Most people are surprised to learn that very little food absorption takes place in the stomach. Most of what the stomach absorbs is a few fat-soluble substances like alcohol and some pain medications. All other food absorption takes place in the small intestine.

Why this is important…Good nutrition is vital to health. Our cells need dozens of nutrients, all of which are supplied by our food and delivered via the bloodstream. Understanding how food crosses this barrier – from digestive tract to bloodstream – is crucial for understanding how our bodies receive nutrition.

Foundations

Breaking down foods into molecules small enough to absorb into the bloodstream requires some extreme chemistry. Enzymes are part of the picture, but so is the extremely acidic environment of the stomach. These extreme chemical environments need to be kept separate from the delicate cells of the body. The entire digestive tract is a tube that runs through your body, like a tunnel through a mountain. While food is in the digestive system, it is not actually inside the body – it is not mixing with blood or cellular fluids.

There is a substantial barrier between our digestive tract and our body fluids. Food molecules must cross this barrier, but harmful substances (acids, bacteria, toxins, dirt) remain in the digestive tract. In this way, it acts a lot like the skin. Unlike the skin, intestines move most of the molecules across the barrier.

Selective membranes are the key to this task. We know that most chemicals cannot cross cell membranes without special channels. We also know that molecules tend to diffuse down their concentration gradients. Cells lining the small intestine have two main jobs to get food molecules across: (1) make transport proteins for each type of food molecule and (2) create concentration gradients that move food molecules out of the intestine. The cells lining the small intestine first actively load the food molecules into their cells, then they leak them into the bloodstream, letting diffusion do the work.

Key terms

Lumen – inner space of a tube. Examples include intestine, blood vessel, sweat gland.

Cotransport – moving two molecules together, as in movement through a membrane protein where two different molecules move through at the same time

Primary active transport – movement of particles (usually ions) across the cell membrane and against their concentration gradient with the help of an ATP-powered protein. These are often referred to as “pumps” or sometimes “ATPases.”

Secondary active transport – method of moving particles across a cell membrane where one particle moves down its concentration gradient (supplying energy) and another particle moves against its concentration gradient

Facilitated diffusion – movement of particles across a cell membrane, when the particle moves down its concentration gradient, but must pass through a membrane transport protein (channel or carrier protein)

Details

Absorption of food requires a large surface area. Once liquified food enters the small intestine, it is moved along by muscles in the intestinal walls, squeezing the food onward. It only has a few hours to finish breaking down food and absorbing the resulting biomolecules. That may seem like a luxury of time, but each food molecule is absorbed only one molecule at a time. To cram as many transport proteins into the lining of the 5-meter-long small intestine, it is highly folded (Figure 5-20). This increases the surface area to 200m² (almost half the floor of a basketball court). This allows enough food molecules to make their way into the bloodstream to provide us with the energy and raw materials needed to sustain life.

Multiple views of small intestine. 1. Tube cut open to exposed wrinkled interior. 2. detail of ridges and internal vascularization. 3. Cell detail with microvilli on top.

FIGURE 5-20. Small intestine lining. The lining of the small intestine is folded several times, adding surface area. Even the cell surfaces lining the lumen are highly folded. (“Histology of small intestine” by OpenStax College is licensed under CC BY 3.0)

Active Transport

Absorption of food requires a lot of energy. Some substances are absorbed passively, but molecules like glucose and amino acids need to be transported actively. Remember that substances naturally spread out (diffusion). Active transport works against this natural process and creates concentration gradients.

The body has many different custom types of molecular pumps in cell membranes. These fall into two categories: primary active transport and secondary active transport.

Primary active transport is a method where molecular pumps in cell membranes use ATP energy to move specific substances against their gradient (Figure 5-21). Some pumps move only one type of type of ion while others move two. Some examples:

Calcium ion pumps bring calcium ions into the cells lining the small intestine.
H+ ion pumps (or proton pumps) come in many forms. In special cells lining the stomach, H⁺ ions are pumped into the stomach, making it extremely acidic. (In the stomach, this pump also brings K⁺ ions into the cells from the stomach. It is known as the hydrogen-potassium ATPase.)
Sodium-potassium pumps (Na/K pumps) operate in the membranes of all cells of animal bodies. They are more complex than the previous two examples because they pump 3 Na⁺ ions out of the cell for every 2 K⁺ they pump into the cell (Figure 5-22). For cells lining the small intestine, this makes sure the cells have a very low internal concentration of Na⁺ This sets the stage for secondary active transport.

FIGURE 5-21. Primary active transport. Protein pump (blue protein with the gear symbol) is powered by ATP and moves ions across the membrane against their concentration gradient. (“Primary active transport” by J. Buegge is in the Public Domain )

VIDEO 5i: Primary active transport

After watching this video, you should be able to:

describe the mechanism of primary active transport
give examples of where primary active transport is used
differentiate between primary and secondary active transport

Secondary active transport takes advantage of the concentration gradients set up through primary active transport. Molecules like amino acids and glucose can be moved against their concentration gradient by allowing sodium in with them (Figure 5-22). Here is a summary of how it works:

Sodium ion (Na⁺) concentrations are kept low in the cells lining the lumen (by primary active transport, as described above).
The sodium ion concentration gradient has potential energy. Sodium ions entering the cell, down their gradient, can be used to do work.
Cotransport proteins allow sodium into the cell from the lumen. At the same time, they drag another molecule – like an amino acid or glucose – along with them. Usually, these biomolecules are moving against their gradient. Of course, each biomolecule needs its own cotransport protein, but the process works the same for each of them.

The end result is a high concentration of biomolecules in the cells lining the small intestine. Sodium ions are continually pumped out of the cell, so their concentrations remain low. But with a high concentration of biomolecules, this sets up a new flow-down gradient that makes them “want” to move out of the cell.

VIDEO 5j: Secondary active transport

After watching this video, you should be able to:

explain how secondary active transport works
recognize examples of secondary active transport

Secondary active transport in the intestines. Transport proteins shown in cell membranes.

FIGURE 5-22. Secondary active transport. On the right, sodium enters a cell through a green cotransport protein along with glucose. On the left, the glucose leaves by facilitated diffusion (through glucose transporter) and sodium is pumped out by the sodium-potassium pump. ("Secondary active transport in small intestine" by Eunice Laurent is licensed under CC BY-SA 4.0)

Stop & Think #17: Explain how secondary active transport works. Pay particular attention to explaining how the sodium ions provide an energy source to power this process.

Passive Transport: Facilitated Diffusion

Once food molecules are concentrated inside these cells, they can leave the cell by diffusion, moving down their gradient. Moving glucose and amino acids into cells by secondary active transport allows them to build up a high concentration inside cells. Once they build up a higher concentration than exists in the bloodstream, they can diffuse out of the cell into the bloodstream.

Each type of molecule needs its own custom-made transport protein. You should remember that polar molecules and ions cannot cross the phospholipid bilayer of the cell membrane. A membrane protein is needed for them to exit the cell. Each protein allows its specific molecule to enter or leave the cell freely. No work or energy input is required – molecules just move down their concentration gradient (Figure 5-23). The transport protein just facilitates the movement of the molecules, which is why this type of transport is called facilitated diffusion (or facilitated transport). They are like a automatic doors that allow people to enter and leave a building. In both cases, the passageway closes behind the person or molecule, leaving the integrity of the barrier intact.

VIDEO 5k: Facilitated diffusion

After watching this video, you should be able to:

determine what types of particles move across the membrane this way
explain the role of gradients and protein channels
explain how changing the number of channels affects flow of particles

Diagram of facilitated diffusion. Particles move across membrane through either protein channels or carrier proteins.

FIGURE 5-23. Facilitated diffusion. Ions can move through protein channels, as shown on the left. Other molecules use carrier proteins, shown in different configurations since they change shape, like swinging doors, to let molecules through the membrane. ("Facilitated diffusion" by LadyofHats Mariana Ruiz Villarreal is in the Public Domain, CC0)

Absorption of Other Molecules

Not every type of food molecule enters the cells lining the intestine the same way. Here are some examples of molecules that take a different path into the bloodstream.

Fructose (a simple sugar) has its own transport channel facing the lumen and enters cells by facilitated diffusion alone.
Minerals in our diet, such as calcium or potassium, have their own transport channels or pumps.
Lipids use a combination of simple diffusion and other types of transport because they’re nonpolar.

Lipids always seem to be the oddball. Since they are nonpolar, lipids have to be dealt with differently in the watery environment of the body. The gall bladder releases bile acids, which break up fats into tiny droplets that stay dissolved in water. Extremely small droplets of lipids enter cells by simple diffusion, since they are nonpolar. They are repackaged into different small clusters which are sent into a series of tubes that are part of the lymphatic system (tan vessels in Figure 5-20) before being dumped into the bloodstream.

Water is also absorbed through the intestinal lining. The liquified food in the small intestine has a high water content. With all the sugars and salts leaving the lumen, water is pulled along with them by osmosis. Water moves through cell membranes and even between cells, ending up in the bloodstream to hydrate the rest of our cells. (More about osmosis in the next chapter.)

Stop & Think #18: If you consumed a high-sugar (or high-carbohydrate) meal and all those sugars were transported into the bloodstream, how would that affect the water content of your blood?

Learning objectives

Describe the process of secondary active transport including the different concentration gradients
Explain how energy stored in a concentration gradient can be used to do work
Describe the process of facilitated diffusion and why transport proteins are needed
Explain why a large surface area is necessary to absorb food molecules efficiently
Explain why lipids require different mechanisms of digestion and transport

Stop & Think Questions Explained

17. This is just a reminder to do more than reading about complex topics. Once you understand it yourself, try explaining it to someone else. Your first explanation may not be perfect, but test yourself to see how much of it you can get. It could make interesting dinner conversation!

18. Sugar is a solute and can drive osmosis. Extra sugar in the bloodstream will draw in water with it.