Growth

Growth is the irreversible increase in an organism's dry mass, size, and complexity. Effective growth requires that material buildup (anabolism) exceeds material breakdown (catabolism). In plants, growth is continuous and occurs at the apex, while in animals, growth is more uniform and has defined stages.

Growth occurs through three main processes: cell division, cell enlargement, and cell differentiation. Life begins as a single fertilized cell that divides into two, then four, and so on. After division, the cells increase in size (enlargement). Eventually, each cell develops into a specialized form (specialization), adapting its shape and structure for specific functions. Most mature specialized cells lose their ability to divide.

Cell Division

Cell division, primarily through mitosis, underpins growth in all multicellular organisms.

Types of Cell Division

  1. Mitosis: Produces cells identical to the parent cell, essential for growth, repair, and asexual reproduction.
  2. Meiosis: Reduces chromosome numbers by half and enables genetic variation, important in gamete formation in animals and spore formation in diploid plants.

Stages of mitosis

  1. Interphase: This is referred to as the resting stage where the chromosomes becomes elongated and form a network of fine threads called chromatids
  2. Prophase: in early prophase chromosomes become visible, nucleolus shrinks, centrioles start moving away from each other in opposite direction and the formation of spindle fibres begins. During the late prophase, chromosomes become shorter, thicker and visible. Each chromosome now forms two distinct chromatids joined by a centromere. Nucleolus and nuclear membrane disappear entirely.
  3. Metaphase: The nuclear membrane disappears, spindle forms, the chromosome move up across the middle of the cell and become attached to the spindle fibres at their centromeres.
  4. Anaphase: The sister chromatids are pulled apart to opposite ends (poles) of the cell as the spindle fibres contract.
  5. Telophase: the cell starts dividing into two by constricting at the equator, nucleolus and nuclear membrane are reform in each daughter cell, spindle fibres degenerates and chromosomes eventually regain their threadlike form
Mitosis Diagram

Aspects of Growth

  1. Mass: Refers to the total matter in an organism's body, measurable as wet or dry mass.
  2. Size: Includes dimensions like length, height, and volume, which can be recorded at intervals.
  3. Cell Number: Involves counting the cells in an organism at various stages of growth.

Regions of Rapid Growth in Plants

In plants, rapid growth areas are found at the root and stem tips:

  1. Region of Cell Division (Apical Meristem): Contains cells actively dividing.
  2. Region of Maturation: Cells here reach full development and begin specific functions.
  3. Region of Cell Elongation: Cells in this area enlarge to their maximum size.

Growth Curve

Organism growth rates can be measured using a growth curve, such as the sigmoid curve observed in humans. This curve has three phases:

  1. Lag Phase: Initial slow growth, where cells accumulate essential materials.
  2. Log or Exponential Phase: Period of rapid growth.
  3. Stationary Phase: No observable growth occurs during this phase.

Factors Affecting Growth

Factors influencing growth include:

External Factors

Internal Factors

Hormones and enzymes are primary internal growth regulators. Hormones, chemical substances produced by ductless glands, are transported to various body parts where they facilitate growth.

Plant Growth Hormones

Animal Growth Hormones