AL Bio notes Autotrophic nutrition. P.1
Autotrophic Nutrition
Autotrophic nutrition involves the use of an external en_____ source to make the complex org_____ moleculesfrom simple in______substances.
Photosynthesisoccurs in plants containing ch______.It involves the formation of organic compounds, generally from carbon dioxide and water, using sunlight as an e______source.
Chemosynthesisis performed by some b______. The raw materials are c______d______and w_____ and the energy comes from the ox______of various in_____ compounds. E.g. the nitrifying bacteria, oxidise amm_____ to n_____ and nitrite_____ to n_____.
Q1Look for other examples of chemosynthesis. What are the importance of these chemosynthesizers in the environment?
Photosynthesis is vital as the means of making complex organic substances available to animals, hence a photosynthesising plant at the beginning of almost* all food ch____. The oxygen released during photosynthesis is essential to support the resp______ of other organisms and to keep a bal_____ between carbon dioxide and oxygen in our atmosphere. The appearance of photosynthesizers in the early history of the Earth is an important step leading to the evolution of aer_____ organisms.
*Give an example of a food chain which is not based on green plants as the producer.
I)To MEASURE THE RATE OF PHOTOSYNTHESIS
1)by Measuring the Rate of Oxygen Released
The evolution of oxygen by aquatic plants can be measured by counting either the number of bubbles they produce per unit time, or by measuring the volume of gas produced. Q.2.0 What is/are the assumption(s) behind the methods?
2)by Measuring the Rate of Carbon Dioxide Uptake.
A control apparatus without a green plant is used to measure the amount of carbon dioxide present in a known volume of air drawn through the apparatus. In the apparatus shown, the green plant will absorb some of the carbon dioxide passing it. Equal volumes of air are drawn through both sets of apparatus simultaneously.
Q. 2.1Suggest a way to find the amount of CO2 taken up by the barium hydroxide solution.
Q. 2.2The result obtained is only a measure of the ‘apparent rate of photosynthesis’. What does it mean by the ‘apparent rate of photosynthesis’ ? What further steps should be taken to find the true rate of photosynthesis?
II)SITES OF PHOTOSYNTHESIS
In plants photosynthetic pig______are located within chloroplastsfound in leaves and green stems. The chloroplasts of many of the higher plants are able to move in cytoplasmic streaming of the cell and may alter their position as the light intensity varies. Many of the green algae are motile and can swim to orientate themselves with respect to light.
A)Leaf Anatomy
B)The Structure of the Chloroplast
In higher plants the chloroplasts are usually disc-shaped biconvex structures about 5 min diameter. Around the outside is a dou___membrane enclosing a str____.
This stroma appears to have no organised structure, but contains st____ grains and enz____ which can catalyse the reduction of carbon dioxide.
Embedded in the stroma are lam____ arranged one above the other and consisting of g_____ and intergranalregions. The grana are composed of a stack of membranes with chl______and electron car____ attached.
a)The chloroplast pigments
All molecules absorb electromagnetic radiation. Molecules that can absorb wavelengths in the vis___ region of the spectrum are called pigments. The color of leaves comes from the wavelength or combinations of wavelengths of light refl______off the pignents.
The two primary pigments involved in photosynthesis are chlorophyll a and chlorophyll b. These two molecules efficiently absorb light at the r__ and b___ end of the visible spectrum and not very efficiently at the middle of the spectrum. When chlorophyll a and b absorb both the blue and red light, we see what is reflected which is primarily g_____.
The existence of a number of pigments in green leaves can be demonstrated by chromat______:
i)Chlorophylls
The green pigments called chlorophylls are the ones chiefly concerned in the photosynthetic process. Chlorophyll a and b are the most common and the only forms found in the higher plants.
The molecule has a porphyrin ring with a mag______atom at its centre and a phytol (long chain alcohol) tail.
/ Absorption SpectrumThe percentage absorption of different wavelengths by an extracts of a pigment is called an ab______spectrum.
The absorption spectrum of Chlorophyll show absorption peaks at the b___-v____ and r__ ends of the visible spectrum.
Action Spectrum
When the rate of photosynthesis is measured for light of different wavelengths
the activity is plotted against the wavelength the result is an a_____ spectrum, which corresponds very closelyto the absorption spectrum of chlorophyll extracts.
Q.3What does this suggest about the role of chlorophylls in promoting photosynthesis?
ii)Carotenoids
Carotenoidsare lipid compounds widely distributed amongst plants and animals, they range in colour from yellow through orange and brown to purple.
The role of the carotenoids in the chloroplast appears to be two-fold. They ab____ light energy and tran___ it to the chlorophylls and secondly they pro____ chlorophyll against photo-oxidation. Mutant algae lacking carotenoids have been shown to die when grown in the light because their chlorophyll oxidises in its absence.
# Photosynthesis has optimized its light-absorbing capabilities by making a series of pigments covering all of the visible spectrum, called accessory pigments. These absorb light in between the red and the blue wavelength and then transfer it to chlorophyll to use in photosynthesis.
# Hydrogen carotenoids (containing only carbon and hydrogen) are calledcarotenes; those that also contain oxygen are called xanthophylls.
b)Relation of pigment to habitat in Algae
It is noticeable, particularly in algae, that the type of pigment present and the habitat of the plant are closely linked. Land plants and sha____ water plants are gr____, absorbing red and blue light for photosynthesis. In deeper water red light is quickly absorbed and only the blue end of the spectrum penetrates. Here brown algae are found, with additional xanthophylls which absorb blue light. Deeper still the red algae become more common, possessing pigments that absorb mainly blue light.
III)FACTORS WHICH AFFECT THE RATE OF PHOTOSYNTHESIS
A)General Factors
a)Light Intensity
In general, increased light intensity increases the rate of photosynthesis until another factor, usually carbon dioxide, becomes lim_____. At the usual carbon dioxide levels in the atmosphere the maximum photosynthetic rate is reached in many plants, at only quarter to half full sunlight.
Leaves of many plants sh____ each other as little as possible. This arrangement is called a leaf mosaic. On the other hand, very high light intensity causes destruction of cell constituents, decolorisation of chlorophyll and ultimately even the death of the cells.
As the light intensity increases, a point is reached when the intake of carbon dioxide for photosynthesis exactly b______ its output in respiration. This is called thec______point.Further increase in light intensity will result in carbon dioxide being ab____ and oxygen ev____.
- Trees in woodland communities are usually s__ plants,adapted to a high light intensity. However, theirseedlings are often capable oftolerating shade conditions.
4.1 Comment on the ecological significance of the different compensation points for sun and shade plants.
4.2Draw a table to show the morphological adaptations in sun and shade plants?
4.3 The utilisation of carbon dioxide in photosynthesis is much less temperature sensitive than the production of carbon dioxide in respiration. This fact has important bearings on glasshouse management. In seasons of low light intensity it is essential to keep temperatures at the safe minimum for the crop concerned. Explain why it is essential to do this.
4.4How could the safe minimum temperature be defined?
b)Quality of light
The action spectrum for photosynthesis has peaks in the red and blue regions, yet the wavelengths available to a plant may vary in different habitats.
Q. 5.1What would you expect to changes in the quality of light available under a woodland? What physiological adaptations are expected in the plants striving at the lower strata inside a woodland?
c)Carbon dioxide Concentration
/ Q.1The normal CO2 concentration in air is ~0.03%. What does the figure indicates about the role of CO2 concentration in the process of photosynthesis ?Under bright sunlight and a temperature of 20--25°C, with a plentiful supply of water, the amount of carbon dioxide in the atmosphere (0.03%)seems likely to be the l______factor for most plants. Increasing the level of the gas up to 1%* shows an increase in the photosynthetic rate of most species.
*Above this level the effect depends on the species. Often there is no corresponding increase in rate and in some plants such as tomatoes the leaves may begin to die as the carbon dioxide level rises.
d)Temperature
/ Q.Comment on the relationship between the rate of photosynthesis and temperature.Since the ‘dark’ reaction of photosynthesis is a series of en______-controlled reactions, it is not surprising that increasing the temperature speeds up the rate of the reaction. Photosynthesis has a Q10 of approximately two up to about 30°C, which is probably the op______temperature for most plants for short periods (about 30 minutes). Rates at higher temperatures for longer periods soon begin to drop, probably due to en_____ destruction, accumulation of e__ products and lim_____ availability of carbon dioxide.
About 40° C the rate falls off rapidly as the enzymes are de______. In temperate regions the range of temperature for photosynthesis for the majority of the plants is probably around 10- 35° C. Tropical species do not usually photosynthesise below about 5° C, whereas some conifers may continue to do so around freezing point.
e)Water
Less than 1% of the water absorbed by most plants is needed for photosynthesis, so it is n__ likely that its deficiency would affect the process directly. The indirect effects of water shortage however, are likely to retard photosynthesis, causing deh______which will affect the structure of protoplasm and hence its metabolism. The structural arrangement of the chloroplast is likely to be affected too.
*
It was originally thought that water deficit closing the stomata. would reduce carbon dioxide absorption, but there is much experimental evidence to suggest that even stomata appearing closed when viewed microscopically are still sufficiently open to admit the gas.
f)Mineral salts
Mineral salts are not directly involved as raw materials for photosynthesis, but shortage of either ir___ or mag______affects the rate of chlorophyll production and hence, indirectly, the photosynthetic rate. Some ions, such as potassium and some trace elements, such as molybdenum, are also required, though the exact function of some of them is obscure.
Macronutrients are required in relatively largeramountCarbon / C / Component of all organic compounds
Oxygen / O / supplied by air & water
Hydrogen / H / Component of all organic compounds
Nitrogen* / N / Part of chlorophyll, amino acids, proteins, nucleic acids
Phosphorus* / P / Used in photosynthesis and almost all aspects of growth, ATP, NADP+, nucleic acids
Potassium* / K / Activates enzymes, used in formation of sugar and starch
Calcium / Ca / Used in cell growth and division, part of cell wall
Magnesium# / Mg / Part of chlorophyll, activates enzymes
Sulfur / S / Part of amino acids and proteins
Micronutrients are required in trace amounts
Boron / B / Affects reproduction
Chlorine / Cl / Aids in root growth
Copper / Cu / Used in chlorophyll, activates enzymes
Iron / Fe / Used in Photosynthesis, cytochromes
Manganese / Mn / activates enzymes
Sodium / Na / Used for water movement
Zinc / Zn / Part of enzymes, used in auxins
Molybdenum / Mo / Used in nitrogen fixation
Nickel / Ni / Liberates Nitrogen
Cobalt / Co / Fixates Nitrogen
Silicon / Si / Makes tougher cell walls: enhances heat and drought tolerance
g)Oxygen
It has been found that for some plants, such as wheat, the atmospheric concentration of oxygen (21%) sl___ down photosynthesis. It may be that the increased availability of oxygen speeds up the respiratory rate allowing it to compete more favourably with photosynthesis.
B)Limiting Factors
Photosynthesis is a complex process, involving a great many steps and subrates. If any one of these slows down or is scarce then it may hold up the rest of the process and it is called alimiting factor.
Q.The effect of different light intensity on the photosynthetic rate of an intact leaf of a flowering plant measured at two different concentrations of carbon dioxide (300 and 600 parts per million (ppm) was studied and the results summarized in the following table:
Photosynthetic rate (Net C02 fixed in mg per 100 cm2leaf area per hour)
Light intensity (kilolux) / 300 ppm C02 / 600 ppm C020 / -2.0 / -2.0
4 / 1.5 / 2.4
8 / 5.0 / 6.8
14 / 10.4 / 13.4
18 / 14.0 / 17.8
22 / 15.0 / 22.3
28 / 15.0 / 29.0
32 / 15.0 / 30.0
50 / 15.0 / 30.0
80 / 15.0 / 30.0
a) Present the results in graphical form.5m
b) Describe how the light intensity affects the rate of photosynthesis of the leaf.3m
c) How does the performance of the leaf differ when put into the two different carbon dioxide concentrations?4m
d) Account for the photosynthetic rate at zero light intensity. Identify and explain the points at which there is no net fixation of carbon dioxide by theleaf. 4m
e) With light intensity at 10 kilolux and CO2 concentration at 300 ppm, explain how you would expect the photosynthetic rate of the leaf to change under
i) light wind, and
ii) moderate water deficiency conditions. 4m
IV)THE PROCESS OF PHOTOSYNTHESIS
Put in a very simplified form the photosynthetic process begins with the absorption of l____ energy by the chloroplast pig______and this energy is then used to split w____. The h______obtained is used to reduce c_____ d______forming carbohydrate, o_____ is evolved as a waste product.
A)The Evidence for Light and Dark reactions
- Chemical reactions are affected by temperature; a 10C rise will approximately double the rate. Photochemical reactions are not much affected by temperature. Photosynthesis is affected by intensity of li____ and temp______, suggesting both photochemical and chem____ stages in the process.
- In the light, isolated chloroplasts were found to accumulate ATP and reduce a hydrogen acceptor NADPH, Oxygen is also evolved. In the dark, chloroplasts will synthesise carbo______if supplied with ATP, NADPH and carbon dioxide.
B)The light (dependent) Reaction
The light reaction consists of the photochemical spl_____ of water, providing h______atoms for the re______of carbon dioxide in the dark reaction and, secondly, the formation of an e_____ store in ATP, which can be used to power the dark reaction. The production of ATP by photochemical means is called photo-phosphorylation.
The light reaction begins with the absorption of light by chlorophyll. An 'ex_____' electron of the chlorophyll is elevated to a h______energy level than the normal ground state #1.
the excited electron is 'picked up' by an electron acc_____. the excess energy it possesses is available for use by the cell to make ATP either in Cy___ phosphorylation or Non-cyclic Photo-p______.
*If the electron merely lost energy and fell back into the chlorophyll molecule it would fluoresce.--- A chlorophyll extract shows a blood red fluorescence in reflected light.
Cyclic and Non-cyclic Photophosphorylation
In cyclic p______the e______ electrons are picked up by an electron a______.
The e_____e_____ are passed from the acc______to the cy______and then back to ch______.
As the e______electrons pass along the cyto______, energy is released and used up in the synthesis of ___. By the time the electron returns to the chlorophyll molecule it has lost all its excess energy and is back to its normal ground state.
- Cyclic Photophosphorylation
Light Reaction -- Cyclic Photophosphorylation / Dark Reaction
e- acceptor
cytochromes
excited e-
ADP + Pi
e-
Light /
ATP
2.Non-Cyclic Photophosphorylation
In n___-c_____ p______the excited electrons emitted by chlorophyll are picked up by an electron acc______.
Water tends to sp___spontaneously into H + and OH-ions. Some of the H + ions from the water and the excitedelectrons combine with h______acceptor NADP+ which is then re______(NADPH + H+).
In its reduced state NADPH enters the d___ reaction of photosynthesis, providing h______for the red______of carbon dioxide. After passing on its hydrogen the NADP+ can be recy___ and used again.
The hydroxyl ions from the water pass e______back to chlorophyll via the cy______(Electron Transport Chain) with consequent production of A___. The hydroxyl ions which have passed on electrons rearrange forming water and o______.
2 H2O 2H+ + 2OH-
4OH- 2H2O + O2 + 4 e-
This process is non-c_____, since electrons (and protons) are accepted by N____ and replaced from w_____.
Light Reaction -- Non-Cyclic Photophosphorylation / Dark ReactionNADP+
2 H+
2e-
excited e-
Light
2e-
ADP + Pi
2e-
2OH- + 2 H+ 2H2O
O2 / NADPH + H+
ATP
C)The Dark Reaction / the Calvin Cycle
a)The Calvin Apparatus and the elucidation of the dark reaction.
/ The individual steps of the reduction of carbon dioxide and synthesis of carbohydrates were traced by Dr Melvin Calvin, using a unicellular green algae..They supplied CO2 with radioactive isotope 14C to an illuminated suspension of the alga. The algae were exposed to radioactive carbon for a known length of time --from a few seconds to a few minutes, they then killed them rapidly in boiling methanol.
Using chromatography and exposing the chromatogram to sensitive photographic film they were able to discover into what substances the 14C had been incorporated.
They found that after about five seconds exposure to 14CO2 the radioactive carbon was mainly in a 3-carbon compound, phosphoglyceric acid (PGA). After thirty seconds it was chiefly in two 3-carbon sugar phosphate (triose phosphate), phosphoglyceraldehyde (PGAL) and dihydroxyacetone phosphate. Finally after about ninety seconds the carbon dioxide was incorporated into pentoses and hexoses.
b)Biochemistry of the Dark reaction
It is the series of enz______steps by which carbon dioxide is re_____ to sugars. Its occurs in the st______ of the chloroplast. The dark reaction does NOT actually require darkness; it is merely light-in-______. However, it relies on the light reaction to provide re______power and en______(ATP).
CO2