Photosynthesis Chapter 6

Carbon and Energy Sources Photoautotrophs Carbon source is carbon dioxide Energy source is sunlight Heterotrophs Get carbon and energy by eating autotrophs or one another

Linked Processes Photosynthesis Energy-storing pathway Releases oxygen Requires carbon dioxide Aerobic Respiration Energy-releasing pathway Requires oxygen Releases carbon dioxide

Autotrophs are the producers of the biosphere Photoautotrophs capture sunlight energy and use it to carry out photosynthesis. Plants, algae, some protists, and bacteria are photosynthetic autotrophs They are the ultimate producers of food consumed by virtually all organisms

On land, plants such as oak trees and cacti are the predominant producers

In aquatic environments, algae and photosynthetic bacteria are the main food producers

Photosynthesis occurs in chloroplasts In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts A chloroplast contains: Stroma (fluid) grana (stacks of thylakoids) The thylakoids contain chlorophyll Chlorophyll is the green pigment that captures light for photosynthesis

Location and Structure of Chloroplasts

Photosynthesis Equation 12H 2 O + 6CO 2 6O 2 + C 2 H 12 O 6 + 6H 2 O LIGHT ENERGY

Photosynthesis is a redox process, as is cellular respiration Water molecules are split apart and electrons and H + ions are removed, leaving O 2 gas These electrons and H + ions are transferred to CO 2, producing sugar Reduction Oxidation

Two Stages of Photosynthesis

Visible Light Humans perceive different wavelengths as different colors Violet (380 nm) to red (750 nm) Longer wavelengths, lower energy Shorter wavelengths, higher energy

Pigments Light-absorbing molecules Absorb some wavelengths and transmit others Color you see are the wavelengths NOT absorbed Wavelength (nanometers) chlorophyll a chlorophyll b

Excitation of Electrons Excitation occurs only when the quantity of energy in an incoming photon matches the amount of energy necessary to boost the electrons of that specific pigment Amount of energy needed varies among pigment molecules

Pigments in Photosynthesis Bacteria Pigments found in plasma membranes Plants Pigments embedded in thylakoid membrane system Pigments and proteins organized into photosystems Photosystems located next to electron transport systems Pigments include chlorophyll a, chlorophyll b and carotenoids

Light-Dependent Reactions Pigments absorb light energy, give up e - which enter electron transport systems Water molecules are split, ATP and NADPH are formed, and oxygen is released Pigments that give up electrons get replacements from photosystem

Photosystem Function: Pigments When excited by light energy, these pigments transfer energy to adjacent pigment molecules Each transfer involves energy loss

Photosystem Function: Reaction Center Energy is reduced to level that can be captured by molecule of chlorophyll a This molecule (P700 or P680) is the reaction center of a photosystem Reaction center accepts energy and donates electron to acceptor molecule

Light Dependent Reactions

Making Sugar from CO 2 : The Calvin Cycle The Calvin–Benson cycle makes sugar from CO 2. Overall reactants Carbon dioxide ATP NADPH Overall products Glucose ADP NADP +

Making Sugar from CO 2 : The Calvin Cycle The Calvin cycle has four phases: fixation of CO 2 Energy consumption and reduction carbohydrate production (release of G3P) regeneration of RuBP.

Calvin Cycle

Using the Products of Photosynthesis Phosphorylated glucose is the building block for: sucrose The most easily transported plant carbohydrate starch The most common storage form in plants

Photosynthesis Summary

Photorespiration in C 3 Plants On hot, dry days stomata close Inside leaf O 2 levels rise CO 2 levels drop Rubisco attaches RuBP to oxygen instead of carbon dioxide Only one PGA forms instead of two

Photorespiration in C 4 Plants Carbon dioxide is fixed twice In mesophyll cells, carbon dioxide is fixed to form four-carbon compound (oxaloacetate) Carbon dioxide is released and fixed again in Calvin cycle

Photorespiration in CAM Plants Carbon is fixed twice (in same cells) Night Carbon dioxide is fixed to form organic acids Day Carbon dioxide is released and fixed in Calvin cycle