Cellular Biology: Tension-Cohesion, DNA, Reproduction, and Cellular Evolution

Posted on Feb 4, 2025 in Biology

The Tension-Cohesion-Adhesion Hypothesis

Tension-Cohesion-Adhesion Hypothesis:

  • Mesophyll cells lose water, creating a water deficit in the top floor, which results in negative pressure, also known as tension. The solute concentration in these cells increases, consequently increasing the osmotic pressure.
  • Mesophyll cells become hypertonic in relation to the xylem, drawing water molecules into the cells.
  • Water molecules are linked together due to cohesion forces (between water molecules) and adhesion forces (between water and xylem), forming a continuous column adhering to the vessel walls.
  • The movement of water in the mesophyll causes the water column to move (transpiration stream). The faster the transpiration, the faster the ascension.
  • A water deficit is created in the root xylem, increasing the water flow from outside to inside the floor.
  • There is a passive flow of water from areas of higher water potential to areas where it is lower.

DNA Chemistry: Nature and Structure

Nucleotide:

  • Phosphoric acid
  • Pentose (deoxyribose)
  • Nitrogenous Bases: adenine (A), cytosine (C), guanine (G), thymine (T), and uracil (U)
  • Pyrimidine bases: C, T, and U
  • Purine bases (double ring): A and G

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Sexual Reproduction and Genetic Variability

Chromosome Segregation – Independence of the two counterparts:

  • The two chromosomes migrate randomly to the cell poles. Thus, in the gametes formed, chromosomes are randomly combined with one parent or another, causing a great variety of possible combinations.

Crossing-over:

  • The counterpart chromosomes exchange segments. Therefore, the two chromatids of each chromosome are different and are randomly separated in anaphase II.

Fertilization:

  • The random union of a female gamete and a male gamete increases variability.

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Life Cycles

Diplontic Life Cycle (e.g., Humans)

  • Only gametes are haploid cells.
  • Pre-gametic meiosis occurs.
  • The zygote is diploid and originates, through mitosis, all somatic cells of the organism.

Haplontic Life Cycle (e.g., *Spirogyra*)

  • Gametes are haploid and fuse, forming the zygote.
  • The zygote undergoes meiosis and gives rise to cells that, through successive mitosis, form a haploid organism that produces gametes by mitosis.
  • The only diploid structure is the zygote, and meiosis is post-zygotic.

Haplodiplontic Life Cycle (e.g., Fern)

  • Alternating stages with multicellular haploid and diploid generations.
  • Diploid generation: sporophyte – produces haploid spores by meiosis.
  • Pre-sporic meiosis.
  • Spores germinate and divide by mitosis, originating a haploid generation: the gametophyte.
  • The gametophyte produces gametes by mitosis.
  • Formation of a diploid zygote, which undergoes mitosis and develops into a sporophyte.

Cellular Evolution Models

Autogenic Model:

  • The endomembrane system of eukaryotic cells evolved from invaginations of the plasma membrane of a specialized prokaryotic cell.
  • This model is supported by the fact that the face of the membrane facing the interior of the intracellular compartments is similar to the outer face of the plasma membrane and vice-versa.

Endosymbiotic Model:

  • Eukaryotic cells are the result of the symbiotic association of several prokaryotic ancestors.
  • This model proposes that the endomembrane system was caused by invaginations of the cytoplasmic membrane and that mitochondria and chloroplasts developed from prokaryotic cells that established an endosymbiotic relationship with larger host cells, living within them.
  • The ancestors of mitochondria would be aerobic heterotrophic prokaryotes, and the ancestors of chloroplasts would be photosynthetic prokaryotes.

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