Growth

Growth refers to the development of a species from emergence or birth to maturity, and for certain species, beyond maturity to eventual senescence or death. Growth also refers to an increase in size as a result of cell multiplication and expansion, as well as tissue maturation. Although growth emphasizes increased cell number and size, it also necessitates programmed cell death, which leads to the formation of the final body structure. As a result, development is a highly dynamic phenomenon involving changes in body shape, metabolism, and bodily processes.

The Process of Growth

Growth is rarely imprecise. Rather, it happens in accordance with a scheme that ultimately defines the individual's size and form. Growth can be limited to specific parts of the organism, such as the layers of cells that divide and grow in size near the plant shoot's tip. Alternatively, as in the human embryo, the cells involved in development may be widely distributed across the organism's body. In the above case, the rates of cell division and cell growth vary in various parts of the body. The types of adults show that the pattern of growth in plants and animals is predetermined and normal. However, in some species, especially slime moulds, there is no normal pattern of growth, and the result is a formless cytoplasmic mass.

The rate of growth of different components of an organism can have a significant impact on its ability to adapt to its surroundings, and thus can influence evolution. An increase in the rate of growth of the fleshy parts of a fish fin, for example, would enable fish to adapt to terrestrial locomotory life more easily than a fish without this modified fin. The evolution of limbs through natural selection would have been unlikely without disproportionate fin development, which eventually resulted from random changes in the genetic material.

Types of Growth in Biology

In Cells: The number and size of cells that make up a person determine the size and shape of a developing organism. Mitosis is a precise cellular reproductive process that results in an increase in cell number. During mitosis, the genetic material-carrying chromosomes are duplicated in the nucleus, and the doubled chromosomes are distributed evenly to the two daughter cells, with one of each chromosome group going to each daughter cell. Before the ends of the dividing cell divide, each end receives a full collection of chromosomes. A pinching off of the cell membrane occurs in animal cells; in plant cells, a new cellulose wall forms between the new cells. The mother cell also expands to twice its original size during the time of cell existence preceding the actual distribution of chromosomes. 

As a result, a cycle of cell growth and division is created. Cell growth, which results in an increase in cytoplasmic mass, chromosome number, and cell surface, is followed by cell division, in which the cytoplasmic mass and chromosomes are transferred to daughter cells. During cell division processes, however, an increase in cytoplasmic mass does not always occur. The original egg cell, which is normally a very large cell, undergoes repeated series of cell divisions without any intervening growth cycles during the early development of an embryo, resulting in the original egg cell dividing into thousands of small cells. The embryo's normal pattern of growth and mitosis begins only after it is able to receive food from its surroundings.

In Plants: The fact that most plant cells grow in size without dividing is a significant difference between plant and animal development. The number of daughter cells arising from cell division behind the tip of the plant root or shoot can increase dramatically. This is achieved by the cells absorbing water, which is then deposited in a central cavity called a vacuole. Water intake creates a pressure that, when combined with other influences, pushes against the cellulose walls of plant cells, lengthening, girthening, and stiffening (turgor) the cells and plant. Most of the size increase in plants occurs after cell division and is largely due to an increase in cell water content rather than a significant increase in dry weight. 

Many cells are distributed around the mass of a developing plant embryo and go through the cycle of growth and cell division. However, once the locations of the root tip, shoot tip, and embryonic leaves are identified, cell division is restricted to cells in specific regions known as meristems. Both changes in the number of cells in the primary root occur at one meristematic centre, which is located just beneath the surface of the developing root. Some of the daughter cells begin to divide at the elongating tip. Other daughter cells in the root continue to grow in length, allowing the new root to push deeper into the soil. 

A restricted meristematic area at the tip of the developing shoot is responsible for the development of the cells of the leaves and stem; cell elongation occurs behind this meristematic centre. Cells associated with the vascular strands of phloem and xylem tissues that bring water from the soil to the leaves and sugar from the leaves to the rest of the plant—develop secondarily in the young seedling. These cells will divide again, supplying new cell material for the formation of a woody covering and more complex vascular strands.  As a result, higher plant growth, that is, the aspects affecting both the pattern of stems, leaves, and roots as well as the rise in bulk, is mainly the result of cell division at the meristem, accompanied by a secondary increase in size due to water uptake. These events take place during the plant's life cycle.

In Animals: Animals' development is more limited in time than that of plants, but cell division is more evenly spread across the organism's body. Although the rate of cell division varies by region, the developing embryo's cell division ability is widely distributed. The embryonic stage is marked by rapid growth, which slows down in adolescents and then disappears entirely. Even after an increase in overall body size no longer exists, cell division and size increase continue. Since cell death counteracts these events, post-juvenile cell growth is mainly a replacement phenomenon. The cessation of cell division and bone deposition in the long bones limits the growth of mammals. 

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Humans have an unusually long juvenile period of growth, with most higher animals reaching maturity shortly after the end of embryonic development. After birth, certain organ systems experience little cell division and growth; for example, all of a female's germ cells (precursors to egg cells) are created by the time she is born. Similarly, by the end of the embryonic phase, all of the brain's nerve cells have grown. An outgrowth of nerve fibres and accumulation of fatty insulation material over them causes the nervous system to grow much larger. While, as in plant cells, the greatest increase in size occurs after cell division stops, nerve fibre outgrowth in animals reflects a true increase in the amount of cytoplasm and cell surface, not just a water uptake.

Some organs continue to have the ability to expand and divide cells throughout the animal's life. The liver, for example, continues to produce new cells to replace those that have become senescent or died. While cell division and growth occur in the liver, other organs have a specific population of cells known as stem cells that can divide. Only the marrow of the long bones contains the cells that produce mammalian blood's circulating red cells. They form a permanent population of dividing cells to replace the red cells that die and vanish from circulation on a regular basis.

Different sections of the body have different rates of growth and cell division. This difference in size is a major factor in determining an organism's form.

Normal and Abnormal Growth

Anomalies and tumours may occur when growth is not properly managed. Hepatic tumours, or hepatomas, may develop if the increase in the number of liver cells is abnormal. Indeed, one of the characteristics of malignant tumours, or cancers, is the lack of normal growth patterns and rates. Malignant tumour cells, in addition to having irregular growth rates, have altered adhesive properties that allow them to easily detach from the tumour, allowing them to spread to other parts of the body (metastasize) and expand in unusual places. The death of an organism is normally caused by the development of tumours in areas other than the organ of origin. 

Tumours may have a wide range of growth rates. They can develop very quickly or very slowly, reaching the rate of cell division in adult tissues. Tumours are described not only by an increased rate of cell division but also by irregular growth patterns. The tumour’s new cells are not organised or incorporated into the organ's structure, and they can form large nodules. These irregular growths can cause no medical problems (e.g., moles) or can have serious consequences, such as the pressure on the brain caused by a tumorous mass of the brain's meningeal covering.

Regeneration

Not all abnormal growths can be determined as tumours. Cells under the bark of a partially burned tree provide a new coating for the exposed vascular strands. It's possible that the growth isn't natural, and there's a visible scar or new bark growth. Similarly, if a mammal's skin is seriously burned, the repair, though irregular and incomplete, does not give the organism any physiological problems. Many species have the ability to regrow or regenerate, parts of the body that have been damaged or injured to various degrees of perfection. Salamanders have extraordinary healing abilities, forming new eyes or limbs when the original is lost. 

Lizards can regenerate their tails, and humans can regenerate portions of their liver. The reasons for differences in regenerative abilities among animals are still a fascinating mystery with significant practical implications. When regeneration occurs, some specialized cells lose their specialised characteristics and enter a cycle of increased cell division; new cells then specialise into tissues of the original body portion. Plants that lose their tops due to pruning may often form new meristematic centers and grow new shoots from dormant tissues.

Growth Rate Biology

The rate, or speed, with which a population's number of organisms grows. This is determined by dividing the change in the number of organisms from one point in time to the next by the amount of time between the two points in time. The term is most often used to describe the growth of cells or microorganisms in laboratory cultures, and it is typically expressed in terms of generation time.

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Stationary Phase

Anything good must come to an end. When a bacterial population runs out of an important nutrient/chemical, or when its growth is slowed by its own waste products (remember, it's a closed container), or by a lack of physical space, the cells enter the stationary process. The number of new cells created equals the number of cells dying off at this stage, or growth has completely stopped, resulting in a flattening of the growth curve.

At this stage, the cells' physiology changes dramatically as they attempt to adapt to their new starvation conditions. Bacilli become almost spherical in shape as a result of the few new cells that are formed. Their plasma membrane becomes less permeable and fluid, with more hydrophobic molecules on the surface that encourage cell adhesion and aggregation. To protect the DNA from injury, the nucleoid condenses and the DNA becomes bound with DNA-binding proteins from starved cells. The changes are intended to allow the cell to live in less-than-ideal conditions for longer while waiting for more favourable conditions to occur. Cells in oligotrophic or low-nutrient environments use similar strategies. Cells in the natural world are thought to live in oligotrophic environments for long periods of time, with only occasional infusions of nutrients returning them to exponential growth for very short periods of time.

Cells are also vulnerable to producing secondary metabolites, or metabolites released after active development, such as antibiotics, during the stationary process. During this level, cells capable of producing an endospore will activate the necessary genes to begin the sporulation process.

Lag Phase Biology

The lag phase is a time of adaptation during which bacteria adjust to their new environment. The duration of the lag period depends on how different the conditions are from those in which the bacteria originated, as well as the state of the bacterial cells themselves. The lag period is shortest when actively growing cells are moved from one form of media to another type of media under the same environmental conditions. Damaged cells would have a long lag time before they can reproduce because they must repair themselves first.

During the lag period, cells typically synthesize RNA, enzymes, and basic metabolites that may be lacking in their new environment (such as growth factors or macromolecules), as well as adapting to changes in temperature, pH, or oxygen availability. They may also be repairing any damaged cells that are needed.

Population Growth Biology 

To explain the rate of change in the size of a population over time, the two simplest models of population growth use deterministic equations (equations that do not account for random events). The first of these models, exponential growth, describes imaginary populations that rise in size without ever reaching a limit. The second model, logistic development, imposes reproductive growth constraints that become more severe as the population grows. Neither model accurately represents natural populations, but they do include comparisons.

Exponential Growth

The English clergyman Thomas Malthus inspired Charles Darwin in establishing his theory of natural selection. In 1798, Malthus published his book, claiming that populations with abundant natural resources grow rapidly, but that further growth is limited by resource depletion. Exponential growth is the term for the early pattern of increasing population size.

Bacteria provide the best example of exponential growth in species. Bacteria are prokaryotes that depend on binary fission to replicate. For several bacterial species, this division takes about an hour.

 If 1000 bacteria are put in a large flask with an abundant supply of nutrients (so that the nutrients do not become exhausted quickly), the number of bacteria would have doubled from 1000 to 2000 in just one hour. Each of the 2000 bacteria will divide in an hour, yielding 4000 bacteria. After the third hour, the flask should contain 8000 bacteria. The key concept of exponential growth is that the rate of growth — the amount of species added with each reproductive generation — is increasing; in other words, the population size is growing at an ever-increasing rate. The population would have grown from 1000 to more than 16 billion bacteria after 24 cycles.

Logistic Growth

Only infinite natural resources allow for extended exponential growth; however, this is not the case in the real world. Individuals will compete (with members of their own or other species) for limited resources, as stated by Charles Darwin in his definition of the "struggle to survive." The good ones have a higher chance of surviving and passing on the qualities that made them successful to the next generation (natural selection). The logistic growth model was created by population ecologists to simulate the reality of limited resources. Exponential development cannot occur indefinitely in the real world due to limited resources. 

Exponential growth can be possible in areas with few individuals and plenty of resources, but as the population becomes large enough, resources will be depleted, and the rate of growth will slow. The rate of growth would eventually slow or stop. The carrying capacity is the population size determined by the maximum population size that a given ecosystem can support. In actual populations, an increasing population often exceeds its carrying capacity, and the death rate exceeds the birth rate, causing the population size to fall down to or below the carrying capacity. Rather than existing right at the carrying capacity, most populations fluctuate around it in an undulating fashion.

Chemotropism Biology

Tropism is described as an organism's involuntary orienting response to a stimulus. It often requires an organism's development rather than its movement. In tropism, the organism's response is often its growth rather than its movement. It may develop in the direction of the stimulus or away from it. Tropisms come in several different ways, one of which is chemotropism. A chemotropism is a form of chemotropism in which an organism's growth is determined by its response to a chemical stimulus. The entire organism or parts of the organism may be involved in the growth response. The answer to growth can be either positive or negative. Positive chemotropism occurs when the growth response is directed toward the stimulus, while negative chemotropism occurs when the growth response is directed away from the stimulus.

Chemotropism can be detected as the pollen tube grows into the ovules. This is because the ovary releases chemicals that affect pollen tube growth. Chemotropism, or positive chemotropism, is often seen in the roots that develop toward useful minerals. However, there are times when the roots expand away from toxic acids, which is known as negative chemotropism. Chemotaxis and chemokinesis are not the same things.

Conclusion

Growth is the development of a species from emergence or birth to maturity. Growth also refers to an increase in size as a result of cell multiplication and expansion. In some species, especially slime moulds, there is no normal pattern of growth, and the result is a formless cytoplasmic mass. The rate of growth of different components of an organism can have a significant impact on its ability to adapt to its surroundings. This can influence evolution, such as the evolution of limbs through natural selection. The process of growth is rarely imprecise and happens in accordance with a scheme that ultimately defines the individual's size and form.