Photosynthesis Diagram: From Beginning To End

Photosynthesis is the process that allows plants to gather energy from the sun and transform it into energy they can use. How exactly does the process of photosynthesis work? Creating a model of a cell or examining a diagram can help you understand the process of photosynthesis. The diagram found above will give you some quick intuition about the process, letting you see its primary components. However, to gain a thorough understanding of the photosynthesis process, we’ll want to go over the process in more detail and examine how each of the component pieces operates.

Photosynthesis Reactants

Let’s begin by taking a look at the reactants of photosynthesis. The necessary components or ingredients for photosynthesis include light energy, oxygen, carbon dioxide, and water. These are referred to as the reactants photosynthesis. The cells of the plant will take in carbon dioxide, water, and sunlight and convert it into usable energy through photosynthesis. Given that carbon dioxide and water are the necessary ingredients for the creation of glucose, or sugar, the chemical equation for photosynthesis can be represented in this form:

6 CO2 + 6 H2O → C6H12O6 + 6 O2

In natural language, this means that water/H2O and carbon dioxide/CO2 are converted into oxygen or O2 and glucose or C6H1206. The carbon dioxide that is used to create glucose come from heterotrophic organisms, organisms which cannot produce their own energy unlike plants can. Heterotrophic organisms emit carbon dioxide during the process of cellular respiration or another process known as fermentation. Plants pull the carbon dioxide from the atmosphere through structures called stomata, which are small holes found within the leaves of the plant. The chloroplasts found within the plant cell will then utilize the carbon dioxide in order to create carbohydrates.

Water, as you know, is found abundantly on Earth and different plants have different methods of absorbing water. Many plants absorb water through their roots, but they can also absorb them through leaves and other structures.

Water is also a byproduct of cellular respiration, much like carbon dioxide. In fact, the equation for cellular respiration can be represented like this:

C6H12O6 + 6O2 → 6CO2 + 6H2O

You may have noticed that the products of cellular respiration and the products of photosynthesis are the inverse of one another. So while cellular respiration takes in glucose and oxygen and releases carbon dioxide and water, photosynthesis carries out the opposite transformation. Animal cells utilize oxygen and the hydrogen found within glucose and they form water as a byproduct.

Photo: Photo: Cellular respiration and phtosynthesis are opposite of one another and part of the carbon cycle. Photo: OpenStax College, Biology/CC-BY 4.0

The ATP is created in animal cells by the transformation of glucose into carbon dioxide. The fact that photosynthesis and cellular respiration represent opposite reactions means they both constitute part of the cycle referred to as the Carbon Cycle, which is the system that lets carbon move from animals to the atmosphere to plants and back through again.

Water and carbon dioxide are merely the ingredients necessary for photosynthesis, but in order to transform these ingredients into glucose, the plant needs something else. It needs an energy catalyst, which is obtained by absorbing the energy that radiates from the sun, by absorbing sunlight. The structures within plant cells that absorb light energy are called pigments.

Pigments And Photosynthesis

The process of photosynthesis is managed by specific organelles within the cell. The organelles that are primarily responsible for photosynthesis are called chloroplasts, and they are filled with chlorophyll, a type of pigment. Pigments absorb certain wavelengths of light or portions of the electromagnetic radiation spectrum. Because different pigments absorb different portions of the light spectrum, different pigments will have different colors. Chlorophyll is the pigment responsible for giving many plants their green coloration. The photosynthetic process happens within the middle layer of the leaf, a region referred to as the mesophyll. The leaves of plants are made out of many different layers stacked on top of one another, but the outermost layers contain the stomata that are responsible for exchanging gases, for exchanging carbon dioxide and oxygen. The process of photosynthesis ends by releasing oxygen into the atmosphere.

The chlorophyll is contained within the chloroplast, the actual organelle responsible for the conversion of carbon dioxide and water into sugar. The light energy is used to create ATP or adenosine triphosphate, a form of energy usable by the cells of the plant. Chlorophyll isn’t the only pigment capable of carrying out photosynthesis, as carotenoids are another kind of pigment which can create nicotinamide adenine dinucleotide phosphate (NADPH), which serves to transport electrons throughout the cell. The electrons that NADPH carries with it will eventually be utilized to form carbohydrates during the Calvin Cycle. The process that creates carbohydrates by using electrons is called CO2 fixation.

Photo: Photo: By Kelvinsong – Own work, CC BY 3.0,

You may know that the mitochondria within cells have two membranes, and this is also true of chloroplasts. The inner membrane of the chloroplast is full of cylindrical structures called thylakoids, and this region is called the stoma. The thylakoids are what actually contain the chlorophylls, and many thylakoids are stacked on top of one another, forming a structure referred to as a granum. Thylakoids can be divided into different classes like stroma thylakoids and granal thylakoids. Nestled within the chloroplasts of the cell are nucleoid, ring-shaped regions of genetic material. Nucleoids are found in most prokaryotic cells. The chloroplast’s interior is also home to plastoglobules, small lipid structures that are responsible for the synthesis of tocopherols. Finally, the chloroplast contains starch granules, semi-crystalline structures comprised of various glucose polymers. These starch granules are where much of the carbon is stored in plants.

The chloroplast’s stroma is where the production of carbohydrates takes place. This region contains DNA chunks and ribosomes within it for this reason. The presence of DNA and ribosomes within chloroplasts are some of the reasons scientists theorize that chloroplasts are the result of a symbiotic relationship between two separate cells that are merged together over a long evolutionary process. It is suspected that cyanobacteria were the precursors to chloroplasts and that they lived inside cells, giving the cell extra energy in return for protection from the outside environment. Furthermore, chloroplasts reproduce through a process known as binary fission, and bacteria reproduce in precisely this way.

As a side note, organisms capable of utilizing chemical energy from sunlight are referred to as photoautotrophs, and this term is intended to distinguish them of from chemoautotrophs, which are types of bacteria capable of deriving their energy from inorganic compounds (from the synthesis of sugars).

Different Types Of Plastids

Pigments operate by absorbing wavelengths of light, and different kinds of pigments can capture different wavelengths. A plant’s color is influenced by the type of pigment it has. Most plants are green and they get their coloration from chlorophyll, but there are other pigments such as carotenoids and phycobilins that give plants different colors. Phycobilins absorb the red, orange, and blue parts of the light spectrum, meaning that every wavelength except these colors is reflected back at the eye. In contrast, the blue and green wavelengths of the electromagnetic spectrum are absorbed by carotenoids which are orange, red, or yellow.

Photosystem I and Photosystem II

The light-dependent reactions in the photosystem happen at the membrane of the thylakoid. Photo: By Somepics – Own work, CC BY-SA 4.0,

There are different kinds of chemical reactions that take place in plant cells, light-dependent reactions, and light-independent reactions. As you may be able to guess, light-independent reactions don’t need sunlight to be carried out, while light-dependent reactions do need sunlight. In the case of light-dependent photosystems, the thylakoids found in the chloroplasts absorb the sunlight and convert it into energy that can be stored in the form of the molecule ATP, or in NADPH (the electron carrier molecule).

The cellular process that is responsible for converting light energy into ATP or NADPH occurs within a complex made out of multi-proteins, a structure called a photosystem that carries out various chemical transformations. The chloroplast is home to two different photosystems: photosystem one and photosystem two. Excited electrons release energy, and this is the energy that comes from sunlight. Both of the photosystems will absorb this energy. The energy captured by the photosystem can then be transported, by moving electrons, to other regions of the cell where they can catalyze light-independent reactions.

Differences Between Plant And Animal Cells

There are organelles within both plant and animal cells that handle the production of energy but these organelles are different between plant cells and animal cells. In the case of animal cells, the mitochondria within them create energy for the cell to use utilizing glucose and oxygen and emit carbon dioxide and water. The chloroplasts in plant cells are what create energy for the cell, even though plant cells also possess mitochondria. The mitochondria found within plant cells operate a little differently to how they operate in animal cells. Animal cell mitochondria are responsible for both aerobic respiration and the production of energy, but only the process of respiration is carried out by mitochondria in plant cells. Meanwhile, animal cells lack chloroplasts entirely.