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Thursday, November 27, 2014

GROWTH

1. Growth is a:
  • characteristic of all living organisms.
  • permanent irreversible increase in size of an organism
  • due to increase in its dry mass.
  • continue throughout the life of an organism (eg: woody perennial plants)
  • may cease when organism reach maturity (eg: man  or other mammals)
2. Result is irresversible:
  • increase in volume of cytoplasm
  • Increase in number of cells
  • increase in differentiation of cells
3. Growth in unicellular organisms:

  • reaches certain size – divide to form two separate cells
  • size limited - due to distance of cell over which a nucleus can exert its controlling influence.
  • division - results in growth of populations.
4. Growth in multicellular organisms:
  • originate from a single cell (the zygote)
  • cell division – increase in number of new cells produced by mitosis
  • cell expansion (cell enlargement) – increase in size, volume + mass (due to assimilation of food materials +  metabolism)
  • cell differentiation:
           i. cells undergo progressive changes in metabolic activities:
               – lead to changes in cell structures
               – produce diff specialized cells/tissues for dif functions
           ii. in higher animals:
                – produce tissues (epithelial tissue, nervous tissue, muscle tissue + connective tissue)
           iii. in higher plants:
                – produces tissues (meristematic tissue, ground tissue +  vascular tissue).
 
5.   Morphogenesis
      a. result from:
  • development (growth+ differentiation) of overall form of organs
  • formation of multicellular organism
      b. cell differentiation cause by:
  • differential expression of genes.
  • certain metabolites activate/repress gene in dif. cells.
  • some hormones are able to switch genes on/off (help to determine overall pattern of development)
 

FACTORS AFFECTING THE PATTERN + RATE OF GROWTH (PLANTS + ANIMALS)

A.    External factors affecting the rate of growth in plants are:

  1. light (intensity, wavelength and photoperiod) – affects photosynthesis, photomorphogenesis, meristemic activities and flowering.
  2. Temperature which affects enzyme activities.
  3. Oxygen – require for aerobic respiration and seed germination.
  4. Carbon dioxide – for photosynthesis, synthesis of organic molecules.
  5. Water – essential for many metabolic processes (photosynthesis, seed germination, transportation of mineral salts/soluble organic material, vacuolation, maintenance of plant cell turgidity
  6. Relative humidity of atm – affects rate of transpiration.
  7. Minerals (K+, Na+, Ca2+, NO3+) for plant metabolism/growth/development.
  8. Gravity – affects distribution of growth regulators for geotropism
  9. Seasonal influences – affect dormancy, abscission, reproduction and photoperiodism
  10. Biotic factors -pathogen-caused plant diseases, intraspecific and interspecific competition for space, sunlight, water and mineral salt).
 
B. Internal factors are:

  1. Genotype of the plant – determines the synthesis of specific proteins and enzymes, the type of metabolism and size.
  2. presence and absence of growth regulators/plant hormones (auxins, gibberellins, cytokinins, absciccis acid, ethene)
 C. Animal growth + development are controlled mainly by:

  1. Genotype of the animal
  2. Animal hormones (thyroxine and somathrophin).
  3. Environmental conditions (lights intensity and photoperiod, temperature, oxygen, availability of water and seasonal influences.
  4. Biotic factors.

 
Parameter
Advantages + disadvantages
A
Height and length
Advantages
·      easy + quick to measure.
·      growth can be measured continually
Disadvantages
·       only measures the length in one dimension + does not take into account growth in other directions.
·       An organism not increasing in height - may continue to grow in girth or grow sideways (as in plant)
B
Surface area
Advantages
·      more accurate measurement.
·      eg: surface area of a leaf attached to a plant can be measure at regular intervals over a period of time.
Disadvantages
·       Dif. + impractical to carry out if the organism is irregular in shape.
·       It’s assumed that plant growth is directly proportional to the increase in the surface area of its leaves.
 
C
Volume, size
Advantages
·       Suitable for measuring organisms that are spherical or cylindrical in shape.
Disadvantages
·       Living organisms are rarely exactly spherical and cylindrical in shape
·       Not accurate measurement.
·       Dif. to carry out – complicated calculation for organism with irregular shape.
 
D
 
Dry mass
Advantages
·       More accurate measurement of the organic materials (protein present in the body.
Disadvantages
·       Water has to be removed by drying the organism.
·       This kills the organism, so continuous growth of the same organism cannot be measured
·       Have to use a large number of individuals from the same population which have similar age and size.
·       Dif. plants used may have dif, growth rates.
E
Wet mass or fresh mass
Advantages
·      Easy to measure
·      No need to kill the organism.
·      Growth can be measured continuously as repetitive measurements can be carried out.
Disadvantages
·       Measurement may be inaccurate due to fluctuations in water content in the body.
·       May be more suitable for measuring growth in animals but not for a living tree.