In this section, we’ll be looking at how all the different mechanisms of transport allow different types of organisms to live and function in the environment. This is section 2.9 of the AP Biology curriculum. We will start with a quick review of active transport, passive transport, endocytosis, and exocytosis. Then, we’ll see how it takes many different mechanisms of transport to complete the process of creating chemical energy in the form of ATP. After we look at this universal process, we’ll see how different organisms use carrier proteins, ion channels, uniporters, symporters, and antiporters to complete a variety of cellular tasks. Specifically, we’ll look at how different mechanisms of transport are at play in bacteria, eukaryotes, plants, fungi, and animals!

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

LEARNING OBJECTIVE
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Describe the processes that allow ions and other molecules to move across membranes.

ESSENTIAL KNOWLEDGE
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A variety of processes allow for the movement of ions and other molecules across membranes, including passive and active transport, endocytosis, and exocytosis.

2.9 Mechanisms of Transport Overview

As if the basics of membrane transport weren’t complex enough, when you start to consider how organisms actually implement these strategies to maintain homeostasis and grow, the overall processes can get mind-boggling. For instance, did you know that a mushroom uses active transport, passive transport, exocytosis, and endocytosis to just to grow and reproduce? Or that a fish’s gills use ion channels, carrier proteins, and modes of active transport to help the fish regulate its water content? Even a plant, which may seem simple, uses all the different mechanisms of transport available. Understanding how these mechanisms of transport work together will definitely be on the AP Test. So, follow along with us as we cover everything you need to know about Mechanisms of Transport!

This section is all about how different mechanisms of transport work together to create living cells, tissues, organs, and organisms.

In previous sections, we covered the basics of cellular membranes and the integral membrane proteins that create the fluid mosaic model. Then, we looked into all the different types of membrane transport. We saw the uniporters, symporters, and antiporters of active transport, and we saw how diffusion, carrier proteins, and channel proteins contribute to passive transport. We even looked at some simple systems involving active and passive transport that work together to create ATP.

These complex processes are constantly active in organisms to help them react to changing environmental conditions. Let’s start with a short review of transport mechanisms and how they can work together.

This is going to be a very quick review of the different mechanisms of transport before we go into the complex processes of how these mechanisms work together to maintain homeostasis in different organisms. If any of these terms are completely unfamiliar or you are having trouble remembering these concepts, please review previous sections to get up to speed. Ready? Let’s go!

There are two basic types of transport that happen across the cell membrane.

Passive transport includes simple diffusion and facilitated diffusion – neither of which requires an input of energy. Small, uncharged molecules can move through the membrane easily via diffusion. The carrier proteins and protein channels of facilitated diffusion are needed for ions and larger molecules. Remember that passive transport always moves substances down their concentration gradient – from high to low.

By contrast, the methods of active transport require energy to move substances against their concentration gradients. Active transport can be primary when they are powered by the chemical energy stored in ATP, or they can be secondary if they are powered by the energy stored in an ion gradient. There are three types of active transport proteins: uniporters, symporters, and antiporters – all of which use energy in some form to pump a substance into an area of higher concentration.

Further, cells can import and export large amounts of substance through endocytosis and exocytosis! Endocytosis can take in very large objects via phagocytosis, large amounts of a solution via pinocytosis, or even bulk import smaller substances via receptor-mediated endocytosis. With exocytosis, the opposite process takes place by merging vesicles with the cell membrane. Large amounts of a specific chemical or large structures can be expelled from the cell through exocytosis.

Both endocytosis and exocytosis rely on complex signaling within cells and the activation of the cytoskeleton to manipulate the cell membrane into forming vesicles that can be drawn into or expelled from the cell. Together, modes of passive and active transport can form systems within cells that complete incredibly complex tasks!

One of the most ubiquitous processes in life is the generation of Adenosine Tri-phosphate molecules. ATP stores energy in the bonds between phosphate groups. When ATP is used, one of the phosphate groups breaks off and the energy from the bond can be applied to a number of other processes. This leaves a molecule of Adenosine Di-Phosphate, which can become ATP if energy is used to add another phosphate group.

In all organisms, the process of creating ATP molecules uses both active and passive transport.

ATP synthase – the enzyme that adds phosphate groups to ADP – is an integral membrane protein that harvests the energy present in the passive transport of hydrogen ions.

For this to happen, a hydrogen ion gradient must be established. This gradient is created in the intermembrane space of chloroplasts and mitochondria, and in the periplasmic space between the two membranes present in bacteria. To establish a gradient like this, cells and organelles need forms of active transport – like a proton pump.

A proton pump is the simplest form of active transport that can create a gradient. This simple system is found in many bacteria and uses the energy created by the breakdown of glucose and other molecules. The enzymes that break down glucose put the energy into a number of electron carriers such as NADH, which can then transfer that energy to the proton pump. The proton pump then uses the energy to pump hydrogen ions (or protons) into the intermembrane space.

While chloroplasts and mitochondria increase the efficiency of this process to create more ATP, each of these systems is essentially just a proton pump used to power ATP synthase. Chloroplasts simply use this ATP energy to generate more stable glucose molecules that can be stored and transferred between cells, while mitochondria break down the stored glucose molecules to create ATP on demand for the rest of the cell!

Bacterial cells use a number of different mechanisms of transport to import and export substances from their cells. Since bacterial cells are already so small, they do this mostly through the use of integral membrane proteins using both active and passive forms of transport. For instance, we’ve already seen how bacterial cells can create ATP using these types of transport. However, bacterial cells use thousands of different protein channels to carry out the functions of life.

For example, bacteria need to gather nutrients and expel waste products in order to grow and reproduce. If bacteria live in a hypotonic environment, they may need to actively transport things like glucose, amino acids, and other molecular building blocks into the cell. But, even bacterial cells use active and passive transport for more than just collecting nutrients.

Consider the flagella – even this mobility structure is driven by interactions between active and passive transport systems. On the inner membrane of the bacteria are a number of active transport proteins that are constantly pumping protons into the intermembrane space. This builds up a gradient, which stores energy. Then, some of these hydrogens are allowed to passively move through the motor proteins. As they do so, they transfer energy to these motor proteins. The motor proteins transfer this energy in order to spin the flagella, allowing the cell to move!

When you get to the level of Eukaryotic cells, the only real difference between these cells and bacteria is the presence of the endomembrane system and organelles found in eukaryotes. The endomembrane system is really like a cell within a cell. Consider a simple food vacuole.

A food vacuole is formed through endocytosis. After the process of phagocytosis, the food vacuole is moved inside the cell. A lysosome merges with the food vacuole, and the contents are digested. While we often visual food vacuoles as simple lipid bilayers, they are in fact embedded with tons of integral membrane proteins. Some of these proteins allow ions and molecules in the food vacuole to pass out of the lysosome down their concentration gradient via passive transport. Other proteins use energy via active transport to actively move substances like amino acids and glucose out of the food vacuole and into the cell.

Remember that food vacuoles are just one small example of the many different active and passive transport processes that take place in a eukaryotic cell. They are also essential for creating ATP energy, maintaining the cell’s water balance, and many other processes!

Plants and fungi, while they are very different types of organisms, use the mechanisms of transport in similar ways.

Plants and fungi both operate on the principle of turgor pressure. This internal cell pressure pushes against the cell walls, creating a rigid structure for the organism. In order to create and maintain turgor pressure, plants and fungi have to maintain their cells at a lower water potential than the surrounding environment in order for water to continuously flow into the cell. Since water potential can be lowered by adding solutes, plants and fungi pack their vacuoles with ions and solutes using active transport. Then, using a series of aquaporins and passive transport, these cells allow water to flow easily into the vacuole from outside the cell.

The turgor pressure that is created allows plant roots and fungi mycelium to push through the soil, while it also allows above-ground growth for both plants and mushrooms! Turgor pressure provides the rigidity these organisms need, while other active and passive mechanisms of transport allow the cell to utilize energy, reproduce DNA and cells, and grow larger!

When we look at the mechanisms of transport in animals, the only big difference seen in animals is the lack of a cell wall. But, the cells must use many different forms of active and passive transport to maintain the overall organism through processes that involve multiple cell types.

Animals use nerves to transfer signals. First, the nerve signal hits passive, voltage-gated ion channels in the sending nerve. This causes vesicles full of neurotransmitters to merge with the cell membrane, dumping the neurotransmitter molecules into the synaptic space via exocytosis. These neurotransmitters hit ligand-gated ion channels on the receiving proteins – causing them to open, cause an action potential, and send the signal through the receiving nerve.

An animal also needs to transport substances like oxygen and glucose to all the cells in its body. Oxygen and carbon dioxide are small, uncharged molecules that can easily diffuse through the cell membranes. But, larger, polar molecules like glucose need specific carrier proteins to carry them across the cells.

Animals also use complex patterns of active and passive transport in order to filter waste products out of their bodies. The nephrons in your kidneys are constantly manipulating water potential and ion concentrations in order to remove urea from your body and concentrate it into urine. In fact, the entire nephron is like a giant concentration gradient. Water and ions pass easily through the cell membranes in the Bowman’s capsule. As they descend into the Loop of Henle, they enter a much more concentrated region of the nephron. Cells in the downward Loop of Henle allow the passage of water, while cells in the ascending loop block the passage of water. This allows the urine to become very concentrated as it enters the collecting duct and heads toward the bladder.