Unit 4: Cellular Respiration

Overview

Cellular respiration is a series of oxidative reactions by which cells gradually release energy from glucose and transfer it to molecules of ATP (adenosine triphosphate). energy stored in ATP is immediately available for cellular activities such as muscle contraction, transmitting nerve impulse and active transport. Cellular respiration is a very critical biological process in any organisms because it breaks down organic food to produce energy for any activity within the organism. There are two kinds of respiration, aerobic and anaerobic.

Energy currency-ATP (adenosine triphosphate)

ATP is the special high-energy molecule that stores energy for immediate use in the cell. It consists of adenosine (adenine + ribose) and 3 phosphate groups.

Structure of ATP

The removal of one phosphate group from ATP results in the formation of a more stable and lower energy molecule, ADP. Energy is released as ATP converts to ADP. Energy is absorbed to add phosphate to ADP to produce ATP.

Interconversion of ATP and ADP

Electron carriers - NAD⁺ and FAD

Electron carriers, also called electron shuttles, are small organic molecules that play key roles in cellular respiration. Their job is to pick up electrons from one molecule and drop them off with another. The electron carriers shuttle electrons from the glucose breakdown reactions to the electron transport chain (ETC). The electron that arrive at the ETC is then used to convert ADP to ATP, the process is discussed in later sections.

Redox reactions are reduction-oxidation reactions. In a biology point of view, reduction is the gain of electron of a molecule and oxidation is the lost of electron.

There are two types of electron carriers that are particularly important in cellular respiration: NAD⁺ and FAD.

When NAD⁺ and FAD pick up electron from glucose breakdown reactions (glycolysis and Kreb’s Cycle), they also gain hydrogen protons, becoming NADH and FADH₂ respectively. Below are the redox equations of NAD⁺ to NAD and FAD to FADH₂.

NAD⁺ + 2e^-+ 2H⁺→ NADH + H⁺

FAD + 2e^-+ 2H⁺→ FADH₂

The electrons in FAD to FADH₂ are dropped off at the ETC. Below are the redox equations of FAD to FADH₂ and NAD⁺ to NAD.

NADH → NAD⁺ + 2e^-+ H⁺

FADH₂ → FAD + 2e^-+ 2H⁺

Mitochondrion

Mitochondrion is an organelle which has a nickname of ‘powerhouse’. This is because major steps of aerobic respiration takes place in the mitochondrion.

Structure of mitochondrion

Outer membrane

The outer membrane is differentially permeable. It controls the movement of substances across the membrane.

Inner membrane (Cristae)

The electron transport chain happens on the cristae. The cristae is highly folded, which provide a large surface area to pack enzymes for the reaction.

Matrix

The Kreb’s Cycle takes place in the matrix, that an space filled with fluid inside the membranes. the fluid median contains enzymes that catalyse that Kreb’s Cycle.

Intermembrane space

The gap between the two membranes is small. This enables the rapid movement of materials across the membranes.

Mitochondrial DNA (mtDNA)

Mitochondria have their own DNA that encodes the proteins inside the mitochondrion. Encoding only 37 genes, it is clear that mtDNA does not contain the whole genome of the organism. In most species, including humans, mtDNA is inherited solely from the mother. Any failure in expressing mtDNA can be fatal to the organism.

Aerobic respiration

Here is the equation for the complete aerobic respiration of one molecule of glucose:

https://www.slideshare.net/elmochem/chapter-nine-cellular-respiration-fermentation

The aerobic respiration consists of three processes, glycolysis, the Kreb’s Cycle and oxidative phosphorylation.

Glycolysis

Glycolysis is the anaerobic phase of aerobic respiration which occurs in the cytoplasm of a cell. One molecule of glucose (a 6-C compound) breaks apart into two molecules of pyruvate (a 3-C compound). There is a net gain of 2 ATP through out glycolysis.

Kreb’s Cycle

The Kreb’s Cycle, also known as the citric cycle, is the first stage of the aerobic phase of cellular respiration.

Oxidative phosphorylation

In glycolysis and the Kreb’s Cycle, sometimes ATP is directly produced. Other times, NAD⁺ is reduced to NADH. NADH is then oxidized back to NAD⁺ later in the electron transport chain (ETC), releasing energy in ATP. The process of which energy is released in oxidation is called oxidative phosphorylation.

The ETC is made up of a series of proteins that oxidize NADH. When the NADH loses electrons, energy is released. The energy is used to pump hydrogen protons across the cristae from the matrix to the intermembrane space through the proteins. After a series of oxidative reactions, the concentration of protons in the intermembrane space is higher than that in the matrix, creating a proton gradient.

https://courses.lumenlearning.com/wm-biology1/chapter/reading-electron-transport-chain/

When a proton gradient exist, protons in the intermembrane space prone to flow across the cristae to the matrix through an enzyme called ATP synthase. ATP synthase works like a turbine. When hydrogen proton flow through the protein, the axle of ATP synthase spins and it keep changing its conformation. On the matrix end of the ATP synthase, ADP and phosphate groups are embedded on special locations where when the synthase spin, ADP and phosphate groups are forced to be jammed together, forming ATP. ATP is then released to be directly used. This is an animation showing how ATP works.

The hydrogen protons and the electrons in the matrix then combine with oxygen molecules to form water molecules, as shown in the following equation:

2e^-+ 2H⁺ + ½ O₂→ H₂O

This is a reductive reaction. Is this said that oxygen is the final electron acceptor. This is why aerobic respiration requires oxygen.

FAD and other co-enzymes go through the similar processes. However, the ETC each FAD goes through is shorter than that of NADH. Generating less energy, less protons are pumped through hence less ATP is produced. More specifically, each NADH produces 3 ATP and each FAD only produces 2 ATP.

Anaerobic Respiration

Anaerobic respiration happens when there is no or not enough oxygen getting to the cells for aerobic respiration, or, the cells need more energy than the amount aerobic respiration can provide. When there is little oxygen to be the final electron acceptor, the electron carriers cannot be regenerated for the Kreb’s Cycle. ATP is produced in alternative pathways anaerobically. There are two pathways of anaerobic respiration, alcoholic fermentation and lactic acid fermentation. The pathway used in organism is determined by the enzymes organism.

Alcoholic fermentation

Alcoholic fermentation happens in yeast and plants when oxygen is in short supply. Seeds and root cells of some plants in waterlogged soil. Glucose is partially broken down. Therefore a large amount of chemical energy is trapped in ethanol. Alcoholic fermentation is applied to produce alcohol.

Pathway of alcoholic fermentation

Lactic acid fermentation

Lactic acid fermentation consists of less steps thus is relatively simple than aerobic respiration. The absence of complication enable the reaction to produce additional energy in a short time. In human, lactic acid fermentation allow muscles to contract more powerfully at a higher rate.

Pathway of lactic acid fermentation

Oxygen debt

After strenuous exercise, breath is fast and deep for a period of time to take in extra oxygen. The extra oxygen is used to break down lactic acid into carbon dioxide and water, or to convert lactic acid to glycogen in the liver.

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