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