NOTES FOR B9: Photosynthesis

9.1   Write the word equation and the balanced chemical symbol equation for photosynthesis

PHOTOSYNTHESIS: the fundamental process (comprising of a complex set of various reactions) by which plants manufacture the complex organic substance glucose (their ‘food’ – autotrophic) + oxygen (waste product) from the simple inorganic molecules carbon dioxide + water in the presence of chlorophyll + light

§  happens within illuminated chloroplasts: light energy is transferred to organic compounds during photosynthesis

§  photosynthesis is essential to life: chemical energy produced is hereby passed along the food chain; plants = producers; consumers (e.g. humans, other animals) release this chemical energy during respiration

Chloroplasts

·       organelles found in green parts of plants

·       contain chlorophyll (green photosynthetic pigment in chloroplasts + occurring in grana sing. granum of plant cells) + enzymes + electron-transport proteins used in the reduction of carbon dioxide to sugars + ATP formation (using light energy)

Role of chlorophyll

·       absorb light energy needed to activate the reaction

·       chlorophyll reflects green light + absorbs the other colours of white light

Building up various biological molecules using glucose

-        disaccharide sucrose (sugar transported in phloem)

-        polysaccharide cellulose (cell walls, growth of new cells)

-        starch (insoluble): for storage (osmosis not affected – water potential of cell contents); plants that contain a lot of storage starch: potatoes, wheat, barley, rice

-        plant fats + lipids: carbon-rich triglycerides allow a minimum of volume to stock a maximum of energy content; plants that contain a lot of storage lipids: pulps (palm, olive, avocado) + seeds (nuts etc.)

-        proteins (CHON): for growth + repair of plant tissues; plants need the element nitrogen – absorbed from the soil solution or fertilisers containing ammonium salts (NH₃⁺)/nitrates (NO₃^-)

9.2   Understand how varying carbon dioxide concentration, light intensity and temperature affect the rate of photosynthesis

LIMITING FACTOR: the component of a reaction that is in ‘shortest supply’ so that it prevents the rate of the reaction increasing + sets a limit to it

Light

·       essential to activate the reaction of photosynthesis

·       green plants in the dark are unable to photosynthesize + only aerobic respiration takes place during night; light intensity = 0, the rate of photosynthesis = 0

·       the rate of photosynthesis increases linearly with light intensity to begin with

·       eventually, at high light intensity a plateau is reached – light intensity is no longer the limiting factor

Carbon dioxide:

·       essential for photosynthesis (one of the two reactants); carbon dioxide is reduced to sugars during photosynthesis

·       carbon dioxide = 0, rate of photosynthesis = 0

·       the rate of photosynthesis increases linearly with carbon dioxide concentration to begin with

·       eventually, at high carbon dioxide concentration a plateau is reached – carbon dioxide is no longer the limiting factor

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