ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Review Questions and Answers		AP BIOLOGY: Chapter Forty-Four Outline MOBILITY OF ANIMALS Most Animals Move From Place to Place fig 44.1 Only animals explore environment via locomotion Plants and fungi move by growing Animals Use Contraction of Muscles to Move The Mechanical Problems Posed by Movement Motion Requires Countering the Force of Gravity Chemical energy in the form of ATP provides force ATP 9 ADP + Pi Releases 7.3 kcal of energy per mole Protists wave cilia resulting in movement Animals compress and shorten structural elements in muscle cells Vertebrate Locomotion Results When Force of Muscle Contraction Moves Bones at Joints BONE: THE STRUCTURAL MATERIAL OF THE VERTEBRATE SKELETON Structure of Bone Special form of connective tissue Organic extracellular matrix of collagen fibers Impregnated with hydroxyapatite (calcium phosphate) Collagen fibers run in all directions Hydroxyapatite crystals aligned with long axes and curved ends of bones Composition is unique Hydroxyapatite is strong and rigid but brittle Collagen is flexible but weak If hydroxyapatite crystal breaks, it runs into collagen before another crystal Collagen distorts and dissipates stress Adjacent crystals not exposed to same stress Formation of Bone A bone is living, dynamic tissue New bone formed by osteoblast cells Secrete collagen fibers that are subsequently calcified Osteocytes: mature osteoblasts trapped within bone Lamellae: concentric layers of bone surrounding Haversian canals Haversian canals interconnect, carry blood vessels and nerve cells Blood flow allows osteocytes to remain alive when embedded in calcified matrix Two types of bone formation Flat bones like skull Osteoblasts located in web of dense connective tissue Produce bone within that tissue Long bones Cartilage skeleton initial template for bone formation Bone formed as cartilage degenerates Bones of vertebrate skeleton composed of two elements fig 44.2 Ends and interiors are open lattice of spongy bone tissue Spaces contain marrow Most blood cells formed in bone marrow Surrounded by concentric layers of compact bone tissue Bone is much denser Gives bone strength to withstand mechanical stress JOINTS: SITES OF ATTACHMENT BETWEEN BONES fig 44.3 Bones Interact at Joints or Articulations Three Kinds of Joints fig 44.3 Immovable joints Called sutures Example: cranial bones Open areas of dense connective tissue in fetus as skull is not fully formed Slightly moveable joints Bones bridged by cartilage Example: vertebral bones in spine Pads of cartilage are intervertebral disks Cushion and allow flexibility Also called cartilaginous joints Freely moveable joints Called synovial joints Articulated end located within synovial capsule with lubricating fluid Ends of bone capped with cartilage Bones move in direction dictated by structure of joint Arm-shoulder joint has ball-and-socket structure Elbow joint has hinge-like movement THE HUMAN SKELETON Endoskeleton of Humans Composed of 206 Bones fig 44.4 Axial skeleton: supports the main body axis Appendicular skeleton: supports arms and legs fig 44.5 Motor control systems system control two divisions independently The Axial Skeleton 80 bones compose skull, backbone and rib cage Skull: 28 bones include cranium, facial, middle-ear and hyoid bones Vertebral column = spine = backbone 33 vertebrae compose flexible column that protects spinal cord 12 pairs of ribs attach in front at breastbone (sternum) to protect heart and lungs The Appendicular Skeleton 126 bones attached to axial skeleton at shoulders and hips Pectoral girdle: shoulders Shoulder blades connected to breastbone by collarbones (clavicles) Attach to arms with 32 bones each, most in hands Pelvic girdle connects to legs, 30 bones each including foot MUSCLES: HOW THE BODY MOVES Animals Possess Specialized Cells Devoted Exclusively to Contraction Vertebrate Muscle Cells Composed of filaments of actin and myosin proteins Vertebrates possess skeletal, cardiac and smooth muscle cells THE STRUCTURE OF SKELETAL MUSCLE Skeletal Muscles Produce Movement of Skeleton fig 44.6 Muscles attach to bones Are usually attached to two different bones May be attached to another structure like skin Connection of muscle to bone called tendon Attachment at origin remains relatively stationary during contraction Insertion end of muscle is attached to bone that moves Muscles May Work in Groups Synergists produce same action at joint Antagonists produce opposing actions Example: lower leg muscles fig 44.7 Quadriceps group cause lower leg to extend, leg moves away from thigh Flexor muscles of thigh (hamstrings) contract and bring lower leg toward thigh Quadriceps muscles are synergists Quadriceps and hamstrings are antagonists Muscles that antagonize are relaxed when opposing set is contracted Microscopic Anatomy of Skeletal Muscle Each muscle contains numerous muscle fibers Cells specialized for rapid contraction and production of large force fig 44.8 Each fiber encloses bundle of 4-20 myofibrils Have cross-striations that produce alternating light-dark appearance Muscle fiber itself has striated appearance Skeletal muscles thus are striated as are cardiac muscles Myofibrils built of long chains of repeating sarcomeres Sarcomere subunits bounded on each end by Z line disk of protein Light and dark banding results from thin and thick myofilaments Thin filament: globular actin proteins twisted into double helix fig 44.9 Thick filament: myosin protein each with a protruding head fig 44.10 Thin and thick filaments interdigitate Occurs near border between light and dark bands Myosin heads extend toward thin filaments CONTRACTION OF SKELETAL MUSCLE Molecular Aspects of Muscle Contraction Muscle contraction associated with cleaving ATP to ADP + Pi At rest myosin heads function as ATPase enzymes Hydrolysis activates myosin heads In this orientation, they can bind to sites on actin filaments Myosin and actin bind when muscle is stimulated to contract fig 44.11 Binding constitutes formation of a cross-bridge between actin and myosin Cross-bridge formation causes conformational change Pulls thin filament toward center of sarcomere fig 44.11b Binding another ATP detaches myosin head from actin Lack of ATP in dead animal causes myosin to remain bound to actin Causes stiffened condition called rigor mortis Cleaving that molecule activates myosin head again Myosin head is slightly closer to the Z line at the next cycle fig 44.12 Repetition of many cycles causes sarcomeres and myofilaments to shorten Thin filaments slide between thick filaments fig 44.13 Process called sliding filament mechanism of contraction Shortening of myofibrils produces tension in muscle fibers and whole muscle Will cause motion if force is greater than opposing forces, like gravity Muscle generates maximum tension if it contracts when at normal resting length Optimal overlap of thin and thick filaments Permits formation of maximum number of cross-bridges At very long length no cross-bridges can form since no overlap of thin and thick filaments At short lengths thick filaments collide with Z line, preventing further shortening Initiation of Skeletal Muscle Contraction Does not occur spontaneously, stimulated by nervous system Five step process Motor neuron produces electrical impulse carried to ends of axon Forms synapses called neuromuscular junctions with one or more muscle fibers Neuron releases acetylcholine as chemical neurotransmitter Excites muscle fiber, stimulates it to produce impulses Muscle fiber impulses carried along sarcolemma (plasma membrane) Also carried along infoldings called transverse tubules fig 44.14 Tubules extend deep into muscle fiber Closely apposed to sarcoplasmic reticulum, specialized ER that surrounds myofibrils Impulses along transverse tubules stimulate release of Ca++ Calcium ions stored in sarcoplasmic reticulum Released into cytoplasm Involves regulatory proteins troponin and tropomyosin Tropomyosin lies against thin filament Troponin bound to tropomyosin fig 44.15 In resting fiber Ca++ in cytoplasm is low Tropomyosin located close to thin filament myosin-binding site Troponin blocks myosin heads from binding to actin Prevents contraction In stimulated fiber Ca++ released by sarcoplasmic reticulum binds to troponin Ca++-troponin complex pulls tropomyosin from myosin-binding sites on actin Cross-bridges can form Cross-bridge cycle continues if Ca++ stays attached to troponin (ATP available) When nerve activity stops so do muscle fiber impulses Ca++ actively transported back to sarcoplasmic reticulum Ca++ released from troponin, tropomyosin returns to position on thin filament Prevents myosin heads from binding to actin Muscle fiber relaxes Process called excitation-contraction coupling Neurons produce electrical excitation of muscle fiber Electrical excitation indirectly produces myofilament sliding and contraction Coupled to contraction through action of Ca++ Summation Twitch: single brief contraction Muscle fiber stimulated by single impulse on motor neuron Fiber contracts rapidly and relaxes Summation Result of repetitive firing of motor neuron innervating muscle fiber Insufficient time for relaxation between twitches Second twitch adds to first, fiber contracts further Tetanus: no visible relaxation between twitches Produces smooth, sustained contraction Recruitment Each skeletal muscle fiber innervated by only one motor neuron One motor neuron may innervate many muscle fibers Motor unit: set of muscle fibers controlled by one neuron fig 44.16 Motor unit with few fibers requires lowest level of activation Results in small contractile force For greater force more motor units are activated Isometric and Isotonic Contractions Isometric contraction: constant length contraction Muscle length cannot shorten with internal contraction Example: trying to lift an immovable object Increases tension of muscle Isotonic contraction: constant tension contraction Muscle shortens under constant load Can change to isometric and back Muscle Energy Consumption Formation of cross-bridges requires large amounts of ATP Isometric contractions have higher rate of energy use than isotonic ATP production by glycolysis Rapid but less efficient Produces lactic acid ATP production by oxidative phosphorylation Produces greater amounts of ATP Requires constant source of oxygen to cells Rapidly contracting muscle starts with oxidative phosphorylation, switches to glycolysis The Oxygen Debt Oxygen consumption remains high at end of strenuous exercise Extra oxygen consumed refer to as oxygen debt Some oxygen associated with metabolism of lactic acid Accumulated lactic acid must be metabolized to CO2 and H2O Cori cycle fig 44.17 Lactic acid converted to glucose in liver Returned to muscle Muscle Fatigue Use-dependent decrease in ability to generate force Mainly occurs from operating under anaerobic conditions High activity causes buildup of lactic acid Acid conditions interfere with cross-bridge formation Also depletes stores of glycogen in muscle and liver Energy production then comes from fat Production half that of glucose energy production Marked decrease in muscle performance Cardiac Muscle Composed of striated fibers, orientation different than skeletal fibers Composed of chains of single cells with individual nuclei Electrically coupled to neighbors by gap junctions Form single, functioning unit called myocardium Structure critical to heart muscle function Contraction initiated at one location called pacemaker Not initiated by impulses in motor neurons Impulses spread from pacemaker throughout myocardium via gap junctions Cells in each chamber of heart contract in synchrony Molecular mechanism of force generation is same as in skeletal muscle Contraction ejects blood from heart chamber, relaxation allows chamber to fill Impulses last longer than in skeletal muscle, allow for blood to be forced out Cardiac muscle does not produce summated contractions or tetanus Smooth Muscle Surrounds hollow internal organs like stomach, intestines, bladder, uterus, blood vessels (except capillaries) Long, spindle-shaped cells with individual nucleus Individual myofibrils of actin and myosin not organized into sarcomeres Parallel arrangements of thick and thin filaments cross diagonally Thick filaments attached to dense bodies or plasma membrane Have 10-15 thin filaments per thick filament Striated muscle fibers have 3 thin filaments per thick filament Smooth muscle cells do not have sarcoplasmic reticulum Ca++ comes from extracellular space Ca++ combines with calmodulin Complex activates myosin light chain kinase (MLCK) MLCK phosphorylates myosin heads, permitting formation of cross-bridges Strength of contraction increases with amount of Ca++ that enters cytoplasm Drugs can block entry of Ca++ into cells, causing vascular smooth muscles to relax Blood vessels dilate Reduces work heart must do to pump blood through them Some smooth muscles contract only when stimulated by nervous system Example: muscles lining walls of blood vessels, in iris of eye Called multiunit smooth muscle Cells not coupled together, must be activated as separate units Other smooth muscle like gut lining can contract spontaneously Contain special cells that produce electrical impulses Spread impulses to adjacent cells through gap junctions Leads to slow, steady contraction of tissue Called unitary smooth muscle, electrical coupling causes muscle to contract as unit Smooth muscle can contract even when greatly stretched Example: uterus Internal organs are frequently stretched, must still be able to contract