Autotrophic Nutrition: Photosynthesis and Chemosynthesis

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

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Academic Questions in Biology

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Food Pyramid - Len Academy

The above diagram shows a _____.

  • A. Food Chain

  • B. Food Web

  • C. Food Flow

  • D. Food Energy

  • E. Food Pyramid

  • F. Food Hierarchy

Anything that elicits a physiological response from an organism is termed _____.

  • A. Response

  • B. Irritability

  • C. Adaptation

  • D. Competition

  • E. Stimulus

  • F. Action potential

Which of the following molecule is utilized directly as energy in the human body?

  • A. Glucose

  • B. Carbohydrate

  • C. Starch

  • D. Adenosine Mono Phosphate

  • E. Adenosine Di Phosphate 

  • F. Adenosine Tri Phosphate

Which of the following is not a basic characteristics of life?

  • A. Locomotion

  • B. Competition

  • C. Adaptation

  • D. Irritability

  • E. Nutrition

  • F. Death

The final organisms in a food chain are best referred to as _____.

  • A. Saprophytes

  • B. Parasites

  • C. Commensals

  • D. Decomposers

  • E. Tertiary consumers

  • F. Quatenary consumers

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. C2C5OH
  • C. H2O
  • D. CO2
  • E. C6H12O6
  • F. C2H5COOH

Food web - Len Academy

The above diagram illustrates a _____.

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



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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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For a thing to be considered Living, it must possess all the Basic Characteristics of Life. These include:

  1. Movement

  2. Respiration

  3. Nutrition

  4. Irritability or Sensitivity

  5. Growth

  6. Excretion

  7. Reproduction

  8. Death

  9. Adaptation

  10. Competition

Please read the explanations on all the Basic Characteristics of Life here

During strenuous exercises like sprinting or 🏃 running, your body may actually work in the shortage of oxygen. During this time, your body owes oxygen (called oxygen debt) and it must be replenished or repayed after the exercise is stopped. This is why you breathe excess of oxygen after you stopped the exercise.

This is the concept of anaerobic respiration

Food Chain - Len Academy

A food Chain is a diagram that shows the relationship between plants and animals within a habitat on the basis of how they rely on each other for food.

Note: Habitat is a place where an organism live. It is of three types:

  • Terrestrial (Land)
  • Aquatic (Water)
  • Arboreal (🌲 Treetops)

You can read on Food Chain here

The term Biology originated from two Greek words. They are:

  • Bios: means life.
  • Logos: means knowledge or to study.

Biology can therefore be defined is the study of life

Anything that has life is generally considered as a living organism (or organism) 🌱🐐🐓. All living organisms must be able to carry the following activities:

  1. Movement
  2. Respiration
  3. Nutrition
  4. Irritability or Sensitivity
  5. Growth
  6. Excretion
  7. Reproduction
  8. Death
  9. Adaptation
  10. Competition

The above listed points are often regarded as the basic characteristics of life. You can memorize them using the acronym (MR. NIGER DAC).

You can read on the introduction to Biology and the basic characteristics of life here

The gas used in the manufacture of food by plants is Carbon dioxide or Carbon(IV)oxide or CO2

Plants manufacture food through a process called photosynthesis.

Please read on photosynthesis here

Note: Plants are the only orgnisms that take away CO2 from the atmosphere