Introduction to Photosynthesis

Chapter 8

OCC BIO-161

Dave Werner

Essential Questions

Why study Photosynthesis? What will you gain from understanding

Photosynthesis? How does Photosynthesis relate to

Cellular Respiration?

Animations

Light & Dark Reactions Podcast – Overview of Photosynthesis Photosynthesis Rap

1.ALL LIFE REQUIRES ENERGY

2.Animals, fungi, and most protists obtain their energy by consuming, directly or indirectly, organic food from their environment (heterotrophs)

3.Some organisms (autotrophs) have the ability to capture the energy of the sun to synthesize their own organic food (green plants, algae)

4.THE ULTIMATE SOURCE OF ALL ENERGY ON EARTH IS THE SUN

5.PHOTOSYNTHESIS is the link between life on earth and the sun

6.It is a set of reactions which convert light energy from the sun into chemical bond energy of glucose and ATP

Light and Pigments

The Nature of Sunlight

The Nature of Sunlight

light is a form of energy known as electromagnetic radiation

light travels in rhythmic waves which are disturbances of electrical and magnetic fields

The Nature of Sunlight

the distance between crests of electromagnetic waves is called the wavelength

the entire range of radiation is known as the electromagnetic spectrum

Light Energy (Fig.8.4) the narrow range from about 380 to 750nm in wavelength

is detectable by the human eye and is called visible light the model of light as waves explains many of its

properties, but in certain respects it behaves as though it consists of discrete particles

these particles called photons act like objects in that each of them has a fixed quantity of energy

the amount of energy is inversely related to the wavelength of light (shorter wavelengths have more energy)

Why are plants green?

Photosynthetic Pigments

as light meets matter, it may be reflected, transmitted or absorbed

substances that absorb light are called pigments

if a pigment is illuminated in white light, the color we see is the color most reflected or transmitted by the pigment

Light perception

the major pigment in leaves, chlorophyll, appears green because it absorbs red and blue light while transmitted and reflecting green

chlorophyll is actually a family of pigments with similar chemical structures

Photoexcitation of Chlorophyll

when energy is absorbed by a molecule of pigment, one of the molecules electrons is elevated to from its ground state to a higher orbital around the nucleus (excited state)

Photoexcitation of Chlorophyll the only photons absorbed are those whose

energy is exactly equal to the energy difference between the ground state and an excited state

the energy of the photon is converted to the potential energy of an electron, making the electron less stable

generally, when pigments absorb light, their excited electrons drop back down to the ground state very quickly releasing their energy as heat and/or light (fluorescence)

Overview of Photosynthesis Reactions

Photosynthesis can be summarized with this chemical equation:

• 6CO2 + 12H2O + LIGHT ENERGY/ENZYMES --> C6H12O6 + 6O2+ 6H2O

6CO2 + 12H2O + LIGHT ENERGY -> C6H12O6 + 6O2 + 6H2O

The chemical change is the reverse of cellular respiration

The low energy inorganic compounds (CO2 and water) are converted into the high potential organic molecule (glucose)

The Chloroplasts: Sites of Photosynthesis

The primary function of this specialized organelle is to convert light energy into ATP and NADPH (nicotinamide adenine dinucleotide phosphate)

Chloroplasts are found mainly in the cells of the mesophyll (about 50/cell), the green tissue on the interior of the leaf

Leaf (Fig.8.1)

Carbon dioxide enters the leaf, and oxygen exits, by way of microscopic pores called stomata

The double membrane of the chloroplast regulates transport of materials in and out

Chloroplasts are filled with an aqueous solution called the stroma which contains all the necessary enzymes for photosynthesis

Chloroplast (Fig. 8.1)

The conversion from light energy to ATP and NADPH occurs in the thylakoid membranes within the stroma

The thylakoid membranes contain all of the pigments involved in the process including chlorophyll (green pigment) and other carotenoids

The thylakoids are organized into closely packed stacks called grana

Choloroplast

Within these thylakoids and grana, light energy is converted into ATP and NADPH – these are said to be LIGHT-DEPENDENT REACTIONS

The reactions that actually convert CO2 to carbohydrate are LIGHT-INDEPENDENT REACTIONS or DARK REACTIONS

The Light Reactions - Video

Must take place in the presence of light Steps that convert solar energy to

chemical energy Light absorbed by chlorophyll drives a

transfer of electrons from water to an acceptor named NADP+ which temporarily stores the energized electrons

Light Reactions Water is split in the process and thus it is the light

reactions of photosynthesis that give off O2 as a by-product

The light reactions also generate ATP by powering

the addition of a phosphate group to ADP, a process called photophosphorylation. This is done w/ the help of ATP Synthase b/c H+ cannot diffuse through the membrane.

THE LIGHT REACTIONS PRODUCE NO SUGAR  

Electron Transport (Fig.8.2)

chloroplasts and mitochondria generate ATP by the same basic mechanism of chemiosmosis

an electron transport chain (ETC) embedded in the thylakoid membrane pumps protons across the membrane as electrons are passed through a series of carriers producing a proton-motive force (potential energy stored in the proton gradient)

ETC: ATP Synthesis in Chloroplasts ATP synthase in the membrane couples the diffusion of

hydrogen ions down their gradient to the phosphorylation of ADP

in contrast to oxidative phosphorylation in mitochondria, chloroplasts use light energy (not chemical energy in food) to drive electrons to the top of the transport chain

the proton pump of the thylakoid membrane moves hydrogen ions from the stroma to the thylakoid space which functions as the H+ reservoir

the membrane makes ATP in the stroma as hydrogen ions diffuse back down their gradient through ATP synthase

Restoring PSII (Fig. 8.13)

2H2O → 4H + 4e- + O2

Can you explain how PSII is restored?

Chemiosmosis

Happens during Light Rxns. Concentration gradient of protons across

thylakoid membrane. Where do the protons come from? ATP Synthase – protein that harnesses

energy. Some Protons are used to make NADPH from

NADP+. NADPH & ATP drive next set of rxns.

Energy Flow in Photosynthesis in its native environment of the thylakoid membrane,

chlorophyll is organized along with proteins, pigments,  and other kinds of smaller organic molecules into photosystems

a photosystem has a light gathering "antenna complex" consisting of a few hundred chlorophyll a, chlorophyll b, and carotenoid molecules

all of the antenna molecules absorb photons of light and the energy is transmitted from pigment molecule to pigment molecule until it reaches the reaction center

Photosystems II and I (Fig.8.13)

there are two photosystems in the thylakoid membranes, photosystem II and photosystem I

the chlorophyll a in both photosystems is identical, it is their association with different proteins that affects their light absorbing properties

 

The Calvin Cycle (Dark Reactions)

The "Dark Reactions" include the biochemical, enzyme-catalyzed reactions involved in the synthesis of carbohydrate from carbon dioxide; these are collectively know as the Calvin-Benson cycle

The Dark Reactions (Calvin Cycle)Video

Light is not required directly for these reactions to occur

These reactions incorporate CO2 from the air into organic material through a process known as carbon fixation

The fixed carbon is then reduced to carbohydrate by the addition of electrons

The reducing power is provided by NADPH and ATP provided by the light reactions

Dark reactions in most plants occur during daylight so that the light reactions can regenerate NADPH and ATP

These reactions occur in the stroma

The Reactions In the Stroma (Fig.8.16) 1. CO2 combines with RuBP to form 2 molecules of

PGA. 2. Each molecule of PGA is converted into a

molecule of PGAL. 3. Most of the PGAL is converted back into RuBP,

but some PGAL can be used to make a variety of organic compounds.

For every three CO2 that enter the Calvin-Benson

cycle via rubisco, a total of six molecules of 3-phosphoglyerate (PGA) are made

The Balance Sheet for Photosynthesis

Each turn of the Calvin Cycle fixes one CO2 molecule.

It take 3 turns to produce each PGAL molecule.

3 turns uses nine ATP molecules and 6 NADPH molecules. Why nine?

A variety of organic compounds are created: amino acids, lipids, and carbohydrates.

The Metabolic Fates of Glucose: About 50% of the glucose formed is used immediately

to meet the plants energy needs Excess glucose can be converted to starch within the

stroma of the chloroplast or in specialized storage cells of roots, tubers, seeds, and fruits

REMEMBER, plants actively metabolize glucose (cellular respiration) and grow in the dark and in the light

The glucose may be converted to sucrose (glucose + fructose) for transport (via the phloem cells) to the non-photosynthetic leaves, roots, and stems

The glucose may be converted to CELLULOSE, to build cell walls, especially in plant cells that are still growing and maturing

Alternative Pathways (Fig.8.20)

Calvin Cycle=most common pathway for carbon fixation.

Plants in harsh conditions fix carbon through alternate pathways and then release it to enter the Calvin cycle.

C3 Plants

Most plants are C3 plants C3 plants exclusively fix carbon through the

Calvin cycle b/c of 3-carbon PGA. Build up of O2, which slows down sugar

production.

C4 Pathway - Video

Corn, sugar cane, crabgrass. Fix CO2 into 4-Carbon compounds = C4

pathway. Partially close stomata during hottest

part of the day. Lose about ½ as much water as C3

plants when producing same amount of carbs.

CAM Pathway CAM plants have adapted to dry conditions by opening

their stomata during the night and closing them during the day, opposite to how other plants behave

when the stomata are open CO2 is incorporated into a variety of organic acids in a method of carbon fixation call crassulacean acid metabolism (CAM)

the mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning when the stomata close

CO2 is released from the acids during the day for incorporation into the Calvin cycle

Lose less water than C3 & C4 plants. Desert plants - Cactus

Rate of Photosynthesis

Now let's revisit the summary equation for photosynthesis & note how each of the raw materials end up in the chloroplasts so that the whole photosynthesis deal can go

down.

Now let's do the same, except pay attention to what

happens to the products of photosynthesis.