How Things Get In and Out of Cells, or gummy bears, zip lock bags or whatever!
SC.912.L.14.3 Benchmark Clarifications: Students will compare and/or contrast the structures found in plant cells and in animal cells. Students will compare and/or contrast the structures found in prokaryotic cells and in eukaryotic cells. Students will describe how structures in cells are directly related to their function in the cell. Students will explain the role of the cell membrane during active and passive transport. Content Limits: Items will not address protists or fungi or assess cellular structures unique to protists or fungi. Items referring to the role of the cell membrane may address hypotonic, hypertonic, and/or isotonic solutions; however, the assessment should be on processes and not terminology.
1. A membrane s molecular organization results in selective permeability A steady traffic of small molecules and ions moves across the plasma membrane in both directions. So what exactly needs to get across??
What determines what kind of things can pass through? What does a membrane have that might help?
Transport proteins Channel proteins Pump Proteins What does SPECIFICITY mean??
2. Passive transport is diffusion across a membrane Diffusion is the tendency of particles of any substance to spread out in the available space Why would they move at all? And who is this Brown guy? And what does this movement have to do with a net?
Let s look at this little system. Does equilibrium mean equal necessarily? At this dynamic equilibrium,as many molecules pass one way as cross the other direction. It s all math!! Fig. 8.10a
Now let s look at several particles at once. The direction of diffusion is always from more to less concentrated. NET movement is the key. Fig. 8.10b
Diffusion is passive transport because it requires no energy from the cell to make it happen. It is driven by the kinetic energy of the particles. When would there be no diffusion? What if you can t get through the bilayer? This is called facilitated diffusion. Much of the glucose that enters a cell does so by facilitated diffusion.
3. Osmosis is the passive transport of water We give the following names to 2 solutions of different concentration: hypertonic. hypotonic. These are comparative terms. Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water. Solutions with equal solute concentrations are isotonic.
How about water? Can it pass through? Help??? Diffusion of water across a selectively permeable membrane is a special case of passive transport called osmosis. Osmosis continues until the solutions are isotonic. Or will it? Do these particles try? Do they know? Fig. 8.11
What can Affect the SPEED of Diffusion? Temperature: if particles are going faster or slower, then they are spreading out faster or slower. Pressure: More pressure makes them move faster, therefore diffuse faster. Concentration gradient: The steeper the gradient, the faster diffusion will occur. This is just math let s take a look
4. Cell survival depends on balancing water uptake and loss An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane. Water flows across the membrane, through aquaporins, but at the same rate in both directions. The volume of the cell is stable.
The same cell in a hypertonic environment will loose water, shrivel, and probably die. A cell in a hypotonic solution will gain water, swell, and burst. Fig. 8.12
For example, Paramecium, a protist, is hypertonic when compared to the pond water in which it lives. In spite of a cell membrane that is less permeable to water than other cells, water still continually enters the Paramecium cell. To solve this problem, Paramecium have a specialized organelle, the contractile vacuole, that functions as a bilge pump to force water out of the cell. What cell cause it to contract? parts Fig. 8.13
The cells of plants, prokaryotes, fungi, and some protists have walls that contribute to the cell s water balance. A plant cell in a hypotonic solution will swell until the elastic cell wall opposes further uptake. At this point the cell is turgid, a healthy state for most plant cells. Fig. 8.12
Turgid cells contribute to the mechanical support of the plant. If a cell and its surroundings are isotonic, there is no movement of water into the cell and the cell is flaccid and the plant may wilt. Fig. 8.12
In a hypertonic solution, a cell wall has no advantages. As the plant cell loses water, its volume shrinks. Eventually, the plasma membrane pulls away from the wall. This plasmolysis is usually lethal. Now it REALLY wilts. Fig. 8.12
In passive diffusion, substances always move A. from an area with fewer of the molecules to an area with more of the molecules B. from an area of higher concentration to an area of lower concentration C. from an area of lower concentration to an area of higher concentration D. from an area with more of the molecules to an area with fewer of the molecules Let s rap it up with those gummy bears.
6. Active transport is the pumping of solutes against their gradients Some transport proteins can move solutes against their concentration gradient, from the side where they are less concentrated to the side where they are more concentrated. They are often called pumps. This active transport requires the cell to expend its own metabolic energy, usually in the form of ATP. Active transport often counteracts diffusion that the cell can t stop, just like the contractile vacuole did.
The sodium-potassium pump actively maintains the gradient of sodium (Na + ) and potassium ions (K + ) across the membrane. Typically, an animal cell has higher concentrations of K + and lower concentrations of Na + inside the cell. Even though they don t diffuse easily because they are charged, some of these ions do diffuse down their gradient (Na in, K out) with the help of channel proteins. The sodium-potassium pump uses the energy of one ATP to pump three Na + ions back out and two K + ions back in.
In plants, bacteria, and fungi, a proton pump is the major electrogenic pump, actively transporting H + out of the cell. Proton pumps in the cristae of mitochondria and the thylaloids of chloroplasts, concentrate H + behind membranes. These electrogenic pumps store energy that can be accessed for cellular work. Lysosomes have them too Fig. 8.17
Fig. 8.16 Both diffusion and facilitated diffusion are forms of passive transport of molecules down their concentration gradient, while active transport requires an investment of energy to move molecules against their concentration gradient.
9. Exocytosis and endocytosis transport large molecules by active transport Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins. Large molecules, such as polysaccharides and proteins, cross the membrane with the help of vesicles. During exocytosis, a transport vesicle budded from the Golgi apparatus is moved by the cytoskeleton to the plasma membrane. When the two membranes come in contact, the bilayers fuse and spill the contents to the outside. Let s watch
During endocytosis, a cell brings in macromolecules and particulate matter by forming new vesicles from the plasma membrane. Endocytosis is a reversal of exocytosis. A small area of the plasma membrane sinks inward to form a pocket As the pocket into the plasma membrane deepens, it pinches in, forming a vesicle containing the material that had been outside the cell
One type of endocytosis is phagocytosis, cellular eating. Here s a cool one. In phagocytosis, the cell engulfs a particle by extending pseudopodia around it and wrapping it in a vacuole. What cell parts make this happen? The contents of the vacuole are digested when the vacuole fuses with a lysosome. Fig. 8.19a
In pinocytosis, cellular drinking, a cell creates a vesicle around a droplet of extracellular fluid. This is a non-specific process. The fluidity of the membrane allows for this type of transport involving the forming and combining of vesicles. Fig. 8.19b