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.