High school biology
|Passive transport||Type of transport that does not require energy to occur|
|Concentration gradient||A region of space over which the concentration of a substance changes|
|Permeability||The quality of a membrane that allows substances to pass through it|
|Equilibrium||The state at which a substance is equally distributed throughout a space|
Types of passive transport
During diffusion, substances move from an area of high concentration to an area of low concentration, until the concentration becomes equal throughout a space.
Image showing the process of diffusion across the plasma membrane. Initially, the concentration of molecules is higher on the outside. There is net movement of molecules from the outside to the inside of the cell until the concentrations are equal on both sides.
This is also true for some substances moving into and out of cells. Because the cell membrane is semipermeable, only small, uncharged substances like carbon dioxide and oxygen can easily diffuse across it. Charged ions or large molecules require different kinds of transport.
Although gases can diffuse easily between the phospholipids of the cell membrane, many polar or charged substances (like chloride) need help from membrane proteins. Membrane proteins can be either channel proteins or carrier proteins.
Even though a concentration gradient may exist for these substances, their charge or polarity prevents them from crossing the hydrophobic center of the cell membrane. Substances transported through facilitated diffusion still move with the concentration gradient, but the transport proteins protect them from the hydrophobic region as they pass through.
Left side: Image of a channel protein, which forms a tunnel allowing a specific molecule to cross the membrane (down its concentration gradient).
Right side: Diagram showing how a carrier protein can bind a target molecule on one side of the membrane, undergo a shape change, and release the target molecule on the other side of the membrane.
Common mistakes and misconceptions
- Not everything enters the cell through passive transport. Only the smallest molecules like water, carbon dioxide, and oxygen can freely diffuse across cell membranes. Larger molecules or charged molecules often require an input of energy to be transported into the cell.
- Even when equilibrium is reached, particles do not stop moving across the cell membrane. Although it may seem as if the concentrations are not changing, nearly equal numbers of particles cross the membrane in both directions. This means that there is no net change in the concentration of the substances.
Want to join the conversation?
- Hi, what exactly is facilitated diffusion?(16 votes)
- When the molecules pass through the membrane via protein. (passive/active transport) Usually large and charged molecules use facilitated diffusion.(26 votes)
- How can water pass through the hydrophobic part of the cell without breaking the cell membrane?(15 votes)
- I think they have passage ways so the water doesn't touch the hydrophobic parts of the cell membrane.(15 votes)
- so if I understand, channel proteins do not require ATP. Am I right?(12 votes)
- can cholesterol pass?(7 votes)
- Cholesterol is part of the phospholipid bilayer membrane, it doesn't pass through because it is part of the membrane(19 votes)
- What direction in a gradient does passive transport go?(5 votes)
- what does it mean to be semi-permeable(1 vote)
- What is the main difference between simple and facilitated diffusion?(2 votes)
- Simple diffusion occurs without directly through the membrane, whilst facilitated diffusion occurs with the help of channels, which increase the permeability of the membrane to certain solutes.(3 votes)
- what is osmosis and what are some examples for it?(1 vote)
- Osmosis is the flow of water or other fluids across a semi-permeable membrane to balance chemical concentrations on both sides of the membrane. Try these Khan Academy videos and articles for more information.
If you really want to understand osmosis, try this experiment. Take a normal egg. submerge it in vinegar for 24-48 hours, or until the shell has dissolved. Then, take your egg without a shell and submerge it in syrup for 24 hours. Observe. Then, take your egg and put it in distilled or bottled water for 24 hours. Observe again.
I know that this is a very vague answer, but I suggest going back and re-reading the article to try to understand why this is happening. I've found that sometimes knowledge gained by oneself when possible is better than getting an answer. Also, the practical application of osmosis in the experiment also helps understand it better.(5 votes)
- Why does the hydrophobic center of a cell membrane prevent polar substances from diffusing through it?(1 vote)
- What if equal Librium can not be reached?(2 votes)
- Usually, cells are always trying to achieve equilibrium. But, if it is not able to do so, the cell is damaged or killed because it is unable to get the nutrients it needs.(1 vote)