Cell level systems Study Guide

Deoxyribonucleic acid, commonly known as DNA, is the fundamental genetic material found in all living organisms. It carries the instructions for an organism's development, functioning, growth, and reproduction. The structure of DNA is often described as a 'double helix', which resembles a twisted ladder. This double helix is formed from two long strands that are wound around each other. Each individual strand of DNA is a polymer, meaning it is made up of many repeating smaller units called nucleotides. Each nucleotide consists of three components: a deoxyribose sugar, a phosphate group, and a nitrogen-containing base. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G), and cytosine (C). The two strands of the double helix are held together by weak hydrogen bonds that form between specific pairs of these bases. Adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C). This specific pairing is called complementary base pairing. The phosphate and sugar components form the 'backbone' of each strand, providing structural support, while the bases project inwards and interact to form the 'rungs' of the ladder. This unique and precise structure allows DNA to store vast amounts of genetic information and to be accurately replicated when cells divide.

Photosynthesis is the process by which plants convert light energy into chemical energy, in the form of glucose, using carbon dioxide and water. The rate at which photosynthesis occurs is influenced by several environmental factors. These factors are known as limiting factors because if any one of them is in short supply, it will restrict the overall rate of photosynthesis, even if the other factors are abundant. The key limiting factors are temperature, light intensity, and carbon dioxide concentration. When light intensity is low, the rate of photosynthesis is limited by the amount of light energy available to drive the reactions. As light intensity increases, the rate generally increases until another factor becomes limiting. Similarly, if carbon dioxide concentration is low, the plant cannot produce glucose efficiently, despite sufficient light and warmth. An increase in carbon dioxide concentration usually boosts the photosynthetic rate. Temperature also plays a crucial role; enzymes involved in photosynthesis work more efficiently at their optimum temperature. Below this temperature, enzyme activity is slow, reducing the rate. Above the optimum, enzymes begin to denature, and the rate rapidly declines. It's important to understand that these factors interact; for example, increasing light intensity will only increase the rate of photosynthesis if carbon dioxide and temperature are not already limiting. Growers often manipulate these factors in greenhouses to maximise crop yield.