerobic respiration Specialized, allowing them to make use of a steady energy supply Many mitochondria, rich blood supply, large amount of myoglobin (O2 storing protein) Myoglobin binds O2 more tightly than hemoglobin, allowing oxidative fibers to extract O2 from the blood efficiently Glycolytic fibers Larger diameter Less myoglobin Use glycolysis as primary ATP source Fatigue more readily than oxidative fibers In poultry and fish, dark meat is made up of oxidative fibers and light meat is made up of glycolytic fibers FastTwitch and SlowTwitch Fibers Fasttwitch fibers Develop tension 23x faster than slowtwitch fibers Enable brief, rapid, powerful contractions Slow fibers have less sarcoplasmic reticulum and pumps Ca2+ more slowly Since Ca2+ remains in the cytosol longer, muscle twitch in a slow fiber lasts about 5x as long as a fast fiber Difference in speed between the two reflects the rate at which their myosin heads hydrolyze ATP All slow twitch fibers are oxidative while fast twitch can be glycolytic or oxidative Muscles of eye and hand are exclusively fast twitch but most skeletal muscle contains both (relative proportions of each is genetically determined) If a muscle is repeatedly used for high endurance activities, some fast glycolytic fibers will develop into fast oxidative fibers Since oxidative fibers do not fatigue as easily Some vertebrates have skeletal muscle fibers that twitch as rates far faster than any human muscle Rattlesnake’s rattle, dove’s coo, mating call of the male toadfish Other Types of Muscle Cardiac muscle is found only in the heart Striated like skeletal muscle Skeletal muscle fibers require motor neuron input but cardiac muscle ion channels cause rhythmic depolarizations that trigger action potentials without nervous system input Intercalated disks = regions that electrically couple adjacent cardiac muscle cells Smooth muscle Found in the walls of hollow organs, blood vessels and organs of the digestive tract Smooth muscle lacks striations Thick filaments are scattered throughout the cytoplasm and thin filaments are attached to structures called dense bodies (some tethered to plasma membrane) Less myosin than in striated muscle fibers Some smooth muscle cells contract only when stimulated by autonomic nervous system neurons Ca2+ regulates smooth muscle contraction but the mechanism is different than in skeletal or cardiac muscle Smooth muscle has no troponin complex or T tubules and the SR is not well developed Calcium ions cause contraction by binding to the protein calmodulin, activating an enzyme that phosphorylates the myosin head Invertebrates have muscle cells similar to vertebrate skeletal and smooth muscle cells The flight muscles of insects are capable of independent, rhythmic contraction The wings of some insects can beat faster than action potentials can arrive from the CNS Muscles that hold a clam shell closed have thick filaments that contain a protein called paramyosin that enables the muscles to remain contracted for as long as a month with low energy consumption Lecture 19: Chapter 36 pp. 757759 and Chapter 42 pp. 916920 Common Types of Plant Cells Cell differentiation: cells become specialized in structure and function over the course of their development May involve changes in cytoplasm, organelles, cell wall Parenchyma Thin/flexible primary walls Lack secondary walls Generally have a large central vacuole Perform most of the metabolic functions of the plant Synthesizing/storing various organic products Photosynthesis occurs within the chloroplasts of parenchyma cells Retain the ability to divide and differentiate into other types of plant cells under particular conditions (ie. wound repair) Collenchyma Help support young parts of the plant shoot Generally elongated cells with thicker primary walls than parenchyma Young stems often have strands of collenchyma just below the epidermis Provide flexible support without restraining growth Sclerenchyma More rigid than collenchyma, but still function as supporting elements Secondary cell wall is thick and contains large amounts of lignin Lignin present in all vascular plants but not bryophytes Unlike collenchyma, mature sclerenchyma cannot elongate Dead at functional maturity Sclereids and fibers Sclereids are boxier than fibers and irregular in shape Fibers are long, slender, and tapered WaterConducting Cells of the Xylem Tracheids and vessel elements are both water conducting cells Tubular and elongated Dead at functional maturity Tracheids occur in the xylem of all vascular plants Most angiosperms, some gymnosperms, and a few seedless vascular plants have vessel elements When living contents of a tracheid or vessel element disintegrate, the cell’s thickened walls remain behind, forming a nonliving conduit thru which water can flow Water can migrate laterally between neighboring cells through pits Tracheids are long, thin, with tapered ends while vessel elements are wider, shorter, thinner walled, and less tapered Vessel elements are aligned end to end forming long pipes known as vessels SugarConducting Cells of the Phloem Alive at functional maturity (unlike xylem) Sugars and other organic nutrients are transported through sieve cells In the phloem of angiosperms, sieve tubes consist of chains of sieve tube elements Sieve tube elements lack a nucleus, ribosomes, distinct vacuole, and a cytoskeleton Sieve plates: end walls between sieve tube elements Companion cell: non conducting cell alongside each sievetube element → connected to the sieve tube element by plasmodesmata Nucleus and ribosomes of the companion cell serve the adjacent sieve tube elements and sometimes help load sugars into sieve tube elements, transporting the sugars to other parts of the plant Circulatory systems link exchange surfaces with cells throughout the body O2 and CO2 can move between cells and their immediate surroundings by diffusion Time for a substance to diffuse from one place to another = proportional to the square of the distance Net movement by diffusion only rapid over very small distances Two basic adaptations that permit effective exchange: Body plan that places many or all cells in direct contact with the environment Each cell can exchange materials directly w/ surrounding medium Found in invertebrates, ie. cnidarians and flatworms Circulatory system Found in all other animals System moves fluid between each cell’s immediate surroundings and body tissues where exchange with the environment occurs Gastrovascular Cavities Hydras, jellies, and cnidarians have a central gastrovascular cavity Functions in the distribution of substances throughout the body and in digestion Opening at one end connects cavity to surrounding water Allows fluid to bathe both inner + outer tissue layers, facilitating exchange of gases + cellular waste Flatworms have gastrovascular cavities and flat bodies that allow for exchange w the environment (increases surface area) Open and Closed Circulatory Systems Three basic components Circulatory fluid Interconnecting vessels Muscular pump (ie. the heart) Powers circulation by using metabolic energy to elevate the fluid’s hydrostatic pressure (pressure fluid exerts on vessels) Circulatory system connects the aqueous environment of the body cells to organs that exchange gases, absorb nutrients, and dispose of wastes Open circulatory system Circulatory fluid = hemolymph, interstitial fluid that bathes body cells Arthropods (ie. grasshoppers), molluscs (ie. clams) Heart contraction pumps hemolymph through circulatory vessels into interconnected sinuses (spaces surrounding organs) Chemical exchange occurs in the sinuses Relaxation of the heart draws hemolymph back in thru pores Body movements squeeze the sinuses, helping to circulate hemolymph Closed circulatory system Circulatory fluid = blood, confined to vessels and is distinct from interstitial fluid One (or more) hearts pump blood into large vessels → branch into smaller ones that infiltrate organs Chemical exchange occurs between blood + interstitial fluid, and between interstitial fluid + body cells Annelids, cephalopods, vertebrates Each system offers evolutionary advantages Open circulatory system = lower hydrostatic pressure, therefore less energy consuming Closed circulatory system = relatively high blood pressure, enabling effective delivery of O2 in larger and more active animals Good for regulating the distribution of blood to different organs Organization of Vertebrate Circulatory Systems Cardiovascular system Arteries Carry blood from the heart to organs Branch into arterioles within organs Capillaries Microscopic vessels with thin porous walls Receive blood from arterioles Capillary beds Networks of capillaries that infiltrate tissues Capillaries converge into venules → veins → carry blood back to the heart Arteries and veins are distinguished by the direction in which they carry blood Arteries: away from the heart toward capillaries, Veins: toward the heart away from capillaries Portal veins: carry blood between capillary beds Vertebrate hearts contain two or more chambers Receive blood: atria, pumping blood out: ventricles Single Circulation Bony fishes, rays, sharks have a two chamber heart Single circulation: blood passes through the heart once in a complete circuit through the body Blood entering heart collects in atrium → transfers to ventricle → contraction of ventricle pumps blood to capillary bed in gills → diffusion of O2 into blood and CO2 out → capillaries converge into a vessel that carries O2 rich blood to capillary beds → blood returns to the heart When blood flows through a capillary bed, blood pressure drops substantially Drop in BP in the gills limits the rate of blood flow, but as the animal swims the contraction + relaxation of muscles helps accelerate the pace of circulation Double Circulation Amphibians, reptiles, mammals Double circulation: two circuits with a combined pump (the heart) Both pumps combined into one simplifies coordination of the pumping cycles R side of the heart delivers O2 poor blood to the capillary beds of gas exchange tissues (net movement of O2 into blood and CO2 out of blood) Pulmonary circuit if capillary beds involved are all in the lungs (reptiles + mammals) Pulmocutaneous circuit if it includes capillaries in both lungs and skin (amphibians) After O2 rich blood leaves gas exchange tissues, it enters L side of the heart Contraction propels this blood to capillary beds in organs and tissues throughout body O2 poor blood returns to the heart, completing systemic circuit Double circulation provides vigorous blood flow to brain/muscles/organs since heart re pressurizes the blood BP is often much higher in systemic circuit than gas exchange circuit Single circulation blood flows under reduced pressure directly from gas exchange to other organs Evolutionary Variation in Double Circulation Some vertebrates are intermittent breathers Amphibians and reptiles fill their lungs with air periodically Passing long periods without gas exchange or relying on another gas exchange tissue Frogs & other amphibians have a 3 chambered heart (2 atria, 1 ventricle) Ridge within the ventricle diverts most (90%) of the O2 rich blood from left atrium into systemic circuit and most of O2 poor blood from right atrium into pulmocutaneous circuit This incomplete division allows the frog to adjust its circulation, shutting off most blood flow to its lungs (which are ineffective underwater) and blood flow continues to the skin which acts as a site of gas exchange 3 chambered heart of turtles, snakes, lizards Incomplete septum partially divides the single ventricle into R and L chambers 2 aortas lead to the systemic circulation Enables control of the relative amount of blood flowing to lungs and the rest of the body Alligators, caimans, other crocodilians, ventricles are divided by a complete septum but the pulmonary and systemic circuits connect where the arteries exit the heart This connection allows arterial valves to shunt blood flow away from the lungs temporarily In birds and mammals, heart has 2 atria and 2 ventricles L side receives and pumps O2 rich R side receives and pumps O2 poor Cannot vary blood flow to the lungs like amphibians and reptiles Birds and mammals need 10x as much energy as ectotherms and therefore need circulatory systems to deliver 10x as much fuel and O2 to their tissues (and remove 10x as much wastes) Traffic of substances is made possible by separate and independently powered systemic and pulmonary circuits & large hearts Lecture 20: Chapter 36 p. 781797 Different mechanisms transport substances over short or long distances Apoplast and symplast are two major pathways of transport in plants The Apoplast and Symplast: Transport Continuums Apoplast: consists of everything external to the plasma membranes of living cells (incl. cell walls, extracellular spaces, interior of dead cells like vessel elements and tracheids) Symplast: entire mass of cytosol of all the living cells in a plant and plasmodesmata Three routes for transport within a plant tissue Apoplastic → water and solutes move along the continuum of cell walls and extracellular spaces Symplastic → water and solutes move along the continuum of cytosol, requires substances to cross a plasma membrane once (when they first enter the plant), then moving cell to cell via plasmodesmata Transmembrane routes → water and solutes move out of one cell, across the cell wall, and into the neighboring cell, which may pass them to the next cell in the same way, requires repeated crossings of plasma membranes Three routes are not mutually exclusive, some substances may use more than 1 ShortDistance Transport of Solutes Across Plasma Membranes Selective permeability of the plasma membrane controls the short distance movement of substances into and out of cells Active and passive transport mechanisms occur in plants Cell membranes equipped with pumps and transport proteins H+ (instead of Na+ as in animals) plays a primary role in basic transport processes Plant cell membrane potential (voltage across membrane) is established mainly through the pumping of H+ by proton pumps (not pumping Na+ by Na/K pumps) In cotransport, plant cells use H+ gradient energy to drive the active transport of many different solutes Cotransport with H+ is responsible for absorption of neutral solutes (ie. sucrose) by phloem cells/other plant cells H+/sucrose cotransporter coupes movement of sucrose against its concentration gradient with movement of H+ down its electrochem gradient Facilitates the movement of ions Most channels are gated, open and close in response to stimuli (chemicals, pressure, voltage) K+ ion channels in guard cells function in opening and closing stomata ShortDistance Transport of Water Across Plasma Membranes Osmosis powers the absorption or loss of water (free water → not bound to solutes) Water potential: physical property that predicts the direction in which water will flow Free water moves from regions of high WP to low WP (if there is no barrier) WP is denoted by psi (ѱ ) measured in units of megapascals (MPa) How Solutes and Pressure Affect Water Potential WP equation: ѱ = ѱs + ѱp ѱ = WP ѱs = solute potential, directly proportional to its molarity, also called osmotic potential Solutes typically mineral ions and sugars ѱ s of pure water = 0 increase in solute concentration has a negative effect on water potential this is why ѱs of a solution is always expressed as negative ѱp = pressure potential physical pressure on a solution can be positive or negative relative to atmospheric pressure Water in living cells usually under + pressure Protoplast: living part of the cell (incl. membrane) Protoplast presses against the cell wall creating turgor pressure Helps maintain stiffness of plant tissues Driving force for cell elongation Water moves from regions of higher water potential to regions of lower water potential Water Movement Across Plant Cell Membranes Flaccid: limp, can be a result of losing water ѱp of 0 MPa If this cell is bathed in a solution of higher solute concentration than itself, water will diffuse out of the cell because the external solution has lower water potential Undergoes plasmolysis (shrinks and pulls away from the cell wall) In pure water the cell has a lower water potential than water and water enters the cell by osmosis Contents of the cell begin to swell and press the plasma membrane against cell wall When this pressure is enough to offset the tendency for water to enter due to the solutes in the cell, then p and ѱ s are = and ѱ =0 Turgid cells are firm Effects of turgor loss are seen during wilting Aquaporins: Facilitating Diffusion of Water Difference in WP determines direction of water movement across membranes Aquaporins facilitate the transport of water molecules across plant cell plasma membranes LongDistance Transport: The Role of Bulk Flow Diffusion is too slow for long distance, so it is completed by bulk flow Always occurs from higher to lower pressure Independent of solute concentration (unlike osmosis) Occurs within the tracheids and vessel elements of the xylem Occurs within the sieve tube elements of the phloem These structures facilitate bulk flow Mature tracheids and vessel elements are dead cells and therefore have no cytoplasm Cytoplasm of sieve tube elements is almost devoid of organelles Absence or reduction of cytoplasm in a plant’s “plumbing” facilitates bulk flow through the xylem and phloem Also enhanced by perforation plates at the ends of vessel elements & sieve plates connecting sieve tube elements Bulk flow is used in long distance transport of sugars in the phloem but active transport of sugar at a cellular level maintains this pressure difference Transpiration drives the transport of water and minerals from roots to shoots via the xylem Absorption of Water and Minerals by Root Cells Cells near the tips of roots are particularly important in absorbing nutrients because most absorption of water and minerals occurs there In this region, epidermal cells are permeable to water Many are differentiated into root hairs → modified cells that account for much of the absorption of water by roots Root hairs absorb soil solution (consists of H2O molecules and dissolved mineral ions) Drawn into hydrophilic walls of epidermal cells and passes into root cortex This flow enhances exposure of cortex cells to soil solution, greater surface area for absorption Transport of Water and Minerals into the Xylem Water and minerals must enter the xylem of the vascular cylinder (stele) to be transported to the rest of the plant Endodermis (innermost layer of root cortex cells) functions as a last checkpoint for the selective passage of minerals from cortex to vascular cylinder Minerals in the symplast when they reach the endodermis continue thru plasmodesmata of endodermal cells and pass into the vascular cylinder Minerals that reach the endodermis via apoplast encounter a dead end (Casparian strip) Casparian strip = belt made of suberin (waxy material) that is impervious to water and dissolved minerals Prevents water and minerals from crossing the endodermis and entering the vascular cylinder via the apoplast Instead, water and minerals must cross the selectively permeable plasma membrane of an endodermal cell before they can enter the vascular cylinder Endodermis transports needed minerals from soil → xylem and keeps unneeded/toxic substances out Also prevents solutes in xylem from leaking back into soil Passage of water/minerals into tracheids and vessel elements is the last segment in the soil → xylem pathway Endodermal cells discharge minerals from their protoplasts into their own cell walls Bulk Flow Transport via the Xylem Water + minerals from soil enter the plant through the epidermis of roots, cross the root cortex, and pass into the vascular cylinder Xylem sap: water and dissolved minerals in the xylem, transported long distances by bulk flow to leaf veins Transporting xylem sap involves the loss of water by transpiration Pushing Xylem Sap: Root Pressure At night (when there is no transpiration) root cells continue pumping mineral ions into the xylem of vascular cylinder Casparian strip prevents the ions from leaking back out into the cortex and soil Accumulation of minerals lowers the water potential within the vascular cylinder Water flows in (from root cortex), generating root pressure, pushing xylem sap Root pressure can sometimes cause more water to enter the leaves that is transpired, causing guttation (exudation of water droplets) Many plants do not generate any root pressure, or only during part of the growing season Root pressure cannot keep pace with transpiration after sunrise Xylem sap is mainly pulled up, not pushed from below by root pressure Pulling Xylem Sap: The CohesionTension Hypothesis Root pressure is only a minor force in the ascent of xylem sap Most xylem sap does not even require living cells to rise through a tree Cohesiontensions hypothesis: transpiration provides the pull for the ascent of xylem sap and cohesion of water molecules transmits this pull along the entire length of the xylem from shoots to roots Xylem sap is normally under negative pressure/tension Transpiration = “pulling” process Rise of xylem sap by cohesiontension begins not with roots but with the leaves (where the driving force for transpirational pull begins) Transpirational Pull Stomata lead to a maze of internal air spaces that expose mesophyll cells to CO2 for photosynthesis Negative pressure potential causes water to move up thru the xylem and develops at the surface of mesophyll cell walls in the leaf As more water evaporates from the cell wall, the curvature of the airwater interface increases and the pressure of the water becomes more negative Water from the more hydrated parts of the leaf are pulled toward this area, reducing tension and the pulling forces are transferred to the xylem due to the cohesion between water molecules Negative pressure potential is consistent with the water potential equation because negative pressure potential (tension) lowers water potential Negative water potential of leaves provides the “pull” in transpirational pull Adhesion and Cohesion in the Ascent of Xylem Sap Adhesion and cohesion facilitate the transport of water by bulk flow Adhesion: attractive force between water molecules and other polar substances Water and cellulose are both polar, so there is a strong attraction between water molecules and cellulose molecules in xylem walls Cohesion: attractive force between molecules of the same substance Water has a high cohesive force due to H bonds Water’s cohesive force gives it tensile strength and allows a column of xylem sap to be pulled up without the water molecules separating Strong adhesion of water molecules to the xylem walls help offset force of gravity Upward pull on sap creates tensions within vessel elements and tracheids Positive pressure → pipe swells, tension → pipe walls pull inward Thick secondary walls prevent vessel elements + tracheids from collapsing under tension Tension lowers WP in root xylem allowing water to flow from soil across root cortex and into vascular cylinder Cavitation: formation of a water vapor pocket, can break the chain of water molecules More common in wide vessel elements than tracheids Can occur during drought stress/when xylem sap freezes in winter Xylem Sap Ascent by Bulk Flow: A Review Cohesiontension mechanism that transports xylem sap against gravity In long distance transport of water from roots → leaves by bulk flow, movement is driven by a water potential difference at opposite ends of xylem tissue WP difference is created at the leaf end by evaporation of water (which lowers water potential, generating tension that pulls water through the xylem) Bulk flow in the xylem differs from diffusion because It is driven by differences in pressure potential WP gradient within xylem is also a pressure gradient Flow does not occur across plasma membranes, but instead through hollow, dead cells Moves the entire solution together and at a greater speed than diffusion Plant expends no energy to lift xylem sap by bulk flow Absorption of sunlight drives most of transpiration by causing water to evaporate from the moist walls of mesophyll cells (lowering WP in the air spaces within a leaf) The rate of transpiration is regulated by stomata Leaves generally have large surface areas and high surface to volume to ratios Large surface area enhances light absorption for photosynthesis High surface to volume ratio aids in CO2 absorption as well as release of O2 After diffusing through stomata, CO2 enters a honeycomb of air spaces formed by spongy mesophyll cells Although large surface area increases rate of photosynthesis, it also increases water loss via stomata Stomata: Major Pathways for Water Loss 95% of water lost escapes thru stomata Each stomata is flanked by a pair of guard cells which control the diameter of the stoma by changing shape (widening or narrowing the gap between the guard cell pair) Amount of water lost depends largely on # of stomata and average size of their pores Stomatal density of a leaf is under genetic and environmental control Desert plants have lower stomatal densities than marsh plants High light exposures and low CO2 levels during leaf development can lead to increased density in many species Mechanisms of Stomatal Opening and Closing Guard cells take in water from neighboring cells → turgid Lecture 21: Chapter 42 pp. 920927 Coordinated cycles of heart contraction drive double circulation in mammals Timely delivery of O2 to the body’s organs is critical, cells die after prolonged lack of O2 Mammalian Circulation Right ventricle pumps blood to the lungs via the pulmonary arteries Blood flows through capillary beds in the left and right lungs, loading O2 and unloading CO2 O2 rich blood returns from the lungs via the pulmonary veins to the left atrium Then flows into the left ventricle, pumping O2 rich blood out to body tissues thru systemic circuit Blood leaves left ventricle via the aorta, conveys blood to arteries leading throughout the body First branches leading from aorta are coronary arteries, supplying blood to the heart itself Then to capillary beds in the head and arms Aorta descends into the abdomen, bringing blood to arteries leadings to capillary beds in abdominal organs and legs Within capillaries there is a net diffusion of O2 from blood to the tissues and of CO2 into the blood Capillaries rejoin, forming venules, conveying blood to veins O2 poor blood from head, neck, forelimbs channel into the superior vena cava Inferior vena cava drains blood from the trunk and hind limbs Two venae cavae empty their blood into the right atrium, from which it flows into the right ventricle The Mammalian Heart: A Closer Look Located behind the sternum Mostly cardiac muscle Two atria have thin walls and serve as collection chambers for blood returning to the heart Ventricles have thicker walls and contract more forcefully, esp. left ventricle which pumps blood to systemic circuit Although L ventricle pumps with greater force, it is the same volume of blood as the R ventricle Contracts → pumps blood, relaxes → chambers fill with blood One complete sequence = cardiac cycle Contraction phase = systole Relaxation phase = diastole Cardiac output = volume of blood each ventricle pumps per minute Determined by heart rate (rate of contraction) and stroke volume (amount of blood pumped by a ventricle in a single contraction) 4 valves in the heart prevent backflow and keep blood moving in the correct direction Valves open when pushed from one side and close when pushed from the other Atrioventricular (AV) valve lies between each atrium and ventricle, anchored by strong fibers Pressure from contractions close AV valves, keeping blood from flowing back into the atria Semilunar valves are located at the 2 exits of the heart, where the aorta leaves the L ventricle and where the pulmonary artery leaves the right ventricle These are pushed open by the pressure generated during ventricle contraction When ventricles relax, BP built up in the aorta/pulmonary artery closes the semilunar valves and prevents significant backflow Heart murmur: abnormal sound created by blood squirting backward through a defective valve Maintaining the Heart’s Rhythmic Beat Heartbeat originates in the heart itself (in vertebrates) Some cardiac muscle cells are autorhythmic = can contract and relax repeatedly without any signal from the nervous system Each of these cells has its own intrinsic contraction rhythm Coordinated by a group of autorhythmic cells in the wall of the right atrium → the sinoatrial (SA) node (or pacemaker) Sets the rate and timing of cardiac muscle cell contractions Produces electrical impulses which spread rapidly within heart tissue (since cardiac cells are electrically coupled through gap junctions) These impulses generate currents conducted to the skin via body fluids Electrocardiogram (ECG/EKG) records currents through electrodes on the skin Impulses from SA node first spread rapidly through walls of the atria (causing both atria to contract in unison) During atrial contraction, the impulses originating at the SA node reach other autorhythmic cells located between the L and R atria → relay point called atrioventricular (AV) node AV node delays impulses by 0.1 second before spreading to heart apex Signals from AV node are conducted to the heart apex and ventricular walls by specialized structures called bundle branches and Purkinje fibers Physiological cues can alter heart tempo by regulating the SA node Sympathetic NS speeds up heart rate and parasympathetic slows it down Patterns of blood pressure and flow reflect the structure and arrangement of blood vessels Blood Vessel Structure and Function Blood vessels contain a central lumen (cavity) lined with endothelium (single layer of flattened epithelial cells) Smooth endothelium surface minimizes resistance to blood flow Surrounding endothelium are tissue layers that differ in capillaries/arteries/veins Capillaries are the smallest vessels Very thin walls (just endothelium and a surrounding layer called basal lamina) Exchange of substances between blood and interstitial fluid only occurs in capillaries Walls of arteries and veins have more complex organization than capillaries Both have two layers of tissue around endothelium Outer layer = formed by connective tissue that contains elastic fibers (allows vessel to stretch/recoil) and collagen (strength) Layer next to endothelium = smooth muscle and elastic fibers Walls of arteries and veins differ Arteries: thick and strong, accommodating blood pumped at high pressure, elastic When heart relaxes, arterial walls recoil, maintaining BP and flow to capillaries Dilating and constricting vessels helps control blood flow to different body parts Veins do not require thick walls (only ⅓ as thick as an artery) Unlike arteries, veins contain valves, maintaining unidirectional blood flow Lower BP in veins than arteries Blood Flow Velocity Blood slows as it moves from arteries → arterioles → capillaries Because # of capillaries is enormous (~7 billion) Total cross sectional area is much greater in capillary beds than in arteries or any other part of circulatory system Decrease in velocity from arteries → capillaries (500x more slowly in caps than aorta) Speeds up from capillaries → venules/veins Blood Pressure Blood flows from areas of higher pressure to lower pressure Contraction of a ventricle generates BP, exerting force in all directions Resistance of arterioles and capillaries caused by their narrow diameter decreases pressure and by the time blood reaches veins, much of the pressure generated by the heart has dissipated Changes in Blood Pressure During the Cardiac Cycle Arterial BP is highest when heart contracts during ventricular systole Pressure called systolic pressure Each spike in BP caused by ventricular contraction stretches the arteries Pulse = rhythmic bulging of the artery walls w/ each heartbeat During diastole, elastic walls of the arteries snap back → lower but still substantial BP when ventricles are relaxed (diastolic pressure) Arteries remain pressurized throughout cardiac cycle → blood continuously flows into arterioles and capillaries Regulation of Blood Pressure Arterial BP is regulated by altering diameter of arterioles Vasoconstriction: smooth muscles in arteriole walls contract, arterioles narrow, increases blood pressure in the arteries Vasodilation: smooth muscles relax, increase in diameter that causes BP in arteries to fall Nitric oxide = a major inducer of vasodilation and endothelin as the most potent inducer of vasoconstriction Vasoconstriction and dilation are often coupled to changes in cardiac output that also affect blood pressure Coordination maintains adequate blood flow as the body’s demands on the circulatory system change During exercise, arterioles dilate causing a greater flow of O2 rich blood to the muscles This would cause BP to drop, but cardiac output increases in response to keep BP up and supporting the increase in blood flow Blood Pressure and Gravity BP generally measured for an artery at the height of the heart Gravity has a significant effect on BP Standing lowers BP in your head and if your head needs higher BP you will faint to allow your head to be level with your heart (increasing blood flow) Athletes need to follow heavy exercise with moderate exercise to allow their heart to go back to resting level to prevent future heart failure if they stop regular exercise Capillary Function Only 510% of caps have blood flowing through them at any given time Each tissue has many capillaries Brain, heart, kidney, and liver caps are usually filled to capacity Blood flow to the skin is regulated to control body temp. Blood flow to digestive tract increases after a meal Blood flow to skeletal muscles and skin is increased during exercise Constriction or dilation of arterioles that supply capillary beds controls the blood flow in capillary beds Precapillary sphincters → rings of smooth muscle located at the entrance of capillary beds Opening and closing these muscular rings regulates/redirects the passage of blood into particular capillaries Nerve impulses, hormones in the bloodstream, and chemicals produced locally signal blood flow regulation Histamine released by cells at a wound site triggers vasodilation Exchange of substances between blood and interstitial fluid takes place across capillary walls Some substances are carried across in vesicles Sometimes O2/CO2 diffuse through microscopic pores in capillary wall Also provide route of transport of small solutes (sugars, salts, urea) and bulk flow of fluid into tissues BP tends to drive fluid out of caps and blood proteins tend to pull fluid back Many blood proteins & all blood cells are too large to pass through the endothelium, causing them to remain in caps Osmotic pressure: pressure produced by the difference in solute concentration across a membrane Dissolved proteins are responsible for much of osmotic pressure Difference in osmotic pressure between blood and interstitial fluid opposes fluid movement out of the capillaries Net loss is generally greatest at the arterial end of the vessels (where BP is highest) Lecture 22: Chapter 42 pp. 933938 Gas exchange occurs across specialized respiratory surfaces Gas exchange: uptake of molecular O2 from the environment and the discharge of CO2 to the environment (not to be confused with cellular respiration) Partial Pressure Gradients in Gas Exchange Partial pressure: Pressure exerted by a particular gas in a mixture of gases Hemoglobin has a high affinity for O2 in the lungs and a lower affinity outside the lungs (to take O2 in and then give it up) Gas always undergoes net diffusion from higher partial pressure → lower partial pressure Partial pressure apply to gases dissolved in liquid When water is exposed to air, equilibrium is reached where partial pressure of each gas in the water = the partial pressure of that gas in the air Respiratory Media Air is less dense and less viscous than water, so it’s easier to move and force through small passageways Humans extract 25% of O2 in inhaled air Gas exchange using water is much more demanding (less O2 dissolved in it than in an equivalent amount of air) Warmer and saltier water = less dissolved O2 Respiratory Surfaces Structure of respiratory surface shows specializations for gas exchange Cells that carry out as exchange have a plasma membrane that must be in contact with an aqueous solution, and therefore respiratory surfaces are always moist Movement of O2 and CO2 across respiratory surfaces takes place by diffusion Rate of diffusion is proportional to the square of the distance through which molecules must move Fast when area for diffusion is large and the path for diffusion is short Causes respiratory surfaces to be large & thin In simple animals (sponges, cnidarians, flatworms) every cell in the body is close enough to the environment that gases can diffuses between any cell & the external In animals where most of the cells lack access to the environment, the respiratory surface is a thin, moist epithelium In earthworms and some amphibians, the skin is a respiratory organ Dense network of capillaries just below the skin facilitates the exchange of gases btwn circulatory system and environment Most animals, general body surface lacks sufficient area to exchange gases for the whole organism → solution is a respiratory organ that is extensively folded/branched to enlarge surface area (gills, tracheae, and lungs) Gills in Aquatic Animals Gills: outfoldings of the body surface that are suspended in the water Often have total surface area much greater than rest of body’s exterior Ventilation: movement of the respiratory medium (ie. water) over the respiratory surface Maintains the partial pressure gradients of O2 and CO2 across the gill that are necessary for gas exchange Gillbearing animals move their gills through the water or move water over their gills to promote ventilation Current of water enters the mouth of the fish → passes through slits in the pharynx → flows over gills → exits body Countercurrent exchange: exchange of a substance or heat between two fluids flowing in opposite directions (in fish gills, blood and water) As blood enters a gill capillary, it encounters water that is completing its passage through the gill Causes a partial pressure gradient favoring the diffusion of O2 from water to blood exists along the entire length of the capillary Tracheal Systems in Insects In most terrestrial animals, respiratory surfaces are enclosed within the body (exposed to the atmosphere only through narrow tubes) Most common system is the insect tracheal system: network of air tubes that branch throughout the body Largest tubes, called tracheae, open to the outside Smallest branches extend close to the surface of nearly every cell, where gas is exchanged by diffusion across moist epithelium For small insects, diffusion through the tracheae brings in enough O2 and removes enough CO2 to support cellular respiration Larger insects ventilate their tracheal systems with body movements that compress and expand the air tubes In flying insects, alternating contraction and relaxation of the flight muscles pumps air rapidly through the tracheal system Lungs Localized respiratory organs (unlike how tracheal systems branch throughout the insect body) Typically divided into numerous pockets Surface of the lung is not in direct contact with all other parts of the body, gap must be bridged by the circulatory system to transport gases between lungs and rest of body Amphibians rely heavily on diffusion across external body surfaces (lungs, if present, are small) Most reptiles + birds and all mammals depend entirely on lungs Turtles are an exception: supplement lung breathing with gas exchange across moist epithelial surfaces (mouth or anus) Mammalian Respiratory Systems: A Closer Look System of branching ducts conveys air to the lungs, located in the thoracic cavity Air enters nostrils → filtered by hairs, warmed, humidified, sampled for odors in the nasal cavity → pharynx, where paths for air and food cross When food is swallowed, larynx (upper part of respiratory tract) moves upward and tips the epiglottis over the glottis (opening of the trachea), allowing food to go down the esophagus to the stomach When glottis is open, enables breathi
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