Transport Across Cell Membrane

There are two types of gradients that condition the direction in which molecules will try to move across a semipermeable membrane like the plasma membrane: chemical and electrical gradients.

• Chemical gradients, also known as concentration gradients, are spatial differences in the concentration of a substance. When talking about chemical gradients in the context of the cell membrane, we are referring to a different concentration of certain molecules on either side of the membrane (inside and outside of the cell or organelle).
• Electrical gradients are generated by differences in the amount of charge on either side of the membrane. The resting membrane potential (usually around -70 mV) indicates that, even without a stimulus, there is a difference in charge on the inside and outside of the cell. The resting membrane potential is negative because there are more positively charged ions outside of the cell than inside, i.e. the inside of the cell is more negative.

When the molecules that cross the cell membrane are not charged, the only gradient we need to consider when working out the direction of movement during passive transport (in the absence of energy) is the chemical gradient. For example, neutral gases like oxygen will travel across the membrane and into the cells of the lung because usually there is more oxygen in the air than within the cells. The opposite is true of CO₂, which has higher concentration within the lungs and travels towards the air without needing extra mediation.

When the molecules are charged, however, there are two things to take into account: the concentration and the electrical gradients. Electrical gradients are only about charge: if there are more positive charges outside the cell, in theory, it doesn't matter if it is sodium or potassium ions (Na⁺ and K⁺, respectively) that travel into the cell to neutralise the charge. However, Na⁺ ions are more abundant outside the cell and K⁺ ions are more abundant inside the cell, so if the appropriate channels open to allow charged molecules to cross the cell membrane, it would be Na⁺ ions that flow more easily into the cell, as they would be travelling in favour of their concentration and electrical gradient.

Why are gradients important?

Gradients are crucial to the cell's functioning because the differences in concentration and charge of different molecules are used to activate certain cellular processes.

The fact that the membrane is semipermeable and not fully permeable allows stricter regulation of the molecules that can cross through the membrane. Charged molecules and large molecules cannot cross on their own, and so will need help from specific proteins that allow them to travel through the membrane either in favour or against their gradient.

There are three main ways in which molecules are transported across the cell membrane: passive, active and secondary active transport. We will have a closer look at each type of transport in the article but first let's look at the main difference between them.

The main difference between these modes of transport is that active transport requires energy in the form of ATP, but passive transport does not. Secondary active transport does not directly require energy but uses the gradients generated by other processes of active transport to move the molecules involved (it indirectly uses cellular energy).

Whether a molecule requires energy to be transported from one side of the membrane to the other depends on the gradient for that molecule. In other words, whether a molecule is transported via active or passive transport depends on if the molecule is moving against or in favour of its gradient.

What are the passive cell membrane transport methods?

Passive transport refers to transport across the cell membrane that does not require energy from metabolic processes. Instead, this form of transport relies on the natural kinetic energy of molecules and their random movement, plus the natural gradients that form on different sides of the cell membrane.

All molecules in a solution are in constant movement, so just by chance, molecules that can move across the lipid bilayer will do so at one time or another. However, the net movement of molecules depends on the gradient: even though molecules are in constant movement, more molecules will cross the membrane to the side of less concentration if there is a gradient.

There are three modes of passive transport:

Simple diffusion

Fig. 1. Simple diffusion: there are more purple molecules on the upper side of the membrane, so the net movement of molecules will be from the top to the bottom of the membrane.

Facilitated diffusion

Channel proteins provide a hydrophilic channel for the passage of charged and polar molecules, like ions. Meanwhile, carrier proteins change their conformational shape for the transport of molecules.

Fig. 2. Facilitated diffusion: it is still a form of passive transport because the molecules are moving from a region with more molecules to a region with less molecules, but they are crossing through a protein intermediary.

Osmosis

Although the correct terminology to use when talking about osmosis is water potential, osmosis is commonly described using concepts related to concentration as well. Water molecules will flow from a region with a low concentration (high amounts of water compared to the low amounts of solutes) to a region with a high concentration (low amount of water compared to the amount of solutes).

Water will flow freely from one side of the membrane to the other, but the rate of osmosis can be increased if aquaporins are present in the cell membrane. Aquaporins are membrane proteins that selectively transport water molecules.

Fig. 3. The diagram shows the movement of molecules through the cell membrane during osmosis

Fig. 4. The diagram shows the movement of molecules in active transport: note that the molecule is moving against its concentration gradient, and so ATP is broken into ADP to release the necessary energy.

Even though the usual active transport we refer to concerns a molecule directly being transported by a carrier protein to the other side of a membrane through the use of ATP, there are other types of active transport that differ slightly from this general model: co-transport and bulk transport.

Bulk transport

As the name indicates, bulk transport is the exchange of a large number of molecules from one side of the membrane to the other. Bulk transport requires a lot of energy and is quite a complex process, as it involves the generation or fusion of vesicles to the membrane. The transported molecules are carried inside the vesicles. The two types of bulk transport are:

• Endocytosis - endocytosis is intended to transport molecules from the outside to the inside of the cell. The vesicle forms towards the inside of the cell.
• Exocytosis - exocytosis is intended to transport molecules from the inside to the outside of the cell. The vesicle carrying the molecules fuses with the membrane to expel its contents outside the cell.

Fig. 5. Endocytosis diagram. As you can see, endocytosis can be divided into further subtypes. Each of these has its own regulation, but the common point is that having to generate a whole vesicle to transport molecules in or out is extremely energy-costly.

Fig. 6. Exocytosis diagram. As with endocytosis, exocytosis can be subdivided into further types, but both are still extremely energy-consuming.

Secondary active transport

Fig. 7. Co-transport of sodium and glucose. Notice that both molecules are transported in the same direction, but they each have different gradients! Sodium is moving down its gradient, while glucose is moving up its gradient.

We hope that with this article you got a clear idea of the types of transport across the cell membrane that there are. If you need more information, check out our deep-dive articles on each type of transport also available at StudySmarter!