• All cells respond to signals in their environment through specific receptors which can either be located on the cell surface or present inside the cytoplasm or the nucleus. A ligand is a chemical messenger which is specific to a receptor. Once bound to its ligand, the receptor undergoes conformational changes, allowing the signal to be interpreted. There are three types of signaling- Paracrine signaling, Autocrine signaling and Endocrine signaling. 

• Apoptosis refers to programmed cell death which occurs as a part of an organism’s growth. It is a normal and highly regulated process. 

• Living organisms take up and utilise free energy through metabolism.

 • Glucose acts as an important metabolic fuel in animal. The conversion of glucose into pyruvate occurs through glycolysis.

 • Under aerobic conditions, pyruvate enters the citric acid cycle in the mitochondrial matrix and gets converted into acetyl CoA. The final products of Citric Acid Cycle is CO2 and H2O. 

• Oxidation of fatty acids to acetyl CoA occurs through the β-oxidation pathway. 

• Transamination of amino acids involves transfer of the amino nitrogen from an amino acid to a carbon skeleton to form other amino acids. Deamination involves excretion of the amino nitrogen as urea. 

• Photosynthesis is the process by which light energy gets converted into chemical energy. Photosynthesis occurs inside the chloroplast in plants and some photosynthetic bacteria. During the light reactions, solar energy is captured and splitting of water occurs along with the release of oxygen. Dark reactions (or calvin cycle) involve assimilation of carbon into trioses and hexoses, leading to synthesis of sugar. 

• Cell division gives rise to two daughter cells which carry the same genetic makeup as the parent cell. Interphase and Mitotic phase are important phases of cell cycle. Interphase is further divided into G1 phase, S phase and G2 phase. During interphase, the cells growth and replicate their genetic content. M phase in mitosis consists of four stages: prophase, metaphase, anaphase and telophase.

Unlike mitosis, meiosis has a single interphase which is followed by two nuclear divisions named as Meiosis I (reductional division) and Meiosis II (equational division). M-phase is followed by cytokinesis (division of cytoplasm).

 • The process by which an unspecialised cell achieves a specialised state is referred to as cell differentiation. Stem cells are undifferentiated and unspecialised cells. The ability of Stem Cells to become differentiated into other cell types is known as cell potency. Stem cells are characterised into three types depending on their potency: totipotent cells, pluripotent cells and multipotent cells. 

• Cell migration is the movement of cell from one place to another. It is important at various stages of development of a living organism, such as embryogenesis, organogenesis, regeneration, immune responses, etc.