All DONE
Key questions
1. Why do membranes form spontaneously, and in doing so, form a barrier that large hydrophilic molecules have difficulty crossing? DONE
Firstly, other than the properties of the phospholipids, no energy, genes, or other properties are needed in order to form plasma membranes. Phospholipids are negatively charged modified triglycerides with a hydrophilic head and hydrophobic fatty acid tails that when shaken in water will arrange themselves surrounding the H2O bubbles. Surrounding these water bubbles the hydrophilic heads will link together through a weak bond in a circular pattern. Then the hydrophobic tails will cluster together to another set of tails forming a lipid bilayer consisting of two circle shaped walls. The circular hydrophilic heads create a barrier that molecules cross in order to give our membrane proper nutrients. The hydrophobic layer of fatty acids in the center of the bilayer, by not attracting the hydrophilic molecules outside of the membrane, impedes the movement of the large hydrophilic molecules through the membrane. Smaller molecules, even though they may be polar (like water), can still diffuse down their concentration gradient through the lipid bilayer. Despite that large polar hydrophilic molecules, such as amino acids, monosaccharides, and ions, struggle to pass through the wall, sometimes they are provided with a hydrophilic transport protein that gives a specific solute molecule a channel to pass through across the membrane.
2. How does the principle of diffusion explain the movement of particles across a membrane through passive transport? DONE
Because molecules are not at absolute zero, they are in motion. The relatively high temperatures that we have in our world mean that molecules are moving very rapidly and in random directions. with this energy with random direction, these particles diffuse, or spread out though a medium from a region of high concentration to low concentration untill the concentration is even. with this random energy, molecules bump into cells with enough force to punch through the plasma membrane due to the weak bonds between the phospholipids that make up the membrane. untill equilibrium is reached, the particles will continue to enter the cell via the plasma membrane. if a molecule is too big or has insufficient energy, it cannot penetrate the plasma membrane, making it necessary for a transport protein.
3. How are transport proteins selective in what they allow through them? DONE
The hydrophobic plasma membrane in a cell provides a barrier, formed spontaneously and made up of 2 layers of phospholipids, that only lets in certain materials that it needs to function. Nonpolar, hydrophobic molecules can dissolve in the lipid bilayer of the membrane and cross it with ease but there are many molecules that cannot cross the membrane, like monosaccharides, amino acids, and ions, either because of their charge, polarity or size. A transport protein constructs a "hydrophilic tunnel" with a specific shape, like a child's shape sorter toy, in the phospholipids and allows only the specific solute molecules that fit to get through. This selectiveness of the transport proteins is called “selective permeability'” meaning it is selective in what it lets through. It is selective because the R-Groups in the protein form a specific shape because of their charges. Positively charged R-Groups attract to negatively charged R-Groups which creates the shape. The solute molecules will only fit in transport proteins that match their specific shape and size. Just like enzymes, when we excercise we change the pH in our body which affects the R-Groups and causes the transport proteins to change shape.
4. How do cells use energy to move Na+ and K+ across membranes against their concentration gradient?
DONE
The sodium-potassium pump actively maintains the gradient of sodium ions (Na+) and potassium ions (K+) across the plasma membrane of animal cells. Typically, K+ concentration is low outside an animal cell and high inside the cell, while Na+ concentration is high outside an animal cell and low inside the cell. The sodium-potassium pump maintains these concentration gradients, using the energy of one ATP to pump three Na+ out and two K+ in. ATP supplies the energy for most active transport. ATP can power active transport by transferring a phosphate group from ATP (forming ADP) to the transport protein. Once the phosphate group attaches itself to the transport protein it changes the shape of the protein and therefore releases the ions that no longer fit in the protein. The phosphate group is attached with a negative charge and it releases heat. Hydrogen bonds break because of the release of heat. The charges on the R-groups are altered because of the negative charge on the phosphate group. This action moves the ions against their concentration gradient. The pump releases these ions, then attracts more that fit in the new shape of the protein. Once the phosphate group is released, the shape is changed once again and returns to its original shape and begins the process again.
5. How do cells move particles that are too large to move through transport proteins into and out of them? DONE
When particles are too large to move across a transport protein, cells use a process called endocytosis. There are 3 types of endocytosis: phagocytosis, pinocytosis, and receptor mediated endocytosis. The first type, phagocytosis, is the process that amoebas use to eat. The cell membrane engulfs the particle and makes a food vacuole. The food is then digested in the food vacuole while the phospholipids in the cell membrane simply form back together without that section of the membrane. The second type, pinocytosis, is on a much smaller molecular level. The particles that are just big enough to not be able to fit through transport proteins are engulfed by the cell membrane to form a vesicle inside the cell which transports them into or across the cell. Both pinocytosis and receptor mediated endocytosis are on a molecular level, however, receptor mediated endocytosis is different from pinocytosis in that it has receptors in a coated pit that act as lock and key mechanisms. The right particles fit inside the receptors and the wrong particles are not allowed through. Then these receptors with the right particles attached form a vesicle to transport the particles. Sometimes though, viruses can trick the receptors with an attachment that “feels” like a key and the viruses can get in the cell that way and spread like wildfire.
Practice: write a sentence (or 2) to show a relationship between the following sets of terms.
1. Lipid bilayer, polar molecule, transport protein DONE
Transport proteins are proteins located in the cell membrane, which is composed of a lipid bilayer, that allow specific molecules to move in and out of the cell. The purpose of the transport protein is to allow one polar molecule to pass through it, such as a monosaccharide, amino acid, or water (H2O).
2. Tertiary structure, selectively permeable, diffuse DONE
The tertiary structure of a transport protein is arranged so that a specific molecule can pass through it, a quality otherwise known as selective permeability. This allows molecules to diffuse from an area where it is highly concentrated (such as the outside of a cell) to the inside of the cell where it is less concentrated.
3. Shape, transport protein, poison DONE
Although the shape of a transport protein is specific to a certain molecule, some molecules that are smaller than the opening of the transport protein, such as small poisons like hydrogen peroxide, can enter the cell and cause harm to the organism.
4. Osmosis, turgor pressure, plant cells DONE
Water moves in and out of a cell by means of osmosis, the spontaneous movement of water through a cell membrane from an area of high concentration to low concentration. When water moves into plant cells, it exerts pressure on the cell wall called Turgor pressure, which is what causes a plant to stand up straight on its stem and why a plant wilts when deprived of water.
5. Passive transport, active transport, ATP DONE
Within a cell, passive transport is the diffusion of substances across a plasma membrane that requires no work or cell energy since it is going from a high concentration to low concentration. On the other hand, active transport requires the cell to use energy because the substance is going against the concentration gradient. In order for this to take place, ATP needs to provide the energy. ATP (which consists of 3 phosphate groups, adenosine and ribose) transfers 1 phosphate group to the transport protein, which then changes shape and allows the substance to get the other side of the membrane.
6. Active transport, concentration gradient DONE
In order to have any flow of material move across membranes in the direction of low to high concentration, or "against" the usual concentration gradient from high concentration to low, active transport is necessary.
7. Phosphate group, ATP, ADP, energy DONE
ATP supplies the energy for most active transport. ATP can power active transport by transferring a phosphate group from ATP (forming ADP) to the transport protein.
8. Protein pump, phosphate group, active transport DONE
When sodium needs to be transported out of a cell faster than it is leaking in, and potassium needs to be transported in faster than it is leaking out, the cell uses active transport in the form of a protein pump. The protein pump uses an ATP molecule to move the sodium molecules out and the potassium molecules in. Three sodium molecules attach to the protein pump. Then an ATP molecule reacts with the pump and when the ATP loses a phosphate group by attaching it to the protein pump and turns into ADP, the energy released and the charges of the phosphate group change the shape of the protein pump so that it releases the sodium outside the cell against its concentration gradient and bonds with potassium molecules. Then the phosphate group detaches and the pump releases the potassium molecules against their concentration gradient, on the inside of the cell. (probably could have been shorter but i wanted to be thorough - good idea and good job)
9. Endocytosis, phagocytosis, pinocytosis DONE
Endocytosis is the process of 1 cell consuming a molecule or group of molecules through active transport and has 3 subcategories, two of which are phagocytosis and pinocytosis. In phagocytosis, a whole organism is consumed and in pinocytosis, molecular groups are consumed.
10. Coated pit, shape, receptor: DONE
When large particles need to get across the cell membrane, the cell uses endocytosis. One form of endocytosis is receptor mediated endocytosis. In this form, there is a coated pit in which receptors attract particles with a lock and key sort of mechanism. The shape of the particles is important because these receptors only pick up shapes that fit perfectly into their “locks” to transport them through the membrane.
