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Explain how photosynthesis and respiration are linked in order to provide you with energy from the food you eat

It is known that photosynthesis is the most significant biological process on our planet where the light energy is converted into chemical energy by the organisms. Plants and some bacterias use the sun energy for production sugar which is converted to ATP by means of cellular respiration.

Aerobic respiration is a chemical reaction where glucose is broken down into water and carbon dioxide. Oxygen is used in this reaction.

It is necessary to say that photosynthesis needs the products of respiration and respiration needs the products of photosynthesis. These two processes make it possible to store the energy in the cells and regulate the concentration of oxygen and carbon dioxide. (Audesirk et al., 2008, p.24)

The energy is transferred from sunlight to ATP, from ATP to sugars and from sugars to the cells in the following way:

During the process of photosynthesis where there are two types of reactions which take place – light reactions and Calvin Cycle. The sunlight energy is transferred to chemical energy in the cells which contain chlorophyll. These reactions occur in chloroplast where chlorophyll absorbs the sun light. The sunlight is transferred to ATP, the major energy molecule in the organism. Then, ATP is carried to chloroplast and is used for production chemical energy which is very important for other metabolic reactions in the organism. Some ATP is used to convert CO2 into sugar. From sugar the energy is transferred to the cell during the process of glycolysis. (Hall & Rao, 1994, p.65)

In the absence of oxygen some cells and organisms can use glycolysis coupled to fermentation to produce energy from the sugar created by photosynthesis. Explain the role of fermentation in allowing the organism to generate energy for its cells in the absence of oxygen. Include any reactions required for this process, and explain how the energy from the sun ends up as chemical energy for the anaerobic organism or cell.

Although the aerobic respiration creates a great deal of ATP, oxygen plays an important role too. Many different organisms can metabolize glucose in order to produce ATP with the help of fermentation. Fermentation is a process which permits respiration to take place in the absence of oxygen. Due to fermentation the cells can take energy from glucose anaerobically. (Hall & Rao, 1994, p.41)

The form of fermentation is glycolysis, the process of breaking down glucose during which a molecule of glucose which has 6 carbon atoms is transformed into 2 molecules of pyruvate with 3 carbon atoms each. During glycolysis the pyruvate acid is produced. It can be transformed to lactic acid or to ethanol. (Hall & Rao, 1994, p.54)

Fermentation stands for the metabolic processes which are responsible for releasing energy from sugar and other organic compounds, molecular oxygen is not necessary as well as an electronic transport system.

The reactions of fermentation can be different. They depend on the sugar used and the product which is produced in this process. One of the fermentation reactions includes the sugar -C6H12O6- glucose and the product – 2C2H5OH- ethanol. (Knobloch, 1997, p.89)

Cells use enzymes as biological catalysts to increase or accelerate the rate of reactions, such as those in photosynthesis and glycolysis. This allows reactions to occur under conditions that sustain life. Explain how enzyme catalyzes a reaction. Include three main steps of the cycle of enzyme-substrate interactions. How is enzyme activity regulated by the cell?

It is known that enzymes are the biological catalysts which play an important role in numerous biochemical reactions. There are different types of enzymes: lyases, hydrolases, ligases and others. They are classified according to the type of reaction that will be catalyzed. Some of them are metabolism, DNA synthesis, digestion, RNA synthesis, protein synthesis. Enzymes increase the rate of the reaction in the cell. The enzyme binds temporary to substrate and creates a enzyme-substrate complex. The enzyme-substrate interactions can weaken or stress the appropriate chemical bonds in the substrates which are linked and form a new molecule. Due to the chemical interactions a new product is produced and released from the enzyme’s active site.

So, three main steps of the cycle of enzyme-substrate interactions include the following ones:

  1. enzyme and substrate;
  2. enzyme-substrate complex;
  3. enzyme and product. (Knobloch, 1997, p.97)

Enzyme activity is regulated by the cell in the following way:

  • some of the enzymes can be inserted into the cell membranes such as plasma membrane, mitochondria membranes, chloroplast membranes and others which reduce the activity of enzymes.
  • some enzymes which can easily attack the cell are inhibiting in the cell which synthesizes all of them.

Pepsin is one of them. It is synthesized in chief cells. It is inactive precursor in this case. An active pepsin will be produced in case the number of inhibiting molecules is removed from the cell. Low pH should be outside the cell.

  • In case the product produced from a number of enzymatic reactions starts accumulation in the cell, the action of the enzymes which are synthesized may be inhibited, and further production of enzymes will be stopped.
  • In case of precursor activation, the activity of enzyme will be regulated by the inhibitor molecule which will bind to enzyme at another site. It will change the shape of enzyme and reduce its activity. It’s “an allosteric effect”. (Knobloch, 1997, p.72)


Audesirk, T., Audesirk, G., Byers, B. (2008) Biology – Life on Earth with Psychology. 8-th Edition. San Francisco. CA: Benjamin Cummings.
Hall, D., Rao, K. (1994) Photosynthesis. 5 Edition, Cambridge.
Knobloch, I. (1997) Readings in Biological Science. New York: Meredith Corp.