The Working Cell:Energy from Food

Chapter 7

Sunlight Powers Life

• Autotrophs: self-feeders– Photosynthesis– Producers

• Heterotrophs: other eaters– Consumers

• Cellular Respiration: chemical process that uses oxygen to convert chemical energy stored in organic molecules into another form of energy– Where does this occur in animal cells?

Food Stores Chemical Energy• Kinetic energy: energy of motion

• Potential energy: energy that is stored due to an object’s position or arrangement

• Thermal energy: (heat) total amount of energy associated with the random movement of atoms and molecules in a sample of matter

• Chemical energy: potential to do work due to the arrangement of atoms with the molecules

• Working cells are similar to a car engine…they produce carbon dioxide and water as their “exhaust”

• Cells are much more efficient than automobile engines- they convert about 40% of the energy from food into useful work…the other 60% is converted to thermal energy, which is lost from our bodies in the form of heat

• calorie: amount of energy required to raise the temperature of 1 gram of water by 1o Celsius

• A calorie is such a tiny unit of energy so people usually express the energy in food in kilocalories

• 1 kilocalorie= 1000 calories• The calories shown on a food label are

actually kilocalories

You can calculate the number of calories in a peanut

• First you dry the peanut, then burn it under an insulated container of water

• Burning the peanut converts its stored energy to thermal energy, releasing heat

• Then you measure the increase in water temperature

• A peanut has about 5,000 calories…what is that in kilocalories?

5 kcal

• Cells use enzymes to break down organic molecules through the more controlled process of cellular respiration…the released energy is easier to manage for work

• Just a handful of peanuts provides enough fuel to power an hour-long walk

ATP provides energy for cellular work

• ATP: adenosine triphosphate

• The triphosphate tail is the business end of ATP…it is the source of energy used for most cellular work

More pictures of ATP

Examples of Cellular Work

• Chemical work: building large molecules such as proteins

• Mechanical work: contraction of a muscle cell

• Transport work: pumping solutes across a cell membrane

• ATP is continuously converted to ADP as your cells do work

• ATP is “recyclable”!!• ADP can be converted back to ATP by adding a

third phosphate group…this requires energy…the source of energy is the organic molecules in food

• This cycle is fast repeating…a working muscle cell recycles all of its ATP molecules about once each minute!! (that’s 10,000,000 ATP molecules spent and regenerated per second

We know that energy stored in food is converted to energy stored in ATP…but, how is

Oxygen involved?

Electrons “fall’ from food to oxygen during cellular

respiration

• Cellular respiration is an aerobic process…it requires oxygen!

• Respiration is used to describe breathing

• Breathing for a whole organism is not the same as cellular respiration, but the two processes are related

Overall equation for cellular respiration

Why does the process of cellular respiration release energy?

• An atom’s positively charged nucleus exerts an electrical “pull” on negatively charged electrons

• When an electron “falls” toward the nucleus, potential energy is released

• Oxygen attracts electrons very strongly…it is sometimes called an “electron grabber”

• Carbon and hydrogen atoms exert much less pull on electrons

• Cellular respiration is a controlled fall of electrons…like a step-by-step “walk” of electrons down an energy staircase

• Cellular respiration unlocks the energy in glucose in small, manageable amounts…the formation of ATP molecules

• Oxygen only comes in as an electron acceptor at the end

Cellular respiration converts energy in food to energy in ATP

• Structure of mitochondria

• All the chemical processes that take place in cells make up the cell’s metabolism…cellular respiration is one type of chemical process

• Cellular respiration consists of a series of reactions and is referred to as a metabolic pathway

“Road Map” of cellular respiration

Stage I: Glycolysis• Glycolysis is the chemical break down of a glucose molecule

– Splitting of sugar– Takes place outside the mitochondria in the cytoplasm of the cell– Two ATP molecules are used as an “investment”– Glucose is split into two three-carbon sugars, each with a

phosphate group– Each of these pyruvate molecules then transfers electrons and

hydrogen ions to a carrier molecule called NAD+– NAD+ is converted to NADH– Pyruvates lose the phosphate groups to form two pyruvic acids– Four new ATP molecules are produced…a net gain of two ATP

molecules (“payment”)

Fermentation

• Fermentation= cellular process of making ATP without oxygen

• Makes ATP entirely from glycolysis… remember that glycolysis does not use oxygen

• Doesn’t seem very efficient, but if enough sugar is burned, fermentation can regenerate enough ATP molecules for short bursts of activity

• Lactic acid is a waste product of fermentation

• Temporary build up of lactic acid in muscles contributes to fatigue after exercising

• Our bodies consume oxygen to convert lactic acid back to pyruvic acid

• We gain the oxygen supply by breathing heavy or stop exercising

Fermentation can be yummy!!

• Anaerobic= with out oxygen• Yeast are forced to ferment sugar when they are placed

in an anaerobic environment• Fermentation in yeasts produces alcohol, alcoholic

fermentation, and carbon dioxide• Bread, beer, wine, etc.

• Some bacteria found in stagnant ponds or deep in the soil are actually poisoned if they come into contact with oxygen

• All of their ATP is generated by fermentation

Stage II: The Krebs Cycle• Hans Krebs 1930s• Krebs Cycle finishes the breakdown of pyruvic

acid molecules to carbon dioxide• More energy is released• The fluid matrix in the inner membrane of the

mitochondrion contains the enzymes for the Krebs cycle

• The pyruvic acid molecules diffuse into the mitochondrion

• They then lose a CO2 molecule…the resulting molecule is then converted to a two-carbon compound called acetyl coenzyme A (also known as acetyl CoA)

• The acetyl CoA then enters the Krebs Cycle

• Once in the Krebs cycle, each acetyl CoA joins a four-carbon acceptor molecule

• The Krebs cycle produces two CO2 molecules and one ATP molecule per acetyl CoA molecule

• But…electron carriers trap most of the energy

• At the end of the cycle the four-carbon acceptor molecule has been regenerated and the cycle can continue

• Since each turn of the Krebs cycle breaks down one acetyl CoA molecule, the cycle actually turns twice for each glucose molecule, producing a total of four CO2 molecules and two ATP molecules

Stage III: Electron Transport Chain and ATP Synthase Action

• Final stage of cellular respiration• Occurs in the inner membrane of mitochondria• Electron transport chain= sequence of

electron carrier molecules that transfer electrons and release energy during cellular respiration

• Carrier molecule (NADH) transfers electrons from the original glucose to an electron transport chain

• Electrons are transported through the chain being pulled to oxygen at the end of the chain

• At the end of the chain the oxygen and hydrogen ions combine to form water

• Each transfer in the chain releases a small amount of energy

• This energy is used to pump hydrogen ions across the membrane

• This pumping action stores potential energy• Mitochondria have protein structures called ATP

synthases

• Hydrogen ions pumped by electron transport rush back through the ATP synthase

• The ATP synthase uses energy from the flow of hydrogen ions to convert ADP back to ATP

• Generates up to 34 ATP molecules for every one glucose molecule

• This process is similar to how a dam works!

Adding up the ATP molecules

• Glycolysis = 2 ATP

• Krebs cycle = 2 ATP

• ATP synthase = 34 ATP

Total 38 ATP

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