The nucleotides in DNA are composed of bonded pairs. Which nucleotide pairs with cytosine?

Cellular respiration is the metabolic process by which cells convert biochemical energy stored in nutrients such as glucose into adenosine triphosphate (ATP), the universal energy currency of cells. Cellular respiration occurs primarily in the mitochondria, which are membrane-bound organelles found in the cytoplasm of eukaryotic cells. Mitochondria are often referred to as the "powerhouses" of the cell due to their role in ATP production through aerobic respiration. The process of cellular respiration involves several interconnected metabolic pathways, including glycolysis, the citric acid cycle (Krebs cycle), and the electron transport chain (oxidative phosphorylation). In glycolysis, glucose is partially oxidized to produce pyruvate and a small amount of ATP in the cytoplasm. Pyruvate is then transported into the mitochondria, where it enters the citric acid cycle, generating more ATP and reducing equivalents (NADH and FADH2). The reduced coenzymes NADH and FADH2 donate electrons to the electron transport chain located in the inner mitochondrial membrane, leading to the production of ATP through oxidative phosphorylation. Overall, cellular respiration is essential for providing energy for cellular processes such as biosynthesis, muscle contraction, active transport, and maintenance of cellular homeostasis.

Uracil is a nitrogenous base that is unique to RNA (ribonucleic acid) and is not found in DNA (deoxyribonucleic acid). In RNA, uracil pairs with adenine through complementary base pairing during transcription, which is the process of copying genetic information from DNA to RNA. In DNA, thymine pairs with adenine, while cytosine pairs with guanine. RNA plays various roles in the cell, including serving as a messenger molecule (mRNA) that carries genetic information from the nucleus to the ribosomes for protein synthesis, as well as functioning in other types of RNA such as transfer RNA (tRNA) and ribosomal RNA (rRNA) involved in translation and protein synthesis.

Carbohydrates are the macromolecules primarily used by cells to provide immediate energy in metabolism. When carbohydrates are broken down through processes like glycolysis, glucose molecules are converted into ATP (adenosine triphosphate), which is the primary energy currency of the cell. ATP provides energy for various cellular processes, including muscle contraction, active transport, biosynthesis, and cell division. Carbohydrates are readily available as a source of energy because they can be quickly broken down into glucose, which can then be utilized by cells to produce ATP. This rapid energy release makes carbohydrates crucial for activities requiring immediate energy, such as muscle contraction during exercise or brain function. Additionally, carbohydrates can be stored in the form of glycogen in animals or starch in plants, providing a readily accessible reserve of energy for future use. Overall, carbohydrates play a central role in cellular metabolism and are essential for sustaining life processes.

The immune system is the body's defense mechanism against infectious agents, foreign substances, and abnormal cells. It consists of a complex network of organs, tissues, cells, and molecules that work together to identify, neutralize, and eliminate pathogens (such as bacteria, viruses, fungi, and parasites) and other harmful invaders. The immune system provides both immediate and slower responses tailored to specific infecting agents. Immediate responses, such as inflammation, phagocytosis, and the release of antimicrobial proteins (e.g., interferons), serve to contain and eliminate pathogens rapidly upon their entry into the body. Slower responses, involving adaptive immunity, take several days to develop but provide long-term protection against specific pathogens. Adaptive immunity involves the activation of lymphocytes (B cells and T cells) and the production of antibodies (humoral immunity) or the activation of cytotoxic T cells (cell-mediated immunity) to target and destroy pathogens. The immune system has memory, allowing it to recognize and mount a more rapid and robust response upon re-exposure to previously encountered pathogens. Dysfunction or dysregulation of the immune system can lead to immunodeficiency (increased susceptibility to infections) or autoimmunity (immune reactions against self-antigens), contributing to various diseases and disorders.