Video 1
Haemoglobin
Haemoglobin is made up of globin proteins and an iron rich compound known as haem. It is found in red blood cells and is involved in the transport of oxygen around the body. Red blood cells do not have a nucleus, increasing the amount of space for haemoglobin. There are approximately 300 million haemoglobin molecules in each red blood cell.
The structure of haemoglobin enables it to carry oxygen with high efficiency. It has a quaternary structure made up of 4 globin subunits, most commonly 2 alpha and 2 beta subunits. In the center of each globin subunit is a haem group which is where oxygen binding takes place. Each haemoglobin molecule can therefore bind with 4 molecules of oxygen, one on each haem group. Oxygen binds to haemoglobin in the lungs where the concentration of oxygen is high.
Binding of oxygen is a cooperative process. This means that when an oxygen molecule binds to one of the haem groups, it causes a conformational change in the protein which makes it easier for oxygen to bind to the other haem groups. The cooperative binding process of haemoglobin is highlighted in the oxygen dissociation curve. This graph shows the partial pressure of oxygen (a way of measuring the concentration of oxygen) on the x-axis; and the percentage saturation of haemoglobin on the y-axis. The graph is sigmoidal, or S-shaped.
Once oxygen has loaded onto haemoglobin, it is transported by the red blood cells to tissues around the body, and unloaded into cells which need oxygen for respiration. Carbon dioxide in the tissues created an acidic environment, affecting the structure of haemoglobin and lowering its affinity for oxygen. A small decrease in the pH results in a large decrease in the percentage saturation of oxygen. The oxygen dissociation curve is affected by levels of carbon dioxide in the blood, for example when carbon dioxide levels go up after exercising. The presence of carbon dioxide helps the release of oxygen from haemoglobin, so the curve shifts to the right. This is known as the Bohr effect.
Oxygen unloads from haemoglobin one molecule at a time and haemoglobin returns to its deoxyhaemoglobin structure. A developing fetus also needs oxygen from its mother’s blood. Fetuses and young infants have a different type of haemoglobin from adults. Fatal haemoglobin has 2 alpha and 2 gamma globin chains and this structure enables it to bind oxygen with a greater affinity. This means that the fatal haemoglobin will always load oxygen from the mother’s haemoglobin. Haemoglobin is found in the red blood cells of most vertebrates and some invertebrates. Different organisms have different types of haemoglobin and their dissociation curves reflect this. For example, a llama living at high altitude, where oxygen levels are reduced, needs to be able to pick up oxygen even at low levels so the curve shifts to the left.
Video 2
Sliding filament theory
Skeletal muscles are attached to bones by tendons. Each skeletal muscle in your body is made up of bundles of muscle fibres. Muscle fibres are specialised cells. They contain multiple nuclei and are made up of myofibrils. The stripy appearance of the fibres results from the regular arrangement of the myofilaments within the myofibrils.
In the myofibril arrangement, the sarcomere is one of the segments into which a fibril of muscle is divided…The M Line provides an attachment for myosin filaments… The Z Line provides an attachment for actin filaments… The A band is the darker band of the myofibril containing the myosin filaments…. The H band is the lighter section in the middle of the A band where only myosin is present… The I band is the lighter band of the myofibril containing only the actin filaments.
When the muscle is relaxed the protein tropomyosin blocks the myosin-binding site on actin. Muscle contraction occurs when the muscle receives an impulse from a nerve cell in the form of an action potential. The impulse penetrates to the centre of the muscle fibre via transverse tubules, deep infoldings of the sarcolemma. This causes calcium ion channels in the sarcoplasmic reticulum membranes to open. Calcium ions diffuse out rapidly into the myofibrils to bind to the troponin molecules. The troponin molecules change shape, causing the tropomyosin to move to a different position on the actin filaments. The myosin-binding site on actin is now free and the extended myosin heads bind to actin.
The bound ADP and phosphate are released causing the myosin heads to change back to their relaxed shape, so pulling the actin filaments towards the centre of the sarcomere. The binding of another ATP to each myosin head causes them to let go of the actin. Each myosin head acts as an ATPase. The breakdown of the ATP to ADP and phosphate in the myosin heads releases energy which extends the head once again. During muscle contraction the actin and myosin filaments do not shorten but they slide past each other. If calcium is still present and there is still a supply of ATP this process will repeat again and again. After the action potential has passed, the calcium ion channels close and the calcium ions are pumped back into the sarcoplasmic reticulum. This allows the troponin to return to its previous state and causes the muscle to relax.
