Ch 10 NOTES (part 1): Photosynthesis in Nature

10.1 – Photosynthesis converts light energy to the chemical energy of food.

PHOTOSYNTHESIS:

● PHOTOSYNTHESIS = the capture of light energy & the conversion of this energy to stored chemical energy (in the form of sugar)

-AUTOTROPHS: “producers”; capable of producing their own food PHOTOAUTOTROPHS: use light energy to synthesize food (e.g. plants)

● Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes

● These organisms feed not only themselves but also the entire living world… thank you plants!

Plants

Unicellular protist

Multicellular algae Cyanobacteria

Purple sulfurbacteria

10 µm

1.5 µm

40 µm

● HETEROTROPHS: “consumers”; live on compounds produced by other organisms

-animals that eat plants or other animals

● Almost all heterotrophs, including humans, depend on photoautotrophs for food and oxygen

**some heterotrophs are:

-decomposers: live on dead matter

Chloroplasts: The Sites of Photosynthesis in Plants

● Leaves are the major locations of photosynthesis

● Their green color is from CHLOROPHYLL, the green pigment within chloroplasts

● Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast

● Through microscopic pores called STOMATA, CO2 enters the leaf and O2 exits

CHLOROPLASTS:

● organelles in which photosynthesis occurs● found primarily in leaves (about 500,000

per mm2)

● contain green pigment CHLOROPHYLL

(absorbs light energy)

● found mainly in mesophyll (tissue inside leaf)● a typical mesophyll cell has 30-40

chloroplasts

Leaf cross sectionVein

Mesophyll

Stomata CO2O2

Mesophyll cellChloroplast

5 µm

Outermembrane

Intermembranespace

Innermembrane

Thylakoidspace

Thylakoid

GranumStroma

1 µm

● CO2 enters and O2

leaves through

STOMATA

● water is delivered to

leaves from the roots

via veins

CHLOROPLAST:

● enclosed by 2 membranes

● STROMA = fluid within the chloroplast

● THYLAKOID MEMBRANES are

interconnected & contain the thylakoid space,

or lumen (are usually in stacked columns

called GRANA)

● chlorophyll is stored in the thylakoid

membranes

PATHWAYS OF PHOTOSYNTHESIS

● TOTAL equation for photosynthesis:

6CO2 + 12H2O + light C6H12O6 + 6O2 + 6H2O energy

● NET EQUATION: 6CO2 + 6H2O + light C6H12O6 + 6O2

energy

The Splitting of Water

● Chloroplasts split water into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

Reactants:

Products:

6 CO2 12 H2O

C6H12O6 6 H2O 6 O2

Tracking Atoms Through Photosynthesis:

Photosynthesis as a Redox Process

● Photosynthesis is a redox process in which water is oxidized and carbon dioxide is reduced

● like respiration, photosynthesis is a series of redox reactions…BUT, the electron flow is reversed!…the electrons increase in potential energy as they move from water sugar… an uphill process!

(the required energy boost comes from light energy)

Two Stages of Photosynthesis: 1) the LIGHT REACTIONS

 

2) the CALVIN CYCLE (a.k.a. the “dark reactions”

or light-independent

reactions)

The Two Stages of Photosynthesis: A Preview

● Photosynthesis consists of the light reactions (the photo part) and the Calvin cycle (the synthesis part)

LIGHT REACTIONS:

● convert solar energy chemical energy

● light is absorbed by chlorophyll and drives a transfer of electrons and H from water to an acceptor called: NADP+

● water is split and O2 is produced as a “waste

product”

● NADP+ is reduced to NADPH…ALSO, ATP is generated via phosphorylation of ADP

(photophosphorylation!)

CALVIN CYCLE:

● CO2 from air is incorporated into organic

molecules by carbon fixation

● NADPH and ATP from the light reactions power the production of sugar

● also called: the “dark” reactions or light-independent reaction

Locations of the Photosynthesis Reactions:

● Light reactions occur in the THYLAKOIDS

● Calvin cycle occurs in the STROMA

H2O

LIGHTREACTIONS

Chloroplast

Light

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

H2O

LIGHTREACTIONS

Chloroplast

Light

ATP

NADPH

O2

NADP+

CO2

ADPP+ i

CALVINCYCLE

[CH2O](sugar)