12th Class Biology Organisms and population Growth

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.

Growth at different levels

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

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

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

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

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

(iv) Matrix formation : Addition of intercellular materials, termed apoplasmatic substances, secreted by the cells between them. The apoplasmatic substances include the matrix of connective tissues and intercellular fluid.

(3) 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.

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.

(1) 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.

(2) 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.

(3) 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 degrowth.

States of growth : Two states of growth are :

(1) 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.

(2) 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

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

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

(i) \[{{G}_{1}}-\]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.

(ii) 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.

(iii) \[{{G}_{2}}-\]phase : It involves, the cell grows further, synthesizing more protein and RNAs and doubling the organelles such as centrioles, mitochondria, Golgi apparatus. The \[{{G}_{2}}\] phase prepares the cell for its division.

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

(2) 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.

Strategies of growth : Growth is accompalished by three strategies :

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

(2) 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.

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

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

(1) 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. Muscular hypertophy is also the auxetic growth.

(2) 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.

(3) 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

(4) 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.

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 :

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

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

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

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

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

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.

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

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

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

(iii) Its rise again slow down

(iv) 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.

(2) 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.

(3) 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.

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.

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.,

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

(2) During the late childhood period, growth rate becomes faster as along with thymosine, two more hormones start operating. Thyroxine hormone of thyroid gland and somatotrophic hormone (STH) or Growth hormone (GH) of anterior pituitary.