Question

How can you determine whether a molecule will be able to cross the membrane by passive transport?

• Which 2 of the following ions (Na⁺, K⁺, Ca⁺²) are more concentrated outside of a cell than inside a cell?• What words are associated with active transport, and what is active transport used for?
• Is the entry of Na⁺ into a cell during an action potential through voltage-gated channels active or passive transport? What about K⁺ leaving the cells through voltage-gated channels?

Answer 1

Biological membranes are made up of phospholipid bilayer. So, for molecules to passively diffuse across the cell membrane, the minimum reqiurement is, they should be lipophilic (lipid loving). Example is steroid hormones. On the other hand ions like Na⁺, K⁺ are polar highly hydrophilic ( that is they are water loving and not lipid loving). Hence they cannot passively diffuse. In addition gases, they can easily diffuse across the cell membrane.

Among the three ions, both Na⁺ and Ca²⁺ concentrations are always higher outside the cell than inside. But in case of pottasium ions, their concentration is higher inside the cell.

Transport across a membrane can be either passive or active. Active transport refers to the movement of molecules from a region of lower concentration to a region of higher concentration (that is against the concentration gradient). So, inorder to pump molecules against the concentration gradient, always energy is required.

The significance of active transport is that, sometimes the concentration of some molecules will be higher inside the cell and lower outside. But, as these molecules are highly essential to the body , they should be absorbed maximum in to the cell. In such cases, molecules has to be pumped from outside of the cell to inside where their concentrations are already higher. This can be used to accumulate essential molecules like glucose, amino acids etc.

The entry of Na⁺ into a cell during an action potential through voltage gated cahnnel is passive transport. It is because, here, sodium ions enter into the cell through a channel in the hydrophobic cell membrane which is made up of membrane protiens. This is called as facilitated diffusion where the diffusion of molecules is faciliataed through a channel ( diffusion- movement of ions from a region of higher concentration to a region of lower concentration). So, as there is no any contribution of energy, it can be referred to passive transoort.