What is growth and development in Biology

What is growth and development in Biology

Growth 

Growth is the permanent and irreversible increase in dry mass and size of an organism. Dry mass is the mass of the organisms when all the moisture content has been removed. Growth involves the following processes: Cell division, Cell expansion and Cell differentiation.

Cell division

Cell division is the division of the parent cells to form daughter cells. Cell division here, involves parent cells dividing mitotically to form daughter cells with the same chromosome number. This process results into increase in number of cells and division of cells.

Cell expansion/elongation/enlargement 

This is the irreversible increase in the size of the cells as a result of osmotic in flow of water and synthesis of cellular materials. It involves assimilation where absorbed soluble materials are utilized by the cells to form or synthesis of new cellular materials.

Cell differentiation 

This is the specialization of the cells to perform particular functions. During differentiation, cells attain and synthesize different structures and become modified to perform different functions. As growth takes place cell development also occurs. 

Development 

Development is the change in shape, form, structure and physiology resulting in an increase in the degree of complexity of an organism during growth.

Morphogenesis

This is the development of structure and form of individual organs and the whole organism through its growth and development. It is the origin and development of morphological characteristics. During growth, there is positive and negative growth.

Positive growth: Occurs when the rate of anabolic reactions is higher than the rate of catabolic reactions resulting into a new increase in dry mass.

Negative growth: This is when there is net decrease in dry mass of the body of an organism due to the rate of catabolic reactions being higher than the rate of anabolic reactions. E.g. during senesce and during utilization of the stored organic food substance at the onset of seed germination. 

Factors affecting Growth

These are: Oxygen, Temperature, Nutrients, Carbon dioxide, Water, Metabolic wastes, Light (intensity, Wave length and duration), Seasonal changes, pH. 

Oxygen: Increase in oxygen concentration increases the rate of growth of aerobes while decrease in oxygen concentration reduces growth of aerobes. This is because optimum oxygen is used for oxidative breakdown of soluble absorbed food substances to release energy in form of adenosine triphosphate needed for anabolic growth processes and active uptake of nutrients required for growth. Obligate anaerobes however, are poisoned and killed by presence of oxygen hence no growth.

Temperature: At optimum temperature the growth rate is high because of maximum enzyme activity for the growth process to occur. When the temperature is above optimum growth decreases or many completely cease due to denaturing of the growth enzymes and below optimum temperature, the rate of growth is low because growth enzymes are inactivated e.g., below 0oC growth stops due to inactivation of growth enzymes.

Note: Different species have different ranges of optimum temperature needed for their maximum growth e.g. tropical plants need a higher range of temperature of 30- 40oC while the temperate plants have an optimum temperature of 20 – 30oC. 

Excessive high temperature is lethal to organisms, causes complete enzyme denaturation and drying out (desiccation).

Treatment of some dormant seeds with low temperature (pre-chilling) can break seed dormancy and induces the seed embryo to start germinating. 

Temperature may also affect growth in plants by altering the rate of photosynthesis which provides metabolites for growth processes.

Nutrients: When the nutrient level is high, the growth rate of organisms is high because nutrients are used in different growth processes such as; synthesis of growth metabolites, photosynthesis in plants e.g. nutrients likecalcium is used for root, bone and teeth formation.

Magnesium is used for chlorophyll information, nitrates are used for synthesis of proteins including enzymes, phosphorus is used for formation of ATP and certain enzymes. Lack of such adequate nutrients results into low growth rate, deficiency diseases and even death of organisms.

Carbon dioxide: This is of a very profound influence towards growth of autotrophs and shelled animals. Because carbon dioxide is a metabolite for photosynthesis its presence increases the photosynthetic rate to a certain level leading to formation of sugars which are used for growth processes.

Complete absence of carbon dioxide ceases photosynthesis because leaves depend on optimum amount of carbon dioxide for the large amount of food needed for high growth rate. But when carbon dioxide is of low level, photosynthesis rate is reduced thereby reducing growth rate. 

In shelled organisms a high carbon dioxide concentration results in high rate of shell formation hence high rate of growth and when carbon dioxide is in low concentration, low rate of shell formation and low growth rate occurs because carbon dioxide is a metabolite.

Note: When the level of carbon dioxide in organisms is exceedingly high, their growth reduces because it can lower the pH of the body from the normal which lowers the activity of several body enzymes.

Water: When enough water is provided to the body of an organism, its growth rate increase  at a high rate and when there is insufficient supply of water to the body , it growth rate reduces.

This is because water is used for the following: It dissolves the solute metabolites of growth into a solution or suspension a form in which materials are made available for use by the cells for their growth processes;.

Water used as a medium for transport/translocation of growth metabolites/substances like hormones from the storage/site of production to the growth sites of the body;.

Water is used during hydrolysis of large organic molecular compound needed for growth into simple soluble forms;. It is used for cell elongation;.

It activates hydrolytic enzymes of seed embryo for germination to occur;.

Water activates the seed embryo to secrete gibberrellic acid which stimulates synthesis of hydrolytic enzymes, used to hydrolyse the stored food materials into simpler soluble and usable forms;.

It is used to soften and rupture the seed coat for the plumule and radicle to emerge at the onset of seed germination;. It is used as a metabolite for photosynthesis. 

Metabolic wastes: These mainly affect growth of lower and primitive organisms e.g. the yeast cells, when they accumulate ethanol in high concentrations, the growth rate of these primitive organisms reduces because ethanol poison up the cells e.g. when the level of ethanol accumulates to 15%, yeast cells are poisoned. When the level of metabolic wastes is low, the rate of growth is high in such organisms. 

Advanced plants and animals are not affected by metabolic wastes, because the animals have effective mechanisms of getting rid of the wastes and the plants produce few or less toxic wastes in small amounts and such wastes are either deposited in tissues which fall off the plant or diffuse out of the plants.

Light, Light intensity, Wave length, Duration (photo period) and Light intensity, mainly affects plants and photosynthetic organisms which carry out photosynthesis.

When light intensity is high, the rate of growth is high due to high rate of photosynthesis and large amounts of food accumulate which is used for fast growth processes. When the intensity of light is low, there is low rate of growth due to low rate of photosynthesis.

Some seeds germinate in absence of light and cannot germinate in presence of light thus light hinder growth in such plant seeds. Light indifferent seeds germinate in both darkness and light conditions.

Light wavelength/equality: When green plants are provided with blue and red wave length of light, they grow at high rate because they tend to photosynthesize at a high rate using those two suitable wavelength of light and when other wave length of light other than red and blue light are provided, photosynthesis occurs at low rate or can hardly take place leading to low growth rate.

Most short wavelength of light inhibit growth from taking place and cause death of organisms e.g., ultraviolet radiation, infra-red etc. Red and far–red light have antagonistic influence towards growth of plants. Red light: inhibits stem elongation, stimulates leaf expansion in dicots, stimulates greening of the leaves by promoting the conversion of protochlorophyll to chlorophyll, and stimulates development of chloroplasts by promoting the convention of etioplasts into chlorophyll, promotes flowering in long day plants and inhibits in short day plants, stimulates unrolling of leaves in grasses, stimulates seed germination in lettuce, inhibits lateral root growth, it unhooks the plumule.

Light enable Photomorphogenesis i.e. light influence change in form and structure of the plants hence growth. 

Light Duration: Some plants flower when exposed to a longer duration of light than darkness in a 24 hour cycle. These are referred to as long day plants. Some plants flowers when the duration of darkness is longer than its critical length in a 24 hour have cycle are called short day plants.

However there is another group of plants which the flower independent of photo period and they are called day neutral plant. Therefore, growth/flowering is influenced if a particular plant is exposed to the necessary light duration.                                                                                  

Seasonal influence on growth: Seasons do influence growth in different ways: Some seasons e.g. winter result into cessation of growth of some organisms as they undergo dormancy. Some plants growth reduces in winter and during hot and dry conditions where some plants shed off their leaves to survive extreme coldness and minimize water loss by transpiration respectively. In spring and autumn growth rate in perennial plants is low due to increase in temperature and light intensity. In summer, the growth rate of perennial plants is high due to higher temperature and light intensity used for high rate of photosynthesis. In hot and dry seasons the growth of some animals e.g. lung fish stops since they undergo aestivation.

During winter season or very low environmental temperatures, some plants are induced to flower due to low temperature i.e. vernation (the arrangement of bud scales or young leaves in a leaf bud before it opens) also occurs while also dormancy of some seeds is broken due to cold treatment or chilling of the seeds which reduces the level of growth inhibitors of the embryo.

pH: Most of the growth processes are enzymes catalyzed and thus there is need to provide an optimum pH for the maximum enzyme activity which leads into high growth rate in the presence of an appropriate pH. pH medium mainly affects  the growth of lower and primitive plants and animals which cannot efficiently maintain  a constant  pH medium of their body e.g., removal of carbon dioxide which lowers the pH.

Measurement of Growth 

This can occur at three levels which include: Growth of the cell, Growth of an organism and Growth of a population.

The cell undergoes growth change by undergoing cell division cell expansion and becomes specialized to perform a particular functions i.e. cell differentiation. The growth of unicellular organisms is measured by counting the number of cells in a small known volume of a medium within a particular period of time.

Growth determining the growth of the shoot, root, and length is the best parameters to use. When change in mass or length of an organism is plotted against time, a sigmoid growth curve is obtained. When mass is used to measure growth in multicellular organisms, two of its types may be considered which include: Fresh mass and Dry mass. 

Fresh mass: is the mass of the living organisms under normal conditions including its moisture.

Advantages of using fresh mass:

It involves use of one organism for several measurement carried out;, it does not cause death of an organism;, it does not need elaborate preparation of the specimen or organism. 

Disadvantages:

It provides non reliable and inconsistent results due to changes in the level of water within the body. When we use fresh mass method, the organism is weighed every after specific time interval say of 24 hours and change in mass is recorded. 

Dry mass: Is the mass of organic matter of an organism after all the water has been removed. The organism is kept in an oven at 110oC to remove its moisture thereafter it is cooled in a desiccators. Further progressive heating and cooling of the body occurs, unit its constant mass is obtained. This method involves using many organisms of the same age and size every after a specific period of time.

Advantages of dry mass

  • It provides reliable, consistent and accurate results.

Disadvantages

  • It leads into injury and death of an organism;, it is tedious and difficult to carry out;, involves use of many organisms of the same age and size;, it needs elaborate preparation of specimen or sample of the organism;, the organism is used once.

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