Passage of molecules into and out of cells

Cell membrane is a semi permeable membrane act as a barrier to most molecules. Cell membrane do it through the following phenomena:

  • Facilitated diffusion.
  • Active transport.

Diffusion: Diffusion is the movement of molecules from an area of higher concentration to the area of lower concentration i.e. along concentration gradient.

Diffusion is a type of passive transport. Passive transport is movement of molecules from a region of its higher concentration to a region of its lower concentration without expenditure of energy.

Through diffusion carbon dioxide, oxygen, glucose transported through diffusion.


2) Facilitated diffusion:- The movement of molecules from a region of higher of higher to a region of lower concentration with the help of transport proteins is called Facilitated diffusion.


The rate of facilitated diffusion is higher than simple diffusion. Many molecules do not diffuse freely across cell membranes because of their size or charge. Such molecules are taken into or out of the cells with the help of transport proteins present in cell membranes.



Osmosis is the movement of water across a semi-permeable membrane from a solution of lesser solute concentration to a solution of higher solute concentration.

The rules of osmosis can be best understood through the concept of tonicity of solutions. The term tonicity refers to the relative concentration of solutes in the solutions being.

The rules of osmosis can be best understood through the concept of tonicity of solutions. The term tonicity refers to the relative concentration of solutes in the solutions being compared.

  • A Hypotonic solution has relatively less solute.
  • Isotonic solutions have equal concentrations of solutes.
  • A Hypertonic solution has relatively more solute.


Water balance problems or effect of different solutions on plant cell and animal cell:- In a solution with higher water potential than the plant cell (hypotonic solution) water enter the cell by osmosis and the cell become turgid but the animal cell a solution of equal water potential (isotonic solution) the cell remains normal. In a solution with lower water potential (hypertonic solution) , water leaves the cell by osmosis and the living part of the cell, protoplast, shrinks.


Turgor Pressure:-Pressure exerted by fluid in a cell that presses the cell membrane against the cell wall. Turgor is what makes living plant tissue rigid. Loss of turgor, resulting from the loss of water from plant cells, causes flowers and leaves to wilt. Turgor plays a key role in the opening and closing of stomata. This pressure, turgidity, is caused by the osmotic flow of water from an area of low solute concentration outside of the cell into the cell’s vacuole, which has a higher solute concentration. Healthy plant cells are turgid and plants rely on turgidity to maintain rigidity. In contrast, this phenomenon is not observed in animal cells which have no cell walls to prevent them from being burst by the flow of water into the cell and must either continually pump out water, with a contractile vacuole, or live in  an isotonic solution where there is no osmotic pressure

Importance of turgor:-1) maintain shape of plant cell. 2) Helps in opening and closing of stomata. 3) Opening and closing of flowers and leaves.


Plasmolysis: The loss of water by osmosis from a plant cell to the extent that the cytoplasm shrinks away from the cell wall. This happens when the cell is placed in a solution that has a higher solute concentration than that of the cell sap, i.e. it has a lower (more negative) water potential, since water always moves from an area of high water potential to an area of low water potential.

Deplasmolysis (Flaccidity):The entrance of water into a plasmolysed  plant cell, causing the cell membrane to return to the cell wall.

Osmosis and Guard cells:

Stomata (openings) in leaf epidermis are surrounded by guard cells. During daytime guards cells are making glucose and so are hypertonic (have higher concentration of glucose) than their nearby epidermis cells. Water enters them from other cells and they swell. In this form, they assume a rigid bowed shape and a pore is created between them. At night when there is low solute concentration in guard cells, water leaves them and they become flaccid. In this form, both guard cells rest against one another and the opening is closed.

Application of knowledge about semi-permeable membranes:

The knowledge about semi-permeable membranes is applied for various purposes. We know that semi-permeable membrane is capable of separating substances. Artificially synthesized semi-permeable membranes are used for the separation of bacteria from viruses, because bacteria cannot cross a semi-permeable membrane. In advanced water-treatment technologies, membrane-based filtration systems are used. In this process, semi-permeable membranes separate salts from water (reverse osmosis).


Filtration is a process by which small molecules are forced to move across semipermeable membrane with the aid of hydrostatic (water) pressure or blood pressure. In animals molecules are transported by blood and in plants by water. In this process only small molecules transported.


Active transport:-Active transport is the movement of molecules from an area of lower concentration to the area of higher concentration. This movement against the concentration gradient requires energy in the form of ATP.

In this process, carrier proteins of cell membrane use energy to move the molecules against the concentration gradient. For example, the membranes of nerve cells have carrier proteins in the form of “sodium-potassium pump”. In a resting (not conducting nerve impulse) nerve cell, this pump spends energy (ATP) to maintain higher concentrations of K+ and lower concentrations of Na+ inside the cell. For this purpose, the pump actively moves Na+ to the outside of the cell where they are already in higher concentration. Similarly this pumps moves K+ from outside to inside the cell where they are in higher concentration



It is the process of cellular ingestion of bulky materials by the infolding of cell membrane.

The two forms of endocytosis are phagocytosis (cellular eating) and pinocytosis (cellular drinking). In phagocytosis cell takes in solid material while in pinocytosis cell takes in liquid in the form of droplets.



It is the process through which bulky material is exported .. This process adds new membrane which replaces the part of cell membrane lost during endocytosis.

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