Cell level systems Study Guide

Photosynthesis is a vital chemical process used by plants, algae, and some bacteria to convert light energy into chemical energy. This chemical energy is stored in glucose, a type of sugar. The process primarily takes place in the chloroplasts, specialized organelles found within plant cells, which contain a green pigment called chlorophyll. Chlorophyll is essential as it absorbs light energy, particularly from the red and blue parts of the light spectrum, and reflects green light, which is why most plants appear green.

The raw materials for photosynthesis are carbon dioxide and water. Carbon dioxide is absorbed from the atmosphere through small pores on the leaves called stomata, while water is absorbed from the soil by the roots and transported to the leaves. Light energy provides the necessary power to drive the reaction. The products of photosynthesis are glucose and oxygen. Glucose is used by the plant for energy through cellular respiration, to build new cells for growth, or converted into starch for storage. Oxygen is released into the atmosphere as a waste product, which is crucial for the respiration of most living organisms. The overall word equation for photosynthesis is: Carbon dioxide + Water → (light energy) → Glucose + Oxygen. Without photosynthesis, the food chains on Earth would collapse, and the atmosphere would lack the oxygen needed to sustain aerobic life.

DNA, or deoxyribonucleic acid, serves as the instruction manual for building proteins within an organism. Its structure is crucial for this function, particularly the sequence of its nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C). These bases are arranged in a specific order along the DNA double helix. A gene is a segment of DNA that contains the instructions for making a specific protein. The information within a gene is encoded in a triplet code, where each sequence of three bases (a codon) specifies a particular amino acid. During protein synthesis, this genetic information flows in two main stages: transcription and translation.

In transcription, the DNA double helix unwinds, and one strand serves as a template. An enzyme, RNA polymerase, synthesizes a complementary messenger RNA (mRNA) molecule. Importantly, thymine (T) in DNA is replaced by uracil (U) in mRNA. So, if the DNA sequence is 'TAC', the mRNA codon will be 'AUG'. The mRNA then detaches and travels from the nucleus to a ribosome in the cytoplasm.

In translation, at the ribosome, the mRNA sequence is read in codons. Transfer RNA (tRNA) molecules, each carrying a specific amino acid and possessing a complementary anticodon, bind to the appropriate mRNA codons. For instance, if the mRNA codon is 'AUG', a tRNA with the anticodon 'UAC' carrying the amino acid methionine will bind. The ribosome facilitates the formation of peptide bonds between successive amino acids, linking them together in the order specified by the mRNA sequence. This chain of amino acids folds into a specific three-dimensional structure, forming a functional protein. Therefore, any change in the DNA base sequence (a mutation) can alter the mRNA codons, subsequently changing the amino acid sequence, which can lead to a different protein structure and potentially a different or non-functional protein. This demonstrates how the precise structure and sequence of DNA directly dictate the types and functions of proteins produced.