Topic 7: Run for your Life Flashcards
7.1 The ——– membrane secretes ——– fluid in a ——– joint. What does this fluid enable?
What is the role of the cartilage?
The synovial membrane secretes synovial fluid, which allows the bones of a synovial joint to move freely. The synovial fluid acts as a lubricant, reducing the friction between the cartilage of the joints during movement, and thereby minimizing wear and tear.
The cartilage protects bones within joints, by absorbing synovial fluid and acting as a shock absorber.
7.1 What is the role of ligaments? Which properties allow them to be effective?
Ligaments attach bones to other bones, controlling and restricting the movement in a joint. They are therefore strong and flexible (or elastic).
7.1 What is the role of tendons? Which properties allow them to be effective?
(Skeletal) muscles are attached to bones by tendons, which tend to be inelastic.
7.1 What is the role of skeletal muscles? What are antagonistic muscle pairs?
Skeletal muscle is the type of muscle used for movement. These muscles contract and relax to move bones at a joint. Muscles that work together to move a bone are called antagonistic pairs. Muscles can only pull through contraction - they can not push. Therefore two muscles are required, creating opposite forces, to move a bone.
7.1 How do extensors and flexors interact to enable movement?
A muscle that contracts to cause extension of a joint (so that it straightens) is called an extensor, while the corresponding relaxing muscle will be a flexor. A muscle that contracts to cause a joint to bend is called a flexor, with the corresponding relaxing muscle an extensor. So when an extensor contracts, the flexor will relax and the joint will straighten. When a flexor contracts, the extensor will relax and so the joint will bend.
7.2 Explain the structure of a sarcomere.
Within each muscle fibre, there are myofibrils. A myofibril is made up of many short contractile units called sarcomeres. The sarcomere is made up of two protein molecules: a thick myofilament, called myosin, and a thin myofilament called actin. Under an electron microscope, the myofibril exhibits a striped appearance. Dark bands contain the thick myosin filaments, and some overlapping thin actin filament (A-bands). Light bands contain thin actin filaments alone (I-bands) and intermediate-coloured bands contain thick myosin filaments alone (H-zone). In the middle of each sarcomere, and the middle of the myosin filaments, is the M-line. The ends of each sarcomere are marked with a Z-line. Sarcomeres are joined together lengthways at their Z-lines.
7.2 Give an overview of how muscle contraction works.
Myosin and actin filaments slide over one another to make the sarcomeres contract - this is the sliding filament theory. The myofilaments themselves don’t contract, remaining the same length. This simultaneous contraction of many sarcomeres results in a contraction of myofibrils and muscle fibres.
7.2 Explain the structure of actin and myosin.
Two other proteins called tropomyosin and troponin are attached to each other, found between actin filaments. Actin filaments have binding sited for myosin heads, called actin-myosin binding sites. Myosin filaments have globular heads that are hinged, enabling them to move back and forth. Each myosin head has a binding site for actin and a binding site for ATP.
7.2 In a resting muscle, what prevents it from contracting?
In an unstimulated muscle, the actin-myosin binding site is blocked by tropomyosin, which is held in place by troponin. So filaments can’t slide past each other because myosin heads can’t bind to the actin-myosin binding site on the actin filament.
7.2 Explain how an influx of calcium ions is triggered in a muscle cell.
When an action potential from a motor neurone arrives at the neuromuscular junction, it stimulates the muscle cell, depolarising the sarcolemma (the membrane of the muscle fibre). Depolarisation spreads down the tranverse (T) tubules (where the sarcolemma folds, spreading electrical impulses to all parts of the muscle fibre) to the sarcoplasmic reticulum - this releases stored calcium ions into the sarcoplasm (the cytoplasm of a muscle cell).
7.2 Explain the process of contraction of skeletal muscle in terms of the sliding filament theory.
When the calcium ions are released, they attach to the troponin, causing it to change shape and move. As a result, the attached tropomyosin also moves, exposing the actin-myosin binding sites and enabling the myosin head to bind - actin-myosin cross bridges are formed. When the myosin head binds to the actin (with ADP and Pi still attached), stored energy (from a previous hydrolysis of ATP into ADP and Pi) is released and the myosin head changes shape - it moves forwards, pulling the actin filament to the center of the sarcomere. The ADP and Pi molecules are also released from the myosin head, allowing it to change shape for an ATP molecule to attach. The ATP molecule causes it to change shape once again, releasing it from the actin-myosin binding site. Calcium ions also activate the enzyme ATPase, which breaks down the ATP into ADP and Pi (attaching them to the myosin head), releasing energy. The hydrolysis causes a change in shape in the myosin head, returning it to its upright position (with stored energy). The myosin head will reattach to a different binding site, and so the cycle will continue as long as calcium ions are present.
7.3 i) Explain the overall reaction of aerobic respiration?
C6H12O6 + 6O2 -> 6CO2 + 6H2O
In aerobic respiration, a respiratory substrate (such as glucose) is split, releasing carbon dioxide as a waste product and reuniting hydrogen with atmospheric oxygen, resulting in a large release of energy.
[During photosynthesis, water is split to form oxygen and hydrogen - the hydrogen is stored in a carbohydrate (such as glucose), but is brought together with oxygen to form water during aerobic respiration]
7.3 ii) What is respiration?
Respiration: a many-stepped process with each step controlled and catalysed by a specific intracellular enzyme.
7.4 Explain the role of glycolysis in aerobic respiration.
Glycolysis (the first stage in both aerobic and anaerobic respiration) happens in the cytoplasm of cells [stores of glycogen must first be converted to glucose - the main respiratory substrate].
1. Glucose is phosphorylated: 2 phosphates are removed from 2 ATP molecules, and added to the glucose - this results in 2 ADP molecules, and 2 molecules of triose phosphate (an intermediate phosphorylated 3C compound).
2. The 2 molecules of triose phosphate are oxidised, forming 2 molecules of the 3C pyruvate. The coenzyme NAD collects 2 hydrogens, forming 2 reduced NAD molecules.4 inorganic phosphates are removed from the triose phosphates, reacting with 4 ADP molecules to form 4 ATP molecules {in substrate-level phosphorylation}.
Glycolysis results in a net gain of 2 ATP molecules, and 2 reduced NAD molecules.
7.5 Explain the role of the link reaction in aerobic respiration.
The link reaction takes place in the mitochondrial matrix (because the required enzymes and coenzymes are there, and the reduced NAD produced by the link reaction are in the right place for oxidative phosphorylation, which occurs across the inner mitochondrial membrane). Each molecule of 3C pyruvate is decarboxylated (releasing 1 molecule of CO2) and dehydrogenated (releasing two hydrogen atoms, and therefore reducing 1 NAD coenzyme). The resulting 2C acetyl compound then combines with coenzyme A to form acetyl coenzyme A (acetyl CoA). So 2 pyruvate molecules produce 2 CO2 molecule, 2 reduced NAD molecules, and 2 molecules of acetyl CoA (so there are 2 link reaction per glucose molecule).
7.5 Explain the role of the Krebs cycle in aerobic respiration.
The Krebs cycle is controlled by a specific intracellular enzyme, which is found in the matrix of the mitochondria (so this is where the Krebs cycle takes place).
- Each 2C acetyl CoA molecule from the link reaction combines with a 4C compound to form a 6C compound.
- This 6C compound is decarboxylated (releasing 1 molecule of CO2) and dehydrogenated (releasing two hydrogen atoms, and therefore reducing 1 NAD coenzyme), forming a 5C compound.
- The 5C compound is then decarboxylated (releasing 1 molecule of CO2), and dehydrogenated 3 times (releasing 6 hydrogens, which form 2 reduced NAD molecules and 1 reduced FAD molecule). One molecules of ATP is also produced (by substrate-level phosphorylation). This regenerates the 4C compound.
So each acetyl CoA molecule produces 2 CO2 molecules, 3 reduced NAD molecules, 1 reduced FAD molecule, and 1 ATP molecule.
The Krebs cycle happens once per acetyl CoA molecule, or once per pyruvate molecule (so 2 Krebs cycles take place per glucose molecule).
7.5 What is substrate-level phosphorylation?
When a phosphate group is directly transferred from one molecule to another it’s called substrate-level phosphorylation: the energy for the formation of ATP comes from the substrates.
- 6 What is oxidative-level phosphorylation?
Oxidative-level phosphorylation is the process where the energy carried by electrons, from reduced coenzymes, is used to make ATP: it involved the electron transport chain and chemiosmosis.
7.6 How is ATP synthesised by oxidative-level phosphorylation?
The reduced coenzymes carry hydrogen atoms to the mitochondrial matrix, where the hydrogen atoms split into protons (H+) and electrons (e-). The electrons move down the electron transport chain (in the inner mitochondrial membrane), going from one electron carrier to another in a series of redox reactions, releasing energy. This energy is used by the electron carriers to pump protons from the mitochondrial matrix into the the intermembrane space. This creates a steep electrochemical gradient across the inner membrane. Protons move down the electrochemical gradient, back into the mitochondrial matrix, via the enzyme ATP synthase: this synthesises ATP from ADP and Pi (this process is known as chemiosmosis). Within the mitochondrial matrix, the protons, electrons and oxygen combine to form water (1/2 O2 + 2e- + 2H+ → H2O). The oxygen is said to be the final electron acceptor.
7.7 Outline what happens after a period of anaerobic respiration in animals.
In anaerobic respiration, glycolysis takes place to produce 2 pyruvate molecules, 2 reduced NAD molecules and 2 ATP molecules (per one glucose molecule). The 2 reduced NAD molcules then transfer their hydrogens to the 2 pyruvate molecules: the pyruvate is reduced into lactate, and the oxidised NAD is formed.
Lactate builds up in the muscles, and forms lactic acid in solution. This decreases the pH of the cells, and inhibits the enzymes that catalyses the glycolysis reaction (the hydrogen ions from the lactic acid neutralise the negatively charged groups in the active site, affecting the attraction between the substrate and the enzyme).
The lactate can be converted back into pyruvate, which is oxidised directly to CO2 and H2O via the Krebs cycle (in aerobic respiration), thus releasing energy to synthesis ATP. As a result, oxygen uptake is greater than normal in the recovery period after exercise (i.e. the oxygen debt). Some lactate can be converted into glucose, which is stored in the liver as glycogen.
7.8 i) Outline the myogenic nature of cardiac muscles.
The heart is myogenic: it can contract without external nervous stimulation.
7.8 ii) Explain how the normal electrical activity of the heart coordinates the heart beat.
Contraction of the cardiac muscles is initiated by a change in polarity, which spreads like a wave of depolarisation from cell to cell.
1) Depolarisation begins at the sinoatrial node (SAN), also known as the pacemaker, in the wall of the right atrium.
2) The SAN generates an electrical impulse, which travels to the right and left atria, causing them to contract simultaneously.
3) A band of non-conducting collagen tissue in the walls prevents the wave of deploarisation from being passed directly from the atria to the ventricles.
4) Instead, the impulse is passed from the SAN to the atrioventricular node (AVN). There is a slight delay, ensuring that the atria have finished contracting and that the ventricles have filled with blood before they contract.
5) The electrical impulse travels from the AVN to the bundle of His (a group of large, specialised muscle fibres), which is then conducted along the (finer) Purkyne fibres to the right and left ventricle walls.
6) The first ventricular cells to be depolarised are in the apex of the heart, so that the wave of contraction travels upwards, allowing the blood is pushed into the aorta and the pulmonary artery.
7.8 iii) How can electrocardiograms (ECGs) be used to aid the diagnosis of cardiovascular disease (CVD) and other heart conditions?
An electrocardiograph records changes in polarisation using electrodes placed on the chest and limbs. During the cardiac cycle, the heart muscle depolarises when it contracts, and repolarises when it relaxes. This produces an electrocardiogram:
1) P wave: depolarisation of the atria, leading to contraction of the atria (atrial systole).
2) PR interval: the time taken for the impulse to be conducted from the SAN across the atria to the ventricles, through the AVN.
3) QRS complex: the wave of depolarisation resulting int the contraction of the ventricles (ventricular systole).
4) T wave: repolarisation (relaxation) of the ventricles (diastole).
The ECG does not show atrial repolarisation because the signals generated are small and hidden by the QRS complex.
Doctors compare their patients’ ECGs with a normal trace, allowing them to diagnose any problems with the heart’s rhythm.
7.9 i) Explain how to calculate cardiac output.
cardiac output (cm3 min-1) = heart beat (beats per minute) x stroke volume (cm3)
Cardiac output is the total volume of blood pumped by the heart in one minute.
Stroke volume is the volume of blood pumped by one ventricle each time it contracts.
