Cell Biology: Regulation, Organelles, Division, and Photosynthesis

Posted on Mar 21, 2025 in Biology

Cellular Regulation and Maintenance

Regulation of pH is essential for maintaining life. It requires that cellular and organic fluids stay within certain limits. Otherwise, structural changes in biomolecules and alterations in chemical reactions may occur. In aqueous solutions, living beings use weak acid-base buffer systems and their conjugates. Types include:

  • Inorganic bicarbonate: HCO3– + H+ ⇌ H2CO3 ⇌ CO2 + H2O
  • Phosphate: HPO42- + H+ ⇌ H2PO4–

Cellular Organelles and Their Functions

Ribosomes: Non-membranous organelles where protein synthesis and translation of genetic messages in mRNA occur in all cells.

Endoplasmic Reticulum: A complex, interconnected network of membranes extending throughout the cytoplasm.

  • Rough ER: Involved in protein synthesis and glycosylation.
  • Smooth ER: Involved in the synthesis of phospholipids, cholesterol, and steroid hormones, and in the detoxification of liposoluble substances.

Golgi Complex (Dictyosomes): Consisting of cis and trans compartments, with associated vesicles involved in secretion processes, recycling of the plasma membrane, glycosylation, and the formation of lysosomes and vacuoles.

Lysosomes: Membrane-bound vesicles containing hydrolytic enzymes responsible for intracellular digestion of molecules (heterophagy – exogenous; autophagy – endogenous).

Vacuoles: Large vesicles filled with fluid and surrounded by a membrane. They store a variety of substances, have digestive activity, regulate turgor pressure, and contribute to cell size increase in plants.

Peroxisomes: Membranous organelles containing enzymes (oxidase and catalase) to carry out oxidative reactions. They are involved in detoxification reactions. Glicoxisomes (in fat seeds) facilitate sugar development in the embryo.

Cell Division: Mitosis and Meiosis

Mitosis: Forms daughter nuclei with the same number of chromosomes as the mother cell. Each cell produces two identical nuclei. Its biological significance includes tissue repair, growth in multicellular organisms, and cellular reproduction. Mitosis does not generate variability.

Meiosis: Forms daughter nuclei with half the number of chromosomes as the parent cell. Each diploid maternal nucleus produces four haploid daughter nuclei. Its biological significance includes maintaining a constant number of chromosomes in a species and generating evolutionary variability.

Cell Theory

Schleiden and Schwann (1838-1839) proposed that the cell is the structural and functional unit of living organisms. Principles:

  • Living organisms are formed by a single cell or multiple cells.
  • The cell is the most elemental unit of life, possessing all the functions of a living organism.

Factors Modulating Photosynthetic Activity

Factors include light intensity, CO2 concentration, temperature, photoperiod, humidity, and light color.

The Problem of Photorespiration: The enzyme Rubisco, which normally catalyzes the joining of CO2 to Rubisco, can also combine with O2. This prevents the Calvin cycle from producing glucose and results in a loss of energy for the plant. Rubisco oxidizes instead of carboxylates. This occurs when the concentration of O2 increases significantly above that of CO2. As a result, the plant closes its stomata to prevent excessive water loss through transpiration, but this also prevents the entry of CO2 and the exit of O2.

Genetic Code

The genetic code is a set of correlations between each triplet of bases in mRNA and the corresponding amino acid in a protein. Features:

  • It is almost universal.
  • It includes a start codon.
  • It is degenerate but not ambiguous (each triplet encodes a single amino acid).
  • Triplets are juxtaposed from beginning to end.