PLANT LIFE PROCESSES

 Manufacture of sugars and their precursors by green plants in the presence of light and chlorophyll

 Carbon dioxide is taken from the air through the stomata, while water is absorbed from the soil by the roots and is transported in the xylem to sites of photosynthesis

 Leaf is the main organ for photosynthesis

 Chloroplast is the main organelle involved.

 Leaf features that make it an ideal organ for photosynthesis:

 It is typically expanded form

 It is usually perpendicular angle to incident light

 It is extensive internal surface with an efficient vascular system for channeling the reactants and end products of photosynthesis

 its pigment for light absorption

CHLOROPLAST

 usually lens-shaped, bounded by a double

membrane

 the inner membrane invaginates parallel to the surface and becomes organized into specialized cytoplasmic body consisting of a stack of thylakoids called granum which are embedded in

a proteinaceous matrix called stroma

CHLOROPHYLL

 principal pigment in photosynthesis located in the partition between two adjacent thylakoids

 chlorophyll a occurs in all higher plants, but other isomers like chlorophyll b, c, d etc. may also be found

 in higher plants, the two main isomers are

chlorophyll a and chlorophyll b, in a 3:1 ratio

 its basic unit is the porphyrin ring system, a structure

made up of four simpler pyrrole nuclei joined by carbon linkage

 the center of porphyrin is occupied by a single magnesium atom

SIGNIFICANCE OF PHOTOSYNTHESIS

 converts light energy into chemical energy in the form of organic nutrients

 supplies oxygen to the atmosphere

 produces food

COMPONENT REACTIONS OF PHOTOSYNTHESIS

Light/Light Dependent/Photochemical Phase

 Light energy is harvested by two photosystems

 Oxidation of water and generation of NADPH and ATP by the chloroplast thylakoids

 The lights induce the splitting of H2O to produce oxygen and the NADPH and ATP (reducing power)

 Rapid process and requires the presence

of light

 Composed of:

 Non-cyclic photophosphorylation

 Cyclic photophosphorylation

 The end products of light reaction, ATP and NADPH, are used to fix CO2

 Dark or Light Independent/Biochemical or CO2 assimilation phase or photosynthetic carbon reduction cycle

 Primary process by which inorganic carbon is converted to organic compounds

 Use of reducing power to reduce CO2 to carbohydrates and water

 Occurs both in the presence or absence of light

 A slow process

 Uses ATP and NADPH

THREE PATHWAYS IN THE FIXATION OR REDUCTION OF CO2 INTO CARBOHYDRATES:

1. CALVIN BENSON CYCLE/REDUCTIVE PENTOSE PATHWAY

 fixation and reduction of one molecule of CO2 requires three molecules of ATP and 2 NADPH (coming from light reaction)

 occurs in the mesophyll cell chloroplast

 CO2 acceptor is RUBP

 RUBP carboxylase enzyme is needed

 the first product is 3-PGA

2. C4 OR HATCH SLACK PATHWAY

 occurs in the mesophyll cell

 CO2 acceptor is PEP, catalyzed by PEP carboxylase enzyme

 products are 4-carbon organic acids (oxaloacetic acid at the mesophyll cells)

 oxaloacetic acid is converted to malate and aspartic acid

 malic acid is decarboxylated to produce CO2

 the 3-carbon compound goes back the mesophyll cells

 the CO2 released enters the Calvin cycle for sugar/starch production

 the 3-carbon compound combined with 1-carbon from the atmosphere to form again into 4-carbon compound

3. CRASSULACEAN ACID METABOLISM (CAM) PATHWAY

 found in succulent plants (cactus, pineapple)

 during the night, CO2 is fixed

 during the day, malic acid is decarboxylated where CO2 is fixed through the C3 pathway

FACTORS AFFECTING PHOTOSYNTHESIS

Internal

 Enzymes – biological catalysts/agents of life

 Genetic factor – chlorophyll, kind of plant, etc.

 Leaf age

 Demand of sinks for photosynthesis

 Water content of the plant

 Amount of plant regulates

External

 Light

 Quality

 Intensity

 Duration

 CO2 and H2O availability

 Temperature

 Wind velocity

An enzyme-catalyzed reaction involving the transformation of organic substrate into carbon dioxide and water accompanied by energy release.

STAGES OF RESPIRATION

 Glycolysis

 Occurs in cytoplasm

 Partial oxidation of a glucose molecule (6C) yields two molecules of pyruvic acid (3C). In the process, substrate phosphorylation of the sugar molecule results in a net production of 2 ATPs

 Krebs Cycle

 Pyruvic acids produced in the cytosol during glycolysis are imported into the mitochondrial matrix, the site of Krebs cycle.

 Pyruvic acid is first oxidized to acetyl co-enzyme A and subsequently converted to CO2.

 For every glucose molecule (2 pyruvic acids) entering the mitochondrion, the Krebs cycle generates 6 NADH and 2 FADH2 and yield 2 ATP via substrate level phosphorylation.

 Electron Transport System (ETS)

 occurs in the inner mitochondrial membrane

 NADH (from glycolysis and Krebs cycle) and FADH2 (from Krebs cycle) are oxidized to yield ATP

 ATP is generated in ETS via oxidative phosphorylation

FACTORS AFFECTING RESPIRATION

 Age and tissue type

 large, young tissues respire more strongly than old ones

 developing tissues respire more than mature ones

 tissues undergoing metabolic processes respire more than resting ones

 Temperature

 enzyme activity doubles for energy; 10°C rise in temperature within certain limits

 more rapid breakdown of respiration as temperature increases above 35°C due to heat destruction of enzymes

 Oxygen

 presence of oxygen is essential for oxidative metabolism

 CO2

 high level (higher than normal atmosphere) inhibits respiration

 high concentration causes the stomata to close

 Physiological status of plant or plant parts

 dormant state respires less than active parts of the plant

 Moisture content of tissues

 seeds with higher moisture content respire more than seeds with drier tissues

TRANSPIRATION

The loss of moisture from plants in the form of water vapor. This evaporative process is dependent on energy, the heat of vaporization (539 cal per gram) which is required to convert water from liquid state to gaseous state

Considered as “necessary evil”

a. it keeps the cells hydrated

b. it maintains favorable turgor pressure for the transport of nutrients absorbed by the roots from the soil

c. it serves as a cooling process

TYPES OF TRANSPIRATION

 Cuticular transpiration – loss of water through the epidermis, usually covered with a cuticle. In some temperate plants, about 5-10% of the water lost from plants may be lost through this pathway.

 Lenticular transpiration – loss of water through numerous pores in the outer layer of a woody plant stem, called lenticels. In deciduous species and in some fruits, water loss through lenticels maybe quite substantial.

 Stomatal transpiration – loss of water through the stomata, accounting for as much as 90% of water loss from plants

Two stages involved in transpiration

 Evaporation of water from the moist cell walls into the substomatal air space

 Diffusion of water vapor from the substomatal space into the atmosphere

FACTORS AFFECTING TRANSPIRATION

 Relative humidity

 Temperature

 Wind velocity

 CO2 concentration – higher concentration will close the stomata

 Light intensity

 Morphology of leaf stomatal modification

SOIL-PLANT-AIR CONTINUUM OF WATER

1. Movement of water from the soil to the root xylem

a. Extracellular or apoplastic route - water moves through non-living parts, e.g. capillary

spaces of the cell walls and intercellular spaces

b. Intracellular route

 Symplastic pathway – plasmodesmata

 Transmembrane or transcellular pathway - vacuolar membrane (tonoplast) and the plasma membranes

2. Movement of water from root xylem to leaf xylem

 transpiration-cohesion-adhesion theory

3. Movement of water from leaf xylem to the air

 influenced by RH and VPD

 towards lower water potential (Ψ; expressed in MPa)

TRANSLOCATION

 long-distance transport of photoassimilates

 transport of solutes by the roots to the other plant parts passing the dead conduits or dead xylem vessels (apoplastic transport)

 transport of photosynthates in living conduits or phloem vessel (symplastic transport)

 transport of solution from the roots to the upper parts through the xylem of the stem (transpirational stream); transpiration or loss of water in plant is the cause of the movement

 tissues involved are the phloem and the xylem

 sucrose is the main photosynthates being translocated

 the translocation is from the sources to the sinks

SOURCE

 an organ or tissue that produces more assimilates than what it requires for its own metabolism and growth exporter organ

SINK

 importer or consumer of assimilate

FACTORS AFFECTING TRANSLOCATION

 Temperature

 rate of translocation increases with temperature to a maximum and then decreases due to hazardous effect of high temperature

 Light

 CO2 assimilation increases as light intensity increases

 Metabolic inhibitors

 Concentration gradient

 Mineral deficiencies

 sucrose movement can be aided by boron

 Hormones

 associated with the active parts, hence growing parts (sinks) greatly influence translocation

ASSIMILATION

 The process of utilizing food (photoassimilates and other solutes) for growth

 During the early stage, food substances are converted into simpler compounds (enzymes are needed, nutrients are necessary for normal action of enzymes) and used as building blocks for more complex substances

 In the later stage, simple and complex compounds are integrated into the living substances of the cells

FACTORS THAT DETERMINE ASSIMILATE PARTITIONING IN A CROP

 Sink strength – ability of a sink to accumulate assimilates; a function of sink size and sink activity

 Proximity of the sink to the source organ – assimilates move preferentially toward sink leaves above and in line with the source leaf. Lower mature leaves feed mainly the roots, the higher mature leaves feed mainly the young leaves and the shoot apex

 Stage of development – developing flowers and fruits become dominant sinks during the reproductive stage of a crop. On the other hand, storage roots used as planting materials export assimilates to developing vegetative tissues.

 Nature of vascular connections between source and sinks – each leaf is connected to the main vascular system of the stem by a vascular trace, which diverts from the vascular tissue of the stem into the petiole.