Photosynthesis Flashcards
Photosynthesis
The process by which light energy is converted to chemical bond energy and carbon is fixed into organic compounds. (6CO2 + 12H2O = C6H12O6 + 6H2O + 6O2)
Two Processes of Photosynthesis
Light-Dependent/Light Reactions and Light-Independent Reactions
Photosynthesis: Light Reactions
Reactions that use light energy directly to produce ATP that powers the light-independent reactions. Light must be present for this reaction to occur.
Photosynthesis: Light-Independent Reactions
Reactions that consist of the Calvin cycle which produces sugar. To power the production of sugar, the Calvin cycle uses the ATP produced by the light reactions. This reaction only occurs when light is present.
Photosynthesis: Light-Independent Reactions
Reactions that consist of the Calvin cycle which produces sugar. To power the production of sugar, the Calvin cycle uses the ATP produced by the light reactions. This reaction only occurs when light is present.
Photosynthetic Pigments
Absorb energy and use it to provide energy to carry out photosynthesis. Two major groups: chlorophylls and carotenoids.
Photosynthetic Pigments: Chlorophyll
Chlorophyll a and chlorophyll b are green and absorb red, blue, and violet range lengths of light waves.
Photosynthetic Pigments: Cartenoids
They are yellow, orange and red. They absorb light in the blue, green and violet range.
Photosynthetic Pigments: Xantholophyll
A carotenoid with a slight chemical variation.
Photosynthetic Pigments: Phycobilins
Found in red algae; are reddish and absorbs light in the blue and green range.
Antenna Pigments
Chlorophyll b, the carotenoids, and the phycobilins are known by this name because they capture light in wavelengths other than those captured by chlorophyll a. They also absorb photons of light and pass the energy along to chlorophyll a.
Photosynthetic Pigments: Chlorophyll A
Directly involved in the transformation of light energy to sugars.
Chlorophyll A: Structure
It’s a large molecule with a magnesium atom in the head surrounded by alternating double and single bonds. The head, called the porphyrin ring, is attached to a long hydrocarbon tail. The double bonds play a critical role in the light reactions because they;re the source of the electrons that flow through the electron transport chains during photosynthesis.
Chlorophyll A: Structure
It’s a large molecule with a magnesium atom in the head surrounded by alternating double and single bonds. The head, called the porphyrin ring, is attached to a long hydrocarbon tail. The double bonds play a critical role in the light reactions because they;re the source of the electrons that flow through the electron transport chains during photosynthesis.
The Chloroplast: Structure
Contains grana, where light reactions occur, and stoma, where light-independent reactions occur. The grana consists of layers of membranes called thylakoids, the site of photosynthesis I and II. It is enclosed by a double membrane.
The Chloroplast: Structure
Contains grana, where light reactions occur, and stoma, where light-independent reactions occur. The grana consists of layers of membranes called thylakoids, the site of photosynthesis I and II. It is enclosed by a double membrane.
Photosystems
Light-harvesting complexes in the thylakoid membraned of the chloroplasts. There are a few hundred in each thylakoid.
Photosystems: Structure
Consists of a reaction center containing chlorophyll a and region contain several hundred antenna pigment molecules that funnel energy into chlorophyll a.
Photosystems: PS I & PS II
PS II operates first followed by PS I. PS I absorbs light best in the 700nm range (it is also called P700). PS II absorbs light in the 680nm range (it is also called P680).
Light Reaction Process Overview
Light is absorbed by PS I and PS II in the thylakoid membranes and electrons flow through the electron transport chain. Afterwards, there are two possible routes for electron flow: noncyclic and cyclic photophosphorylation.
Light Reaction: Cyclic Photophosphorylation
Electrons enter two electron transport chains and ATP and NADPH are formed. Electrons flow from water to P680 to P700 to NADP which carries them to the Calvin cycle.
1st Step of Light Reaction
Energy is absorbed by P680/PS OO. Electrons from the double bonds at the head of chlorophyll a become energized and move to a higher energy level. They are captured by a primary electron accepter.
1st Step of Light Reaction
Energy is absorbed by P680/PS OO. Electrons from the double bonds at the head of chlorophyll a become energized and move to a higher energy level. They are captured by a primary electron accepter.
2nd Step of Light Reaction
Photolysis: Water gets split apart, providing electrons to replace those lost from chlorophyll a in P680. Photolysis splits water into two electrons, two protons (H+) and one oxygen atom. Two oxygen atoms combing to form O2 and is released as a waste product.
3rd Step of Light Reaction
Electron Transport Chain: Electrons from P680 pass along an ETC consisting of plastoquinone and ultimately end up in P700. This flow of electrons is exergonic and provides energy ti produce ATP by chemiosmosis. This is called photophosphorylation because ATP synthesis is powered by light.
ETC: Plastoquinone
The ETC consists of this. A complex of two cytochromes and several other proteins.
3rd Step of Light Reaction
Electron Transport Chain: Electrons from P680 pass along an ETC consisting of plastoquinone and ultimately end up in P700. This flow of electrons is exergonic and provides energy to produce ATP by chemiosmosis. This is called photophosphorylation because ATP synthesis is powered by light.
ETC: Plastoquinone
The ETC consists of this. A complex of two cytochromes and several other proteins.
4th Step of Light Reactions
Chemiosmosis: The process by which ATP is formed during light reactions of photosynthesis. Protons that were released from water during photolysis are pumped by the thylakoid membrane into the thylakoid space (lumen). ATP is formed as these protons diffuse down the gradient from the thylakoid space, through ATP-synthase channels, into the stroma. The ATP produced here powers the Calvin Cycle.
4th Step of Light Reactions
Chemiosmosis: The process by which ATP is formed during light reactions of photosynthesis. Protons that were released from water during photolysis are pumped by the thylakoid membrane into the thylakoid space (lumen). ATP is formed as these protons diffuse down the gradient from the thylakoid space, through ATP-synthase channels, into the stroma. The ATP produced here powers the Calvin Cycle.
5th Step of Light Reaction
NADP becomes reduced when it picks up two protons that were released from water in P680. Newly formed NADPH carries hydrogen to the Calvin cycle to make sugar in the light-independent reactions.
6th Step of Light Reaction
Energy is absorbed by P700. Electrons from the head of chlorophyll a becomes energized and are captured by a primary electron receptor. This process is similar to the way is happens in P680. One difference is that the electrons that escape from chlorophyll a are replaces with electrons from P680, instead of water. Also, the ETC contains ferrodoxin and ends with production of NADPH not ATP.
6th Step of Light Reaction
Energy is absorbed by P700. Electrons from the head of chlorophyll a becomes energized and are captured by a primary electron receptor. This process is similar to the way is happens in P680. One difference is that the electrons that escape from chlorophyll a are replaces with electrons from P680, instead of water. Also, the ETC contains ferrodoxin and ends with production of NADPH not ATP.
Overview of Non-cylic Phosphorylation
light → P680 oxygen released → ATP produced → P700 → NADPH produced (NADPH carries H+ to the Calvin cycle)
Cyclic Phosphorylation
The sole purpose of this process is to produce ATP. No NADPH is produced and no oxygen is released.
Cyclic Phosphorylation
The sole purpose of this process is to produce ATP. No NADPH is produced and no oxygen is released. The Calvin cycle requires enormous amounts of ATP so when the chloroplast runs low the chloroplast carries our cyclic phosphorylation.
Overview of Cyclic Phosphorylation
Cyclic electron flow takes out photo excited electrons on a short-circuit pathway. Electrons travel from the P680 ETC to P700 to a primary electron acceptor then back to the cytochrome complexes in the P680 ETC. The cytochromes phosphorylate ADP to ATP.
The Calvin Cycle
A cyclical process that produces the 3-carbon sugar PGAL. Carbon enters the stomates of a leaf in the form of CO2 and becomes fixed/incorporated into PGAL.
Important Facts about the Calvin Cycle
The process that occurs is carbon fixation. It is a reduction reaction because carbon gains hydrogen. The Calvin cycle depends on products of light reactions such as ATP and NADPH. This only occurs in the light.
Calvin Cycle Process
CO2 enters the Calvin cycle and becomes attached to a 5-carbon sugar RuBP, forming a 6-carbon molecule. The 6-carbon molecule is unstable and immediately breaks down into two 3-carbon molecules of 3-PGA. The enzyme that catalyses the first step is rubisco.
Calvin Cycle Process
CO2 enters the Calvin cycle and becomes attached to a 5-carbon sugar RuBP, forming a 6-carbon molecule. The 6-carbon molecule is unstable and immediately breaks down into two 3-carbon molecules of 3-PGA. The enzyme that catalyses the first step is rubisco.
Photorespiration in C-3 Plants
CO2 enters the Calvin cycle and is fixed into 3-phosphoglycerate by the rubsico enzyme. The plants are called C-3 because the first step produces 3-PGA which contains 3 carbons. This process isn’t very efficient because rubisco binds with both O2 and CO2.
What happens when O2 binds with rubisco in photorespiration?
When rubisco binds with O2, photorespiration is diverted in two ways. Either, unlike respiration, no ATP is produced or, unlike photosynthesis, no sugar is formed. Instead peroxisomes break down the products of photorespiration. This process is probably a vestige from when the atmosphere had little to no oxygen.
C-4 Photosynthesis
A modification for dry environments. They exhibit modified anatomy that enable them to minimize excess water loss and maximize sugar production. Examples are corn, sugar cane and crabgrass. In C-4 plants a series of steps precedes the Calvin cycle the steps pump CO2 that entered the leaf away from the air spaces near the stomates.
C-4 Photosynthesis
A modification for dry environments. They exhibit modified anatomy that enable them to minimize excess water loss and maximize sugar production. Examples are corn, sugar cane and crabgrass. In C-4 plants a series of steps precedes the Calvin cycle the steps pump CO2 that entered the leaf away from the air spaces near the stomates.
1st Step that Precedes the Calvin Cycle in C-4 Plants
CO2 enters the mesophyll cell of the leaf and combines with a 3-carbon
C-4 Photosynthesis
A modification for dry environments. They exhibit modified anatomy (Kranz Anatomy) that enable them to minimize excess water loss and maximize sugar production. Examples are corn, sugar cane and crabgrass. In C-4 plants a series of steps precedes the Calvin cycle the steps pump CO2 that entered the leaf away from the air spaces near the stomates.
1st Step that Precedes the Calvin Cycle in C-4 Plants
CO2 enters the mesophyll cell of the leaf and combines with a 3-carbon molecule PEP to form a 4-carbon molecule oxaloacetate. This is why they are called C-4. The enzyme that catalyzes this reaction PEP carboxylase does not bind with oxygen and can therefore fix CO2 more efficiently than rubisco.
2nd Step that Precedes the Calvin Cycle in C-4 Plants
From oxaloacetate the mesophyll cell produces malic acid which it pumps through plasmodesmata into the adjacent bundle sheath cells. Once in the bundle sheath cell, it releases CO2, which gets incorporated into PGAL by the Calvin cycle. Because the bundle-sheath cell is deep within the lead and little oxygen is present, rubisco can fix CO2 without binding with O2. This is called the Hatch-Sack Pathway and it diverted CO2 from air near stomates.
3rd Step that Precedes the Calvin Cycle in C-4 Plants
When CO2 is sequestered inside bundle-sheath cells there is a steep CO2 gradient between the airspace in the mesophyll near the stomates and the atmosphere. Thus, C-4 plants can maximize the amount of CO2 that diffuses into the air space in the leaf and minimize the length of time the stomates must be open.
Kranz Anatomy
The anatomy that differentiates as C-4 leaf from a C-3 leaf. In C-4 leaves the bundle sheath cells lie under the mesophyll cells, deep where CO2 is sequestered. The light reactions occur in the mesophyll cells and the dark reaction occur in the bundle sheath cells. In C-3 laves, all photosynthetic cells have direct access to CO2.
CAM Plant Photosyntehsis
Called crassulacean acid metabolism which is another adaptations for dry conditions. These plants keep their stomates closed during the day and open at night (reverse of most plants). The mesophyll cells store CO2 in organic compounds they synthesize at night. During the day, when light reactions can supply energy for the Calvin cycle , CO2 is released from organic acids made the night before to become incorporated into sugar.
