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.

water is a polar molecule. a chemical bond is a force of attraction between atoms or ions. bonds form when atoms share or transfer valence electrons. atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons. so, false

explanation:

in h2o molecule, two water molecules are bonded by a hydrogen bond but the bond between two h - o bonds within a water molecule are covalent. the greater the electronegativity difference, the more ionic the bond is. bonds that are partly ionic are called polar covalent bonds. nonpolar covalent bonds, with equal sharing of the bond electrons, arise when the electronegativities of the two atoms are equal.