- The size of an organism increases as a result of a growth in the number of its cells, its protoplasmic material, or both.
- Not usually do cell quantity and protoplasmic content grow simultaneously; cell division can occur without an increase in protoplasm, resulting in a greater number of smaller cells (e.g., cleavage).
- Alternately, protoplasm can be generated without cell division, resulting in bigger cells.
- Any increase in protoplasm necessitates the production of cell components including mitochondria, cell membranes, enzymes, and other proteins.
- Thus, growth involves an increase in the organism’s size and mass due to the synthesis of additional protoplasm.
- It also involves a growth in the number of apoplasmatic components, such as the fibres and matrix of connective tissues in mammals and other higher animals.
- When the anabolic process dominates the metabolic activity, the expansion of the body of a higher organism occurs through the addition of new components, protoplasmic and apoplasmatic.
- In contrast, when deconstruction surpasses synthesis, the internal food store (such as fat in adipose tissues) is depleted first in order to power the body’s machinery, and subsequently energy is gained at the price of protoplasmic proteins. This results in the depletion of life stuff and degrowth.
Levels of Growth
There are two distinct stages of growth in living organisms:
A. Cell growth
- There are two basic cellular activities that govern the growth of all multicellular organisms. This refers to the reproduction and expansion of individual cells within the body.
- In my last essay on Cell Division, I stated that during the interphase phase, cells produce new materials such as nucleic acids (DNA, RNA) and proteins, resulting in their expansion.
- The multiplication of individual body cells is the most important and fundamental aspect of growth in all multicellular organisms.
- In tissue culture or the culture of unicellular organisms, it is possible to evaluate the cyclicity of cell multiplication and growth with precision.
B. Growth of multicellular organisms
The growth of multicellular animals and plants in connection to the growth and multiplication of their individual cells can be divided into three distinct categories:
(1) Auxetic growth (Auxesis = growth resulting from increase in cell size)
- In this form of growth, the body’s volume rises due to the proliferation of body cells, but the number of cells does not increase.
- Infrequent examples of auxetic growth include worms, rotifers, and tunicates.
(2) Multiplicative growth
- This form of expansion is caused by an increase in the number of bodily cells.
- Mitotic divisions are responsible for the rise in cellular number.
- In this instance, however, the average size of the cells remains unchanged or increases little.
- This sort of growth is known as multiplicative growth, and it happens throughout morphogenesis in embryos. It also has a role in the prenatal development of higher animals.
(3) Accretionary growth
- Generally, post-embryonic growth in animals and plants is caused by the mitotic multiplication of specific types of cells in particular body sites.
- The differentiated cells of the body’s organs and tissues lose their ability to divide and grow (e.g., muscles, nerve cells, osteocytes of bone, fat cells, xylem, phloem, parenchymal cells, etc.).
- Special cells exist in an undifferentiated condition as reserve cells, including meristematic cells in angiosperms, stem cells such erythropoietic tissue of red bone marrow, periosteum cells of bone, ciliary body cells of vertebrate eye, and epidermal cells of stratum germinativum.
- These reserve cells reinforce and replace worn-out differentiated cells if necessary. In such circumstances, they develop into the sort of cells they will reinforce and replace. This sort of expansion is known as accretionary expansion.
- According to Green and Taylor (1990), the growth of a multicellular organism can be split into three phases, beginning with a single cell: I cell division or hyperplasia, i.e., an increase in cell number due to mitotic division; (ii) cell expansion or hypertrophy, i.e., an irreversible increase in cell size due to the uptake of water or the synthesis of living material; and (iii) cell differentiation, i.e., the specialisation of cells; in its broadest sense, growth also encompasses this phase of cell development (viz., differentiation).
Limited and Unlimited Growth
- According to studies of the duration of plant and animal growth, there are two fundamental growth patterns: limited (definite or determinate) growth and unbounded (indefinite or indeterminate) growth.
- After a period of maximum growth, during which the plant matures and reproduces, there follows a period of negative growth or senescence before the plant dies.
- Several plant parts, including fruits, vegetative propagation organs, dicotyledonous leaves, and stem internodes, exhibit reduced development but do not experience a phase of negative growth. Included among animals with limited growth are insects, birds, and mammals.
- In contrast, woody perennial plants exhibit infinite growth.
- Fungi, algae, monocotyledonous leaves, and numerous animals, especially nonchordates, fishes, and reptiles, also exhibit unrestricted growth.
- The cell is a dynamic system exhibiting a unique growth phenomenon. A cell grows by consuming food resources from its environment and transforming them into cellular components.
- The increase in active cell mass is the simultaneous result of synthetic and degenerative processes.
- When a cell reaches its size limit, it divides into two daughter cells. These cells are in a growth-duplication cycle.
- Single-celled organisms such as E. coli, yeast, and Amoeba, as well as cultured somatic cells, are instances of a growth-duplication cycle.
- A newly-formed cell grows by macromolecular synthesis, achieves species-specific division size, and replicates to continue the cycle.
- Mitosis (or nuclear division) concludes the growth duplication cycle of eukaryotic cells and is typically preceded by DNA replication.
Kinetics of Cell Growth
- Determining the kinetics of cell development between divisions is extremely difficult because the parameters for measuring growth (such as dry mass, volume, and linear dimensions) do not behave consistently even within the same cell. Studies of both fixed and living cells have revealed two distinct growth patterns: linear and exponential.
- A pattern of exponential growth indicates that the growth rate is proportional to the total mass; as the mass grows, so does the growth rate.
- The growth rate is proportional to the amount of active protoplasm or replicating organisms.
- A linear growth pattern indicates that the growth rate remains constant and does not rise during the cell cycle.
- In this case, growth rate is independent of cell mass and is instead proportional to a constant number of elements (synthetic sites) whose activities remain constant during the growth cycle.
- Not only does a growing cell increase in size, but it also gains mass. This indicates that growth is linear and may be studied as a function of time.
- On a well-defined medium, E.coli has been extensively utilised to research growth under laboratory conditions.
- The nutritional medium contains glucose as the carbon source and a number of dissolved inorganic ions.
- The optimal temperature for studying growth is 370 degrees Celsius, and it takes approximately 60 minutes to double the cell mass.
- However, the development of E.coli can be accelerated by 20 minutes by adding different amino acids, purine, and pyrimidine bases to the medium.
- Observations of the growth of a single E.coli cell have revealed that the cell develops and splits into daughter cells after a unit of time, a constant factor for each generation.
- This time unit is known as the generation time. The number of bacteria multiplies exponentially, with growth and division occurring simultaneously.
- Twenty, twenty-one, twenty-two, twenty-three, twenty-four…cells develop as an exponent of two, and the pattern is represented by a growth curve.
- The linear component of the curve depicts exponential growth, showing a time-dependent increase in the number of cells. The subsequent flattening of the growth curve corresponds to a phase of maximum stationary growth.
- During this time, cell division is slow and growth is no longer exponential. Several conditions, including nutritional depletion and oxygen deficiency, have been linked to the cessation of exponential development.
- Due to the diminished viability of the DNA, cells cannot divide indefinitely.
- Mitchison (1963) measured the dry mass and volume of a budding yeast cell and demonstrated that mass increases linearly after division without a lag and continues to increase at a constant rate until it doubles before the next division; that is, the combined growth rate of daughter cells is twice that of the mother cell.
- Cell volume, on the other hand, follows a broadly exponential curve for the first three quarters of the cell cycle, plateauing before to division.
- Yeast growth dynamics highlights several characteristics shared by all cells that take nutrients through their surface: (1) growth rate is constant between divisions and doubles immediately after; (2) cell mass doubles between divisions; (3) mass increase is connected with nuclear rather than cytoplasmic change; and (4) there is no lag period – linear growth commences immediately after division.
- There are exceptions to this tendency in Amoeba. Even though the mass of an amoeba doubles between divisions, its growth slows and reaches a plateau many hours before mitosis.
- Clearly, genetic factors impact growth patterns, but nutritional disparities also exist. Amoeba ingests solid food, but yeast cells absorb foreign nutrients straight through the cell membrane.
- Contractile vacuole variations in Amoeba introduce extraneous disturbances during cell mass measurements. In addition, a decreasing growth rate and a time of stability towards the conclusion of Amoeba development indicate a drop in feeding before to division.
Mechanisms Involved in Cell Growth
In the majority of instances, the kinetics of mass gain is matched by the parallel synthesis of RNA, protein, and membrane. Being discontinuous, DNA synthesis is not directly related to the kinetics of cell growth. However, it is involved in controlling cell size.
1. RNA synthesis and cell growth
- In most cases, ribosomal RNA and tRNA are constantly generated throughout the eukaryotic cell cycle.
- During the cell cycle, the rate of synthesis may rise; in mammalian cells, the rate of rRNA synthesis doubles following S phase.
- Different types of mRNA are generated throughout varied phases of the cell cycle and at different rates, hence the mRNA pattern is unknown.
- The association between overall cell growth patterns and rRNA synthesis shows that the synthesis of ribosomes may be a crucial growth-regulating site.
- Clearly, the total number of ribosomes in a bacterial cell determines the rate of protein synthesis during growth, i.e., the ratio of ribosomes per DNA genome to the rate of growth and protein synthesis (O. Maaloe).
- The nucleolus, the site of ribosome production, regulates the total quantity of cytoplasmic ribosomes per cell, which determines the growth rate of eukaryotic cells.
2. Nucleolus and cell growth
- The nucleolus undergoes cyclic alterations during the cell cycle and is connected to cell proliferation in some way.
- During interphase, when cells are rapidly dividing, nucleoli are abundant and produce ribosomal RNA at an accelerated rate.
- In prophase, when growth ceases, nucleoli vanish and their contents are emptied into the nucleoplasm.
- In metaphase and anaphase, nucleoli are missing, but they resurface early in telophase at twice their original number as new nucleoli organise at nucleolar organiser sites in each daughter nucleus.
- Even though overall protoplasmic mass has not changed, the aggregate growth rate of daughter cells is double that of the mother cell.
- The change from physiological “oneness” to physiological “twoness” is most closely synchronised with the duplication of nucleolar organisers and the development of nucleoli.
- Thus, the growth rate doubles after the replication of nucleolar organisers during the S phase, when twice as many ribosomal RNA genes begin to transcribe rRNA.
- In fact, the nucleolus is a dynamic organelle that responds swiftly to changing needs for novel growth patterns and rates in response to metabolic demands.
- It gets huge and metabolically active in the majority of developing and proliferating cells, such as tumour cells, but vanishes in non-protein-synthesis-active cells.
- When there is a high demand for ribosomes, as in mature oocytes, nucleolar function is dramatically expanded by the production of numerous extra nucleoli (up to 1,000 in certain species), each of which is supplied with a DNA segment containing multiple copies of the rRNA genes.
- Consequently, growth regulation appears to be dependent on the nucleolus and its regulation of ribosome production. Ribosome production and turnover may dictate cell development, with the nucleolus serving as the primary “flow-through” centre or “valve” that controls the entire process.
3. Protein synthesis and cell growth
- The vast majority of eukaryotic cell growth is the result of total protein accumulation, which is the net balance between total protein synthesis and protein degradation.
- Both processes are subject to distinct types of control, with the former being mostly dependent on ribosome availability.
- In addition, while total cell protein may appear to increase continually during the cell cycle, certain individual proteins may be stable, while others may be declining and still others may be gradually rising.
- During the cell cycle, enzymes, for instance, exhibit the following three patterns of synthesis: 1. Synthesis may be periodic, similar to DNA synthesis, and increase rapidly during one phase of the cycle; for example, enzymes involved in DNA synthesis, such as thymidine kinase, exhibit a stepwise pattern of synthesis. Numerous respiratory enzymes in mouse fibroblasts are characterised by continuous enzyme synthesis, either linear or exponential. Several enzymes have a peak pattern.
- They rise rapidly during the cycle and then vanish, most likely due to degradation and turnover. Each protein inside the same cell can be separately controlled.
- However, some collections of proteins, particularly those that define a cell’s phenotypic, may be coordinatedly controlled.
Two hypotheses or models have been offered to explain the varied growth-related patterns of protein synthesis:
1. Oscillatory repression model
- According to this paradigm, periodic enzyme synthesis triggers the suppression of end products by negative feedback.
- When the enzyme pool is high, enzyme synthesis is inhibited; when the enzyme pool is low, enzyme production is enhanced.
- This pattern will result in oscillations whose frequency need not correlate to other activities in the cell cycle, such as DNA synthesis.
2. Sequential gene expression model
- A chromosome is programmed for the sequential expression of genes at various stages of the cell cycle, according to this paradigm.
- Here, an ordered interpretation of genes is highlighted.
- The RNA polymerase proceeds around the genome, sequentially transcribing genes. Therefore, genes are only accessible for transcription during specific phases of the cell cycle.
- For instance, linear gene reading has been observed in synchronised yeast growth. The timing of the production of twelve distinct enzymes in yeast corresponds to the location of their respective genes on the chromosomes.
- Each enzyme is created sequentially, with the gene’s location presumably influencing the order of its expression during the cell cycle.