Photosynthesis Occurs in the Green Parts of Plants

Photosynthesis

BIO Chp 6

Overview of photosynthesis

•  Flowering plants as photosynthesizers

–  Photosynthesis occurs in the green parts of plants

–  Leaves contain mesophyll tissue specialized for photosynthesis

–  Water is taken up by roots and transported to leaves by veins

–  Carbon dioxide enters through openings in the leaves called stomata

–  Light energy is absorbed by photopigments in thylakoids of chloroplasts

Plants as photosynthesizers

Overview of Photosynthesis

•  Photosynthetic reaction

Solar energy + 6CO2 + 6H2O à C6H12O6 + 6O2

–  Carbon dioxide is reduced to form glucose

–  Water is oxidized to form oxygen, therefore the oxygen given off by photosynthesis comes from water

•  Two set of reactions occurs in photosynthesis

–  Light reactions occur first followed by the Calvin cycle

–  NADP+ carries hydrogens from the light reactions to the Calvin cycle

–  ATP formed in the light reactions is used in the Calvin cycle

Overview of photosynthesis

Captured Solar Energy

•  Visible light

–  Visible light is a part of the electromagnetic spectrum

–  Visible light includes the colors blue, violet, green, yellow, orange, and red

–  Blue-violets have the shortest wavelengths and the highest energy content

–  Photopigments chlorophyll a and b and carotenoids absorb specific portions of the light spectrum

–  Blue, violet, and red are best absorbed

–  Green is reflected

Solar energy capture

•  Light reactions

–  2 types of photosystems composed of photopigments

–  Reaction center of each photosystem is chlorophyll a

–  Electrons of chlorophyll a absorb light energy and move to an electron acceptor molecule

–  Electrons then pass down one of 2 paths-noncyclic or cyclic

•  The noncylic pathway

–  Produces ATP and NADPH

–  Light energy is absorbed by chlorophyll a in PSII

–  Excited electrons are removed & replaced by splitting a water molecules

H2O à 2H+ + 2e- + ½O2

–  ATP is produced

•  Organization of thylakoid

–  Thylakoid space acts as reservoir of hydrogen ions

–  Hydrogen ions flow down gradient through an ATP synthase complex in thylakoid membrane to produce ATP

–  Thylakoid membranes contain the following complexes:

•  PS I and II

•  ATP synthase complexes

Organization of the Thylakoid

Carbohydrate synthesis

Stages of the Calvin cycle

Carbon fixation

•  carbon fixation occurs when a reaction occurs that attaches CO2 to an organic compound

•  Ribulose biphosphate (RuBP) combines with CO2 (carbon dioxide) to form a 6C molecule

•  Enzyme which catalyzes this reaction is RuBP carboxylase

The Calvin cycle reactions

Carbohydrate synthesis

•  Occurs by reduction of carbon dioxide

•  Importance of the Calvin cycle

–  G3P is the Calvin cycle product that can be converted to glucose phosphate

–  Glucose phosphate can then be converted into many different organic molecules

Carbohydrate synthesis

•  Occurs by reduction of carbon dioxide

•  Importance of the Calvin cycle

–  G3P is the Calvin cycle product that can be converted to glucose phosphate

–  Glucose phosphate can then be converted into many different organic molecules

Photosynthesis vs. Cellular Respiration

•  Photosynthesis vs cell respiration

–  Both plant and animal cells carry out cell respiration

–  Only plant cells photosynthesize

–  Both processes utilize an electron transport chain and chemiosmosis for ATP production

–  Photosynthesis reduces CO2 to carbohydrates and releases O2

–  Respiration utilizes O2 and gives off CO2

Photosynthesis vs Cell Respiration