Another huge source of water vapour in the air is plants. This is called transpiration. Transpiration is the evaporation of water from plants. It occurs chiefly at the leaves while their stomata are open for the passage of CO2 and O2 during photosynthesis. Air that is not fully saturated with water vapour (100% relative humidity) will dry the surfaces of cells with which it comes in contact, so the photosynthesising leaf loses substantial amount of water by evaporation. This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem of the roots
The Carbon Cycle is a complex series of processes through which all of the carbon atoms in existence rotate. All life is based on the element carbon. Carbon is the major chemical constituent of most organic matter, from fossil fuels to the complex molecules (DNA and RNA) that control genetic reproduction in organisms yet carbon is not one of the most abundant elements within the Earth's crust. Carbon is stored on our planet in major sinks. It is stored as organic molecules in living and dead organisms found in the biosphere; as the gas carbon dioxide in the atmosphere; as organic matter in soils; in the lithosphere as fossil fuels and sedimentary rock deposits such as limestone, dolomite and chalk; and in the oceans as dissolved atmospheric carbon dioxide and as calcium carbonate shells in marine organisms.
Ecosystems gain most of their carbon dioxide from the atmosphere. A number of autotrophic organisms have specialized mechanisms that allow for absorption of this element into their cells. These organisms use photosynthesis to chemically convert the carbon dioxide to carbon-based sugar molecules. These molecules can then be chemically modified by these organisms through the metabolic addition of other elements to produce more complex compounds like proteins, cellulose, and amino acids.
Carbon dioxide enters the waters of the ocean by diffusion. Once dissolved in seawater, the carbon dioxide can remain as is or can be converted into carbonate or bicarbonate. Certain forms of sea life produce calcium carbonate. This substance is used to produce shells and other body parts by organisms. When these organisms die, their shells and body parts sink to the ocean floor where they accumulate as carbonate-rich deposits. After long periods of time, these deposits are physically and chemically altered into sedimentary rocks. Ocean deposits are by far the biggest sinks of carbon on the planet.
Carbon is released from ecosystems as carbon dioxide gas by the process of respiration. Respiration takes place in both plants and animals and involves the breakdown of carbon-based organic molecules into carbon dioxide gas and some other compound by products.
Carbon is stored in the lithosphere in both inorganic and organic forms. Inorganic deposits of carbon in the lithosphere include fossil fuels like coal, oil, and natural gas, oil shale, and carbonate based sedimentary deposits like limestone. Organic forms of carbon in the lithosphere include litter, organic matter, and substances found in soils. Some carbon dioxide is released from the interior of the lithosphere by volcanoes.
The nitrogen cycle is the complex series of reactions by which nitrogen is slowly but continually recycled in the atmosphere, lithosphere and hydrosphere. The nitrogen cycle represents one of the most important nutrient cycles found in the earths ecosystems and almost all of the nitrogen found in any ecosystem originally came from the atmosphere. Nitrogen is used by living organisms to produce a number of complex organic molecules such as amino acids, proteins, and nucleic acids. The largest store of nitrogen is found in the atmosphere where it exists as a gas. Other major stores of nitrogen include organic matter in soil and the oceans. Despite its abundance in the atmosphere, nitrogen is often the most limiting nutrient for plant growth as they can only take up nitrogen in certain forms. Most plants obtain the nitrogen they need as inorganic nitrate from the soil solution. Animals receive the required nitrogen they need for metabolism, growth, and reproduction by the consumption of living or dead organic matter containing molecules composed partially of nitrogen.
In most ecosystems nitrogen is primarily stored in living and dead organic matter. This organic nitrogen is converted into inorganic forms when it re-enters the biogeochemical cycle via decomposition. Decomposers, found in the upper soil layer, chemically modify the nitrogen found in organic matter.
Eutrophication alters the way in which the nutrient cycles occur. Natural eutrophication is the process by which lakes gradually age and become more productive. It normally takes thousands of years to progress. However, humans, through their various cultural activities, have greatly accelerated this process in thousands of lakes around the globe. Water pollution occurs caused by excessive plant nutrients. Humans add excessive amounts of plant nutrients (primarily phosphorus, nitrogen, and carbon) to streams and lakes in various ways. Runoff from agricultural fields, field lots, urban lawns, and golf courses is one source of these nutrients. Untreated, or partially treated, domestic sewage is another major source. Sewage was a particular source of phosphorus to lakes when detergents contained large amounts of phosphates. The phosphates acted as water softeners to improve the cleaning action, but they also proved to be powerful stimulants to algal growth when they were washed or flushed into lakes.
The excessive growth of algae promoted by these phosphates changed water quality in many lakes. This led to oxygen depletion resulting in the death of many fish. Species resistant to these conditions replaced the native fish. Beaches and shorelines were fouled by masses of rotting, stinking algae. A means to control this problem became a paramount need. Testing concluded that phosphorus was the key nutrient for the control of eutropication.
There are a number of cycles that take place in humans. The menstrual cycle is a cycle that only occurs in females. It involves the preparation of the uterus each month for a fertilised egg. Hormones control the process that occurs and forms a monthly cycle. The hypothalamus is a gland in the brain responsible for regulating the body's thirst, hunger; sleep patterns, libido and endocrine functions. It releases the chemical messenger Follicle Stimulating Hormone Releasing Factor (FSH-RF) to tell the pituitary gland in the brain to secrete Follicle Stimulating Hormone (FSH) and a little Leutenising Hormone (LH) into the bloodstream, which cause the follicles to begin to mature.
The maturing follicles then release another hormone, oestrogen. As the follicles mature over a period of about seven days, they secrete more and more oestrogen into the bloodstream. Oestrogen causes the lining of the uterus to thicken. When the oestrogen level reaches a certain point it causes the hypothalamus to release Leutenising Hormone Releasing Factor (LH-RF) causing the pituitary to release a large amount of Leutenising Hormone (LH). This surge of LH triggers the mature follicle to burst open and release an egg. This is called ovulation.
Inside the Fallopian tube, the egg is carried along by tiny, hair-like projections, called "cilia" toward the uterus. Fertilization occurs if sperm are present as the live egg reaches the uterus.
If fertilisation occurs, the female becomes pregnant and initially the corpus luteum secretes sufficient progesterone to maintain the uterus lining and sustain the developing embryo. After this, the placenta takes over, where progesterone (and some oestrogen) from the placenta, maintain the uterine lining and inhibit the development of further egg production, so during pregnancy the menstrual cycle stops as the specific hormones are not produced. At the end of pregnancy, progesterone levels fall, and high oestrogen levels trigger the onset of labour. After pregnancy, the menstrual cycle resumes as the correct hormones are being produced for its onset.
The Calvin cycle is a series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast.
Krebs cycle or the citric acid cycle or tricarboxylic acid cycle, occurs in mitochondria, is the common pathway to completely oxidize fuel molecules, which mostly is acetylcholine, the product from the oxidative decarboxylation of pyruvate. It enters the cycle and passes ten steps of reactions that yield energy and CO2
The Krebs cycle is also known as the tricarboxylic acid (TCA) cycle and as the citric acid cycle. The Krebs cycle takes place in the mitochondria and consists of eight steps. The first reaction of the cycle occurs when acetylcholine transfers its two-carbon acetyl group to the four-carbon compound oxaloacetate, forming citrate, a six-carbon compound. The citrate then goes through a series of chemical transformations, losing first one and then a second carboxyl group as carbon dioxide. Most of the energy made available by the oxidative steps of the cycle is transferred as energy-rich electrons to NAD+, forming NADH. For each acetyl group that enters the Krebs cycle, three molecules of NAD+ are reduced to NADH. In Step 6, electrons are transferred to the electron acceptor FAD rather than to NAD+.
In one turn of the citric acid cycle, two molecules of carbon dioxide and eight hydrogen atoms are removed, forming three NADH and one FADH2. The carbon dioxide produced accounts for the two carbon atoms of the acetyl group that entered the citric acid cycle. These hydrogens come from water molecules that are added during the reactions of the cycle.
Because two acetylcholine molecules are produced from each glucose molecule, the cycle must turn twice to process each glucose. At the end of each turn of the cycle, the four-carbon oxaloacetate is left, and the cycle is ready for another turn. After two turns of the cycle, the original glucose molecule has lost all of its carbons and may be regarded as having been completely consumed. Only one molecule of ATP is produced directly with each turn of the citric acid cycle. The rest of the ATP that is formed during aerobic respiration is produced by the electron transport system.