The Importance Of Cycles In Biology

Cycles are important in several metabolic and chemical reactions in organisms and allow for the continued production of useful substances in organisms. For example, one cycle is the Calvin cycle which occurs in the stroma of chloroplasts in photosynthesising plant cells and is also known as the light independent stage of photosynthesis. In the Calvin cycle the enzyme rubisco combines one carbon dioxide molecule with a ribulose bisphosphate compound as part of carbon fixation- this forms an unstable 6 carbon compound which then breaks down to form 2 glycerate-3-phosphate molecules. Energy from ATP and hydrogen from NADPH formed in the light dependent stage of photosynthesis are utilised to reduce glycerate 3 phosphate to 2 triose phosphates. The reduction of each glycerate 3 phosphate to triose phosphate requires one ATP and one NADPH. 1/6 of the triose phosphates is utilised to make glucose and the remaining five sixth of the triose phosphates are used to regenerate ribulose biphosphate, requiring ATP. As such this process is cyclical because the starting compound ribulose bisphosphate is remade in the final stage of the cycle. Since ribulose bisphosphate is constantly remade, more glycerate 3 phosphates can be produced and be reduced to triose phosphates so more glucose can be made continuously in the cycle. The continued production of glucose is thus important because glucose can be utilised by plants as a respiratory substrate in glycolysis of aerobic respiration, enabling ATP to be produced, which can further provide energy for processes such as active loading of sucrose into phloem cells to enable sucrose transportation by translocation.

Another important cycle is the Krebs cycle. This cycle is part of aerobic respiration and occurs in the matrix of mitochondria. The role of the Krebs cycle is to produce ATP, NADH and FADH which are involved in oxidative phosphorylation. In the Krebs cycle, acetyl coenzyme A (which was produced in the link reaction previously) combines with oxaloacetate, a 4-carbon compound to form citrate, a 6-carbon compound. Citrate then forms alpha ketoglutarate producing carbon dioxide and NADH. This alpha ketoglutarate then forms succinate producing NADH, ATP and carbon dioxide. The succinate forms malate producing FADH and then malate reforms oxaloacetate producing NADH. As a result, the oxaloacetate is regenerated in the cycle and can continue to combine with more acetyl CoA to undergo a series of enzyme catalysed reactions to produce more NADH, FADH and ATP. The cyclical process and continued production of these coenzymes is important as NADH and FADH can release protons and electrons. The electrons can travel along the electron transport chain from a high energy to a low energy state releasing energy which can then be utilised to pump the protons actively. This leads to a build-up of protons and a proton gradient is established. The protons can move by chemiosmosis via ATP synthase leading to the bulk production of ATP that occurs in oxidative phosphorylation. As a result, due to the Krebs cycle more coenzymes are produced enabling more ATP to be synthesised in oxidative phosphorylation. The ATP can itself have several uses such as enabling the active transport of sodium, potassium and chloride ions out of the thick ascending limb into the interstitial fluid in the loop of Henle to establish a concentration gradient for more reabsorption of water.

The nitrogen cycle is another key cycle that involves many different organisms which allow different reactions to be carried out so nitrogen can be taken up by organisms. Firstly, nitrogen fixing bacteria convert nitrogen in the atmosphere into ammonium compounds. As they are soluble, they can easily be taken up from the soil by plants. When organisms die, saprobionts aerobically break down protein or DNA present in dead material using enzymes, converting these nitrogen-containing compounds back to ammonium compounds by ammonification. Nitrosomonas are then involved which first oxidise ammonium to nitrites and then nitrobacter bacteria convert the nitrites to nitrate ions and these nitrate ions are then able to be converted back into nitrogen gas through denitrification which is carried out by anaerobically respiring micro-organisms. This cycle involves nitrogen being converted into different compounds enabling several uses. One such importance of the nitrogen cycle is that by enabling nitrogen to be converted into ammonium compounds plants have an easily accessible source of nitrogen. Plants utilise this nitrogen to make many vital proteins such as IAA. This is a hormone responsible for stimulating cell elongation and enabling plant shoots to exhibit positive phototropism towards a directional stimulus of a light source, enabling optimum light absorption for photosynthesis. DNA can also be produced from this nitrogen source and the synthesis of DNA nucleotides enables cell division and growth of cells which further aids in cell elongation and causing a shoot to grow towards a light source.

The cell cycle is an important cycle that takes place in several organisms. The cell cycle enables continued cell division by mitosis so more cells can be produced at a fast rate. The first stage of the cell cycle is interphase, and this is made-up of an initial Gap 1 phase where organelles are replicated and the cell grows. DNA replication then occurs in the synthesis phase where DNA replicates by semiconservative replication to form many copies of DNA molecules. Gap 2 phase then follows when the cell and the DNA is checked before beginning mitosis to ensure there is no faulty DNA present, which can prevent cell division to form tumour cells. Mitosis is made of four stages: prophase, metaphase, anaphase, and telophase. In prophase, the chromosomes condense to form sister chromatids joined together at the centre by the centromere. The nuclear membrane and the nucleolus also disintegrate, and spindle fibres begin to form. In metaphase, the chromosomes line up at the equator and the spindle fibres attach to the chromosomes at the centromere. In anaphase the sister chromatids are pulled apart at the centromere and get separated to the opposite poles by contraction of the spindle fibres. In telophase the nuclear membrane begins to reform around the two groups of chromatids separated at opposite poles. Cytokinesis then occurs which is when the cytoplasm and the cell membrane divide to form 2 genetically identical daughter cells. The cell cycle enables continued production of cells and is important in instances such as humoral immunity. When antigens are present and displayed on macrophages, T helper cells with complementary receptors bind to these antigens and stimulate B lymphocytes. These B lymphocytes then produce complementary antibodies which then divide by mitosis leading onto clonal selection and expansion. Hence the cell cycle is very important here to produce many identical B lymphocytes which can further divide to produce plasma cells, secreting a large concentration of complementry antibody to destroy antigens (e.g. by agglutination).