Chapter 8.2

Overview of Photosynthesis

Overview of Photosynthesis

1.) The process by which autotrophs convert sunlight energy into chemical energy for use by their cells is called

2.) How can it be said that photosynthesis “supports most life on Earth?”

With the exception of chemoautotrophs, organisms that make food directly from sunlight (photoautotrophs) form the basis of all food chains on Earth.

3.) Visible light or white light is made up of a spectrum of colors. Each color has a different wavelength and each wavelength contains a different amount of energy.a. Which color (wavelength) contains the most energy? b. Which color contains the least energy? c. Which color is reflected by plants that appear green?

photosynthesis.

greenred

violet / blue

4.) Photoautotrophs obtain energy from light wavelengths that they __________, but not from colors that they ________.

5.) A __________ is a molecule that absorbs certain wavelengths (colors) of

light and reflects others.

Overview of Photosynthesis

absorb reflect

pigment

Photosynthetic pigment Colors/Wavelengths of Light Absorbed

Colors/Wavelengths of Light Reflected

chlorophyll a

chlorophyll b

carotenoids

violet / blue / red green / yellow

blue / orange

blue

green / yellow

green / yellow / orange / red

6b.) What color(s)/ wavelength(s) of visible light are absorbed by all three of the photosynthetic pigments in the chart?

Why might this be so?

6c.) What color(s)/ wavelength(s) of visible light are reflected by all three of these photosynthetic pigments?  

Overview of Photosynthesis

violet / blue

green

Violet/ blue light contains the most energy, therefore plants might have adapted pigments that can capture this energetic light.

Overview of Photosynthesis7.) Which of the three photosynthetic pigments in the table is most

commonly found in plants? ____________ 8.) Given the information from questions #6 and #7, why do most plants

appear green?

9.) Photosynthetic pigments are located in an organelle called a ____________.

chlorophyll a

Most plants appear green because they contain chlorophyll a, a pigment that reflects green light.

chloroplast

10.) Below is a diagram of a chloroplast. Label the following parts:

Overview of Photosynthesis

granum

intermembrane space

outer membrane

lumen

stroma

inner membrane

thylakoid

interior of the thylakoid

Overview of Photosynthesis• Photosynthesis occurs in two phases:

1. Light dependent reactions• Occur in the thylakoids• Light energy is converted into chemical energy (ATP and NADPH)

2. Light independent reactions / Calvin cycle• Sometimes called dark reactions, but this is inaccurate• Occur in the stroma• ATP and NADPH used to make glucose

Light dependent reactions

Calvin cycle

Typical chloroplast

Glucose

Overview of Photosynthesis

Calvin Cycle

Overview of Photosynthesis

ADP + P NADP+

Calvin Cycle

Light Dependent Reactions

Light-Dependent Reaction Cast of Characters• Electron transport chain –

– located in the thylakoid membrane; transports energized electrons along photosystems and proteins

• Photosystem – – Holds chlorophyll; absorbs light energy and passes energy along to electrons

• Water (H2O) – – Splits to release energized electrons to photosystem, hydrogen inside the thylakoid,

and oxygen as a waste product• Ferrodoxin –

– Product that transfer energized electrons to NADP+ to make NADPH• NADPH -

– Chemical that accepts electrons and stores energy from electrons• Concentration Gradient –

– Difference in concentration of H+ ions; high concentration inside the thylakoid membrane and low concentration outside the membrane

• ATP Synthase– Enzymes that allow H+ ions to pass through and initiate ATP synthesis

Light-Dependent Reaction1. Light energizes electrons in photosystem II.

Light energy is used to split water molecules releasing an electron into the electron transport chain. When water splits, protons (H+ hydrogen ions) stay in the thylakoid space and oxygen is released from the cell.

Light-Dependent Reaction2. An energized electron moves down the

electron transport chain to the photosystem I. As electrons are passed down the chain, H+ ions are pumped across the membrane, increasing the concentration of H+ in the thylakoid

Light-Dependent Reaction3. Electrons are reenergized at photosystem I.

Electrons are transferred to the protein, ferrodoxin. These energized electrons are them passed onto the NADP+ to make NADPH. The electron carrier NADPH stores the energy to be used to make glucose.

Light-Dependent Reaction

4. Chemiosmosis – production of ATP molecules. The high concentration of H+ ions in the thylakoid diffuses through the ATP synthase . The diffusion of H+ ions supplies energy to the ATP synthase to produce ATP.

Calvin Cycle(a.k.a. dark reactions or light independent reactions)

Carbon fixation: Six CO2 molecules combine with six 5-carbon compounds and are rearranged into twelve 3-carbon compounds (PGA).

STEP 1.

ATP and NADPH from the LD rxn are used to convert twelve PGA molecules into twelve G3P molecules.

STEP 2.

Two G3P molecules leave the cycle and are converted into glucose or other compounds.

STEP 3.

An enzyme (Rubisco) converts the remaining ten G3P back into six 5-carbon compounds.

STEP 4.

Calvin cycle (recap)

1. Carbon fixation: Six CO2 molecules combine with six 5-carbon compounds and are rearranged into twelve 3-carbon compounds (PGA).

2. ATP and NADPH from the LD rxn are used to convert twelve PGA molecules into twelve G3P molecules.

3. Two G3P molecules leave the cycle and are converted into glucose or other compounds.

4. An enzyme (rubisco) converts the remaining ten G3P back into six 5-carbon compounds.

Carbon accounting

(6*1) + (6*5) =

6 + 30 = 36

12 * 3 = 36

36 – (2 * 3) =

36 – 6 = 30

(10 * 3) = 30

(6 * 5) = 30

C4 pathway

• Fix carbon into 4-carbon compounds instead of 3-carbon compounds.

• Keep the pores in their leaves (called stomata) closed during hot days.

• Minimizes water loss while allowing these 4-carbon compounds to enter the Calvin cycle

• Occurs in sugar cane and corn.

CAM plants• Crassulacean acid metabolism (CAM)

photosynthesis• Allow CO2 to enter leaves only at night.• CO2 is fixed into organic compounds.• During the day CO2 can be released

from these compounds and used in the Calvin cycle.

• Water-conserving plants that live in harsh environments (salt marshes, deserts, high altitude)

• Cacti, orchids, pineapple