Join Date: Mar 2009
Mesophyll: The ground tissue (parenchyma) of a leaf, located between the layers of epidermis; it is structured into pallisade- and spongy parenchyma, housing chloroplasts which contain chlorophyll (site of photosynthesis).
Motility of Plants: Even though plants cannot move actively across a given area (except adventitious root development) it possesses various ways of motion, which requires ATP: Insectivory: Feeding upon animals; plants that are able to utilize proteins obtained from trapped animals, chiefly insects; e.g. venus fly trap, in which inward-lying cells are elongated by virtue of the high vacuole- turgor-pressure; once an insect touches the trigger-hairs the flaps snap shut rapidly (deflation of stretched cells consumes ATP), pressing the insect against digestive glands on the inner surface of the trap. The trapping mechanism is so specialized that it can distinguish between living prey and inanimate objects.
Tropism: (Gk. tropes, turning) A growth response involving bending, curving of a plant part towards (positive) or away (negative) from an external stimulus that determines the direction of movements. • Gravitropism: (L. gravis, heavy; also geotropism) Response of a shoot or root to the pull of the earth’s
gravity. Roots grow downwards, positive gravitropism (high levels of auxin inhibit growth in roots), whereas shoot grows upwards, negative gravitropism. Roots are more sensitive to the response of auxin than stems (high auxin concentration, inhibits root growth). Perception of gravity is correlated with sedimentation of amyloplasts (starch containing statolithic plastids) w/n specific cells of shoot and root.
• Heliotropism: (Gk. helios, the sun) Ability of the leaves and flowers of many plants to move diurnally, orienting themselves either perpendicular or parallel to the sun’s direct rays; also called heliotropism. Diaheliotropism: the movement of the leaves is such that the broad surfaces of the blades remain perpendicular to the sun’s direct rays, resulting in a higher photosynthetic rate.
Paraheliotropism: Avoiding direct sunlight during periods of drought by orienting leaf blades parallel to
the sun’s rays. Minimizes absorption of solar radiation, lowers leaf temp., and transpirative water loss. • Phototropism: (Gk. photos, light) Growth movement of cells or organs in which light plays a decisive
role, and is related to the direction of light as the controlling external factor; e.g.: growth of a plant toward a light source by the influence of auxin; the cells of the shaded side of the tip migrates from the light-side to the dark-side, resulting in a turn or bend (compare photoperiodism). Cytoplasmic streaming: The plasma in the cell rotates and circulates actively under the influence of light. Particles in the protoplast such as nucleus, mitochondria, and plastids are often carried passively. Chloroplastic Orientation: Active repositioning of chloroplasts in the cytoplast of the mesophyll cells.
i) Diastrophic: Dark-light position, with max. light absorption i.e.: perpendicular to incoming light.
i) Parastrophic: Bright-light position; reduced light absorption; i.e.: almost parallel to incoming light. • Thigmotropism: (Gk. thigma, to touch) Response to contact with a solid object as seen in tendrils. They
wrap around any object with which they come in contact, and so enable the plant to cling and climb. NADx: see coenzymes.
Oxaloacetate: ): The C4-molecule formed in C4- plants, i.e.: the krebs cycle; the product when Pi is split off from PEP. Depending on the species, oxaloacetate is either reduced to malate or converted to asparate (additional amino group NH2), both C4 -compounds. Malate or asparate is a mediator transporting CO2 used in the calvin cycle.
Pathogen: (Gk. pathos, suffering + genesis, beginning) An organism that causes disease. PEP or Phosohophenolpyruvate: The compound to which CO2 binds in C4-plants. PEP converts under the influence
of CO2 to oxaloacetate, which is either reduced to malate (malic acid) or asparate (extra amino group NH2). Peroxisome: A microbody that plays an important role in glycolic acid metabolism associated with photorespiration. PGA (3-phoshoglycocerate): The C3-sugar formed in C3- plants, i.e.: the calvin cycle. Phosphoryltation: (Gk. phosphorous, bringing light) A reaction in which phosphate is added to a compound; e.g.:
the formation of ATP from ADP (PSI), NADPH from NADP+ (PSII) and inorganic phosphate; see photosystem. Photo-P.: (Gk. photos, light) The formation of ATP in the chloroplast during photosynthesis. Cyclic Photo-Phosphorylation: A photosystem lacking PSII, yielding only small amounts of ATP an no NADPH; a very ancient type of photosynthesis.
Noncyclic Photo-Phosphorylation: The modern type of plant having both PS I & II - the zigzag scheme providing both ATP, and the more efficient energy carrier NADPH.
Light dependent reaction: Light is required to generate ATP and NADPH.
PSII: 2H2O → (Elight = h⋅f) → O2 + 4H+ + 4e- ADP+Pi →(Elight =h⋅f)→ATP
Photoperiodism: Response to duration and timing of day and night; a mechanism evolved by organisms for measuring seasonal time. Plants that flower only under certain day-length conditions; and is a biological response to a change in the proportions of light and dark in a 24 hour daily cycle. • Short Day Plants: Flower in early spring or fall when days are shorter with less intense light.
• Long Day Plants: Flower in the summer, when the light periods are longer than the critical length.
• Day Neutral Plants: Flower without respect to length of the day. Photorespiration: see plants, types of - C3-plant. Photosynthesis: (Gk. photos, light + syn, together + tithenai, to place) Conversion of light energy to chemical energy;
PLANTS: Solar radiation is used by plants to oxidize water, reduce CO2, and release of O2. Plants use <5% of the radiant energy, the majority is reflected or lost as heat; see also cycle-carbon. Chloroplast: A two-membranous plastid containing pigments (chlorophyll and carotenoids); most active photosynthetic tissue in higher plants is found in the pallisade parenchyma of leaves (mesophyll).
• Granum: Stacks of thylakoids seen with electron microscopes, and as green granules with light microscopes; grana contain chlorophylls and carotenoids; see also photosynthetic pigments.
• Stroma: (Gk. stroma, to spread out) Ground substance, aqueous region of plastids; the site of reactions by which photochemical energy is used to synthesize carbon containing compounds (site of dark reaction). • Thylakoid: (Gk. thylakos, sac + oides, like) A saclike membranous structure in cyanobacteria and grana in
eukaryotic autotrophic organisms (stacks of thylakoids form grana); chlorophyll pigments are found within
the thylakoid membranes (site of light dependent reaction). Intense photosynthesis, causes some of the photosynthate to be stored temporarily in chloroplast as starch- grains; sugars are transported via phloem to target cells (root) for nourishment and to produce cellulose.
Light independent reaction (dark reaction in the stroma of photosynthetic cells), the enzymatic reactions resulting in the synthesis of glucose from CO2, ATP; and NADPH;
PSI: 2NADP+ + 2H+ → (Elight = h⋅f) → 2NADPH Nitrate (NO2-) conversion to ammonium (NH4+) occurs in chloroplasts as well, hence, in stark competition for ATP and free electrons with the carbon fixing process! Allover oxygenic photosynthesic reaction: CO2 + 12H2O → (Elight = h⋅f) → C6H12O6 + 6H2O + 6O2 An/oxic autotrophic organisms use light energy to satisfy their energetical requirements; anoxic bacteria derive their H+ protons from chemical compounds other than H2O requiring less energy to be split); see bacterium. Phototrophic anoxic reaction: CO2 + H2S → (Elight = h⋅f) → CH2O + H2O + S
2NADPH → 2CH2O≈P → 2NADP+ + 2H+
Photosynthetic Pigments: A substance that absorbs light, often selectively. P. Spectrum: The spectrum of light waves absorbed by a particular pigment eliciting a certain reaction; gamma rays (short wavelength) are too energetic (destroy biological molecules), radio waves don’t excite them at all. Carotenoids: A class of fat-soluble pigments (yellow and orange pigments); found in chloro- and chromo- plasts of plants. Carotenoids act as accessory pigments in photosynthesis. • β-C.: A yellow to red carotenoid with the empirical formula C40H56, found in fruits e.g.: pericarp of
tomatoes. • Fucoxanthin: (Gk. phykos, seaweed + xanthos, ye/br) Brownish pigment of brown algae and
chrysophytes. • Xanthophyll: (Gk. xanthos, yellowish-brown + phyllon, leaf) A yellow fat-soluble light shielding pigment.
Light Shielding: Besides cytoplasmic streaming, xanthophyllic carotenoid absorb excessive light (electromagnetic radiation) to protect the photosynthetic complex from overexposure by converting EMR it into heat; light converts violaxanthin into zeaxanthin which deactivates the antenna complex of the photosystem, preventing photodistruction of chlorophyll by overexposition to pure O2.
Chlorophyll: (Gk. chloros, green + phyllon, leaf) The green pigment of plant cells, which is the receptor of light energy in photosynthesis; a tetrapyrrole ring structure on top with 4 internally placed N-atom, itself facing towards the centrally located Mg-atom; the entire complex is attached to a hydrophobic C20H39 phytol tail, which anchors the molecule into the photosynthetic thylakoid membrane; see table and scan below.
• C. a: blue-green; with an extra CH3 molecule attached at the opposite end of the tetrapyrrole. • C. b: yellow-green; aldehyde (CHO) instead of CH3 attached at the opposite end of the tetrapyrrole. Phycobilins: A group of water-soluble accessory pigments, which occur in the red algae and cyanobacteria. Phytochrome: A phycobilin-like pigment (photoreceptor for red and far-red light) found in the cytoplasm of plants and a few green algae; phytochromes do not participate in photosynthetic reactions. Plants contain phytochrome in two different inter-convertible forms: • Pr absorbs red light (660 nm), the biological inactive form of the protein (inhibits reactions, but allows
plant to grow pale and spindly, changes to Pfr (pigment fully reactive) when exposed to (red) light.
Phytochrome is continuously synthesized as Pr from its amino acid precursors. • Pfr absorbs far-red light (730 nm), the biological active form triggering reactions, e.g.: induces germination
flowering, dormancy, leaf formation, seed germination, shade detection, etc. by triggering the release of plant hormones via a cis-trans isomerisation (cascade amplification). Pfr is converted back to Pr when exposed to far-red light (darkness, dark reversion) or lost through hydrolysis by proteases.