Transcript
Overview of Photosynthesis Photosynthesis All living organisms consist of one or more cells Each cell runs on the chemical energy found mainly in carbohydrate molecules (food) Majority of these molecules are produced by photosynthesis Convert solar energy into chemical energy – used to build carbohydrate molecules, then released when organism breaks down food Cells use this energy to perform work Also results in the release of oxygen in the atmosphere Solar dependence and food production Autotroph – an organism that can produce its own food Plants Certain types of bacteria Algae Photoautotrophs – a type of autotroph that uses sunlight and carbon from carbon dioxide to synthesize energy in the form of carbohydrates Heterotrophs – organisms incapable of photosynthesis that must obtain energy and carbon from food by consuming other organisms Main structures and summary of photosynthesis Photosynthesis requires sunlight, carbon dioxide, and water (start) Photosynthesis releases oxygen Photosynthesis produces carbohydrate molecules (most common – glucose) Photosynthesis takes place (in plants) in leaves Mesophyll – middle layer of the leaf Stomata – where the gas exchange of carbon dioxide and oxygen occurs through, small, regulated openings Chloroplasts – organelle in which photosynthesis takes place -exist in mesophyll have double membrane – inner/outer thylakoids- disc-shaped structures that in stacked form that make up the third chloroplast membrane granum – stack of thylakoids stroma – space surrounding the granum chlorophyll – embedded in thylakoid membrane, a pigment (absorbs light) in which photosynthesis begins responsible for green color Two parts of photosynthesis light dependent reactions take place at the thylakoid membrane chlorophyll absorbs energy from sunlight converts it into chemical energy with the use of water release oxygen from the hydrolysis of water as a byproduct Calvin cycle Takes place in the stroma Chemical energy derived from the light dependent reactions drives both the capture of carbon in carbon dioxide molecules and the assemble of sugar molecules Two reactions use carrier molecules to transport energy from one reaction to another Light dependent to Calvin = full, bring energy Calvin to light dependent = empty, return to obtain more energy The light dependent reactions of photosynthesis What is light energy? Sun emits enormous amount of electromagnetic radiation (solar energy) Waves – way solar energy travels Wavelength – amount of energy of a wave; distance between two consecutive similar points in a series of waves (crest to crest, or trough to trough) Electromagnetic spectrum – range of all possible wavelengths of radiation Each wavelength corresponds to a different amount of energy carried Each type of electromagnetic radiation has a characteristic wavelength Longer the wavelength, less energy is carried Shorter the wavelength, more energy is carried Absorption of light Light energy enters the process of photosynthesis when pigments absorb the light In plants, only absorb visible light Visible light seen by humans as white light is actually a rainbow of colors Violet/blue – shorter wavelength Red – longer wavelength Understanding pigments Different kinds of pigments exist Each absorb only certain wavelength of visible light Chlorophyll a – Photosynthetic organisms, common green color (plants) Absorbs wavelength from either end of the visible spectrum (blue/red) except green Chlorophyll b – absorbs blue and red-orange Absorption spectrum – each type of pigment can be identified by the specific pattern of wavelengths it absorbs from visible light How light-dependent reactions work Overall purpose of light-dependent reactions is to convert light energy into chemical energy This energy will be used by the Calvin cycle to fuel the assembly of sugar molecules Steps: Begin in a photosystem – grouping of pigment molecules and proteins, exist in thylakoid membrane A pigment molecule in the photosystem absorbs one photon – a quantity of light energy – at a time Photon of light energy travels until it reaches a molecule of chlorophyll The photon causes an electron in chlorophyll to become excited The energy given to the electron allows it to break free from a atom of the chlorophyll molecule Chlorophyll donates an electron To replace the electron in the chlorophyll, a water molecule is split This slitting releases an electron and results in the formation of oxygen and hydrogen ions in the thylakoid space Each breaking of water molecule release a pair of electrons therefore can replace two donated electrons The replacing of the electron enables chlorophyll to respond to another photon. The oxygen molecules produced as byproducts find their way to the surrounding environment. In eukaryotes and some prokaryotes, two photosystems exist After the photon hits, photosystem II transfers the free electron to the first in a series of proteins inside the thylakoid membrane called the electron transport chain. As the electron passes along these proteins, energy from the electron fuels membrane pumps that actively move hydrogen ions against their concentration gradient from the stroma into the thylakoid space. After the energy is used, the electron is accepted by a pigment molecule in the next photosystem, which is called photosystem I Generating an energy carrier: ATP In the light-dependent reactions, energy absorbed by sunlight is stored by two types of energy-carrier molecules ATP and NADPH. energy that these molecules carry is stored in a bond For ATP, it is a phosphate atom for NADPH, it is a hydrogen atom When these molecules release energy into the Calvin cycle, they each lose atoms to become the lower-energy molecules ADP and NADP+ Steps: The buildup of hydrogen ions in the thylakoid space forms an electrochemical gradient because of the difference in the concentration of protons (H+) and the difference in the charge across the membrane that they create This potential energy is harvested and stored as chemical energy in ATP through chemiosmosis, the movement of hydrogen ions down their electrochemical gradient through the transmembrane enzyme ATP synthase, just as in the mitochondrion The hydrogen ions are allowed to pass through the thylakoid membrane through an embedded protein complex called ATP synthase This same protein generated ATP from ADP in the mitochondrion The energy generated by the hydrogen ion stream allows ATP synthase to attach a third phosphate to ADP, which forms a molecule of ATP in a process called photophosphorylation. The flow of hydrogen ions through ATP synthase is called chemiosmosis because the ions move from an area of high to low concentration through a semi-permeable structure Generating another energy carrier: NADPH The remaining function of the light-dependent reaction is to generate the other energy-carrier molecule, NADPH Steps: As the electron from the electron transport chain arrives at photosystem I, it is re-energized with another photon captured by chlorophyll The energy from this electron drives the formation of NADPH from NADP+ and a hydrogen ion (H+) Now that the solar energy is stored in energy carriers, it can be used to make a sugar molecule The Calvin cycle The Calvin cycle After the energy from the sun is converted and packaged into ATP and NADPH, the cell has the fuel needed to build food in the form of carbohydrate molecules. Calvin cycle - reactions of photosynthesis that use the energy stored by the light-dependent reactions to form glucose and other carbohydrate molecules. The interworking’s of the Calvin cycle The reactions are named after the scientist who discovered them Calvin-Benson cycle - in plants carbon dioxide (CO2) enters the chloroplast through the stomata diffuses into the stroma of the chloroplast—the site of the Calvin cycle reactions where sugar is synthesized The Calvin cycle reactions can be organized into three basic stages: Fixation Reduction Regeneration Steps: Carbon fixation In the stroma, in addition to CO2, enzyme RuBisCO, and the molecule ribulose bisphosphate (RuBP) are present to initiate the Calvin cycle RuBP has five atoms of carbon and a phosphate group on each end. RuBisCO catalyzes a reaction between CO2 and RuBP, which forms a six-carbon compound that is immediately converted into two three-carbon compounds CO2 is “fixed” from its inorganic form into organic molecules Reduction reaction ATP and NADPH use their stored energy to convert the three-carbon compound, 3-PGA, into another three-carbon compound called G3P A reduction is the gain of an electron by an atom or molecule. The molecules of ADP and NAD+, resulting from the reduction reaction, return to the light-dependent reactions to be re-energized. Regeneration One of the G3P molecules leaves the Calvin cycle to contribute to the formation of the carbohydrate molecule, which is commonly glucose (C6H12O6). Because the carbohydrate molecule has six carbon atoms, it takes six turns of the Calvin cycle to make one carbohydrate molecule (one for each carbon dioxide molecule fixed). The remaining G3P molecules regenerate RuBP, which enables the system to prepare for the carbon-fixation step. ATP is also used in the regeneration of RuBP. Photosynthesis in prokaryotes Prokaryotes lack membrane bound organelles Have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis The energy flow Carbohydrates are storage molecules for energy in all living things. Although energy can be stored in molecules like ATP, carbohydrates are much more stable and efficient reservoirs for chemical energy. Photosynthetic organisms also carry out the reactions of respiration to harvest the energy that they have stored in carbohydrates Photosynthesis produces oxygen as a byproduct, and respiration produces carbon dioxide as a byproduct. In nature, there is no such thing as waste. Every single atom of matter is conserved, recycling indefinitely. Substances change form or move from one type of molecule to another, but never disappear
