Created by: CK-12/Adapted by Christine Miller
Letting in the Light
Look at the big windows in this house (Figure 4.7.1). Imagine all the light they must let in on a sunny day. Now imagine living in a house that has walls without any windows or doors. Nothing could enter or leave. Or imagine living in a house with holes in the walls instead of windows and doors. Things could enter or leave, but you couldn’t control what came in or went out. Only when a house has walls with windows and doors that can be opened or closed, can you control what enters or leaves. Windows and doors allow you to let in light and the family dog and keep out rain and bugs, for example.
Transport Across Membranes
If a cell were a house, the plasma membrane would be walls with windows and doors. Moving things in and out of the cell is an important function of the plasma membrane. It controls everything that enters and leaves the cell. There are two basic ways that substances can cross the plasma membrane: passive transport — which requires no energy expenditure by the cell — and active transport — which requires energy from the cell.
Transport Without Energy Expenditure By The Cell
Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is required because the substances are moving from an area where they have a higher concentration to an area where they have a lower concentration. Concentration refers to the number of particles of a substance per unit of volume. The more particles of a substance in a given volume, the higher the concentration. A substance always moves from an area where it is more concentrated to an area where it is less concentrated.
There are several different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion. Each type is described below.
Simple Diffusion
Diffusion is the movement of a substance due to a difference in concentration. It happens without any help from other molecules. The substance simply moves from the area where it is more concentrated to the area where it is less concentrated. Picture someone spraying perfume in the corner of a room. Do the perfume molecules stay in the corner? No, they spread out, or diffuse throughout the room until they are evenly spread out. Figure 4.7.2 shows how diffusion works across a cell membrane. Substances that can squeeze between the lipid molecules in the plasma membrane by simple diffusion are generally very small, hydrophobic molecules, such as molecules of oxygen and carbon dioxide.
Osmosis
Osmosis is a special type of diffusion — the diffusion of water molecules across a membrane. Like other molecules, water moves from an area of higher concentration to an area of lower concentration. Water moves in or out of a cell until its concentration is the same on both sides of the plasma membrane. In Figure 4.7.3, the dotted red line shows a semi-permeable membrane. In the first beaker, there is an uneven concentration of solutes on either side of the membrane, but the solute cannot cross — diffusion of the solute can’t occur. In this case, water will move to even out the concentration as has happened on the beaker on the right side. The water levels are uneven, but the process of osmosis has evened out the concentration gradient.
Facilitated Diffusion
Water and many other substances cannot simply diffuse across a membrane. Hydrophilic molecules, charged ions, and relatively large molecules (such as glucose) all need help with diffusion. This help comes from special proteins in the membrane known as transport proteins. Diffusion with the help of transport proteins is called facilitated diffusion. There are several types of transport proteins, including channel proteins and carrier proteins. Both are shown in Figure 4.7.4.
- Channel proteins form pores (or tiny holes) in the membrane. This allows water molecules and small ions to pass through the membrane without coming into contact with the hydrophobic tails of the lipid molecules in the interior of the membrane.
- Carrier proteins bind with specific ions or molecules. In doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane.
Transport and Homeostasis
For a cell to function normally, the inside of it must maintain a stable state. The concentrations of salts, nutrients, and other substances must be kept within certain ranges. The state in which stable conditions are maintained inside a cell (or an entire organism) is called homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The transport of substances into and out of cells as described in this section plays an important role in homeostasis. By allowing the movement of substances into and out of cells, transport keeps conditions within normal ranges inside the cells and throughout the organism as a whole.
Watch this video “Cell Transport,” by the Amoeba Sisters:
Cell Transport with the Amoeba Sisters, 2016.
4.7 Summary
- Controlling the movement of things in and out of the cell is an important function of the plasma membrane. There are two basic ways that substances can cross the plasma membrane: passive transport — which requires no energy expenditure by the cell — and active transport — which requires energy.
- No energy is needed from the cell for passive transport because it occurs when substances move naturally from an area of higher concentration to an area of lower concentration.
- Simple diffusion is the movement of a substance due to differences in concentration. It happens without any help from other molecules. This is how very small, hydrophobic molecules (such as oxygen and carbon dioxide) enter and leave the cell.
- Osmosis is the diffusion of water molecules across a membrane. Water moves in or out of a cell by osmosis until its concentration is the same on both sides of the plasma membrane.
- Facilitated diffusion is the movement of a substance across a membrane due to differences in concentration, but it only occurs with the help of transport proteins (such as channel proteins or carrier proteins) in the membrane. This is how large or hydrophilic molecules and charged ions enter and leave the cell.
- Processes of passive transport play important roles in homeostasis. By allowing the movement of substances into and out of the cell, they keep conditions within normal ranges inside the cell and the organism as a whole.
4.7 Review Questions
- What is the main difference between passive and active transport?
- Summarize three different ways that passive transport can occur. Give an example of a substance that is transported in each way.
- Explain how transport across the plasma membrane is related to homeostasis of the cell.
- In general, why can only very small, hydrophobic molecules cross the cell membrane by simple diffusion?
- Explain how facilitated diffusion assists with osmosis in cells. Define osmosis and facilitated diffusion in your answer.
- Imagine a hypothetical cell with a higher concentration of glucose inside the cell than outside. Answer the following questions about this cell, assuming all transport across the membrane is passive, not active.
- Can the glucose simply diffuse across the cell membrane? Why or why not?
- Assuming that there are glucose transport proteins in the cell membrane, which way would glucose flow — into or out of the cell? Explain your answer.
- If the concentration of glucose was equal inside and outside of the cell, do you think there would be a net flow of glucose across the cell membrane in one direction or the other? Explain your answer.
- What are the similarities and differences between channel proteins and carrier proteins?
-
4.7 Explore More
Osmosis and Water Potential, Amoeba Sisters, 2018.
Structure Of The Cell Membrane – Active and Passive Transport, Professor Dave Explains, 2016.
Attributions
Figure 4.7.1
Windows/ The Oyster Suite in Eureka, CA by Drew Coffman on Unsplash is used under the Unsplash License https://unsplash.com/license).
Figure 4.7.2
Diffusion/ Scheme simple diffusion in cell membrane by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
Figure 4.7.3
Osmosis by OpenStax on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0) license.
Figure 4.7.4
Scheme facilitated diffusion in cell membrane by Mariana Ruiz Villarreal [LadyofHats] is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).
References
Amoeba Sisters. (2016, June 24). Cell transport. YouTube. https://www.youtube.com/watch?v=Ptmlvtei8hw&feature=youtu.be
Amoeba Sisters. (2018, June 27). Osmosis and water potential. YouTube. https://www.youtube.com/watch?v=L-osEc07vMs&feature=youtu.be
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, April 25). Figure 3.7 Osmosis [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane
Professor Dave Explains. (2016, September 5). Structure of the cell membrane – Active and passive transport. https://www.youtube.com/watch?v=AcrqIxt8am8&feature=youtu.be
A semi-permeable lipid bilayer that separates the interior of all cells from their surroundings.
a type of movement of substances across the cell membrane which does not require energy because the substances are moving with the concentration gradient (from high to low concentration).
The movement of ions or molecules across a cell membrane into a region of higher concentration, assisted by enzymes and requiring energy.
The ability to do work.
The amount of particles of a substance in a given amount of solution.
The movement of a substance from an area of high concentration to an area of low concentration.
The movement of water or other solvent through a plasma membrane from a region of low solute concentration to a region of high solute concentration.
The passive movement of molecules across the cell membrane with the aid of a membrane protein.
The semipermeable membrane surrounding the cytoplasm of a cell.
Attracted to water.
A membrane protein involved in the movement of ions, small molecules, or macromolecules, such as another protein, across a biological membrane.
The smallest unit of life, consisting of at least a membrane, cytoplasm, and genetic material.
The ability of an organism to maintain constant internal conditions despite external changes.
Repelled by water.
