Photosynthesis: The Process of Converting Light Energy into Chemical Energy and Oxygen - P, Study notes of Biology

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Photosynthesis

Photosynthesis

• (^) Photosynthesis is the process by which organisms use the energy of the sun to synthesize organic compounds (sugars) from inorganic compounds (CO 2 and water)
• (^) Photosynthesis transforms the energy of the sun into chemical energy (glucose)
• (^) Provides the oxygen we breath, removes CO 2 , and provides food, energy and shelter!

Carbon dioxide C 6 H 12 O 6 Photosynthesis CO 2 H 2 O O 2 Water 6 + 6 Light energy Glucose^ **Oxygen gas

• 6**

Photosynthesis

• (^) Carbon dioxide and water are waste products of cellular respiration! Photosynthesis takes these products and converts them to the glucose and O 2 necessary for cellular respiration

Photosynthesis

• (^) Photosynthesis occurs on a cellular level
• (^) Chloroplasts are organelles which carry out photosynthesis
• (^) Chloroplasts contain chlorophyll , a light- absorbing pigment which give autotrophs their distinctive color

The Chloroplast

• (^) A chloroplast contains two membranes (as do mitochondria)
• (^) A thick fluid called the stroma fills the inner compartment of the chloroplast
• (^) Suspended in the stroma are the thylakoids , a system of interconnected membranous sacs, which enclose another compartment known as the thylakoid space

Chloroplast Outer and inner membranes Intermembrane space Stroma Granum Thylakoid space Thylakoid

Make like a tree and…

• (^) Leaves are designed to capture light and increase the absorption of carbon dioxide
• (^) Carbon dioxide enters the leaf (and oxygen exits the leaf) via the stomata, tiny pores protected by guard cells
• (^) Water is supplied to the tree via its roots, but may exit the leaves when the stomata are open (a catch 22!); why stomata open at night in many plants

Pigments

• (^) Pigments are light-absorbing molecules built into the thylakoid membranes
• (^) Pigments absorb some wavelengths of light, but reflect others
• (^) We do not see the absorbed wavelengths (because their energy has been absorbed by the pigment molecules); we see the wavelengths that the pigment reflects!

Wavelength (nm) 10

- nm Increasing energy Visible light 650 **nm 10

nm 1 nm 10 3 nm 10 6 nm 1 m**^10 3 m 380 400 500 600 700 750 Radio waves Micro- waves X-rays UV^ Infrared Gamma rays

Pigments

• (^) Chlorophyll a (a type of chlorophyll pigment) absorbs light mainly in the blue-violet (high energy) and red (low energy) wavelengths Light Chloroplast Thylakoid Absorbed light Transmitted light Reflected light

Autumn color change

• (^) In addition to chlorophyll, other pigments, known as accessory pigments are present in plants; these include carotene, and cyanins
• (^) When chlorophyll concentrations decrease at the end of summer, some of these other pigments – which are usually masked by chlorophyll – reveal their colors
• (^) Carotene, for example, is especially long-lasting

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Photosynthesis

• (^) Photosynthesis occurs in 2 stages, each with multiple steps
  1. Light reactions convert light energy into chemical energy, and produce O 2.
  2. Light-independent reactions (the Calvin Cycle) assembles glucose molecules using CO 2 (carbon fixation) and the energy-rich products of the light reactions

1. Light (dependent) Reactions

• (^) Light reactions occur in the thylakoid membranes of the chloroplast
• (^) Water is split, providing a source of electrons and giving off O 2 as a by-product H 2 O  2H+^ + 1/2 O 2 + 2e-
• (^) Light energy absorbed by chlorophyll molecules is used to drive the transfer of electrons and H+ from water to NADP + and generate ATP