CH. 9 - Photosynthesis

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1. General

1. Autotroph is any organism that manufactures its own food from inorganic

2. Photosynthesis - autotrophs harnessing the energy from the sun

3. Chemosynthesis - autotrophs harnessing energy from chemicals to obtain energy

Keywords

1. Photosynthesis

1. Chlorophyll - the pigments where the magic happens

2. Cyclic Electron Flow - excited electron of P700 moves along series of electron carriers, producing ATP from ADP.

3. Dark Reactions - incorporates CO2 into organic molecule via carbon fixation

4. Ferrodoxin - one of the early electron carriers in the electron chain transport

5. Grana - stacks of thyladoid sacs

6. Light Reaction - also called photolysis.  Convert solar energy to ATP and NADPH

7. Photoionization - escape of high energy electrons from chlorophyll molecule

8. Photosystem - light capturing unit of the thylakoid

9. Ribulous biphosphate - start of calvin cycle, 5 carbon sugar that CO2 fixed to

10. Stroma - the fluid matrix of the chloroplast

11. Thylakoid membranes - where the chlorophyll resides

Photosynthesis

1. General

1. Take place in plants in a specialized organelle called chloroplast

2. Photosynthetic bacteria that lack chloroplast have cell membranes that function the same way

2. Structure of a chloroplast

1. Chloroplast is really a plastid containing the chlorophyll pigment

2. Bound by two membranes

1. Contains within a network of membranes called thylakoid membranes

2. Chlorophyll found within the thylakoid membrane

3. Thylakoid sacs are stacked into columns called grana

3. The fluid matrix of the chloroplast is called the stroma

4. Chlorophyll is usually complexed with metal magnesium

5. Light and its role

1. When chlorophyll absorbs light, electrons are excited and harnessed to drive reactions of photosynthesis

2. Chlorophyll absorbs in the red and blue wavelengths (thus green)

6. Two main types of chlorophyll

1. Chlorophyll A

2. Chlorophyll B

7. Chlorophyll molecules are part of 2 different photosystems

8. Photosystem

1. Light capturing unit of the thylakoid membrane

2. Center of the photosystem is a single chlorophyll molecule coupled to proteins that is excited by the absorbed photons

3. Photosystem I

1. chlorophyll A absorbs best at 700 nm, called P700

4. Photosystem II

1. Chlorophyll B absorbs best at 680 nm, called P680

3. Overview of Photosynthesis

1. Reduces CO2 to carbohydrate

1. Releases oxygen from water as byproduct

2. Reaction is the reverse of respiration

1. 6 CO2 + 12 H2O + light -> C6H12O6 + 6 O2 + 6 H2O

3. Can be divided into two reactions

1. Light reaction - converts solar energy into chemical in form of ATP and NADPH

2. Dark reaction - coupled to light reaction and incorporates CO2 into organic molecules called carbon fixation.

1. Also called reduction synthesis because carbohydrates are produced by reducing CO2

3. Both happens in chloroplasts

4. Light Reactions

1. Begins with absorption of light by chlorophyll molecule

2. The excited electron of the chlorophyll can flow along 2 pathways

3. Cyclic Electron Flow

1. The excited electron of P700 move along a chain of electron carriers

2. Via a series of redox reaction, electron eventually goes back to P700

3. Produces ATP in the process called cyclic photophosphorylation

4. Uses a coenzyme carrier called ferrodoxin, an early electron carrier in this chain

4. Noncyclic electron flow

1. Key pathway of the light reaction and involves both photosystem

2. Instead of electron returning to P700, it goes to electfon acceptor NADP+. 

1. P700 is left with electron "holes" and becomes powerful oxidizing agent

3. When light strikes P680 in photosystem 2, electrons are excited again.

1. The electrons would fill the holes in the P700

2. P680 is strong enough to oxidize water and fill its hole

3. Water is split into two hydrogen ions and oxygen atom.

1. Oxygen combine to form O2

4. The net result is the production of NADPH and ATP and break down of water, releasing oxygen.

5. Chemical Aspects of Photosynthesis

1. Oxygen produced in photosynthesis comes from water, not carbon dioxide.

2. Photoionization - escape of high energy electrons from chlorophyll

6. The Dark Reaction

1. Uses ATP and NADPH from light reaction to reduce CO2 to carbohydrate

2. Although doesn't directly require light, it only happens during day

3. Also called calvin cycle, carbon-fixation/reduction synthesis

4. Product of the cycle is three carbon sugar Phosphoglyceraldehyde (PGAL)

1. Cycle must take place three times to make 3 carbon sugar

5. Cycle begins with CO2 added to ribulous biphosphate

1. This produces 6-carbon intermediate

2. Splits into 2 3-carbon molecules

1. 3 carbon molecules called 3-phosphoglyceric acid

2. Phosphorylated by ATP, reduced by NADPH to give glyceraldehyde 3-phosphate (PGAL)

3. 2 PGAL converted to glucose

7. Summary of Calvin Cycle

1. In 6 turns of Calvin Cycle

1. 12 PGAL form 6 ribulose biphosphate

2. 1 molecule of glucose

Plant Structure

1. The Leaf

1. Waxy cuticle - reduce transpiration and conserve water on upper surface.

2. Palisade - layer of elongated chloroplast-containing cells spread over a large surface area.  Well exposed to light.

3. Spongy Layer - stomata opens into air spaces that allows more air into internal moist surface with loosely packed spongy layer cells.

4. Guard Cells - surround each of the stomata on the lower surface of the leaves (opens or closes the stomata)

1. During the day, produces glucose

1. High glucose content causes swelling and produce a curvature of opening

2. When photosynthesis stops, cell turgor decreases and stomate closes

5. Stomata - openings in the lower epidermis that permits diffusion of carbon dioxide, water vapor, and oxygen between leaf and the atmosphere

1. Size of stomate opening regulated by guard cells

2. Opens during day to admit CO2, close at night to limit loss of water vapor

6. Vascular Bundle

1. Veins containing xylem and phloem to bring water to leaf (xylem) and carry food out of leaf (phloem)

2. The Root

Specialized root hairs found in root and increases surface for absorption of water and minerals by diffusion and active transport