Final Study Sheet - AP1 2024 Biology Exam Prep PDF

# Cell Organelles and Functions | Cell Organelle | Function | |---|---| | Nucleus | DNA, controls cell activities, regulates gene expression | | rER | Synthesizes proteins | | SER | Synthesizes lipids, detoxifies toxins, stores Ca++ ions | | Mitochondria | Produces ATP, "powerhouse" | | Golgi appar...

# Cell Organelles and Functions | Cell Organelle | Function | |---|---| | Nucleus | DNA, controls cell activities, regulates gene expression | | rER | Synthesizes proteins | | SER | Synthesizes lipids, detoxifies toxins, stores Ca++ ions | | Mitochondria | Produces ATP, "powerhouse" | | Golgi apparatus | Modifies, sorts, packages proteins/lipids for secretion | | Lysosome | Digests macromolecules, microorganisms | | Centrosomes | Organizes microtubules, helps in cell division | | Proteosomes | Gets rid of unneeded or damaged proteins | | Cytokeleton | Provides structural support, microtubules, helps in facilitates movement | ## Final Exam Study Guide - AP1 2024 ### Ribosomes (free) - Synthesize proteins - Used within the cytosol ### Vesicles - Transports materials - Secretes substances - Digests materials ## Homeostasis - Stable internal environment, even when external conditions occur changes. - Maintain body structure and function. - Ex. regulate body temp, maintain blood glucose levels, etc. ### Positive Feedback - Moves variable in the same direction until event occurs - body returns to homeostasis. - Amplifies change, moves it further away from set point. - Ex. blood clotting, childbirth, fruit ripening. ### Negative Feedback - Moves variable back towards its set point. Counteracts a change. - Maintains stability. - Resulting action is in the opposite direction of stimulus. - Ex. body temp regulation, control of blood glucose levels. ## Atoms - Smallest particle of a substance, cannot be broken down chemically. - Contains nucleus, electrons, neutrons, protons. ## Elements - Simple substance made of only 1 type of atom, cannot be broken down into smaller parts or changed into another substance. - All atoms with the same element have the same # of protons. ### Electrons - (-) charged, orbits nucleus ### Protons - (+) charged, in nucleus ### Neutrons - (neutral) charged, in nucleus ### Atomic Mass - # of protons + # of neutrons ### Atomic # - # of protons = # of electrons ### Cations - (+) charged, produced by ↓ of electrons (-) ### Anions - (-) charged, produced by ↑ of electrons (-) ## Organic Compounds - Contain carbon - part of living organism. - Ex. sugar, cellulose, carbs, proteins, lipids, nucleic acids (DNA, RNA), etc. ## Inorganic Compounds - No carbon - found in non-living matter. - Ex. H2O, salts, acids, bases, etc. ## Metabolism - All chemical reactions that occur within a living organism. ## Cellular Respiration - Metabolic pathway where cells break down glucose to produce ATP. ### 4 Stages: 1. **Glycolysis** - located inside cytoplasm - input: 1 glucose, 2 ATP, 2 NAD+ - output: 2 pyruvate, 2 ATP, 2 NADH 2. **Intermediate Stage (Pyruvate Oxidation)** - located within mitochondria - input: 2 pyruvate, 1 NAD+, 1 Coenzyme A - output: 2 AcetylcoA, 2 NADH, 1 CO2 3. **CAC (Krebs Cycle)** - located in mitochondria - input: 2 AcetylcoA - output: 2 ATP, 6 NADH, 2 FADH2 4. **ETC (Electron Transport Chain)** - located in mitochondria - input: NADH, FADH2 - output: 4 ATP, 10 NADH, 2 FADH2 ## Enzymes - Catalysts, accelerate chemical reactions. - Regulated by other molecules that ↑ or ↓ their activity: - **activators** - **inhibitors** ### Competitive Inhibitor - Resembles substrate. - Binds to active site of enzyme. ### Non-Competitive Inhibitor - Does not resemble substrate. - Binds to site other than active site (allosteric site). ## Exergonic vs. Endergonic Reactions ### Exergonic - Releases energy - negative - spontaneous reaction. - + of energy. - Ex. cellular respiration ### Endergonic - Absorbs energy - positive. - Non-spontaneous reaction. - - of energy. - Ex. photosynthesis ## Anabolism vs. Catabolism ### Anabolism - Small molecules joined to form larger ones. ### Catabolism - Large molecules broken down into smaller ones. ## Organization of Living Things (smallest->largest) 1. Chemical level - atoms + molecules 2. Cellular level - cells 3. Tissue level - tissues 4. Organ level - organs 5. Organ system level - multiple organs that work together 6. Organismal level - all body star systems function independently in an organism. ## Functions of Each Body System 1. **Integumentary System** - Hair, skin, nails - Protection, sensation, temperature regulation. 2. **Skeletal System** - Bones for support of softer parts. - Support, movement, protection, mineral storage, blood cell production. 3. **Muscular System** - Produces body movements. - Heart, digestive organs, blood vessels 4. **Nervous System** - Eye, brain, spinal cord, nerves. - Controls muscles + some glands. 5. **Endocrine System** - Hypothalamus, glands, thymus, pancreas. - Secretes hormones, controls digestive processes + reproductive functions, maintains homeostasis of blood composition + volume. 6. **Cardiovascular System** - Heart, blood vessels. - Delivers O2, nutrients, hormones. 7. **Lymphatic System** - Tonsils, thymus, nodes, spleen. - Transports & filters lymph. 8. **Respiratory System** - Lungs, nose, trachea, bronchi. - Exchanges gases between blood + air in lungs 9. **Urinary System** - Kidneys, ureters, urethra, bladder. - Filters blood, removes waste products. 10. **Digestive System** - Mouth, esophagus, stomach, intestines, anus. - Digests, absorbs, & expels products. 11. **Male Reproductive System** - Testes, prostate gland, penis, scrotum. - Produces sex cells + hormones. 12. **Female Reproductive System** - Ovaries, uterus, vagina, fallopian tubes. - Produces sex cells + hormones. ## Common Elements In Body 1. Oxygen (O) 2. Carbon (C) 3. Hydrogen (H) 4. Nitrogen (N) ## Plasma Membrane - Phospholipid bilayer - Hydrophilic heads, hydrophobic tails. - Small, nonpolar, hydrophobic molecules: - Ex. O2, CO2 - **Fluid Mosaic Model:** The structure of the PM. - **What can affect fluidity of PM?:** Temperature, cholesterol, composition of fatty acids in phospholipids. ## Polar vs. Non-Polar, Hydrophilic, Hydrophobic ### Polar Molecules - Uneven distribution of charge (1+, 1-). - Asymmetric. ### Nonpolar Molecules - Even distribution of charge. - Symmetric. ### Hydrophilic - "Water loving", polar ### Hydrophobic - "Water fearing", nonpolar ### Amphipathic - Both hydrophilic + hydrophobic. ## Macromolecules ### Major Macromolecules - Carbohydrates, proteins, lipids, nucleic acids. ### Single unit - Monomer - Small molecules/atoms that bond together to form complex structures (polymer). ### Polymer - Large molecule, composed of monomers. ### Bonds (chemical Bonds) - Ionic bonds, covalent bonds, hydrogen bonds. ## REDOX Reactions (Oxidation-Reduction Reaction) - Electrons moved from 1 chemical structure to another. ### LEO the lion goes GER - **LEO:** Structure that **loses** an electron = **oxidation** - **GER:** Structure that **gains** an electron = **reduction** ## Endomembrane System - A group of membranes & organelles in eukaryotic cells that work together to modify, package, and transport lipids/proteins. - Includes: nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles. - Smooth - Rough ## Membrane Transport ### Passive Transport - No energy (ATP). - ↓ the [] gradient [A] to [+] - Ex. diffusion O2, CO2, osmosis H2O - Ex. diffusion O2, CO2, osmosis H2O ### Active Transport - Requires energy (ATP). - Against [] gradient [↓] to [↑] - Ex. pumps (sodium-potassium pump) ## How the Concentration [] gradient Determines Transport - Dictating that molecules will move from ↑ concentration to ↓ concentration. ## Osmosis & Diffusion ### Osmosis - Movement of H2O ### Isotonic - Cytosol + solution have the same concentration of solutes. - Ex: normal saline, with a concentration of 0.9% NaCl ### Hypotonic - Solution has a lower concentration of solutes, a higher concentration of H2O than cytosol. - Ex. erythrocytes in pure H2O ### Hypertonic - Solution has a higher concentration of solutes than cytosol. - Ex. erythrocytes in 3% NaCl H2O solution ### Diffusion - Movement of ions/molecules from ↑ concentration to ↓ concentration. - Movement of molecules across a cell membrane without help of proteins. ### Facilitated - Movement of molecules across a cell membrane with help of proteins: -Carrier -Channel ## Symport vs. Antiport ### Symport - 2 or more molecules transported in the same direction. - Ex. sodium-glucose transporter ### Antiport - 2 or more molecules transported in the opposite direction. - Ex. sodium-calcium exchanger ## Vesicular Transport ### Out - **Exocytosis:** Cell releases substances from the inside by fusing vesicles with the cell membrane. ### Inside - **Endocytosis:** Cell takes in substances from outside by engulfing them with the cell membrane. - 2 types: - **Phagocytosis** (cell eating) - **Pinocytosis** (cell drinking) ## Receptor-Mediated Transport (Endocytosis) - Allows cells to import large amounts of molecules from outside the cell. - Ex. transport of cholesterol from blood cells bound to LDL ("bad" cholesterol). ## Membrane Potential - Nerve Signaling - **RMP (resting membrane potential):** Difference in electrical charge between the inside & outside of the cell when the cell is at rest. - **Na+, K+:** A neuron sends info to its axon, away from the cell body. ### RMP Sequence of Events 1. **↑ K+ ions are inside than outside**, ↑ Na+ ions are outside than inside. 2. **Cell membrane is more permeable to K+ ions than Na+ ions**, allowing more K+ to leak out of the cell - concentration (gradient) of K+ ions makes them move out of the cell. 3. **The (-) charge in the cell attracts them back in - active transport protein pumps Na+ out of the cell & K+ back in**, maintains ion gradients + RMP. ### AP Sequence of Events 1. **Resting potential** → 2. **Depolarization (Na+ influx)** → peak potential → 3. **Repolarization (K+ efflux)** → hyperpolarization (below resting potential). ### Depolarization - MP becomes + (-), (-) charge inside neuron (Na+ channels) ### Hyperpolarization - MP becomes ↑ (-), A(-) charge inside neuron (K+ channels) ### Concentration & Electrical [] gradient - Dictates the direction of ion movement based on abundance in or outside the cell. - Pulls ions towards opposite charges, balancing these forces to maintain a stable MP or trigger rapid changes in AP. ## Nucleus Structure ### DNA - In the nucleus, monomers - deoxyribonucleotides. ### Chromatin - Made up of DNA- wrapped around proteins (histones) ### Chromosomes - Thread-like - in nucleus. ## DNA Structure ### Monomer - Nucleotide ### Polymer - DNA ### Bonds - Phosphodiester ### Nucleotide - Nitrogeneous base (ATCG), 5 sugar molecule (deoxyribose), phosphate group. ## Transcription of RNA ### Steps 1. **Initiation:** RNA polymerase binds to the promoter region on DNA. 2. **Elongation:** RNA polymerase reads DNA template & builds the RNA molecule. 3. **Termination:** RNA polymerase reaches a terminator sequence & releases the completed RNA transcript ### Enzymes involved - RNA polymerase - Unwinds DNA double helix. - Builds RNA molecule by linking ribonucleotides together ### Modification of mRNA - Initially synthesized mRNA: pre-mRNA - Undergoes changes before leaving nucleus. - Formation of mature mRNA. - Used to make protein. ### Capping - Adds special structure to the start of the mRNA. ### Splicing - Removes unnecessary parts & can create different versions of proteins. ### Adding of poly A tail - Adds a tail to the end of the mRNA (Polyadenylation). ## Translation of mRNA - Protein - tRNA align a.a. with their corresponding codons in mRNA. ### Anticodon - tRNA ### Types of Codons - Start codon - Stop codon - (AUG), signal to begin protein synthesis - Where mRNA reading ends. ## 3 Steps: Translation of mRNA - Protein 1. **Initiation:** The small (30S) subunit binds to mRNA, tRNA molecule bind to start codon, large (50S) subunit joins to form a functional ribosome. 2. **Elongation:** Ribosome moves along the mRNA, reads the codons, adds the corresponding a.a. to a growing protein chain. 3. **Termination:** Ribosome reaches stop codon. New synthesized protein is released. ## Cell Division: Phases of Mitosis 1. **Interphase:** DNA replication occurs, preparing the cell for mitosis. 2. **Prophase:** Chromatin condenses into chromosomes, nuclear membrane breaks, centrioles more to opposite poles, spindle forms. 3. **Metaphase:** Chromosomes line along center of cell, spindle fibers are attached to centromeres of each chromosome. 4. **Anaphase:** Sister chromatids separate and are pulled apart by the spindle fibers to opposite poles. 5. **Telophase:** Nuclear membrane reforms around each chromosome at poles, chromosomes decondense, 2 separate nuclei form. 6. **Cytokinesis:** Cell cytoplasm divides into 2 daughter cells. ## Cell Cycle ### Interphase - Time the cell prepares for division. - 3 Phases: - G1 - S - G2 ### Mitosis - Occurs in somatic cells (all cells other than sex cells) ### G1 - Growth + production of new organelles. - Replication of centrioles to produce 2 pairs. ### G2 - Centriole replication completes. - Enzymes are synthesized ### Cell Aging - Cells lose the ability to divide/replicate ### Death (Apoptosis) - Programmed cell death. ## 3 Functions of Integument (skin) 1. Secretion 2. Protection from External Environment. 3. Temperature Regulation ## 3 Regions of Integument (skin) 1. **Epidermis:** Top layer, barrier, prevents water loss + entry of substances/organisms. - Ex. Stratified squamous epithelium. 2. **Dermis:** Deeper layer, contains blood + lymph vessels, nerves, hair follicles, sweat glands. - Ex. Dense irr. CT 3. **Hypodermis (Subcutaneous):** Deep to dermis, layer of fat + CT, not considered part of true skin or integumentary system. - Ex. Alveolar CT ## 5 Layers of Epidermis ### Superficial 1. **Stratum corneum:** Keratinocytes __Come__ 2. **Stratum lucidum:** Keratinocytes __Lets__ 3. **Stratum granulosum:** Keratinocytes ___Get___ 4. **Stratum spinosum:** Keratinocytes __Sun__ ### Deep 5. **Stratum basale:** Melanocytes, ___Burnt___ Keratinocytes. ## Composition of Dermis - **CT proper:** Blood vessels, sweat glands, sebaceous glands, hair follicles, nail roots, arrector pili. - **Papillary layer:** Superficial region - deep to epidermis. - Areolar CT. - Dermal papillae. - **Reticular layer:** Deeper - major portion. - Dense irr. CT - Collagen, elastic fibers. ## Arp of Skin - Skin = Organ - Integumentary system = Organ system. - Includes: skin, hair, nails. * **Stratum corneum:** Outermost layer of epidermis, protects against abrasion & infection. * **Stratum granulosum:** Thin later, located between "spinosum" and "lucidum", lots of keratin. * **Stratum basale (germinativum):** Deepest layer, produces keratinocytes, melanocytes, and tactile cells. ## Melanocytes - Cells that produce melanin. - Gives skin, hair, nails, eyes color. ## Sweat Glands - Produce sweat to regulate body temp and keep hydrated. - 2 types: - **Eccrine:** Open on skin, most common. - **Apocrine:** Open into hair follicles (armpits, groin, scalp) ## Nerve Receptors (Sensory Receptor) - Thermoreceptors - Nociceptors ## Sebaceous Glands - Holocrine glands that produce sebum. - Secretory products. - Only secretion. ## Arrector Pili Muscle - Thin smooth muscle, extend from hair follicle dermal papillae "goosebumps" ## Hair Follicle & Hair - Hair found almost everywhere. - Keratinized cells - grows from hair follicles. ## General Function of NS - Control + integration of electrical impulses. - Neurons ## CNS - Brain/spinal cord. ### Skull - Houses and protects. ### Vertebral canal: - Houses and protects (spinal cord). ## PNS - Nerves, ganglia. ### Bundles of axons of neurons - Cluster of neuron cell bodies along neuron. ## General organization of NS ### Sensory neurons - Receptors to CNS. - Afferent. - Sensory input. - "Arrives" ###### 2 types: - Somatic: Conscious. - Ex. The 5 senses. - Visceral: Unconscious. ### Motor Neuron - CNS to effectors. - Efferent - "Exits" - Somatic (voluntary) - Autonomic (involuntary) - Motor output. - Skeletal muscle (neurotransmitter ACH) - Cardiac muscles - Smooth muscles - Glands. ## Structure of Nerves + Ganglia ### Picture in Notes: - **Nerves:** Organs- cable-like bundles of axons. - Based on CNS structural origin - cranial/spinal. - **Ganglia:** Cluster of neuronal cell bodies (soma). - Surrounded by glial cells. - Enclosed within CT. - Picture of the ganglion in lab notes. ## Epineurium (1st CT layer of nerve) - Thick/dense CT - Nerves - Protects/supports nerve. ## Perineurium (2nd CT layer of nerve) - Dense irr. CT - fascicle (nerve). - Bundle of axons. ## Endoneurium (3rd CT layer of nerve) - Areolar CT - 1 axon ## Fascicle - Bundle of axons - surrounded by endoneurium + perineurium. - Provides protection/support to nerve fibers. ## PM of Neurons - Sodium-potassium (Na+/K+) actively pumping. - 3 out - 2 in - Maintains RMP ## Channels: Ion - Allow specific ions to pass through membrane based on size/charge. - Types: - Voltage-gated - Ligand-gated ## Function of Receptors, Diff. Types of Effectors in NS ### Receptors Function - Detect changes in the environment (stimuli). - Convert them into electrical signals. - Send info to CNS. ### Photoreceptors - Eye, ear, nose - Hair cells - Chemoreceptors - Through nerve impulses - external or internal. ## Types of Effectors Controlled by NS: - Muscles: Skeletal muscles for voluntary movement. - Smooth muscles in organs (ANS). - Glands: Secrete hormones in response to nervous signals (ANS)- ## Functions of Neuron Structures ### Dendrites - Receives electrical signals from other neurons, collects info through synapses & transmitting it to the cell body (soma). ### Axon - Transmits electrical impulses or AP away from the cell body (soma). - Carries signal to other neurons, muscle cells, or glands. - Sends info out from neuron to its target destination. ### Synaptic Vesicles - Store neurotransmitters, release them at the synapse. ### Neurofibrils - Provide structural support, maintain cells shape (provides framework for organelles to pass), propagation of nerve impulses. ## Ions, Phospholipid Bilayer, PM Channels in Neurons ### Role of ions - Create electrical signals through their movement across membrane, enables neurons to transmit info by generating AP. - Ex. Na+, K+ are involved ### Role of Phospholipid Bilayer - Selective barrier, forms structure of membrane, controls what substances can enter & exit the cell, maintains electrical potential + signal transmission. ### Role of PM Channels - Allows the movement of ions across membrane. - 3 types: 1. Chemically gated: Na+, K+, Cl 2. Voltage-gated: Na+, K+ (voltage) 3. Ca2+ pumps/channels (voltage) ## Electrical Current - **Voltage (V):** mV - amount in e- charge - **Current (I):** Movement of ions - **Resistance (R):** Opposition of charged ions ## Conditions of Neuron at Rest in terms of RMP - **Concentration (gradient) for Na+: ↑ outside the cell than inside.** - Favors the entry of Na+ into cell. - **Concentration (gradient) for K+: ↑ inside the cell, ↓ outside the cell.** - **Concentration (gradient) for Cl-: ↑ outside the cell than inside.** - **Concentration (gradient) for Ca++ at synaptic knob: ↑ outside the cell, ↓ inside the cell.** ### Voltage-gated channels State - **Closed state:** But can transition to an "open" state when stimulated by change in MP (voltage) - axon hillock ## Significance of Treshold MP in Initial Segment of the Axon - It's the "decision point" for whether or not an AP will be generated + propagated down the axon - axon hillock/terminal end ## Depolarization & Repolarization Occur in Conductive Segment - **When voltage-gated Na+ channels open:** - Rapid influx of Na+ into the cell. - MP is more (+) - **When voltage-gated Na+ channels close, voltage-gated K+ channels open:** - K+ ions flow out of the cell. - Restores (-) MP to its resting state. ## Propagated AP at Synaptic Knob - Synaptic Cleft Events - **Triggers the opening of voltage-gated Ca2+ channels:** - Ca2+ ions flow into terminal. - **Releases neurotransmitter into synaptic cleft:** - Causes synaptic vesicles containing neurotransmitters to fuse with presynaptic membrane. ## AP vs. GP - **AP:** Long-distance signaling. - **GP:** Local signal transmission (short-distance). ## Neurotransmitters ### Structural - A.a. (excitatory/inhibitory), monoamines. - Ach, glutamate - GABA - Dopamine, norepinephrine - (EPSP) (IPSP) ### Functional - Excitatory, inhibitory - ↑ likelihood of firing AP - ↓ likelihood of firing AP ## How RMP IS Established + Maintained - Distribution of charged particles - cations, anions. - Phosphate ions (-) charged & more frequent in cytosol - proteins. ## 3 Main Functions of CSF 1. **Bouyancy:** Weight of brain by 95%. 2. **Protection:** As a cushion "mums buffer". 3. **Environmental stability:** Transports nutrients/waste. ## General Function of Cerebrum - Conscious thought, intellectual function. - Both halves receive sensory & motor input. ## Functions of Hypothalamus 1. **Control of autonomic (HR, BP, respiration, digestion):** Hypothalamus is the primary command center. 2. **Control endocrine**. 3. **Regulates body temp**. ## Major Functions of CNS - **Temporal lobe:** - Processes auditory info, memory formation, visual info, memory formation. - **Occipital lobe:** - Processes visual info, memory formation. - **Frontal lobe** - Voluntary movement, language processing, social behavior, emotional regulation, etc. - **Parietal lobe:** - Processes sensory info (touch, temp, pressure, pain) - **(L) (R) Cerebral Hemisphere:** - Spatial relationships, musical ability, emotional processing, language, verbal memory, analytical thinking. - **Brainstem:** - Breathing, consciousness, BP, HR, sleep. - **Corpus callosum:** Connects brain to spinal cord. - **Facilitates communication between L+R hemispheres.** ## Midbrain - Motor movement (eye & reflexes), relays sensory info, vision, hearing, pain, sleep/wake cycle. ## Medulla Oblongata - Breathing, HR, BP, swallowing, coughing, sneezing, vomiting. ## Olfactory Nerve/Bulb - Sense of smell ## Optic Nerve - Transmits visual info from retina to brain, eye reflexes, sleep/wake cycles ## Hippocampus - Memory, learning, spatial navigation. ## Prefrontal Cortex - Attention regulation, memory processing, response inhibition. ## Pituitary Gland - Produces & releases hormones, regulates growth, metabolism, reproduction, stress. ## Pineal Gland - Controls sleep/wake cycle by secreting melatonin ## Cerebellum - Coordinates movement + balance, language, attention, vision + eye movement. ## Osteoblast - Building of bone ## Osteoclast - Bone breakdown ## Osteoprogenitor Cells - Bone stem cells ## Anatomy of Long Bone - **Bone growth and repair** - **Articular cartilage** - **Spongy bone** - **Epiphyseal line** - **Periosteum** - **Endosteum** - **Medullary cavity** - **Compact bone** - **Proximal epiphysis** - **Metaphysis** - **Diaphysis** - **Metaphysis** - **Distal epiphysis** ## Anatomy of Muscles Tissue - **Skeletal striated:** Allows for movement, posture, breathing. - Most abundant - arrangement of sarcomeres. - **Smooth involuntary:** Lines the walls of hollow organs, thick & thin filaments. - Not striated. - Not arranged of sarcomeres. - Stomach, intestines, etc. - **Cardiac:** Walls of heart, striated, involuntary. - Pumps blood. ## Process of Muscle Contraction 1. **Nerve impulse:** Motor neuron sends electrical signal (AP) "arrives" - muscle fiber. 2. **AP travels along muscle fiber's membrane (sarcolemma):** Through T-tubules. 3. **Excitation-contraction coupling:** - Causes Ca++ ions to release from the sarcoplasmic reticulum. 4. **Ca++ binds to troponin:** - Released Ca++ ions bind to troponin on actin filaments. - Exposes myosin binding sites. 5. **Myosin heads attach to exposed binding sites on actin:** - Cross-bridges form. 6. **Myosin heads pivot, pulling actin filaments towards the center of the sarcomere:** - Generates tension and shortens the muscle fiber. 7. **ATP binds to myosin head - myosin detaches from the actin filament:** 8. **Nerve signal stops, Ca++ ions pump back into sarcoplasmic reticulum:** - Causes troponin-tropomyosin complex to block actin binding sites - muscle relaxes ## End of Document