Subtopic 5

Isotonic, Hypertonic and Hypotonic

Hypertonic solutions

Hypertonic solutions are solutions where the water potential of the solution outside the cell is lower than the water potential of the solution inside the cell. There is a concentration gradient (difference in concentration) which causes water to move from an area of high water potential, which is in the cell, to an area of high water potential, which is outside the cell, down a concentration gradient. 

The net movement of water out of the cell could result in 2 things happening, depending on the type of cell from which water were moving out from. If water moves out of an animal cell, the cell shrivels and shrinks (animal cell shrinking due to water movement out of the cell is called crenation). 

If this took place in a plant cell, the plant cell would not shrink but the cell membrane and the cytoplasm move away from the cell wall; this process of moving away from the cell wall is called plasmolysis. It causes the cell to be flaccid (limp) and the plant cell to wilt.

Hypotonic solutions

Hypotonic solutions are solutions where the water potential (concentration) of the solution inside the cell is lower than the water potential outside the cell. Because water moves from an area of high water potential to an area of low water potential, down a concentration gradient, water moves into the cell from an area of high water potential outside the cell to an area of low water potential inside the cell. The net (overall) movement is into the cell.

In animal cells, the large movement of water into the cell causes the cell to swell up and burst (lysis). In plant cells, the movement of water into plant cells causes the plant cell to swell up (become turgid) and the cell membrane and cytoplasm to push against the cell wall. The plant cell does not burst because the rigid cell wall surrounds the cell, resisting pressure and preventing the cell from bursting.

Think of ‘hypo’ and ‘low’ to remember it if you’re confuse d between hypotonic and hypertonic solutions. Hypotonic solutions have a lower water potential.

Isotonic solutions

Isotonic solutions are solutions where the water potential of the solution inside the cell is the same as the water potential outside the cell. Because the water potential inside and outside of the cell is the same, there is no change in the concentration gradient meaning there is there is no net (overall) movement of particles either into or out of a cell. Water diffuses into and out of the cell by osmosis but there is still no net (overall) movement of particles into and out of the cell.

Subtopic 4

Active Transport

Active Transport Active transport is the energy-demanding transfer of a substance across a cell membrane against its concentration gradient, i.e., from lower concentration to higher concentration. Special proteins within the cell membrane act as specific protein ‘carriers’. The energy for active transport comes from ATP generated by respiration (in mitochondria).

Major examples of Active Transport

1. Re-absorption of glucose, amino acids and salts by the proximal convoluted tubule of the nephron in the kidney. 

2. Sodium/potassium pump in cell membranes (especially nerve cells).

Endocytosis / Exocytosis


Pinocytosis (‘cell drinking’) This is the uptake of large molecules (DNA, protein) from solution, by a form of endocytosis – the vesicles formed are minute and short-lived. 

Phagocytosis (‘cell eating’) This is the uptake of solid particles by a cell e.g. Amoeba feeding, phagocytes engulfing bacteria

Subtopic 3

Osmosis

Osmosis is the movement of water through a semipermeable membrane according to the concentration gradient of water across the membrane, which is inversely proportional to the concentration of solutes. Semipermeable membranes, also termed selectively permeable membranes or partially permeable membranes, allow certain molecules or ions to pass through by diffusion.

While diffusion transports materials across membranes and within cells, osmosis transports only water across a membrane. The semipermeable membrane limits the diffusion of solutes in the water. Not surprisingly, the aquaporin proteins that facilitate water movement play a large role in osmosis, most prominently in red blood cells and the membranes of kidney tubules.

Mechanism of Osmosis

Osmosis is a special case of diffusion. Water, like other substances, moves from an area of high concentration to one of low concentration. An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane (otherwise the concentrations on each side would be balanced by the solute crossing the membrane). If the volume of the solution on both sides of the membrane is the same but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane. If there is more solute in one area, then there is less water; if there is less solute in one area, then there must be more water.

To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.

Osmosis: In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can.

Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of passing through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system. Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up.

For futher understanding please do watch this video

https://m.youtube.com/watch?v=KmQyVWtxeqM

Subtopic 2

Facilitated Diffusion

1.The movement of hydrophilic molecules or ions across the plasma membrane with the help of transport proteins.

2.Substances that are not soluble in lipids do not pass readily through the phospholipid bilayer.

3.Substances such as glucose, amino acids, proteins and nucleic acids pass through the membrane by facilitated diffusion.

Facilitated diffusion involves 2 types of protein

1. Channel proteins

Provide functional pore in the membrane for the diffusion of ions. The pore are selective about which ions can pass through.

2. Carrier proteins

Pick up the diffusion molecules on one side of the membrane and release them on the other side.

- Absorption of certain nutrients through the villi of the small intestine.

- Carrier proteins found on the membrane of cells that line the intestinal wall transport small    molecules such as amino acids and glucose into the blood capillaries of the villi

Subtopic 1

What is plasma membrane?
Many biochemical reactions take place in a cell.The cells require many substances to carry out these biochemical reaction.'Waste products that are formed during biochemical reactions within the cells must be eliminated because they are poisonous.      

 The structure of the plasma membrane

The plasma membrane has 3 components

1. Double layer of phospholipids molecules.

2. Pore

3. Carrier Proteins

.The phospholipids bilayer, proteins and other parts are not rigid or static, but form a dynamic and flexible structure.

•The protein molecules float about in the phospholipid bilayer to form a mosaic pattern that is always changing like fluid.

•Singer and Nicolson call it ‘fluid-mosaic model’

Movement across plasma membrane