1. Introduction
Membranes are selectively permeable. This means that membranes allow the cell to “select” what can pass through the membrane. This, in turn, allows the cell to determine what can enter or leave the cell, and what can’t.
In what follows, we’ll look at a few ways that things make their way across the cell membrane, and how the membrane determines what’s permitted to enter and leave, and what’s not.
2. Diffusion across the membrane
Video Introduction: What are the Differences Between Simple Diffusion, Facilitated Diffusion, and Active Transport?
2a. Diffusion and Concentration Gradients.
Diffusion is the tendency of molecules to spread out from where they’re more concentrated to where they’re less concentrated.
- Put a drop of ink into a cup of water. Over time, you’ll see the ink slowly spread out until it’s uniformly distributed throughout the cup. That’s diffusion.
Diffusion (by Blausen via Wikipedia) - A fragrance (or bad odor) released in one part of a room spreads throughout the entire room. That’s diffusion.
A key idea related to diffusion is the idea of a concentration gradient. A gradient is a slope. In the same way that a ball will roll down a hill, molecules in a fluid will diffuse down a concentration gradient, spreading out from higher concentration to lower concentration. So when you think of diffusion, just imagine that the diffusing molecules are flowing down a slope, moving from where they’re more concentrated (on the left side of this diagram) to less concentrated (on the right side).
The -ΔG indicates that this process is spontaneous. Think about a ball positioned near the top of a hill. Once you give it an initial push to overcome its inertia, you don’t have to provide additional energy to keep the ball rolling. Similarly, you don’t have to add any outside energy to power the diffusion of molecules down their concentration gradient. Their own kinetic energy does all the work.
2b. Simple and Facilitated Diffusion
Diffusion can also occur across a membrane. All that’s required is that the membrane be permeable to the diffusing substances.
You can see diffusion across a membrane in the animation on the right: the molecules, represented by cubes, are flowing down their concentration gradient as they diffuse from where they’re in higher concentration (below the membrane) to where they’re in lower concentration (above the membrane).
Diffusion through a membrane can be of two types: simple diffusion or facilitated diffusion.
Simple diffusion is diffusion through the phospholipid portion of the membrane. This works for small, nonpolar substances such as carbon dioxide or oxygen molecules. It also works for some larger, nonpolar molecules, such as triglycerides or steroids.
Facilitated diffusion is diffusion through a protein channel. Why is a protein channel needed? It’s because of the properties of the cell’s phospholipid bilayer. The interior of the bilayer consists of the hydrophobic fatty acid tails of the phospholipids that make up the bilayer. This layer is nonpolar and excludes any molecule that’s polar (such as glucose) or charged (including amino acids, hydrogen ions, or ions of sodium, potassium, or calcium). Larger molecules, such as proteins, are also unable to diffuse through the bilayer.
The protein channels used during facilitated diffusion are specific. For example, a channel that lets a sodium ion through won’t let a calcium ion through.
In addition, the protein channels used for facilitated diffusion are often gated. For example, the membrane channel (“c”) that allows a molecule like glucose (represented by “d”) to diffuse into cells is usually closed, and won’t allow glucose to diffuse through. However, when the hormone insulin (represented by “a”) binds with a membrane receptor, it sets off changes in the membrane channel that cause the channel to open, allowing glucose to diffuse into the cell.
2c. Water and Aquaporins
The diffusion of water is so important in the life of a cell that we’ll devote the next tutorial to exploring how it works. It’s worth mentioning now, however, that water, as a polar molecule, diffuses poorly through the phospholipid bilayer. To facilitate water’s diffusion, cells provide special channels called aquaporins. These transmembrane proteins are composed of numerous alpha-helices, with a core of hydrophilic amino acid residues that allow water molecules to stream through at a much higher rate compared to water’s ability to diffuse through the lipid bilayer.
3. Active Transport
Everything discussed so far can be classified as passive transport. The cell provides a pathway — either through the phospholipid bilayer or through a membrane channel — and the molecules flow down their concentration gradients and enter or leave the cell.
Cells, at times, need to move substances from lower concentration to higher concentration. Unlike diffusion, which occurs spontaneously, this requires the cell to expend energy as it pumps materials up a concentration gradient.
This type of energy-requiring movement of materials from lower to higher concentration is called active transport.
In diagrams like the one above, two clues tell you that it’s about active transport. First, follow the arrow. If the molecules are moving from where there are very few of them to where there are a lot of them, it’s active transport. Also, if you see “ATP” going into a pump or protein channel, you know that the cell is expending its energy (in the form of ATP) to do the work of pumping molecules up a concentration gradient.
4. Checking Understanding: Passive and Active Transport
[qdeck random = “true;” style=”width: 528px; border: 2px solid black; ” bold_text=”false” qrecord_id=”sciencemusicvideosMeister1961-Passive and Active Transport FC (2.0)”]
[h] Flashcards: Passive and Active Transport
[i] If you haven’t used a set of flashcards on Learn-Biology.com before, here’s what you need to know.
- Click ‘Check Answer’ to see the answer to each card.
- If you know it, click ‘Got it.”
- If you don’t know it as well as you’d like, click ‘Need more practice,’ and that card will go to the bottom of the deck so you can practice it again.
- ‘Shuffle’ lets you shuffle the deck.
[q]In the context of membranes, define what it means to be selectively permeable.
[a] Membranes allow certain molecules to pass through but not others. In other words, membranes are selectively-permeable. (note that semi-permeable means just about the same thing).
[q]What’s happening in the image below?
[a]The image shows diffusion: the movement of molecules from where they’re more concentrated to where they’re less concentrated.
[q]What’s the relationship between diffusion and concentration gradients?
[a]One way to talk about diffusion is to say the molecules in a fluid will move down a concentration gradient, moving from high concentration to low concentration.
[q]Define simple diffusion, and draw a simple sketch.
[a]Simple diffusion is when substances diffuse directly through a membrane’s phospholipid bilayer. Your sketch should look like this:
[q]Define facilitated diffusion, and draw a simple sketch.
[a]Facilitied diffusion is when substances diffuse into a cell by passing through a protein channel. A sample sketch should look like this:
[q]Define active transport, and draw a simple sketch.
[a]Active transport is when cells expend energy to move a substance from lower concentration to higher concentration.
[q]What types of molecules can pass through the membrane via simple diffusion?
[a]Simple diffusion works for small nonpolar molecules such as oxygen (O2) and carbon dioxide (CO2), which can diffuse directly through the phospholipid bilayer. So can larger nonpolar molecules such as lipids.
[q]What types of molecules can only pass through the membrane via facilitated diffusion (and why).
[a]The nonpolar interior of the phospholipid bilayer is impermeable to charged or polar molecules. For these molecules to diffuse into or out of the cell, a protein channel is needed. Examples of such molecules include amino acids, mono- and disaccharides, ions, and larger molecules (such as proteins).
[q]How does water diffuse into and out of cells.
[a]Because water is polar, it can only diffuse through the phospholipid portion of the bilayer at a very slow rate. To facilitate water’s diffusion, cells provide special channels called aquaporins.
[q]Compare and contrast passive and active transport.
[a]In passive transport, the cell doesn’t have to expend any of its own energy. Instead, molecules move down their concentration gradient and diffuse into or out of the cell. In active transport, the cell has to use its own energy, usually in the form of ATP, to pump molecules up a gradient from lower to higher concentration.
[q]Describe what this diagram is saying in relation to facilitated diffusion.
[a]This diagram is saying that not only can the cell use membrane channels to determine what can enter the cell, but also when something can enter the cell. In the diagram, the membrane channel (“c”) is closed until a signal binds with a receptor that’s attached to the channel. Once binding occurs, the channel opens, allowing some needed molecule to diffuse in.
[x]
If you want more practice, please press the restart button below. Otherwise, follow the links below.
[restart]
[/qdeck]
5. Endocytosis and Exocytosis
All of the transport we’ve discussed so far has been on the molecular level. In what follows, we’ll look at ways in which the cell can use its membrane to move larger particles and greater volumes of substances in and out.
In endocytosis, the membrane pinches in. This creates a pocket that surrounds material outside the membrane. Like active transport, this requires cell energy.
There are three types of endocytosis:
| Pinocytosis | Phagocytosis | Receptor-Mediated Endocytosis |
 |
 |
 |
| In pinocytosis, the membrane pinches in (1). The pinching in continues (2) until a vesicle forms (3), surrounding some of the extracellular fluid and whatever was inside it. Think of pinocytosis as the cell taking a small “sip” of the material outside the membrane. | During phagocytosis, the cell uses its membrane to surround a particle (or even another cell). The membrane pinches in (1) to form a vesicle that enters the cytoplasm (2). Phagocytosis is used by white blood cells in the immune response to swallow invaders. Single-celled organisms like amoebas use phagocytosis to eat. | In receptor-mediated endocytosis, a piece of the membrane pinches in response to some molecule that binds with a receptor embedded in the membrane. |
| Images from www.boundless.com. Used under Creative Commons 4.0 license | ||
Exocytosis is the opposite of endocytosis. A vesicle filled with a molecule that the cell needs to release (1) fuses with the cell membrane (2). As the membrane of the vesicle fuses with the membrane of the cell, the vesicle’s contents are released into the fluid outside the cell.
Exocytosis is how cells, such as B cells (a type of white blood cell), export the proteins they’ve manufactured. For more detail about the cellular machinery involved, take a look at our tutorial on the endomembrane system.
Two of the processes described above have played important roles in the COVID-19 Pandemic.
1. During a SARS-CoV-2 infection, the virus’s spike protein (at “A”) binds with a membrane-bound enzyme on the surface of the cells that line our respiratory tissues (in the nose and lungs), as well as other tissues throughout the body. The enzyme is at “B,” and it’s called ACE.
When the spike protein binds with ACE, the enzyme acts as a receptor. The binding initiates the epithelial cells to start receptor-mediated endocytosis. That brings the virus into the cell, after which it uses the cell’s metabolic machinery to create more viruses. That causes the virus to spread throughout the body, and to other people.
Why would our cells have a receptor for a harmful viral protein? The answer is that our cells have been tricked. The virus, through natural selection, evolved to have a spike protein with a shape that binds with ACE, and tricks our cells into inviting the virus in. Once that mutation evolved, the pandemic took off.
2. The vaccines created by Pfizer, Moderna, and other companies teach the immune system to create antibodies that can neutralize the virus by binding to its outer surface (as shown on the right) This reduces the virus’s ability to infect cells, which is why infections in people who have been vaccinated are far milder (resulting in fewer deaths and hospitalizations) than infections in people who have not been vaccinated. In the diagram called “Exocytosis in action” above, you can see what the B cells of a vaccinated person can do: release antibodies targeted toward fighting off the virus.
6. Endocytosis and Exocytosis Flashcards
[qdeck bold_text=”false” qrecord_id=”sciencemusicvideosMeister1961-Endocytosis/Exocytosis FC (2.0)”]
[h]Endocytosis and exocytosis
[q]Describe exocytosis, and draw a simple sketch representing the process.
[a]Exocytosis occurs when a vesicle filled with some substance fuses with the membrane, releasing that substance outside the cell.
[q]Descrbe endocytosis.
[a]During endocytosis, the cell membrane pinches in, taking in something outside of the cell and enclosing it within a vesicle.
[q]Describe pinocytosis, and draw a sketch representing the process.
[a]Pinocytosis is a kind of endocytosis in which a small portion of the membrane pinches in, allowing the cell to “take a sip” of the fluid outside of the cell.
[q]Describe phagocytosis.
[a]Phagocytosis is a type of endocytosis in which a large portion of the membrane surrounds a food particle (or even another cell), and then brings that particle into the cell.
[q]Describe receptor-mediated endocytosis.
[a]In receptor-mediated endocytosis, the membrane only pinches in when a molecule outside the cell binds with a receptor molecule in the membrane
[/qdeck]
7. Quiz: Cell Membrane Transport
[qwiz random = “false;” style = “border: 3px solid black; ” qrecord_id=”sciencemusicvideosMeister1961-Cell Membrane Transport Quiz”]
[h] Quiz: Cell Membrane Transport
[i]This activity tests your understanding of membranes and transport. Here’s how the quiz works:
- Each question is multiple-choice, but the entire quiz is like a series of flashcards.
- If you get the question right, it comes off the deck.
- If you get the question wrong, it goes to the bottom of the deck, so you can try it again.
[q labels = “top”]
[l]lower concentration
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]higher concentration
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[q labels = “top”]
[l]ink molecules in high concentration
[fx] No. Please try again.
[f*] Excellent!
[l]ink molecules spread out
[fx] No, that’s not correct. Please try again.
[f*] Great!
[q labels = “top”]
| _____________________ | ___________________ | _____________________ |
|
|
|
|
[l]simple diffusion
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]active transport
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]facilitated diffusion
[fx] No. Please try again.
[f*] Great!
[q labels = “top”]
[l]active transport
[fx] No. Please try again.
[f*] Great!
[l]simple diffusion
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]facilitated diffusion
[fx] No. Please try again.
[f*] Excellent!
[q labels = “top”]
| ____________________________ | _________________________ |
[l]exocytosis
[fx] No. Please try again.
[f*] Good!
[l]endocytosis
[fx] No. Please try again.
[f*] Great!
[q labels = “top”]
[l]pinocytosis
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]phagocytosis
[fx] No. Please try again.
[f*] Great!
[l]receptor-mediated endocytosis
[fx] No. Please try again.
[f*] Good!
[q labels = “top”]
[l]molecules released from the cell
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]vesicle
[fx] No. Please try again.
[f*] Great!
[l]molecules inside the vesicle
[fx] No, that’s not correct. Please try again.
[f*] Great!
[q] When a cube of sugar is placed in a beaker of water, sugar molecules spread through the beaker by
[c]ZW5kb2N5dG9zaXM=[Qq]
[c]ZGlmZn VzaW9u[Qq]
[c]ZmFjaWxpdGF0ZWQgZGlmZnVzaW9u[Qq]
[c]YWN0aXZlIHRyYW5zcG9ydA==[Qq]
[f]Tm8uIEVuZG9jeXRvc2lzIGludm9sdmVzIGEgbWVtYnJhbmUgcGluY2hpbmcgaW4gdG8gYnJpbmcgb3V0c2lkZSBtYXRlcmlhbCBpbnNpZGUgdGhlIGNlbGwuIFRoZSBzcHJlYWRpbmcgb3V0IG9mIHRoZXNlIHN1Z2FyIG1vbGVjdWxlcyBpcyBhbiBleGFtcGxlIG9mIHN1YnN0YW5jZXMgbW92aW5nIGZyb20gaGlnaGVyIGNvbmNlbnRyYXRpb24gdG8gbG93ZXIgY29uY2VudHJhdGlvbi4gV2hhdCYjODIxNztzIHRoYXQga2luZCBvZiBtb3ZlbWVudCBjYWxsZWQ/[Qq]
[f]RXhjZWxsZW50LiBUaGlzIGlzIGFuIGV4YW1wbGUgb2YgZGlmZnVzaW9uLg==[Qq]
[f]Tm8uwqBGYWNpbGl0YXRlZCBkaWZmdXNpb24gaW52b2x2ZXMgbW9sZWN1bGVzIG1vdmluZyBhY3Jvc3MgYSBtZW1icmFuZSB0aHJvdWdoIGEgcHJvdGVpbiBjaGFubmVsLiBEcm9wIHRoZSAmIzgyMjA7ZmFjaWxpdGF0ZWQmIzgyMjE7IHBhcnQsIGFuZCB5b3UmIzgyMTc7bGwgaGF2ZSB0aGUgYW5zd2VyLg==[Qq]
[f]Tm8uIEFjdGl2ZSB0cmFuc3BvcnQgaW52b2x2ZXMgY2VsbHMgdXNpbmcgZW5lcmd5IHRvIHB1bXAgbWF0ZXJpYWxzIGZyb20gbG93ZXIgdG8gaGlnaGVyIGNvbmNlbnRyYXRpb24uIFRoZSBzcHJlYWRpbmcgb3V0IG9mIHRoZXNlIHN1Z2FyIG1vbGVjdWxlcyBpcyBhbiBleGFtcGxlIG9mIHN1YnN0YW5jZXMgbW92aW5nIGZyb20gaGlnaGVyIGNvbmNlbnRyYXRpb24gdG8gbG93ZXIgY29uY2VudHJhdGlvbi4gV2hhdCYjODIxNztzIHRoYXQga2luZCBvZiBtb3ZlbWVudCBjYWxsZWQ/
Cg==[Qq]
[q] Diffusion is a term for the movement of molecules from
[c]IGFuIGFyZWEgb2YgbG93IGNvbmNlbnRyYXRpb24gdG8gYW4gYXJlYSBvZiBoaWdoIGNvbmNlbnRyYXRpb24u[Qq]
[c]IG91dHNpZGUgb2YgdGhlIGNlbGwgdG8gaW5zaWRlIHRoZSBjZWxs[Qq]
[c]IGFuIGFyZWEgb2YgaGlnaCBjb25jZW50cmF0aW9uIH RvIGFuIGFyZWEgb2YgbG93IGNvbmNlbnRyYXRpb24u[Qq]
[c]IGEgbnVjbGV1cyB0byB0aGUgbWl0b2Nob25kcmlhLg==[Qq]
[f]Tm8uIExvb2sgYXQgdGhpcyBkaWFncmFtLCBhbmQgc2VlIGlmIHlvdSBjYW4gY29tZSB1cCB3aXRoIHRoZSByaWdodCBleHBsYW5hdGlvbi4=
Cg==[f]Tm90IG5lY2Vzc2FyaWx5LiBEaWZmdXNpb24gbWlnaHQgbW92ZSBtYXRlcmlhbHMgaW50byBhIGNlbGwsIGJ1dCBpdCBjYW4gYWxzbyBtb3ZlIG1hdGVyaWFscyBvdXQgb2YgYSBjZWxsLiBMb29rIGF0IHRoaXMgaW1hZ2UgYW5kIHNlZSBpZiB5b3UgY2FuIGZpZ3VyZSBvdXQgdGhlIHJpZ2h0IGFuc3dlci4=
Cg==[f]RXhhY3RseS4gRGlmZnVzaW9uIGlzIHRoZSBtb3ZlbWVudCBvZiBhIHN1YnN0YW5jZSBmcm9tIGhpZ2ggdG8gbG93IGNvbmNlbnRyYXRpb24u[Qq]
[f]Tm8uIERpZmZ1c2lvbiBpcyBub3QgYWJvdXQgbW92aW5nIGZyb20gb25lIHNwZWNpZmljIHBsYWNlIHRvIGFub3RoZXIuIEl0JiM4MjE3O3MgbXVjaCBtb3JlIGdlbmVyYWwuIExvb2sgYXQgdGhpcyBpbWFnZSBhbmQgc2VlIGlmIHlvdSBjYW4gZmlndXJlIG91dCB0aGUgcmlnaHQgYW5zd2VyLg==
Cg==[q] Diffusion is about the random movement of
[c]IGNlbGxz[Qq]
[c]IG9yZ2FuaXNtcw==[Qq]
[c]IGNlbGwgbWVtYnJhbmVz[Qq]
[c]IG1vbGVj dWxlcw==[Qq]
[f]Tm8uIERpZmZ1c2lvbiBpcyBhYm91dCByYW5kb20gbW92ZW1lbnQgb24gYSB2ZXJ5IHNtYWxsIGxldmVsLiBOZXh0IHRpbWUsIGNob29zZSBhIGxldmVsIG11Y2ggc21hbGxlciB0aGFuIGNlbGxzLg==[Qq]
[f]Tm8uIERpZmZ1c2lvbiBpcyBhYm91dCByYW5kb20gbW92ZW1lbnQgb24gYSB2ZXJ5IHNtYWxsIGxldmVsLiBOZXh0IHRpbWUsIGNob29zZSBhIGxldmVsIG11Y2ggc21hbGxlciB0aGFuIG9yZ2FuaXNtcy4=[Qq]
[f]Tm8uIERpZmZ1c2lvbiBpcyBhYm91dCByYW5kb20gbW92ZW1lbnQgb24gYSB2ZXJ5IHNtYWxsIGxldmVsLiBOZXh0IHRpbWUsIGNob29zZSBhIGxldmVsIG11Y2ggc21hbGxlciB0aGFuIGV2ZW4gYSBjZWxsIG1lbWJyYW5lLg==[Qq]
[f]WWVzLiBEaWZmdXNpb24gaXMgYWJvdXQgdGhlIHJhbmRvbSBtb3ZlbWVudCBvZiBtb2xlY3VsZXMu
Cg==[Qq]
[q] For diffusion to occur, there must be:
[c]IGEgbWVtYnJhbmU=[Qq]
[c]IGEgZ3Jh ZGllbnQ=[Qq]
[c]IHdhdGVy[Qq]
[c]IEFUUA==[Qq]
[f]Tm8uIERpZmZ1c2lvbiBkb2VzIG5vdCByZXF1aXJlIGEgbWVtYnJhbmUuIEl0IGNhbiBvY2N1ciBpbiBhIGN1cCBvZiB3YXRlciBvciBhIHJvb20uIEluIHRlcm1zIG9mIHRoZSBtb2xlY3VsZXMgdGhhdCBhcmUgZGlmZnVzaW5nLCB3aGF0IGRvZXMgZGlmZnVzaW9uIHJlcXVpcmU/[Qq]
[f]WWVzLiBEaWZmdXNpb24gcmVxdWlyZXMgYSBncmFkaWVudCYjODIxMTthIGRpZmZlcmVuY2UgaW4gY29uY2VudHJhdGlvbiBsZXZlbHMu[Qq]
[f]Tm8uIERpZmZ1c2lvbiBjYW4gb2NjdXIgaW4gd2F0ZXIsIGFuZCB3YXRlciBjYW4gZGlmZnVzZSwgYnV0IHdhdGVyIGlzIG5vdCA=cmVxdWlyZWQ=IGZvciBkaWZmdXNpb24uwqBJbiB0ZXJtcyBvZiB0aGUgbW9sZWN1bGVzIHRoYXQgYXJlIGRpZmZ1c2luZywgd2hhdCBkb2VzIGRpZmZ1c2lvbiByZXF1aXJlPw==[Qq]
[f]Tm8uIEFUUCBpcyB0aGUgY2VsbCYjODIxNztzIGltbWVkaWF0ZSBlbmVyZ3kgc291cmNlLCBhbmQgY2VsbCBlbmVyZ3kgaXMgbm90IHJlcXVpcmVkIGZvciBkaWZmdXNpb24uIEluIHRlcm1zIG9mIHRoaXMgcXVlc3Rpb24sIHdoYXQgZG9lcw==IGRpZmZ1c2lvbiByZXF1aXJlPw==
Cg==[Qq]
[q] Cell membrane channels are made of
[c]IHBob3NwaG9saXBpZHMu[Qq]
[c]IGNhcmJvaHlkcmF0ZXMu[Qq]
[c]IEROQS4=[Qq]
[c]IFJOQS4=[Qq]
[c]IHByb3Rl aW5zLg==[Qq]
[f]Tm8uIFBob3NwaG9saXBpZHMgbWFrZSB1cCB0aGUgYmlsYXllciwgd2hpY2ggaXMgdGhlIGtleSBzdHJ1Y3R1cmFsIHBhcnQgb2YgdGhlIG1lbWJyYW5lLiBUaGUgY2hhbm5lbHMgYXJlIG1hZGUgb2Ygc29tZXRoaW5nIGVsc2Uu[Qq]
[f]Tm8uIENhcmJvaHlkcmF0ZXMgYXJlIHBvc2l0aW9uZWQgb24gdGhlIG91dGVyIHN1cmZhY2Ugb2YgdGhlIG1lbWJyYW5lLCBwcm92aWRpbmcgZm9yIGNlbGwgcmVjb2duaXRpb24uIFRoZSBjaGFubmVscyBhcmUgbWFkZSBvZiBzb21ldGhpbmcgZWxzZS4=[Qq]
[f]Tm8uIEROQSBpcyBub3QgcGFydCBvZiB0aGUgbWVtYnJhbmUuIFdoYXQgY291bGQgdGhlc2UgY2hhbm5lbHMgYmUgbWFkZSBvZj8=[Qq]
[f]Tm8uIFJOQSBpcyBub3QgcGFydCBvZiB0aGUgbWVtYnJhbmUuIFdoYXQgY291bGQgdGhlc2UgY2hhbm5lbHMgYmUgbWFkZSBvZj8=[Qq]
[f]RXhjZWxsZW50ISBNZW1icmFuZSBjaGFubmVscyBhcmUgbWFkZSBvZiBwcm90ZWlu
Cg==[Qq]
[q] The cell membrane lets only some things in and out. The best way to describe that is to say that the membrane is
[c]IHNlbWktcG VybWVhYmxl[Qq]
[c]IGltcGVuZXRyYWJsZS4=[Qq]
[c]IHBlcm1lYWJsZQ==[Qq]
[c]IGltcGVybWVhYmxl[Qq]
[f]IFRoYXTigJlzIHJpZ2h0LiBNZW1icmFuZXMgbGV0IG1hdGVyaWFscyBwYXNzIGluIGFuZCBvdXQgb2YgdGhlIGNlbGwsIGJ1dCB0aGV5IGRvIHNvIGluIGEgdmVyeSBzZWxlY3RpdmUgd2F5LiBUaGlzIG1ha2VzIHRoZSBtZW1icmFuZSBzZW1pLXBlcm1lYWJsZS4gTm90ZSB0aGF0ICYjODIyMDtzZWxlY3RpdmVseSBwZXJtZWFibGUmIzgyMjE7IG1lYW5zIGp1c3QgYWJvdXQgdGhlIHNhbWUgdGhpbmcu[Qq]
[f]IE5vLiDigJhJbXBlbmV0cmFibGXigJkgbWVhbnMgdGhhdCBub3RoaW5nIGNhbiBwYXNzIHRocm91Z2guIElmIG1lbWJyYW5lcyB3ZXJlIGltcGVuZXRyYWJsZSwgdGhleSB3b3VsZG7igJl0IGJlIGFibGUgdG8gbGV0IGZvb2QgYW5kIG5lZWRlZCBzdWJzdGFuY2VzIGludG8gdGhlIGNlbGwsIGFuZCBsZXQgd2FzdGVzIG91dC4gTmV4dCB0aW1lLCBzZWxlY3QgYSBjaG9pY2UgdGhhdCByZWZsZWN0cyB0aGUgbWVtYnJhbmXigJlzIGFiaWxpdHkgdG8gc2VsZWN0IHdoYXQgc3Vic3RhbmNlcyB3aWxsIGVudGVyIHRoZSBjZWxsLCBhbmQgd2hhdCBzdWJzdGFuY2VzIHdpbGwgbGVhdmUu[Qq]
[f]IE5vLiBNZW1icmFuZXMgYXJlIHBlcm1lYWJsZSB0byBzb21lIHN1YnN0YW5jZXMsIGJ1dCBub3QgdG8gZXZlcnl0aGluZy4gSWYgbWVtYnJhbmVzIHdlcmUgZnJlZWx5IHBlcm1lYWJsZSwgdGhleSB3b3VsZG7igJl0IGJlIGFibGUgdG8ga2VlcCBuZWVkZWQgc3Vic3RhbmNlcyBpbiB0aGUgY2VsbCwgbGV0IHVud2FudGVkIHdhc3RlcyBvdXQsIGFuZCBrZWVwIHVud2FudGVkIHN1YnN0YW5jZXMgZnJvbSBlbnRlcmluZy4gTmV4dCB0aW1lLCBzZWxlY3QgYSBjaG9pY2UgdGhhdCByZWZsZWN0cyB0aGUgbWVtYnJhbmXigJlzIGFiaWxpdHkgdG8gc2VsZWN0IHdoYXQgc3Vic3RhbmNlcyB3aWxsIGVudGVyIHRoZSBjZWxsLCBhbmQgd2hhdCBzdWJzdGFuY2VzIHdpbGwgbGVhdmUu[Qq]
[f]IE5vLiAmIzgyMjA7SW1wZXJtZWFibGUmIzgyMjE7IG1lYW5zIHRoYXQgbm90aGluZyBjYW4gcGFzcyB0aHJvdWdoLiBJZiBtZW1icmFuZXMgd2VyZSBpbXBlcm1lYWJsZSwgdGhleSB3b3VsZG7igJl0IGJlIGFibGUgdG8gbGV0IGZvb2QgYW5kIG5lZWRlZCBzdWJzdGFuY2VzIGludG8gdGhlIGNlbGwsIGFuZCBsZXQgd2FzdGVzIG91dC4gTmV4dCB0aW1lLCBzZWxlY3QgYSBjaG9pY2UgdGhhdCByZWZsZWN0cyB0aGUgbWVtYnJhbmXigJlzIGFiaWxpdHkgdG8gc2VsZWN0IHdoYXQgc3Vic3RhbmNlcyB3aWxsIGVudGVyIHRoZSBjZWxsLCBhbmQgd2hhdCBzdWJzdGFuY2VzIHdpbGwgbGVhdmUu
Cg==[Qq]
[q]When molecules diffuse, they flow
[c]dXAgdGhlaXIgY29uY2VudHJhdGlvbiBncmFkaWVudA==[Qq]
[c]ZG93biB0aGVpciBjb25jZW 50cmF0aW9uIGdyYWRpZW50[Qq]
[f]Tm8uIEZsb3dpbmcgdXAgYSBjb25jZW50cmF0aW9uIGdyYWRpZW50IGlzIHdoYXQgaGFwcGVucyBpbiBhY3RpdmUgdHJhbnNwb3J0Lg==[Qq]
[f]WWVzLiBEdXJpbmcgZGlmZnVzaW9uLCBtb2xlY3VsZXMgZmxvdyBkb3duIHRoZWlyIGNvbmNlbnRyYXRpb24gZ3JhZGllbnQu
Cg==[Qq]
[q] True or false: In active transport, a cell uses the kinetic energy in molecules and lets them flow down their concentration gradient.
[c]IFRSVUU=[Qq]
[c]IEZB TFNF[Qq]
[f]Tm8uIEluIGFjdGl2ZSB0cmFuc3BvcnQsIGEgY2VsbCBpcyAmIzgyMjA7ZmlnaHRpbmcmIzgyMjE7IGRpZmZ1c2lvbiwgd2hpY2ggaXMgZHJpdmVuIGJ5IG1vbGVjdWxlcyYjODIxNzsga2luZXRpYyBlbmVyZ3kuIMKgV2hhdCB0aGF0IG1lYW5zIGlzIHRoYXQgY2VsbHMgbmVlZCB0byB1c2UgdGhlaXIgb3duIGVuZXJneSB0byBtb3ZlIGEgc3Vic3RhbmNlIHVwIGEgY29uY2VudHJhdGlvbiBncmFkaWVudC4=[Qq]
[f]Q29ycmVjdC4gVGhpcyBzdGF0ZW1lbnQgaXMgZmFsc2UgYmVjYXVzZSwgaW4gYWN0aXZlIHRyYW5zcG9ydCwgdGhlIGNlbGwgaGFzIHRvIHVzZSBpdHMgb3duIGVuZXJneS4=
Cg==[Qq]
[q] If you were a molecule, then facilitated diffusion would be most like
[c]IEJlaW5nIGNhcnJpZWQgYWNyb3NzIGEgc3RyZWFtLg==[Qq]
[c]IFdhbGtpbmcgYWNyb3NzIGEgcm9vbS4=[Qq]
[c]IENvYXN0aW5nIGFjcm9zcyBhIG JyaWRnZSBvbiB5b3VyIGJpa2Uu[Qq]
[c]IFJpZGluZyBhIHNraSBsaWZ0Lg==[Qq]
[f]Tm8uIEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgdGhlIGNlbGwgbGV0cyBhIG1vbGVjdWxlIHBhc3MgdGhyb3VnaCBpdHMgbWVtYnJhbmUgYnkgcHJvdmlkaW5nIGEgY2hhbm5lbC4gRnJvbSBhIG1vbGVjdWxlJiM4MjE3O3MgcGVyc3BlY3RpdmUsIGJlaW5nIGNhcnJpZWQgYWNyb3NzIGEgc3RyZWFtIHNlZW1zIG1vcmUgbGlrZSBhY3RpdmUgdHJhbnNwb3J0LCBzaW5jZSB0aGUgY2FycmllciBoYXMgdG8gZXhwZW5kIGVuZXJneS4gV2hpY2ggY2hvaWNlIGludm9sdmVzIGEgcGFzc2FnZXdheSB0aGF0JiM4MjE3O3MgZmFjaWxpdGF0aW5nIHlvdXIgY3Jvc3Npbmc/[Qq]
[f]Tm8uIEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgdGhlIGNlbGwgbGV0cyBhIG1vbGVjdWxlIHBhc3MgdGhyb3VnaCBpdHMgbWVtYnJhbmUgYnkgcHJvdmlkaW5nIGEgY2hhbm5lbC4gRnJvbSBhIG1vbGVjdWxlJiM4MjE3O3MgcGVyc3BlY3RpdmUsIHdhbGtpbmcgYWNyb3NzIGEgcm9vbSBzZWVtcyBsaWtlIGRpZmZ1c2lvbiAobW92aW5nKSBidXQgbm90IGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiAobW92aW5nIGFjcm9zcyBhIGJhcnJpZXIpLsKgV2hpY2ggY2hvaWNlIGludm9sdmVzIGEgcGFzc2FnZXdheSB0aGF0JiM4MjE3O3MgZmFjaWxpdGF0aW5nIHlvdXIgY3Jvc3Npbmc/[Qq]
[f]WWVzISBJbiBmYWNpbGl0YXRlZCBkaWZmdXNpb24sIHRoZSBjZWxsIGxldHMgYSBtb2xlY3VsZSBwYXNzIHRocm91Z2ggaXRzIG1lbWJyYW5lIGJ5IHByb3ZpZGluZyBhIGNoYW5uZWwuIElmIHlvdSBjb2FzdCBhY3Jvc3MgYSBicmlkZ2Ugb24geW91ciBiaWtlLCB5b3UmIzgyMTc7cmUgdXNpbmcgeW91ciBraW5ldGljIGVuZXJneSB0byBjcm9zcyBhIGJhcnJpZXIsIHVzaW5nIGEgcGFzc2FnZXdheSAodGhlIGJyaWRnZSkgdGhhdCBmYWNpbGl0YXRlcyB5b3VyIGNyb3NzaW5nLg==[Qq]
[f]Tm8uIEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgdGhlIGNlbGwgbGV0cyBhIG1vbGVjdWxlIHBhc3MgdGhyb3VnaCBpdHMgbWVtYnJhbmUgYnkgcHJvdmlkaW5nIGEgY2hhbm5lbC4gRnJvbSBhIG1vbGVjdWxlJiM4MjE3O3MgcGVyc3BlY3RpdmUsIHJpZGluZyBhIHNraSBsaWZ0wqBzZWVtcyBtb3JlIGxpa2UgYWN0aXZlIHRyYW5zcG9ydCwgc2luY2UgdGhlIGNhcnJpZXIgKHRoZSBza2kgbGlmdCkgaGFzIHRvIGV4cGVuZCBlbmVyZ3kuIFdoaWNoIGNob2ljZSBpbnZvbHZlcyBhIHBhc3NhZ2V3YXkgdGhhdCYjODIxNztzIGZhY2lsaXRhdGluZyB5b3VyIGNyb3NzaW5nPw==
Cg==[Qq]
[q] Molecules that diffuse through a cell membrane by facilitated diffusion:
[c]IFJlcXVpcmUgYW4gZXhwZW5kaXR1cmUgb2YgY2VsbCBlbmVyZ3k=[Qq]
[c]IE1vdmUgZnJvbSBhbiBhcmVhIG9mIGxvdyBjb25jZW50cmF0aW9uIHRvIGFuIGFyZWEgb2YgaGlnaCBjb25jZW50cmF0aW9u[Qq]
[c]IENhbiBmbG93IGRpcmVjdGx5IHRocm91Z2ggdGhlIGNlbGwmIzgyMTc7cyBwaG9zcGhvbGlwaWQgYmlsYXllci4=[Qq]
[c]IFJlcXVpcmUgdGhlIGFpZCBvZi B0cmFuc3BvcnQgcHJvdGVpbnM=[Qq]
[f]Tm8uIExpa2UgYWxsIGZvcm1zIG9mIGRpZmZ1c2lvbiwgZmFjaWxpdGF0ZWQgZGlmZnVzaW9uIGludm9sdmVzwqB0aGUga2luZXRpYyBlbmVyZ3kgaW4gdGhlIGRpZmZ1c2luZyBtb2xlY3VsZXMuIE5vIGV4dHJhIGVuZXJneSBleHBlbmRpdHVyZSBieSB0aGUgY2VsbCBpcyBpbnZvbHZlZC4=[Qq]
[f]Tm8uIExpa2UgYWxsIGZvcm1zIG9mIGRpZmZ1c2lvbiwgZmFjaWxpdGF0ZWQgZGlmZnVzaW9uIGludm9sdmVzIHRoZSBtb3ZlbWVudCBvZiBtYXRlcmlhbHMgZnJvbSBoaWdoZXIgdG8gbG93ZXIgY29uY2VudHJhdGlvbi4=[Qq]
[f]Tm8uIFRvIA==ZmFjaWxpdGF0ZQ==IG1lYW5zIHRvIGFzc2lzdA==LiBUaGUgcG9pbnQgb2YgZmFjaWxpdGF0ZWQgZGlmZnVzaW9uIGlzIHRoYXQgaXQgaW52b2x2ZXMgbW9sZWN1bGVzIHRoYXQgY2FuJiM4MjE3O3QgZGlmZnVzZSB0aHJvdWdoIHRoZSBwaG9zcGhvbGlwaWQgYmlsYXllciwgYW5kIG5lZWQgc29tZSBraW5kIG9mIGFzc2lzdGFuY2UgdG8gbWFrZSB0aGVpciB3YXkgdGhyb3VnaC4=[Qq]
[f]RmFidWxvdXMhIE1vbGVjdWxlcyB0aGF0IHBhc3MgdGhyb3VnaCB0aGUgbWVtYnJhbmUgYnkgZmFjaWxpdGF0ZWQgZGlmZnVzaW9uIGFyZSBkb2luZyBzbyB3aXRoIHRoZSBhc3Npc3RhbmNlIG9mIGEgcHJvdGVpbiBjaGFubmVsIG9yIHRyYW5zcG9ydCBwcm90ZWluIHRoYXQgZmFjaWxpdGF0ZXMgdGhlaXIgcGFzc2FnZS4=
Cg==[Qq]
[q] A molecule that can diffuse freely through a phospholipid bilayer is probably
[c]wqBwb2xhcg==[Qq]
[c]IFBvc2l0aXZlbHkgY2hhcmdlZA==[Qq]
[c]IG5vbn BvbGFy[Qq]
[c]wqBhIGxhcmdlIHByb3RlaW4=[Qq]
[f]Tm8uIFBvbGFyIG1vbGVjdWxlcyBjYW4mIzgyMTc7dCBwYXNzIHRocm91Z2ggdGhlIGh5ZHJvcGhvYmljLCBub25wb2xhciBwYXJ0IG9mIHRoZSBwaG9zcGhvbGlwaWQgYmlsYXllci4gSnVzdCByZW1lbWJlciB0aGF0IG9pbCAobm9ucG9sYXIpIGFuZCB3YXRlciAocG9sYXIpIGRvbiYjODIxNzt0IG1peC4=[Qq]
[f]Tm8uIENoYXJnZWQgbW9sZWN1bGVzIGNhbiYjODIxNzt0IHBhc3MgdGhyb3VnaCB0aGUgaHlkcm9waG9iaWMsIG5vbnBvbGFyIHBhcnQgb2YgdGhlIHBob3NwaG9saXBpZCBiaWxheWVyLg==[Qq]
[f]WWVzLiBOb25wb2xhciBtb2xlY3VsZXMgKGZhdHMsIG9pbHMsIHN0ZXJvaWQgaG9ybW9uZXMpIGNhbiBmcmVlbHkgcGFzcyB0aHJvdWdoIGEgcGhvc3Bob2xpcGlkIGJpbGF5ZXIu[Qq]
[f]Tm8uIExhcmdlIHByb3RlaW5zIGNhbiYjODIxNzt0IHBhc3MgZGlyZWN0bHkgdGhyb3VnaCBhIHBob3NwaG9saXBpZCBiaWxheWVyLiBJZiB0aGV5IGVudGVyZWQgYSBjZWxsIGF0IGFsbCwgaXQgd291bGQgaGF2ZSB0byBiZSB0aHJvdWdoIGEgdHJhbnNwb3J0IHByb3RlaW4u
Cg==[Qq]
[q] Which of the following best describes what occurs during facilitated diffusion?
[c]IEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgc3Vic3RhbmNlcyBkaWZmdXNlIHRocm91Z2ggdGhlIHBob3NwaG9saXBpZCBiaWxheWVyIGFzIHRoZXkgZW50ZXIgb3IgbGVhdmUgdGhlIGNlbGwuIE5vIGNlbGwgZW5lcmd5IGlzIHJlcXVpcmVkLg==[Qq]
[c]IEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgdGhlIGNlbGwgdXNlcyBpdHMgb3duIGVuZXJneSB0byBwdW1wIHN1YnN0YW5jZXMgZnJvbSBsb3dlciBjb25jZW50cmF0aW9uIHRvIGhpZ2hlciBjb25jZW50cmF0aW9uLg==[Qq]
[c]IEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgbW9sZWN1bGVzIGRpZmZ1c2UgaW50byB0aGUgY2Vs bCB0aHJvdWdoIHByb3RlaW4gY2hhbm5lbHMuIE5vIGNlbGwgZW5lcmd5IGlzIHJlcXVpcmVkLg==[Qq]
[c]IEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgdGhlIGNlbGwgdXNlcyBpdHMgb3duIGVuZXJneSB0byBwdW1wIG1hdGVyaWFscyB0aHJvdWdoIHRoZSBwaG9zcGhvbGlwaWQgYmlsYXllci4=[Qq]
[f]IE5vLiBJbiBmYWNpbGl0YXRlZCBkaWZmdXNpb24sIHRoZSBjZWxsIHByb3ZpZGVzIHByb3RlaW4gY2hhbm5lbHMgb3IgY2FycmllciBwcm90ZWlucyB0byBhbGxvdyBzdWJzdGFuY2VzIHRvIGRpZmZ1c2UgaW50byBvciBvdXQgb2YgdGhlIGNlbGwuIE5vIGVuZXJneSBpcyByZXF1aXJlZCBvbiB0aGUgY2VsbOKAmXMgcGFydC4=[Qq]
[f]IE5vLiBJbiBmYWNpbGl0YXRlZCBkaWZmdXNpb24sIHRoZSBjZWxsIHByb3ZpZGVzIHByb3RlaW4gY2hhbm5lbHMgb3IgdXNlcyBjYXJyaWVyIHByb3RlaW5zIHRvIGFsbG93IHN1YnN0YW5jZXMgdG8gZGlmZnVzZSBpbnRvIG9yIG91dCBvZiB0aGUgY2VsbC4gTm8gZW5lcmd5IGlzIHJlcXVpcmVkIG9uIHRoZSBjZWxs4oCZcyBwYXJ0LiBJbnN0ZWFkLCB0aGUgZW5lcmd5IGlzIHByb3ZpZGVkIGJ5IHRoZSBraW5ldGljIGVuZXJneSBvZiB0aGUgZGlmZnVzaW5nIG1vbGVjdWxlcy4=[Qq]
[f]IEV4Y2VsbGVudCEgRmFjaWxpdGF0ZWQgZGlmZnVzaW9uIGlzIHN0aWxsIGEgZm9ybSBvZiBkaWZmdXNpb24sIGJ1dCByYXRoZXIgdGhhbiBzaW1wbHkgZGlmZnVzaW5nIHRocm91Z2ggdGhlIGxpcGlkIGJpbGF5ZXIsIHRoZSBwYXNzYWdlIG9mIG1vbGVjdWxlcyBpcyBmYWNpbGl0YXRlZCAobWFkZSBlYXNpZXIpIHRocm91Z2ggcHJvdGVpbiBjaGFubmVscyBvciBjYXJyaWVyIHByb3RlaW5zIHRoYXQgbGV0IHNwZWNpZmljIG1vbGVjdWxlcyBwYXNzIHRocm91Z2ggdGhlIG1lbWJyYW5lLg==[Qq]
[f]IE5vLiBJbiBmYWNpbGl0YXRlZCBkaWZmdXNpb24sIHRoZSBjZWxsIHByb3ZpZGVzIHByb3RlaW4gY2hhbm5lbHMgb3IgY2FycmllciBwcm90ZWlucyB0byBhbGxvdyBzdWJzdGFuY2VzIHRvIGRpZmZ1c2UgaW50byBvciBvdXQgb2YgdGhlIGNlbGwuIE5vIGVuZXJneSBpcyByZXF1aXJlZCBvbiB0aGUgY2VsbOKAmXMgcGFydC4gSW5zdGVhZCwgdGhlIGVuZXJneSBpcyBwcm92aWRlZCBieSB0aGUga2luZXRpYyBlbmVyZ3kgb2YgdGhlIGRpZmZ1c2luZyBtb2xlY3VsZXMu
Cg==[Qq]
[q]When a substance created inside the cell is exported outside the cell, it leaves the cell through
[c]ZW5kb2N5dG9zaXM=[Qq]
[c]ZXhvY3l0 b3Npcw==[Qq]
[c]cGhhZ29jeXRvc2lz[Qq]
[f]Tm8uIEVuZG9jeXRvc2lzIG1lYW5zICYjODIyMDtpbnRvIHRoZSBjZWxsLiYjODIyMTsgV2hhdCBpcyB0aGUgdGVybSBmb3IgJiM4MjIwO2xlYXZpbmcgdGhlIGNlbGw/JiM4MjIxOw==[Qq]
[f]WWVzLsKgV2hlbiBhIHN1YnN0YW5jZcKgY3JlYXRlZCBpbnNpZGUgdGhlIGNlbGwgaXMgZXhwb3J0ZWQgb3V0c2lkZSB0aGUgY2VsbCwgaXQgbGVhdmVzIHRoZSBjZWxsIHRocm91Z2ggZXhvY3l0b3Npcy4=[Qq]
[f]Tm8uIFBoYWdvY3l0b3NpcyBpbnZvbHZlcyBjZWxscyBlbmd1bGZpbmcgZm9yZWlnbiBvYmplY3RzIGFuZCBicmluZ2luZyB0aGVtIGludG8gdGhlIGNlbGwuIEhlcmUsIHlvdSYjODIxNztyZSBsb29raW5nIGZvciBhIHRlcm0gdGhhdCBtZWFucyAmIzgyMjA7bGVhdmluZyB0aGUgY2VsbC4mIzgyMjE7
Cg==[Qq]
[q] Passive transport is defined as the movement of any substance across a membrane without the use of
[c]IGNlbGwg ZW5lcmd5[Qq]
[c]wqBtZW1icmFuZSBjaGFubmVscw==[Qq]
[c]IGRpZmZ1c2lvbg==[Qq]
[f]Q29ycmVjdC7CoFBhc3NpdmUgdHJhbnNwb3J0IGlzIGRlZmluZWQgYXMgdGhlIG1vdmVtZW50IG9mIGFueSBzdWJzdGFuY2UgYWNyb3NzIGEgbWVtYnJhbmUgd2l0aG91dCB0aGUgdXNlIG9mIGNlbGwgZW5lcmd5Lg==[Qq]
[f]Tm8uIFBhc3NpdmUgdHJhbnNwb3J0IGluY2x1ZGVzIGRpZmZ1c2lvbiB0aHJvdWdoIG1lbWJyYW5lIGNoYW5uZWxzLiBDb25zaWRlciB0aGUgZmFjdCB0aGF0ICYjODIyMDtwYXNzaXZlIHRyYW5zcG9ydCYjODIyMTsgaXMgdGhlIG9wcG9zaXRlIG9mICYjODIyMDthY3RpdmUgdHJhbnNwb3J0LCYjODIyMTsgYW5kIHlvdSYjODIxNztsbCBoYXZlIHlvdXIgYW5zd2VyLg==[Qq]
[f]Tm8uIFRoZSB0ZXJtIA==cGFzc2l2ZSB0cmFuc3BvcnQ=IGhhcyBhIGxvdCBpbiBjb21tb24gd2l0aCB0aGUgdGVybSA=ZGlmZnVzaW9uLsKgQ29uc2lkZXIgdGhlIGZhY3QgdGhhdCAmIzgyMjA7cGFzc2l2ZSB0cmFuc3BvcnQmIzgyMjE7IGlzIHRoZSBvcHBvc2l0ZSBvZiAmIzgyMjA7YWN0aXZlIHRyYW5zcG9ydCwmIzgyMjE7IGFuZCB5b3UmIzgyMTc7bGwgaGF2ZSB5b3VyIGFuc3dlci4=
[Qq][q] Which of the following terms describes the process by which the cell membrane moves substances from lower concentration to higher concentration.
[c]IGZhY2lsaXRhdGVkIGRpZmZ1c2lvbg==[Qq]
[c]IG9zbW9zaXM=[Qq]
[c]IGRpZmZ1c2lvbg==[Qq]
[c]IGFjdGl2ZSB0 cmFuc3BvcnQu[Qq]
[f]IE5vLiBJbiBmYWNpbGl0YXRlZCBkaWZmdXNpb24sIG1vbGVjdWxlcyBhcmUgc3RpbGwgZGlmZnVzaW5nIGZyb20gaGlnaGVyIHRvIGxvd2VyIGNvbmNlbnRyYXRpb24uIEhvd2V2ZXIsIHJhdGhlciB0aGFuIHNpbXBseSBkaWZmdXNpbmcgdGhyb3VnaCB0aGUgbGlwaWQgYmlsYXllciwgdGhleSBwYXNzIHRocm91Z2ggYSBwcm90ZWluIGNoYW5uZWwuIE5leHQgdGltZSwgY2hvb3NlIGEgcHJvY2VzcyB0aGF0IHJlZmxlY3RzIHRoZSBuZWVkIGZvciBjZWxscyB0byBhY3RpdmVseSBwdW1wIG1hdGVyaWFscyBmcm9tIGxvd2VyIHRvIGhpZ2hlciBjb25jZW50cmF0aW9uLg==[Qq]
[f]IE5vLiBPc21vc2lzIGlzIHRoZSBkaWZmdXNpb24gb2Ygd2F0ZXIuIExpa2UgYWxsIGRpZmZ1c2lvbiBwcm9jZXNzZXMsIHdhdGVyIG1vdmVzIGZyb20gd2hlcmUgaXQgaXMgbW9yZSBjb25jZW50cmF0ZWQgdG8gd2hlcmUgaXQgaXMgbGVzcyBjb25jZW50cmF0ZWQu[Qq]
[f]IE5vLiBEaWZmdXNpb24gaXMgdGhlIG1vdmVtZW50IG9mIHN1YnN0YW5jZXMgZnJvbSBoaWdoZXIgY29uY2VudHJhdGlvbiB0byBsb3dlciBjb25jZW50cmF0aW9uLg==[Qq]
[f]IENvcnJlY3QhIEFjdGl2ZSB0cmFuc3BvcnQgaW52b2x2ZXMgY2VsbHMgaGF2aW5nIHRvIHB1bXAgbWF0ZXJpYWxzIGZyb20gbG93ZXIgdG8gaGlnaGVyIGNvbmNlbnRyYXRpb24sIGFnYWluc3QgdGhlIGRpcmVjdGlvbiBvZiBkaWZmdXNpb24u
Cg==[Qq]
[q]In nerve cells, the amount of positively charged potassium ions inside the cell is much higher than the amount outside the cell. For the cell to take up more potassium, which process is required?
[c]YWN0aXZlIHRy YW5zcG9ydA==[Qq]
[c]cGFzc2l2ZSB0cmFuc3BvcnQ=[Qq]
[c]ZmFjaWxpdGF0ZWQgZGlmZnVzaW9u[Qq]
[c]c2ltcGxlIGRpZmZ1c2lvbg==[Qq]
[f]WWVzLiBUaGUgY2VsbCBpcyBnb2luZyB0byBoYXZlIHRvIGV4cGVuZCBpdHMgb3duIGVuZXJneSB0byBtb3ZlIHRoZSBwb3Rhc3NpdW0gZnJvbSBhIGxvd2VyIGNvbmNlbnRyYXRpb24sIG91dHNpZGUgdGhlIGNlbGwsIHRvIGEgaGlnaGVyIGNvbmNlbnRyYXRpb24gaW5zaWRlIHRoZSBjZWxsLg==[Qq]
[f]Tm8uIFBhc3NpdmUgdHJhbnNwb3J0IGlzIGVzc2VudGlhbGx5IHRoZSBzYW1lIGFzIGRpZmZ1c2lvbi4gSXQgcmVzdWx0cyBpbiBhbiBlcXVhbCBjb25jZW50cmF0aW9uIG9mIHN1YnN0YW5jZXMgYWNyb3NzIG1lbWJyYW5lcy4gV2hhdCBwcm9jZXNzIHdvcmtzIGFnYWluc3QgZGlmZnVzaW9uPw==[Qq]
[f]Tm8uIEZhY2lsaXRhdGVkIGRpZmZ1c2lvbiByZXN1bHRzIGluIGFuIGVxdWFsIGNvbmNlbnRyYXRpb24gb2Ygc3Vic3RhbmNlcyBhY3Jvc3MgbWVtYnJhbmVzLiBXaGF0IHByb2Nlc3Mgd29ya3MgYWdhaW5zdCBkaWZmdXNpb24/[Qq]
[f]Tm8uIFNpbXBsZSBkaWZmdXNpb24gaXMgZXNzZW50aWFsbHkgdGhlIHNhbWUgYXMgZGlmZnVzaW9uLiBJdCByZXN1bHRzIGluIGFuIGVxdWFsIGNvbmNlbnRyYXRpb24gb2Ygc3Vic3RhbmNlcyBhY3Jvc3MgbWVtYnJhbmVzLiBXaGF0IHByb2Nlc3Mgd29ya3MgYWdhaW5zdCBkaWZmdXNpb24/
Cg==[Qq]
[q]In the diagram below, number 1 is
[c]YWN0aXZlIHRyYW5zcG9ydA==[Qq]
[c]ZmFjaWxpdGF0ZWQgZGlmZnVzaW9u[Qq]
[c]c2ltcGxlIGRp ZmZ1c2lvbg==[Qq]
[f]Tm8uwqBJbiBhY3RpdmUgdHJhbnNwb3J0LCB0aGUgY2VsbCBtb3Zlc8KgbW9sZWN1bGVzIGZyb20gbG93ZXIgdG8gaGlnaGVyIGNvbmNlbnRyYXRpb24uIEluIG51bWJlciAxLMKgwqB0aGUgbW92ZW1lbnQgaXMgZnJvbSBoaWdoZXIgdG8gbG93ZXIu[Qq]
[f]Tm8uIEluIGZhY2lsaXRhdGVkIGRpZmZ1c2lvbiwgbWF0ZXJpYWxzIGFyZcKgYWxsb3dlZCB0byBkaWZmdXNlIGRvd24gdGhlaXIgY29uY2VudHJhdGlvbiBncmFkaWVudHMsIGJ1dCBhIHByb3RlaW4gY2hhbm5lbCBpcyBwcm92aWRlZCBmb3IgdGhlIG1hdGVyaWFscyB0byBwYXNzIHRocm91Z2guIFRoZXJlJiM4MjE3O3Mgbm8gY2hhbm5lbCBpbiBudW1iZXIgMS4=[Qq]
[f]WWVzLiBOdW1iZXIgMSBpcyBzaW1wbGUgZGlmZnVzaW9uIHRocm91Z2ggdGhlIHBob3NwaG9saXBpZCBiaWxheWVyLiBJbiBudW1iZXIgMywgdGhlIG1hdGVyaWFscyBhcmUgbm90IGdvaW5nIHRocm91Z2ggdGhlIHBob3NwaG9saXBpZCBiaWxheWVyLCBidXQgdGhyb3VnaCBhIGNhcnJpZXIgcHJvdGVpbi4=
Cg==[Qq]
[q] As part of the immune response, a white blood cell swallows an invading bacteria. Another name for that process is
[c]IGV4b2N5dG9zaXMu[Qq]
[c]IHBhc3NpdmUgdHJhbnNwb3J0Lg==[Qq]
[c]IHBoYWdvY3 l0b3Npcy4=[Qq]
[c]wqBmYWNpbGl0YXRlZCBkaWZmdXNpb24=[Qq]
[f]Tm8uIEV4b2N5dG9zaXMgbW92ZXMgdGhpbmdzIG91dCBvZiB0aGUgY2VsbC4=[Qq]
[f]Tm8uIFBhc3NpdmUgdHJhbnNwb3J0IGluY2x1ZGVzIHByb2Nlc3NlcyB0aGF0IGRvbiYjODIxNzt0IHJlcXVpcmUgYW55IGNlbGwgZW5lcmd5LCBsaWtlIHNpbXBsZSBhbmQgZmFjaWxpdGF0ZWQgZGlmZnVzaW9uLiBTd2FsbG93aW5nIGFuIGVudGlyZSBjZWxsIHJlcXVpcmVzIGEgbG90IG9mIGVuZXJneS4=[Qq]
[f]WWVzLiBQaGFnb2N5dG9zaXMgaXMgdGhlIG5hbWUgZm9yIHRoZSBwcm9jZXNzIGluIHdoaWNoIGNlbGxzIHN3YWxsb3cgcGFydGljbGVzIG9yIG90aGVyIGNlbGxzLg==[Qq]
[f]Tm8uwqBGYWNpbGl0YXRlZCBkaWZmdXNpb24gaW52b2x2ZXMgbW9sZWN1bGVzIGRpZmZ1c2luZyB0aHJvdWdoIHByb3RlaW4gY2hhbm5lbHMuIFdoYXQmIzgyMTc7cyBhYm92ZSBpcyBhIGZvcm0gb2YgZW5kb2N5dG9zaXMu
Cg==[Qq]
[q]The type of endocytosis in which the membrane pinches in and takes a small “sip” of whatever is outside the membrane is called
[c]cGhhZ29jeXRvc2lz[Qq]
[c]cGlub2N5 dG9zaXM=[Qq]
[c]cmVjZXB0b3IgbWVkaWF0ZWQtZW5kb2N5dG9zaXM=[Qq]
[f]Tm8uIFBoYWdvY3l0b3NpcyBpbnZvbHZlcyBhIHNpZ25pZmljYW50IGV4dGVuc2lvbiBvZiB0aGUgbWVtYnJhbmUuIFRoZXNlIGV4dGVuc2lvbnMsIGNhbGxlZCBwc2V1ZG9wb2RzLCBlbmd1bGYgbGFyZ2UgcGFydGljbGVzIG9yIGVudGlyZSBjZWxscy4=[Qq]
[f]WWVzLsKgV2hhdCYjODIxNztzIHNob3duIGhlcmUgaXMgcGlub2N5dG9zaXMsIGEgc21hbGwgcGluY2hpbmcgaW4gb2YgdGhlIGNlbGwgbWVtYnJhbmUgdGhhdCBlbmFibGVzIHRoZSBjZWxsIHRvICYjODIyMDt0YWtlIGEgc2lwJiM4MjIxOyBvZiB0aGUgc3Vycm91bmRpbmcgZXh0cmFjZWxsdWxhciBmbHVpZC4=[Qq]
[f]Tm8uIFJlY2VwdG9yLW1lZGlhdGVkIGVuZG9jeXRvc2lzIA==ZG9lcw==IGludm9sdmUgcGluY2hpbmcgb2YgdGhlIG1lbWJyYW5lLCBidXQgdGhlIHByb2Nlc3MgaXMgaW5pdGlhdGVkIGJ5IHRoZSBiaW5kaW5nIG9mIGNlcnRhaW4gc3Vic3RhbmNlcyB0byByZWNlcHRvcnMsIHdoaWNoIHlvdSBkb24mIzgyMTc7dCBzZWUgaGVyZS4=[Qq]
[q]One way to talk about diffusion is to say the molecules in a fluid will move down a [hangman] [hangman].
[c]Y29uY2VudHJhdGlvbg==[Qq]
[c]IGdyYWRpZW50[Qq]
[q]When substances diffuse into a cell by passing through a protein channel, the process is called [hangman] [hangman].
[c]ZmFjaWxpdGF0ZWQ=[Qq]
[c]ZGlmZnVzaW9u[Qq]
[q]When cells expend energy to move a substance from lower concentration to higher concentration, this process is known as [hangman] [hangman].
[c]YWN0aXZl[Qq]
[c]dHJhbnNwb3J0[Qq]
[q]A vesicle filled with some substance fuses with the membrane, releasing that substance outside the cell. This process is called [hangman].
[c]ZXhvY3l0b3Npcw==[Qq]
[q]The cell membrane pinches in, taking in something outside of the cell and enclosing it within a vesicle. This process is called [hangman].
[c]ZW5kb2N5dG9zaXM=[Qq]
[q]The kind of endocytosis in which a large portion of the membrane surrounds a food particle (or even another cell), and then brings that particle into the cell is called [hangman].
[c]cGhhZ29jeXRvc2lz[Qq]
[q]The best name for the process shown in this diagram is [hangman] [hangman],
[c]c2ltcGxl[Qq]
[c]ZGlmZnVzaW9u[Qq]
[q]The best name for the process shown in this diagram is [hangman] [hangman],
[c]YWN0aXZl[Qq]
[c]dHJhbnNwb3J0[Qq]
[q]The best name for the process shown at “A,” “B,” and “C” in this diagram is [hangman],
[c]ZW5kb2N5dG9zaXM=[Qq]
[q]The specific type of endocytosis shown in this diagram is
[hangman]-[hangman] [hangman]
[c]cmVjZXB0b3I=[Qq]
[c]bWVkaWF0ZWQ=[Qq]
[c]ZW5kb2N5dG9zaXM=[Qq]
[q][hangman] are channels that allow water to diffuse through the cell membrane.
[c]YXF1YXBvcmlucw==[Qq]
[q]Na+, Cl, Ca++, Cl– or H+ can only pass through membranes through protein channels because they’re
[c]YmlnLg==[Qq]
[f]Tm8gKGJ1dCB0aGlzIGlzIGEgY29tbW9uIG1pc3VuZGVyc3RhbmRpbmcpLiBBbGwgb2YgdGhlc2UgaW9ucyBhcmUsIGNvbXBhcmVkIHRvIG9yZ2FuaWMgbW9sZWN1bGVzIG9yIGV2ZW4gdG8gbW9sZWN1bGVzIGxpa2Ugd2F0ZXIgb3IgY2FyYm9uIGRpb3hpZGUsIHZlcnkgc21hbGwuIEhlcmUmIzgyMTc7cyBhIGhpbnQ6IG5vdGljZSB0aGUgc3VwZXJzY3JpcHRzLg==[Qq]
[c]Y2hhcm dlZC4=[Qq]
[f]RmFuc3Rhc3RpYy4gVGhlIGh5ZHJvcGhvYmljIG1pZGRsZSBvZiB0aGUgcGhvc3Bob2xpcGlkIGJpbGF5ZXIgcmVwZWxzIGNoYXJnZWQgcGFydGljbGVzLiBUaGUgb25seSB3YXkgdGhleSBjYW4gcGFzcyB0aHJvdWdoIHRoZSBtZW1icmFuZSBpcyBieSBmYWNpbGl0YXRlZCBkaWZmdXNpb24gdGhyb3VnaCBhIHByb3RlaW4gY2hhbm5lbC4=[Qq]
[c]cG9sYXIu[Qq]
[f]Tm8sIGJ1dCB5b3UmIzgyMTc7cmUgb24gdGhlIHJpZ2h0IHRyYWNrLiBJbiB0ZXJtcyBvZiBlbGVjdHJvbmVnYXRpdml0eSwgd2hhdCYjODIxNztzIHRoZSBuZXh0IHN0ZXAgdXAgYWZ0ZXIgcG9sYXI/[Qq]
[x]
[restart]
[/qwiz]
8. What’s Next
- Proceed to Topic 2.8, Osmosis and Water Potential (the next topic in AP Bio Unit 2)