Photosynthesis: Light and Dark Reactions

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Questions and Answers

What process occurs when a water molecule is split during the light reactions?

It produces ATP.

It forms glucose.

It releases electrons, H+ ions and O2. (correct)

It captures sunlight energy.

Which of the following best describes the purpose of the proton gradient created during the light reactions?

It drives ATP synthesis. (correct)

It is used to reduce NADP+.

It helps in the formation of RuBP.

It increases the rate of photosynthesis.

What is the main function of NADP+ in the light reactions?

To absorb photons.

To transport carbon molecules.

To store energy from ATP.

To accept energized electrons. (correct)

What can inhibit the electron transport in photosystem II (PSII)?

Herbicides. Signup and view all the answers

Which step in the Calvin cycle involves the reduction of 3CO2 molecules?

Carbon reduction. Signup and view all the answers

What inefficiency is associated with rubisco in C3 plants?

It reacts with oxygen instead of CO2. Signup and view all the answers

What is the immediate product of the dark reactions in the Calvin cycle?

1 G3P. Signup and view all the answers

How does water stress affect photosynthesis in C3 plants?

It leads to stomatal closure. Signup and view all the answers

What occurs to animal cells in a hypotonic solution?

The cells burst due to excess water intake. Signup and view all the answers

Which statement accurately describes isotonic conditions in plant cells?

There is no net water movement in and out of the plant cells. Signup and view all the answers

In which scenario does endocytosis occur?

When the cell membrane infolds to take in substances. Signup and view all the answers

What is the primary difference between exocytosis and endocytosis?

Exocytosis expels materials, whereas endocytosis takes materials in. Signup and view all the answers

Which of the following describes a hypertonic environment's effect on plant cells?

The cells will undergo plasmolysis and shrink. Signup and view all the answers

Which of the following is considered cellular work?

Active transport of ions across membranes Signup and view all the answers

What aspect of the fluid mosaic model describes the positioning of phospholipid molecules?

Head-groups towards the surface; fatty acids inwards Signup and view all the answers

Which factor does NOT affect diffusion?

Color of the particles Signup and view all the answers

Osmosis specifically refers to the diffusion of which substance?

Water Signup and view all the answers

What characteristic of fatty acids affects the packing of phospholipids in cell membranes?

Saturation level of the fatty acids Signup and view all the answers

Which of the following processes is a spontaneous movement of particles from high to low concentration?

Diffusion Signup and view all the answers

Which process is considered NOT cellular work?

Spontaneous formation of a lipid bilayer Signup and view all the answers

Which of the following proteins is embedded in the phospholipid bilayer?

Integral proteins Signup and view all the answers

What is the primary role of ATP in cellular processes?

Providing energy currency for cellular activities Signup and view all the answers

Which of the following best describes the process of ATP hydrolysis?

It releases energy, making it exergonic. Signup and view all the answers

What characterizes enzymes as biological catalysts?

They lower the activation energy needed for reactions. Signup and view all the answers

What is the primary function of the active site of an enzyme?

To bind substrates and facilitate the catalytic reaction Signup and view all the answers

During cellular respiration, what is generated as a byproduct?

Carbon dioxide Signup and view all the answers

Which process converts solar energy into chemical energy in plants?

Photosynthesis Signup and view all the answers

What is the relationship between cellular respiration and ATP molecules?

Cellular respiration converts chemical energy into ATP. Signup and view all the answers

In the context of enzyme activity, what does the term 'induced fit' refer to?

The structural change in the enzyme upon substrate binding Signup and view all the answers

What is produced during the light reactions of photosynthesis?

Oxygen gas Signup and view all the answers

What role do carotenoids play in photosynthesis?

They help extend the useful range of light. Signup and view all the answers

Where does the Calvin cycle take place in the chloroplast?

In the stroma Signup and view all the answers

Which statement correctly describes the process of electron transfer in photosystems?

Energized electrons are transferred to primary receptors in both photosystems. Signup and view all the answers

What happens to light energy absorbed by chlorophyll during the light reactions?

It raises an electron to a higher energy orbital. Signup and view all the answers

What is the primary purpose of the electron transport chain in photosynthesis?

To generate a proton gradient for ATP synthesis. Signup and view all the answers

What does the splitting of water during photosynthesis produce?

Electrons, hydrogen ions, and oxygen Signup and view all the answers

During photosynthesis, which pigment molecule is responsible for reflecting green light?

Chlorophyll a Signup and view all the answers

Study Notes

Photosynthesis: Light Reactions

Light-harvesting pigment molecules absorb photons of light and become energized.
This energy is transferred to the reaction center.
An energized electron is captured by the primary receptor.
A water molecule is split, releasing electrons, H+ ions and O2. These electrons replace those lost by the reaction centers.
Energized electrons from PSII's primary acceptor are transferred to PSI, replacing lost electrons. This process creates a proton gradient.
Pigment molecules in PSII absorb light and become energized, similar to PSI. Resonance energy transfer occurs. Energized electrons are captured by PSI's primary acceptor.
Energized electrons from PSI are transferred to NADP+ via a short electron transport chain.
Herbicides can block electron transport in PS.

Photosynthesis: Dark Reactions (Calvin Cycle)

Carbon fixation: CO2 is incorporated into an organic molecule.
Carbon reduction: The incorporated CO2 is reduced using ATP and NADPH.
Synthesis and release of G3P: A three-carbon sugar (G3P) is produced and released.
Regeneration of RuBP: The starting molecule (RuBP) is regenerated for the next cycle.
The "sugar" product in chloroplast is a triose.
Trioses are exported and used to synthesize hexoses.

Summary Reactions for Photosynthesis

Light reactions: 6H2O + 9ADP+Pi +6NADP+ -> 12H+ + 12e- +3O2
Dark reactions: 3CO2 + 9ATP + 6NADPH -> C3H6O3 + 3H20 + 9ADP+Pi + 6NADPH+

Rubisco and C3 Plants

Rubisco can sometimes react with oxygen instead of CO2.
This occurs in the light and consumes oxygen.
Carbon dioxide is released, ATP is used, and sugar is not produced.
This is a major inefficiency in photosynthesis.
The reaction depends on the ratio of CO2 to O2.

Factors Affecting CO2 Availability

C3 photosynthesis can quickly deplete CO2 in the air.
Water stress causes stomatal closure, reducing both CO2 entry and O2 escape from leaves.

Cellular Work

Cellular work includes:
- Active transport of ions across membranes.
- Beating of cilia or flagella.
- Muscle contraction.
- Polymer synthesis from monomers.
- Metabolic generation of heat.
- Bioluminescence.
Cellular work does NOT include:
- Diffusion of solutes.
- Evaporation of water.
- Formation of hydrogen bonds.
- Spontaneous formation of lipid bilayers or protein folding.

ATP Cycle

The energy currency of the cell.
Catabolic processes (e.g., glycogen breakdown, cellular respiration) release energy.
Anabolic processes (e.g., protein synthesis, DNA replication) require energy.
ATP hydrolysis is exergonic (releases energy).
ATP synthesis is endergonic (requires energy).

Enzymes

Biological catalysts that are not consumed in a reaction.
Usually proteins, sometimes RNA molecules.
Very specific: catalyze only one specific reaction.
Reversible: can carry out a reaction in the forward or reverse direction.
Active site: the part of the enzyme where the substrate binds and the catalytic reaction occurs.
Induced fit: when a substrate enters the active site, the enzyme changes shape slightly.
Enzymes lower the activation energy required to start a reaction.
Substrate: the specific reactant that an enzyme acts on.

The Catalytic Cycle

Enzyme with an empty active site.
Substrate binds to the enzyme with induced fit.
Substrate is converted to products.
Products are released.

Cellular Respiration

The breakdown of sugars and other food molecules into carbon dioxide and water in the presence of oxygen to generate ATP.
Equation: C6H12O6 + 6O2 -> 6CO2 +6H2O

Photosynthesis Equation

6C02 + 6H20 -> C6H12O6 +6O2

Energy Flow in Ecosystems

All life requires energy.
The sun is the ultimate source of energy for most ecosystems.
Photosynthesis in chloroplasts converts solar energy, CO2, and H2O into chemical energy in sugars.
Cellular respiration converts the chemical energy of food molecules into ATP.

Cellular Membranes

Composed of a fluid mosaic bilayer of phospholipids.
Double bonds in fatty acids prevent tight packing.
Head-groups face the surface, fatty acids face inwards.
Integral proteins are embedded in the membrane.
Peripheral proteins are loosely attached to the membrane.
Proteins and lipids move freely within the membrane.

Diffusion

The spontaneous movement of a substance from a region of high concentration to a region of low concentration.
Passive process due to random molecular motion.
Occurs down a concentration gradient.
One molecule moves independently of another.
Continues until equilibrium is achieved.
Affected by temperature, area of diffusion, molecular size, resistance of the diffusion medium, and distance.

Osmosis

Diffusion of water across a selectively permeable membrane from a region of low solute concentration to a region of high solute concentration.
Affected by osmotic pressure, concentration gradient, water potential, surface area, and temperature.

Tonicity

Hypotonic: Solute concentration outside the cell is less than inside the cell. Water enters the cell.
- Animal cells: lysis (burst).
- Plant cells: turgid (normal).
Isotonic: Solute and water concentrations are equal inside and outside the cell. No net water movement.
- Animal cells: normal.
- Plant cells: flaccid.
Hypertonic: Solute concentration outside the cell is greater than inside the cell. Water leaves the cell.
- Animal cells: crenated (shriveled).
- Plant cells: plasmolyzed (shriveled).

Active Transport

Energy-dependent transport of a substance across a biological membrane.
Involves hydrolysis of ATP and specific transport proteins.
Results in transport and accumulation against a concentration gradient.
Includes endocytosis and exocytosis.

Exocytosis

Release of intracellular vesicle contents to the outside of a cell by fusion with the plasma membrane.

Endocytosis

Uptake of extracellular materials into the cell via infolding of the plasma membrane and formation of vesicles.

Similarities Between Exocytosis and Endocytosis

Both involve vesicles and the plasma membrane.
Both are essential for maintaining cellular functions and homeostasis.

Differences Between Exocytosis and Endocytosis

Endocytosis: Brings materials into the cell. Used for nutrient reuptake, immune responses, and receptor-mediated processes.
Exocytosis: Expels materials out of the cell. Used for secretion of hormones, neurotransmitters, and waste removal.

Photosynthetic Pigments

Electromagnetic radiation: the main form of energy emitted by the sun.
Travels in waves of different lengths and units of energy called photons.
A continuous spectrum of wavelengths exists.
Only a small portion of the spectrum is visible.
Chlorophyll b has an aldehyde group in ring 2.
Carotenoids: Accessory pigments that help photosynthesis in two ways:
- Extend the useful range of light into blues.
- Act as antioxidants to protect the chloroplast from reactive oxygen species.

Photosystems

A collection of light-harvesting complexes comprised of pigment molecules bound to proteins.
Embedded in the thylakoid membrane.
Light absorption raises an electron to a higher energy orbital.
Return to ground state releases light energy.

Photosystems I and II

PSII and PSI work together to capture light energy and transfer electrons.
Photosystem II (PSII):
- Absorbs light energy and splits water molecules, releasing electrons, H+ ions, and oxygen.
- Passes these electrons to a primary acceptor.
Photosystem I (PSI):
- Uses light energy to energize electrons from PSII.
- Passes these electrons to NADP+, creating NADPH.

Electron Transport Chain

Occurs in both PSII and PSI.
Essential for transferring electrons and generating a proton gradient.
During this process, energy is released and used to pump protons across the thylakoid membrane, creating a proton gradient.
This gradient is then used by ATP synthase to produce ATP.

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Description

Explore the intricate processes of photosynthesis, including both the light-dependent reactions and the Calvin cycle. Understand how light energy is converted into chemical energy and how carbon dioxide is fixed into organic molecules. Test your knowledge on key concepts, mechanisms, and the role of pigments in photosynthesis.