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Chapter 14 Autotrophic Nutrition. Autotrophic organisms use an inorganic form of carbon, e.g. carbon dioxide , to make up complex organic compounds, with energy from two sources: (1) light and (2) chemicals .
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Autotrophic organisms use an inorganic form of carbon, e.g. carbon dioxide, to make up complex organic compounds, with energy from two sources: (1) light and (2) chemicals. When using light, the process is photosynthesis, as practised by all green plants. When using chemicals, the process is chemosynthesis, as practised by certain bacteria.
Photosynthesis is more common and important because: • It is the means by which the sun's energy is captured by plants for use by all organisms. • It provides a source of complex organic molecules for heterotrophic organisms. • It releases oxygen for use by aerobic organisms.
Equation for photosynthesis: Adaptations of the leaf for photosynthesis: 1 To obtain light (sunlight) 2 To obtain & remove gases (carbon dioxide & oxygen) 3 To obtain & remove liquids (water & sugar solution) 6CO2 + 6H2O C6H12O6 + 6O2 chlorophyll
14.1.1 Adaptations for obtaining energy (sunlight) To ensure plants are efficient to absorb sunlight, a leaf shows many adaptations: 1.Phototropism causes shoots to grow towards the light to allow the leaves to obtain maximum illumination 2.Etiolation causes rapid elongation of shoots which are in the dark, to ensure that the leaves are brought up into the light as soon as possible
3. Leaves arrange themselves into amosaic to minimize overlapping 4.Leaves havea large surface areato capture as much light as possible 5. Leaves are thin to reduce filtration of light into the lower layers 6. Cuticle and epidermis aretransparentto allow light through the photosynthetic mesophyll beneath
7.The palisade mesophyll are packed with chloroplasts and arranged with their long axes perpendicular to the surface to trap most light 8. Chloroplast within the cells can move – This allows them to arrange themselves into the best positions within a cell for efficient absorption of light
9. The chloroplasts hold the chlorphyll in a structured way – The chlorophyll is contained within the grana on the sides of a series of unit membranes. This presents the maximum amount of light and close proximity to other pigments.
14.1.2 Adaptations for obtaining and removing gases To ensure rapid diffusion of gases: 1 Numerous stomata are present in the epidermis of leaves. 2Stomata can be opened and closed by differential expansion of the cell walls of the guard cells surrounding the stoma 3 Spongy mesophyll possesses many airspaces to provide uninterrupted diffusion of gases between the atmosphere and the palisade mesophyll
14.1.3 Adaptations for obtaining and removing liquids 1 A large central midrib containing a large comprising xylem and phloem tissue. Xylem transports water and minerals to the while phloem conducts away food, usually in the form of sucrose. 2 A network of small veins to ensure a constant supply of waterand removing the sugars. Its sclerenchyma associated provides a frame work of support to the leaves to present maximum surface area to the light.
14.2 Mechanism of light absorption 14.2.1 The nature of light There are 3 features of light which make it biologically important: 1 spectral quality (colour) 2 intensity (brightness) 3 duration (time)
14.2.2 The photosynthetic pigments Most important are chlorophylls a and b which absorb light in the blue and the red regions of the visible spectrum. Green is reflected thus gives chlorophyll its characteristic colour.
Structure of chlorophyll: a porphyrin ring (hydrophilic) lies on the thylakoid membrane surface, a long hydrocarbon tail (hydrophobic) embedded in thylakoid membrane
carotinoids Other pigments: carotenoids – carotenes xanthophylls • colour ranges from yellow, through orange to red, • depends on number of double bonds (deeper colour with more double bonds) • colour usually masked by chlorophylls but apparent when chlorophylls break down in autumn,
OR in many flowers and fruits -they absorb lights in the blue-violet spectrum --carotene as orange colour in carrots & a good source of vitamin A
14.2.3 Absorption and Action Spectra for common plant pigments
14.2.3 Absorption and action spectra An absorption spectrum is the degree of absorption at each wavelength by a pigment An action spectrum is the effectiveness of different wavelengths of light in bringing about photosynthesis Results show that the action spectrum for photosynthesis is closely related to the absorption spectra for chlorophylls a and b and carotenoids. This suggests that these pigments are those responsible for absorbing the light used in photosynthesis.
Raw materials: carbon dioxide and water Main product: carbohydrates; By-product: oxygen Light energy is changed into chemical energy trapped in the carbohydrate formed The nature of photosynthesis
Photosynthesis: an anabolic process It takes place in chloroplasts of green plants Chlorophyll (a green pigment) in chloroplasts absorbs lightas energy to drive the reactions of photosynthesis The nature of photosynthesis
Light reaction (in light only) & Dark reaction (in light or darkness) Light Reaction: water is split by light into hydrogen & oxygen (gas) Water hydrogen + oxygen sunlight chlorophyll The process of photosynthesis:
Dark Reaction: Hydrogen from light reaction combines with carbon dioxide to form carbohydrates (glucose) Water is produced as a by-product The process of photosynthesis: carbon dioxide + hydrogen carbohydrate (glucose) + water
14.3 Mechanism of photosynthesis Overall equation Experiments showed that rate of photosynthesis is affected by both light intensity and temperature. As temperature does not affect processes such as the action of light on chlorophyll, thus temperature only affects a purely chemical stage. 6CO2 + 6H2O C6H12O6 + 6O2
Photosynthesis is a process of energy transduction. Light energy is firstly converted into electrical energy and finally into chemical energy. It has three main phases: 1. Light harvesting in which light is captured by the plant using a mixture of pigments including chlorophyll. 2. The light dependent stage (photolysis) in which a flow of electrons results from the effect of light on chlorophyll and so causes the splitting of water into hydrogen ions and oxygen. 3. The light independent (dark) stage during which these hydrogen ions are used in the reduction of carbon dioxide and hence the manufacture of sugars.
14.3.2 Light stage (photolysis) - occurs in the grana of the chloroplast - Photolysis means the splitting of water by light - Photophosphorylation means light is involved in the addition of phosphorus (phosphorylation)
Process of photolysis: 1. Light energy is trapped in pigment system II and boost electrons to a higher energy level.
Process of photolysis: 1. Light energy is trapped in pigment system II and boost electrons to a higher energy level. 2. The electrons are received by an electron acceptor.
Process of photolysis: 1. Light energy is trapped in pigment system II and boost electrons to a higher energy level. 2. The electrons are received by an electron acceptor. 3. The electrons are passed from the electron acceptor along a series of electrons carriers to pigment system I which is at a lower energy level.
Process of photolysis: 1. Light energy is trapped in pigment system II and boost electrons to a higher energy level. 2. The electrons are received by an electron acceptor. 3. The electrons are passed from the electron acceptor along a series of electrons carriers to pigment system I which is at a lower energy level. The energy lost by the electrons is captured by converting ADP to ATP. Energy has thereby been converted to chemical energy.
4.Light energy absorbed by pigment system I boosts the electrons to an even higher energy level.
4.Light energy absorbed by pigment system I boosts the electrons to an even higher energy level. 5.The electrons are received by another electron acceptor.
4.Light energy absorbed by pigment system I boosts the electrons to an even higher energy level. 5.The electrons are received by another electron acceptor. 6. The electrons which have been removed from the chlorophyll are replaced by pulling in other electrons from a water molecule.
4.Light energy absorbed by pigment system I boosts the electrons to an even higher energy level. 5.The electrons are received by another electron acceptor. 6. The electrons which have been removed from the chlorophyll are replaced by pulling in other electrons from a water molecule. 7.The loss of electrons from the water molecule causes it to dissociate into oxygen gas and protons.
