bio Lecture 8: Cellular Respiration: Obtaining Energy from Foods

All life requires

energy

In almost all ecosystems on Earth, this energy originates with

the sun

During___ plants convert the energy of sunlight to the chemical energy of sugars and other organic molecules

photosynthesis

Humans and other animals depend on this (photosynthesis) conversion for

for our food and more

From an animal’s point of view, photosynthesis is primarily

about providing food

Plants and other autotrophs (“self-feeders”) are

organisms that make all their own organic matter (including carbohydrates, lipids, proteins, and nucleic acids) from nutrients that are entirely inorganic (CO2 from the air, and H2O and minerals from the soil)

Autotrophs make their own food;

they don’t need to eat to gain energy to power their cellular processes

humans and other animals are

heterotrophs (“other-feeders”), organisms that cannot make organic molecules from inorganic ones

we must eat ___ our nutrients and provide energy for life’s processes

organic material

Most ecosystems depend entirely on

photosynthesis for food. For this reason, biologists refer to plants and other autotrophs as producers

Heterotrophs, are

consumers, because they obtain their food by eating plants or by eating animals that have eaten plants

We animals and other heterotrophs depend on autotrophs for organic fuel and for

the raw organic materials we need to build our cells and tissues

The chemical ingredients for photosynthesis are CO2,

a gas that passes from the air into a plant via tiny pores,

H2O, which is absorbed from the

soil by the plant’s roots

Inside leaf cells, organelles called chloroplasts use

light energy to rearrange the atoms of these ingredients to produce sugars – most importantly glucose (C6H12O6) – and other organic molecules

A by-product of photosynthesis is O2

that is released through pores into the atmosphere

Both animals and plants use the organic products of photosynthesis as

sources of energy

A chemical process called __uses O2 to convert the energy stored in the chemical bonds of sugars to another source of chemical energy called __

cellular respiration, ATP

Cells expend ATP for

almost all their work

n both plants and animals, the production of ATP during cellular respiration occurs mainly in

the organelles called mitochondria

The waste products of cellular respiration

(CO2, H2O) are the very same ingredients used as inputs for photosynthesis

Plants store chemical energy via

photosynthesis and then harvest this energy via cellular respiration

Plants usually make more organic molecules than they need for fuel

This photosynthetic surplus can be stored (as starch in potatoes, for example

Respiration on the organismal level should not be confused with

cellular respiration, though they are similar

Cellular respiration requires a cell to

exchange two gases (CO2, O2) with its surroundings

Respiration, or breathing, results in the exchange of

these same gases between your blood and the outside air

O2 is present in the air you inhale, and CO2 in your bloodstream diffuses

and exits your body when you exhale

Cellular respiration is the main way that chemical energy is harvested from

food and converted to ATP energy

Cellular respiration is an

aerobic process, which is just another way of saying that it requires oxygen

define cellular respiration as the

aerobic harvesting of chemical energy from organic fuel molecules

The conversion of energy in fuel (food molecules) to a form that cells can use directly

Most often, the fuel molecule used by cells is C6H12O6, a simple sugar (monosaccharide)

• This equation summarizes the transformation of glucose during cellular respiration

The many chemical reactions that make up cellular respiration can be grouped into three main stages

1. Glycolysis

2. Citric acid cycle

3. Electron transport

During __, a molecule of glucose is split into

two molecules of a compound called pyruvic acid

glycolysis

The enzymes for glycolysis are located

in the cytoplasm

The __completes the breakdown of glucose all the way to CO2, which is then released as a waste product

Citric acid cycle (also called the Krebs cycle)

The enzymes for the Citric acid cycle are

dissolved in the fluid within mitochondria

Glycolysis and the citric acid cycle generate a

small amount of ATP directly

They generate much more ATP indirectly, via reactions that transfer electrons from fuel molecules to a molecule called NAD+ (nicotinamide adenine dinucleotide) that cells make from niacin, a B vitamin

The electron transfer forms a molecule called

NADH that acts as a shuttle carrying electrons through the cell

The third stage of cellular respiration is

electron transport

electron transport:

electrons captured from food by the NADH formed in the first two stages are stripped of their energy, a little bit at a time, until they are finally combined with oxygen to form water

The proteins and other molecules that make up electron transport chains are

embedded within the inner membrane of the mitochondria

The transport of electrons from NADH to oxygen releases

the energy your cells use to make most of their ATP

The overall equation for cellular respiration shows

that the atoms of the reactant molecules glucose and oxygen are rearranged to form the products carbon dioxide and water

The main function of cellular respiration is to

generate ATP for cellular work

cellular respiration can produce around 32 ATP molecules for

each glucose molecule consumed

During glycolysis, a

six-carbon glucose molecule is broken in half, forming two three-carbon molecules

This initial split requires an energy investment of

two ATP molecules per glucose

The three-carbon molecules then donate

high-energy electrons to NAD+, forming NADH

In addition to NADH, glycolysis also makes

four ATP molecules

Glycolysis thus produces a

net of two molecules of ATP per molecule of glucose

What remains of the fractured glucose at the end of glycolysis are two molecules of

pyruvic acid that goes into the citric acid cycle

The two molecules of pyruvic acid from glycolysis, are

not quite ready for the citric acid cycle (CAC

The pyruvic acid must be converted to a form the

citric acid cycle (CAC)

Through several reactions, the pyruvic acid is converted to

acetic acid and attached to a molecule called coenzyme A (CoA) forming acetyl CoA

The CoA escorts the acetic acid into the

first reaction of the CAC. The CoA is then stripped and recycled

The citric acid cycle finishes extracting the energy of sugar by

dismantling the acetic acid molecules all the way down to CO2

Acetic acid joins a

four-carbon acceptor molecule to form a six- carbon product called citric acid (for which the cycle is named)

The citric acid cycle harvests energy from the fuel, some of which is used to produce

ATP directly

The CICTRIC ACID cycle captures much more energy in the form of NADH and a second, closely related electron carrier called

FADH2

Because glycolysis splits glucose in two, the citric acid cycle occurs

twice for each glucose molecule that fuels a cell

Each link in an electron transport chain is actually a molecule

usually a protein

n a series of reactions, each member of the chain

transfers electrons

With each transfer, the electrons give up a small amount of energy that can then be used

indirectly to generate ATP

Electron Transport Chain The first molecule of the chain accepts

electrons from NADH

Electron Transport Chain: NADH carries electrons from

glucose and other fuel molecules and deposits them at the top of an electron transport chain

Electron Transport Chain: The electrons cascade down the chain, from molecule to molecule

like an electron bucket brigade

Electron Transport Chain: The molecule at the bottom of the chain finally

“drops” the electrons to oxygen

Electron Transport Chain: At the same time, oxygen picks up

hydrogen, forming water

The overall effect of all this transfer of electrons during cellular respiration is a

“downward” trip for electrons from glucose to NADH to an electron transport chain to oxygen

During the stepwise release of chemical energy in the electron transport chain, our cells make

most of their ATP. It is actually oxygen at the end, that makes it all possible

By pulling electrons down the transport chain from fuel molecules, oxygen

functions somewhat like gravity pulling objects downhill

Electron Transport Chain

• This role as a final electron acceptor is how the

oxygen we breathe functions in our cells and why we cannot survive more than a few minutes without it

The molecules of electron transport chains are built

into the inner membranes of mitochondria

Electron Transport Chain: Because these membranes are highly folded, their large surface area can

accommodate thousands of copies of the electron transport chain

Electron Transport Chain: Each chain acts as a chemical pump that uses the energy released by the

“fall” of electrons to move hydrogen ions (H+) across the inner mitochondrial membrane

This pumping causes ions to become more concentrated on one side of the membrane than on the other

There is a tendency for hydrogen ions to gush back to where they are less concentrated,

just as there is a tendency for water to flow downhill. The inner membrane temporarily “dams” hydrogen ions

Your mitochondria have structures that act like

turbines

Each of these miniature machines, called an ATP synthase, is

constructed from proteins built into the inner mitochondrial membrane, adjacent to the proteins of the electron transport chains

The H+ concentrated on one side of the mitochondrial membrane rushes back

“downhill” through an ATP synthase

This action spins a component of the ATP synthase, just as water turns the turbines in a dam. The rotation activates parts of the synthase molecule that attach phosphate groups to ADP molecules to generate ATP

Fermentation is the anaerobic

harvest of food energy

Although you must breathe to stay alive, some of your cells

can work for short periods without oxygen by utilizing fermentation

Under strenuous conditions, your muscles can

spend ATP faster than your bloodstream can deliver O2. This causes your muscle cells to work anaerobically

After functioning anaerobically for about 15 seconds

muscle cells will begin to generate ATP by the process of fermentation

Fermentation relies on

glycolysis, the first stage of cellular respiration

Glycolysis does not require O2 but does produce

two ATP molecules for each glucose molecule.

This is not as efficient as the 32 or so ATP molecules each glucose molecule generates during cellular respiration, but it can energize muscles for a short burst of activity

However, in such situations your cells will have to consume more glucose fuel per second because

so much less ATP per glucose molecule is generated under anaerobic conditions

Fermentation: Since there is no O2 in glycolysis to accept the electrons from NADH; the NAD+ is regenerated when

NADH transfers the electrons it removed from food to pyruvic acid

The addition of electrons to pyruvic acid produces a

waste product called lactic acid

The lactic acid by-product is eventually transported to the

liver, where liver cells convert it back to pyruvic acid

Our muscles cannot rely on lactic acid

fermentation for very long

However, the two ATP molecules produced per glucose molecule during fermentation is

enough to sustain many other microorganisms

Fermentation in Microorganisms: We have domesticated such microbes to transform milk into cheese, sour cream, and yogurt

• These foods owe their sharp or sour flavor mainly to

lactic acid

Fermentation in Yeast

• Yeast

a microscopic fungus, is capable of both cellular respiration and fermentation

When kept in an anaerobic environment

yeast cells ferment sugars and other foods to stay alive

the yeast produce

ethyl alcohol as a waste product instead of lactic acid

This alcoholic fermentation also releases CO2

For thousands of years, people have put yeast to work producing alcoholic beverages such as

BEER AND WINE