Cellular Respiration Details

Section 2: Cellular Respiration: How do your cells get energy?

Objectives:

• Illustrate the cycling of matter and the flow of energy through photosynthesis and respiration
• Measure the production of one or more products of either photosynthesis or respiration

 

Introduction

How does the food you eat provide energy?

• When you need a quick boost of energy, you might reach for an apple or a candy bar.
• Although foods with sugars can give you a quick boost of energy, they cannot be used for energy directly by your cells.
• Energy is simply stored in these foods.

 

Through the process of cellular respiration, the energy in food is converted into energy that can be used by the body's cells.

• In other words, glucose (and oxygen) is converted into ATP (and carbon dioxide and water).
• ATP is the molecule that provides energy for your cells to perform work, such as contracting your muscles as you walk down the street or performing active transport.
• Cellular respiration is simply a process that converts one type of chemical energy, the energy stored in sugar, into another type, ATP.

 

Overview of Cellular Respiration

Most often, cellular respiration proceeds by breaking down glucose into carbon dioxide and water.

• As this breakdown of glucose occurs, energy is released.
• The process of cellular respiration includes the conversion of this stored energy into ATP.
• The overall reaction for cellular respiration is as follows:

 

C₆H₁₂O₆ + 6O_2--------->6CO₂ + 6H₂O

 

Notice that the equation for cellular respiration is the direct opposite of photosynthesis.

• Water was broken down to free hydrogen and oxygen during photosynthesis.
  • photosynthesis requires carbon dioxide and releases oxygen
• In cellular respiration oxygen is combined with hydrogen to form water.
  • cellular respiration requires oxygen and releases carbon dioxide
• This exchange of carbon dioxide and oxygen helps to balance atmospheric oxygen and carbon dioxide.

 

Cellular respiration doesn’t happen all at once.

• Glucose is broken down slowly so that cells convert as much sugar as possible into the usable form of energy, ATP.
  • Still, some energy is lost in the process in the form of heat.
• When one molecule of glucose is broken down, it can be converted to a net total of 36 or 38 molecules of ATP.

Cellular respiration can be divided into three phases.

• Glycolysis: the breakdown of glucose – in the cell cytoplasm
• The Kreb Cycle (citric acid cycle): the formation of electron carriers – in the mitochondria
• The electron transport chain: use of oxygen to make ATP – in the mitochondria

 

The mitochondria is known as the “powerhouse” of the cell because this is the organelle where the ATP that powers the cell is produced.

 

Glycolysis

The first step of cellular respiration is glycolysis.

• During glycolysis, glucose is converted into Pyruvate and energy.
• For more information, check out the website below - http://tinyurl.com/UTBio2-2d (Links to an external site.)

 

An Overview of Glycolysis
Inputs		Outputs
1 glucose	2 Pyruvate
2 NAD+	2 NADH
2 ATP (energy)	2 ADP
4 ADP	4 ATP

 

After glycolysis, the pyruvate can go down several different paths.

• If there is oxygen available, the pyruvate moves inside the mitochondrion to produce more ATP during further break-down stages.
• In the absence of oxygen, the fermentation process begins.

 

Before the Kreb’s cycle can take place, the pyruvate molecules need to be converted into two Acetyl CoA molecules (two-carbon molecule)

• This takes place in the Mitochondrial Matrix

 

An Overview of the Pre-Kreb’s Cycle
Inputs	Outputs
2 pyruvate	2 CO2
2 NAD+	2 NADH (electron carrier)
2 CoA	2 Acetyl CoA

 

Inside the Mitochondria

If oxygen is available, the next step of cellular respiration is moving the pyruvate into the mitochondria.

• The mitochondria’s unique shape makes it possible to carry out the Krebs Cycle.

The Kreb cycle starts with pyruvic acid (pyruvate) and produces energy.

• What are the final products of the Kreb cycle?

Within the mitochondria the Kreb’s Cycle or citric acid cycle occurs.

• The citric acid cycle is a series of steps that produce Carbon dioxide, ATP, NADH and FADH2.

 

An Overview of the Citric Acid Cycle
Inputs	Outputs
2 Acetyl CoA molecules	4 CO2
6 NAD+	6 NADH (electron carrier)
2 FAD+	2 FADH2 (electron carrier)
2 ADP	2 ATP (energy)

 

In the final steps of cellular respiration, the electron transport chain converts NADH and FADH2 into ATP.

• During this energy conversion oxygen, which is necessary, combines with hydrogens and becomes water.
• That is the key reason why this process only occurs in the presence of oxygen.
• This is known as aerobic respiration, and is illustrated in the figure.

 The Electron Transport Chain takes place in the Mitochondria and changes NADH and FADH in to ATP. What is the final product of the ETC?

 

An Overview of the Electron Transport Chain
Inputs	Outputs
6 O2	6 H2O
8 NADH	8 NAD+
2 FADH2	2 FAD+
32-34 ADP	32-34 ATP

 

However, there is not always enough oxygen present for aerobic respiration to occur.

• In this case, the next step after glycolysis will be fermentation instead of the citric acid cycle

ONE ATP SYNTHASE CAN PRODUCE 100 to 150 ATP PER SECOND- some cells like mature red blood cells have NO mitochondria, but liver cells have up to 2000 mitochondria.  The human body has Billions of Mitochondria.  

 

FERMENTATION

Sometimes cellular respiration is anaerobic, occurring in the absence of oxygen.

• In the process of fermentation, the NAD+ is recycled so that is can be reused in the glycolysis process.
• No additional ATP is produced during fermentation, so the organism only obtains the two net ATP molecules per glucose from glycolysis.
• Yeasts (single-celled eukaryotic organisms) carry on alcoholic fermentation in the absence of oxygen, making ethyl alcohol (drinking alcohol) and carbon dioxide.
• Alcoholic fermentation is central to bread baking.
  • The carbon dioxide bubbles allow the bread to rise, and the alcohol evaporates.
• In wine making, the sugars of grapes are fermented to produce the wine.

 

Animals and some bacteria and fungi carry out lactic acid fermentation.

• Lactate (lactic acid) is a waste product of this process.
• Our muscles undergo lactic acid fermentation during strenuous exercise, when oxygen cannot be delivered to the muscles quickly enough.
  • The buildup of lactate is what makes your muscles sore after vigorous exercise.
• Bacteria that produce lactate are used to make cheese and yogurt.
• Tooth decay is also accelerated by lactate from the bacteria that use the sugars in your mouth.
• In all these types of fermentation, the goal is the same: to recycle NAD+ for glycolysis.

Products of fermentation include cheese (lactic acid fermentation) and wine (alcoholic fermentation).

 

 

Lesson Summary

• Cellular respiration is the breakdown of glucose to release energy in the form of ATP.
• Glycolysis, the conversion of glucose into two 3-carbon pyruvate molecules, is the first step of cellular respiration.
• If oxygen is available, the pyruvate enters the mitochondria and goes through a series of reactions, including the citric acid cycle, to produce more ATP.
• If oxygen is not available, the pyruvate is reduced during the process of fermentation to free up more NAD+ for glycolysis, and there is no net gain of ATP.
• Watch this video:  

   

  https://www.youtube.com/watch?v=ZYUdDjyNe3o (Links to an external site.)