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    I know about competition for light etc/ photosynthesis bla bla but its a whole coursework and I'm stuck on what exactly to write..

    Is there anyone who has detailed notes on this or knows where I could find some please?

    Totally lost here!!



    Have a look at the revision guides for AS foundation and A2 central concepts - lots of good info on there.

    Also a simple google search should yield helpful results

    See below pasted from some old notes i had..

    Effect of light intensity on the rate of photosynthesis.

    · Refer to practical on measuring rate of photosynthesis using a Photosynthometer.

    · At zero light intensity (in darkness) the rate of photosynthesis is zero. As light intensity increases the rate of photosynthesis also increases, but only up to a certain point. At this point the CO2 concentration or temperature, must be limiting the rate of photosynthesis.

    Example :

    · In curve C, CO2 concentration (0.01%) is the limiting factor.
    · In the curve B, temperature (15 °C) is the limiting factor.
    · In the curve A, the rate becomes constant because it has reached the light saturation point. This is the maximum rate of photosynthesis. Exposure to light intensities for prolonged periods can destroy Chloroplasts and causes to become bleached.

    Effect of CO2 concentration on the rate of photosynthesis

    CO2 is needed in the light independent stage of photosynthesis ( Calvin cycle ).

    The CO2 concentration of the atmosphere is about 0.035% or 350ppm ( parts per million ) by volume.

    This is far less than the optimum CO2 concentration for photosynthesis. Thus CO2 acts as a limiting factor. It has been shown that by increasing the CO2 concentration in greenhouses has increased the yield of tomatoes and lettuces.

    However, prolonged exposure to CO2 concentration of above 0.5% can cause closure of stomata.

    Effect of temperature on rate of photosynthesis.

    Temperature affects the enzymes involve in the Calvin cycle.

    Thus it influence the rate of photosynthesis. If other factors are not limiting then a 10°C temperature increase ( Within the range 10-35°C ) will lead to a doubling of the rate of photosynthesis.

    Usually a temperature of about 25°C is considered as the optimum temperature for photosynthesis.

    Increasing the CO2 concentration and temperature in a glasshouse can be achieved by burning high quantity paraffin (fuel). This burns without producing unwanted fumes and produces CO2 and heat at the same time.

    Effect of wavelength of light on rate of photosynthesis.


    8. Understand the concept of limiting factors; compensation point.

    Light intensity and compensation point

    The point at which the rate of photosynthesis is equal to the rate of respiration is called the light compensation point.

    O2 / sugar produced in photosynthesis synthesis = O2 / sugar used by respiration

    Law of limiting factors

    When a process is influenced by several factors, the rate at which the process proceeds is determined by the factor in the shortest supply.

    9. Appreciate uptake by roots of mineral ions; understand the function of nitrate, phosphate and magnesium ions.

    Uptake of mineral ions by roots - Refer to Unit 2 notes – in particular the diff pathways that nutrients can take in a root.

    Roles of mineral ions

    Nitrates: (NO3-) ;
    Essential for synthesis of amino acids, proteins, Nucleic acids, pigment molecules, coenzymes. Deficiency leads to reduced growth.

    Phosphate (PO43-) ;
    Required for synthesis of nucleic acids, phospholipids: component of nucleotides (ATP); Phosphate groups included in phosphorylation of intermediates in metabolism. Deficiency leads to retarded growth.

    Magnesium (Mg2+) ;
    Constituent of chlorophyll molecule; Activation of enzymes. Deficiency leads to chlorosis ( yellowing of leaves).

    10. Recall the detection of light in flowering plants by phytochrome pigments; understand the effect of light on the growth of plants;

    Refer to UNIT 4 - Phytochromes and photoperiodism

    11. Understand the nature of plant growth substances; explain the effects of auxins, cytokinins, gibberellins, abscisic acid and ethene on plant growth; understand the terms synergism and antagonism; understand the commercial applications of auxins.

    Growth in plants is coordinated by plant growth substances (PGS). These are produced in certain areas of the plant and transported to other parts where they can affect the cell division, cell elongation and cell differentiation.
    Plant growth substances are not specific and can affect different tissues and organs in contrasting ways.
    For example, High concentration of auxins stimulates cell elongation (growth ) in shoots, but, inhibits cell elongation (growth) in roots.
    However, low concentration of auxins stimulate growth in roots.

    Effect of Auxins on plant growth and commercial applications of Auxins.

    Promotes cell elongation

    auxins in coleoptile tips cause the coleoptiles to bend towards light (positive phototropism). The auxins move away from the illuminated side of the coleoptile and accumulate on the darker side, where it stimulates cell elongation (growth).

    The auxins soften the cell wall. The cell becomes less turgid and takes up more water, resulting in expansion of the cell. Due to the orientation ( pattern of arrangement ) of cellulose micro fibrils in the cell wall, cell elongation occurs in the longitudinal direction.

    Apical dominance:

    Auxins from the main apical bud (terminal bud) inhibits the growth of side branches from lateral buds (axillary buds) on stems. This is known as apical dominance. Gardeners usually cut off the apical buds to make plants grow bushy. This process is called pruning.

    Rooting in stem cuttings:

    Auxins like Naphthalene acetic acid (NAA) and Indole butyric acid (IBA) stimulate the growth of adventitious roots from a stem cutting. Dipping the end of a stem cutting in auxin containing powder dramatically increases the chances of the cutting developing roots and ‘taking’. However, excess rooting hormone may inhibit lateral root growth.

    Auxins help fruit to set :

    spraying of flowers with auxins greatly increase the natural success rate for pollination and fertilization. It can also bring about fruit formation even in the absence of fertilization. This is called parthenogenesis or parthenocarpy, which results in the formation of seedless fruit.

    Abscission of leaves and fruits:

    Auxin delays ‘falling off’ of fruits from the plant. This ensures that the fruit remains on the plant until it is harvested. It also delays abscission in leaves during the early stages but promotes abscission in later stages. If the supply of auxins from the leaves/ fruits exceeds that from the stem, the fruit/leaf remains intact.

    Synthetic auxins as weed killers:

    Some synthetic auxins disrupt the growth of plant cell. They increase the rate of cellular respiration and cause abnormal growth of internodes and rooting system, leading to death of the plant. These auxins are absorbed much more effectively by broad-leaved (dicotyledonous) plants than by monocotyledons and are therefore particularly useful for removing broad-leaved weeds from monocotyledonous cultures such as lawns and wheat fields.

    Effect of Gibberellins on plant growth and commercial applications of Gibberellins.

    This was first isolated in the 1930s from a fungus (Gibberella fujikuroi) growing on rice plants.
    The effects of gibberellins are:

    Reversing of genetic dwarfism:

    Dwarf varieties of peas and maize plants grow to their normal size when gibberellic acid is applied to it. However, it has no effect on the normal tall variety.

    Promotes cell elongation:

    Gibberellins increase the length of internodes if sprayed on sugarcane. This increases the yield of sugarcane.

    Promotes bolting in long day plants during short days:

    Chinese cabbage is the rosette form of the plant which develop during short days. During long days the rosette forms develops into a long flower stalk (bolt). By using gibberellic acid we can bring about bolting during short days.

    Remove need for cold period in vernalization:

    Carrots need to be subjected to a period of cold for them to flower. Application of gibberellins can remove the need for the cold period to induce flowering (vernalization).

    Breaks dormancy in seeds and buds:

    Germination of seeds and development of buds is stimulated by gibberellins.

    Cytokinins :

    These are growth regulators found particularly in regions of very active cell division.

    They are mostly extracted from seeds ( e.g. Zeatin from endosperm of maize/coconut ) where they seem to be involved in the growth of the embryo. They stimulate cell division, only in the presence of auxins.
    As auxins equally cannot stimulate cell division without Cytokinins, it appears that the two substances interact. They work together to affect the divisions of cells.
    This is an important difference between plant and animal hormones, which work independently.
    Some other processes, apart from promoting growth by cell division are:

    · Delay in leaf senescence (ageing).
    · Stimulate bud development.
    · Breaks dormancy in both seeds and buds.

    Abscisic acid:

    This is a growth inhibitor. It has an inhibitory affect on auxins, gibberellins and Cytokinins, and seems to be involved in the production of a weakened area of cells (abscission layer) at the base of a fruit or leaf which finally breaks as the fruit or leaf falls off (abscission).

    Other functions involve:

    · Retardation of growth in most plants.
    · Induces dormancy in seeds and buds.
    · Closes stomata in times of water stress.


    This is a gas produced in small amounts by plants from the amino acid methionine.
    Some of the functions of ethene are:

    · Ripens fruits by increasing the rate of respiration.
    · Breaks dormancy in buds in some plants.
    · Induces flowering in pineapples.
    · Promotes abscission in leaves.


    If two growth regulators act together and give a greater response than each regulator alone, the interaction is known as synergism.

    For example, the effect of auxins on growth is much more dramatic if gibberellins are present as well.


    If two growth substances have opposite effects then it is called antagonism. e.g.: Abscisic acid is usually antagonistic to the effects of auxins.
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