A leaf is the main organ of a plant that carries out photosynthesis. (p.28)
The lamina is flat, thin and green to expose maximum surface to sunlight and allow efficient gas diffusion. (p.28)
The petiole is the leaf stalk that connects the lamina to the stem and extends into the lamina as midveins for support. (p.28)
The cuticle is a waxy, waterproof, transparent layer covering the upper and lower epidermis; it reduces transpiration and lets sunlight pass through. (p.29)
The upper epidermis has no chloroplasts and is transparent; the lower epidermis contains stomata, each guarded by a pair of guard cells. (p.29)
Palisade mesophyll cells are vertical and closely packed, with many chloroplasts, as the main site of photosynthesis. (p.30)
Spongy mesophyll cells are irregular and loosely arranged with many air spaces, fewer chloroplasts, and aid gaseous exchange. (p.30)
Xylem transports water and mineral salts from roots to leaves and has lignified walls for support. (p.30)
Phloem transports organic substances produced in photosynthesis from leaves to other parts of the plant. (p.30)
2.2 Main Organ for Gaseous Exchange
Gaseous exchange of oxygen and carbon dioxide occurs through stomata, mainly on the lower epidermis. (p.31)
Stomatal opening depends on guard cell turgidity, which is controlled by potassium ion uptake and sucrose concentration. (p.31)
In light, guard cells take in K+ or accumulate sucrose, lowering water potential; water enters by osmosis, guard cells become turgid and the stoma opens. (p.32)
In darkness, K+ leaves the guard cells or sucrose falls, water leaves by osmosis, guard cells become flaccid and the stoma closes. (p.32)
Cacti open their stomata at night to reduce water loss. (p.31)
Thick, less elastic inner cell walls and thin, elastic outer cell walls make guard cells curve outwards when turgid, opening the stoma. (p.35)
When the plant lacks water, guard cells become flaccid and the stoma closes. (p.35)
2.3 Main Organ for Transpiration
Transpiration is water loss as water vapour through evaporation from the plant to the atmosphere. (p.36)
About 90% of transpiration occurs through stomata in leaves. (p.36)
Transpiration cools the plant, and creates a pulling force that moves water and mineral salts through the xylem. (p.36)
Higher light intensity increases transpiration until other factors become limiting. (p.37)
Higher temperature increases kinetic energy of water molecules and increases transpiration. (p.37)
Lower relative air humidity increases transpiration. (p.37)
Faster air movement carries away water vapour and increases transpiration. (p.37)
2.4 Main Organ for Photosynthesis
Plants are autotrophs that produce their own food through photosynthesis; glucose is used by organisms to produce energy. (p.40)
Van Helmont (1640s) showed plant growth came mainly from something other than soil. (p.40)
Priestley (1772) showed that plants release oxygen. (p.40)
Photosynthesis needs chlorophyll to absorb light energy, carbon dioxide from the air and water from the soil; oxygen is released as a by-product. (p.41)
A chloroplast contains thylakoids, grana, stroma and lamellae. (p.42)
Granum is a stack of thylakoids that increases surface area; thylakoids contain photosynthetic pigments; stroma is the site of light-independent reactions. (p.42)
Chromatography separates pigments by solubility; Rf = distance travelled by pigment / distance travelled by solvent. (p.43)
Rf values: chlorophyll a 0.60 (blue/greenish), chlorophyll b 0.50 (green), carotenoid 0.95 (orange), xanthophyll 0.35 (yellow), pheophytin 0.70 (grey). (p.43)
Light-dependent reactions occur in the thylakoids: light excites electrons, ATP and NADPH are formed, and photolysis of water releases oxygen. (p.44)
Light-independent reactions occur in the stroma: CO2 is fixed by 5-carbon compounds, reduced by NADPH and ATP to glucose, and glucose forms starch stored in the stroma. (p.44)
Overall equation: 12H2O + 6CO2 gives C6H12O6 + 6O2 + 6H2O, using light energy and chlorophyll. (p.44)
Light-dependent reactions need light and use water; light-independent reactions use CO2 and do not directly need light. (p.45)
Increasing carbon dioxide concentration increases the rate of photosynthesis until another factor becomes limiting. (p.46)
Increasing light intensity increases the rate of photosynthesis until light saturation point. (p.46)
Photosynthesis enzymes have an optimum temperature of about 25 to 30 degrees Celsius; very high temperatures denature enzymes. (p.47)
Red and blue light give the highest rate of photosynthesis because chlorophyll and carotenoid absorb them. (p.50)
Green light is reflected by chlorophyll, which is why leaves look green. (p.51)
2.5 Compensation Point
Compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration. (p.52)
Above the compensation point, photosynthesis exceeds respiration; excess CO2 is taken up, excess O2 is released, and excess glucose is stored as starch. (p.52)
For growth and reproduction, photosynthesis must exceed respiration every day. (p.53)
Photosynthesis occurs in chloroplasts and is anabolic; respiration occurs in mitochondria in all cells and is catabolic. (p.53)