Photosynthesis and Chlorophyll Quiz

Photosynthesis

• Photosynthesis is the process by which plants, some bacteria and some protistans use sunlight energy to produce glucose from carbon dioxide and water.
• Glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration.
• Oxygen is also formed during photosynthesis.

Chlorophyll

• Chlorophyll is a complex molecule essential for photosynthesis.
• All photosynthetic organisms have chlorophyll a.
• Accessory pigments, including chlorophyll b, xanthophylls, and carotenoids, absorb energy that chlorophyll a does not.
• Chlorophyll a absorbs energy from the violet-blue and reddish orange-red wavelengths, and little from the green-yellow-orange wavelengths.
• All chlorophylls have a lipid-soluble hydrocarbon tail and a flat hydrophilic head with a magnesium ion at its centre
• Different chlorophylls have different side-groups on the head.

Leaves

• Leaves are specialized structures for photosynthesis in plants, acting like "solar collectors" packed with photosynthetic cells.
• Water enters the root and is transported to the leaves via xylem vessels.
• Land plants have evolved specialized structures called stomata to allow gas exchange.
• Carbon dioxide enters and oxygen exits the leaf through the stomata.
• Water is also lost through the stomata during gas exchange.

Chloroplast Structure

• The thylakoid is the structural unit of photosynthesis, found in both prokaryotes and eukaryotes.
• Thylakoids are flattened sacs containing photosynthetic chemicals.
• Eukaryotes have chloroplasts surrounded by a membrane.
• Thylakoids are stacked in grana, with the areas between grana referred to as stroma.
• The chloroplast has three membrane systems, forming three compartments.

Stages of Photosynthesis

• Photosynthesis is a two-stage process.
• The light-dependent reactions occur in the grana and require direct sunlight energy to make energy-carrier molecules.
• The light-independent reactions occur in the stroma and use the energy-carrier molecules to create carbohydrates from carbon dioxide.

Light-Dependent Reactions

• Light energy is trapped by chlorophyll to make ATP (photophosphorylation).
• Water is split into oxygen, hydrogen ions and free electrons (photolysis).
• The electrons react with NADP+, changing it to its reduced state (NADPH).

Light-Independent Reactions

• Carbon dioxide from the atmosphere is captured and modified by the addition of hydrogen to form carbohydrates.
• This process is called carbon fixation.
• Energy for this process comes from the light-dependent reactions.

Electron Transfer System

• Chlorophyll a absorbs light energy, exciting its electrons.
• The excited electrons are transferred to a primary electron acceptor.
• The chlorophyll molecule is oxidized and becomes positively charged.
• Energy from photoactivation is transferred to ATP and NADPH.
• Photoionisation of chlorophyll releases electrons that are transferred to an electron acceptor.
• The positively charged chlorophyll ion receives electrons from an electron donor, such as water.

Photosystems

• Two photosystems, Photosystem II (PSII) and Photosystem I (PSI), are involved in electron transfer.
• PSII occurs before PSI, though it was discovered second.
• Energy changes accompanying the electron transfer create a "Z scheme".
• Energy released during electron transfer produces ATP.

Non-cyclic phosphorylation

• Both ATP and NADPH are produced.
• Photoionisation of chlorophyll in PSII transfers electrons to an electron acceptor.
• Photolysis of water produces oxygen, hydrogen ions, and electrons, which replace those lost from chlorophyll.
• Electrons are transferred through the electron transport chain to PSI.
• Light energy further excites electrons in PSI, leading to the reduction of NADP+ to NADPH.

Chemiosmosis

• Electrons passing through the transport chain provide energy to pump H+ ions from the stroma into the thylakoid compartment.
• The electrochemical gradient drives the diffusion of H+ ions back into the stroma.
• This diffusion powers the production of ATP.

Cyclic phosphorylation

• Only PSI is involved, generating excited electrons.
• Electrons are transferred to the electron transport chain between PSII and PSI, but not to NADP+.
• No NADPH is formed.
• Electrons are transported back to PSI, completing the cycle.

The Calvin cycle

• Carbon dioxide combines with RuBP, a five-carbon sugar.
• An unstable six-carbon sugar is formed, which breaks down into two glycerate 3-phosphate (GP) molecules.
• ATP phosphorylates GP into glycerate diphosphate molecules.
• NADPH reduces glycerate diphosphate to two molecules of glyceraldehyde 3-phosphate (GALP).
• One GALP molecule is used to create glucose and other carbohydrates, lipids, or amino acids.
• The other GALP molecule is converted back into RuBP, initiating the cycle again.

Factors affecting photosynthesis

• Light intensity, carbon dioxide concentration, and temperature are key factors.
• As light intensity increases, the rate of photosynthesis generally increases.
• An increase in carbon dioxide concentration increases the rate of carbon fixation.
• Photosynthesis is dependent on temperature, as it is driven by enzymes.