Osmosis and diffusion

  MEANS OF TRANSPORT

Diffusion

• Passive movement and may be from one part of the cell to the other, or from cell to cell, or over short 

distances, say, from the intercellular spaces of the leaf to the outside. 

• No energy expenditure takes place. 

• Molecules move in a random fashion, the net result being substances moving from regions of higher 

concentration to regions of lower concentration. 

• Diffusion is a slow process and is not dependent on a 'living system'. 

• Diffusion is obvious in gases and liquids, but diffusion in solids rather than of solids is more likely. 

• Diffusion is very important to plants since it the only means for gaseous movement within the plant body. 

• Diffusion rates are affected by the gradient of concentration, the permeability of the membrane separating them, size of the substances, temperature and pressure.

Facilitated Diffusion : 

• The diffusion of any substance across a membrane also depends on its solubility in lipids, the major 

constituent of the membrane. Substances soluble in lipids diffuse through the membrane faster. 

Substances that have a hydrophilic moiety, find it difficult to pass through the membrane; their 

movement has to be facilitated. Membrane proteins provide sites at which such molecules cross the membrane. 

• They do not set up a concentration gradient: a concentration gradient must already be present for molecules to diffuse even if facilitated by the proteins. This process is called facilitated diffusion. 

• In facilitated diffusion special proteins help move substances across membranes without expenditure of ATP energy. 

• Facilitated diffusion cannot cause net transport of molecules from a low to a high concentration - this would require input of energy. 

• Transport rate reaches a maximum when all of the protein transporters are being used (saturation). 

• Facilitated diffusion is very specific: it allows cell to select substances for uptake. 

• It is sensitive to inhibitors which react with protein side chains. 

• The proteins form channels in the membrane for molecules to pass through. Some channels are 

always open; others can be controlled. Some are large, allowing a variety of molecules to cross. 

• The porins are proteins that form huge pores in the outer membranes of the plastids, mitochondria 

and some bacteria allowing molecules up to the size of small proteins to pass through. 

• When an extracellular molecule bound to the transport protein; the transport protein then rotates and 

releases the molecule inside the cell, e.g., water channels (made up of eight different types of aquaporins.)

Passive symports and antiports 

• Some carrier or transport proteins allow diffusion only if two types of molecules move together. 

• In a symport, both molecules cross the membrane in the same direction; in an antiport, they move inopposite directions. 

• When a molecule moves across a membrane independent of other molecules, the process is called uniport.

Osmosis 

• In plant cells, the cell membrane and the membrane of the vacuole, the tonoplast together are 

important determinants of movement of molecules in or out of the cell. 

• Osmosis is the term used to refer specifically to the diffusion of water across a differentially or semi-

permeable membrane. Osmosis occurs spontaneously in response to a driving force. 

• The net direction and rate of osmosis depends on both the pressure gradient and concentration gradient. 

• Water will move from its region of higher chemical potential (or concentration) to its region of lower 

chemical potential until equilibrium is reached. At equilibrium the two chambers should have the same water potential.

pressure required to prevent water from diffusing is the osmotic pressure and this is the function 

of the solute concentration; more the solute concentration, greater will be the pressure required to prevent water from diffusing in. 

Numerically osmotic pressure is equivalent to the osmotic potential, but the sign is opposite. Osmotic 

pressure is the positive pressure applied, while osmotic potential is negative.