Test: Diffusion & Active Transport

• Uphill transport is a process in which diffusion of a component occurs from a less concentrated stream to a more concentrated permeable stream.
• Facilitated transport is a form of passive transport in which materials are moved across the plasma membrane by a transport protein down their concentration gradient.
• It requires integral membrane proteins and highly selective biological membranes to cross.
• Saturation occurs in facilitated, diffusion because not enough carriers may be available to handle all the free solute molecule the rate of movement may reach a maximum. Thus, uphill transport does not pertain to facilitated transport.

• Polysaccharides and disaccharides must be digested to monosaccharides prior to absorption and the key players in these processes are the brush border hydrolases, which include maltase, lactase and sucrase. Lactase cleaves lactose into a glucose and a galactose
• Glucose and galactose are taken into the enterocyte by cotransport with sodium using the same transporter. Fructose enters the cell from the intestinal lumen via facilitated diffusion through another transporter. Similarly after digestion of nucleic acids the nucleotides are absorbed in the small intestine by facilitated diffusion.

So the correct option is 'Absorption of fructose and nucleotides in the small intestine'.

Active absorption is the only absorption method that can move species against their concentration gradient (from low to high concentration). Facilitated diffusion only moves species down their concentration gradient from high to low concentration.

Transport systems that use carrier molecules but which do not require energy to proceed are called facilitated diffusion.

• Facilitated transport does not require ATP energy.
• In facilitated transport, transport saturates.
• It does not require Uphill Transport.
• A chemical first binds to the carrier protein in the cell membrane and then diffuses through the membrane.
• Because no energy is used, facilitated transport into the cell cannot proceed, if the concentration of that chemical is greater inside the cell membrane than outside.
• The involvement of carriers means that the process is also subject to competitive inhibition and saturation.

Hence, the correct answer is option E: (II), (III) and (IV) are relevant, but (I) and (V) are irrelevant.

• Turgor pressure is the positive hydrostatic pressure applied by protoplasm on the cell wall when protoplasm swells due to an increase in volume caused due to endosmosis. 
• When the protoplasm applies turgor pressure on the wall, the wall applies an equal and opposite wall pressure on to the protoplasm. 
• Diffusion is a slow physical process, which describes the movement of substances along the concentration gradient. It occurs in all three states of matter and its rate does not change with the physical state of the substance. 
• Imbibition is the physical surface adsorption of solvent onto the matrix or adsorbing surface. 
• The (V) statement is wrong because osmosis is the movement of water not substances.