Cell level systems Study Guide

DNA, or deoxyribonucleic acid, is a biological macromolecule that carries genetic information. It is described as a polymer because it is made up of many smaller, repeating units called monomers. In the case of DNA, these monomers are called nucleotides. Each nucleotide consists of three main components: a deoxyribose sugar, a phosphate group, and a nitrogen-containing base. There are four types of nitrogenous bases in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).

These individual nucleotide monomers link together to form long chains. The phosphate group of one nucleotide forms a strong covalent bond with the sugar of the next nucleotide, creating a sugar-phosphate backbone. This backbone is a strong, stable structure that provides structural integrity to the DNA molecule. A single strand of DNA is therefore a polymer formed by the sequential linkage of many nucleotides.

Typically, DNA exists as a double helix, meaning it consists of two such polynucleotide strands coiled around each other. The two strands are held together by weaker hydrogen bonds that form between specific pairs of nitrogenous bases: adenine always pairs with thymine (A-T), and guanine always pairs with cytosine (G-C). This specific pairing rule is crucial for DNA's role in storing and transmitting genetic information, as it ensures that the sequence of one strand can be precisely replicated or transcribed from the other.

Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize nutrients from carbon dioxide and water. This process is vital for life on Earth. The rate at which photosynthesis occurs is influenced by several environmental factors: temperature, light intensity, and carbon dioxide concentration. These are often referred to as limiting factors because, if any one of them is in short supply, it will restrict the rate of photosynthesis, even if the other factors are abundant. For example, if light intensity is low, increasing the temperature or carbon dioxide concentration will not significantly increase the rate of photosynthesis until the light intensity is no longer the limiting factor.

Temperature directly affects the enzymes involved in photosynthesis. As temperature increases, the kinetic energy of reactant molecules and enzyme molecules increases, leading to more frequent collisions and a faster rate of reaction, up to an optimal temperature. Beyond this optimum, the enzymes begin to denature, losing their specific three-dimensional shape, which reduces their efficiency and causes the rate of photosynthesis to decrease rapidly.

Light intensity provides the energy for the chemical reactions of photosynthesis. An increase in light intensity generally leads to an increased rate of photosynthesis, as more chlorophyll molecules are excited and supply energy to convert carbon dioxide and water into glucose and oxygen. However, there is a saturation point where increasing light intensity further will no longer increase the rate because another factor, such as carbon dioxide concentration, becomes limiting.

Carbon dioxide concentration is a reactant in photosynthesis. An increase in the concentration of carbon dioxide generally leads to an increased rate of photosynthesis, as more raw material is available for the process. Similar to light intensity, there is a point where further increases in carbon dioxide concentration will not increase the rate because another factor, such as temperature or light intensity, becomes limiting.