Biology notes - topic 7 [UNFINISHED]


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Biology notes - topic 7 [UNFINISHED]

  1. 1. Biology Unit 5<br />Joints and Movement<br />1943735876300Muscles bring about movement at a joint, at least two are needed to move a bone to and fro because muscles can only pull. A pair of muscles working in this way are described as antagonistic. A muscle that causes contraction is called an extensor while the flexor muscle contracts in the reverse movement.<br />Joint Structure<br />Other types of jointsBall-and-socket jointA round head fit into a cup-shaped socket, e.g. the hipGliding jointTwo flat surfaces slide over one another, e.g. the articulating surfaces between neighbouring vertebraeHinge jointA convex surface fits into a concave surface, e.g. the elbowPivot jointPart of one bone fits into a ring-shaped structure and allows rotation e.g. the joint at the top of the spineThe plates of the skull are fixed together by fibrous tissue so very little movement occurs, protecting the brain. In the skull of a newborn baby the joints are not yet fixed, allowing the plates to move and the skill deform (reversibly) during birth. There are spaces between the skull bones, which fill in as the plates grow and fuse together.Pelvic bones are joined together by cartilage, so slight movement is possible during childbirth. This is a cartilaginous joint. There are also saddle joints in which more complex concave and convex surfaces articulate.This is an example of a synovial joint, such as those of the kip, knee and ankle. The bones that articulate in the joint are separated by a cavity filled with synovial fluid, which enables them to move freely. <br />Muscles<br />How muscles work<br />Muscle is made up of bundles of muscle fibres. Each fibre is a single muscle cell. Each cell has several nuclei, referred to as multinucleate. This is because one nucleus does not effectively control the metabolism of such a long cell. During prenatal development, several cells fuse together to form the length of muscle fibres. The muscle cells are stripped which is important for them to be able to contract.<br />Tendon<br />Tendons at each end of the muscle connect the muscle to bone.<br />Bundle of Muscle Fibres<br />The muscle is made up of bundles of muscle fibres up to 2cm across. These are bound together by connective tissue, which is continuous with the tendons.<br />Muscle Fibre<br />Each muscle fibre is a single muscle cell surrounded by a cell surface membrane. Each muscle fibre may be several centimetres long, but is less than 0.1mm in diameter. Inside the muscle fibre is the cytoplasm containing mitochondria and other organelles.<br />Myofibrils<br />Within each muscle fibre there are numerous myofibrils; each is composed of repeated contractile units called sarcomeres.<br />101917538735<br />Inside muscle fibres<br />Inside each muscle fibre are numerous myofibrils which are made of a series of contractile units called sarcomeres.<br />Sarcomeres are made of types of protein molecules called actin and myosin. Actin normally makes up the thin filaments whereas myosin is mainly for thicker filaments. Contractions are produced by co-ordinating the sliding of the filaments in the sarcomeres. The proteins overlap and give the muscles fibres the striated characteristic.<br />2981325789305-257175789305Where actin filaments occur on their own, there is a light band on the sarcomeres. Where both actin filaments and myosin filaments occur, there is a dark band. Where only myosin filaments occur, there is an intermediate coloured band. <br />2. Actin and myosin filaments when the muscle is contracted1. Actin and myosin filaments when the muscle is relaxed<br />-93345241935<br />3. The banding patterns created on an extended muscle myofibril<br />How sarcomeres shorten<br />Actin is associated with the proteins troponin and tropomyosin. The club shafts of myosin lie together as a bundle, with heads protruding along their length. In contraction, the change in orientation of the myosin heads brings about the movement of actin. The myosin heads attach to the actin and dip forward, sliding actin over the myosin; this is the sliding filament theory.<br />The sliding filament theory<br />When a nerve impulse arrives at a neuromuscular junction, calcium ions are released from the sarcoplasmic reticulum. This is a specialised type of endoplasmic reticulum: a system of membrane-bound sacs around the myofibrils. The calcium ions diffuse through the sarcoplasm. This initiates the movement of protein filaments.<br /><ul><li>Ca2+ attaches to the troponin molecule, causing it to move
  2. 2. As a result, the tropomyosin on the actin filament shifts its position, exposing myosin binding sites on the actin filaments
  3. 3. Myosin heads bind with myosin binding sites on the actin filament, forming cross-bridges
  4. 4. When the myosin head binds to the actin, ADP and Pi on the myosin head are released
  5. 5. The myosin changes shape, causing the myosin head to nod forward. This movement results in the relative movement of the filaments; the attached actin moves over the myosin
  6. 6. An ATP molecule binds to the myosin head. This causes the myosin head to detach
  7. 7. An ATPase on the myosin head hydrolyses the ATP, forming ADP and Pi
  8. 8. This hydrolysis causes a change in the shape of the myosin head. It returns to its upright position. This enables the cycle to start again</li></ul>When a muscle relaxes, it is no longer being stimulated by nerve impulses. Calcium ions are actively pumped out of the muscle sarcoplasm, using ATP. The troponin and tropomyosin move back, once again blocking the myosin binding sites on the actin. In the absence of ATP, the cross-bridges remain attached. This is what happens in rigor mortis when the muscles that are contracted become rigid.<br />Other types of muscleMuscles found in the gut wall, blood vessels and the iris of the eye are known as smooth muscle as their fibres do not appear to be striped. These are small cells with a single nucleus. They have a similar mechanism of contraction to skeletal muscle, using myosin and actin protein filaments. However, they are not arranged in the same way as they have gap junctions. These intercellular channels less than 2nm in diameter, and are between the smooth muscle cells to give cytoplasmic continuity between the cells. This allows chemical and electrical signals to pass between adjacent cells, and so allows synchronised contraction. Contractions in smooth muscle fibres are slower and longer lasting and the fibres fatigue very slowly if at all.The heart walls are made of specialised muscle fibres called cardiac muscle. These are striped and interconnected to ensure that a co-ordinated wave of contraction occurs in the heart. Cardiac muscle fibres do not fatigue. Neither smooth nor cardiac muscles are under conscious control.<br />Energy for Action<br />The minimum energy requirement is called the basal metabolic rate (BMR), measured in kJ g-1 h-1. It is used to measure the minimum energy requirement of the body at rest to fuel basic metabolic processes. BMR is measured by recording oxygen consumption under strict conditions of no food consumption for 12 hours before measurement; the body had to be totally at rest in a thermostatically controlled room. BMR is roughly proportional to the body’s surface area. It varies between individuals depending on their age and gender. Percentage body fat seems to be important in accounting for these differences.<br />Physical activity increases the body’s total daily energy expenditure. Energy is needed for muscle contraction to move the body but the energy can vary on how the muscles are used. For example, an elite marathon runner uses energy at almost half the rate of a sprinter.<br />Releasing energy<br />Food is the source of energy for all animal activity. The main energy sources are carbohydrates and fats that have been absorbed or stored around the body. Respiration is linked to ATP synthesis as the cells use the molecule ATP as an energy carrier molecule. <br />ATP is created from ADP by the addition of Pi. In solution, phosphate ions are hydrated and so the phosphate needs to be separated from these water molecules to make ATP, requiring energy. ATP in water is higher in energy than ADP and phosphate ions in water, so ATP is water is a way of storing chemical potential energy. ATP keeps the phosphate separated from the water, but they can be brought together in an energy-yielding reaction every time energy is needed for reactions within the cell.<br />When one phosphate group is removed from ATP by hydrolysis, ADP forms. A small amount of energy is required to break the bond holding the end phosphate in the ATP. Once removed, the phosphate group becomes hydrated. A lot of energy is released as bonds form between water and phosphate. This energy can be used to supply energy-requiring reactions in the cell. Some of the energy transferred during hydration of phosphate from ATP will raise the temperature of the cell; some is available to drive other metabolic reactions such as muscle contraction, protein synthesis or active transport. The hydrolysis of ATP is coupled to these other reactions:<br />ATP in water->ADP in water+hydrated Pi+energy transferred<br />Carbohydrate oxidation<br />If exercise is low intensity, enough oxygen is supplied to cells to enable ATP to be regenerated through aerobic respiration of fuels. Fates and carbohydrates are oxidised to carbon dioxide and water. A summary of the equation for aerobic respiration:<br />C6H12O6+6O2->6CO2+H2O+energy released<br />Photosynthesis was described as a process that separates hydrogen from oxygen by photolysis. The hydrogen from water is stored by combining it was carbon dioxide to form carbohydrate. In aerobic respiration, the hydrogen stored in glucose is brought together with oxygen to form water again. The bonds between hydrogen and carbon atoms in glucose are not as strong as the bonds between hydrogen and oxygen atoms in water. Therefore, the input of energy needed to break the bonds in glucose and oxygen is not as great as the energy released when the bonds in carbon dioxide and water are formed. Overall, there is a release of energy and this can be used to generate ATP.<br />Glucose and oxygen are not brought together directly, as this would release large amounts of energy too quickly and therefore damage the cell. Glucose is split apart in a series of small steps with carbon dioxide as the waste product. Hydrogen from the glucose is eventually reunited with oxygen to release large amounts of energy as water is formed.<br />3199130295275Glycolysis first<br />The initial stage of carbohydrate breakdown, known as glycolysis, occurs in the cytoplasm of cells and the sarcoplasm of muscle cells.<br />Stores of glycogen in muscle or liver cells must first be converted to glucose. It is a good fuel but it is quite stable and unreactive. Therefore, the first reactions of glycolysis need an input of energy from ATP to get started. Two phosphate groups are added to the glucose from two ATP molecules increasing the reactivity of glucose. It can then be split into two 3-carbon molecules.<br />Each intermediate 3C sugar is oxidised to produce pyruvate, a 3C compound. Two hydrogen atoms are removed during the reaction and taken up by the coenzyme NAD, which is a non-protein organic molecule. Glucose is at a higher energy level than the pyruvate and so some energy becomes available for the direct creation of ATP. Phosphate from the intermediate compounds is transferred to ADP to create ATP, known as the substrate-level phosphorylation.<br />In summary, glycolysis reactions yield a net gain of two ATPs, two pair of hydrogen atoms, and two molecules of 3C pyruvate.<br />The fate of pyruvate if oxygen is available<br />If oxygen is available, the pyruvate created passes into the mitochondria where it is completely oxidised to form carbon dioxide and water.<br />4362450114935The link reaction<br />In the first step, pyruvate is decarboxylated, when carbon dioxide is released as a waste product, and dehydrogenated, where two hydrogens are removed and taken up by the coenzyme NAD. The resulting 2C molecule combines with coenzyme A to form acetyl coenzyme A (acetyl CoA). Two hydrogen atoms released are involved in ATP formation. The coenzyme A carries the 2C acetyl groups to the Krebs cycle.<br /><ul><li> Understanding the Chemistry of RespirationThe chemical reactions inside cells are controlled by enzymes. There are four important types of reaction in the Krebs cycle:Phosphorylation reactionsDecarboxylation reactionsRedox reactions</li></ul>Krebs cycle<br />Each 2-carbon acetyl CoA combines with a 4-carbon compound to create 6 carbons. Two steps are decarboxylation with the formation of carbon dioxide. Four steps are dehydrogenation, which removes pairs of hydrogen atoms. One of the steps includes substrate-level phosphorylation with direct synthesis of a single ATP molecule. The Krebs cycle takes place in the mitochondrial matrix where the enzymes that catalyse the reactions are located.<br />Each 2-carbon molecule entering the Krebs cycle results in the production of two carbon dioxide molecules, one molecule of ATP by substrate-level phosphorylation, and four pairs of hydrogen atoms, which are taken up by the hydrogen acceptors, the coenzymes NAD and FAD. The hydrogen atoms are subsequently involved in ATP production via the electron transport chain.<br />The Electron Transport Chain<br />For most hydrogen produced, the coenzyme NAD is the hydrogen acceptor but those released in one-step of the Krebs cycle are accepted by the coenzyme FAD rather than NAD.<br />When a coenzyme accepts hydrogen with its electron, the coenzyme is reduced, becoming reduced NAD or reduced FAD. This reduced coenzyme ‘shuttles’ the hydrogen atoms to the electron transport chain on the mitochondrial inner membrane. Each hydrogen atom’s electron and proton then separate, with the electron passing along a chain of electron carriers in the inner mitochondrial membrane.<br />ATP Synthesis by Chemiosmosis<br />Energy is released as electrons pass along the electron transport chain. This energy is used to move hydrogen ions from the matrix, across the inner mitochondrial membrane, and into the intermembrane space. This creates a steep electrochemical gradient across the inner membrane. There is a large difference in the concentration of hydrogen ions across the membrane, and a large electrical difference, making the intermembrane space more positive than the matrix. <br />The hydrogen ions diffuse down this electrochemical gradient through hollow protein channels in stalked particles on the membrane. As the hydrogen ions pass through the channel, ATP synthesis is catalysed by ATPase located in each stalked particle. The hydrogen ions cause a conformational change in the enzyme’s active site, so the ADP can bind.<br />Within the matrix, the hydrogen ions and electrons recombine to form hydrogen atoms. These combine with oxygen to form water. The oxygen, acting as the final carrier in the electron transport chain, is thus reduced. This method of ATP synthesis is known as oxidative phosphorylation.<br />How much ATP is produced?<br />The total number of ATP produced by one glucose molecule can vary according to the efficiency of the cell. A simple explanation would give a maximum number of 38 ATP molecules per glucose molecule. This is based on the assumption that that reduced NAD that is reoxidised results in the formation of three ATP molecules and each reduced FAD results in production of two ATP molecules.<br />Rate of Respiration<br />The rate of aerobic respiration can conveniently be determined by measuring the uptake of oxygen using a respirometer. The rate is determined by the any factor affecting the rate of the enzyme-controlled reactions.<br />The concentration of ATP in the cell has a role in the control of respiration. ATP inhibits the enzyme in the first step of glycolysis, the phosphorylation of glucose. The enzyme responsible for glucose phosphorylation can exist in two different forms. As the ATP is broken down, the enzyme is converted back to the active form and catalysis the phosphorylation of glucose. This is known as end point inhibition. <br />Google: W H Freedman Animations<br />