http://www.thefoodsection.comhttp://www.sciencegear.com

The Working CellLecture 5

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Much of the text material in the lecture notes is from our textbook, “Essential Biology with Physiology” by Neil A. Campbell, Jane B.

Reece, and Eric J. Simon (2004 and 2008). I don’t claim authorship. Other sources were sometimes used, and are noted.

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Outline

• Conservation of energy• Entropy• Chemical energy• Calories and food calories• ATP and cellular work• Enzymes and enzyme inhibitors• Passive transport• Osmosis and water balance• Active transport• Endocytosis and exocytosis• Cell signaling• Words and terms to know• Possible test items

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Hill Climb

http://www.eurocycler.com

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Energy

• Energy is the capacity to perform work.• Work is performed when an object is moved against an opposing force.• For example, to reach the top of the hill, a bicyclist must overcome the

opposing force of gravity.

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Conservation of Energy

• The bicyclist’s energy, derived from food, is converted to kinetic energy, the energy of movement.

• This kinetic energy becomes potential energy as the bicyclist continues to climb the hill.

• During the ascent energy is neither created nor lost, but converted to another form—the principle is known as the conservation of energy.

• The potential energy is converted to kinetic energy during the bicyclist’s descent.

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Bottom of the Hill

• At the bottom of the hill, the bicyclist’s kinetic energy is dissipated, and will require further expenditure of energy to climb the next hill—and the next hill and the next.

• The energy was lost to heat, a type of kinetic energy contained in the random motion of molecules.

• The friction between the bicycle tires and road surface enabled heat build-up in the wheels, and even to some extent to the roadway and surrounding air.

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Another Example

http://www.coastergallery.com

A roller coaster car pulled to the top of the track accumulates potential energy, which is converted to kinetic energy during

the descent.

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Entropy

• Energy conversion generates heat, which is energy in a much less useful form.

• Heat is the energy of random molecular movement, and entropy is the measure of this disorder or randomness.

• Everything that happens in nature increases the entropy of the universe.

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Department of Entropy

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Chemical Energy

• Food, gasoline, and other fuels are forms of potential energy that can be concerted to chemical energy.

• Carbohydrates, fats, and gasoline consist of carbon structures that make them especially rich in chemical energy.

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Harvesting of Chemical Energy

• Cells and automotive engines use similar physical processes to perform work.

• An engine mixes oxygen with octane in an explosive reaction that breaks down the fuel molecules and rapidly liberates energy.

• All cells use oxygen to harvest chemical energy from food molecules in a controlled process known as cellular respiration.

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Cellular Powerhouses

An electron micrograph of a mitochondrion

http:/biology.unm.edu

Artist’s conception

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Efficiency

• The waste, or byproducts, of the internal combustion engine and cellular respiration are primarily CO2 and water.

• Automobile engines extract about 25 percent of the chemical energy to produce kinetic energy for driving the wheels (the remainder is converted to heat).

• Cells extract about 40 percent of the chemical energy from food molecules for useful work.

http://hardtop.org

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Useful Work

• Useful work includes maintaining the body’s metabolism and allowing for muscle contractions.

• The remaining 60 percent of the chemical energy generates body heat to keep the body at a constant temperature (98.6oF or 37oC) even when the surrounding air is much colder.

• Sweating and other cooling mechanisms enable the body to lose excess heat such in high-temperature environments and during physical exercise.

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All Life Requires Chemical Energy

Coco

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Calories

• Calories are a unit of energy used in the physical and biological sciences.• A calorie is the amount of energy required to raise the temperature of one

gram of water by one degree Celsius (oC).• The caloric content of a food is measured by burning it completely to ashes

under a container of water, and measuring the increase in water tempera-ture.

• A handful of peanuts contains enough chemical to boil more than a quart of water if all could be converted to heat.

• The equipment used by food scientists to measure caloric content is known as the bomb calorimeter.

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Bomb Calorimeter

http://chemistry.umeche.maine.edu

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Food Calories

• The use of calories for the energy content of food is not practical since calorie is a very small unit of measurement.

• A daily recommended diet for adults would be about 2.0 x 106, or two million calories.

• Kilocalories (kcal) are instead used, where a kilocalorie equals 1000 calories.

• Calories listed on commercial food packages are always expressed in kilocalories.

• One way to avoid confusion is to refer to kilocalories as food calories.

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Caloric Accounting

• Although foods may have approximately equal food calorie content, they can have different caloric densities—for example, an orange and a donut.

• Caloric accounting considers a person’s food intake and physical activity, which are both measured in food calories.

• Caloric imbalances between food intake and physical activity will result in weight gain or weight loss.

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Avery scales used for weighing tea in the 1800s.

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Caloric Density

http://www.asymptotia.com

http://www.wisegeek.com

Each plate contains about 200 food

calories

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Nutrition Label

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‘Burning-off’ Calories

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600 kcal (fast), 200 kcal (slow)

535 kcal (2 mph)

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160 kcal (3 mph)

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510 kcal (fast) �170 kcal (slow)

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Cellular Work

• Cellular work is not produced directly by carbohydrates, fats, and other food molecules.

• Chemical energy—released by the breakdown of these molecules during cellular respiration—is used to generate ATP, or adenosine triphosphate.

Epithelial cellshttp://www.covingtoninnovations.com

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ATP and ADP

http://www.dickspsortinggoods.com

• The molecular tail of ATP provides the energy needed for all cellular work.• Each of the three phosphates group is negatively charged, and they tend

to repel each other.• The crowding of negative charges in ATP is analogous storing energy by

compressing a spring.• When released a spring can perform useful work—the release of a single

phosphate group from the molecular tail also makes energy available for cellular work.

• The molecule has two phosphate groups—it becomes ADP or adenosine diphosphate.

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ATP Molecule

http://biology.clc.uc.edu

The three phosphate groups are shown in green.

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Phosphate Transfer

• The phosphate group released from ATP is transferred to other molecules.• This transfer enables the cell to perform three types of work—mechanical,

transport, and chemical.• For the bicyclist, phosphate groups from ATP molecules are transferred to

motor proteins that change their shape and allow muscles cells to contract.• ATP provides energy for dehydration synthesis in the formation of macro-

molecules such as starches and proteins.• In the nervous system, ATP enables cells to pump ions across the plasma

membranes to allow for transmission of signals along their fibers known as axons.

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ATP Cycle

• Cells continuously expend ATP molecules to release phosphate groups for performing work.

• One muscle cell can recycle ATP molecules about 600 million (6.0 x 108) times per minute.

• ATP is restored by adding a phosphate group to ADP using the chemical energy that cellular respiration harvests from fats and carbohydrates.

• This entire process is known as the ATP cycle.

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ATP Cycle

The circle turns

clockwise

ATP

ADP +

Energy fromfood

Energy forcellular work

P

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Metabolism and Enzymes

• The sum of all chemical reactions in an organism is called its metabolism.• Enzymes are specialized proteins that speed-up the chemical reactions.• Covalent bonds in molecules must be broken to initiate a chemical reaction.• Bond breakage occurs, for example, when a disaccharide is hydrolyzed into

two monosaccharides.

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Enzymes

• Energy, usually in the form of heat, is needed for a chemical reaction to occur—the threshold amount of heat is known as its activation energy.

• Adding substantial amounts of heat is often not possible or desirable with living cells.

• Enzymes enable metabolism to occur at lower temperatures by reducing the amount of activation energy required to break the molecular bonds.

• Enzymes are catalysts that lower the barriers for chemical reactions to occur.

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Induced Fit

• An enzyme is very specific in the chemical reaction it catalyzes—many thousands of different metabolic reactions occur in the human body.

• The active site of an enzyme has a shape that fits a portion of the substrate molecule, much like the correct key readily opens a lock.

• When an enzyme attaches to the substrate, the enzyme changes its shape slightly to allow for an embrace between the molecules known as ‘induced fit.’

http:/www.carefreeenzyme.com

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Breaking of Covalent Bonds

• The enzyme places the substrate under physical or chemical stress, making it easier to break the covalent bonds and initiate the chemical reaction.

• Once the bonds are broken the enzyme can bind with another molecule.

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Enzyme Inhibitors

• Some types of molecules can inhibit metabolic reactions by binding to enzymes and disrupting their functions.

• Enzyme inhibitors are specific to the enzymes they target.• Some inhibitors are imposters of substrates that bind to enzymes. • Other inhibitors bind to a different part of the enzyme and change the

shape of the active site so that it can no longer bind to the substrate.• Organisms produce enzyme inhibitors to control the overproduction of

enzymes.• For example, an enzyme inhibitor assures that not too much lactase is

present in the small intestine.

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Enzyme Inhibitors

• Enzymes inhibitors are manufactured for various medical and biological purposes.

• Malathion, an insecticide, inhibits an enzyme needed for the functioning of insect nervous systems.

• Aerial spraying of malathion was used to control Mediterranean fruit fly infestations in California.

• Penicillin, an antibiotic, inhibits an enzyme needed to form cell walls in bacteria.

http

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Transport

• Cells control the flow of materials across their plasma membranes.• The primary function of the plasma membrane, in addition to providing

the cell boundary, is regulating the movement of molecules into and out of the cell.

• The three forms of movement are: diffusion, osmosis, and active trans-port.

http://upload.wikimedia.org

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Diffusion

• The heat energy generated by molecules makes them vibrate and move randomly—this is known as Brownian motion.

• The result of the motion is diffusion, the tendency of molecules to spread into the available space.

• Although each molecule moves randomly, the overall movement is in one direction, along the concentration gradient.

• If the top is removed from a perfume bottle the net direction of molecules is out of the bottle and into the room—a few molecules may re-enter the bottle.

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Diffusion and Directional Movement

• Directional movement can also be shown with movement of dye across a permeable membrane in a container of water.

• An equilibrium exists once the dye is evenly diffused in the water—the number of molecules moving across the membrane is about the same in both directions.

• Two different types of molecules will diffuse along their own concentration gradients.

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Semi-Permeable Membrane

Passage of time

Concentration gradient

Separate concentration gradientsRed

Green

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Passive Transport

• Diffusion across a membrane is known as passive transport since it does not require expenditure of energy.

• The cell’s plasma membrane is selectively permeable—for example, it allows small ions to pass (such as Na+ and Cl-), but not large molecules.

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Importance to Life

• Passive transport is important to all living cells.• It is the means by which O2 enters the hemoglobin of red blood cells to

nourish tissues and carry-away CO2, a waste product of cellular respira-tion.

• The plasma membrane has hydrophobic molecules, phospholipids, that slow the passage of hydrophilic molecules including H+.

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Osmosis

• The passive transport of water across a permeable membrane is known as osmosis.

• Consider a membrane between two compartments that is permeable to H2O but not to C6H12O6 (glucose).

• The solution with a higher concentration of solute is ‘hypertonic’ and the solution with a lower concentration is ‘hypotonic.’

• H2O will diffuse across the membrane from the hypotonic solution to the hypertonic solution until an equilibrium is established.

• The solutions are ‘isotonic’ once they reach equal sugar concentrations.

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Osmosis Experiment

H20 molecules are small enough to pass through the semi-permeable membrane, but the sugar molecules cannot since they are much larger.

http

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Water Regulation

• The survival of cells depends on the ability to regulate water uptake and water loss.

• When red blood cells are immersed in an isotonic solution, the volume of the cells remains constant.

• The cells are isotonic since the cell interior and the blood have the same concentration of solutes.

• Hypotonic and hypertonic environments can cause a cell to expand and burst, or shrivel and die.

http

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Red Blood Cells

http://www.ccs.k12.in.us

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Osmoregulation

• Organisms have mechanisms to balance the excessive uptake or loss of water.

• Mammals have complex water regulation mechanisms involving the brain, hormones, and kidneys.

• Many marine animals are isotonic to seawater allowing them to survive in an environment where land animals could not live.

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Plant Cells

http://www.bbc.co.uk

Plants have different water balance challenges from animal cells because of their semi-rigid walls.

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Active Transport

• Active transport requires that a cell expend energy to move molecules across the plasma membrane.

• A transport protein, using ATP as its energy source, pumps the solute across a membrane against its concentration gradient.

• Active transport enables cells to maintain concentrations of molecules that differ from the surrounding environment.

• For example, an animal cell has a higher concentration of potassium (K+) ions and lower concentration of sodium (Na+) ions inside the cell.

• The concentration differences are controlled by the sodium-potassium pump consisting of transport proteins.

• This pump is vital to enabling neurons in the brain to send impulses or signals.

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Endocytosis and Exocytosis

• Large molecules including many proteins are too large to fit through the plasma membrane.

• Transport vesicles carry proteins manufactured by the ribosomes, and fuse with the membrane to empty their contents outside of the cell.

• The process is known as exocytosis.• The opposite process is endocytosis—cells take in materials such as

food molecules and water in vesicles that bud inward from the plasma membrane.

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Exocytosis

http://www.linkpublishing.com

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Receptor-Mediated Endocytosis

• Another type of endocytosis is when certain external molecules bind with receptor proteins in the plasma membrane to be transported into the cell.

• This mechanism is known as receptor-mediated endocytosis.• It is how the liver takes up cholesterol in low-density lipoproteins (LDLs)

circulating in the blood.• In a genetic disorder, the plasma membrane of liver cells cannot take-up

cholesterol.• It will result in high LDL levels and possibly cardiovascular problems if

left untreated.• We will discuss the genetic basis of this disorder later in the semester.

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Endocytosis

http://www.pigur.co.il

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Cell Signaling

• Cells can communicate with each other across their plasma membranes.• A signal originating outside of the cell—such as a hormone—is received

by receptor proteins in the plasma membrane.• The signal triggers a chemical chain reaction inside the cell (known as a

signal transduction pathway).• The signal can lead to responses such as metabolic changes in the cell

or rearrangement of the cytoskeleton.

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Words and Terms to Know

• Activation energy• Active transport• ATP and ADP• ATP cycle• Calorie• Cell signaling• Cellular respiration• Chemical energy• Concentration gradient• Conservation of energy• Diffusion• Endocytosis• Entropy

•Enzyme•Enzyme inhibitor•Exocytosis•Food calorie•Induced fit•Kinetic energy•Osmoregulation•Osmosis•Passive transport•Potential energy•Signal transduction pathway•Substrate•Transport protein

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Possible Test Items

1. Describe the conservation of energy and entropy, and give an example related to biology.

2. Describe food calories, what they represent, and the role they have in diet and physical exercise.

3. Describe ATP and ADP in cellular work and how ATP is replenished in the ATP cycle.

4. Describe how enzymes work, and how enzyme inhibitors block their functions.

5. Describe diffusion and osmosis in the regulation of a cell’s internal environment.

6. Describe how active transport works, and how it differs from passive transport.