Photosynthesis and its phases (light and dark). Photosynthesis: light and dark phase

Photosynthesis is a set of processes of forming light energy into energy chemical bonds organic substances with the participation of photosynthetic dyes.

This type of nutrition is typical for plants, prokaryotes and some types of unicellular eukaryotes.

During natural synthesis, carbon and water, in interaction with light, are converted into glucose and free oxygen:

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

Modern plant physiology understands the concept of photosynthesis as a photoautotrophic function, which is a set of processes of absorption, transformation and use of light energy quanta in various non-spontaneous reactions, including the conversion of carbon dioxide into organic matter.

Phases

Photosynthesis in plants occurs in leaves through chloroplasts- semi-autonomous double-membrane organelles belonging to the class of plastids. The flat shape of the sheet plates ensures high-quality absorption and full use of light energy and carbon dioxide. The water necessary for natural synthesis comes from the roots through water-conducting tissue. Gas exchange occurs by diffusion through the stomata and partly through the cuticle.

Chloroplasts are filled with colorless stroma and penetrated by lamellae, which, when connected to each other, form thylakoids. It is in them that photosynthesis occurs. Cyanobacteria themselves are chloroplasts, so the apparatus for natural synthesis in them is not separated into a separate organelle.

Photosynthesis proceeds with the participation of pigments, which are usually chlorophylls. Some organisms contain another pigment, a carotenoid or phycobilin. Prokaryotes have the pigment bacteriochlorophyll, and these organisms do not release oxygen after natural synthesis is completed.

Photosynthesis goes through two phases - light and dark. Each of them is characterized by certain reactions and interacting substances. Let's take a closer look at the process of the phases of photosynthesis.

Light

First phase of photosynthesis characterized by the formation of high-energy products, which are ATP, the cellular energy source, and NADP, the reducing agent. At the end of the stage, oxygen is produced as a by-product. The light stage necessarily occurs with sunlight.

The process of photosynthesis occurs in thylakoid membranes with the participation of electron transport proteins, ATP synthetase and chlorophyll (or other pigment).

The functioning of electrochemical chains, through which electrons and partially hydrogen protons are transferred, is formed in complex complexes formed by pigments and enzymes.

Description of the light phase process:

1. When sunlight hits the leaf blades of plant organisms, chlorophyll electrons in the structure of the plates are excited;
2. IN active state the particles leave the pigment molecule and land on the outer side of the thylakoid, which is negatively charged. This occurs simultaneously with the oxidation and subsequent reduction of chlorophyll molecules, which take away the next electrons from the water entering the leaves;
3. Then photolysis of water occurs with the formation of ions, which donate electrons and are converted into OH radicals that can participate in further reactions;
4. These radicals then combine to form water molecules and free oxygen released into the atmosphere;
5. The thylakoid membrane acquires a positive charge on one side due to the hydrogen ion, and on the other side a negative charge due to electrons;
6. When a difference of 200 mV is reached between the sides of the membrane, protons pass through the enzyme ATP synthetase, which leads to the conversion of ADP to ATP (phosphorylation process);
7. With the atomic hydrogen released from water, NADP + is reduced to NADP H2;

While free oxygen is released into the atmosphere during reactions, ATP and NADP H2 participate in the dark phase of natural synthesis.

Dark

A mandatory component for this stage is carbon dioxide, which plants constantly absorb from the external environment through stomata in the leaves. The dark phase processes take place in the stroma of the chloroplast. Since at this stage a lot of solar energy is not required and there will be enough ATP and NADP H2 produced during the light phase, reactions in organisms can occur both day and night. Processes at this stage occur faster than at the previous one.

The totality of all processes occurring in the dark phase is presented in the form of a unique chain of sequential transformations of carbon dioxide coming from the external environment:

1. The first reaction in such a chain is the fixation of carbon dioxide. The presence of the enzyme RiBP-carboxylase contributes to the rapid and smooth course of the reaction, which results in the formation of a six-carbon compound that breaks down into 2 molecules of phosphoglyceric acid;
2. Then a rather complex cycle occurs, including certain number reactions, upon completion of which phosphoglyceric acid is converted into natural sugar - glucose. This process is called the Calvin cycle;

Along with sugar, the formation of fatty acids, amino acids, glycerol and nucleotides also occurs.

The essence of photosynthesis

From the table comparing the light and dark phases of natural synthesis, you can briefly describe the essence of each of them. The light phase occurs in the grana of the chloroplast with the obligatory inclusion of light energy in the reaction. The reactions involve components such as electron transfer proteins, ATP synthetase and chlorophyll, which, when interacting with water, form free oxygen, ATP and NADP H2. For the dark phase occurring in the stroma of the chloroplast, sunlight is optional. The ATP and NADP H2 obtained at the previous stage, when interacting with carbon dioxide, form natural sugar (glucose).

As can be seen from the above, photosynthesis appears to be a rather complex and multi-stage phenomenon, including many reactions that involve different substances. As a result of natural synthesis, oxygen is obtained, which is necessary for the respiration of living organisms and their protection from ultraviolet radiation through the formation of the ozone layer.

- synthesis of organic substances from carbon dioxide and water with the mandatory use of light energy:

6CO 2 + 6H 2 O + Q light → C 6 H 12 O 6 + 6O 2.

In higher plants, the organ of photosynthesis is the leaf, and the organelles of photosynthesis are the chloroplasts (structure of chloroplasts - lecture No. 7). The membranes of chloroplast thylakoids contain photosynthetic pigments: chlorophylls and carotenoids. There are several different types chlorophyll ( a, b, c, d), the main one is chlorophyll a. In the chlorophyll molecule, a porphyrin “head” with a magnesium atom in the center and a phytol “tail” can be distinguished. The porphyrin “head” is a flat structure, is hydrophilic and therefore lies on the surface of the membrane that faces the aqueous environment of the stroma. The phytol “tail” is hydrophobic and due to this retains the chlorophyll molecule in the membrane.

Chlorophylls absorb red and blue-violet light, reflect green light and therefore give plants their characteristic green color. Chlorophyll molecules in thylakoid membranes are organized into photosystems. Plants and blue-green algae have photosystem-1 and photosystem-2, while photosynthetic bacteria have photosystem-1. Only photosystem-2 can decompose water to release oxygen and take electrons from the hydrogen of water.

Photosynthesis is a complex multi-step process; photosynthesis reactions are divided into two groups: reactions light phase and reactions dark phase.

Light phase

This phase occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron transport proteins and the enzyme ATP synthetase. Under the influence of a quantum of light, chlorophyll electrons are excited, leave the molecule and enter the outer side of the thylakoid membrane, which ultimately becomes negatively charged. Oxidized chlorophyll molecules are reduced, taking electrons from water located in the intrathylakoid space. This leads to the breakdown or photolysis of water:

H 2 O + Q light → H + + OH - .

Hydroxyl ions give up their electrons, becoming reactive radicals.OH:

OH - → .OH + e - .

OH radicals combine to form water and free oxygen:

4NO. → 2H 2 O + O 2.

Oxygen is removed in external environment, and protons accumulate inside the thylakoid in a “proton reservoir.” As a result, the thylakoid membrane, on the one hand, is charged positively due to H +, and on the other, due to electrons, it is charged negatively. When the potential difference between the outside and internal sides thylakoid membrane reaches 200 mV, protons are pushed through ATP synthetase channels and ADP is phosphorylated to ATP; Atomic hydrogen is used to restore the specific carrier NADP + (nicotinamide adenine dinucleotide phosphate) to NADPH 2:

2H + + 2e - + NADP → NADPH 2.

Thus, in the light phase, photolysis of water occurs, which is accompanied by three important processes: 1) ATP synthesis; 2) the formation of NADPH 2; 3) the formation of oxygen. Oxygen diffuses into the atmosphere, ATP and NADPH 2 are transported into the stroma of the chloroplast and participate in the processes of the dark phase.

1 - chloroplast stroma; 2 - grana thylakoid.

Dark phase

This phase occurs in the stroma of the chloroplast. Its reactions do not require light energy, so they occur not only in the light, but also in the dark. Dark phase reactions are a chain of successive transformations of carbon dioxide (coming from the air), leading to the formation of glucose and other organic substances.

The first reaction in this chain is the fixation of carbon dioxide; The carbon dioxide acceptor is a five-carbon sugar. ribulose biphosphate(RiBF); enzyme catalyzes the reaction Ribulose biphosphate carboxylase(RiBP carboxylase). As a result of carboxylation of ribulose bisphosphate, an unstable six-carbon compound is formed, which immediately breaks down into two molecules phosphoglyceric acid(FGK). A cycle of reactions then occurs in which phosphoglyceric acid is converted through a series of intermediates to glucose. These reactions use the energy of ATP and NADPH 2 formed in the light phase; The cycle of these reactions is called the “Calvin cycle”:

6CO 2 + 24H + + ATP → C 6 H 12 O 6 + 6H 2 O.

In addition to glucose, other monomers of complex organic compounds are formed during photosynthesis - amino acids, glycerol and fatty acids, nucleotides. Currently, there are two types of photosynthesis: C 3 - and C 4 photosynthesis.

C 3-photosynthesis

This is a type of photosynthesis in which the first product is three-carbon (C3) compounds. C 3 photosynthesis was discovered before C 4 photosynthesis (M. Calvin). It is C 3 photosynthesis that is described above, under the heading “Dark phase”. Features C 3-photosynthesis: 1) the carbon dioxide acceptor is RiBP, 2) the carboxylation reaction of RiBP is catalyzed by RiBP carboxylase, 3) as a result of carboxylation of RiBP, a six-carbon compound is formed, which decomposes into two PGAs. FGK is restored to triose phosphates(TF). Some of the TF is used for the regeneration of RiBP, and some is converted into glucose.

1 - chloroplast; 2 - peroxisome; 3 - mitochondria.

This is a light-dependent absorption of oxygen and release of carbon dioxide. At the beginning of the last century, it was established that oxygen suppresses photosynthesis. As it turned out, for RiBP carboxylase the substrate can be not only carbon dioxide, but also oxygen:

O 2 + RiBP → phosphoglycolate (2C) + PGA (3C).

The enzyme is called RiBP oxygenase. Oxygen is a competitive inhibitor of carbon dioxide fixation. The phosphate group is split off and the phosphoglycolate becomes glycolate, which the plant must utilize. It enters peroxisomes, where it is oxidized to glycine. Glycine enters the mitochondria, where it is oxidized to serine, with the loss of already fixed carbon in the form of CO 2. As a result, two glycolate molecules (2C + 2C) are converted into one PGA (3C) and CO 2. Photorespiration leads to a decrease in the yield of C3 plants by 30-40% ( With 3 plants- plants characterized by C 3 photosynthesis).

C 4 photosynthesis is photosynthesis in which the first product is four-carbon (C 4) compounds. In 1965, it was found that in some plants (sugar cane, corn, sorghum, millet) the first products of photosynthesis are four-carbon acids. These plants were called With 4 plants. In 1966, Australian scientists Hatch and Slack showed that C4 plants have virtually no photorespiration and absorb carbon dioxide much more efficiently. The pathway of carbon transformations in C 4 plants began to be called by Hatch-Slack.

C 4 plants are characterized by a special anatomical structure of the leaf. All vascular bundles are surrounded by a double layer of cells: the outer layer is mesophyll cells, the inner layer is sheath cells. Carbon dioxide is fixed in the cytoplasm of mesophyll cells, the acceptor is phosphoenolpyruvate(PEP, 3C), as a result of carboxylation of PEP, oxaloacetate (4C) is formed. The process is catalyzed PEP carboxylase. Unlike RiBP carboxylase, PEP carboxylase has a greater affinity for CO 2 and, most importantly, does not interact with O 2 . Mesophyll chloroplasts have many grains where light phase reactions actively occur. Dark phase reactions occur in the chloroplasts of the sheath cells.

Oxaloacetate (4C) is converted to malate, which is transported through plasmodesmata into the sheath cells. Here it is decarboxylated and dehydrogenated to form pyruvate, CO 2 and NADPH 2 .

Pyruvate returns to the mesophyll cells and is regenerated using the energy of ATP in PEP. CO 2 is again fixed by RiBP carboxylase to form PGA. PEP regeneration requires ATP energy, so it requires almost twice as much energy as C 3 photosynthesis.

The meaning of photosynthesis

Thanks to photosynthesis, billions of tons of carbon dioxide are absorbed from the atmosphere every year and billions of tons of oxygen are released; photosynthesis is the main source of the formation of organic substances. Oxygen forms the ozone layer, which protects living organisms from short-wave ultraviolet radiation.

During photosynthesis, a green leaf uses only about 1% of the solar energy falling on it; productivity is about 1 g of organic matter per 1 m2 of surface per hour.

Chemosynthesis

The synthesis of organic compounds from carbon dioxide and water, carried out not due to the energy of light, but due to the energy of oxidation of inorganic substances, is called chemosynthesis. Chemosynthetic organisms include some types of bacteria.

Nitrifying bacteria oxidize ammonia to nitrogen, and then to nitric acid(NH 3 → HNO 2 → HNO 3).

Iron bacteria convert ferrous iron into oxide iron (Fe 2+ → Fe 3+).

Sulfur bacteria oxidize hydrogen sulfide to sulfur or sulfuric acid (H 2 S + ½O 2 → S + H 2 O, H 2 S + 2O 2 → H 2 SO 4).

As a result of oxidation reactions of inorganic substances, energy is released, which is stored by bacteria in the form of high-energy ATP bonds. ATP is used for the synthesis of organic substances, which proceeds similarly to the reactions of the dark phase of photosynthesis.

Chemosynthesizing bacteria contribute to the accumulation of minerals in the soil, improve soil fertility, and promote cleaning waste water etc.

Go to lectures No. 11“The concept of metabolism. Biosynthesis of proteins"

Go to lectures No. 13“Methods of division of eukaryotic cells: mitosis, meiosis, amitosis”

How to explain such a complex process as photosynthesis briefly and clearly? Plants are the only living organisms that can produce their own food. How do they do it? For growth and receive all the necessary substances from environment: carbon dioxide - from the air, water and - from the soil. They also need energy, which they get from the sun's rays. This energy triggers certain chemical reactions, during which carbon dioxide and water are converted into glucose (food) and is photosynthesis. The essence of the process can be explained briefly and clearly even to school-age children.

"Together with the Light"

The word "photosynthesis" comes from two Greek words - "photo" and "synthesis", the combination of which means "together with light." The solar energy is converted into chemical energy. Chemical equation of photosynthesis:

6CO 2 + 12H 2 O + light = C 6 H 12 O 6 + 6O 2 + 6H 2 O.

This means that 6 molecules of carbon dioxide and twelve molecules of water are used (along with sunlight) to produce glucose, resulting in six molecules of oxygen and six molecules of water. If you represent this as a verbal equation, you get the following:

Water + sun => glucose + oxygen + water.

The sun is very powerful source energy. People always try to use it to generate electricity, insulate houses, heat water, and so on. Plants figured out how to use solar energy millions of years ago because it was necessary for their survival. Photosynthesis can be briefly and clearly explained this way: plants use the light energy of the sun and convert it into chemical energy, the result of which is sugar (glucose), the excess of which is stored as starch in the leaves, roots, stems and seeds of the plant. The sun's energy is transferred to plants, as well as to the animals that eat these plants. When a plant needs nutrients ah for growth and other life processes, these reserves turn out to be very useful.

How do plants absorb energy from the sun?

Talking about photosynthesis briefly and clearly, it is worth addressing the question of how plants manage to absorb solar energy. This occurs due to the special structure of the leaves, which includes green cells - chloroplasts, which contain a special substance called chlorophyll. This is what gives the leaves green and is responsible for absorbing energy from sunlight.

Why are most leaves wide and flat?

Photosynthesis occurs in the leaves of plants. Amazing fact is that plants are very well adapted to capture sunlight and absorb carbon dioxide. Thanks to the wide surface, much more light will be captured. It is for this reason that solar panels, which are sometimes installed on the roofs of houses, are also wide and flat. How more surface, the better the absorption occurs.

What else is important for plants?

Like people, plants also need beneficial nutrients to stay healthy, grow, and perform their vital functions well. They obtain minerals dissolved in water from the soil through their roots. If the soil lacks mineral nutrients, the plant will not develop normally. Farmers often test the soil to ensure it has enough nutrients for crops to grow. Otherwise, resort to the use of fertilizers containing essential minerals for plant nutrition and growth.

Why is photosynthesis so important?

To explain photosynthesis briefly and clearly for children, it is worth telling that this process is one of the most important chemical reactions in the world. What reasons are there for such a loud statement? First, photosynthesis feeds plants, which in turn feed every other living thing on the planet, including animals and humans. Secondly, as a result of photosynthesis, oxygen necessary for respiration is released into the atmosphere. All living things inhale oxygen and exhale carbon dioxide. Fortunately, plants do the opposite, so they are very important for humans and animals, as they give them the ability to breathe.

Amazing process

Plants, it turns out, also know how to breathe, but, unlike people and animals, they absorb carbon dioxide from the air, not oxygen. Plants drink too. That's why you need to water them, otherwise they will die. With the help of the root system, water and nutrients are transported to all parts of the plant body, and carbon dioxide is absorbed through small holes on the leaves. The trigger for starting a chemical reaction is sunlight. All metabolic products obtained are used by plants for nutrition, oxygen is released into the atmosphere. This is how you can briefly and clearly explain how the process of photosynthesis occurs.

Photosynthesis: light and dark phases of photosynthesis

The process under consideration consists of two main parts. There are two phases of photosynthesis (description and table below). The first is called the light phase. It occurs only in the presence of light in thylakoid membranes with the participation of chlorophyll, electron transport proteins and the enzyme ATP synthetase. What else does photosynthesis hide? Light and replace each other as day and night progress (Calvin cycles). During the dark phase, the production of that same glucose, food for plants, occurs. This process is also called a light-independent reaction.

Light phase

Dark phase

1. Reactions occurring in chloroplasts are possible only in the presence of light. In these reactions, light energy is converted into chemical energy 2. Chlorophyll and other pigments absorb energy from sunlight. This energy is transferred to the photosystems responsible for photosynthesis 3. Water is used for electrons and hydrogen ions, and is also involved in the production of oxygen 4. Electrons and hydrogen ions are used to create ATP (energy storage molecule), which is needed in the next phase of photosynthesis

1. Extra-light cycle reactions occur in the stroma of chloroplasts 2. Carbon dioxide and energy from ATP are used in the form of glucose

Conclusion

From all of the above, the following conclusions can be drawn:

• Photosynthesis is a process that produces energy from the sun.
• Light energy from the sun is converted into chemical energy by chlorophyll.
• Chlorophyll gives plants their green color.
• Photosynthesis occurs in the chloroplasts of plant leaf cells.
• Carbon dioxide and water are necessary for photosynthesis.
• Carbon dioxide enters the plant through tiny holes, stomata, and oxygen exits through them.
• Water is absorbed into the plant through its roots.
• Without photosynthesis there would be no food in the world.

Comparison of the stages of photosynthesis

	Light phase	Dark phase
Place of processes	Thylakoid membranes	Chloroplast stroma
Terms	Light	Light is not required
Necessary substances	Water, carbon dioxide, ADP, NADP	Carbon dioxide, ATP, NADPH,
Processes occurring at this stage	Photolysis of water, Non-cyclic phosphorylation (ATP formation)	Calvin cycle
What is formed?	Oxygen (removed to the atmosphere), ATP, NADP-N.	Glucose, ADP, NADP

As a result, the total equation of the two stages of photosynthesis will look like this:

6CO 2 + 6H 2 O → C 6 H 12 O 6 + 6O 2

1. Light phase of photosynthesis

The light phase is a stage for which reactions require the absorption of a solar energy quantum. Its purpose is to convert the light energy of the sun into the chemical energy of ATP molecules and other energy-rich molecules. These reactions occur continuously, but are easier to study by dividing them into three stages:

1 . a) Light hitting chlorophyll imparts enough energy to it so that one electron can be removed from the molecule;

b) electrons detached from chlorophyll are captured by carrier proteins built into the thylakoid membrane, along with chlorophyll, and carried through the ATP synthetase channel to the side of the membrane facing the stroma;

c) in the stroma there is always a substance that carries hydrogen, NADP + (nicotine amide adenine dinucleotide phosphate). This compound captures e and protons excited by light, which are always present in the stroma, and is reduced, turning into NADP H 2.

2 . Water molecules decompose under the influence of light (photolysis of water): electrons, H + and O 2 are formed. Electrons replace e lost by chlorophyll in stage 1. Protons replenish the proton reservoir, which will be used in stage 3. Oxygen moves outside the cell into the atmosphere.

3 . Protons they try to exit through the ATP synthase channel, but cannot. After some time by force electric current, protons are ejected outward from the thylakoid. protons rush from the thylakoid outward - into the stroma. The output is created high level energy that goes into ATP synthesis - non-cyclic phosphorylation (ADP + Ph n = ATP). The resulting ATP molecules move into the stroma, where they participate in the reactions of carbohydrate formation.

So, the result of the light phase:

the formation of molecules rich in energy ATP and NADP H 2,

by-product – O 2?.

2. Dark phase of photosynthesis

This phase takes place in the stroma of the chloroplast, where CO 2 comes from the air, as well as the products of the light phase ATP and NADP H 2. Here these compounds are used in a series of reactions that accumulate CO 2 in the form of carbohydrates, this process is Calvin cycle(Nobel Prize 1961).

To create one molecule of glucose, the cycle must be repeated six times: each time, one carbon atom from CO 2 is added to the stock of fixed carbon in the plant.

ADP, Phn and NADP + from the Calvin cycle return to the surface of the membranes and are again converted into ATP and NADP H 2.

During the daytime, while the sun is shining, the active movement of these molecules in the chloroplasts does not stop: they scurry back and forth like shuttles, connecting two independent series of reactions. There are few of these molecules in chloroplasts, so ATP and NADP H 2 formed during the day, in the light, and after sunset are quickly consumed in carbon fixation reactions. Photosynthesis then stops until dawn. With sunrise, the synthesis of ATP and NADP·H 2 begins again, and carbon fixation soon resumes.

result of the dark phase: the formation of glucose.

So, as a result of photosynthesis, light energy is converted into the energy of chemical bonds in the molecules of organic substances. And plants, therefore, I act as intermediaries between the Cosmos and life on Earth.” This is the great space role(USE!) green plants!

Every living thing on the planet needs food or energy to survive. Some organisms feed on other creatures, while others can produce their own nutrients. They produce their own food, glucose, in a process called photosynthesis.

Photosynthesis and respiration are interconnected. The result of photosynthesis is glucose, which is stored as chemical energy in. This stored chemical energy results from the conversion of inorganic carbon (carbon dioxide) into organic carbon. The process of breathing releases stored chemical energy.

In addition to the products they produce, plants also need carbon, hydrogen and oxygen to survive. Water absorbed from the soil provides hydrogen and oxygen. During photosynthesis, carbon and water are used to synthesize food. Plants also need nitrates to make amino acids (an amino acid is an ingredient for making protein). In addition to this, they need magnesium to produce chlorophyll.

Note: Living things that depend on other foods are called . Herbivores such as cows, as well as plants that eat insects, are examples of heterotrophs. Living things that produce their own food are called. Green plants and algae are examples of autotrophs.

In this article you will learn more about how photosynthesis occurs in plants and the conditions necessary for this process.

Definition of photosynthesis

Photosynthesis is the chemical process by which plants, some algae, produce glucose and oxygen from carbon dioxide and water, using only light as an energy source.

This process is extremely important for life on Earth because it releases oxygen, on which all life depends.

Why do plants need glucose (food)?

Like humans and other living things, plants also require nutrition to survive. The importance of glucose for plants is as follows:

• Glucose produced by photosynthesis is used during respiration to release energy that the plant needs for other vital processes.
• Plant cells also convert some of the glucose into starch, which is used as needed. For this reason, dead plants are used as biomass because they store chemical energy.
• Glucose is also needed to make other chemicals such as proteins, fats and plant sugars needed to support growth and other important processes.

Phases of photosynthesis

The process of photosynthesis is divided into two phases: light and dark.

Light phase of photosynthesis

As the name suggests, light phases require sunlight. In light-dependent reactions, energy from sunlight is absorbed by chlorophyll and converted into stored chemical energy in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy molecule ATP (adenosine triphosphate). Light phases occur in thylakoid membranes within the chloroplast.

Dark phase of photosynthesis or Calvin cycle

In the dark phase or Calvin cycle, excited electrons from the light phase provide energy for the formation of carbohydrates from carbon dioxide molecules. The light-independent phases are sometimes called the Calvin cycle due to the cyclical nature of the process.

Although dark phases do not use light as a reactant (and, as a result, can occur during the day or night), they require the products of light-dependent reactions to function. Light-independent molecules depend on the energy carrier molecules ATP and NADPH to create new carbohydrate molecules. Once energy is transferred, the energy carrier molecules return to the light phases to produce more energetic electrons. In addition, several dark phase enzymes are activated by light.

Diagram of photosynthesis phases

Note: This means that the dark phases will not continue if the plants are deprived of light for too long, as they use the products of the light phases.

The structure of plant leaves

We cannot fully study photosynthesis without knowing more about the structure of the leaf. The leaf is adapted to play a vital role in the process of photosynthesis.

External structure of leaves

• Square

One of the most important characteristics of plants is the large surface area of ​​their leaves. Most green plants have wide, flat, and open leaves that are capable of capturing as much solar energy (sunlight) as is needed for photosynthesis.

• Central vein and petiole

The central vein and petiole join together and form the base of the leaf. The petiole positions the leaf so that it receives as much light as possible.

• Leaf blade

Simple leaves have one leaf blade, while complex leaves have several. The leaf blade is one of the most important components of the leaf, which is directly involved in the process of photosynthesis.

• Veins

A network of veins in the leaves transports water from the stems to the leaves. The released glucose is also sent to other parts of the plant from the leaves through the veins. Additionally, these leaf parts support and keep the leaf blade flat for greater capture of sunlight. The arrangement of the veins (venation) depends on the type of plant.

• Leaf base

The base of the leaf is its lowest part, which is articulated with the stem. Often, at the base of the leaf there are a pair of stipules.

• Leaf edge

Depending on the type of plant, the edge of the leaf can have different shapes, including: entire, jagged, serrate, notched, crenate, etc.

• Leaf tip

Like the edge of a leaf, the top is various shapes, including: sharp, round, blunt, elongated, drawn out, etc.

Internal structure of leaves

Below is a close diagram internal structure leaf tissues:

• Cuticle

The cuticle acts as the main, protective layer on the surface of the plant. As a rule, it is thicker on the top of the leaf. The cuticle is covered with a wax-like substance that protects the plant from water.

• Epidermis

The epidermis is a layer of cells that is the covering tissue of the leaf. Its main function is to protect the internal tissues of the leaf from dehydration, mechanical damage and infections. It also regulates the process of gas exchange and transpiration.

• Mesophyll

Mesophyll is the main tissue of a plant. This is where the process of photosynthesis occurs. In most plants, the mesophyll is divided into two layers: the upper one is palisade and the lower one is spongy.

• Defense cages

Guard cells are specialized cells in the epidermis of leaves that are used to control gas exchange. They perform a protective function for the stomata. Stomatal pores become large when water is freely available, otherwise the protective cells become sluggish.

• Stoma

Photosynthesis depends on the penetration of carbon dioxide (CO2) from the air through the stomata into the mesophyll tissue. Oxygen (O2), produced as a by-product of photosynthesis, leaves the plant through the stomata. When the stomata are open, water is lost through evaporation and must be replaced through the transpiration stream by water absorbed by the roots. Plants are forced to balance the amount of CO2 absorbed from the air and the loss of water through the stomatal pores.

Conditions required for photosynthesis

The following are the conditions that plants need to carry out the process of photosynthesis:

• Carbon dioxide. A colorless, odorless, natural gas found in the air and has the scientific name CO2. It is formed during the combustion of carbon and organic compounds, and also occurs during respiration.
• Water. Transparent liquid chemical substance odorless and tasteless (under normal conditions).
• Light. Although artificial light is also suitable for plants, natural sunlight tends to create best conditions for photosynthesis, because it contains natural ultraviolet radiation, which has positive influence on plants.
• Chlorophyll. It is a green pigment found in plant leaves.
• Nutrients and minerals. Chemicals and organic compounds that plant roots absorb from the soil.

What is produced as a result of photosynthesis?

• Glucose;
• Oxygen.

(Light energy is shown in parentheses because it is not matter)

Note: Plants obtain CO2 from the air through their leaves, and water from the soil through their roots. Light energy comes from the Sun. The resulting oxygen is released into the air from the leaves. The resulting glucose can be converted into other substances, such as starch, which is used as an energy store.

If factors that promote photosynthesis are absent or present in insufficient quantities, the plant can be negatively affected. For example, less light creates favorable conditions for insects that eat the leaves of the plant, and a lack of water slows it down.

Where does photosynthesis occur?

Photosynthesis occurs inside plant cells, in small plastids called chloroplasts. Chloroplasts (mostly found in the mesophyll layer) contain a green substance called chlorophyll. Below are other parts of the cell that work with the chloroplast to carry out photosynthesis.

Structure of a plant cell

Functions of plant cell parts

• : provides structural and mechanical support, protects cells from, fixes and determines cell shape, controls the rate and direction of growth, and gives shape to plants.
• : provides a platform for most chemical processes controlled by enzymes.
• : acts as a barrier, controlling the movement of substances into and out of the cell.
• : as described above, they contain chlorophyll, a green substance that absorbs light energy through the process of photosynthesis.
• : a cavity within the cell cytoplasm that stores water.
• : contains a genetic mark (DNA) that controls the activities of the cell.

Chlorophyll absorbs light energy needed for photosynthesis. It is important to note that not all color wavelengths of light are absorbed. Plants primarily absorb red and blue wavelengths - they do not absorb light in the green range.

Carbon dioxide during photosynthesis

Plants take in carbon dioxide from the air through their leaves. Carbon dioxide leaks through a small hole at the bottom of the leaf - the stomata.

The lower part of the leaf has loosely spaced cells to allow carbon dioxide to reach other cells in the leaves. This also allows the oxygen produced by photosynthesis to easily leave the leaf.

Carbon dioxide is present in the air we breathe in very low concentrations and is a necessary factor in the dark phase of photosynthesis.

Light during photosynthesis

The leaf usually has a large surface area so it can absorb a lot of light. Its upper surface is protected from water loss, disease and exposure to weather by a waxy layer (cuticle). The top of the sheet is where the light hits. This mesophyll layer is called palisade. It is adapted to absorb large quantity light, because it contains many chloroplasts.

In light phases, the process of photosynthesis increases with more light. More chlorophyll molecules are ionized and more ATP and NADPH are generated if light photons are concentrated on a green leaf. Although light is extremely important in the photophases, it should be noted that excessive amounts can damage chlorophyll, and reduce the process of photosynthesis.

Light phases are not very dependent on temperature, water or carbon dioxide, although they are all needed to complete the process of photosynthesis.

Water during photosynthesis

Plants obtain the water they need for photosynthesis through their roots. They have root hairs that grow in the soil. Roots are characterized by a large surface area and thin walls, allowing water to pass through them easily.

The image shows plants and their cells with enough water (left) and lack of it (right).

Note: Root cells do not contain chloroplasts because they are usually in the dark and cannot photosynthesize.

If the plant does not absorb enough water, it wilts. Without water, the plant will not be able to photosynthesize quickly enough and may even die.

What is the importance of water for plants?

• Provides dissolved minerals that support plant health;
• Is a medium for transportation;
• Maintains stability and uprightness;
• Cools and saturates with moisture;
• Makes it possible to carry out various chemical reactions in plant cells.

The importance of photosynthesis in nature

The biochemical process of photosynthesis uses energy from sunlight to convert water and carbon dioxide into oxygen and glucose. Glucose is used as building blocks in plants for tissue growth. Thus, photosynthesis is the method by which roots, stems, leaves, flowers and fruits are formed. Without the process of photosynthesis, plants will not be able to grow or reproduce.

• Producers

Due to their photosynthetic ability, plants are known as producers and serve as the basis of almost every food chain on Earth. (Algae are the equivalent of plants in). All the food we eat comes from organisms that are photosynthetics. We eat these plants directly or eat animals such as cows or pigs that consume plant foods.

• Base of the food chain

Within aquatic systems, plants and algae also form the basis of the food chain. Algae serve as food for, which, in turn, act as a source of nutrition for larger organisms. Without photosynthesis in aquatic environments, life would not be possible.

• Carbon dioxide removal

Photosynthesis converts carbon dioxide into oxygen. During photosynthesis, carbon dioxide from the atmosphere enters the plant and is then released as oxygen. In today's world, where carbon dioxide levels are rising at alarming rates, any process that removes carbon dioxide from the atmosphere is environmentally important.

• Nutrient cycling

Plants and other photosynthetic organisms play a vital role in nutrient cycling. Nitrogen in the air is fixed in plant tissue and becomes available for the creation of proteins. Micronutrients found in soil can also be incorporated into plant tissue and become available to herbivores further up the food chain.

• Photosynthetic dependence

Photosynthesis depends on the intensity and quality of light. At the equator, where sunlight is plentiful all year round and water is not a limiting factor, plants have high growth rates and can become quite large. Conversely, photosynthesis occurs less frequently in the deeper parts of the ocean because light does not penetrate these layers, resulting in a more barren ecosystem.