Control and Regulation Higher Biology Unit 3
Growth and development Growth patterns in plants and animals Growth is the irreversible increase in the dry mass of an organism. To avoid killing the organism other factors, such as height or fresh weight, may be used to measure.
Growth patterns A graph of growth measurements taken during the life of an organism often shows an “s-shaped” curve.
(b) Meristems A meristem is a group of undifferentiated plant cells which are capable of dividing repeatedly. Animals do not have meristems, growth takes place all over the organisms.
Apical meristems These increase the length of stems and roots. They are found at the tips of stems and roots. Cell division here produces primary tissues.
ROOT TIP ROOT HAIRS Elongation + Vacuolation zone Cell division zone Differentiation zone
Cell division zone: Following mitosis the resulting cells are small cubes with a dense cytoplasm. Elongation and vacuolation zone: The cells absorb water by osmosis causing them to elongate. Many small vacuoles appear in the cytoplasm. They eventually merge to form a large sap vacuole. Differentiation zone: Here, unspecialised cells become altered to perform a special function in a permanent tissue. e.g. Xylem vessels or phloem tubes.
2. Lateral meristems These produce an increase in the thickness of stems and roots. The tissues produced by lateral meristems are called secondary tissues and they cause secondary thickening.
Development of tissues in the stem When primary tissues are fully formed, the stem (in cross section) looks like this:
Inside each vascular bundle a narrow meristem called cambium arises. Cambium is a lateral meristem which produces secondary xylem and secondary phloem.
In time, the cambium extends between the vascular bundles where it continues to divide and produce a complete ring of secondary xylem and phloem.
Each year a new ring of secondary xylem is formed. After 4 years the stem will look like this:
The xylem vessels (cells) produced in the cambium in the spring are larger then those produced in late summer and autumn. This difference shows up as an annual ring. The inner core of xylem is called wood.
Regeneration Regeneration is the process by which an organism replaces lost or damaged parts. The ability to regenerate depends on the presence of relatively undifferentiated cells.
1. Angiosperms (flowering plants) These have extensive powers of regeneration. • Cuttings Sections of plant are cut off and then planted in the soil. The cutting is able to produce shoots and roots by regeneration.
(b) Tissue culture Growers can mass produce identical clones of plants which show desirable features.
2. Mammals Mammals have only limited regenerative powers. • Regeneration is restricted to the healing of wounds • Mending broken bones • The replacement of blood • The regeneration of damaged liver
Genetic control of growth and development • Jacob-Monod hypothesis of gene action in bacteria e.g. Lactose digestion by the bacterium E. coli. Lactose sugar is digested by E. coli into glucose and galactose.
The reaction is controlled by the enzyme ß-galactosidase. ß-galactosidase lactose glucose + galactose The enzyme is only produced by the bacteria when the substrate (lactose) is present.
The substrate therefore acts as an inducer in the following way: On the bacterial chromosome, three genes control the production of the ß-galactosidase enzyme.
Structural gene – codes for the manufacture of ß-galactosidase. • Operator gene – switches on the structural gene. • Regulator gene – produces repressor molecules which stop the operator switching on the structural gene.
Operon = structural gene + operator gene If lactose is absent: Repressor molecules prevent the operator gene from switching on the structural gene. No ß-galactosidase enzyme is produced.
If lactose is present: Repressor molecules are “mopped up” by some of the lactose. The operator gene is now free to switch on the structural gene. ß-galactosidase is produced.
2. Genetic control of metabolic pathways A metabolic pathway consists of several stages, each of which involves the conversion of one molecule to another during a break-down or synthesis process.
Each stage in a metabolic pathway is controlled by an enzyme, as shown in the imaginary example below. GENE 1 GENE 2 GENE 3 ENZYME 1 ENZYME 2 ENZYME 3 Metabolite D Metabolite A Metabolite B Metabolite C
If any one of the 3 genes is faulty then the enzyme is not made and the pathway is blocked. This happens with the illness phenylketonuria (PKU).
In an unaffected person, surplus amounts of an amino acid phenylalanine are converted to harmless substances by a the following pathway:
ENZYME 1 ENZYME 2 Phenylalanine Tyrosine Melanin In a PKU sufferer, a gene mutation means that ENZYME 1 cannot be made. Phenylalanine is then broken down into toxic wastes which can cause brain damage.
Essay practice Write an essay on: • The control of lactose metabolim in E. coli. (6 marks) • Phenylketonuria in humans. (4 marks)
3. Genetic control of cell differentiation Gene Activation Every cell contains every gene but some genes are switched on (activated) in all cells while other genes are switched off in cells where they are not required (e.g insulin formation genes only remain switched on in pancreas cells).
Genetic control of blood cell formation Differentiated red blood cells, phagocytes and lymphocytes are formed from undifferentiated cells by switching on of relevant genes (to make haemoglobin, antibodies etc) and the switching off of irrelevant genes.
Hormonal influences on growth Hormones are chemical “messengers” secreted into the blood by endocrine glands. They travel in the blood to target sites where they have their effect.
(a) Pituitary hormones The pituitary gland produces 2 hormones which affect growth and development:
Growth hormone Promotes growth by increasing amino acid transport into growing tissues, which stimulates protein production. 2) Thyroid-stimulating hormone Stimulates the thyroid gland to produce thyroxin. Thyroxin controls the rate of ATP synthesis in the cytochrome system and therefore the rate of metabolism and growth
(b) Plant growth substances (Plant hormones) Plant growth substances (hormones) which affect the growth and development of plants. Two important growth substances are • Indole acetic acid (IAA) • Gibberellic acid (GA)
(a) Auxins The commonest auxin is indole acetic acid (IAA). IAA is produced in apical meristems.
It moves back from the meristems in two ways: • Diffusion, over short distances, from cell to cell. • Translocation, over longer distances, in the phloem.
IAA affects plant growth in the following ways: At cell level (in meristems) • Increases cell division • Causes cell elongation by making cell walls more “stretchy” • Causes phototropism (shoots growing towards the light) by stimulating growth on the shaded side.