From The Student Room
TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Scottish Higher Biology - Unit 3 Summary
Control and Regulation
Growth differences
Growth- irreversible increase in dry mass of an organism due to an increase in cell number and size.
Meristem- a group of undifferentiated plants cells capable of division
Regeneration- replacement of lost/ damaged parts.
Meristems
Apical meristems
Increase the length of the plant, found in the root tips and shoots.
Activity of apical meristems
- Mitosis and cell division in a meristematic cell. This increases cell number.
- Elongation and vacuolation increases cell size
- Differentiation, the cell specialises to perform different functions
- Permanent tissues- xylem and phloem
Lateral meristems
Increase the girth of a plant, found in the stem – cambium
- Responsible for increasing the thickness of a stem. Known as secondary thickening.
- Cambial division is radial and a complete ring of cambium is formed around the stem
- Tangential division then occurs. Large amounts of Xylem are produced, and small amount of secondary phloem.
Annual rings
Arise form the different sizes of Xylem tissue formed throughout the year
None are formed in winter.
Springwood consists of large vessels with thin walls, and allows increased water flow and minerals for growth.
Autumn wood consists of small, thick walled cells, which provides support.
- A wider ring is usually the result of warm or wet weather
- A narrow ring is due to cold/ dry conditions or leaf loss.
Plants vs animals
Plant growth occurs in meristems throughout life
Animal growth occurs all over the body until adulthood, with growth spurts
Flowering plants have extensive ability of regeneration. Whole plants can be grown from small parts.
Regeneration in mammals is limited and occurs in some tissues only, e.g. liver and skin
Control of differentiation
All cells contain the same genetic info but are not identical
Switching particular genes on or off controls differentiation.
Different sets of genes are active in different types of cells.
Jacob-Monod hypothesis of lactose metabolism in E coli
- In the 1950s Jacob and Monod investigated the idea of switching on and off genes.
- They looked at control of enzyme synthesis in the bacterium E-coli
- E-coli only synthesise enzymes when they are needed
- E.g. if lactose is present in the medium in which the bacterium is growing, B galactosidase is produced.
- Lactose is the inducer
- B galactosidase breaks down lactose into glucose and galactose
- Therefore the gene for this enzyme must be induced by a specific substrate in the environment
Jacob-Monod hypothesis proposes that the structural gene contains the genetic information for the synthesis of the enzyme B glycosidase
The structural gene is under the control of the operator gene, which is under the control of the regulator gene
click here to see this in diagramatic form.
The regulator gene produces a repressor molecule
Lactose absent- repressor molecule binds to the operator gene and prevents it switching on the structural gene
When lactose is present, it binds with the repressor molecule, so that it cannot bind to the operator, so b galactosidase is produced
Click here and here to see the two stages.
The role of genes in phenylketonuria
- A metabolic path way is series of enzyme-controlled steps
- A particular gene codes for each enzyme
- If a fault occurs on the gene as a mutation, the enzymes will not be produced
- Usually mutated alles are recessive
- Only a recessive homozygous person will have a disorder known as an inborn error of metabolism
- Phenylalanine and tyrosine are amino acids obtained from the diet
- Excess phenylalanine is converted into tyrosine by an enzyme
- A sufferer of the hereditary inborn error of metabolism, phenylketonuria does not have the gene to code for the enzyme to convert phenylalanine
- Alternative metabolic pathways result in toxins building up
Hormones
Pituitary hormones
The pituitary releases
Growth hormone, somatatrophin- promotes growth by increasing cell division and amino acid uptake.
- Overproduction leads to enlargement of hands/ feet and jaw in adults and increased overall growth in children. click here for an image.
- Underproduction reduces overall growth in children.
Thyroid stimulating hormone (TSH)- controls thyroxin production by the thyroid gland.
- Over production leads to an increase in thyroxin, resulting in an increase in size and activity of the thyroid gland. This gives a swollen neck and bulging eyes. click here to see a typical goiter.
- Under production decreases thyroxin and decreases mental/ skeletal development in children and induces lethargy ad weight gain in adults.
Plant growth substances
Auxins Ie. Indole Acetic Acid (IAA)
- produced in active root and shoot tips.
- Promote division, elongation and differentiation of cells.
- Auxins can both stimulate and inhibit growth.
- High concentrations stimulate shoot growth and inhibit root growth.
- High concentration of auxin inhibits growth of lateral buds, this is called apical dominance. High concentration induces fruit growth of ovary wall.
- When low auxin concentration is present, abscission of leaves occurs and leaves fall.
Gibberelic acid
- GA overcomes dormancy of seeds and buds. It reverses genetic dwarfism by increasing the length of internodes
- GA induces alpha amylase production in barley grains. A very low GA concentration is produced in the embryo, where it passes to the aleurone layer. In this layer it induces alpha amylase production, this converts starch into maltose and provides energy for growth.
clicke here to see a cross section of a barley grain.
Applications of plant growth substances – Auxins
- Selective weed killer- narrow leaved grasses do not pick up sufficient concentrations to be destroyed, only broad leaved weeds killed
- Rooting powder- growth of adventitious roots stimulated.
Macro elements and plants
Plants for normal growth require macro elements.
Reduced levels of macro elements result in stunted growth and other deficiencies.
- Nitrogen - Needed for amino acid synthesis , Component of protein, nucleic acid and chlorophyll
deficiancy results in:
- Chlorosis (yellowing leaves)
- Red leaf base
- Long, thin roots
- Phosphorus Formation of ATP and nucleic acid, deficiancy results in Red leaf base
- Potassium, Membrane transport, deficiency results in Premature death
- Magnesium, Component of chlorophyll, deficiency results in Chlorosis
Comments
