Photosynthesis occurs within the chloroplasts of plant cells, mainly those of the leaf, and only in daylight hours. leaf cells, both in their structure and arrangement, show adaptations for maximising photosynthesis. Chloroplasts have structural adaptations enabling them to carry out efficiently the chemical reactions of photosynthesis.
Palisade cell layer- cells are rich in chloroplasts for maximum photosynthesis, close to upper surface of the leaf. Chloroplasts near outer region of cell to intercept as much light as possible and reduce the distance carbon dioxide has to diffuse into the stroma. Cells are long, packed for maximum light absorption with large surface area.
Spongy cell layer- involved in gas exchange, contain few chloroplasts, smaller than palisade cells. Cells are loosely packed and spaced out to give maximum surface area and air space to facilitate diffusion of carbon dioxide, oxygen and water vapor to/from cells. Close to lower epidermis allowing rapid diffusion of gases into and out of leaf.
In the stroma, the light-independent (Calvin cycle) photosynthetic reactions take place- the bonding of carbon dioxide with hydrogen (from water), and their rearrangement to form glucose, C6H12O6.
Chloroplasts are also found in the outer layer of cells (cortex) in the stems of non-woody plants. Light is able to penetrate to these cells, allowing photosynthesis to occur to supplement that from the leaves. Pores (lenticels) in the epidermis of the stem allow carbon dioxide and oxygen to diffuse into/out of the cells.
There are two main chemical pathways in which the raw materials- water and carbon dioxide - are joined to produce the sugar glucose (with oxygen being a waste product). The source of energy is solar (from the sun) and its entrapment requires the presence of pigment molecules such as chlorophyll. All chemical pathways in photosynthesis are controlled by enzymes.
The light-dependent reaction takes place on the thylakoid membranes of the grana of chloroplasts.
This step is catalysed by a series of enzymes. Chlorophyll in the thylakoid membrane absorbs solar energy (this gets the chlorophyll's electrons "excited" by solar energy hitting them), which splits water into hydrogen and oxygen. The high energy electrons are used to make the "energy molecule" ATP from ADP. Oxygen is a waste product and is removed form the cell. The hydrogen produced is picked up by NADPH and is carried into the dark/light independent stage.
The light-independent reaction is also known as the Calvin Cycle. This occurs in the liquid matrix of the chloroplast, the stroma.
This stage is likewise controlled by enzymes. The hydrogen from the first stage and carbon dioxide enters a biochemical cycle and react to produce glucose. Carbon dioxide and hydrogen are continuously fed into the cycle. ATP (produced in the light-dependent reaction) is used to run the cycle by supplying the energy needed to bond the carbon dioxide and water in forming glucose. The glucose is used to provide energy, and is changed into other chemicals (e.g cellulose) or is stored as starch.
The rate of photosynthesis is determined by factors such as temperature, light intensity and carbon dioxide concentration.
Temperature- increasing the temperature increases the rate of photosynthesis up to an optimum temperature. When temperatures rise too far above the optimum temperature, the enzymes controlling the reaction denature and can no-longer catalyse the reactions; photosynthesis ceases.
Light intensity- increasing the light intensity increases the rate of photosynthesis up to a maximum. Above the maximum, further increases in light intensity have no further effect on the photosynthesis rate (because either the light-absorbing pigments are saturated and /or the concentration of carbon dioxide is limiting the Calvin cycle and/or the temperature is too low).
Carbon dioxide concentration- increasing carbon dioxide concentration increase the rate of photosynthesis up to a maximum. Above the maximum carbon dioxide concentration, further increases in carbon dioxide concentration have no further effect on the photosynthesis rate (because either the light-absorbing pigments are saturated and /or the carbon dioxide which is saturating the chloroplasts is limiting the Calvin cycle and/or the temperature is too low).
Because these three factors combine to determine the rate of photosynthesis, maximum rates of photosynthesis can alter if one of the other factors changes.
The rate of photosynthesis is typically measured as the amount of oxygen produced or the amount of carbon dioxide consumed in a given time.
Other environmental factors that can influence photosynthesis are the following.
Water- cells, even if dehydrated, contain quantities of water sufficient to allow photosynthesis; however, the closure of the stomata to prevent waterloss will also prevent entry of carbon dioxide which will limit photosynthesis.
Mineral ions- certain mineral ions are essential for photosynthesis to occur (e.g Fe is needed for the synthesis of chlorophyll, Mg is and essential part of the structure of the chlorophyll molecule (a deficiency of Mg causes yellowing leaves)).
Light wavelength- the light-absorbing pigments are most active when absorbing light in the blue and red wavelengths of the electomagnetic spectrum; wavelengths of green are not absorbed, but reflected (hence the green colour of plants).
The importance of photosynthesis is that it converts solar energy into chemical energy as organic molecules (carbohydrates, proteins, fats).
Plants are autotrophs (self-feeders) since they are the producers (of food)- the starting point of food chains. Organisms that feed on other organisms are called heterotophs. The wastes and dead bodies of autotrophs and heterotrophs are broken doen by decomposers (bacteria and fungi), and the chemicals locked in the wastes and bodies are released into the soil. These enter the roots of plants, allowing their growth and new supplies to food chains.
Photosynthesis by plants accounts for the high level (approximately 20%) of oxygen in the atmosphere. This gas is essential for the process of areobic respiratioin.
Without photosynthesis, animal life could not exist.
Respiration occurs in the mitochondria of the cells in all organisms all the time (both day and night). Respiration is the process by which the cell breaks down glucose to produce ATP - adenosine triphosphate; heat energy is a by-product. ATP is the so-called "energy molecule" in all organisms and is used to fuel all the chemical reactions of the cell, including the following.
Active transport of substances across membranes.
Synthesis of molecules -e,g proteins from amino acids.
Movement- e.g phagocytosis, action of cilia and flagella, action of actin and myosin in muscle contraction.
Bioluminescence (light production) in cells of such animals as glow-worms and fireflies.
The breakdown of glucose may be aerobic or anaerobic; aerobic respiration produces much larger amounts of ATP per glucose molecule than anaerobic respiration does.
ATP is constantly made in cells from ADP - adenosine diphosphate. ATP is made up of a molecule called ADP (adenosine diphosphate) bonded with a phosphate ion. Energy is needed to for the bond and is released when the bond is broken. The energy from glucose metabolism adds a high-energy phosphate bond to ADP to make ATP.
When a cell needs energy, the high-energy phosphate bond is broken and ATP returns back to ADP (ADP could be thought of as a "flat battery" and ATP as a "charged battery"). The process is very rapid and on-going.
ATP receives energy from the breakdown of glucose, and then releases energy from the breakdown of ATP.
The heat produced as a by-product of respiration (aerobic respiration is approximately 45% efficient) is used in maintaining body temperature in homeotherms ("warm blooded animals - birds and mammals).
Aerobic respiration needs oxygen for the complete breakdown of glucose into carbon dioxide and water; energy is released in the form of ATP and heat. There are three main enzyme-controlled chemical pathways in aerobic respiration - glycolysis, Krebs cycle and the electron transfer chain.
Glycolysis ("glucose splitting"), the first chemical pathway, occurs in the cytoplasm of the cell.
Each glucose molecule is broken down into two pyruvate molecules.
Two molecules (small amount) of ATP are produced in the breakdown, as well as hydrogen which moves to the cristae.
This stage does not require oxygen.
The Krebs cycle (also known as the Citric acid cycle), is the second chemical pathway of aerobic respiration, and it occurs in the matrix of the mitochondria.
Hydrogen ions are picked up by a carrier molecule (NAD, a co-enzyme) and taken to the third chemical pathway of aerobic respiration.
The electron transfer chain (also known as the Hydrogen transfer chain or the Respiratory chain) is the final chemical pathway of aerobic respiration, and it occurs on the cristae of the mitochondria.
Hydrogen ions and their high-energy electrons are passed along the electron transfer chain, releasing energy which is captured as 34 ATP. Oxygen is required at this stage and Water is produced. Each glucose molecule produces 38 ATP during aerobic respiration. At the end of the electron transfer chain, the electrons are returned to the hydrogen ions, which become atoms again and combine with oxygen to form water.
The third stage of aerobic respiration produces the most ATP. One molecule of glucose that enters glycolysis yields 38 molecules of ATP by the end of the electron transfer chain.
Anaerobic respiration occurs in the absence of oxygen. Only glycolysis takes place, therefore only 2 molecules of ATP are produced from each glucose molecule. This is what happens if the Krebs cycle or electron-transfer chain get backed up because there is no/not enough oxygen available.
Anaerobic respiration in animals occurs when oxygen is short in supply; typically during prolonged exercise (only animals such as endoparasites respire anaerobically as a way of life). The pyyruvate formed in glycolysis is broken down into lactic acid. The build up pf lactic acid causes muscle fatigue. The muscles need to stop working to allow replenished supplies of oxygen to continue the breakdown of the lactic acid into water and carbon dioxide.
In yeast and bacteria, anaerobic respiration is a way of life. In yeast, the pyruvate is broken down into ethanol, C2H5OH, and carbon dioxide. This process is also known as fermentation. The small energy yield of 2 ATP molecules is sufficient to meet the lifestyle needs of these organism.
Fatty acids and glycerol (from digestion of fats), and amino acids (from digestion of proteins), can also be used in respiration (entering Krebs cycle after prior processing) to generate ATP.
The rate of respiration is determined by factors such as temperature and energy demands.
Body'e energy demands- the rate of respiration will increase up to a maximum as the demand from the cells of tissues increases (e.g muscle cells of legs when a human sprints). The amount of oxygen needed and the amount of carbon dioxide produced increase, and breathing rate increases to compensate. The build up of carbon dioxide will slow down the rate of respiration.
The rate of respiration is typically measured by the amount of oxygen consumed or the amount of carbon dioxide produced in a given time.
The full name for respiration is cellular respiration- because it occurs in the cells to release energy, a chemical process. It is not the same process as breathing (inhaling and exhaling of gases into and out of the body, a physical process) or gas exchange (exchange of oxygen and carbon dioxide across a membrane by diffusion, a passive process).
In plants, the effect of respiration in the daytime is typically masked by photosynthesis, with the net result being the consumption of carbon dioxide and the production of oxygen. Therefore, the effects of respiration are typically seen only at night in plants.