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Flashcards in Chapter 10 - Photosynthesis Deck (47):

What is photosynthesis?

The process that feeds the biosphere!

The process that converts light energy into chemical energy; directly or indirectly, photosynthesis nourishes almost the entire living world.

*Photosynthesis occurs in plants, algae, certain and other unicellular eukaryotes, and some prokaryotes


An organism acquires the organic compounds it uses for energy and carbon skeletons by one of which two major modes?

1. Autotroph nutrition (autotrophs)

2. Heterotrophic nutrition (heterotrophs)


What are autotrophs?

Autotrophs are self feeders and can sustain themselves without eating anything derived from other living beings; autotrophs produce their own organic molecules from CO2, and other inorganic raw materials obtained from the environment;

They are the the ultimate sources of organic compounds for all nonautotrophic organisms and are the PRODUCERS of the biosphere


What are photoautotrophs?

Organisms that use light as a source of energy to synthesize organic substances; almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules.


What are heterotrophs?

Organisms that obtain organic material by the second major mode of nutrition; unable to make their own food, they live on compounds produced by other organisms; heterotrophs are the biospheres CONSUMERS; almost all heterotrophs demend on photoautotrophs for food and O2


What are decomposers?

A type of heterotroph that consumes the remains of dead organisms by decomposing and feeding an organic litter such as carcasses, feces, and fallen leaves; most fungi and many types of prokaryotes get their nourishment this way.


What do fossil fuels represent?

Stores of the sun's energy from the distant past


What are the sites of photosynthesis in plants?

Chloroplasts; structurally similar to and likely evolved from photosynthetic bacteria; the structural organization of these organelles allow for the chemical reactions of photosynthesis to take place

*Solar powered chemical factories


What is mesophyll?

The tissue in the interior of the leaf, where chloroplasts are mainly found


What are stomata?

Microscopic pores on the outer layer of leaves where carbon dioxide enters and oxygen exits; how plants "breathe"


What is the stroma?

A chloroplast has an envelope of two membranes surrounding a dense fluid called the stroma


What are thylakoids?

Suspended within the stroma is a third membrane system, made up of sacs called thylakoids, which segregates the thylakoid space inside these sacs. In some places, thylakoid sacs are stacked in columns called grana


What is chlorophyll?

The green pigment that gives leaves their color; resides in the thylakoid membrane of the chloroplast


What are the major organs of photosynthesis in plants?



What is the summarized equation of photosynthesis?

6 CO2 + 6 H2O + Light Energy ---> C6H12O6 + 6 O2

*The overall chemical change during photosynthesis is the reverse of the one that occurs during cellular respiration


How is H2O split during photosynthesis?

Chloroplasts split H2O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules and releasing oxygen as a by product


How is photosynthesis a redox reaction?

Photosynthesis reverses the direction of electron flow compared to respiration; water is split, and electrons are transferred along with hydrogen ions from the water to carbon dioxide reducing it to sugar;

Photosynthesis is a redox process in which H2O is oxidized and CO2 is reduced;

Photosynthesis is an endergonic process, the energy boost is provided by light


What are the two stages of photosynthesis?

1. Light reactions (the photo part)

2. Calvin cycle (the synthesis part)


What are the light reactions? What are the steps?

The light reactions (in the thylakoids) are the steps of photosynthesis that convert solar energy to the chemical energy NADPH and ATP;

1. H2O is split
2. O2 is released
3. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from water to an acceptor called NADP+, where they are temporarily stored
4. The light reactions use solar energy to reduce NADP+ to NADPH by adding a pair of electrons along with an H+
5. The light reactions generate ATP, using chemiosmosis to power the addition of a phosphate group to ADP through photophosphorylation (the process of generating ATP from ADP and phosphate by means of chemiosmosis, using a proton-motive force generated across the thylakoid membrane)

*Light energy is thus initially converted to chemical energy in the form of two compounds: NADPH (reducing power) and ATP


What is the Calvin cycle?

Takes place in the stroma and forms sugar from CO2, using ATP and NADPH; reduces the fixed carbon carbohydrate by the addition of electrons


What is carbon fixation?

In the Calvin cycle, the initial incorporation of carbon into organic compounds


What are pigments?

Light receptors; substances that absorb visible light; different pigments absorb different wavelengths; wavelengths that are not absorbed are reflected or transmitted

*If a pigment absorbs all wavelengths, it appears black

*White light is a mixture of all wavelengths of visible light

*Leaves appear green because chlorophyll reflects and transmits green light


What is light?

Light is a form of energy known as electromagnetic energy, also called electromagnetic radiation and travels in rhythmic waves


What is a wavelength?

The distance between the crests of electromagnetic waves; wavelength determines the type of electromagnetic energy

*The entire range of radiation is known as the electromagnetic spectrum


What is visible light?

A type of radiation that can be detected as various colors by the human eye; consists of wavelengths that produce the colors we can see

*Visible light drives photosynthesis


What are photons?

Not tangible objects, but they act like objects in that each of them has a fixed quantity of energy; light behaves as though it consists of these discrete particles called photons; the amount of energy is inversely related to the wavelength of the light: the shorter the wavelength, the greater the energy of each photon of that light; a photon of violet light packs nearly twice as much energy as a photon of red light


What is a spectrophotometer?

Measures a pigments ability to absorb different wavelengths; this machine sends light through pigments and measures the fraction of light transmitted at each wavelength


What is the absorption spectrum?

A graph plotting a pigment's light absorption versus wavelength


What is chlorophyll a?

The key light capturing pigment that participates directly in the light reactions; the absorption spectrum for chlorophyll a suggests that violet blue and red light works best for photosynthesis


What is chlorophyll b?

A separate group of accessory pigments called carotenoids


What is an action spectrum?

Profiles the relative effectiveness of different wavelengths of radiation in driving a process


What are carotenoids?

Hydrocarbons that are various shades of yellow and orange because they absorb violent and blue green light; carotenoids may broaden the spectrum of colors that can drive photosynthesis

Photoprotection; they absorb excessive light that would damage chlorophyll


What happens when chlorophyll and other pigments absorb light?

When a pigment absorbs light, it goes from a ground state to an excited state; when a molecule absorbs a photon of light, one of the molecule's electrons is elevated to an orbital where it has more potential energy.

*A particular compounds absorbs only photons corresponding to specific wavelengths, which is why each pigment has a unique absorption spectrum

*The excited state is unstable, so generally, when isolated pigment molecules absorb light, their excited electrons drop back down to the ground state orbital, releasing their energy as heat; photons are given off, an afterglow called fluorescence


What is a photosystem?

Composed of a reaction-center complex surrounded by several light-harvesting complexes; each photosystem functions in the chloroplast as a unit. It converts light energy t chemical energy, which will ultimately be used for the synthesis of sugar.

*The photosystem is located in the Thylakoid membrane


What is a reaction-center complex?

An organized association of proteins holding a special pair of chlorophyll a molecules


What is a light harvesting complex?

Consists of various pigment molecules (which may include chlorophyll a, chlorophyll b, and multiple carotenoids) bound to proteins


What is the primary electron acceptor?

Within the reaction-center complex, the primary electron acceptor is a molecule capable of accepting electrons and becoming reduced.

The solar powered transfer of an electron from the reaction-center chlorophyll a pair to the primary electron acceptor is the first step of the light reactions; as soon as the chlorophyll electron is excited to a higher energy level, the primary electron acceptor captures it; this is a redox reaction.


What are the two types of photosystems located within the thylakoid membrane?

1. Photosystem II (PS II) - functions first in the light reactions

2. Photosystem 1 (PS I)


What is the reaction-center chlorophyll a of PS II known as?

P680; this pigment is best at absorbing light have a wavelength of 680 nm


What is the reaction-center chlorophyll a of PS I known as?

P700; it most effectively absorbs light of wavelength 700 nm


What are the two main products of the light reactions?



How does light drive the synthesis of ATP and NADPH?

By energizing the two photosystems embedded in the thylakoid membranes of chloroplasts; the they to this energy transformation is the flow of electrons through the photosystems and other molecular components built into the thylakoid membrane


During the light reactions, what are the two possible routes for electron flow?

Linear and cyclic


What is the linear electron flow?

The primary pathway that occurs during the light reactions of photosynthesis; involves both photosystems and produces ATP and NADPH using light energy


What are the 8 steps in the linear electron flow?

1. A photon of light strikes a pigment molecule, boosting one of its electrons to a higher energy level; it's energy is passed among pigment molecules until it excites the pair of P680 chlorophyll a molecule in the PS II reaction center complex

2. An excited electron from P680 is transferred to the primary electron acceptor (we now call it P680+)

3. An enzyme catalyzes the split of two H2O molecules into two electrons, two hydrogen ions (H+) and an oxygen atom. The electrons are transferred from the hydrogen atoms to P680+, thus reducing it to P680

- P680+ is the strongest biological oxidizing agent known; this greatly facilitates the transfer of electrons from the split water molecule

-O2 is released as a byproduct of this reaction

4. Each electron "falls" down an electron transport chain from the primary receptor of PS II to PS I

5. This "fall" provides energy for the synthesis of ATP; energy released by the exergonic fall drives the creation of a proton gradient by diffusing H+ (protons) across the thylakoid membrane

6. Light energy has been transferred via light harvesting complex pigments to the PS I reaction-center complex, exciting an electron of the P700 pair of chlorophyll a molecules located there and loses an electron to the electron acceptor; P700+ accepts an electron passed down from the electron transport chain in PS II

7. Photoexcited electrons are passed in a series of redox reactions from the primary electron acceptor of PS I down a second electron transport chain through the protein ferredoxin (Fd)

8. The electrons are transferred from Fd to NADP+. Two electrons are required for its reduction to NADPH; the electrons of NADPH are now available for the reactions of the Calvin cycle (this process also removes an H+ from the stroma


What is cyclic electron flow?

In certain cases, photoexcited electrons can take an alternative path called cyclic electron flow, which uses PS I but not PS II; the electrons cycle back from Fd to the cytochrome complex in the PS I reaction center.

- There is no production of NADPH and no release of oxygen that results from this process
- Cyclic flow generates ATP

- Some organisms have PS I but not PS II; i.e. purple sulfur bacteria
- Cyclic electron flow is thought to have evolved before linear electron flow
- Cyclic electron flow may protect cells from light induced damage and may be photoprotective


Compare chemiosmosis in chloroplasts and mitochondria.

- In chloroplasts the high energy electrons dropped down the transport chain come from water, which in mitochondria, they are extracted from organic molecules (which are thus oxidized)

- Chloroplasts do not need molecules from food to make ATP; their photosystems capture light energy and use it to drive the electrons from water to the tope of the transport chain; mitochondria use chemiosmosis to transfer chemical energy from food molecules to ATP

- In the mitochondria, protons diffuse down their concentration gradient from the intermembrane space through ATP synthase back into the mitochondrial matrix, driving ATP synthesis; in the chloroplast, ATP is synthesized as the hydrogen ions diffuse from the thylakoid space back to the stroma through the ATP synthase complexes, whose catalytic knobs are on the stroma side of the membrane

*Thus, ATP forms in the stroma; ATP and NADPH are produced on the side facing the stroma, where the Calvin cycle takes place

*Light reactions generate ATP and increase the potential energy of electrons by moving them from H2O to NADPH