Plant Embryo Development and Cellular Processes

Posted on Jan 24, 2026 in Biology

Embryo Formation in Flowering Plants

The formation of embryos in flowering plants begins after fertilization, when the zygote (formed by the fusion of the male and female gametes) undergoes a series of divisions and differentiations. Though the initial stages of development are similar in both dicots and monocots, the final structure of the embryo differs significantly.

Dicot Embryo Formation

In dicot plants (e.g., bean, pea), the zygote first divides asymmetrically into a small terminal cell and a large basal cell. The terminal cell gives rise to most of the embryo, while the basal cell forms a suspensor that pushes the embryo into the endosperm and helps in nutrient transfer. As the embryo develops, it passes through several stages: globular, heart-shaped, and torpedo-shaped.

During the heart-shaped stage, two lateral outgrowths appear, forming the two cotyledons (seed leaves), which is a defining feature of dicots. The embryo differentiates into:

  • Radicle (future root)
  • Plumule (future shoot)
  • Hypocotyl (region below the cotyledons)
  • Epicotyl (region above the cotyledons)

Monocot Embryo Structure

In monocot plants (e.g., maize, wheat), the early stages of embryo development are similar to those in dicots up to the globular stage. However, only one cotyledon develops, which is often large and specialized. In grasses, this cotyledon is called the scutellum, which functions in absorbing nutrients from the endosperm.

The monocot embryo also forms a coleoptile (a protective sheath covering the plumule) and a coleorhiza (a sheath covering the radicle). The monocot embryo thus differentiates into:

  • Single cotyledon (scutellum)
  • Radicle and coleorhiza
  • Plumule and coleoptile
  • Epiblast (a small outgrowth thought to be a remnant of the second cotyledon)

Cyclic Photophosphorylation Mechanism

Photophosphorylation is the process of ATP formation from ADP and inorganic phosphate (iP) in the presence of sunlight and the ATPase enzyme: Sunlight + ADP + iP $\rightarrow$ ATP.

There are two different pathways of photophosphorylation: cyclic and non-cyclic. The pathway in which an electron moves in a cyclic pattern, where the electron acceptor and donor are the same, and the electron returns to the same reaction center, is cyclic photophosphorylation.

In this process:

  • Electrons expelled by the reaction center of PS I travel through a series of carriers and return to the same center.
  • Only one reaction center is involved.
  • ATP is produced, but NADPH₂ and O₂ are not produced.

The electrons released by P700 of PS I in the presence of light are taken up by the primary acceptor (chlorophyll called A₀) to A₁, then to Fe-S. They pass to Ferredoxin (Fd); after that, the electron is passed to Plastoquinone (PQ), the cytochrome complex (CC), Plastocyanin (PC) respectively, and finally back to P700. This is cyclic photophosphorylation. Energy is produced without the photolysis of water. ATP is produced during the transfer of electrons from Ferredoxin to plastoquinone and between the cytochrome components.

Respiration Comparison

Aerobic Respiration

  1. Occurs in the presence of oxygen.
  2. Takes place in the mitochondria of cells.
  3. Involves the complete breakdown of glucose.
  4. Produces carbon dioxide and water as by-products.
  5. Generates a high amount of energy (approximately 36–38 ATP molecules per glucose).
  6. Includes three main stages: glycolysis, Krebs cycle, and electron transport chain.

Anaerobic Respiration

  1. Occurs in the absence of oxygen.
  2. Takes place in the cytoplasm of the cell.
  3. Involves partial breakdown of glucose.
  4. Produces less energy (only 2 ATP molecules per glucose).
  5. End products vary by organism: ethanol and CO₂ in yeast, lactic acid in animals.
  6. Does not involve the Krebs cycle or electron transport chain.

Nucleic Acid Structures

Structure of RNA

RNA, or ribonucleic acid, is a nucleic acid that plays crucial roles in coding, decoding, regulation, and expression of genes. Its structure is composed of a single strand of nucleotides, each consisting of a ribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), uracil (U), cytosine (C), or guanine (G). This linear sequence forms the primary structure of RNA.

Within the strand, complementary base pairing can occur, leading to the formation of secondary structures such as hairpins, stem-loops, bulges, and internal loops. These features result from hydrogen bonding between bases, most commonly A-U and G-C pairs, though G-U pairs (wobble pairs) are also common. The RNA strand can further fold into a complex three-dimensional shape, forming its tertiary structure, stabilized by interactions including base stacking and hydrogen bonding. In some cases, RNA molecules associate with other RNAs or proteins to form quaternary structures, such as in ribosomes or spliceosomes. Unlike DNA, RNA is usually single-stranded and contains uracil instead of thymine.

Structure of DNA

Deoxyribonucleic acid is the hereditary material in most living organisms and carries the genetic instructions used in growth, development, functioning, and reproduction. Structurally, DNA is composed of two long strands of nucleotides that form a double helix.

Each nucleotide consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C), or guanine (G). The two strands run in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary base pairs: A pairs with T, and C pairs with G. This specific pairing forms the double helix, which is the secondary structure of DNA. The sequence of bases along a DNA strand constitutes its primary structure.

DNA can also form higher-level structures—its tertiary structure—through supercoiling and packaging into chromatin in eukaryotic cells. Unlike RNA, DNA is more chemically stable due to the absence of a hydroxyl group on the sugar and is typically double-stranded, making it ideal for long-term storage of genetic information.

Cellular Reproduction Processes

Sporogenesis

Sporogenesis is the biological process through which spores are produced, typically in plants, fungi, and some protists. It begins with a diploid sporogenous cell, which undergoes meiosis to form haploid spores. In plants, the diploid sporophyte produces spore mother cells within sporangia. These mother cells undergo meiotic division to produce haploid spores (microspores or megaspores), which develop into gametophytes. Sporogenesis is essential for the alternation of generations in plants and introduces genetic variation through meiosis.

Gametogenesis

Gametogenesis is the process by which gametes (sex cells) are formed in animals and plants. It involves the differentiation of germ cells into sperm in males (spermatogenesis) and eggs in females (oogenesis). In both cases, a diploid germ cell undergoes mitosis to form primary spermatocytes or oocytes, which then undergo meiosis.

In spermatogenesis, each primary spermatocyte produces four haploid sperm. In oogenesis, each primary oocyte typically results in one large haploid ovum and smaller polar bodies. Gametogenesis ensures the formation of haploid cells needed for sexual reproduction and contributes to genetic diversity through meiotic recombination.

Types of Epithelium

Simple Epithelium

  1. Simple Squamous Epithelium: Single layer of flat, thin cells. Allows diffusion and filtration. Found in alveoli (lungs), blood vessels (endothelium), and kidney glomeruli.
  2. Simple Cuboidal Epithelium: Single layer of cube-shaped cells. Involved in secretion and absorption. Found in kidney tubules, ducts of glands, and the thyroid gland.
  3. Simple Columnar Epithelium: Single layer of tall, column-like cells. Performs absorption and secretion. Found in the lining of the stomach, intestines, and uterus.
  4. Ciliated Simple Columnar Epithelium: Columnar cells with cilia on their surface. Helps move mucus or ova. Found in the lining of the fallopian tubes and respiratory tract.

Pseudostratified Columnar Epithelium

  1. Appears multi-layered but is a single layer with nuclei at different levels. Often ciliated and involved in secretion and movement of mucus. Found in the trachea and upper respiratory tract.