Cell membranes surround each cell and some organelles, such as the nucleus and the Golgi body. They are comprised of a phospholipid bilayer and this acts as a semipermeable barrier that regulates what enters and exits the cell or organelle. Transport across the cell membrane is a highly regulated process, that sometimes involves investing energy directly or indirectly to get the molecules that the cell needs inside, or the ones that are toxic for it out.
Types of gradients across the cell membrane
To understand how transport across the cell membrane works, first we need to understand how gradients work when there is a semi-permeable membrane between two solutions.
A gradient is just a gradual difference in a variable across space.
In cells, the semipermeable membrane is the plasma membrane with its lipid bilayer, and the two solutions can be:
- The cytoplasm of the cell and the interstitial fluid when the exchange happens between the cell and its exterior environment.
- The cytoplasm of the cell and the lumen of a membranous organelle when the exchange happens between the cell and one of its organelles.
Because the bilayer is hydrophobic (lipophilic), it only allows the movement of small nonpolar molecules across the membrane without any protein mediation. Regardless of if polar or big molecules are moving without the need for ATP (i.e. through passive transport), they will need a protein mediator to get them through the lipid bilayer.
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.
When a molecule travels in favour of its gradient, it is said to travel "down" the gradient. When a molecule travels against its concentration gradient, its said to travel "up" the 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.
For example, the resting membrane potential is especially important in neurones and muscle cells, because the change in charge that happens after neuronal stimulation allows neuronal communication and muscle contraction. If there was no electrical gradient, neurones wouldn't be able to generate action potentials and synaptic transmission wouldn't happen. If there was no difference in Na+ and K+ concentrations on each side of the membrane, the specific and tightly regulated flow of ions that characterises action potentials also wouldn't happen.
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.
Types of transport across the cell membrane
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).
Remember that any mode of transport across a membrane can happen at the cell membrane (i.e. between the inside and outside of the cell) or at the membrane of certain organelles (between the lumen of the organelle and the cytoplasm).
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
Simple diffusion is the movement of molecules from a region of high concentration to a region of low concentration until an equilibrium is reached without the mediation of proteins.
Oxygen can freely diffuse through the cell membrane using this form of passive transport because it is a small and neutral molecule.
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
Facilitated diffusion is the movement of molecules from a region of high concentration to a region of low concentration until an equilibrium is reached with the help of membrane proteins, such as channel proteins and carrier proteins. In other words, facilitated diffusion is simple diffusion with the addition of membrane proteins.
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.
Glucose is an example of a molecule that is transported across the cell membrane through facilitated diffusion.
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
Osmosis is the movement of water molecules from a region of high water potential to a region of lower water potential through a semipermeable membrane.
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
What are the active transport methods?
Active transport is the transport of molecules across the cell membrane using carrier proteins and energy from metabolic processes in the form of ATP.
Carrier proteins are membrane proteins that allow the passage of specific molecules across the cell membrane. They are used in both facilitated diffusion and active transport. Carrier proteins use ATP to change their conformational shape in active transport, allowing a bound molecule to pass through the membrane against its chemical or electrical gradient. In facilitated diffusion, however, ATP is not needed to change the shape of the carrier protein.
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.
A process that relies on active transport is the uptake of mineral ions in plant root hair cells. The type of carrier proteins involved is specific for mineral ions.
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.
Co-transport
Co-transport is a type of transport that does not directly use cellular energy in the form of ATP, but that does require energy nonetheless.
How is energy generated in co-transport? As the name suggests, co-transport requires the
transport of several types of molecules at the same time. In this way, it is possible to use carrier proteins that transport
one molecule in favour of their concentration gradient (generating energy) and
another one against the gradient, using the energy of the simultaneous transport of the other molecule.
One of the most well-known co-transport examples is the Na+/glucose cotransporter (SGLT) of the intestinal cells. The SGLT transports Na+ ions down their concentration gradient from the lumen of the intestines to the inside of the cells, generating energy. The same protein also transports glucose in the same direction, but for glucose, going from the intestines to the cell goes against its concentration energy. Therefore, this is only possible because of the energy generated by the transport of Na+ ions by the SGLT.
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!
Transport Across Cell Membrane - Key takeaways
- The cell membrane is a phospholipid bilayer that surrounds each cell and some organelles. It regulates what enters and exits the cell and organelles.
- Passive transport does not require energy in the form of ATP. Passive transport relies on the natural kinetic energy and random movement of molecules.
- Simple diffusion, facilitated diffusion, and osmosis are forms of passive transport.
- Active transport across the cell membrane requires carrier proteins and energy in the form of ATP.
- There are different types of active transport, such as bulk transport.
- Co-transport is a type of transport that doesn't directly utilise ATP, but that still requires energy. The energy is gathered through the transport of a molecule down its concentration gradient, and is used to transport another molecule against its concentration gradient.
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