Chapter 5 Photosynthesis

ATP Synthase Very Technical https://www.youtube.com/watch?v=kXpzp4RDGJI https://www.youtube.com/watch?v=k_DQ1FjFuYM

Chlorophyll Plant pigment Absorbs certain wavelengths of energy (photons) from the sun Absorbed energy excites electrons The pigments in the human eye absorb many different wavelengths pretty well: that’s why we can see so many colors. When plants use sunlight’s energy to make sugar during photosynthesis, they also use the visible portion of the electromagnetic spectrum. Plant pigments are embedded in the thylakoid membrane. When pigment absorbs energy, electrons are “excited” —light energy is converted to kinetic energy of electrons. Light energy is kinetic energy made of particles called photons. 4

ROY G BIV

The primary photosynthetic pigment, called chlorophyll a, efficiently absorbs blue-violet and red wavelengths of light. Every other wavelength generally travels through or bounces off this pigment. Because chlorophyll a cannot efficiently absorb green light and instead reflects those wavelengths back, our eyes and brain perceive the reflected light waves as green, and so the pigment (and the leaves in which it is found) appears green. Another pigment, chlorophyll b, is similar in structure but absorbs blue and red-orange wavelengths. Chlorophyll b reflects back yellow-green wavelengths. A related group of pigments called carotenoids absorbs blue-violet and blue-green wavelengths and reflects yellow, orange, and red wavelengths. Figure 4-14 (part 1) Plant pigments. 6

Figure 4-14 (part 2) Plant pigments. Each photosynthetic pigment absorbs and reflects specific wavelengths. Why do the leaves of some trees turn beautiful colors each fall? In the late summer, cooler temperatures cause some trees to prepare for the winter by shutting down chlorophyll production and reducing their photosynthesis rates, going into a state that resembles an animal’s hibernation. Gradually, the chlorophyll a and b molecules present in the leaves are broken down and their chemical components are stored in the branches. As the amounts of chlorophyll a and b in the leaves decrease relative to the remaining carotenoids, the striking colors of the fall foliage are revealed. During the rest of the year, chlorophyll a and b are so abundant in leaves that green masks the colors of the other pigments. 7

Chloroplasts The organelle WHERE photosynthesis happens

INSIDE Chloroplasts Thylakoid saclike membranes where photosynthesis happens. They are arranged in stack (the stacks are called grana) Stroma Area inside the chloroplast but outside the thylakoids  

2 Stages of Photosynthesis Light dependent Light independent

LIGHT-DEPENDENT REACTIONS Needed Light Water ADP NADP+

LIGHT-DEPENDENT REACTIONS Produces Oxygen O2 (unnecessary byproduct) ATP NADPH  

LIGHT-DEPENDENT REACTIONS Photosystems (Use Light Energy) Photosystem II Photosystem I

PHOTOSYSTEM chlorophyll and other molecules organized on the thylakoid membrane

Photosystem II Light is Absorbed Energy from light is used by chlorophyll (other pigments too) Energy is ultimately used to excite an electron from a water molecule

The Water Splits This splitting releases O2 The 2 H+ are put into the inside of the thylakoid membrane (they are very useful there, later)

This electron from the water molecule gets passed through an electron transport chain Energy from this electron is used to pump H+ ions and create the H+ ion concentration gradient.

Light is used by chlorophyll molecules Electrons are reenergized in Photosystem I NADP+ picks up these electrons (plus H+ from the stroma) becoming NADPH

What about that Positive + Thylakoid? H+ ions pass from inside the thylakoid to the outside stroma through a protein called ATP SYNTHASE

H+ is moving from area of higher concentration to an area of lower concentration of H+ This passing of H+ ions makes the ATP Synthase protein physically rotate  This physical rotation makes ATP from ADP

An Electron Transport Chain Is important to both photosynthesis and cellular respiration. Think of a pump pushing water into an elevated tank, creating a store of potential energy that can run out of the tank with great force and kinetic energy, which can be harnessed to do work, such as moving a large paddle wheel. Similarly, the protons eventually rush out of the thylakoid sacs with great force—and that force is harnessed to build energy-storing ATP molecules, one of the two products of the “photo” portion of photosynthesis. Figure 4-18 The electron transport chain. Excited electrons pump H+ ions across the membrane, like water behind the dam. The ATP Synthase enzyme is like the hydroelectric generator making electricity – makes ATP. 24

Two photosystems involved: In the first part of photosynthesis, the “photo” part, sunlight hits a plant and, in a three-step process, the energy in this sunlight is ultimately captured and stored in an ATP molecule and another molecule (called NADPH) that stores energy by accepting high-energy electrons. Figure 4-19 Summary of the “photo” reactions. Some excited electrons are used to make ATP and some are passed on to a high-energy electron carrier (NADPH). 25

Light Independent Reactions (aka) Calvin Cycle NO LIGHT NEEDED Produces sugar (glucose) that can be used or stored WHERE In the stroma of the chloroplast  

Calvin Cycle Uses/Needed CO2 from that air ATP & NADPH ****from Light Dependent Reactions***

Calvin Cycle EACH CYCLE PRODUCES ***1 SIX CARBON LONG SUGAR MOLECULE This sugar is stored as is (C6H12O6) OR Used to build bigger and more complex molecules Examples Cellulose Carbohydrates

The Calvin Cycle (Dark reactions) Series of chemical reactions Occurs in stroma Enzymes and starting material (5 carbon molecule) are recycled All the Calvin cycle reactions occur in the stroma of the leaves’ chloroplasts, outside the thylakoids. Plants carry out these reactions using the energy stored in the ATP and NADPH molecules that are built in the “photo” portion of photosynthesis. This dependency links the light-gathering (“photo”) reactions with the sugar-building (“synthesis”) reactions. Figure 4-20 Overview of the “synthesis” reactions of photosynthesis. 31

Summary of Photo-synthesis