Unit 4 NotesPhotosynthesis

Photosynthesis

Converts light energy into food energy

6CO2 + 12H2O light, enzymes, chlorophyll C6H12O6 + 6O2 + 6H2O

Photosynthesis

Two types of autotrophic organisms

Photosynthetic—make food from sun’s energy

Chemosynthetic—make food from other inorganic molecules

2 Steps to Photosynthesis

Light dependent reactions—need light and chlorophyll to make ATP and NADPH

Light independent reactions—take ATP and NADPH + CO2 to make glucose (or other sugars)

Plant StructureLeaf Structure

Plant Structure

Leaf Parts Cuticle—waxy—provides protection from insects,

environmental damage, etc., and keeps water from getting out

Epidermis—defense/barrier—reduces water loss

Palisade Mesophyll—structure (lots of chloroplasts)

Spongy Mesophyll—air flow (lots of chloroplasts)

Xylem—vascular tissue that moves water

Phloem—vascular tissue that moves sugars

Stomata—openings on bottom of leaf—lets CO2 in and O2 and H2O out

Plant Structure

Leaf Parts (continued)

Guard Cells—cells around the opening of the stomata—open when water flows in by osmosis

Open

Triggers = Light / Decrease CO2 / Internal Clock

K+ enters cell water potential decreases osmosis in cells become turgid stomata opens

Close

Triggers = No Light / Increased Temperature / Lose H2O / Increase CO2

K+ leaves osmosis out cells become flaccid stomata close

Plant Structure

Plant Structure Chloroplasts

Site of Photosynthesis

The membranes (thylakoids) within the chloroplast give it the ability to build up concentration gradients for “whooshing”—chemiosmosis

Light Dependent Reactions

Light energy (photons) is absorbed by pigments in the thylakoid membranes

Pigment Absorbs Reflects

Chlorophyll a Red/Blue Green

Chlorophyll b Red/Blue Green

Carotenoids Blue/Green Orange/Yellow

Light Dependent Reactions

Photosystems—Light Harvesting Units

Photosystem I—has chlorophyll p700 (longer wavelength)

Photosystem II—has chlorophyll p680 (shorter wavelength)

Pigments absorb light energy and transfer the energy to other pigments until it is absorbed by a photosystem

The photosystem gathers enough energy to raise 1e- to a higher energy level (can be used for ETS!)

Light Dependent Reactions Cyclic Photophosphorylation—Electron transfer

with PS I

Electrons from PSI go through an ETS and create ATP--electrons then goes back to PSI

Electrons are cycling!

Not enough energy for most plants

Light Dependent Reactions Non-cyclic Photophosphorylation—Electron transfer

with PS I & PS II

PS II gathers light energy which raises electron to ETS—ATP is made by chemiosmosis

Electron goes to PS I and then is raised to a 2nd ETS—NADP is reduced to NADPH

Electrons in PS II are replaced by photolysis of H2O

Light Dependent Reactions

Light Independent Reactions

Reactions occur on the stromal surface of the thylakoid

Need:

Enzymes

Ribulose Biphosphate (RuBP)—a 5-Carbon sugar

CO2 (From Air)

NADPH

ATP

Plant Makes

From Non-Cyclic Photophosphorylation

Light Independent Reactions—Calvin Cycle Carbon Dioxide Fixation:

6CO2 + 6RuBP 6-6C molecules (unstable); so they split into 12-3C molecules called PGA (phosphoglyceric acid)

Reduction

Each PGA is phosphorylated by ATP and reduced by NADPH into 12 molecules of PGAL (phosphoglyceraldehyde)

Regeneration of RuBP

10 of 12 PGAL remake RuBP—need ATP to do it!

2 of 12 PGAL make 6C sugar (glucose)

Light Independent Reactions—Calvin Cycle

Light Independent Reactions—Calvin Cycle

Cellular Respiration & Photosynthesis

Category Cellular Respiration Photosynthesis

Purpose Convert stored chemical energy (glucose) to usable energy (ATP

Convert solar energy to stored chemical energy (glucose)

Who ALL CELLS Photosynthetic Autotrophs

Where Cytoplasm & Mitochondria

Chloroplast

Steps 1. Glycolysis2. Intermediate Step3. Kreb’s Cycle4. ETS

1. Light Dependent2. Light Independent

Cellular Respiration & Photosynthesis

Sun

Photosynthesis

Glucose + Oxygen Gas

Cellular Respiration

Carbon Dioxide

ATPWork

Photosynthesis in Dry Environments

C3 Plants—Plants that fix CO2 with Rubisco & make 3-C PGA

In hot / dry environment, must close stoma to conserve water

Closing of stoma reduces access to CO2

Rubisco fixes O2 instead of CO2—leading to photorespiration—consumes oxygen gas and makes carbon dioxide without making ATP (actually uses ATP)

Photosynthesis in Dry Environments C4 Plants—Plants have an alternative mode

of fixing CO2 that is more efficient

Leaf structure is slightly differentBundle sheath cells packed around veins

Mesophyll cells loosely packed around

Calvin Cycle limited to Bundle Sheath Cells

Process

PEP carboxylase fixes CO2 making a 4C product

4C product is sent to bundle sheath cells and releases CO2 for Calvin Cycle

Process requires ATP but is more efficient than CR

Photosynthesis in Dry Environments

Photosynthesis in Dry Environments

CAM Plants—crassulacean acid metabolism

Plants open stomata at night & close during the day—helps conserve water

CO2 enters stomata at night & is incorporated into organic acids stored in central vacuoles

CO2 is released from acids in the morning to enter Calvin Cycle

Photosynthesis in Dry Environments