OSMOSIS AND ACTIVE TRANSPORT Summarized Notes

This Should Take You Abaut 2 Hours To Learn, If It Takes Longer Than That You Should Not Worry As We All Learn At Different Pace

OSMOSIS

in the previous post we covered, three examples of diffusion were dealt with:

• putting potassium permanganate crystals in distilled water showed us how molecules diffuse through water

• putting a tea bag in warm water showed us how molecules diffuse through water

• learning about ammonia showed us that molecules can diffuse through air.

Living organisms are made of cells. In order for a cell to survive and grow, materials or molecules need to be transported in and out of the cell. In our previous discussion, we read that every cell has an outer cell membrane, which is partially permeable (a membrane that will allow small molecules to pass through but not larger molecules). How do you think this affects diffusion? To investigate the diffusion of atoms and molecules, we use a manufactured membrane called Visking tubing. This looks like transparent sticky-tape, but is really a narrow tube. Visking tubing is like the cell membrane. It allows atoms and small molecules like water, oxygen and carbon dioxide to pass through the tiny openings in the tubing, but prevents larger molecules like sugar molecules and protein molecules from passing through. The cells of all organisms are composed of 80% water, so your body is 80% water. It is vital that water should be able to pass into and out of cells easily. The diffusion of water molecules through a partially permeable membrane is called osmosis. Osmosis has to take place through a partially permeable membrane. This means that the water molecules move from a higher water potential toa lower water potential, through a partially permeable membrane, down a water potential gradient. In other Words, osmosis is a special form of diffusion, which happens in all living cells. The following figure shows two solutions, one dilute (has lots of water molecules and a higher water potential) and one concentrated (has few water molecules and  lower potential) , separated by a partially permeable membrane. A concentration gradient thus exists between the two solutions. The solution on the left is diluted, while the solution on the right is concentrated. The figure below shows a sugar solution separated by a partially permeable membrane 

The water molecules will move from the left-hand side solution where they are very Concentrated to the right-hand side solution where they are of a very low concentauon and so osmosis takes place. If you place an animal cell in distilled water, osmosis will result in the water molecuies moving from the distilled water where they are very concentrated, to the cell where they are of low concentration through the cell surface membrane. The size of the cell increases; we say the cell becomes more turgid. As the volume of the water inside the cell increases, it causes the cell membrane to stretch and the cell to become more turgid. As more water molecules enter the cell, the cell will eventually burst and die.

The partially permeable membrane has holes or pores in it that are very small, allowing the water molecules to pass through, but not the sugar molecules. There is also a concentration gradient for the water molecules, as shown in the above figure. On the left-hand side of the partially permeable membrane, there is a high concentration of water molecules (higher water potential). On the right-hand side of the partially permeable membrane, the concentration of water molecules is lower (lower water potential), because much of the space is taken up by sugar molecules. The water molecules therefore move by diffusion from the left-hand side into the right-hand side.They can do this because the pores in the membrane are large enough for them to pass through.

• The symbol w is used to indicate water potential.

• Pure water has the highest water potential.

Concentration Gradient For Water Molecules 

What is the result of this? Water has diffused from

the pure water (higher water potential), through the partially permeable membrane, into the sugar solution (lower water potential ). The sugar solution will become more dilute because of the extra water molecules coming into it. This process is called osmosis. Osmosis can be defined as the passage of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane, down a water potential gradient. The process of osmosis is usually demonstrated using the apparatus shown below

The beaker contains pure water. The Visking tubing contains a concentrated sugar solution. The glass tubing can be marked to show how the level of the sugar solution rises. Water diffuses from the pure water (higher ) into the sugar solution (lower ), down its Concentration gradient. Sugar molecules cannot diffuse out because the pores in the Visking tubiny are too small for them to do this. Therefore, the level of the sugar solution rises as it is diluted by the water ditfusing into it by osmosis.

Three Types Of Environment Where Osmosis Accurs

Hypotonic Solution

The solution has more free water molecules than the cell, so water moves into the cell causing plant cells to swell and become turgid, and animal cells to swell, burst and become ysed

In Animal Cell

In Plant Cell

Isotonic Solution

The water potential is equal inside and outside the cell, water moves acrosS the membrane in both directions maintaining cell size

In Animal Cell 

In Plant Cell

Hypertonic Solution 

The solution has a higher Solute Concentration than the cell, so water moves out of the cell and into the solution causing the cell to become flaccid and plasmolyse

In Animal Cell

In Plant Cell

The Effects Of Osmosis On Plant And Animal Tissues

The roots of plants are surrounded by soil particles. Each soil particle has a film of water around it. When plants take up water through their roots, the water molecules are moving from where they are concentrated, in the soil, to where they are less concentrated, inside the root. The cell membranes of root hair cells are similar to Visking tubing, because they let some substances through but not others. The cell membranes are described as partially permeable.

They are usually separated by two different solutions:

• the cytoplasm inside the cell

• the solution outside the cell

If the two solutions are of different concentrations, then osmosis will occur.

Water Potential And Osmosis

When moving molecules hit against a membrane, they push against it and exert a pressure. When the molecules are more concentrated there are more of them and they exert greater pressure. Water flows from a higher water potential to a lower water potential we call the pressure that water molecules exert, water potential. 

Look at following figure below

on the left side, the dilute sugar solution, there are more water molecules. This means that the dilute sugar solution hasa higher water potential than the concentrated sugar solution on the right side. The net movement of water molecules is thus from the left to the right, from the higher water potential to the lower water potential.

Osmosis And Animal Cells

Animal cells burst if placed in pure water. The cytoplasm inside the cell is a fairly concentrated solution. The proteins and many other substances dissolved in it are too large to get through the cell membrane. Water molecules, though, can get through. Water molecules will diffuse from the pure water into the cell, down the concentration gradient. As more and more water enters the cell by osmosis, it swells and stretches the cell membrane eventually the strain becomes too much, and the cell bursts and become lysed, down is an example figure of this

Animal cells shrink if placed in concentrated solutions, such as seawater. If the solution is more concentrated than the cytoplasm, the water molecules will diffuse out of the cell, down the concentration gradient. As the water molecules pass through the cell membrane, the cytoplasm shrinks and the cell shrivels up, an example of this is shown in the figure below

When animal cells are in isotonic solution, movement of water out of the cell is exactly balanced by movement of water into the cell. Animal cells remain normal if placed in the same solutions as itself. There is no movement of water molecules, because the concentration is the same, an example is be seen below 

Osmosis And Plant Cells

When plant cells are placed in a hypotonic solution, or a dilute solution, they do not burst. A plant cell in pure water will take in water by osmosis through the partially permeable membrane of the cell. As the water enters, the vacuole swells and pushes the cytoplasm and cell membrane up against the cell wall. The cell is described as turgid. This can be seen in the following figure below

it is only the presence oft the strong cell wall that prevents the cell from bursting. The cell within the cell wall is like a blown up tyre, once again water molecules move from a region of higher water potential to lower water potential, through a partially permeable cell membrane. All plant cells are usually turgid. This helps to support the stem, leaves and flowers, and keep them firm. The leaves are held out flat and can obtain maximum sunlight for photosynthesis. Turgor is very important in plants that are not supported by wood. Turgor pressure pushes the plasma membrane against the cell wall of plant, bacterial and fungal cells, as well as those protist cells that have cell walls. When a plant cell is placed in a hypertonic solution, a concentrated solution, such as seawater or concentrated sugar solution, the reverse can be observed. Here the water potential within the plant cell is greater than the outside solution. Water will pass out of the cell by osmosis, and the cytoplasm and vacuole pulls away from the cell wall. This can be seen in the figure below 

Cells in this state are said to be flaccid. The entire plant will become limp and the plant will eventually die. The same thing happens when a plant does not get enough water. Cells that have lost water are said to be plasmolysed Plasmolysis is the condition vwhen the cell membrane is pulled away from the cell wall. It is not usual for cells to get to this extreme situation, the figure below is an example of a plasmolysed plant cell

ACTIVE TRANSPORT 

So far in this unit, we have seen how molecules and atoms move into and out of cells by the processes of diffusion and osmosis. In both cases, atoms and molecu les move through cell membranes in response to their concentration gradients. This is a passive process, because it requires no energy input. You may have been wondering whether it is possible, in cells, for solutes to move through a membrane against a concentration gradient. Can cells take in solutes that are already concentrated outside their cells? The answer is yes. There is another process in Biology that is used to transport ions, atoms and molecules against a concentration gradient. This process is called active transport. Active transport is almost the exact opposite process to osmosis and diffusion. It is defined as the passage of atoms and molecules from a region of lower concentration to a region of higher concentration, against the concentration gradient. This process is energy-consuming. The energy for active transport is obtained from respiration in the cell, particularly the hydrolysis of ATP in the cells. This energy-Consuming process is Vitally important in the uptake of mineral ions, particularly nitrate ions, by root hair cells and the uptake of glucose by epithelial cells in the villi of the small intestine. Further details of these processes are explained to you in other sections that will follow later.

The Importance Of Active Transport In Plants And Animals

So far we have talked about what active transport is and how it happens. Now we are going to look at why this process is important in plants and animals. Plants and animals cannot always depend on free movement of molecules of substances by diffusion and osmosis trom their area of higher Concentration to their area of lower concentration. Sometimes, substances need to be taken into the cells against their Concentration gradient by moving from their area of lower concentration to their area of higher concentration.

In plants, active transport takes place during the uptake of minerals, such as potassium, magnesium and nitrate ions, from the soil. Plants need to take up these minerals, even though their concentration is already higher in the roots than in the soil. lf diffusion was the only process, minerals would diffuse out of the plant back into the soil. Active transport prevents this from happening. Active transport is enabled in plants as the cells of the root hairs contain special proteins called carrier proteins. These proteins can hold the minerals that are needed and take them into the cell. As it was previously discussed, this process requires energy from cell respiration, hence the term 'active'.

Mineral Uptake Through Active Transport 

In animals, glucose is taken up in the same way by the epithelial cells in the villi of the small intestine. Carrier proteins in the epithelial cells of the villi take up glucose present in the contents of the small intestine and allow it to enter the bloodstream during absorption. The process repeats itself when glucose is taken up from the bloodstream into the liver, where it is converted into glycogen for storage.

Structure of A Villi

The Villi Of The Small Intestine And Kidney Tubules Taking Up Glucose By Active Transport 

Membrane Carrier Proteins

Active transport occurs across membranes as a result of the activity of special protein molecules in the membranes called carrier molecules. These are protein molecules that form part of the structure or the membrane. In the presence of energy, they attach to atoms, ions and molecules on the side of the membrane where they are at a low concentration, and are pumped or "flipped" across the membrane to the side with the high concentration. 

These protein molecules act as carriers of solutes, such as: 

• Small ions, for example, sodium, potassium and nitrate

• Molecules, for example, glucose and amino acids.

The molecules of carrier proteins extend right through the phospholipid bilayer. They are specific, with one kind of carrier protein normally transporting only one kind of solute and only in one direction. The carrier proteins are called protein pumps because they are pumps made of protein, not because they transport proteins. The Five steps below shows how scientists think active transport happens through cell surface membranes.

Step 1

One one surface of the membrane, a protein carrier molecule recognizes it's specific solute

Step 2

The protein carrier binds with the solute

Step 3

ATP then attaches itself to the carrier molecule 

Step 4

The carrier molecule is activated to change shape and it moves with the solute through to the other surface of the membrane where it releases the solute

Step 5

The protein carrier molecule then goes back to it's original shape

You can see now why movement against a concentration gradient is called active transport, the cell surface membrane is actively involved, through its protein carrier molecules, in transporting solutes from one surface to the other surface.

The End, See You In The Next Post, Posted by Mr DeHaan Ahil.