photosynthesis
TRANSCRIPT
Photosynthesis!
Intro
Organisms (autotrophs) obtain energy from the sun and store it in organic compounds (glucose) during photosynthesis
Energy is necessary to do work
Growth and repair, active transport across membranes, reproduction, synthesis, etc.
Carbon dioxide and water are combined to create glucose and oxygen
6CO2 + 6H2O + energy 6O2 + C6H12O6
Chloroplasts!Absorb sunlight during the light dependent reactions
Double membrane organelles with an inner membrane folded into discs called thylakoids
Thylakoids contain chlorophyll and are arranged into stacks called granum
Granum is surrounded by a gel-like material called stroma
Light-capturing pigments in granum are organized into photosystems
Pigments in Chloroplasts!When all colors are absorbed the object appears black
When all colors are reflected the objects appears white
If a color is reflected the object appears that color
Chlorophyll a and chlorophyll b are common types of chlorophyll
The light energy absorbed in chlorophyll b is transferred to chlorophyll a in the light reactions
Overview!Light Reactions (H2O O2 + ATP + NADPH2)
Water splits, giving off oxygen
Dependent on sunlight for activation
Light is absorbed by chlorophyll a and “excites” the electrons in the cholorphyll molecule
Electrons are passed through a series of carriers and ATP is produced
Takes place in thylakoids
Overview (cont.)Dark Reactions (ATP + NADPH2 + CO2 C6H12O6)
Carbon dioxide is split, providing carbon to make sugars
Glucose is the final product
Does not require light energy
Includes the Calvin Cycle
Takes place in the stroma
The Calvin Cycle!STEP 1
CO2 is diffused into the stroma
An enzyme combines CO2 with a five-carbon carbohydrate called RuBP
The resulting six-carbon molecule splits into a pair of three-carbon molecules called PGA
The Calvin Cycle (cont.)
STEP 2
Each PGA molecule receives a phosphate group from ATP
It also receives a proton from NADPH and releases a phosphate group producing PGAL
This produces ADP, NADP+, and phosphate which are used in Light Reactions
The Calvin Cycle (cont.)
STEP 3
Some PGAL is converted to RuBP to continue the cycle
Some PGAL leaves the cycle to create organic compounds
Each turn of the cycle fixes one CO2 molecule and it takes six turns to make one glucose molecule
Photosystems and ETC!Only 1/250 chlorophyll molecules (chlorophyll a) actually convert light energy into usable energy – these are called reaction-center chlorophyll
Other molecules (chlorophyll b, c, d, and carotenoids absorb light and send it to the reaction-center chlorophyll – aka antenna pigments
Photosystem – a unit of hundreds of antenna pigments and a reaction center (Photosystem I and II)
Light is absorbed by the antenna pigments of photosystem I and II
Photosystems and ETC (cont.)
Photosystem I
Energy is moved to the reaction center (P700)
Electrons from the water are moved to the cytochrome complex
Activates P700 which reduces NADP+ to NADPH
Photosystem II
Energy is moved to the reaction center (P680)
P680 loses an electron and becomes positive
It can now split water and release electrons
ChemiosmosisThe making of
Dependent on the concentration gradient of protons (H+) across the thylakoid membrane
Concentration of protons is higher inside the tylakoid
ATP synthase creates ATP by adding a phosphate group to ADP
ATP
Alternate PathwaysC3 plants commonly use the Calvin cycle
Stomata are small openings in the leaves for gas exchange
Guard cells open and close the stomata
C4 plants fix CO2 to four-carbon compounds during the hottest part of the day when the stomata is partially closed
Rate of PhotosynthesisLight intensity – as intensity increases the rate increases and eventually levels off into a plateau
Temperature – only dark reactions are dependent on temperature because of the enzyme
Increasing amount of CO2 increases rate of photosynthesis