What is Cell Growth
Growth is an irreversible, exponential progressive process which results in the increase in the biomass of an organism both quantitatively and qualitatively over a period of time thereby making an organism fit for survival in every better possible way.
Growth is a net result of accumulation of small changes initiating at cellular level and contributing in large scale over a period of time.
Plants show a remarkable feature of infinite cell divisions at the specialized region of meristems where the stem cells are present waiting for an internal stimulator to initiate growth.
When a sapling starts its growth period the meristems were present all over the plant diving continuously and increasing exponentially the number of cells in plant.
Later, the region of meristem gets restricted to specific parts such as the Apex, Laterals, and Intercalary.
Phases of Growth
With appropriate nutrients and favorable environmental condition, a plant undergoes desirable change is gradual steps. The changes initiates from cells forms 3 phases of growth.
A. Cell Division
B. Cell Enlargement
C. Cell Differentiation
A. Cell Division
The initial process of plant growth is the division of cells by the process of DNA replication and cell growth.
Cell cycle mitosis becomes very essential as it is associated with somatic cell proliferation.
The division takes place in meristems where the stem cell proliferates to increase the length and girth of a plant.
Mitosis involves the 4 stages of cell division producing 2 cells by single division. The four stages are Prophase, Metaphase, Telophase and Anaphase followed by Cytokinesis and G0 Phase (i.e.) Quiescent phase.
The mitosis cycle is dependent on the Cyclin Dependent Kinase (CDK) that regulates the synthesis, replication and cell division.
I. Prophase: Initiation of Karyokinesis
1. DNA synthesis is complete at this stage
2. Condensation of chromosome
3. Chromosome has 2 chromatids connected by a centromere
4. Duplicated centrosome migrates to the poles of the nuclear membrane
5. Centrosome releases microtubules forming aster. The microtubules reach centromere of each chromosome by forming spindle fibers
6. The disintegration of nuclear membrane initiates at the end of the prophase
7. The cell organelles start to disappear
II. Metaphase: Chromosomal Alignment
1. Nuclear membrane disintegration initiates the stage of metaphase
2. Chromosome becomes distinct with sister chromatids held together by centromere
3. The centromere surface has kinetochores. Kinetochores are binding region for the spindle fibers and are disc shaped
4. Spindle fibers align the chromosomes at the equator of the cell called metaphase plate
5. The chromosome is now arranged equatorially in the cytoplasm attached by spindle fibers
III. Anaphase: Sister Cell is Formed
1. The third stage of mitosis carries one half of chromosome to respective poles
2. Centromere drags the chromatids to opposite poles
IV. Telophase: Separation of Daughter Cells
1. Chromatids groups at the poles of the cell
2. Nuclear membrane is generated around the chromatids
3. Chromatids decondense to become chromatin reticulum
4. Reappearance of lost cell organelles – Golgi apparatus, Endoplasmic reticulum, etc.,
1. The process of division of one cell into two daughter cells after mitosis is cytokinesis
2. A cell plate is formed in between nucleus
3. The cell plate formation begins with small furrow in between 2 daughter nuclei
4. The furrow extends further to the lateral sides and the cells separate from each other
5. Cell plate is formed by the Golgi Body vesicles namely phragmoplast.
6. The orientation of nuclei is controlled by actin, myosin II and regulatory proteins forming a contractile ring
7. Cell division is accomplished by expansion in size of daughter cells
8. The new plasma membrane is formed by the fusion of intracellular vesicles
The cell division or the cell formation phase takes place at the region of meristems in the plant.
B. Cell Enlargement
Enlargement of cell is the maturation phase where cell size increases to acquire nutrient for the coordinated growth.
The enlargement takes place horizontally where the inner cell wall has high solute content increasing the osmotic pressure for the water to enter.
The entry of water is stored in the vacuolated cells, which increase in size.
High water quantity makes the cell diluted and turgid.
The cell wall now becomes thin. Golgi apparatus has a clear role in the formation of the pressure inside the cell.
This function is also regulated by the hormonal influences of the body along with cytoskeleton.
C. Cell Differentiation
In this phase the cell completely matures and retains stem cell for dedifferentiation.
The phase provides a clear distinction between permanent tissue and meristematic tissues.
The sigmoid curve representing the rate of growth is the growth curve. The overall growth of the plant is simply represented in the curve.
Four phases of curve are plotted. Namely: Lag Phase, Log Phase, Diminishing Phase and Stationary Phase.
1. Lag Phase: the initial growth period is referred as lag phase. In this phase each cell starts to divide continuously and make itself easily available to uptake of nutrients and increase cell mass. The phase involves gradual increase in cell growth.
2. Log Phase: the rapid cell growth period is the log phase. Under Favorable environmental condition the cell growth increases exponentially in large scale by the multiplication of cell division. Simultaneous nutrient input and maturation takes place in this stage. However, the cell division exceeds the maturation
3. Diminishing Phase: the cells start maturation providing a higher yield of cellular metabolites. The growth or new cell formation is confined to certain region of meristems which divides but to keep up with overall plant growth. Reduces the rate of formation of new cells
4. Stationary Phase: A final stage of plant growth where the meristematic regions constantly produce new cells and old cells are removed. This constant maintenance of cell cycle is the Stationary Phase.
Phases of Growth Citations
- Linear growth retardation in relation to the three phases of growth. Eur J Clin Nutr . 1994 Feb;48 Suppl 1:S25-43; discussion S43-4.
- The diagnostic performance of dental maturity for identification of the circumpubertal growth phases: a meta-analysis. Prog Orthod . 2013 May 23;14:8.
- The influence of growth patterns on sexual size monomorphism in lemurs. J Evol Biol . 2015 Sep;28(9):1670-81.