chem i osmosis

8/11/2019 Chem i Osmosis

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ChemiosmosisFrom Wikipedia, the free encyclopediaChemiosmosisis the movement of ions across a selectively permeable membrane,down their electrochemical gradient. More specifically, it relates to the generationofATPby the movement ofhydrogenions across amembraneduring cellularrespiration.

An Ion gradient haspotential energyand can be used to power chemical reactions when the ions pass

through achannel(red).

Hydrogen ions (protons) willdiffusefrom an area of high proton concentration to anarea of lower proton concentration.Peter Mitchellproposed that anelectrochemical

concentration gradientof protons across a membrane could be harnessed to makeATP.He linked this process toosmosis,the diffusion of water across a membrane, which iswhy it is called chemiosmosis.

ATP synthaseis the enzyme that makes ATP by chemiosmosis. It allows protons topass through the membrane and uses thekinetic energytophosphorylateADP, making

ATP. The generation ofATPby chemiosmosis occursinchloroplastsandmitochondriaas well as in mostbacteriaandarchaea.

Contents

[hide]

1 The Chemiosmotic Theory

2 The proton-motive force

o 2.1 Equations

3 In mitochondria

4 In plants

5 In prokaryotes

6 See also

7 References cited
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This was a radical proposal at the time, and was not well accepted. The prevailing viewwas that the energy of electron transfer was stored as a stable high potentialintermediate, a chemically more conservative concept.

The problem with the older paradigm is that no high energy intermediate was everfound, and the evidence for proton pumping by the complexes of theelectron transfer

chaingrew too great to be ignored. Eventually the weight of evidence began to favor thechemiosmotic hypothesis, and in 1978, Peter Mitchell was awarded theNobel Prize inChemistry.[3]

Chemiosmotic coupling is important for ATP production inchloroplasts[4]andmanybacteriaandarchaea.

[5]

The proton-motive force[edit]

Energy conversion by the inner mitochondrial membrane and chemiosmotic coupling between the

chemical energy of redox reactions in therespiratory chainand theoxidative

phosphorylationcatalysedby theATP synthase[6][7]

(sometimes called as "mitochondrial mushrooms").

The movement of ions across the membrane depends on a combination of two factors:

1. Diffusionforce caused by concentration gradient - allparticles including ions tend to diffuse from higherconcentration to lower.

2. Electrostatic force caused by electrical potentialgradient -cationslike protons H+tend to diffuse downthe electrical potential,anionsin the opposite direction.

These two gradients taken together can be expressed as anelectrochemical gradient.

Lipid bilayersofbiological membraneshowever are barriers for ions. This is why energycan be stored as a combination of these two gradients across the membrane. Onlyspecial membrane proteins like for exampleion channelscan sometimes allow ions to
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move across the membrane (see also:Membrane transport). In chemiosmotic theorytransmembraneATP synthasesare very important. They convert energy ofspontaneous flow of protons through them into chemical energy ofATPbonds .

Hence researchers created the term proton-motive force(PMF), derived fromtheelectrochemical gradientmentioned earlier. It can be described as the measure of

the potential energy stored as a combination of proton and voltage gradients across amembrane (differences in proton concentration and electrical potential). The electricalgradient is a consequence of the charge separation across the membrane (when theprotons H+move without acounterion,such as chloride Cl-).

In most cases the proton motive force is generated by an electron transport chain whichacts as a proton pump, using the energy of electrons from an electron carrier (Gibbsfree energyofredoxreactions) to pump protons (hydrogen ions) out across themembrane, separating the charge across the membrane. In mitochondria, energyreleased by the electron transport chain is used to move protons from the mitochondrialmatrix to the intermembrane space of the mitochondrion. Moving the protons out of themitochondrion creates a lower concentration of positively charged protons inside it,resulting in a slight negative charge on the inside of the membrane. The electricalpotential gradient is about -170 mV

[6].These gradients - charge difference and the

proton concentration difference both create a combined electrochemical gradient acrossthe membrane, often expressed as the proton motive force (PMF). In mitochondria, thePMF is almost entirely made up of the electrical component but in chloroplasts the PMFis made up mostly of the pH gradient because the charge of protons H +is neutralized bythe movement of Cl-and other anions. In either case, the PMF needs to be about 50kJ/mol for the ATP synthase to be able to make ATP.

Equations[edit]

The proton-motive force is derived from theGibbs free energy:[6]

G is the Gibbs free energy change during transfer of 1mol ofcationsXm+

from thephase A to B down the electrical potential, is the electrical potential difference (mV)between phases P and N (A and B), [Xm+]Aand [X

m+]Bare our cation concentrations onopposite sides of the membrane, F is theFaraday constant,Rgas constant.The Gibbsfree energy change here is expressed frequently also as electrochemical ion gradient

m+

In case of the electrochemical proton gradientthe equation can be simplified to:

where

(pH in phase P - pH in phase N)
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Mitchell defined the proton-motive force(PMF) as

H+= 1 kJmol corresponds to p= 10.4 mV. At 25 C (298K) this equation takes the

form:

The energy expressed here as Gibbs free energy, electrochemical proton gradient, orproton-motive force (PMF), is a combination of two gradients across the membrane:

concentration gradient expressed here as pH

electrical gradient

When a system reaches equilibrium, G (m+, p) = 0, but it doesn't mean thatconcentrations are equal on both sides of the membrane. The ions' electrical gradient,in addition to the concentration difference, affects spontaneous movement across the

membrane.

Sample values:[6]

A diagram of chemiosmotic phosphorylation

Membrane

(mV)pH

p

(mV)

Gp

(kJmol1)H+/ ATP
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mitochondrial,inner (liver) 170 0.5 200 66 3.4

chloroplast,thylakoid 0 3.3 195 60 3.1

E. colicells, pH 7.5 140 0.5 170 40

Gpis the Gibbs free energy of ATP synthesis,

ADP + Pi ATP

also called phosphorylation potential. The H+/ ATP ratio values in the table above canbe calculated by comparison of p and Gp, for example:

H+/ ATP = 66 kJmol

1/ (200 mV / 10.4 kJmol

1/mV) = 66 / 19.2 = 3.4 (mitochondrion)

For mitochondria, Gptakes here into account 1 H+consumed to transfer a phosphatemolecule (Pi) across the inner membrane into the matrix by thephosphate carrier(PiC).Otherwise it would be lower. In E. colithe H+/ ATP ratio is difficult to determine (markedas ).

The energy of more than 3 H+is required to generate the chemical energy to convert asingleATP.This value is slightly lower than the theoretical number of 4 H +involvedinoxidative phosphorylationof one ADP molecule to ATP duringcellular respiration(3H

+flowing through theATP synthase/ 1 ATP + 1 leaking from the cytoplasm through the

phosphate carrier PiC).[6][7]

In mitochondria[edit]

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Directions of chemiosmotic proton transfer in themitochondrion,chloroplastand ingram-negative

bacterialcells (cellular respirationandphotosynthesis). The bacterialcell wallis omitted,gram-positive

bacterialcells don't have outer membrane.[6]

The complete breakdown ofglucosein the presence ofoxygenis calledcellularrespiration.The last steps of this process occur in mitochondria. The reducedmoleculesNADHandFADH2are generated by theKrebs cycle,glycolysis, andpyruvate processing. These molecules pass electrons to anelectron transport chain,which uses the energy released to create a proton gradient across theinnermitochondrial membrane.ATP synthasethen uses the energy stored in thisgradient to make ATP. This process is called oxidative phosphorylation because oxygenis the final electron acceptor and the energy released by reducing oxygen to water isused to phosphorylate ADP and generate ATP.

In plants[edit]

Thelight reactionsofphotosynthesisgenerate energy by chemiosmosis. Light energy(photons) are received by the antenna complex of Photosystem 2, which excites a pairof electrons to a higher energy level. These electrons travel down an electron transportchain, causing H+ to diffuse across the thylakoid membrane into the inter-thylakoidspace. These H+ are then transported down their concentration gradient through anenzyme called ATP-synthase, creating ATP by phosphorylation of ADP to ATP. Theelectrons from the initial light reaction reach Photosystem 1, then are raised to a higherenergy level by light energy and then received by an electron receptor and reduceNADP+ to NADPH+H. The electrons from Photosystem 2 get replaced by the splitting ofwater, called "photolysis." Two water molecules must be split in order to gain 4

electrons (as well as O2, the oxygen eudicots require for survival).

In prokaryotes[edit]
http://en.wikipedia.org/wiki/Mitochondrionhttp://en.wikipedia.org/wiki/Mitochondrionhttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Gram-negative_bacteriahttp://en.wikipedia.org/wiki/Gram-negative_bacteriahttp://en.wikipedia.org/wiki/Gram-negative_bacteriahttp://en.wikipedia.org/wiki/Gram-negative_bacteriahttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Gram-positive_bacteriahttp://en.wikipedia.org/wiki/Gram-positive_bacteriahttp://en.wikipedia.org/wiki/Gram-positive_bacteriahttp://en.wikipedia.org/wiki/Gram-positive_bacteriahttp://en.wikipedia.org/wiki/Chemiosmosis#cite_note-Nicholls92-6http://en.wikipedia.org/wiki/Chemiosmosis#cite_note-Nicholls92-6http://en.wikipedia.org/wiki/Chemiosmosis#cite_note-Nicholls92-6http://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/NADHhttp://en.wikipedia.org/wiki/NADHhttp://en.wikipedia.org/wiki/NADHhttp://en.wikipedia.org/wiki/FADH2http://en.wikipedia.org/wiki/FADH2http://en.wikipedia.org/wiki/FADH2http://en.wikipedia.org/wiki/FADH2http://en.wikipedia.org/wiki/Citric_acid_cyclehttp://en.wikipedia.org/wiki/Citric_acid_cyclehttp://en.wikipedia.org/wiki/Citric_acid_cyclehttp://en.wikipedia.org/wiki/Electron_transport_chainhttp://en.wikipedia.org/wiki/Electron_transport_chainhttp://en.wikipedia.org/wiki/Electron_transport_chainhttp://en.wikipedia.org/wiki/Mitochondrial_membranehttp://en.wikipedia.org/wiki/Mitochondrial_membranehttp://en.wikipedia.org/wiki/Mitochondrial_membranehttp://en.wikipedia.org/wiki/ATP_synthasehttp://en.wikipedia.org/wiki/ATP_synthasehttp://en.wikipedia.org/wiki/ATP_synthasehttp://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=5http://en.wikipedia.org/wiki/Light-dependent_reactionhttp://en.wikipedia.org/wiki/Light-dependent_reactionhttp://en.wikipedia.org/wiki/Light-dependent_reactionhttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=6http://en.wikipedia.org/wiki/File:Chemiosmotic_proton_transfer.gifhttp://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=6http://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Light-dependent_reactionhttp://en.wikipedia.org/w/index.php?title=Chemiosmosis&action=edit&section=5http://en.wikipedia.org/wiki/ATP_synthasehttp://en.wikipedia.org/wiki/Mitochondrial_membranehttp://en.wikipedia.org/wiki/Electron_transport_chainhttp://en.wikipedia.org/wiki/Citric_acid_cyclehttp://en.wikipedia.org/wiki/FADH2http://en.wikipedia.org/wiki/NADHhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Glucosehttp://en.wikipedia.org/wiki/Chemiosmosis#cite_note-Nicholls92-6http://en.wikipedia.org/wiki/Gram-positive_bacteriahttp://en.wikipedia.org/wiki/Gram-positive_bacteriahttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Cellular_respirationhttp://en.wikipedia.org/wiki/Gram-negative_bacteriahttp://en.wikipedia.org/wiki/Gram-negative_bacteriahttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Mitochondrion


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Biology - ReproductionCells normally reproduce by dividing. The division is called mitosis. You should

know thatmitosishas four stages, and you have to know what goes on in each stage.

You should also know what goes on before mitosis.

Before a cell undergoes mitosis, every single chromosome in its nucleus has to

reproduce itself. That is called interphase. It is called interphase because it takes

place in between two mitotic divisions. It happens after a previous mitotic division

has occurred and before the next one occurs. Most of the cells life is spent in

interphase.

After interphase, each chromosome and its duplicate are joined in the middle by acentromere, so they make one discrete physical structure called a chromosome made

of two chromatids. So for example in a human cell, even though

all chromosomeshave been duplicated, we say that after interphase the cell still has a

total of 46 chromosomes.

After interphase, the cell is ready to begin mitosis. When the

cellschromosomeshave fully replicated, the cell is ready to begin mitosis. The

following are the four steps of mitosisobserved under the microscope.

In step one or prophase, the chromosomesthicken and become visible. Meanwhile,

the centrioles move to opposite poles of the cells, and next a bunch of lines called

tubules form a spindle. Aster fibers appear. Aster fibers form around each centriole.

Finally, the nuclear membrane begins to fall apart.

In step two or metaphase, the double-strandedchromosome line up in a column at

the center of the cell.

In step three or anaphase, each double-strandedchromosome divides at the

centromere. The chromatids separate. The separated chromatids then move toward

opposite poles along the spindle. Each chromatid has now become a single-

strandedchromosome.
http://www.freemcatprep.com/2011/02/biology-reproduction.htmlhttp://www.freemcatprep.com/2011/02/biology-reproduction.html


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In step four or telophase, the cytoplasm divides. This is called cytokinesis. The

nuclear membranes re-form and two daughter cells result, each with single-

strandedchromosomes.

Now, we will talk a little about meiosis. Meiosis is the process from which sperm and

ova arise. Ova and sperm are haploid, not diploid. When we think about humans, we

remember that ova and sperm are the only haploid cells in the body and they only

arise through meiosis. That means meiosis must somehow start with a diploid cell

and produce a haploid cell. The easiest way to understand meiosis is to compare it

to mitosisand see how it is similar and how it is different.

Let us talk about the formation of sperm cells or spermatozoa. Before meiosis begins,

the diploid cell goes through interphase just as it does before it begins mitosis. It is

then called a primary spermatocyte. In humans, the spermatogonium has 46 single-

strandedchromosomes, and the primary spermatocyte has 46 double-


Meiosis features the same four stages that are associated with mitosis: prophase,

metaphase, anaphase and telophase. However, in meiosis during prophase, synapsis

occurs. The two pairs of homologous chromosomescome together and form a

tetrad. When synapsis occurs, pieces of DNA can be exchanged. This exchange ofgenetic material is called crossing-over. This results in genetic recombination.

In mitosis, every double-strandedchromosome lines up on the spindle, and then in

anaphase the centromere separates the two single-strandedchromosomes. In

meiosis, however, the two pairs of homologous chromosomesa tetradline up

together on the spindle. Then in anaphase, the centromere does not break up. The

two identical pairs of double-strandedchromosomesseparate, each with its

centromere intact.

At telophase, two daughter cells are formed. Both cells are now called second

spermatocytes, each containing 23 double-strandedchromosomesand is called a

haploid cell. Each daughter now contains duplicate chromosomesstill joined by a

centromere.


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Each of the two daughter cells goes through a second division that is just like mitosis.

There is a prophase, metaphase, anaphase and telophase. In each cell, a spindle forms

and the 23double-strandedchromosomesjoined by a centromere line up and then

separate at the centromeres. The result is four daughter cells, each one with 23 single-

strandedchromosomes. At this stage, they are called spermatids.

So actually, there are two cell divisions. The first division is called the first meiotic

division. It is also called the reduction division since the two

daughter chromosomesend up with 23 double-strandedchromosomesinstead of 46

single-strandedchromosomes.

The second division is very much like mitosis. In each daughter cell, a spindle forms

and the 23 centromeres separate. Then the centromeres divide and each daughter cell

in turn gives rise to two new daughter cells, each one having 23 single-


The testis is where meiosis takes place in male. The testis has a lot of little tubes

called seminiferous tubules in it. These seminiferous tubules get together to form one

big tube called the vas deferens. The wall inside of the seminiferous tubule is made of

cells, and the cells are called spermatogonia. Spermatogonia, like most cells, arediploid but they undergo meiosis and produce haploid cells, which are sperm cells or

spermatozoa.

Once spermatozoa are produced, they move through the seminiferous tubules to a

structure called the epididymis. When spermatozoa leave the epididymis, they are

mature. Then they pass into the vas deferens. The vas deferens leads to the urethra.

Fluid is added by two accessory glands, the seminal vesicles and the prostate. The

urethra is the way out, and it runs the length of the penis.

Ovaries are the site of meiosis in females. Females have two ovaries. When they

undergo meiosis, they produce egg cells. One egg cell is called an ovum, and the

plural for ovum is ova. The formation of ova is called oogenesis. So the cells inside

the female ovary undergo meiosis and produce ova.


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In the male, the cell that undergoes meiosis is called a spermatogonium. In the

female, it is called an oogonium. The oogonium, like most other cells, is diploid but it

undergoes meiosis and produces a haploid ovum. The one major difference is that

oogenesis produces only one ovum, not four. In spermatogenesis, one

spermatogonium undergoes meiosis and produces four spermatozoa. In oogenesis,

one primary oocyte produces only one ovum.

The reason for this is that the primary oocyte undergoes the first meiotic division just

like the spermatogonia. It produces two daughter cells called secondary oocytes, and

each one has 23 double-strandedchromosomes. Each daughter cell goes on into the

second meiotic division and produces two new haploid daughter cells, but three of

those cells get only a tiny amount of cytoplasm and degenerate. So we are left with

only one haploid daughter cell, and that is called an ovum.

The three tiny cells that degenerate during oogenesis are called polar bodies.

Oogenesis produces only one ovum because it wants to conserve as much cytoplasm

as possible.

There is one more difference between oogenesis and spermatogenesis. At birth, all

the oogonia are present and arrested in prophase of the first meiotic division. At thetime of ovulation, the ovum is sent out of the ovary into the fallopian tube, which

connects the ovary to the uterus. If at the time the ovum is released there happens to

be a spermatozoan in the fallopian tube, fertilization occurs. Fertilization means that

the sperm cell penetrates the ovum.

The sperm has enzymes in a portion of its head called the acrosome which enable the

sperm to penetrate the layers of cells that cover the ovum. The outer layer is called

the corona radiata, and the inner layer is called the zones pellucida. The result is azygote, which develops into an embryo.


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Why Can't We Drink The

Ocean?Ugg!! Gross!!! The salt makes it tastes bad, but it also makes it impossible to drink.

Why? In order to understand the answer to this question, you must first understand

diffusion, osmosis, and osmoregulation.

Diffusion and OsmosisDiffusionis the process by which molecules or ions spontaneously move from regions

of higher concentration to regions of lower concetration. Molecules naturally moveand constantly collide with each other, which provides a mixing action. These

molecules are always trying to reach a state of equilibrium, a uniform distribution of

molecules.

(1) The container is seperated into two compartments by a membrane that is permeable to water and sugar

molecules. Compartment A contains both types of molecules, while compartment B contains only sugar molecules.

(2) As a result of molecular motions, sugar molecules tend to diffuse from compartment A into compartmetn B.

Water molecules diffuse from Compartment B to Compartment A. (3) Eventually equilibrium is reached.

Osmosisis the diffusion of water through a selectively permiable membrane in

response to a concentration gradient. A concentration gradientis the situation in

which different areas have different levels of concentration. Osmosis is a special case

of diffusion. It occurs whenever water molecules diffuse from a region of higher

concentration to a region of lower concentration across a selectively permeable

membrane.


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Link to Diffusion and Osmosis

OsmoregulationCell membranes are usually permeable to water. Water is equilibriated by osmosis

throughout the body. This makes the concentration of water and solutes

in intracellular(in the cell) and extracellular(around the cell) essentially the same.

This creates a situation where the osmotic pressure in the intracellular and

extracellular fluid is the same. This means it is an isotonic solution.

An isotonicsolution is a solution that has the same concentration of disolved particles

as the solution with which it is compared. In this situation, water enters and leave the

cells in equal amounts and the cell remains unchanged.

Hypertonicis a condition in which a solution contains a greater concentration of

dissolved particles than the solution with which it is compared. In other words, this is

a situation where cells (high osmotic pressure) are placed in a solution with a lower

osmotic pressure. This creates a net water movement out of the cells and into the fluid

surrounding them, causing the cells to shrink.

Hypotonicis a condition in which a solution contains a lesser concentration of

dissolved particles than the solution to which it is compared. Cells with this condition

tend to gain water by osmosis and swell. Even though cell membranes are somewhatelastic, the cells may swell so much that they burst.

(a) If red blood cells are placed in a hypertonic solution, more water leaves than enters, and the cells shrink. (b) In a

hypotonic solution, more water enters than leaves, and the cells swells and may burst. (c) In a isotonic solution,

water enters and leaves cells in equal amounts, and their sizes remain unchanged.
http://www.kghc.org/biology/diffus.htmhttp://www.kghc.org/biology/diffus.htmhttp://www.kghc.org/biology/diffus.htm


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The swelling or shrinking of cells can damage them. Human cells are about .9% saline

(salt water). So cells are isotonic to a .9% saline soltion. Placing cells in distilled

water (0% saline) causes water to diffuse into the cells and they hemolyze, or burst.This is an example of a hypotonic solution. But if cells are placed in a soltion that is

higher than .9% saline, water will diffuse from the cells causing them to shrink. This

hypertonic solution is caused by the need for the solutions to always be at

equilibrium.

Drinking Salt WaterDrinking water is normally quenches your thirst. But when you drink salt water it

seems to make you more thirsty. The water in your mouth creates a situation in which

the cells in your mouth are in a hypertonic solution. In order to reach an equilibrium, a

net water movement out of the cells takes place. Now the cells have even less water

than before, and you feel even more thirsty.

Salt water not only dries out your mouth but the cells in your body too!! As it enters

your body, the cells near it release water to reach equilibrium with the surroundingfluid. The cells shrink and may become damaged. This is a condition called

dehydration, or excessive water loss.

In order to make ocean (salt) water drinkable, a system has been divised to remove the

salt. It is called distillation.

chem i osmosis

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