In this section of the AP Biology curriculum, we start to look at how cell membranes operate to maintain solute and water balance, and how they deal with importing and exporting biological macromolecules. Cells do this mainly through two methods: passive transport and active transport. Passive transport includes the diffusion of substances along their concentration gradient, without the need to add energy. Active transport involves methods that require energy to move molecules against their concentration gradient. We will also look at exocytosis and endocytosis, two methods cells use for exporting and importing large quantities of material at once.

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ENDURING UNDERSTANDING
ENE-2
Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments.

LEARNING OBJECTIVE
ENE-2.E
Describe the mechanisms that organisms use to maintain solute and water balance.

ENE-2.F
Describe the mechanisms that organisms use to transport large molecules across the plasma membrane.

ESSENTIAL KNOWLEDGE
ENE-2.E.1
Passive transport is the net movement of molecules from high concentration to low concentration with the direct input of metabolic energy.
ENE-2.E.2
Passive transport plays a primary role in the import of materials and the export of wastes.
ENE-2.E.3
Active transport requires the direct input of energy to move molecules from regions of low concentration to regions of high concentration.
ENE-2.F.1
The selective permeability of membranes allows for the formation of concentration gradients of solutes across the membrane.
ENE-2.F.2
The process of endocytosis and exocytosis require energy to move large molecules into and out of cells–

1. In exocytosis, internal vesicles fuse with the plasma membrane and secrete large macromolecules out of the cell.
2. In endocytosis, the cell takes in macromolecules and particulate matter by forming new vesicles derived from the plasma membrane.

2.6 Membrane Transport Overview

This section of the AP Biology curriculum – 2.6 Membrane Transport – covers the basics of how cells import and export the substances they need. We’ll start by looking at the differences between active and passive transport. Then, we’ll take a specific look at both passive transport (including diffusion and facilitated diffusion), and the energy-dependent modes of active transport. We’ll also take a look at how cells can take in large amounts of material via endocytosis and how cells can export large amounts of material via exocytosis.

The difference between active transport and passive transport is simple – active transport requires energy. As we will see, active transport can get this energy from ATP or it can utilize the potential energy stored in a concentration gradient. Active transport requires energy because it is moving a substance against the concentration gradient. In other words, the molecules are moving from an area of low concentration to an area of high concentration.

By contrast, passive transport does not require energy. No energy is needed because all forms of passive transport are moving molecules from an area of high concentration to an area of low concentration. Passive transport includes simple diffusion through the plasma membrane as well as facilitated diffusion through ion channels and carrier proteins. Let’s take a closer look at each of these modes of transport.

Passive transport does not require energy simply because molecules are moving in the direction they would be moving anyway – from high concentration to low concentration. There are two basic types of passive transport: simple diffusion and facilitated diffusion. Let’s take a closer look at simple diffusion.

Some molecules (like oxygen, water, and carbon dioxide) are small enough that they can pass right through the plasma membrane. Oxygen and carbon dioxide are nonpolar, uncharged molecules. This means that the hydrophobic core of the lipid bilayer does not effectively block them from passing through. While water is a polar molecule, it does not carry a charge. So, water can still slip through the plasma membrane when concentration gradients or pressure changes force it to move. When water moves across the membrane it is called osmosis, and we will take a closer look at this phenomenon in section 2.8. Now, let’s take a look at Facilitated Transport.

Facilitated transport is required for ions and large molecules. Ions cannot pass through the plasma membrane because they carry a charge and are blocked by the hydrophobic core. So, they must pass through hollow proteins known as channel proteins. Large molecules, such as glucose, are simply too large and polar to pass through the small gaps in the plasma membrane. These molecules are also too large for channel proteins, so they require a special carrier protein. These large molecules enter the carrier protein and bind to the active site – which changes the conformation of the protein. This change causes the protein to open on the other side of the membrane, releasing the molecule and resetting the process. We will cover both of these transport proteins further in section 2.7.

Active Transport requires energy because it is moving molecules from an area of low concentration to an area of high concentration. Unlike most forms of passive transport, active transport is directional – that is, it transports a specific substance in only one direction. There are three main types of proteins that engage in active transport.

A uniport (or sometimes uniporter) uses energy to actively pump 1 type of substance against its concentration gradient.

A symport (or symporter) moves two substances at the same time, in the same direction across the cell membrane. Some symporters are moving both molecules against their gradient, while others use the energy from one substance’s gradient to power the movement of another molecule against a gradient.

An antiport (or antiporter) moves two substances across the membrane but in opposite directions. Antiporters can also use one molecule’s gradient to power the movement of another molecule against the gradient.

There are two types of energy that can be used to power active transport: primary and secondary.

Primary active transport requires chemical energy from ATP or other energy-transporting molecules. The ATP molecule reacts with the transporter protein, removing a phosphate group and releasing energy into the protein’s molecular structure. This allows the protein to grab onto a substrate molecule and move it through the membrane against the concentration gradient.

By contrast, secondary active transport does not rely on chemical energy molecules like ATP. Instead, secondary active transport relies on the potential energy stored in a concentration gradient. For example, a sodium/calcium antiporter is using the energy stored in the sodium concentration gradient to move calcium against its concentration gradient. Three sodium molecules move into the antiporter, pushed by the concentration gradient. The antiporter then takes up one calcium ion. The energy from the sodium gradient forces a conformational change, forcing the calcium ion out of the cell against its concentration gradient!

Cells use a wide variety of integral membrane proteins to build up these chemical gradients and use them to power the movement of other substances across their cell membranes!

Next up, let’s look at some forms of membrane transport that are on a much larger scale than individual membrane proteins. Endocytosis and exocytosis are how the cell can import or export large amounts of material at the same time using large folds of the plasma membrane. The difference is simple to remember if you break down the words.

“Endo” means within or into, whereas cytosis refers to cells. So, Endocytosis means “into the cell”. Cells use endocytosis to take in large molecules, create food vesicles, and even “eat” smaller cells.

By contrast, “exo” means external or out of. So, Exocytosis means out of the cell. Cells use exocytosis to dump entire vesicles into the external environment.

Endocytosis and exocytosis are both forms of active transport because it takes a lot of energy to form vesicles and move them around the cell using the cytoskeleton. Let’s take a look at the different kinds of endocytosis and exocytosis.

There are three main types of endocytosis that cells use to intake large quantities of material: phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Phagocytosis is how cells take in very large macromolecules and even smaller cells. For instance, entire bacterial cells can be eaten by white blood cells. The cell membrane wraps itself around the large object, then pinches off into a food vacuole. A lysosome will merge with the food vacuole, digesting its contents so the cell can use them.

Similarly, pinocytosis takes in a large quantity of water and substances by creating an inward fold of the cell membrane. The folds are generally much smaller than with phagocytosis. In this case, the cell simply sucks in water and smaller substances that are dissolved in water. This is a good way for a cell to take in a large quantity of water and nutrients at the same time.

But, cells can use receptor-mediated endocytosis to take in a large quantity of very specific substances. For instance, this is how your body transfers and recycles molecules like cholesterol, which would otherwise get stuck in the plasma membrane. Cholesterol is bonded to protein molecules, making lipoproteins. These lipoproteins can bind to specific receptors on the cell’s surface. When enough receptors have been activated, this entire portion of the cell membrane undergoes endocytosis. The vacuole merges with a liposome, it is digested completely, and the components of the original cholesterol can be recycled.

For the same reason that cells need to use entire portions of the cell membrane to intake substances, there are many uses for expelling substances with a similar process. This process is exocytosis. For instance, this is exactly what happens in your neurons every time they transfer a signal to the next neuron.

The nerve impulse comes through the presynaptic neuron, ending at the axon terminal. This causes vesicles full of neurotransmitters to bind with the cell membrane. These neurotransmitters are dumped into the synaptic space via exocytosis. The neurotransmitters quickly reach the next neuron and open ion channels. This disrupts the electrical balance of the cell membrane, causing a new nervous impulse to travel through the post-synaptic neuron.