The main function of a cell membrane is to allow substances to enter and leave the cell. Which is a correct description of transport through a cell membrane

The question is incomplete, below is the completed question:

The main function of a cell membrane is to allow substances to enter and leave the cell. Which is a correct description of transport through a cell membrane ?

a. Diffusion, or passive transport, occurs when molecules diffuse from a lower concentration to a higher concentration.

b. Facilitated transport occurs when proteins that extend through the cell membrane provide a channel through which certain substances can diffuse more rapidly than by passive diffusion.

c. The lipid bilayer opens up and allows the ions or molecules to enter the cell, then closes.

d. Active transport occurs when molecules diffuse from a higher concentration to a lower concentration.

b. Facilitated transport occurs when proteins that extend through the cell membrane provide a channel through which certain substances can diffuse more rapidly than by passive diffusion.

Explanation:

Transport through cell membranes can be broadly classified as active transport or passive transport.

Active transport requires energy in the form of ATP for the transportation of substances across the cell membrane.

Option b correctly describes facilitated diffusion, which is a passive-mediated diffusion process, where channels in the cell membrane mainly composed of proteins called transmembrane integral proteins, allow for the movement of certain molecules (mostly polar/hydrophilic) molecules that will not otherwise be transported through simple diffusion because of the hydrophobic nature of the cell membrane.

The other options are wrong because:

a. diffusion or simple transport involves the movement of substances from regions of higher to a lower concentration not lower to higher as suggested here.

c. The lipid bi-layer does not allow ions or hydrophilic molecules because it is hydrophobic in nature, and it is not a channel, so it does not open and closes

d. In active transport, transfer of molecules is from regions of lower to higher concentration and not higher to lower concentration as suggested here.

he forelimbs below belong to four species that descended in a direct line from the ancestral mammal shown above. of the four descendants shown, which species most likely appeared earliest in evolutionary history?