Photosynthesis is a process used by plants and other organisms to convert light energyinto chemical energy that can later be released to fuel the organisms' activities. This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – hence the name photosynthesis, from the Greek φῶς, phōs, "light", and σύνθεσις, synthesis, "putting together".[1][2][3] In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth.[4]
Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centresthat contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate(NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.
In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporatedinto already existing organic carbon compounds, such as ribulose bisphosphate(RuBP).[5] Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reducedand removed to form further carbohydrates, such as glucose.
Lithified ash (or ash mixed with pyroclastic fragments) forms a volcaniclastic rock called a Tuff.
A form of rock called tuff is created when volcanic ash is blasted from a vent during an eruption.
The ash is transformed into a rock after ejection and deposition. Tuff is defined as rock with an ash content of more than 75%, whereas tuffaceous refers to rock with an ash content of 25% to 75%.
The thickness of tuff often decreases with distance from the volcano and is usually greatest close to the volcanic vent. The typical shape of a tuff deposit is that of a "lens," not a "layer."
Tuff may also be thickest on the vent's side that faces away from the wind or on the side facing the direction of the blast.
a. the first methionine in eukaryotic translation contains a formyl group.
Explanation:
Protein synthesis begins when the initiator aminoacyl tRNA carrying the initiator amino acid joins the mRNA-small ribosomal subunit complex. Procaryotes and eucaryotes differ from each other in the identity of the first initiator amino acid. In both types of organisms, "AUG" serves as an initiation codon. However, procaryotes have formyl-methionine joined to the amino acid arm of the initiator tRNA while eucaryotic initiator tRNA carries methionine as the first amino acid to be added to the protein.
DNA synthesis is performed by the enzyme DNA polymerase. However, DNA polymerase requires the presence of a free 3' OH on the existing DNA or RNA segment. The enzyme primase forms small RNA segments that serve as primers. Primers are formed by using the DNA template strands and have free 3' OH ends. DNA polymerase extends the primers by adding deoxyribonucleotides according to the sequence of the DNA template strand. Therefore, DNA polymerases are the enzymes of primer elongation.
