NOTES FOR BIOLOGY 1001

SECTION 005

Spring 2005

DR. STEVEN POMARICO

CHAPTER 7

HOW CELLS ACQUIRE ENERGY

Photosynthesis transforms light energy trapped by chloroplasts into chemical bond energy and stores that energy in sugar and other organic molecules

          -Synthesis of energy-rich organic molecules from energy-poor molecules

                     (CO₂ and H₂O)

          -Uses CO₂ as a carbon source and light-energy as the energy source.

          -Directly or indirectly supplies energy to most living organisms.

>>>>>Plants and other autotrophs are the producer of the biosphere

Organisms acquire organic molecules used for energy and carbon skeletons (a.k.a. food) by one of two nutritional modes:

                     1. Autotrophic nutrition

                     2. Heterotrophic nutrition

---Autotrophic nutrition is a nutritional mode in which the energy to live (and produce molecules) comes from some source other than organic molecules.

---Photoautotrophic nutrition is an autotrophic nutritional mode in which the energy to live (and produce molecules) comes from some light.

---Heterotrophic nutrition is a nutritional mode in which the energy to live (and produce molecules) comes from organic molecules.

>>>>>Chloroplasts are the site of photosynthesis in plants

---Photosynthesis transforms light energy trapped by chloroplasts into chemical bond energy and stores that energy in sugar and other organic molecules

          -Synthesis of energy-rich organic molecules (glucose) from energy-poor               molecules (CO₂ and H₂O)

          -Uses CO₂ as a carbon source and light-energy as the energy source.

          -Directly or indirectly supplies energy to most living organisms.

Leaves are the major organs of photosynthesis (See fig. 7.3)

          -Chlorophyll is the green pigment that gives a leaf its color

                     chlorophyll is also responsible for the absorption of the light energy                                 that drives photosynthesis

          -Chloroplasts are primarily in cells of mesophyll (in the leaf interior)

                                           (See fig 7.3b)

          -CO₂ and H₂O enter the leaf through pores called stomata

          -Water is absorbed by the roots and transported to the leaves through the                      vascular bundles.

Chloroplasts contain the thylakoids and the stroma (See fig 7.3d and e)

---Thylakoids are flattened membranous sacs inside the chloroplast.

          Chlorophyll is located in the thylakoid membrane.

          The thylakoids are arranged in stacks called grana.

          The thylakoids are where the light-dependent reactions occur.

---Grana are stacks of thylakoids in a chloroplast.

---Light-dependent reaction is the reaction of photosynthesis that converts light energy to chemical bond energy in ATP and NADPH

          -Solar energy to chemical energy

          -Occurs in the thylakoid membranes

          -NADP⁺ to NADPH

          -H₂O split, O₂ byproduct

In the light-dependent reaction:

                     H₂O + NADP⁺ + ADP => O₂ + NADPH + ATP

---Stroma is the fluid-filled space outside the thylakoids and inside the inner chloroplast membrane.

          The stroma is the site of the light-independent reaction

In the light-independent reaction:

          CO₂ + ATP + NADPH + H₂O => C₆H₁₂O₆ + ADP + NADP⁺

>>>>>Capturing The Light

>The nature of sunlight

          Light is electromagnetic energy

          Electromagnetic energy travels in waves.

                     -Wavelength and the electromagnetic spectrum (gamma rays) 10^-3 nm                                            to 10³ m (radio waves) (See fig 7.5)

                                -Visible light 400 nm to 750 nm

          Light also has particle like properties

                     -discrete particles called photons

>>>>Photosynthetic pigments

---Pigments are substances that absorb visible light

          -Chlorophyll is the pigment which is the key light-capturing molecule in                          thylakoid membranes.

          -Other pigments (carotenoids and phycocyanins) are called accessory                                 pigments

          -Different pigments absorb different wavelengths of light

          -Absorption versus reflection (or transmittance)

          -The pattern of absorption is called the absorption spectrum

                                                                (See fig. 7.6)

>Photoexcitation of chlorophyll (see fig. 10.10)

          An electron in a chlorophyll molecule is boosted to an excited state from its                      ground state by the absorbed light energy (photons).

          The excited electron may:

                     -be transferred to an electron carrier molecule

          or 

                     -fall back to the ground state, releasing its energy as:

                                Heat or light (fluorescence)

>>>Photosystems are the assemblies which turn light energy into chemical energy in the thylakoid membranes.

Chlorophyll and the accessory pigments are arranged into photosystems (See fig. 7.10)

          Components of a photosystem:

                     1. Light-harvesting complex

                     2. Reaction-center chlorophyll

                     3. Electron transport system

There are two types of photosystems

                                1. Photosystem I (PS-I)

                                2. Photosystem II (PS-II)

>Noncyclic electron flow (See fig. 7.13)

The electrons that are excited from each of the photosystems can be donated to a primary electron acceptor.

          -Both photosystems are active

          -Electrons flow in a path from one electron carrier to the next.

                     Light excites electrons in PS-II (P680)

                     Electrons transferred to the primary electron acceptor

                     Electrons flow down the electron transport chain

                     Electrons transferred to PS-I (P700)

                     Light excites electrons in PS-I (P700)

                     Electrons transferred to the primary electron acceptor

                     Electrons transferred to NADP⁺

                     Electrons (and reducing power) are stored in NADPH

          -Electrons from the splitting of H₂O (photolysis), replace the electrons lost from PS-II and

                      generate O₂ as a byproduct.

As the electrons flow through this non-cyclic pathway they are gaining energy (See fig. 7.13)

The flow of electrons and the splitting of water also creates a proton gradient across the thylakoid membrane, which then is used to synthesize ATP by chemiosmosis.

This process is known as noncyclic photophosphorylation

>Cyclic electron flow (see fig. 10.14)

          -Only photosystem I is active

          -Electrons flow in a path form one electron carrier to the next.

                     Light excites electrons in PS-I (P700)

                     Electrons transferred to the primary electron acceptor

                     Electrons flow down the electron transport chain

                     Electrons transferred to PS-I (P700)

          -Electrons from the electron transport chain replace electrons lost from PS-I.

          -No H₂O split and no O₂ generated

-The flow of electrons creates a proton gradient across the thylakoid membrane, which is used to synthesize ATP by chemiosmosis.

This process is known as cyclic photophosphorylation.

>>The Light-Independent Reaction

In the light-independent reaction the chemical energy stored from the light-dependent reaction is used to make glucose.

CO₂ + ATP + NADPH + H₂O => C₆H₁₂O₆ + ADP + NADP⁺

---Calvin-Benson cycle (a.k.a. The C₃ cycle) is the cycle of reactions in photosynthesis in which atmospheric carbon CO₂ is fixed (carbon fixation) using ATP and NADPH

          -Occurs in the chloroplast stroma

          -No direct light energy required

          -NADPH and ATP provide the chemical energy.

The Calvin-Benson Cycle is divided into 3 phases

          1. Carbon fixation

          2. PGAL synthesis

                                ATP is used

                                NADPH is used

          3. Regeneration of ribulose bisphosphate

                                Sugar carbons are shuffled around to make 3 5-carbon sugars                                                                 from 5 3-carbon sugars

Note: The section of alternative mechanisms of carbon fixation (pages 126-127) will not be covered in lecture. You will be responsible for this material and should know the MAIN POINTS ONLY.