The uptake of l-ascorbate (vitamin C) and its oxidized form, dehydro-l-ascorbic acid (DHAA), was evaluated in brush border membrane vesicles isolated from adult human small intestine. Ascorbate uptake was Na+-dependent and potential-sensitive (Km, 200 umol/L), whereas DHAA transport occurred through Na+-independent facilitated diffusion (Km, 800 µmol/L). If the Vmax of vitamin C import through channels is 401µmol/min/cm², what import rate would |you expect if the lumen contained 133 µmol/L of the version of vitamin C that moves through them?

Human Anatomy & Physiology (11th Edition)
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Author:Elaine N. Marieb, Katja N. Hoehn
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Chapter1: The Human Body: An Orientation
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The uptake of l-ascorbate (vitamin C) and its oxidized form, dehydro-l-ascorbic acid
(DHAA), was evaluated in brush border membrane vesicles isolated from adult human
small intestine. Ascorbate uptake was Na+-dependent and potential-sensitive (Km, 200
umol/L), whereas DHAA transport occurred through Na+-independent facilitated
diffusion (Km, 800 µmol/L).
If the Vmax of vitamin C import through channels is 401µmol/min/cm², what import rate would you
expect if the lumen contained 133 µmol/L of the version of vitamin C that moves through them?
Transcribed Image Text:The uptake of l-ascorbate (vitamin C) and its oxidized form, dehydro-l-ascorbic acid (DHAA), was evaluated in brush border membrane vesicles isolated from adult human small intestine. Ascorbate uptake was Na+-dependent and potential-sensitive (Km, 200 umol/L), whereas DHAA transport occurred through Na+-independent facilitated diffusion (Km, 800 µmol/L). If the Vmax of vitamin C import through channels is 401µmol/min/cm², what import rate would you expect if the lumen contained 133 µmol/L of the version of vitamin C that moves through them?
Expert Solution
Step 1: Background

Cell membrane or plasma membrane is a phospholipid bilayer and is selectively permeable. Small, nonpolar solutes freely pass across the membrane, down their concentration gradient. The hydrophobic phospholipid bilayer prevents the passage of large, polar and charged solutes across it. Passage of these large and polar solutes is facilitated by special proteins. 

The mechanisms by which solutes cross the membrane are called membrane transport mechanisms. These mechanisms can be divided into two major categories - passive transport and active transport. Passive transport is transport of solutes without any energy expenditure. Active transport is energy-dependent transport of solutes.

Passive transport can be through simple diffusion or facilitated diffusion. Simple diffusion is unassisted movement of solutes across the membrane down their concentration gradient. No protein is needed for simple diffusion; solutes just diffuse through the phospholipid bilayer. Facilitated diffusion is movement of a solute across the membrane down its concentration gradient, with the help of a carrier or channel protein. Solutes that cannot cross the hydrophobic phospholipid bilayer have to move down their concentration gradient across the membrane with the help of carrier or channel proteins.

During active transport, solutes are moved against their concentration gradient. This requires expenditure of energy. The energy is generally provided by ATP hydrolysis. Active transport across the membrane usually makes use of carrier proteins.

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