- Describe different types of passive transport.
- Explain how different types of active transport occur.
- Outline the role of cell transport in homeostasis.
- active transport
- movement of substances across a plasma membrane that requires energy
- type of passive transport that does not require the help of transport proteins
- type of vesicle transport that moves substances into a cell
- type of vesicle transport that moves substances out of a cell
- facilitated diffusion
- diffusion with the help of transport proteins
- diffusion of water molecules across a membrane
- passive transport
- movement of substances across a plasma membrane that does not require energy
- sodium-potassium pump
- type of active transport in which sodium ions are pumped out of the cell and potassium ions are pumped into the cell with the help of a carrier protein and energy from ATP
- transport protein
- protein in a plasma membrane that helps other substances cross the membrane
- vesicle transport
- type of active transport in which substances are carried across the cell membrane by vesicles
Imagine living in a house that has walls without any windows or doors. Nothing could enter or leave the house. Now imagine living in a house with holes in the walls instead of windows and doors. Things could enter or leave the house, but you wouldn’t be able to control what came in or went out. Only if a house has walls with windows and doors that can be opened or closed can you control what enters or leaves. For example, windows and doors allow you to let the dog in and keep the bugs out.
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 role 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 and active transport.
Passive transport occurs when substances cross the plasma membrane without any input of energy from the cell. No energy is needed 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. It’s a little like a ball rolling down a hill. It goes by itself without any input of extra energy.
There are several different types of passive transport, including simple diffusion, osmosis, and facilitated diffusion. Each type is described below. You can also watch a video of passive transport:
Passive Transport in Cells: Simple and Facilitated Diffusion and Osmosis
Diffusion is the movement of a substance across a membrane, due to a difference in concentration, without any help from other molecules. The substance simply moves from the side of the membrane where it is more concentrated to the side where it is less concentrated. Figure below shows how diffusion works. 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.
Take a look at this video that shows diffusion across a membrane:
Here’s a Video recap worksheet to fill out and put in your biology notebook:
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.
Diffusion and osmosis are discussed at http://www.youtube.com/watch?v=aubZU0iWtgI.
Khan Academy: Diffusion and Osmosis:
Here is an animation that shows how osmosis works:
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. The 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 below.
- 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. They are like tunnels that only allow certain molecules to go through. “Sorry buddy, you can’t use this chunnel, it’s only for these fellows over here. You’ll have to go to the carrier protein and he’ll let you through.”
- Carrier proteins bind with specific ions or molecules, and in doing so, they change shape. As carrier proteins change shape, they carry the ions or molecules across the membrane. Don’t they look like crocodile mouths picking up a piece of stuff from the outside of the cell and then spitting it into the inside? Carrier crocodile proteins. Ok, that’s not really their name, but it might help you remember.
Watch this helpful animation that shows how facilitated diffusion works:
Active transport occurs when energy is needed for a substance to move across a plasma membrane. Energy is needed because the substance is moving from an area of lower concentration to an area of higher concentration. This is a little like moving a ball uphill; it can’t be done without adding energy. The energy for active transport comes from the energy-carrying molecule called ATP. Like passive transport, active transport may also involve transport proteins. You can watch a video of active transport here:
An example of active transport is the sodium-potassium pump. When this pump is in operation, sodium ions are pumped out of the cell, and potassium ions are pumped into the cell. Both ions move from areas of lower to higher concentration, so ATP is needed to provide energy for this “uphill” process. Figure below explains in more detail how this type of active transport occurs.
A more detailed look at the sodium-potassium pump is available at http://www.youtube.com/watch?v=C_H-ONQFjpQ
Khan Academy: Sodium Potassium Pump:
Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport. Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis. Both types are shown in Figure below and described below.
- Endocytosis is the type of vesicle transport that moves a substance into the cell. The plasma membrane completely engulfs the substance, a vesicle pinches off from the membrane, and the vesicle carries the substance into the cell. When an entire cell is engulfed, the process is called phagocytosis. When fluid is engulfed, the process is called pinocytosis.
- Exocytosis is the type of vesicle transport that moves a substance out of the cell. A vesicle containing the substance moves through the cytoplasm to the cell membrane. Then, the vesicle membrane fuses with the cell membrane, and the substance is released outside the cell. You can watch an animation of exocytosis at the link below.
Review the types of transport with a helpful interactive animation:
Homeostasis and Cell Function
For a cell to function normally, a stable state must be maintained inside the cell. For example, the concentration of salts, nutrients, and other substances must be kept within a certain range. The process of maintaining stable conditions inside a cell (or an entire organism) is homeostasis. Homeostasis requires constant adjustments, because conditions are always changing both inside and outside the cell. The processes described in this lesson play important roles in homeostasis. By moving substances into and out of cells, they keep conditions within normal ranges inside the cells and the organism as a whole.
Click here to practice what you’ve learned in this chapter with an interactive online activity.
- A major role of the plasma membrane is transporting substances into and out of the cell. There are two major types of cell transport: passive transport and active transport.
- Passive transport requires no energy. It occurs when substances move from areas of higher to lower concentration. Types of passive transport include simple diffusion, osmosis, and facilitated diffusion.
- Active transport requires energy from the cell. It occurs when substances move from areas of lower to higher concentration or when very large molecules are transported. Types of active transport include ion pumps, such as the sodium-potassium pump, and vesicle transport, which includes endocytosis and exocytosis.
- Cell transport helps cells maintain homeostasis by keeping conditions within normal ranges inside all of an organism’s cells.
Lesson Review Questions
1. What is osmosis? What type of transport is it?
2. Describe the roles of transport proteins in cell transport.
3. What is the sodium-potassium pump?
4. Name two types of vesicle transport. Which type moves substances out of the cell?
5. Assume a molecule must cross the plasma membrane into a cell. The molecule is a very large protein. How will it be transported into the cell? Explain your answer.
6. The drawing below shows the fluid inside and outside a cell. The dots represent molecules of a substance needed by the cell. The molecules are very small and hydrophobic. What type of transport will move the molecules into the cell?
7. Compare and contrast simple diffusion and facilitated diffusion. For each type of diffusion, give an example of a molecule that is transported that way.
8. Explain how cell transport helps an organism maintain homeostasis.
Points to Consider
All cells share some of the same structures and basic functions, but cells also vary.
- Plant cells have structures that animal cells lack. What important process takes place in plant cells but not in animal cells that might explain their differences?
- All cells, including both plant and animal cells, need energy for processes such as active transport. How do cells obtain the energy they need?
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