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Biology

Can someone please help me with this question?how do the following substance pass through plasma membrane.
steroids,
vitamin A or D
Large protein
Water
Ions

Reply 1

macpatgh-Sheldon

vitamin A or D: these are 2 of the fat-soluble vitamins (the others being E and K), so they will be hydrophobic (lipophilic) and will easily pass through the lipoprotein nature of the cell membrane by diffusion.

Large protein: Some very large proteins will not pass through the cell membrane AT ALL!. However, the fact that some people are allergic to certain proteins tells us that these proteins are capable of passing through the epithelial cells of the villi in the small intestine (as intact proteins) - this process involves special movement across membranes by active transport via specialized channels; this is usually an abnormal phenomenon e.g. the passage of protein in the urine (=protenuria) is not good news!

Water: Osmosis

Ions: Some by simple diffusion; others by the presence of an electrochemical gradient; still others by active transport e.g. Na+ reabsorption in the proximal tubule of the kidney using Na+, K+ - ATPase (these cells have many mitochondria (why??) and have microvilli (again, why??) OR by co-transport e.g. glucose transporter (GLUT1) can also carry some ions jointly with the glucose carried across.

Hope this helps!
M (specialist biology tutor)