4.1 Cellular Transport

Learning Objectives

• Describe methods of transporting molecules into and out of the cell.
• Distinguish between active and passive transport.
• Explain how diffusion and osmosis work.
• Explain how endocytosis and exocytosis work.

Introduction

Cells must obtain raw materials and energy and remove waste. How might these substances enter and exit the cell?

Cells are found in all different types of environments, and these environments are constantly changing. One-celled organisms, like bacteria, can be found on your skin, or in the ground, or in all different types of water. The cells of your body interact with the food you eat, and also with other cells in your body. All cells need a way to protect themselves. This job is done by the cell membrane.

The cell membrane is semipermeable, or selectively permeable, which means that only some molecules can get through the membrane. If the cell membrane were completely permeable, the inside of the cell would be the same as the outside of the cell. It would be impossible for the cell to maintain homeostasis. Homeostasis means maintaining a stable internal environment. For example, if your body cells have a temperature of 98.6 degrees Fahrenheit and it is freezing outside, your cells will maintain homeostasis if the temperature of the cells stays the same and does not drop.

How does the cell ensure it is semipermeable? How does the cell control what molecules enter and leave the cell? The ways that cells control what passes through the cell membrane will be the focus of this lesson.

Guided Learning

What is Transport?

Molecules in the cell membrane allow it to be semipermeable. The membrane is made of a double layer of phospholipids (a "bilayer") and proteins (Figure below). A single phospholipid molecule has two parts:

1. A head that is hydrophilic, or water-loving.

2. A tail that is hydrophobic, or water-fearing.

There is water found on both the inside and the outside of cells. Since hydrophilic means water-loving and they want to be near water, the heads face the inside and outside of the cell where water is found. The water-fearing, hydrophobic tails face each other in the middle of the cell membrane because water is not found in this space. An interesting quality of the plasma membrane is that it is constantly moving, like a soap bubble. Water and small molecules such as oxygen and carbon dioxide can pass freely through the membrane, but larger molecules cannot easily pass through the plasma membrane. Some molecules need a special way to get across the membrane.

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The cell membrane is made up of a phospholipid bilayer, two layers of phospholipid molecules.

Diffusion

Small molecules can pass through the plasma membrane through a process called diffusion. Diffusion is the movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance (Figure below). The amount of a substance in relation to the total volume is the concentration.

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Diffusion is the movement of a substance from an area of a higher amount towards an area of lower amount. Equilibrium is reached when there is an equal amount on both sides of the membrane.

The diffusion of water across a membrane because of a difference in concentration is called osmosis. Let's explore three different situations and analyze the flow of water.

1. A hypotonic solution means the environment outside of the cell has a lower concentration of dissolved material than the inside of the cell. If a cell is placed in a hypotonic solution, water will move into the cell. This causes the cell to swell, and it may even burst.

2. A hypertonic solution means the environment outside of the cell has more dissolved material than inside of the cell. If a cell is placed in a hypertonic solution, water will leave the cell. This can cause a cell to shrink and shrivel.

3. An isotonic solution is a solution in which the amount of dissolved material is equal both inside and outside of the cell. Water still flows in both directions, but an equal amount enters and leaves the cell.

How do marine animals keep their cells from shrinking? How do blood cells keep from bursting? Both have to do with the cell membrane and transport of materials. Marine animals live in salt water, which is a hypertonic environment; there is more salt in the water than in their cells. To prevent losing too much water from their bodies these animals intake large quantities of salt water and secrete salt by active transport; this will be discussed later in this lesson. Red blood cells can be kept from bursting or shriveling if put in a solution that is isotonic to the blood cells. If the blood cells were put in pure water, the solution would be hypotonic to the blood cells, so water would enter the blood cells and they would swell and burst. This is represented in Figure below.

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Osmosis causes these red blood cells to change shape by losing or gaining water.

Passive Transport

Diffusion is called passive transport. This means it does not require energy to move molecules. For example, oxygen diffuses out of the air sacs in your lungs into your bloodstream because oxygen is more concentrated in your lungs than in your blood. Oxygen moves from the high concentration of oxygen in your lungs to the low concentration of oxygen in your bloodstream. Sometimes, special proteins are needed to help molecules move across the membrane. These are called channel proteins or carrier proteins (Figure below). Watch passive transport in action.

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Protein channels and carrier proteins are involved in passive transport.

Active Transport

During active transport, molecules move from an area of low concentration to high concentration. This is the opposite of diffusion. Active transport is called "active" because this type of transport requires energy to move molecules. A protein in the membrane carries the molecules across the membrane. These proteins are often called "pumps," because like other pumps they use energy to move molecules. There are many cells in your body that use pumps to move molecules. For example, your nerve cells would not send messages to your brain unless you had protein pumps moving molecules by active transport. The sodium-potassium pump (Figure below) is an example of an active transport pump. Watch active transport in action.

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The sodium-potassium pump moves sodium ions to the outside of the cell and potassium ions to the inside of the cell. ATP is required for the protein to change shape. As ATP adds a phosphate group to the protein, it leaves behind adenosine diphosphate (ADP).

Two types of active transport are endocytosis and exocytosis. Endocytosis is the process of cells bringing in large particles from the outside of the cell to the inside of the cell. The particles will gather at receptor sites around the cell membrane and then a coated vesicle will form to bring the particles into the cell without the membrane having to open for the particles. Exocytosis is the process of the cell bringing large particles from the inside of the cell to the outside of the cell.This process is the exact opposite of endocytosis. During endocytosis, a vesicle will form within the cell, and then will release the particles to the outside of the cell. Watch endocytosis and exocytosis in action. 

Review

• The plasma membrane is semipermeable, meaning that some molecules can move through the membrane easily, while others cannot.

• Passive transport, such as diffusion and osmosis, does not require energy.

• Active transport moves molecules in the direction of the higher concentration and requires energy and a carrier protein. Two forms of active transport are endocytosis and exocytosis.

Vocabulary

active transport

A type of transport where molecules move from an area of low concentration to high concentration; requires energy to move molecules (opposite of diffusion).

concentration

The amount of a substance in relation to the total volume.

diffusion

The movement of molecules from an area where there is a higher concentration (larger amount) of the substance to an area where there is a lower concentration (lower amount) of the substance.

Endocytosis

        The process of bringing large particles into the cell.

Exocytosis

        The process of bringing large particles from inside of the cell to the outside of the cell.

hydrophilic

A property of the head of a phospholipid; refers to "water-loving."

hydrophobic

A property of the tail of a phospholipid; refers to "water-fearing."

hypertonic solution

A solution outside of the cell that has more dissolved material than inside of the cell.

hypotonic solution

A solution outside of the cell that has a lower concentration of dissolved material than the inside of the cell.

isotonic solution

A solution in which the amount of dissolved material is equal both inside and outside of the cell.

osmosis

The diffusion of water across a membrane because of a difference in concentration.

passive transport

A type of transport that does not require energy to move molecules.

phospholipid

A type of lipid that is a major part of the cell membrane; forms a lipid bilayer.

selectively permeable

A property that cell membranes possess that allow only some molecules to pass through it while preventing others from doing so.