Biology ms aziz photosynthesis

#shean #NotEssam

Define Electromagnetic Spectrum, how does it relate to photosynthesis?

The electromagnetic spectrum is the range of all types of electromagnetic radiation, photosynthesis depends on the absorption of light by chlorophylls and carotenoids acting in combination. It absorbs specific wavelengths of visible light (400-700nm we don’t need to know that,) at a different efficiency.

What is the absorption Spectrum, what is it’s relation to photosynthesis?

The absorption spectrum is a plot of the absorption light as a function of wavelength, used to determine which wavelength a pigment absorbs.

What is an action spectrum? What is it’s relation to photosynthesis?

An Action spectrum is a plot of the effectiveness of light energy of different wavelengths in driving a chemical process. This is determined by using a suspension of chloroplasts or algal cells and measuring the amount of O2 released by photosynthesis at different wavelengths of visible light.

What colours are absorbed by what?

Chlorophyll a & b absorb blue and red light, making these regions the most effective for photosynthesis. Accessory pigments (e.g. carotenoids): extends the the range of usable light (in this case carrots are orange.)

Pigments: Overall purpose of pigments:

pigment molecules are bound very precisely to a number of different proteins. They do not float freely within the thylakoid membranes, the pigment proteins are organized into photosystems. Their purpose is to…
1) light absorption: absorb light energy, exciting electrons to a higher state
2) energy transfer: accessory pigments transfer absorbed energy to chlorophyll A in the reaction centre
3) Electron excitation: chlorophyll A molecules in the reaction centers of photosynthesis 2 and 1, they excite electrons starting the light dependent reactions.
4) carotenoids protect against oxidative

Types of pigments:

Chlorophyll A: primary pigment, absorbs blue-red, found in photosystems
Chlorophyll B: Accessory pigment, transfers energy for chlorophyll A
Carotenoids: Accessory pigment including carotene and xanthophyll (don’t need to know those 2,) protects the plant from photooxidative damage and extends the range of light absorption.
Phycobilins: Found in certain algae, absorbs light in wavelengths that aren’t efficiently absorbed by chlorophyll such as green light.

Photosystems (structure explained)

Photosystems are composed of the large antenna complex (also called a light harvesting complex) of proteins and some 250-400 pigment molecules surrounding a central reaction centre. The reaction centre is bound to a pair of specialized chlorophyll a molecules as well as the primary electron acceptor. Light energy absorbed by the antenna complex is then transferred to specialized chlorophyll molecules in the reaction centre, then the light energy is converted to chemical energy as a reaction centre chlorophyll donates an electron to the primary electron acceptor. The electron then gets passed along a ETC.

Photosystems purpose

They trap photons of light and use the energy to energize chlorophyll A molecules in the reaction centre. The chlorophyll A molecule is then oxidized as it transfers a high energy electron to the primary electron acceptor.

Types of photosystems

Photosystem 1 and 2 were named in order of discovery, although photosystem 2 comes first in the light dependent reaction (PS2 then PS1.) Both collections of pigment proteins

Photosystem 2:
- A reaction centre, containing P680 (pigment 680, meaning it absorbs light best at 680nm.)
- captures energy and excites electrons
- electrons transferred through ETC
- Splits water molecules to replace lost energy, producing gas as a byproduct
- contributes to the proton gradient of ATP synthesis.

Photosystems 1:
- A reaction center, containing a special pair of chlorophyll A molecules P700 (pigment 700, meaning it absorbs light best at 700nm.)
- It reenergizes electrons from photosystem 2 through ETC
- transfers electrons to NADP+, reducing it to NADPH
- NADPH is used in the calvin cycle to synthesize carbohydrates
- composed of accessory pigments → Antenna complex that absorbs light and transfers it

What is a reaction center?

Contains chlorophyll a molecules and associated proteins, facilitates electron excitation and transforming.

Order of stuff in photosystem 2

• a photon of light strikes the antenna complex, it is absorbed and it’s energy is transferred to the molecule P680, and one of it’s electrons change from the ground state to an excited state

• the excited electron is transferred to the primary acceptor molecule, which becomes negatively charged, while P680 carries a positive charge.

• The resulting positive ion P680+ is now extremely electronegative and can exert forces strong enough to remove an electron from a molecule of water. It is the strongest oxidant known in biology.

• Driven by this powerful electronegative pull, the water-splitting complex oxidizes molecule of water, passing n electron to the P680+ to make it neutral again.

• The acceptor molecule transfers an electron to plastoquinone (PQ) and becomes neutral, allowing the process to restart

• This occurs twice for each water molecule that is completely oxidized

How the photosystems work together:

Z-Scheme: Photosystem 2 absorbs light and excites electrons and drives ATP synthesis → ETC transfers electrons from photosystem 2 to photosystem 1, where it absorbs additional light energy, energizing electrons further and facilitating NADPH production (producing NADPH and ATP for the calvin cycle.)

Intermediate Electron Carrier Definition:

Molecules or protein complexes that transfer electrons between different components of the ETC during photosynthesis, play an important role in transfer of energy and ensuring efficient energy conversion.

Types of intermediate electron carriers

1) Plastoquinone (PQ): accepts electrons from photosystem 2 and carriers them to the cytochrome b6f complex, becomes reduced when it picks up electrons and protons, oxidized when it transfers them.

2) Cytochrome B6f complex: acts as a bridge between plastoquinone and plastocyanin, uses electron transfer to pump protons (H+) into the thylakoid lumen, contributing to the proton gradient for ATP synthesis

3) Plastocyanin (PC): small copper containing protein that transfers electrons from cytochrome b6f complex to photosystem 1, allows electron movement between photosystem 2 and 1.

4) Ferredoxin: transfers electrons from photosystem 1 to NADP+ reductase, where the reduction of NADP+ to NADPH occurs

5) NADP+ Reductase (FNR): final carrier in the chain, catalyzes the transfer of electrons from ferredoxin to NADP+, forming NADPH.

Simple steps for light dependent reactions

• photosystem 2 excites electrons to the PQ

• PQ carriers electrons to the cytochrome b6f complex, which pumps protons into the thylakoid lumen

• The PC then transfers electrons from cytochrome b6f to photosystem 1

• Photosystem 1 then re-energizes electrons using light and transfers them to ferredoxin (Fd.)

• From the ferredoxin to NADP+ reductase, electrons reduce NADP+ to form NADPH
  The energy released during electron transfer drives proton pumping into the lumen, the resulting proton gradient powers ATP synthase, producing ATP through chemiosmosis.

Outline of the sequence of the light reactions (Non-cyclic:)

1) Embedded in the thylakoid membrane are 2 types of photosystems (1 and 2)
2) At the center of each photosystem is a chlorophyll A molecule (P1 → P700, P2 → P680)
3) A photon of light strikes P2 and excites 2 e- of P680
4) The e- are boosted to a higher energy level and snatched awa yby a primary acceptor
5) P680 is now missing electrons
6) A water molecule is split and it’s electrons are given to P680
7) This occurs in the thylakoid compartment H2O → 2e- + 2H+ + ½ O2
8) oxygen diffuses out the chloroplast
9) the excited electrons pass from the primary acceptor along an ETC that includes plastoquinone (PQ), cytochrome complex and plastocyanin (PC)
10) As e- pass along the ETC, proton channels are opened and H+ moves into the thylakoid compartment
11) As H+ diffuse out of the thylakoid compartment, the e- reach photosystem 1 (P1) and replace e- that were missing there.

In:
light
H2O
NADP+

Out:
ATP
O2
NADPH

Detailed version of Non-cyclic:

• Oxidation of P680: absorption of light energy by photosystem 2 results in the formation of an excited state P680 molecule, which is rapidly oxidized resulting in the transfer of a high energy electron to the primary acceptor
  

• Oxidation-reduction of plastoquinone: From the primary acceptor, electrons transfer to the PQ, moving through the lipid bilayer and acts as a shuttle between photosystem 2 and the cytochrome complex. When PQ donates electrons to the cytochrome complex, it releases protons into the lumen increasing the concentration there
  

• Electron transfer from Cytochrome complex and shuttling by plastocyanin: from the cytochrome complex, electrons pass to the mobile carrier plastocyanin, which shuttles electrons from cytochrome complex to photosystem 1
  

• Oxidation-Reduction of P700: when a photon of light is absorbed by photosystem 1, an electron is excited and P700 (excited state) forms, the P700 (excited state) transfers it’s electrons to the primary electron acceptor of photosystem 1 forming P700+. This then acts as an electron acceptor andis reduced back to P700 by the oxidation of plastocyanin
  

• Electron transfer to NADP+ by ferredoxin: the first electron from P700 (excited state) is transported by a sequence of carriers within photosystem 1, which is transferred to photosystem 1 (an iron-sulfur protein.) The oxidation of ferredoxin results in the transfer of the electron to NADP+, reducing it to NADP.
  

• Formation of NADPH: A second electron is transferred to NADP by another molecule of ferredoxin. The second electron and a proton (H+) from the stroma are added to NADP by the NAPD+ reductase to form NADPH. The NADPH is now carrying 2 high energy electrons, the concentration of the protons in the stroma decreases as a result of the NADPH formation, along with the movement of protons from stroma to lumen by plastoquinone and the splitting of water into protons. These three processes create a higher proton concentration inside the lumen than outside the stroma.

Chemiosmosis (synthesis of ATP:)

Refers to the utilization of the energy source of the proton gradient across a membrane to form ATP in the thylakoid membrane.

1) protons taken into lumen by the reduction and oxidation of plastoquinone as it moves from photosystem 2 to the cytochrome complex and back again
2) the concentration of protons inside the lumen increased by the addition of 2 protons for each water molecule split in the lumen
3) the removal of one proton from the stroma for each NADPH molecule formed decreases the concentration of protons in the stroma outside the thylakoid.

the proton gradient created by this creates a force that drives protons out of the lumn back into the stroma. However, the thylakoid membrane allows protons to pass out into the stroma only through the pores in the protein complexes of ATP synthase, which are embedded in the membrane. Protons then move through the channels in the ATP synthase, some of their free energy i captures and used to synthesize ATP from ADP and inorganic phosphate

Oxidation:

When an atom or molecule loses an electron (H2O)

(Oxygen production: 2H2O → 4 (H+) + 4 (e-) + O2

Reduction:

when an atom/molecule/ion gains an electron
(NADP+ + 2 e- + 2H+ → NADPH+ + H) or (reduction of 3PGA to G3P.)

Autotrophs

organisms capable of building their own organic molecules from inorganic molecules (ex/ E + 6CO2 → C6H12O6)

Photoautotrophs

when light is the source of energy

Chemoautotrophs

When another chemical is the source of energy for making organic compounds

Heterotrophs

To eat organisms or other products to get their organic molecules

Role of light energy:

The ETC begins with a electronegative H2O and ends with the high energy NADPH. To drive the photosynthetic electron transport, H2O must be given enough potential energy to establish a proton gradient to form NADPH (done by the actions of Photosystem 1 and 2.)

Energy Level change order

1) Absorbing a photon of light, an electron in P680 gets excited and moves further away from the nucleus

2) The high energy electron is now held less strongly by P680 (excited state) and enables the electron to be transferred to the primary electron acceptor

3) A series of redox reactions begins: Oxidation of the acceptor molecule by plastoquinone and ends with the oxidation of plastocyanin by photosystem 1, which decreases the free energy of the electron as oxidizing agents establish a stronger and stronger force of attachment to the electron. This establishes enough free energy to establish proton gradient, but the electron is now bound to a strongly electronegative P700 chlorophyll molecule in PS1. This is overcome when photosystem 1 absorbs a photon of light producing P700 (excited state) and the electron moves further away from nucleus which is able to be transferred to the acceptor molecule.

Where do light reactions occur?

Thylakoid membrane

Where does the calvin cycle occur?

stroma

Where are the protons pumped to?

Into the thylakoid lumen

Where is ETC located?

Thylakoid membrane

Where ATP synthase is located

Thylakoid membrane

Where water splitting is located

thylakoid lumen (near PS2)

where are Photosystems

Thylakoid membrane

label

thylakoids: hollow on the
granum: stacks of thylakoids
stroma: space inside chloroplast
Lumen: space inside thylakoids

What is the cyclic cycle?

When photosystem 1 functions independently of photosystem 2. The electron transport from photosystem 1 to ferredoxin is not followed by electron donation to the NADP+ reductase complex. Instead, reduced ferredoxin donates electrons bak to plastoquinone; this continually reduces and oxidizes it and keeps moving protons across the thylakoid membrane. The net result of the cyclic cycle is that the energy absorbed from light is converted into chemical energy of ATP without oxidation of water or reduction of NADP+ to NADPH (the light energy captures is used to drive the phosphorylation of ADP to ATP.) Although, this requires more ATP than NADPH and additional AT pmolecules are provided by cyclic electron transport.

Comparison between Cyclic and Non-Cyclic electron flow:

Non-Cyclic:
- H2O needed
- O2 generated
- ATP made
- NADPH made
- Light needed
- needs both photosystem
- doesn’t need CO2

Cyclic:
- ATP made
- Light needed
- photosystem 1 only
- No CO2

3 phases of Calvin Cycle + Input/Output

1) Carbon Fixation: conversion of carbon from an inorganic to an organic form, important for life on earth, every cell of living things takes part in this reaction
2) Reduction: phosphate gets added from the hydrolysis of ATP to 3PGA. This molecule is reduced by electrons from NADPH, producing G3P.
3) Regeneration: G3P molecules are combined and rearranged to regenerate RuBP, that is required to restart the cycle

In:
6 CO2
18 ATP
12 NADPH

Out:
2 G3P
18 ADP + 18 Pi (inorganic phosphate)
12 NADP +

Balanced equation for photosynthesis & identifying the roles of chlorophyll, Co2, H2O, ATP, NADPH, and Light

6CO2 + 6H2O + light energy → C6H12O6 + 6O2

Chlorophyll: Acts as the primary pigment that absorbs light energy (red and blue) converting light to chemical, exciting electrons to drive the light dependent reactions

Co2: inorganic carbon source used in the calvin cycle (light independent reactions) fixed into organic molecules through a series of enzyme mediated steps

H2O: provides electrons for light dependent reactions when oxidized in PS2, produces O2 as biproduct and supplies protons to help create a proton gradient

Light Energy: powers the excitation of electrons in chlorophyll during the light dependent reactions, drives the production of ATP and NADH through photophosphorylation

ATP: Produced in the light-dependent reactions with chemiosmosis, supplies energy for the calvin cycle to convert 3PGA into G3P

NADPH: produced in light dependent reactions from PS1, when NADP+ gets reduced, provides electrons to reduce 3PGA to G3P.

How many cycles for a G3P molecule

three turns of the calvin cycle.

Problem with rubisco

Despite it being the most important/abundant protein in the world, it catalyzes very slow, only 3 CO2 per second and it occasionally binds with oxygen gas instead of CO2.

What is photorespiration?

consumption of oxygen in the presence of light (photon.) It decreases the photosynthetic output and drains away 50% of carbon fixed from Calvin cycle. This occurs in C3 plants on hot or dry days (when stomata is closed to conserve energy.) This creates the consequence of CO2 being unable to enter the leaf and O2 being able to leave the leaf.

- since rubisco can accept oxygen as well in it’s active site, as oxygen levels rise rubisco adds O2 to RuBP instead of CO2. This makes the 5 carbon product split into a 2 carbon compound and 3 carbon compound. Where the 2 carbon compound is exported from the chloroplast and broken down into CO2. This happens since primitive atmosphere had less O2 and more CO2.

C4 Plant avoid it by:

Since the calvin cycle is performed by bundle sheath cells which are surrounded mesophyll cells. This is due to CO2 being fixed in 4 carbon compounds in the mesophyll cells, as PEP carboxylase has a high affinity for CO2.

CAM plants avoid it by:

Opening their stomata at night (when water loss is minimized) to take in CO2, 4 carbon molecules are stored in vacuoles overnight then releases CO2 for calvin cycle (maintaining a high CO2 concentration.)

Night: CO2 → organic acid → stored in vacuoles
Day: Oragnic Acid → CO2 → chloroplasts → C.C

Describe the effect of photosynthesis:

Light Intensity: As light intensity increases, the rate of photosynthesis increases since more energy is available to produce the light dependent reactions. Although this goes to an extent as very high intensity damages the chlorophyll and reduces photosynthesis.

Chlorophyll Concentration: higher Chlorophyll concentration increases the rate of photosynthesis since more light energy can be utilized

CO2 Concentration: increases the photosynthetic rate since more CO2 is available for the calvin cycle to fit into glucose (goes to a certain point since other factors become limited.)

O2 levels: increases photorespiration rate since more, reduces efficiency of calvin cycle, slowing rate of photosynthesis

Temperature: temperature dependent since it involves enzyme reactions, rate increases up to optimal range, too high of a temperature denatures the enzymes

Comparison between cellular respiration and photosynthesis:

Only in photosynthesis:
- Anabolic
- occurs entirely in chloroplasts
- endergonic
- harnesses light energy

Only in Cellular Respiration:
- exergonic
- in mitochondria + cytoplasm
- catabolic


Both:
- has reduction reactions
- same products and reactants involved
- Both produce ATP by chemiosmosis.

Mitochondria vs Chloroplasts

Mitochondria:
- location of most of cellular respiration
- consumes O2
- produces CO2

Chloroplasts:
- has chlorophyll and other pigments, can absorb light energy
- location of photosynthesis
- produces O2
- produces G3p (carbs)
- consumes CO2

Both:
- ribones ad own DNA (circular)
- double membrane
- compartments for H+ to be pumped into
- both make ATP by chemiosmosis
- Both have ETC
- Both have intermediate carriers
- both have ATP synthase

Different ways G3P is used by plant:

G3P can either become glucose or go into the cytoplasm…

If it goes into the cytoplasm, it becomes converted to sucrose and becomes transported to other cells of the plant through phloem. If cellulose → cell wall, if converted to glucose → cellular respiration, if amino acids → proteins, lipids, or stored starch.

If it is directly turned into glucose…

glucose → cytoplasm → glycolysis → ATP
or
Glucose → stroma → polymerized into starch

Describe the evidence for endosymbiosis:

• double membrane (inner resembles plasma membrane of prokaryotes, outer resembles host cell membrane during engulfment)

• Circular membrane (DNA is linear in eukaryotes not circular)

• Ribosomes are smaller than ones in eukaryotic cytoplasm, allowing it to produce it’s own proteins

• Reproduction: replicates through fission similar to prokaryotes, dividing independently.

• Chloroplast DNA similar to photosynthetic Bacteria

• Sensitive to antibiotics that eukaryotes aren’t.

Possible package multiple choices:

• newly formed ATP in stroma

• newly formed NADPH in stroma

• newly formed oxygen in thylakoid space

• water needed in thylakoid space

• photosystems located in thylakoid membrane

• shortest wavelength of electromagnetic energy: gamma

• longest form: radio

• what range can we see? 380-750nm