Photosynthesis Chapter 7

Photosynthesis In photosynthesis, organisms trap radiant energy from sunlight and convert it into the energy of chemical bonds in large molecules. Chlorophyll is a green pigment that absorbs light energy. Photosynthesis transforms light energy into chemical bond energy in the form of ATP. ATP is then used to form complex organic molecules such as glucose.

Chloropasts Chloroplasts are the organelles in which photosynthesis takes place. Two distinct regions in chloroplasts: – Grana – stacks of membranous stacks containing chlorophyll. – Stroma – the spaces between the membranes.

Summary of Photosynthetic Events Light energy + carbon dioxide + water  glucose + oxygen Light energy + 6CO 2 + 6H 2 O  C 6 H 12 O O 2

3 Distinct Events In The Photosynthetic Pathway Light-Capturing events – chlorophyll and other pigments absorb specific wavelengths of light. – Light energy is captured by photosynthetic and other pigments resulting in excited electrons. – When specific amounts of light are absorbed by the photosynthetic pigments, the electrons become “excited” and can enter into chemical reactions responsible for the production of ATP. – Takes place in the grana of the chloroplast.

3 Distinct Events In The Photosynthetic Pathway Light-dependent reactions (light reactions) – Energy from the excited electrons is used to dissociate water molecules into hydrogen and oxygen. – use the excited electrons to produce ATP. – Takes place in the grana of the chloroplast. Light-independent reactions (dark reactions) – Organic molecules such as glucose is produced. – Takes place in the stroma.

Light-Capturing Events Chlorophyll and other pigments such as carotenoids use light energy to drive photosynthesis. Chloroplasts are membranous, saclike organelles containing many thin, flattened sacs called thylakoids.

Light-Capturing Events The thyylakoids contain chlorophyll and are stacked in groups called grana. The pigments embedded in the thylakoids capture light energy and some of the electrons in the pigments become excited.

Light-Capturing Events The chlorophylls are arranged in clusters called photosystems. Photons of light energy  excited electrons from chlorophyll.

Light-Dependent Reactions The excited electrons from the light-capturing stage are passed through a series of electron- transport steps.

Light-Dependent Reactions The chloroplast in the electron transport system regains electrons from water; thereby producing hydrogen ions, electrons, and oxygen gas. Excited electrons + H ADP + NADP +  ATP + NADPH + O 2

Light-Independent Reactions The light-independent reactions are a series of oxidation-reduction reactions, which combine hydrogen from water with carbon dioxide from the atmosphere to form simple organic molecules such as sugar. ATP + NADPH + ribulose + CO 2  ADP + NADP + + complex organic molecule + ribulose

Plant Metabolism Photosynthetic organisms are able to manufacture organic molecules (including fats, proteins, and complex carbohydrates) from inorganic molecules. Vitamins are another important organic molecule derived from plants. Vitamins are organic molecules that we cannot manufacture, but must have in small amounts to maintain health.

Interrelationships Between Autotrophs and Heterotrophs Autotrophs can capture energy and create organic molecules from inorganic molecules. Heterotrophs must have organic molecules as starting points. All organisms must do some form of respiration.

Interrelationships Between Autotrophs and Heterotrophs Plants, like animals, require oxygen for the ETS portion of aerobic cellular respiration. Plants give off more oxygen to the atmosphere than they take in during respiration.

Interrelationships Between Autotrophs and Heterotrophs The surplus oxygen given off is the source of oxygen for cellular respiration in both plants and animals. Animals supply the raw materials of CO 2, H 2 0, and nitrogen needed by plants, and plants supply the raw materials of sugar, oxygen, amino acids, fats, and vitamins needed by animals.