Chapter 4 : Reproduction and Growth

Growth Curve

The Meaning Of Growth: * It can be described as a irreversible increase in body size, mass, and number of cells in an organism* The latter are quantitive changes that occurs during growth* However growth involves qualitative changes to cell

The Parameter Of Measuring Growth

* Growth in multicellular organism can be estimated by measuring some parameters like length,height, volume, dry mass and fresh mass over a certain period of time* Height or length is convenient but the problem is inaccurate. For example, a tree, while not increasing in height, it may continue to grow in diameter by growing sideways* Volume : more accurate but is frequently not practial to measure* Fresh mass : the measurement is inaccurate because involves temporary changes in water content* Dry mass : mass of organism after all moisture has been removed by heating

The Sigmoid Growth Curve Of An Organism* When growth parameter such as height or mass of an organism plotted against time, a growth curve is obtained * The pattern of growth is about the same for most organisms* The curve is S-shaped and known as a sigmoid curve

The Stages In The Sigmoid Growth Curve

* The sigmoid curve can be divided into five stages :

Lag phase - It is the initial stage of growth - The rate of growth is low - Mitotic cell division occurs rapidly but there is no increase in the size of the cell

Exponential phase - The rate of growth is the highest - Cells grow due to the absorption of water and the increase in the protoplasm

Slow Growth Phase - The rate of growth is small - The cells have reached an optimum size and no increase in size will occur again - A bit of cell division will occur to replace any dead or damaged cells

Stationary Phase - The rate of growth has reached zero - The division of cells still occurs to replace damaged or dead cells

Senescence Phase - The rate of growth is negative because the body loses part of its material when it becomes old - This stage ends with death

The General Growth Pattern of Humans

* The actual picture of the growth of an organism is not complete if only the increase in size is used. The growth pattern of an organism is also influenced by factors such as changes in the : - number of cells - shape - rate of cell growth* The life of a human from birth to death can be divided into five phases that involve changes in size, form and features: -infant phase - childhood phase - teenage phase - adult phase - senescenece phase

The Human Growth Curve

* A newborn baby experiences a fast rate of growth* The growth rate decreases in the middle of the childhood phase* At the teenage phase ( sexual maturity ), the rate of growth increses again before decreasing when approaching the adult phase* At the adult phase, the growth of most parts of the body stops except for the skin, nails and hair which keeps on growing until death* At the senescene phase, growth becomes negative ( loss of body weight ) until death

The Growth Patterns At The Embryo, Foetus and Infant Stages

* The growth curve of a man from embryo → foetus → infant which is a sigmoid curve* Around the 5th month, the foetal growth rate increases exponentially* During this time, the cells in the foetus differentiate rapidly to form specialised cells, such as muscle and nerve cells* After this period of time, less specialised cells are formed and this causes the growth rate to decrese

Differences Between The Growth Patterns Of Males And Females

The Growth Curve Of Animals

* The growth curve of a white mouse. The early part of the graph has a sigmoid shape* For some organisms such as the fish and the crustacean, growth continues even after the adult stage but at a slower rate

* The pattern of growth of animals with exoskeletons like insects is in the form of steps as ecdysis ( or moulting ) occurs a number of times before these animals reach the adult stage

Growth Curve Of Plants* The growth curve of an annual plant ( e.g. maize plant ) can be obtained by measuring its dry massStage AB* The weight of the seed decreases during germination as the food in the cotyledons is used for respirationStage BC* The plant carries out photosynthesis* Its weight increases as the synthesised food exceeds food that is oxidisedStage CD* The growth rate slows down when the plant reaches maturityStage DE* The rate at which food is synthesised is the same as the rate at which food is used, which results in zero causing growthStage EF* Weight decreases because the rate of photosynthesis decreases* At point F the plant dies

The Growth Patterns Of Animals With Exoskeleton

* Two types of growth patterns in animals are discussed below :

- Continuous Growth is seen in humans and animals with endoskeletons

- Intermitted Growth ( discontinuous growth ) is seen in animals with exoskeletons such as crabs, prawns, scorpions, and insects. The growth curve of animals which have exoskeletons is in the form of steps. Their hard exoskeletons obstruct continuous growth, so that it is not continuous like the growth of humans and animals with endoskeletons

* The exoskeleton of an insect ( such as the grasshopper ), called cuticle, consists of chitin which is hard and rigid

* Hence, for the young animal with an exoskeleton to grow, it has to change its external skeleton every timeit grows. This process is called ecdysis

ECDYSIS* For an insect such as the grasshopper, ecdysis takes place each time it grows, right until it reaches the adult stage* During ecdysis, the old cuticle softens and a new exoskeleton forms below it. The old cuticle is then discarded* The insect then swallows and retains air to inflate the new soft cuticle until it hardens* While the body of the animal is inflating, the existing exoskeleton break off* Before the new exoskeleton hardens, growth takes place actively to increase the size of the insect* This active growth during each ecdysis gives rise to the growth curve in the shape of a staircase

LIFE CYCLE OF GRASSHOPPER