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The left part of this image shows a plant cell bathed in a hypertonic solution so that the plasma membrane has pulled away completely from the cell wall, and the central vacuole has shrunk. The middle part shows a plant cell bathed in an isotonic solution; the plasma membrane has pulled away from the cell wall a bit, and the central vacuole has shrunk. The right part shows a plant cell in a hypotonic solution. The central vacuole is large, and the plasma membrane is pressed against the cell wall.
The turgor pressure within a plant cell depends on the tonicity of the solution that it is bathed in. (credit: modification of work by Mariana Ruiz Villareal)
The left photo shows a plant that has wilted, and the right photo shows a healthy plant.
Without adequate water, the plant on the left has lost turgor pressure, visible in its wilting; the turgor pressure is restored by watering it (right). (credit: Victor M. Vicente Selvas)

Tonicity is a concern for all living things. For example, paramecia and amoebas, which are protists that lack cell walls, have contractile vacuoles. This vesicle collects excess water from the cell and pumps it out, keeping the cell from lysing as it takes on water from its environment ( [link] ).

A transmission electron micrograph shows an oval-shaped cell. Contractile vacuoles are prominent structures embedded in the cell membrane that pump out water.
A paramecium’s contractile vacuole, here visualized using bright field light microscopy at 480x magnification, continuously pumps water out of the organism’s body to keep it from bursting in a hypotonic medium. (credit: modification of work by NIH; scale-bar data from Matt Russell)

Many marine invertebrates have internal salt levels matched to their environments, making them isotonic with the water in which they live. Fish, however, must spend approximately five percent of their metabolic energy maintaining osmotic homeostasis. Freshwater fish live in an environment that is hypotonic to their cells. These fish actively take in salt through their gills and excrete diluted urine to rid themselves of excess water. Saltwater fish live in the reverse environment, which is hypertonic to their cells, and they secrete salt through their gills and excrete highly concentrated urine.

In vertebrates, the kidneys regulate the amount of water in the body. Osmoreceptors are specialized cells in the brain that monitor the concentration of solutes in the blood. If the levels of solutes increase beyond a certain range, a hormone is released that retards water loss through the kidney and dilutes the blood to safer levels. Animals also have high concentrations of albumin, which is produced by the liver, in their blood. This protein is too large to pass easily through plasma membranes and is a major factor in controlling the osmotic pressures applied to tissues.

Section summary

The passive forms of transport, diffusion and osmosis, move materials of small molecular weight across membranes. Substances diffuse from areas of high concentration to areas of lower concentration, and this process continues until the substance is evenly distributed in a system. In solutions containing more than one substance, each type of molecule diffuses according to its own concentration gradient, independent of the diffusion of other substances. Many factors can affect the rate of diffusion, including concentration gradient, size of the particles that are diffusing, temperature of the system, and so on.

In living systems, diffusion of substances into and out of cells is mediated by the plasma membrane. Some materials diffuse readily through the membrane, but others are hindered, and their passage is made possible by specialized proteins, such as channels and transporters. The chemistry of living things occurs in aqueous solutions, and balancing the concentrations of those solutions is an ongoing problem. In living systems, diffusion of some substances would be slow or difficult without membrane proteins that facilitate transport.

Art connections

[link] A doctor injects a patient with what the doctor thinks is an isotonic saline solution. The patient dies, and an autopsy reveals that many red blood cells have been destroyed. Do you think the solution the doctor injected was really isotonic?

[link] No, it must have been hypotonic as a hypotonic solution would cause water to enter the cells, thereby making them burst.

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Questions & Answers

give me somes of examples.
Manu Reply
give example of what.
Ante
example of what
Elizabeth
what are chromosomes?
Ante
what is Biology
Darkoa
Biology is the study of animals and plants on their characters and death
Okol
No biology is the study of life (living thing's)
Achibiri
Ecology is the study of
Abdul Reply
Ecology is the study of the interaction of organism in each other and their environment.
Ante
Ecology is the study of the interaction of organism in each other and their environment
Everlove
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Great
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Ante
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Ante
a geographical area with dinstic species
Francis
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Ante
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characteristics of living things
Jane Reply
excretion, movement, sensitivity, reproduction,growth, respiration,nutrition
Estel
what are there function
Ante
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Ante
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Ante
sexual reproduction and asexual reproduction
Abdul
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Manu
Hi
Olufunmilokun
What is light independent reaction
Benjamin Reply
correct word or phrase. ______A____ uses carbon dioxide and _____B______ to make sugar during______C______. _____D______ traps light energy during this process. *
Andrea Reply
A_plant ,B_water,C_photosynthesis,D_chlorophyll
Abdul
Plant,sunlight,photosynthesis
Olufunmilokun
by which of the following feature c3 and c4 plant similar A .enzyme used to fix carbon dioxide B.cell where calvin cycle takes place C.cell where carbon dioxide fixation takes place
Kaleab Reply
correct word or phrase. ______A____ uses carbon dioxide and _____B______ to make sugar during______C______. _____D______ traps light energy during this process. *
Andrea
what is biology ?
Bizone Reply
study of organisms
Aurelia
what is link
Lamina Reply
is carbon an organism that matter?
DKP Reply
what is buttress root?
Dora Reply
Buttress root are wide large root
Iyabo
Buttress root/roots are thick roots that emerge out form the base of a large canopy
Musoke
From the base of a large canopy
Musoke
What is biology
Jiboh Reply
Biology is the study of structure and growth of living things and organisms within their biography
Musoke
compare the mechanism of gaseous exchange in an insect and mammal
World Reply
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Joy Reply
Movement Respiration Nutrition Irritability or sensitivity Growth Excretion Reproduction Adaptation Competition Death or Life Span
Emmanuel
Respiration Irritability Movement Excretion Nutrition Growth Reproduction
Amponsah
movement respiration nutrition irritability growth excretion reproduction death
Elizabeth
what is the function of medulla oblongata
mbalenhle Reply
what is the strongest bone in the human body
Chionye
what is biology
Dauda Reply
is a branch of science which deals with the study of living thing
sheka
thanks
Dauda
Biology is got from two main words: bios: means life and Logos: means knowledge therefore, biology is a branch of science that deals with knowledge, life and functioning of living things.
Musoke
what are the characteristics of organism
Seldam
Responds to stimuli
Musoke
Biology is an aspect of science that deals with the. study of living organisms
Amponsah

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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