Light Dependent Reactions - Houston ISD



Photosynthesis Unit

Photosynthesis

= a process that captures energy from sunlight to make sugars that store chemical energy

- plants absorb visible light for photosynthesis

- appears white but is made of a variety of colors (ROYGBIV) = visible spectrum

- pigment = a compound that absorbs light

- different colors absorb different colors of light

- chlorophyll = a molecule in chloroplasts that absorbs visible light

- two types: chlorophyll a (primary pigment)

chlorophyll b (accessory pigment)

- absorb mostly red and blue light

- neither a nor b absorbs green light (this is why plants appear green – color is reflected)

- carotenoids (accessory pigments) – contain yellow, orange, red, brown pigments

- in plants, chlorophylls are much more abundant than other pigments

- in fall, plants lose their chlorophylls (carotenoids then become visible)

Light Reactions

- capture energy in sunlight and transfer it

- take place in thylakoid membrane of chloroplast

- chlorophyll absorbs energy from sunlight

- water is broken down (into H+ ions, electrons, and oxygen)

- oxygen is released as a waste product

- NADPH is formed (functions like ATP = energy) when electrons are added to NADP+

- energy is transferred to make ATP (when H+ ions diffuse)

- overall, oxygen is given off as a waste product, NADPH and ATP are formed

Dark Reactions (Calvin Cycle)

- energy (NADPH and ATP) from light reactions make sugars

- occurs in stroma of chloroplast

- does not need sunlight

- carbon dioxide is needed

- a simple sugar, glucose, is formed from carbon dioxide and energy from ATP and NADPH

- overall, glucose, NADP+, and ADP are created. NADP+ and ADP go back to the light reactions

Equation of Photosynthesis

6CO2 + 6 H2O (((((( C6H12O6 + 6O2

(carbon dioxide) (water) (sugar = glucose) (oxygen)

Functions of Photosynthesis

- ultimately produces food for all organisms

- provides materials for plant growth/development

- simple sugars become starch and cellulose (found in cell walls)

- helps regulate Earth’s environment

- removes carbon dioxide from atmosphere

Factors that Affect Photosynthesis

1) Light Intensity – as light intensity increases, photosynthesis increases to a point

2) CO2 Concentration – as CO2 Concentration, photosynthesis increases to a point

3) Temperature – as temperature increases, photosynthesis will increase to a point, then a decrease will occur if temperatures continue to increase

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