6.12: Osmosis and living with and without a cell wall - Biology

Cells are packed full of molecules. The concentration of water outside of the cell [H2O] out will necessarily be higher than the concentration of water inside the cell [H2O] in. This concentration gradient in solvent leads to the net movement of water into the cells183. A surprising finding, which won Peter Agre a share of the 2003 Noble prize in chemistry, was that the membrane also contains water channels, known as aquaporins184. In addition to water, aquaporin-type proteins can also facilitate the movement of other small uncharged molecules across a membrane.

The difference or gradient in the concentrations of water across the cell membrane, together with the presence of aquaporins, leads to a system that is capable of doing work. The water gradient, can lift a fraction of the solution against the force of gravity, something involved in having plants stand up straight185. How is this possible? If we think of a particular molecule in solution, it will move around through collisions with its neighbors. These collisions drive the movement of particles randomly. But if there is a higher concentration of molecules on one side of a membrane compared to the other, then the random movement of molecules will lead to a net flux of molecules from the area of high concentration to that of low concentration, even though each molecule on its own moves randomly, that is, without a preferred direction [this video 186 is good at illustrating this behavior]. At equilibrium, the force generated by the net flux of water moving down its concentration gradient is balanced by forces acting in the other direction.

The water concentration gradient across the plasma membrane of most organisms leads to an influx of water into the cell. As water enters, the plasma membrane expands; you might want to think about how that occurs, in terms of membrane structure. If the influx of water continued unopposed, the membrane would eventually burst like an over-inflated balloon, killing the cell. One strategy to avoid this lethal outcome, adopted by a range of organisms, is to build a semi-rigid “cell wall” exterior to the plasma membrane. The synthesis of this cell wall is based on the controlled assembly of macromolecules secreted by the cell through the process of exocytosis (see above). As water passes through the plasma membrane and into the cell (driven by osmosis), the plasma membrane is pressed up against the cell wall. The force exerted by the rigid cell wall on the membrane balances the force of water entering the cell. When the two forces are equal, the net influx of water into the cell stops. Conversely, if the [H2O]outside decreases, this pressure is reduced, the membrane moves away from the cell wall and, because they are only semi-rigid, the walls flex. It is this behavior that causes plants to wilt when they do not get enough water. These are passive behaviors, based on the structure of the cell wall; they are built into the wall as it is assembled. Once the cell wall has been built, a cell with a cell wall does not need to expend energy to resist osmotic effects. Plants, fungi, bacteria and archaea all have cell walls. A number of antibiotics work by disrupting the assembly of bacterial cell walls. This leaves the bacteria osmotically sensitive, water enters these cells until they burst and die.

Questions to answer & to ponder:

  • Make a graph of the water concentration across a typical cellular membrane for an organism living in fresh water; explain what factors influenced your drawing.
  • Look at this video: How could you use reverse osmosis to purify water?
  • Where does the energy involved in moving molecules come from?
  • Plants and animals are both eukaryotes; how would you decide whether the common ancestor of the eukaryotes had a cell wall.
  • Why does an aquaporin channel not allow a Na+ ion to pass through it?
  • If there is no net flux of A, even if there is a concentration gradient between two points, what can we conclude?

11 Examples Of Osmosis In Real Life

Osmosis is a simple natural process that occurs all around and inside us, and it’s one of the most vital processes for our survival. Everything tends to reach equilibrium and to reach at equilibrium the most crucial role is played by the water. Even each cell of our body, plants, and animals around us are surviving due to osmosis. Osmosis functions as a Life-Preserver. From helping out cells to survive to the desalination of seawater, the process involved is osmosis. Let’s dig into some interesting examples of osmosis in our daily life, but before that let’s understand, What is Osmosis?

Definition Of Osmosis

Osmosis is the movement of water from less concentrated to the more concentrated solution through a semi-permeable membrane.

1. Crucial In Plant’s Survival

When we water plants, we usually water the stem end and soil in which they are growing. Hence, the roots of the plants absorb water and from the roots water travel to different parts of plants be it leaves, fruits or flowers. Every root acts as a semipermeable barrier, which allows water molecules to transfer from high concentration (soil) to low concentration (roots). Roots have hair, which increases surface area and hence the water intake by the plants.

2. Helps in Regulating Our Cell’s Life

We drink water, but also our cells absorb it by osmosis in the same way that plant roots do. As the concentration of waste products in a cell rises, the osmotic pressure between the inside and outside of the cell wall, which is a semipermeable membrane, increases, and the cell absorbs water from the blood, which is a more dilute solution that of the cell’s cytoplasm. Even the primary nutrients and minerals get transferred through osmosis into the cells. Also, our intestine absorbs nutrients and minerals through osmosis.

3. Ever tried soaking resins or dry fruits in water. what happened?

well, when we soak resins in water they swell up and this is all that happens due to osmosis. Water travels from high concentration to low concentration and keeps moving by osmosis until the equilibrium is reached, that is when the concentration of both solutions is the same.

4. Responsible For Your Pruned or Wrinkled Fingers

When we sit in the bathtub or submerge our fingers in water for a while they got wrinkly. And that is too because of osmosis. The skin of our fingers absorb water and get expanded or bloated leading to the pruned or wrinkled fingers.

5. Osmosis May kill Slugs or Snails

You must have heard about the killing of slugs or snails by putting salt on it. Well, It’s nothing but the process of osmosis which kills them. The liquid inside them comes out and try to dilute the salt concentration and maintain the mucus layer, and hence, they end up shedding water. Too much salt and slugs or snails will dry up and die!

6. Reason Behind To Get Thirsty

We usually feel thirsty after eating salty food because salt is a solute and after consuming lots of salt, our cells become concentrated with salt, which triggers the process of thirst. So, our cells absorb water and we feel thirsty, and hence, we start drinking water.

7. Helps You Get Relieved From Sore Throat

In case you have a sore throat, cells and tissues surrounding the throat are swollen because of the excess of water. The salt water which we use for gargles has a lower concentration of water than the cells of the throat. So, water molecules move from the swollen cells of the throat to the salt water reducing pain and swelling.

8. Helps In Preserving Your Food

The reason why we can enjoy pickles and jams for a longer period of time without any fear of their spoilage is osmosis. Pickles and Jams have been used over decades as quick spreads and ready to eat food for kids as well as adults. They both contain high proportions of salt and sugar, respectively that acts as a natural preservative for fruits and vegetables. Though vegetables and fruits are very prone to bacterial attack but the high salt and sugar concentration is hypertonic to bacteria cells, and bacteria cells lose water and it kills them by dehydration before they can cause the food to get spoiled.

9. Helps You Getting Pure Water

RO’s are installed in almost every home in today’s scenario. Actually, this is not osmosis, strictly speaking, but, Reverse osmosis that is what RO stands for. Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally, without the involvement of energy however, to reverse the process of osmosis, you need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. However, you need to ‘push’ the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process allowing pure water to pass through while holding back a majority of contaminants. Reverse Osmosis is also used in large scale desalination of seawater turning it into drinking water.

10. Saves Eyes From Dry Contact Lenses

Soft contact lenses consist of semipermeable materials. If you wear contacts after storing them in sterile saline solution, the concentration of the saline in the contacts matches the salt content in the natural fluid that moistens your eyes the contacts stay moist, soft and comfortable. If you store contacts in distilled water, the salt concentration is higher in the eye fluid and water flows out of the contacts slowly drying them out.

11. Helps In Maintaining Water Balance In Our Body

Kidneys are the vital organ of our body, which helps in the removal of waste and toxic materials. Osmosis occurs to recover water from waste material. Kidney dialysis is an example of osmosis. In this process, the dialyzer removes waste products from a patient’s blood through a dialyzing membrane(acts as a semi-permeable membrane) and passes them into the dialysis solution tank. The red blood cells being larger in size cannot pass through the membrane and are retained in the blood. Thus, by the process of osmosis waste materials are continuously removed from the blood.

The Difference Between Apoplast and Symplast

Apoplast refers to the non protoplasmic components of a plant, including the cell wall and the intracellular spaces.

Symplast refers to the continuous arrangement of protoplasts of a plant, which are interconnected by plasmodesmata.

Apoplast consists of non protoplasmic parts such as cell wall and intracellular space.

Symplast Consists of protoplast

Apoplast composed of nonliving parts of a plant.

Symplast composed of living parts of a plant.

In apoplast, the water movement occurs by passive diffusion.

In symplast, the water movement occurs by osmosis.

In apoplast, the water movement is rapid.

In the symplast, the water movement is slower.

The metabolic rate of the cells in the root cortex does not affect the water movement.

The metabolic rate of the cells in the root cortex highly affects the water movement.

It shows less resistance to the water movement.

It shows some resistance to the water movement.

With the secondary growth of the root, most of the water moves by the apoplast route.

Beyond the cortex, water moves through the symplast route.

Similarities Between Apoplast and Symplast:

Apoplast and symplast are two ways in which the water moves from root hair cells to the xylem.

Both the apoplast and symplast occur in the root cortex.

Both the apoplast and symplast carry water and nutrients towards the xylem.

Pathways For Root Absorption Through Apoplast:

The apoplastic pathway provides a way towards the vascular cell through free spaces and cell walls of the epidermis and cortex. An additional apoplastic route that allows the direct access to the xylem and phloem is along the margins of the secondary roots. The secondary root is developed from the pericycle, a cell layer just inside the endodermis. The endodermis is characterized by the Casparian strip. Apoplast was previously defined as the whole thing but the symplast, consisting of cell walls and spaces between cells in which water and solutes can move freely.

Types of Solutions

Every solution has a solvent and a solute. When you buy contact lens solution, you are essentially buying saltwater water is the solvent and salt is the solution. The same is basically true of your tears.


To answer the question, what is osmosis in biology, we have to understand the types of solutions in biology. Solutions include isotonic, hypotonic, and hypertonic.

Isotonic solutions have equal amounts of solutes inside and outside the cell. Therefore, isotonic solutions have no net movement because the concentration is already equal.

Hypotonic solutions have lower concentrations of solutes outside of the cell than inside. This causes osmosis as solvents enter the cell to even the concentration.

Hypertonic solutions have higher concentrations of solutes outside the cell causing osmosis as solvents exit the cell to balance the concentration.

Why Is Osmosis Important to Plants?

Osmosis is important to plants because it allows for water uptake, photosynthesis and general stability. Osmosis ensures that all cells and structures within a plant have correct water pressure and volume.

Osmosis is a specific form of diffusion, the movement of molecules from areas of high concentration to areas of low concentration. Osmosis is the diffusion of water molecules through semipermeable membranes. A cell membrane surrounds plant cells, just like animal cells. Plant cells also have an additional outer layer, the cell wall, that is much more rigid than the cell membrane. As water enters a plant cell it collects in fluid-containing organelles, or vacuoles. Pressure inside the cell increases as vacuoles fill with water, lending rigidity to the cell wall and the entire plant. Plants that do not receive enough water wilt because there is not enough pressure in their cells to support the structure of the plant. Conversely, cells that fill with too much water begin to burst, causing brown spots on plant leaves. Osmosis is the control by which cells maintain just the right amount of water. If the concentration of water molecules is higher outside of the cells, water flows into the plant's cells. But, if the concentration is higher inside the cells, excess water flows out. Plants absorb water from the soil through osmosis in their root cells. When water concentrations around a plant's roots are higher than inside the plant, water flows into the roots and then, through continuing osmosis, up the stem into the rest of the plant. During photosynthesis osmotic pressure in the cells of plant leaves causes openings, or stomata, in the leaves to open. Once open the stomata begin to absorb the carbon dioxide that is vital for photosynthesis to occur.

Bet You Didn’t Know the Importance of Osmosis in Living Organisms

In living organisms, nutrients derived from food reach various organs by way of osmosis, carried by fluids. It's the process of flow of liquid through a semi-permeable membrane.

In living organisms, nutrients derived from food reach various organs by way of osmosis, carried by fluids. It’s the process of flow of liquid through a semi-permeable membrane.

Solids and liquids share a simple, yet complex relationship. Simple, because often, solids yield to the solvent power of liquids and both get bound to each other in the form of a solution. Complex, because a lot of times, liquids find it challenging to breach the surface of solids and move ahead. The phenomenon by which a liquid moves towards its destination by breaching a partially permeable or semi-solid surface without the use of any force is known as Osmosis.

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In living organisms, osmosis mostly takes place in the form of a nutrient-laden liquid permeating the cell membrane to move towards the inner cellular anatomy where the solute concentration is higher owing to the presence of various organelles that carry out a number of essential functions necessary for the sustenance of life. By enabling nutrients to reach the cellular interiors and aiding in their absorption, the process of osmosis makes sure that cells get repaired and stay healthy.

Significance of Osmosis in Living Organisms

Besides carrying nutrient-rich liquid across cell membranes, a very important function of osmosis is to maintain an ideal concentration of solute within the cell. Osmosis balances the pressure and concentration of solution on both sides of a semi-permeable membrane by making water molecules move from the high-concentration side to the low concentration side by passing through the wall of semi-absorbent material that lies between both sides. The following points enumerate the comprehensive role osmosis plays in the biology of living organisms:-

– Through the process of osmosis, nutrients get transported to cells and waste materials get moved out of them.
– The pressure within and outside each cell is maintained by osmosis as this process ensures a balance of fluid volume on both sides of the cell wall. If fluid volume within a cell is more than the fluid volume outside it, such pressure could lead the cell to become turgid and explode. On the contrary, if fluid volume outside the cell is more than the fluid volume within, such pressure could lead the cell to cave in. Both cases would be detrimental to normal and healthy cellular function.
– It is via osmosis only that roots of plants are able to absorb moisture from the soil and transport it upwards, towards the leaves where nutrition is created via photosynthesis. Plants wouldn’t exist without osmosis and without plants, no other life could exist as they are a vital link of the entire food chain of the planet.
– Without osmosis, it would be impossible for our bodies to separate and expel toxic wastes and keep the bloodstream free from impurities. The process of blood purification is carried out by the kidneys which isolates the impurities in the form of urine, which is, then, carried to the bladder from where they are, eventually, sent ahead for evacuation from the body.

The role of osmosis is twofold – it helps maintain a stable internal environment in a living organism by keeping the pressure of the inter and intra-cellular fluids balanced, and it allows the absorption of nutrients and expulsion of waste from various bodily organs on the cellular level.

These are two of the most essential functions that a living organism cannot do without, as a steady internal pressure is essential to counter the atmospheric pressure from the outside, and the transfer of nutrients and cellular waste by using the least number of energy units (the process of osmosis does not involve the exertion of any kind of force) ensures that any energy produced or processed by the body is directed towards the performance of more complex biological functions.

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