Chapter 7

Who reduces and oxidizes in photosynthesis?

- CO2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it. - water molecules are oxidized when they lose electrons along with hydrogen ions.

What other features are found on plant leaves?

- Stomata: are tiny pores in the leaf that allow carbon dioxide to enter and oxygen to exit. -veins: in the leaf deliver water absorbed by roots.

Where does photosynthesis occur in plants?

- take place in chloroplasts -converts carbon dioxide and water into organic molecules, and - releases oxygen.

The light energy is passed from molecule to molecule within the photosystem.

-Finally it reaches the reaction center where a primary electron acceptor accepts these electrons and consequently becomes reduced. -This solar‐powered transfer of an electron from the reaction‐center pigment to the primary electron acceptor is the first step in the transformation of light energy to chemical energy in the light reactions.

How does photophosphorylation compare with oxidative phosphorylation?

-Mitochondria use oxidative phosphorylation to transfer chemical energy from food into the chemical energy of ATP. -Chloroplasts use photophosphorylation to transfer light energy into the chemical energy of ATP.

Within a thylakoid membrane, chlorophyll and other pigment molecules

-absorb photons and -transfer the energy to other pigment molecules.

The Calvin cycle makes sugar within a chloroplast to produce sugar The necessary ingredients are

-atmospheric CO2 and -ATP and NADPH generated by the light reactions. The Calvin cycle uses these three ingredients (CO2, ATP and NADPH) to produce an energy‐rich, three‐carbon sugar called glyceraldehyde3phosphate (G3P). A plant cell may then use G3P to make glucose and other organic molecules.

C4 plants have evolved a means of

-carbon fixation that saves water during photosynthesis while -optimizing the Calvin cycle. ‐C4 plants are so named because they first fix CO .2 into a four carbon compound ‐When the weather is hot and dry, C4 plants keep their stomata mostly closed, thus conserving water.

In hot and dry weather, C3 plants

-close their stomata to reduce water loss but -prevent CO2 from entering the leaf and O2 from leaving. -As O2 builds up in a leaf, rubisco adds O2 instead of CO2 to RuBP, and a two‐carbon product of this reaction is then broken down in the cell. -This process is called photorespiration because it occurs in the light, consumes O2, and releases CO2. -But unlike cellular respiration, it uses ATP instead of producing it.

Interestingly, chemiosmosis is the mechanism that

-is involved in oxidative phosphorylation in mitochondria and -generates ATP in chloroplasts. ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions across a membrane.

What are the stages of photosynthesis?

1. light reactions 2. Calvin cycle

What do lightharvesting complexes do in terms of function?

A light‐harvesting complex contains various pigment molecules bound to proteins. Collectively, the light‐harvesting complexes function as a lightgathering antenna.

What are the components of a photosystem?

A photosystem consists of a number of lightharvesting complexes surrounding a reactioncenter complex.

What is an autotroph?

Autotrophs: - make their own food through the process of photosynthesis, - sustain themselves, and - do not usually consume organic molecules derived from other organisms.

What happens to the electrons that were generated?

Between the two photosystems, the electrons -move down an electron transport chain and -provide energy for the synthesis of ATP.

Calvin Cycle

Calvin Cycle occurs in the stroma of the chloroplast. - the calcin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich products of the light reactions. -during the calvin cycle, CO2 is incorporated into organic compounds in a process called carbon fixation. - after carbon fixation, enzymes of the cycle make sugars by further reducing the carbon compounds. -the calvin cycle is often called the dark reactions of light-independent reactions, because none of the steps requires light directly.

How do cellular respiration and photosynthesis redox reactions differ?

Cellular Respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule. - this is accomplished by oxidizing the sugar and reducing O2 to H2O. - the electrons lose potential as they travel down the electron transport chain to O2. - In contrast, the food-producing redox reactions of photosynthesis require energy.

What is present in chloroplasts that absorbs light?

Chlorophyll - is an important light-absorbing pigment in chloroplasts - is responsible for the green color of plants, and - plays a central role in converting solar energy to chemical energy.

Where are chloroplasts found in places?

Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf.

How are chloroplasts organized?

Chloroplasts consist of an envelope of two membranes, which - enclose an inner compartment filled with a thick fluid called stroma and - contain a system of interconnected membranous sacs called thylakoids.

What are the types of pigments involved in photosynthesis?

Chloroplasts contain several different pigments, which absorb light of different wavelengths. -Chlorophyll a absorbs blue‐violet and red light and reflects green. -Chlorophyll b absorbs blue and orange and reflects yellow‐green. -Carotenoids broaden the spectrum of colors that can drive photosynthesis and provide photoprotection, absorbing and dissipating excessive light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules.

How is ATP made in the light reaction?

In photophosphorylation, using the initial energy input from light, -the electron transport chain pumps H+ into the thylakoid space, and -the resulting concentration gradient drives H+ back through ATP synthase, producing ATP.

What occurs in a photosynthesis redox reaction?

In photosynthesis, - light energy is captured by chlorophyll molecules to boost the energy of electrons. - light energy is converted to chemical energy, and - chemical energy is stored in the chemical bonds of sugars.

How do the photosystems ultimately make ATP?

In the light reactions, light energy is transformed into the chemical energy of ATP and NADPH. To accomplish this, electrons are -removed from water, -passed from photosystem II to photosystem I, and -accepted by NADP +, reducing it to NADPH.

How are pigments organized in the thylakoids?

In the thylakoid membrane, chlorophyll molecules are organized along with other pigments and proteins into photosystems.

How do we define light?

Light behaves as discrete packets of energy called photons. -A photon is a fixed quantity of light energy. -The shorter the wavelength, the greater the energy.

How do plants then compensate for the hot and dry weather?

Other methods of carbon fixation have evolved in hot, dry climates. -C4 plants -CAM plants

How do plant pigments interact with light?

Pigments -absorb light and -are built into the thylakoid membrane. -absorb some wavelengths of light and -reflect or transmit other wavelengths. (We see the color of the wavelengths that are transmitted. For example, chlorophyll transmits green wavelengths.)

How do pigments capture solar energy?

Pigments in chloroplasts absorb photons (capturing solar power), which -increases the potential energy of the pigment's electrons and -sends the electrons into an unstable state. -These unstable electrons drop back down to their "ground state," and as they do, release their excess energy as heat.

Visible radiation absorbed by pigments drives the light reactions

Sunlight contains energy called electromagnetic energy or electromagnetic radiation. -Visible light is only a small part of the electromagnetic spectrum, the full range of electromagnetic wavelengths. -Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves.

What are the final products made in the light reactions?

The products of the light reactions are -NADPH, -ATP, and -oxygen.

What are the steps of the Calvin Cycle?

The steps of the Calvin cycle include -carbon fixation, -reduction, -release of G3P, and -regeneration of the starting molecule ribulose bisphosphate (RuBP).

What are features of the thylakoids?

Thylakoids: - are often concentrated in stacks called grana and - have an internal compartment called the thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion. - thylakoid membranes also house much of the machinery that converts light energy to chemical energy. - chlorophyll molecules are built into the thylakoid membrane and capture light energy.

How many photosystems exist in photosynthesis?

Two types of photosystems (photosystem I and photosystem II) cooperate in the light reactions. Each type of photosystem has a characteristic reaction center. -Photosystem II, which functions first, is called P680 because its pigment absorbs light with a wavelength of 680 nm. -Photosystem I, which functions second, is called P700 because it absorbs light with a wavelength of 700 nm.

heterotrophs

are consumers that feed on plants or animals, or decompose organic material.

chemoautotrophs

are prokaryotes that use inorganic chemicals as their energy source.

light reactions

light reactions occur in the thylakoid membranes. In these reactions, -water is split, providing a source of electrons and giving off oxygen as a by-product, -ATP is generated from ADP and a phosphate group, and -light energy is absorbed by the chlorophyll molecules to drive the transfer of electrons and H+ from water to the electron acceptor NADP+ reducing it to NADPH. -NADPH produced by the light reactions provides the electrons for reducing carbon in the calvin cycle.

photoautotrophs

use the energy of light to produce organic molecules.

Another adaptation to hot and dry environments has evolved in the CAM plants, such as pineapples and cacti.

‐CAM plants conserve water by opening their stomata and admitting CO2 only at night ‐CO2 is fixed into a fourcarbon compound, -which banks CO2 at night and -releases it to the Calvin cycle during the day.

Other methods of carbon fixation have evolved in hot, dry climates

‐Most plants use CO2 directly from the air, and carbon fixation occurs when the enzyme rubisco adds CO2toRuBP. ‐Such plants are called C3 plants because the first product of carbon fixation is a three‐carbon compound, 3‐PGA.