Discussion of Cellular Respiration & Photosynthesis; Enzymes; Hydrogen Ion Gradients

1. Why is water so critical to living organisms?

Water is an essential element of live; all living organism require water for survival and functioning. Without water, life is not possible. Survival without water is limited to a few days (Belk & Maier, 2013). Plants, animals and humanity on the planet need water for chemicals processes and other different functions like breathing, digestion, maintaining blood pressure, protective functions, etc. As known, a decrease below the body’s required water level (dehydration) can “lead to muscle cramps, fatigue, headache, dizziness, nausea, confusion, and increase heart rate” (Belk & Maier, 2013, p. 56). Water is very crucial to living organisms as all living organisms on Earth adapt to survival with the help of water.

2. How do large molecules, such as proteins, get into a cell?

Large molecules such as proteins and others like polysaccharides or nucleotides or even whole cells get into cells by using membrane vesicles. In order to get access to cells “nutrients move across the plasma membrane, which functions as a semipermeable barrier that allows some substances to pass and prevents others from crossing” (Belk & Maier, 2013, p. 63)

3. What is the function of an enzyme? Describe the process of how it does this.

Enzymes are proteins that regulate all metabolic reactions; enzymes speed up or catalyze the rate of biological reactions. They break or build substances from simple to more complex; they help to increase the rate of reactions to maintain life. Enzymes do not require heat to catalyze the body’s chemical reactions, so they break chemical bonds without destroying or killing the cells; enzymes speed up the breakdown of the chemical bonds (Belk & Maier, 2013).

4. What happens in a cell if cellular respiration occurs in the absence of oxygen? What are two reasons why this is less advantageous for the cell compared to if oxygen is present?

As known, cellular respiration that occurs in the presence of oxygen is called as aerobic respiration, and that happens in the absence of oxygen is called as anaerobic respiration (Belk & Maier, 2013). In anaerobic respiration the pyruvic acid formed in glycolysis is channeled to the three pathways known as lactate cycle. When muscle cells have low supply of oxygen they must transmit their adenosine tri-phosphate (ATP) from glycolysis, the stage that does not need oxygen. When glycolysis occurs without oxygen, cells have low nicotinamide adenine dinucleotide (NAD); then fermentation occurs to generate NAD. Aerobic respiration’s core benefit is the amount of energy it releases; and aerobic respiration produces more ATP than anaerobic respiration. Thus, anaerobic respiration is less advantageous for the cell compared to if oxygen is present.

5. Thoroughly describe how energy in the bonds of a glucose molecule is used to build a bond in ATP in the mitochondria. Be sure to use the electron transport chain and the ATP Synthesis in your answer.

The energy stored in the bonds of a glucose molecule is used to build a bond in ATP in the mitochondria. As known, the process of cellular respiration undergoes three stages such as glycolysis, the citric acid cycle, and the electron transport and the ATP synthesis. During glycolysis, 6-carbon glucose molecule breaks down into two 3-carbon pyruvic acid; this stage produces two molecules of ATP and two NADH. During the citric acid cycle, the pyruvic acid loses a molecule of carbon dioxide leaving 2-carbon molecule to further metabolize in the mitochondria. Inside the mitochondrion, “the energy is stored in the remains of glucose is converted into the energy stored in the bonds of ATP” (Belk & Maier, 2013, p. 80). The citric acid cycle breaks the remains of carbohydrate, gather its electrons, and release carbon dioxide into the air. During the third stage, from the citric acid cycle electrons are carried by NADH to the electron transport chain; while electrons go through proteins of the electron transport chain, the hydrogen ions are putted into the inner membrane space and then flow back through the ATP synthase protein that transforms ADP to ATP. As seen, in this manner, energy from electrons added to the electron transport chain is used to produce ATP (Belk & Maier, 2013, p. 82)

6. What is the role of hydrogen ion gradients in both cellular respiration in the mitochondria and photosynthesis in the chloroplast?

NAD(P)H and FAD(P)H are both electron transmitters produced in cellular respiration and photosynthesis; both of them enter the electron transport chain and carry their electrons to the next electrons until they come to oxygen. All these movements push H+ from one place to another. In cellular respiration, the electron movements make protons to accumulate somewhere in the middle – between inner and outer membrane of the mitochondria, while in photosynthesis the electron movement cause proton to accumulate inside the thylakoid membranes – “disc-like membranous structures that typically stacked in piles” (Belk & Maier, 2013, p. 99). All these accumulation processes result in proton motive force (PMF) that later use by the ATP synthase that moves protons down.

7. Describe five similarities between cellular respiration and photosynthesis.

As know, cellular respiration means “a series of metabolic reactions that converts the energy stored in chemical bonds of food into energy that cells can use while releasing waste products” (Belk & Maier, 2013, p. 76), while photosynthesis is “the process by which plants and other microorganisms trap light energy from the sun and use it to convert carbon dioxide and water into sugar” (Belk & Maier, 2013, p. 99). Using the metabolic process of photosynthesis, plants make their own food from light, water, and carbon dioxide (Starr, et al. 2007, p. 74). Each year plants produce 220 billion tons of sugar (Starr, et al. 2007). Both cellular respiration and photosynthesis processes are similar as they produce energy but in different forms; both of these processes involve the exchange of gases; both the cellular respiration and photosynthesis take place in cell organelle and have alternate pathway. For both cellular respiration and photosynthesis, the final result is glucose with oxygen being a waste product.


Belk, C. & Maier, V. (2013). Biology: Science for Life with Physiology, 4th ed. Benjamin Cummings. Starr, C., Evers, C.A., & Starr, L. (2007). Biology Today and Tomorrow with Physiology. Second edition, Thompson.

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