Chapter 6_Section 1

Mr. Camm

Biology

Review text pages: 112-129

I. What is Photosynthesis?Definition: “Light” + “Putting Together” The process by which

photoautotrophic cells containing chlorophyll in green plants convert light energy into chemical energy and synthesize organic compounds from inorganic compounds, especially carbohydrates from carbon dioxide and water, accompanied by the simultaneous release of oxygen.

Photosynthetic Equation:

6CO2 +6H20 Light Energy C 6 H 12 06 + 602

Why are Carbohydrates Important…Carbohydrate- An essential structural component of living cells

(cellulose in plants), as well as storage and transport of energy (starch & glycogen)

- Created by photosynthetic process of plants and contain a combination of only carbon, hydrogen, and oxygenSaccharides are a group of carbohydrates that

include sugars and starches. Derived from a Greek word meaning "sugar".

Where does Your Energy Come From?Almost all energy used by organisms to carry out the functions of life comes directly or indirectly from the Sun.

Obtaining Energy: Two Classifications of organismso Autotroph “Self-Nutrition”

- An organism that uses energy from sunlight or from chemical bonds in inorganic substances to make organic compounds. (i.e. Plants & algae)

o Heterotroph “Another Nutrition” - An organism that receives its energy from food instead of

directly from sunlight or inorganic substances. (i.e. Humans, Animals, Fungi, and some Bacteria)

The Kingdom PlantaeGreen algae, mosses, ferns, pine trees, oak trees, shrubs, vines, grasses, herbs

Eukaryotic cells with walls of celluloseStore carbohydrate as starchShare particular photosynthetic pigmentsindeterminate growth

Means they can grow and grow as long as a biotic factors allow.)

II. Two Stages of Photosynthesis:

Light ReactionLight energy from the sun is absorbed by autotrophs and is converted into chemical energy, which is stored in ATP and the energy carrier molecule NADPH.

Calvin CycleOrganic compounds are formed using carbon dioxide and the chemical energy stored in ATP and NAPHD

Light Reactions:First stage of photosynthesis. Requires Light to begin reaction. Occur in Thylakoid membranes or grana, located in Chloroplasts. Chloroplasts- specialized organelles in plants and algae that absorb light. o Thylakoid

-are flattened sacs stacked to form grana. o Stroma -is a solution surrounding the grana.

Properties of LightAlthough the sun’s light appears to be white, it is made of a visible spectrum of colors. (Page 115, Figure 6-4)

Light can be reflected, transmitted, or absorbed when it hits an object. Pigments- compounds that absorb light. Normally absorbing specific colors more strongly than others.

(i.e. Chlorophyll & Carotenoids)

-When colors are absorbed, the remaining reflected or transmitted colors from the spectrum are visible.

What is Chlorophyll, and where is it located in a plant cell?

Chlorophyll is a green pigment located in the thylakoids of a plant cell, that absorbs light most strongly in the blue and red, but poorly in the green portions of the light spectrum.• Responsible for the green appearance of plant leaves because

it does NOT strongly absorb green light.

Two types:• Chlorophyll a

• pigment directly involved in photosynthetic light reactions. [absorbs less blue, but more red]

• Chlorophyll b• an accessory pigment that helps capture light energy but is

not directly involved in photosynthesis.

Properties of Light Continued…

Carotenoids- other accessory pigments [yellow, orange, and brown] that are located in the thylakoids and assist photosynthesis indirectly by absorbing colors that chlorophyll a cannot, therefore capturing more light energy. These are visible in the absence of chlorophylls in the fall, as observable by vibrant fall leaves.

III. Light Energy to Chemical EnergyA. Pigments in chloroplast capture light energy.B. Light energy is converted to chemical energy.C. Chemical energy is stored in ATP and NADPH. D. Oxygen (O2) is released E. Accessory pigment molecules in Photosystems I

and II absorb light. Energy is passed to other pigment molecules until it reaches a specific pair of Chlorophyll a molecules.

Photosystem- cluster of pigment molecules and proteins they are embedded in. There are two types of photosystem (I and II) with similar pigments types, but different light reaction roles.

Light Reaction Steps: p.116

http://www.cnr.vt.edu/dendro/forestbiology/photosynthesis.swf

Steps in Light to Chemical Reaction

1) Light energy forces electrons to enter a higher energy level in the two Chlorophyll a molecules of photosystem II, causing them to reach an excited state.

2) Excited electrons can leave Chlorophyll a molecules, making the molecules undergo an oxidation reaction that must be accompanied by a reduction reaction. The lost electrons are accepted by the primary electron acceptor located in the thylakoid membrane.

3) The primary electron acceptor donates electrons to the electron transport chain, a series of molecules that transfer electrons from one molecule to the next. This transfer causes the electrons to lose most of their excited energy. The lost energy is used to move protons into the thylakoid.

Steps Continued…4. Light is absorbed by photosystem I and II simultaneously. Electrons move

from a pair of chlorophyll a molecules in photosystem I to another primary electron acceptor. Lost electrons from this process are replaced by the electrons that have passed through the electron transport chain from photosystem II.

An enzyme inside the thylakoid splits water molecules into protons, electrons, and oxygen. {For every 2 molecules of water split, 4 electrons are available to replace those lost by chlorophyll molecules in photosystem II. }Protons are left inside thylakoid, and oxygen defuses out of chloroplast and eventually the plant.

5. NADP+ is reduced to NADPH after the primary electron acceptor of photosystem I donates electrons to a different electron transport chain that brings them to the side of the thylakoid that faces the stroma. At this location the electrons combine with a proton and NADP+.

NADP+ - an organic molecule that accepts electrons during oxidation/reduction reactions.

Summary of Processes

Replacing Electrons in Light ReactionsElectrons from photosystem II replace electrons thatleave photosystem I. Replacement electrons forphotosystem II are provided by the splitting of watermolecules.

Oxygen produced when water molecules are split diffuses out of the chloroplast and then leaves the plant.

Step 4

2H20 4H + 4e + 02

Therefore, in light reactions electrons flow fromH2O ® PSII ® PSI ® NADP

one way electron flow, makes ATP & NADPHBecause ATP generated, called NON-CYCLIC PHOTOPHOSPHORYLATION

Making ATP in Light Reactions

An important part of the lightreactions is the synthesis of ATP.

During chemiosmosis, the movementof protons through the enzyme ATP synthase into the stroma releases energy, which is used to produce ATP.

Relies on the concentration gradient of protons across the Thylakoid membrane

Text p. 118

Section 1_Conclusion

The Calvin Cycle

Chapter 6_Section 2 p. 120-124

DARK REACTIONS (Carbon Fixation)

Enzymatic reactions; temperature sensitiveOccur in STROMA of chloroplastATP and NADPH produced in light reactions used to reduce CO2

Occurs via cyclical reaction = CALVIN CYCLE

also known as C3 photosynthesis

Carbon Fixation

The ATP and NADPH produced in the light reactions drive the second stage of photosynthesis, the Calvin cycle.

In the Calvin cycle, CO2 is incorporated into organic compounds, a process called carbon fixation.

The Calvin cycle, which occurs in the stroma of the chloroplast, is a series of enzyme-assisted chemical reactions that produces a three-carbon sugar.

Most of the three-carbon sugars (G3P) generated in the Calvin cycle are converted to a five-carbon sugar (RuBP) to keep the Calvin cycle operating. But some of the three-carbon sugars leave the Calvin cycle and are used to make organic compounds, in which energy is stored for later use.

Alternative Pathways• The C4 Pathway

– Some plants that evolved in hot, dry climates fix carbon through the C4 pathway. These plants have their stomata partially closed during the hottest part of the day.

– Certain cells in these plants have an enzyme that can fix CO2 into four-carbon compounds even when the CO2 level is low and the O2 level is high. These compounds are then transported to other cells, where the Calvin cycle ensues.

Factors that Affect Photosysnthesis• Light Intensity

– The rate of photosynthesis increases as light intensity increases, because more electrons are excited in both photosystems.

– However, at some point all of the available electrons are excited, and the maximum rate of photosynthesis is reached. The rate then stays level regardless of further increases in light intensity.

• Carbon Dioxide Levels– As with increasing light intensity, increasing levels of carbon

dioxide also stimulate photosynthesis until the rate levels off.