Biology

LUMINOUS PHASE

The components that make up the light phase take place inside the photosystems.

A PHOTOSYSTEM is an organized complex of molecules, which has the task of capturing the

energy of solar radiation by converting it into chemical energy, stored in molecules of ATP and

NADPH.

Each photosystem is made up of a pigment molecule and a REACTION CENTER, in turn made

up of a pair of special chlorophyll molecules and a molecule called the primary electron acceptor.

The pigments of the light photosystem from antennas, that is, they capture the solar energy and

transmit it back to the reaction center. Here, the two molecules of chlorophyll a, oxidizing, release

electrons to the primary acceptor, which is reduced. Many steps of photosynthesis are in fact redox

reactions.

Two types of photosystems have been identified, named photosystems I and II, based on the order

in which they were discovered; in reality it is photosystem II that goes into action first, followed by

photosystem I. In the reactions of the light phase, electrons move from one photosystem to another

by means of transporters that oxidize other transporters, that is, through transport chains.

When electrons are transferred from one molecule to another during the redox reactions of the light

phase, they first pass to a higher energy state and then to a lower one. During the descent they

release energy.

In each photosystem, the pigment antenna molecules collect solar radiation and transfer it to the

reaction center. Here, thanks to the energy received, “excited” electrons, ie electrons that are in a

higher energy state, are expelled from the chlorophyll molecules of the reaction center.

These electrons are first captured by the primary acceptor of each photosystem and then pass to the

transport chain where, through successive redox reactions, they lose part of their energy. Energy is

used in photosystem II for the synthesis of ATP, and in photosystem I for the synthesis of NADPH.

The flow of electrons is continuous in the two photosystems and is powered by the splitting reaction

of water molecules: H2O -> 1/2 O2 + 2H + 2 e-

The splitting reaction of water molecules takes place thanks to the energetic contribution of

sunlight, and for this reason it is called photolysis of water.

The migration of electrons

During the stages of the light phase, electrons migrate through the chloroplast. The electrons depart

from the water molecules present in the internal compartment of the thylakoid, penetrate its

membrane, cross it - aided in each phase by various transporters - and finally extend the stroma

linked to a molecule of NADPH.

The two most important events of the light phase:

- The first key step is the splitting of the water molecules. During the breakdown of H2O

molecules, hydrogen atoms are removed from the molecule while oxygen remains free. From the

union of two oxygen atoms in the internal compartment of the thylakoid derives a molecule of

O2, which is the form in which oxygen is present in our atmosphere.

- The second key step of the light phase is the transformation of solar energy into chemical energy.

When the energy of sunlight is transferred to a chlorophyll molecule in the reaction center, an

electron receives the boost needed to move from what is called its ground state to an excited

state. In chloroplasts, on the other hand, the excited electrons pass to another molecule, that is to

the primary acceptor of photosystems II and I. They do not return to the ground state, and do not

release heat or light, but therefore come by means of a redox reaction, which represents a kind of

connection bridge with the living world. In photosynthesis, however, the electrons obtained from

water are pushed by solar radiation towards a higher energy state and transferred to other

molecules.

One of the most important effects of electron transport between photosystem II and photosystem I is

the release of energy, which is used for the production of ATP. Assembled from ADP and phosphate,

through a process of chemosmosis, this ATP will then be used for the reactions of the second part of

photosynthesis.

At the end of the luminous phase, solar energy is then converted into two different forms: the ATP

and the term carrier NADPH, containing excited electrons. Overall, the light phase of

photosynthesis serves to capture energy.

DARK PHASE

In the second phase of photosynthesis, called "dark" because it does not need light, the energy

accumulated in the reactions of the light phase is used to power a cycle of reactions in which carbon

dioxide, obtained from the atmosphere, combines with a low-energy sugar and produces an energy-

rich carbohydrate, i.e. a nutrient. This cyclic series of reactions constitutes the Calvin cycle.

To understand the mechanism of the Calvin cycle, let's resume photosynthesis from the point where

we left it at the end of the light phase, that is, in the chloroplast stroma. NADP + has transformed

into NADPH by accepting excited electrons from photosystem I. In the stroma there is also ATP,

another product of the light phase.

The cycle begins with the CARBON FIXATION process, in which the carbon present in the CO2

gas is fixed, i.e. incorporated into an organic molecule. The reaction occurs when the carbon

dioxide absorbed by the leaves of the plant binds to a low-energy sugar, already present in the

plant's tissues: ribulose diphosphate or RuDP, with five carbon atoms.

For each molecule of RuDP and CO2 an unstable compound with six carbon atoms is generated

which immediately splits into two molecules of 3-phosphoglyceric acid (3-PGA), each with three

carbon atoms. The Calvin cycle is also called the C3 cycle precisely because its first product is a

three-carbon compound.

The subsequent reactions constitute the "energizing" passages of the cycle, in which the 3-PGA and

its derivatives come into contact with the products of the light phase, interacting with the ATP and

subsequently acquiring the electrons of the NADPH rich in energy (those originally obtained from

water molecules and excited by sunlight). In this way, a sugar that is relatively low in energy

increases its energy content and is transformed into a nutrient. The sugar thus obtained,

glyceraldehyde-3-phosphate or G3P, is the main product of photosynthesis. Each Calvin cycle

produces one molecule. In the remainder of the Calvin cycle, a series of reactions produce more

RuDP, which will start a new turn of the cycle.

The final product of photosynthesis, G3P sugar, is now ready to be transformed into many different

products: for example, glucose is obtained from the union of two G3P molecules. Many glucose

molecules can bind themselves to form polysaccharides such as starch, stored as an energy reserve,

or cellulose which will make up many parts of the plant. Furthermore, proteins are also obtained

from sugar, which are then used as structural components of the plant or as enzymes.

​​

NUCLEAR COMPARTMENT

A key difference between prokaryotes and eukaryotes is the presence of a nucleus in which DNA is

compacted into chromatin.

The contents of the nucleus appear as a mass of viscous material, enclosed by a complex nuclear

envelope that marks the boundary.

Inside the nucleus of an iterphase cell we find:

chromosomes: present as extended nucleoprotein fibers, called chromatin

One or more nucleoli: in which the synthesis of ribosomal RNA and the assembly of ribosomes

took place

Nucleoplasm: fluid substance in which isolates of the nucleus are dissolved

Nuclear envelope:

It consists of two cell membranes arranged parallel, which merge at certain sites, forming circular

pores that contain an association of proteins.

The outer membrane is covered with ribososms.

The inner membrane, on the other hand, is linked by integral membrane proteins to a thin fibrillar

network called the nuclear lamina, which acts as a support for the envelope, serves as an attachment

site for chromatin fibers and plays a role in DNA replication and translation. . The filaments of the

nuclear lamina are composed of polypeptides called laminae.

The nuclear envelope is the barrier between the nucleus and the cytoplasm and the nuclear pores are

the passageways through the barrier.

Nuclear pores contain a donut-shaped structure, called the nuclear pore complex (CPN), which

extends the entire length of the pore, protruding both towards the cytoplasm and towards the

nucleoplasm.

CPNs contain about 30 different proteins called nucleoporins, each of which is present in multiple

pairs. At the heart of the CPN there is a central channel delimited by a nucleoporin ring that can

change the opening diameter.

The CPN is not a static structure.

In 1982, it was discovered that nucleoplasmins (nuclear proteins) contain an amino acid segment

near the C-terminus that acts as a nuclear localization signal (NLS), allowing a protein to pass

through nuclear pores and enter the nucleus.

The study of nuclear transport has represented an active research area in recent years.

Through various studies, the researchers were able to identify which proteins are necessary for

importing particular macromolecules into the nucleus.

They discovered a family of proteins that function as transport receptors, capable of transporting

macromolecules through the envelope.

importins: carry macromolecules from the cytoplasm to the nucleus

Esportine: carry macromolecules in the opposite direction

The import begins when the protein to be transported binds to a receptor: importin α / β, located in

the cytoplasm that carries the protein on the external surface of the nucleus, where it attaches to the

cytoplasmic filaments. A series of gold particles linked to these filaments are covered by a protein

that is transported to the nuclear pore complex.

After the cargo has passed through the CPN, a protein called Ran mediates the release of the

transported protein into the nuclear compartment.

When the importin-protein complex enters the nucleus, it encounters and binds to a Ran-GTP

molecule causing its disassembly.

The imported protein is released in the nucleosome and a portion of the NLS receptor is returned to

the cytoplasm panting with Ran-GTP which is then hydrolyzed, resulting in its release from the β

subunit of the importin. At this point, Ran-GTP is brought back to the nucleus, where it is

reconverted to the form linked to GTP making it available for a new cycle.

Chromosomes and chromatin:

A typical human cell contains an amount of DNA, broken down into 46 chromosomes, each of

which contains a single linear DNA molecule.

Chromosomes are composed of DNA and associated proteins, which form chromatin.

The compatibility of DNA within chromatin depends on histones, which is a group of small proteins

that contain a large number of basic amino acids: lysine and arginine.

Isotones are divided into 5 classes and are highly conserved.

In the early 1970s, it was observed that when chromatin is treated with non-specific nuclear, most

of the DNA is converted into fragments with a certain length; if instead a protein-free DNA is