Cycles in Biology

Cycles in Biology

Cycles in biology play a fundamental role in the world that we live in. Cycles occur all around and inside of us in many different forms, from the Krebs to the Calvin cycle. There are large scale cycles happening and they are essential in regulating the nutrients and substances that are around us which without, life on earth could no function.

One of the largest cycles that occurs all around us is in everyday life is the carbon cycle. The current atmospheric composition currently consists of approximately 0.04% of Carbon dioxide. A large proportion of it is found dissolved in the oceans as well as the atmosphere. The carbon cycle consists of 6 stages. Initially the CO2 that is absorbed by plants for the use in photosynthesis becomes carbon compounds in plant tissue. The carbon is moved up the food chain by consumption, a primary consumer. It is passed on to the secondary and tertiary consumers when they eat other consumers. When these organisms die they are digested by microorganisms known as decomposers (bacteria and fungi), when these decomposers feed on the dead organism it is called saprobiotic nutrition. The carbon is then released back into the atmosphere and other living organisms which proceed on to respiring and this causes CO2 to be released. However if the dead organism ends up somewhere were there is no decomposers present, then this matter will turn into fossil fuels over millions of years. We will then extract the fossil fuels and use them for energy and as fuels, this process known as combustion is very widely used, it then releases CO2 back into the atmosphere where it once came from.

Another large scale cycle that occurs is the nitrogen cycle. Plants and animals need nitrogen to make proteins and nucleic acids. In the atmosphere there is roughly 78% nitrogen content, but plants and animals cannot use it in that form. Bacteria are required to convert the nitrogen gas into nitrogen compound. The nitrogen cycle is used to show nitrogen is converted into a usable form and then passed on between different living organisms. The nitrogen cycle includes food chains and four processes that involve bacteria, Nitrogen fixing is caused by a bacteria called Rhizobium that converts nitrogen gas into ammonia; Ammonification occurs when nitrogen compounds from dead organisms are turned into ammonium compounds by decomposers; Nitrification involves changing the ammonium compounds in the soil into nitrogen compounds that can be used by plants, the nitrifying bacteria (such as Nitrosomonas) firstly have to change the ammonium compounds into nitriles then the other nitrifying bacteria called Nitrobacter, change the nitriles into nitrates. The final step is Denitrification when nitrates in the soil are converted into nitrogen gas by denitrifying bacteria by using the nitrates to carry out respiration under anaerobic conditions to produce nitrogen gas. Farmers however use fertilisers on their crops which can leak into ponds and rivers which stimulates the growth of the algae. The algae can block light from reaching the bottom of the pond/river, this causes plants to die and provide dead matter for the bacteria. The increased number of bacteria can reduce the oxygen concentration of the water by multiplying thus killing the fish.

Advances in biochemistry have lead us further to understand the processes which occur within humans, for example how humans make energy. The Krebs cycle allows for the oxidative phosphorilation of glucose to occur to produce ATP. The oxidative phosphorilation in itself involves two processes, the electron transport chain and chemiosmosis, however, the Kreb cycle is a large part of the respiration process of mammals. The cycle is aerobic and requires oxygen. To initiate the Krebs cycle the Dehydration and decarboxilation of acetyl co-enzyme A to form 2 pyruvate molecules from 1 glucose molecule. The 4C acid accepts the acetyl group from the acetyl co-A to form citrate. During the link reaction a series of redox reaction occur to produce ATP which is then used in the Krebs cycle to produce reduced NAD and FAD.

Humans have many cycles happening within us such as the cardiac cycle which uses a myogenic contraction of the cardiac muscle. The impulse initiates at the SAN and travels across the left atria to the AVN where it is delayed to allow atrial systole to occur and to fill up the ventricles. The impulse then travels down the perkinje fibres and the bundle of hiss until it reaches the ventricles where ventricular systole then occurs. Muscles are used by us every day, hour and second of our life. There are 3 different types of muscle, skeletal, myogenic (the heart) and smooth muscle. Skeletal muscle has a cycle within it which allows contraction to occur, it is called the sliding filament hypothesis which explains how muscles contract. Muscle fibres are in cylindrical shape made up of many myofibrils joined together. There are calcium stores within the muscle which are used during contraction. An influx of Ca2+ ions (caused by a depolarisation of the sarcolemma) allows Ca2+ ions to bind to troponin changing its shape which in turn pulls the tropomyosin out of the actin-myosin binding site on the actin filament. The exposed binding site allows the myosin head to bind to the actin filament until ATP is added to the myosin head which will hydrolise it with ATPase and the released energy will break the action-myosin bridge, the myosin head detaches from the actin filament and then the myosin head reattaches to a different binding site further along the actin filament. This cycle is repeated many times over.

Another interesting cycle that occurs within any living organism, the cell cycle. This is what allows our bodies to heal themselves; it explains how we grow and even how we are born! There are two different types of cell division, mitosis and meioses. The cell cycle is broken down into 4 stages, Mitosis (the start and end point), G1 – where cells grow and new organelles and proteins are made, S – cells replicate DNA ready to divide my mitosis, and finally G2 where the cells grown and the proteins needed for cell division are made.

Plants on the other hand have other interesting ways of making energy, they photosynthesise, this requires only water and carbon dioxide as shown in the reaction 6CO2 + 6H2O ( C6H12O6 + 6O2. This involves a different cycle to the human one, this one is called the Calvin cycle. It is the light independent stage of photosynthesis therefore this process is anaerobic. This reaction takes place in the stroma of the chloroplast, a molecule is triose phosphate is made from CO2 and ribulose biphosphate (RuBP). The triose phosphate can be used to make useful substances in plants like glucose. RuBP is regenerated in this cycle, ready to be used again, however this cycle requires ATP and H+ ions to run constantly. The ATP generated for this is made by cyclic photophosphorylation which only uses Photosystem 1 (PS1). The process can be called cyclic as the electrons from the chrolophyll are not passed onto NADP, but they are passed back to PS1 via electron carriers and this allows the electrons to flow repeatedly round, this process only produces a small amount of ATP.