NEET Biology Organisms and population Growth

 

Growth

 

Introduction

An embryo and off springs body gradually enlarges and assumes the form and size characterstic for the adult of its species (growth). The animals carries on the various vital processes to maintain health and keep alive. In its body, the cell organelles are constantly renewed worn out cells are healed up (repair). In certain animals, even the lost organs of the body are regrown (regeneration). Since the animals have limited life span, their body starts undergoing degenerative changes showing sign of old age (ageing). The last events of which is death.

 

(i) Meaning and definition of growth: Growth is an important properties of all living organisms. All organisms grow from a young stage to an adult stage. Growth is a permanent increase in dimensions of the body and its parts. It results from the addition to the body tissues. Cleavage of a zygote produces a multicellular embryo without an increase in size. This process should be regarded growth though it does not confirm to the definition of growth as it is a developmental event and growth and development go together. Moreover, cleavage increase the number of cells. In simple form growth can be defined as “The increase in size and weight of an organism due to synthesis of new protoplasm”

(ii) Growth at different levels:

(a) Molecular level: At molecular level, the growth involves synthesis of new molecules and their aggregation into organelles and storage products in the cells.

(b) Cellular level: At the cellular level, the growth involves.

(1) Cell expansion (hypertrophy): Increase in the size of the cells due to addition of new cell material, called protoplasm.

(2) Cell division (hyperplasia): Increase in the number of cells by cell division.

(3) Cell differentiation: Specialisation of cells for specific roles, in its broad sense, growth includes.

(4) Matrix formation: Addition of intercellular materials, termed Apo plasmatic substances, secreted by the cells between them. The term protoplasm includes the nucleus as well as the cytoplasm and its organelles. The Apo plasmatic substances include the matrix of connective tissues and intercellular fluid.

(c) Individual level: At individual level, the growth is the visible increase in the body, dimension, size volume and weight. Increase in weight will show that the growth has taken place. Growth result from the –

(1) Increase in the protoplasm.

(2) Addition to the Apo plasmatic materials.

(3) Increase in the number of cells.

Each of these processes may occur at separate times. The growth starts in the embryonic period after laying down of the germ layers and continues for a long time in the postembryonic period. In the unicellular organism, such as bacteria and protozoans, cell division results in reproduction (not growth) of the individual and growth of the population.

Differences between Protoplasmic and Aprotoplasmic substances.

S. No.	Characters	Protoplasmic structures	Aprotoplasmic structures
(1)	Nature of structures	Living	Non-living.
(2)	Location	Intracellular	Extracellular.
(3)	Examples	Cytoplasm, cell organelles and nucleus	Matrix, fibres, minerals, etc.
(4)	Growth	Grow	Do not grow.
(5)	Division power	Can divide	Cannot divide

 

(iii) Physiological condition for growth: A variety of chemical reaction occur all the time in the living organisms. These are collectively referred to as metabolism. Metabolism has two phases building up phases or anabolism and break down phase or catabolism. Variation in the rates of the metabolic phases result in three types of growth.

(a) Positive growth:  Anabolism normally out weighs catabolism and this brings about growth during the growing period of the organism and maintain the body thereafter. This is called positive growth.

(b) Zero growth: If the anabolic and catabolic processes are balanced, there is no addition to the bulk of the body and no increase in body size. This is referred to as zero growth.

(c) Negative growth: If catabolism occurs at a faster rate than anabolism, as happens in fasting the organism gradually becomes weak and may finally die. At this time, first the food reserves (glycogen, fat) and then body's own protein are used as sources of energy to run the body machine. This depletes the living material, causing negative growth. Due to this the reserve food and living material decrease in amount and is called regrowth.

 

Differences between growth and regrowth

S. No.	Characters	Growth	DE growth
(1)	Rate of metabolism	Anabolism faster than catabolism.	Catabolism faster than anabolism.
(2)	Fate of living matter	Both protoplasmic and aprotoplasmic structures are synthesized so there is increase in the living matter.	Food reserves (fats and glycogen) are catabolised to provide the required energy so there is decrease in the living matter.

 

(iv) States of growth: Two states of growth are –

(a) Pre-functional state of growth: It is the early embryonic stage during which the developmental processes transform a zygote into an embryo. During this growth period the organ rudiments are established but are not functional.

(b) Functional state of growth: It is the late embryonic and post embryonic developmental stage during which organ rudiments become functional and organogenesis begins.

 

 Differences between Embryonic growth and Post-embryonic growth

 

S. No.	Characters	Embryonic growth	Post-embryonic growth
(1)	Period of occurrence	During pre-natal (before birth) period, e.g. during blastulation and gastrulation.	During post-natal period.
(2)	Cell growth	Does not occur.	Occurs.
(3)	Nature of cells	Cells only divide so size of blastomeres becomes smaller and smaller.	Cell division occurs after the cell growth so size of cells remains nearly same.
(4)	Nature of organs	Only organ rudiments are formed but are non-functional.	Organs have been fully developed and are functional.

 

(v) Biological activities of growth: Growth of a multicellular organism is governed by two main biological activities.

(a) Cell growth    (b) Cell reproduction

(a) Cell growth: Growth and division of a cell occur in three cyclic phases:

(1) G₁-Phase: It involves, pooling of amino acids and nucleotides for the synthesis of protein and nucleic acids. A newly formed cell grows by synthesizing carbohydrates, lipids, proteins, RNAs, ATP and enzymes to loosen and unfold the DNA.

(2) S-phase: It  involves, replication of DNA, so each chromosome now consist of two sister chromatids joined at the centromere and carries a duplicate set of genes. A diploid cell (2n), thus, becomes tetraploid (4n) at the end of S-phase. Synthesis of histone protein of the chromosome.

(3) G₂-phase: It involves, the cell grows further, synthesizing more protein and RNAs and doubling the organelles such as centrioles, mitochondria, Golgi apparatus. The G₂ phase prepares the cell for its division.

The growth of individual cells is most essential factor of growth in all multicellular animals.

(b) Cell reproduction: It occurs during the M-phase of the cell cycle during which a fully formed adult cell undergoes mitosis to produce two genetically similar daughter cells which repeat the process.

 

 

 

 

 

 

 

(vi) Strategies of growth: Growth is accomplished by three strategies:

(a) Cell proliferation: The growth of a structure by cell multiplication due to cell division e.g. growth of lens.

(b) Cell enlargement: In this, cell do not divide but their size increase due to synthesis of more cytoplasm e.g. growth in cardiac muscles, neurons and skeletal muscles.

(c) Growth by accretion: In this, growth occurs due to secretion of large amount of extra-cellular materials e.g. growth of cartilage and bones.

(vii) Types of growth: In animals body four basic types of cellular growth are recognised.

 

(a) Auxetic growth: In some organisms growth occurs as a result of increase in the size of their cells. The number of cells remains the same. It is a rare type of growth and is found in a few nematodes (Ascaris) rotifers, tunicates (Herdmania) etc. Auxetic growth is also found in certain tissues of higher animals. Growth of a body muscle resulting from regular exercise is due to increase in size of the individual muscle fibres rather than to an increase in their number. Infant's heart has the same number of cells as the adult's heart even though it is only about 6% of its size and weight. Increase in the size of an organ due to enlargement of cells is termed hypertrophy.

 

(b) Multiplicative growth: In this, the growth occurs due to an increase in the number of cells of the body by rapid mitosis division an appreciable growth of the cells. It involves both cell growth and cell reproduction. It is found in the embryo. For example an adult human is made up of some 60 trillion (6×10¹³) cells, while the new born baby contains only about 2 trillion \[\left( 2\times {{10}^{12}} \right)\] cells. In this type of growth, the average cell size remain the same or increases insignificantly. Growth of embryo, young ones and prenatal growth in mammals is of this type. Increase in the number of cells in a tissue is called hyperplasia.

 

(c) Accretionary growth: During postembryonic growth and also in the adult, all the body cells are incapable of undergoing division. The differentiated or specialized cells of organ and tissues lose the ability to divide. The undifferentiated cells (reserve cells) present at specific location in the body divide mitotically and help in growth. This kind of growth is called accretionary growth. Malpighian layer of the epidermis in vertebrate skin consist of undifferentiated cells. These cells divide to form new cells that replace the epidermal cells lost to the environment at the surface. Red bone marrow of vertebrates contains unspecialized cells, called pluripotent stem cells, which continuously produce blood cells to replace the worn out ones. Germinal epithelium of the gonads consists of undifferentiated cells. These cells, by mitotic divisions, produce new cells that give rise to the gametes. The archaeocytes of sponges and interstitial cells of coelenterates are also reserve cells meant for replacing the other types of cells in the bodies of these animals.

(d) Appositional growth: It involves the addition of new layers on the previously formed layers. For example, the addition of lamellae in the formation of bone. It is characteristic mode of growth in rigid materials.

 

Differences between auxetic and multiplicative growth

 

S. No.	Characters	Auxetic growth	Multiplicative growth
(1)	Nature of growth	Body growth occurs only due to increase in size of the body cells.	Body growth occurs due to increase in number of the body
(2)	Number of cells	Number of cells remains same, but the size of cells increases.	Number of cells increases but size of cells remains nearly same.
(3)	Occurrence	Nematodes, tunicates, etc.	During embryonic period of development in most of vertebrates.

 

(viii) Growth rate in animals: Growth is the perceptible and measurable increase in the mass of living materials and can be confirmed by an increase in weight of an animal.  All higher animals, including man, grow at a specific rate and rhythm. The growth rate is not uniform but is different at different periods of life, so the growth is differential.

Growth period in human may be divided into 5 stages:

(a) Prenatal stage: It comprises about nine months of embryonic life.

(b) Infantile stage: It extends from birth to 10 months of age.

(c) Early childhood stage: It extends from 10 months to 4 or 5 years of age.

(d) Juvenile stage: It extends from 4 or 5 years to about 14 years of age, i.e., upto the time of puberty.

(e) Adolescent plus post adolescent stage: It extends from 14 years to 20 or 22 years of age.

Maximum growth in human foetus occurs at the age of four months (growth occurs at the rate of 10 cm per month). Growth is rapid in the pre-natal and puberty period (14-18 years); it is slow in the juvenile (5-14 years) and post-adolescent (18-22 years) but is almost nil after the post-adolescent period as there is no addition of living matter in this period. The growth stops at adulthood (22-23 years).

 

(ix) Growth curve: The growth rate in an individual at different periods of life can be represented in a curve by plotting the weight of individual at different time intervals (in years) on graph paper.

 

 

(a) Sigmoid curve: Growth curve of higher animals, including man, is S-shaped and is called sigmoid growth curve. This growth curve proves that:

(1) First rises very slowly, showing a low rate of growth.

(2) Then rises steeply, indicating fast rate of growth.

(3) Its rise again slow down

(4) Finally it starts running horizontally, depicting stoppage of growth.

Its 4 phases are respectively called lag phase, exponential (log) phase, senescent (decelerating) phase and steady (plateau) phase. The point where the exponential growth begins to slow down is known as inflexion point.

(b) Absolute growth: The difference between the initial and final weight (or size) of an individual in a given period of time is called absolute growth.

(c) Variation in steady phase: The nature of the curve during the steady phase may vary in different species. In some cases (many invertebrates, fish and certain reptiles) the curve may continue to rise slightly till the animal dies. This is a case of positive growth. In some cnidarians, the curve flattens out showing stoppage. In many mammals including humans, the curve slowly tails off, showing degrowth or negative growth due to physical weakness caused by ageing.  

(x) Patterns of growth: Growth patterns may be viewed from two aspects: body proportions and duration of growth.

(a) Regarding the body proportions, two patterns of growth are commonly seen in animals:

(1) Isometric growth: In this pattern of growth, an organ grows at the same mean rate as the rest of the body. The external form of the body does not change as the organism grows in size and the form and size remain proportional. Fish and certain insects, such as locust, show isometric growth.

 

 

 

(2) Allometric growth: In this pattern of growth, an organ grows at a rate different from that at which the body grows. The external form of the organism changes as the body grows in dimensions. This type of growth is seen in human body parts.

 

Differences between Isometric growth and Allometric growth

S. No.	Characters	Isometric growth	Allometric growth
(1)	Nature of growth of organs	At same mean rate.	At different rates during different periods.
(2)	Body form and size	Remain proportional.	Becomes unproportional as body parts grow at different rates.
(3)	Examples	Fish and insects (e.g. locust).	Birds and mammals.

 

(b) Regarding duration of growth, animals show two basic patterns:

(1) Limited (definite, determinate) growth: Their growth stops when the size characteristic for the species is attained. Example – Insects, birds and mammals. Arthropods show discontinuous growth due to an inelastic exoskeleton. They grow in spurts for short periods after mounts.

(2) Unlimited (indefinite, indeterminate) growth: They keep growing throughout life. Example – Non-vertebrates, fishes and reptiles.

(xi) Differential growth of human body parts: In human being, similar to other animals, different body organs or body parts (head, neck thorax and limbs etc.) do not grow at the same rate. The growth rate of different body parts is different.

If we keenly observe the growth of these body parts by comparing their photographs from birth for a number of years till these attain their final shape, size and weight e.g. head of a newly born human baby is proportionately larger than the rest of its body. It forms about half of the length of two month old foetus. But its growth stops early in the childhood so the head of an adult person is proportionately smaller (about one-eighth of whole body) than that of a newly born baby (about one-forth of whole body). The arms attain their proportionate size shortly after birth but legs attain their proportionate size only after 10 years of age. Reproductive organs do not grow rapidly until 12 to 14 years after the birth. These show faster growth during puberty (14–18 years). The brain and spinal cord grow rapidly in the childhood period and attain their adult size by nine years of age while thymus attains maximum size at the age of 20. In human beings, the muscles show maximum growth (from 0.8 kg in newly born baby to 30 kg in adult) while brain shows minimum growth (from 0.4 kg in newly born baby to 1 kg in adult) from birth to adulthood and show in table.

 

 

 

Changes in the weights of human body parts from birth to adulthood

S. No.	Body parts	Weight in kilograms
		New-born baby	Adult male
(1)	Muscles	0.8	30
(2)	Skeleton	0.4	10
(3)	Fat	0.8	10
(4)	Brain	0.4	1
(5)	Rest of body	0.9	19
	Total	3.3	70

 

(xii) Control of growth and development: The processes of growth and development are controlled by the information encoded in the genes (DNA). Growth, however, is the result of an interaction between DNA and environmental factors, internal (hormones, growth substances) as well as external (food, oxygen).

(xiii) Hormonal control of human growth rate: Throughout the developmental period from birth to adulthood, the growth is controlled by hormones secreted by endocrine glands in the blood. But different periods of growth are under different hormones e.g.

(a) Growth rate in early childhood period and juvenile period (from 10 months to 14 years) is very slow and is controlled by thymidine hormone secreted by thymus gland. It is a pinkish coloured, beloved gland located in front of heart.

(b) During the late childhood period, growth rate becomes faster as along with thymidine, two more hormones start operating. Thyroxin hormone of thyroid gland and somatotrophic hormone (STH) or Growth hormone (GH) of anterior pituitary. Secretion of GH begins within the first year after birth and is regulated by Growth hormone releasing hormone (increases secretion of GH) and somatostatin (inhibits secretion of GH) of hypothalamus. Growth hormone also controls its own secretion by feedback signals to the hypothalamus.

(c) The growth rate reaches its peak during puberty, 14 to 18 years of age. At this time, the secretion of sex hormones (testosterone in the male; estrogens and progesterone in the female) begins, leading to the development of secondary sex organs and accessory sex characters. The person becomes full grown and sexually mature by the completion of puberty. After 18 years of age, the growth rate begins to fall and the growth almost stops at about the age of 22.

 

Approximate ages of sexual maturity in some mammals

 

S.N0.	Mammals	Age of Maturity
(1)	Human being	11-16 years
(2)	Asiatic elephant	8-16 years
(3)	White-handed gibbon	8 years
(4)	Fin whale	3 years
(5)	Rhesus monkey	2-4 years
(6)	Horse	1 year
(7)	Cat	6-15 months
(8)	Dog	6-8 months
(9)	Rabbit	6 months
(10)	House mouse	35 days