Autotrophic Nutrition: Photosynthesis and Chemosynthesis

len Alfred Ajibola - Sat, 25th May, 2019 @ 19:11: PM

Topics in Biology

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Academic Questions in Biology

Please check out our Test Your Knowledge page to see all Questions and Answers

Which of the following is a category of long bones?

  • A. Stunted long bones
  • B. Reduced long bones
  • C. Giant long bones
  • D. Bended long bones
  • E. Curved long bones
  • F. Elongated long bones

The end product of photosynthesis is _____.

  • A. O2
  • B. C​​​​​​2​​​​C5OH
  • C. H2​​​​​O
  • D. CO2
  • E. C6H12O6
  • F. C2H5COOH

The above diagram illustrates a _____.

  • A. Food Chain
  • B. Food Web
  • C. Food Pyramid
  • D. Food Link
  • E. Food Hub
  • F. Food zone

The living cells that makes up cartilages are called _____.

  • A. osteocytes
  • B. osteoblasts
  • C. osteoclasts
  • D. chondroblasts
  • E. chrondocytes
  • F. Osteoprogenitor cells

Which of the following statement is incorrect with regards to chitin?

  • A. Ecdysis is a term associated with chitin
  • B. It is a skeletal material
  • C. It is a type of skeleton
  • D. It is present outside the organism
  • E. It does not grow when the organism grows
  • It is present in ants

Which of the following is NOT a function of the human skeleton?

  • A. Gives Shape
  • B. Provides Protection
  • C. Aids in Locomotion
  • D. Supports Movement
  • E. Assists in Inhalation and Exahalation
  • F. None of the above
  • G. All of the above

Organisms that manufacture their own food through the help of sunlight or chemicals are best referred to as _____.

  • A. autotrophs
  • B. chemosynthetic
  • C. photosynthentic
  • D. chemoautotrophs
  • E. photoautotrophs
  • F. chemoautotrophs

Organisms that begins a food chain and also manufacture their food through photosynthesis are best referred to as _____.

  • A. heterotrophs
  • B. autotrophs
  • C. homotrophs
  • D. producers
  • E. photosynthetic
  • F. chemosynthetic

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Autotrophic Nutrition:

One of the characteristics of every living organism is their ability to feed. This process of feeding is what nutrition entails. In fact, it is one of the basic characteristics of living organisms.

Please read on the Basic Characteristics of Living Organisms here.

Nutrition can be defined as the intake of food or nutrients by an organism; to be used for its growth, repair and every other necessary process for the orgnism's survival.

Regarding nutrition, an organism will either manufacture its own food or depend on readymade food. For this reason, the mode of nutrition can be grouped into two major classes.

Modes of Nutrition

The modes of nutrition are:

  1. Autotrophic Nutrition
  2. Heterotrophic Nutrition

In this article, we will explain the processes involved in autotrophic mode of nutrition.


Autotrophic mode of Nutrition

In autotrophic nutrition, organisms are able to manufacture or produce their own food.

Organisms that manufacture their own food are referred to as autotrophs.

Autotrophic nutrition can be put into two groups; these are:

a. Photosynthetic nutrition

Photosynthetic nutrition requires that organisms are able to manufacture their own food by utilizing carbon(IV)oxide (CO2) and water (H2O) in the presence of sunlight. This process is known as photosynthesis.

Note: All green plants photosynthesize.

Photosynthesis can be defined as the process whereby green plants manufacture sugars (food) by utilizing carbon(IV)oxide (CO2) and water (H2O) in the presence of light energy (sunlight); while giving off oxygen (O2) as a bye product.

The equation below expresses the definition of photosynthesis.

6CO2 + 6H2O -> C6H12O6 + 6O2

  • CO2 = Carbon(IV)oxide
  • H2O = Water
  • C6H12O6 = Sugar
  • O2 = Oxygen

Please read on Sugars here

Although not seen on the equation; Light energy and Chlorophyll are also involved (necessary) in the photosynthetic reaction.

Photosynthesis occurs in a plant’s leaf. The leaves of plant contain an organelle called chloroplast.

The chloroplast houses the chlorophyll .

The chlorophyll gives the plant’s leaf its green coloration. Also, it is the chlorophyll that traps sunlight energy.

Note: Photosynthesis and Reproduction are two entirely different processes in plants. For instance, sexual reproduction in plants take place in the flower while photosynthesis occurs in the leaf.

Please read on Plants' Sexual and Asexual Reproduction here.


How Photosynthesis works

An endothermic reaction is a reaction that takes in heat from its surroundings.

Photosynthesis is therefore an endothermic reaction since it requires sunlight energy.

Note: It isn't every stage in the process of photosynthesis that requires sunlight.

You can read on the Sun as a part of the Solar System here.

Stages of Photosynthesis

Photosynthetic reaction is put into two stages or reaction. These are:

  1. Light Stage or Light Reaction
  2. Dark Stage or Dark Reaction

  • Light Reaction or Light Stage

This stage of photosynthesis occurs in the presence of sunlight. (That is; during the day).

The chlorophyll (present inside the chloroplast in the plant’s leaf) traps the light or solar energy.

This trapped light energy is used for breaking down or splitting water molecules into hydrogen ion (H+) and hydroxyl ion (OH-).

The process of splitting water into hydrogen ion (H+) and hydroxyl ion (OH-) in the presence of sunlight is referred to as the photolysis of water. The equation below shows this process:

4H2O -> 4H+ + 4OH-

Note: A co-enzyme called NADH is reduced to NADPH. ATP is formed during this process.

  • NADH means Nicotinamide Adenine Dinucleotide
  • NADPH means Nicotinamide Adenine Dinucleotide Phosphate
  • ATP means Adenosine TriPhosphate

The formed hydroxyl ion is further converted into water and oxygen as shown in the equation below.

4OH- -> 2H2O + O2

Note: The oxygen given off as a bye product of photosynthesis comes from the light stage of photosynthesis.

The diagram below explains both the Light and Dark stages of photosynthesis.

Photosynthesis Reactions - Len Academy

Note: The sun is regarded as the ultimate source of energy. This is a statement of fact because the process of photosynthesis depends on it (sunlight energy).

Without the sun, there will be no green plants on earth and without green plants, there will be no animals. (There will be no flow of energy as seen in food chain).

Please read on Food Chain, Food Web and Food Pyramid here.


  • Dark Reaction or Dark Stage

This stage of photosynthesis occurs in the absence of light. (That is; at night).

During the dark stage, The ATP (which is energy itself) is utilized; as carbon(IV)oxide is taken in (through the stomata of the plant’s leaf) to react with the hydrogen ion (4H+) from the light reaction.

After a series of step by step reaction (with each step controlled by a specific enzyme); sugar (which is food) is formed.

The dark reaction is shown in the equation below:

4H+ + CO2 -> C6H12O6 + H2O

Note: C6H12O6 is the chemical formula for simple sugar.

Think of the stomata as an opening in the plants' leaf where gases enter and exit the plant.

Note: The sugar produced in photosynthesis occurs in the dark stage or reaction.

All green plants, blue-green algae, spirogyra and some protists carry out photosynthesis.

Please read more on Protists here.


b. Chemosynthetic Nutrition

Organisms that manufacture food in a chemosynthetic manner do not require sunlight energy; rather chemicals are utilized here.

Chemosynthetic organisms (also referred to as Chemotrophs) synthesize their food from the oxidation of simple inorganic substances (chemicals) like carbon(IV)oxide (CO2), Ammonia (NH3), Nitrite(NO2) and water (H2O).

Note: Think of oxidation as a chemical reaction that involves oxygen (O2).

Some bacteria like Nitrosomonas and Nitrobacter make use of chemosynthesis in the manufacture of food. This is shown in the equation below:

  • Nitrosomonas will convert Ammonia (NH3) to Nitrite (NO2).
  • 2NH3 + 3O2 -> 2HNO3 + H2O + Energy


  • Nitrobacter will convert Nitrite (NO2) to Nitrate (NO3).
  • HNO2 + O2 -> HNO3 + Energy

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Alfred Ajibola is a Medical Biochemist, a passionate Academician with over 7 years of experience, a Versatile Writer, a Web Developer, a Cisco Certified Network Associate and a Cisco CyberOps Associate.

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