GEN BIO 1 | Photsynthesis (Light Dependent Reaction)

Why do leaves change color in the fall?

Chlorophyll breaks down when there is a change in the season. The lower temperature indicates that there is less water and nutrients for the leaves. The carotenoids become visible after being masked by the chlorophyll.

Photosynthesis

Metabolic pathway by which most autotrophs use energy of light to make sugars from CO2 and H2O.

Redox Reaction

Transfer of electrons from one molecule to another is an oxidation-reduction reaction

Reduction

Molecule receives an electron (charge becomes more negative)

Oxidation

molecule loses an electron (charge becomes more positive)

Photosynthesis Reaction

reduction of CO2 and oxidation of water through sun’s energy produces a carbohydrate

Cellular Respiration Reaction

oxidation of carbohydrates produces energy and CO2

Life depends on photosynthesis (Autotrophs)

They (not just plants) make their own food using energy and inorganic molecules

Life depends on photosynthesis (Heterotroph)

organisms that obtain carbon from organic compounds assembled by other organisms

Sunlight as energy source

The light has UV radiation, visible light, and infrared radiation. The visible light is most important and has a band about 380 nm and 750 nm

Chlorophyll

It can only absorb red and violet blue, therefore it reflects the green color (one that they do not absorb)

Carotenoids

It can only absorb green and violet blue, therefore it reflects the warmer colors (one that they do not absorb)

Chloroplasts as Sites of Photosynthesis (Thylakoids and Granum)

It has a double membrane, the granum is a stack of thylakoids, which has has chlorophyll where photosynthesis occurs (light dependent reaction) particularly in thylakoid membranes and spaces

Chloroplasts as Sites of Photosynthesis (Stroma)

It is a fluid filled space where the light independent reaction occurs, also known as the calvin cycle

Light Reactions

pigment molecules capture sunlight energy and transfer it to molecules of ATP (Adenosine Triphosphate) and NADPH (Nicotinamide adenine dinucleotide phosphate).

Calvin Cycle

use energy to build sugar molecules out of CO2

ATP

a nucleotide composed of the nitrogen-containing base adenine and the 5-carbon sugar ribose (together as adenosine) and three phosphate groups; stores energy in the high-energy phosphate bonds

ATP-ADP Cycle (Hydrolysis)

When ATP is used as an energy source, a phosphate group is removed by hydrolysis (breakdown of chemical bond by addition of water to become ADP (Adenosine Diphosophate)

Phosphrylation

adding phosphate group to molecule

ATP-ADP (Reverse Hydrolysis)

releases water and regenerates ATP from ADP in the mitochondria through cellular respiration

NADPH

mobile electron carrier that transport two electron and a protons from Light Dependent to Calvin Cycle

Photosystems

clusters of organized chlorophyll molecules

Transfer of Light

The light hits the chlorophyll and it gets transferred to reaction-center complex

Reaction-Center Complex

Also known as PSII, has a pair of special chlorophyll a molecules and a molecule called a primary electron acceptor. It has higher light and receives the energy.

Light-harvesting complex

contains various pigments bound to proteins

PSII and PS1

Based on discovery, not order. PS1 has more energy (P700) than PSII (P680)

PSII Light Reception

A special pair absorbs energy and emits electrons to the electron-transfer chain

Replacement Electrons

Photosystem pulls replacement electrons from water molecules which split into hydrogen ions and oxygen atoms. Oxygen leaves cell in O2 gas (waste product) while hydrogen is in the thylakoid compartment

Electron entering thylakoid membrane

electrons enter the electron transfer chain in thylakoid membrane

Energy Release and Hydrogen Ion Gradient in Transport Chain

Energy released by electrons allows H+ to accumulate and go through the transport chain from stroma to thylakoid compartment. The hydrogen ion gradient forms across membrane.

Before Electron Transfer Chain

PSII receives light and it reaches the reception-center complex and electron is carried to chain

Electron Transfer Chain

Photoexcited electron passes from PSII primary electron receptor via plastoquione (Pq), cytochrome complex, plastocyanin (Pc). Fall of electron to lower energy level produces ATP

Photosystem I During ETC

it receives light and replacement electron comes from PSII. Light re-energizes PS1 electron through protein called ferredoxin (Fd). NADP+ reductase is an enzyme that catalyzes transfer of electrons from Fd to NADP+. Two electrons are required for NADP+’s reduction.

NADPH Formation

Electrons from PS1 enter an ETC and combine with NADP+ (final electron acceptor) and H+ to form NADPH.

Hydrogen ions in Thylakoid

follow gradient, which has a lot of potential energy, across thylakoid membrane by flowing through ATP synthases. Hydrogen ion flow causes ATP synthases to phosphorylate ADP so ATP forms in stroma

Gradient

In ATP production, H ions moves down a concentration gradient from thylakoid space to stroma through.

Thylakoid Space

it is a reserve for H+ ions. When water is split, it remains in the space. H+ is pumped from stroma to thylakoid space due to ETC.

ATP Production

Channel in ATP synthases allows H+ ions in thylakoid space to return to chloroplast stroma. Gradients dissipates and energy is released. ATP synthase enzyme uses energy to add phosphate group to ADP.

Chemiosmotic Phosphorylation

It is coupling of proton gradient and ATP formation. Energy released in ETC drives the active transport of protons (H+) into thylakoid space. Protons diffuse out through channels in ATP synthase, and the movement powers the phosphorylation of ADP to ATP.