Unit 4, Quest 2: Cell Structure and Function

Dr. Z @Trinity

Unit 4 Packet

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What happens to allow glucose molecules to pass through the cell membrane? What is this process called?

Hormones open the gated channel and allow glucose to go through. Facilitated diffusion.

What is active transport? Describe the process.

ATP binds to an ATP-binding site on the channel protein and leaves behind a phosphate group, becoming ADP. The energy from the phosphate group changes the protein’s shape and thus, the substance being transported goes through. It goes against (up) the concentration gradient.

For each statement, answer if it applies for active transport (AT), diffusion (D) and/or facilitated diffusion (FD).

a) Requires energy input by the cell

b) Molecules move along (down) a concentration gradient

c) Moves molecules against (up) a concentration gradient

d) Always involves channel (membrane-spanning) proteins

e) Molecules pass between the phospholipids

f) Moves ions like Na⁺ and K⁺

g) Moves large molecules

h) Moves small nonpolar and polar molecules

a) AT

b) D, FD

c) AT

d) AT, FD

e) D

f) AT, FD
g) AT, FD

h) D

Why would the line representing facilitated diffusion level off as the concentration gets higher, while the line representing diffusion continues to go up at a steady rate?

Diffusion happens on its own, but facilitated diffusion needs hormones.

Nonpolar molecules, such as hydrocarbons, CO₂ and O₂, are hydrophobic. Explain why these molecules can easily cross the plasma membrane without the aid of proteins.

They’re hydrophobic, so they want to avoid the water prevalent in life, similar to the tails in between the phospholipid bilayer. As such, they’re easily able to pass through. They’re also small enough.

Explain why water would have a difficult time diffusing across the cell membrane. Keep in mind the structure of water in your answer.

The cell membrane is designated to keep water out, forming a phospholipid bilayer so the tails don’t interact with water, sandwiched in the middle of the hydrophilic tails. Therefore, water cannot pass through without the help of aquaporin.

Is the sodium-potassium pump a channel protein or a carrier protein?

A carrier protein like the sodium-potassium pump needs adenosine triphosphate (ATP, energy) to activate it bringing molecules through, not just a passage to pass through like aquaporin.

Inside: 6 sugar, 24 water

Outside: 15 sugar, 15 water

Which is more concentrated?

Inside: 6/(6+24) = 6/30 = 20% sugar concentration

Outside: 15/(15+15) = 15/30 = 50% sugar concentration

Outside is has a higher solute concentration.

The solution inside the cell will become (more/less) concentrated with the net movement of water being out of the cell.

The solution outside the cell will become (more/less) concentrated with the net movement of water being out of the cell.

More (6:24 [6/30 = 20%] → 6:6 [6/12 = 50%], more concentrated)

Less (15:15 [15/30 = 50%] → 15:100 [15/100 = 15%], less concentrated)

Diffusion = movement of molecules across membrane from an area of high concentration to an area of low concentration. Is the cell with 6 sugar, 24 water (inside) and 15 sugar, 15 water (outside), with a net movement outside, undergoing diffusion? Explain.

Yes, the net movement of water is going from an area of higher concentration (inside) to an area of lower concentration (outside).

Inside: 24/(24+6) = 24/30 = 80% water concentration

Outside: 15/(15+15) = 15/30 = 50% water concentration

Inside has a higher solvent concentration, so the cell is undergoing osmosis.

Tonicity

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Animal Cells

Hypertonic (more movement of solution [concentrated] outside than inside) → crenated (shriveled)

Isotonic (equal movement [concentration] of solution) → normal (healthy ovals)

Hypotonic (more movement of solution [concentrated] inside than outside) → lysed (swelled up)

Plant Cells

Hypertonic (more movement of solution [concentrated] outside than inside) → plasmolysed (shriveled cell membrane, cell wall remains)

Isotonic (equal movement [concentration] of solution) → normal

Hypotonic (more movement of solution [concentrated] inside than outside) → turgid (swelled-up vacuole, healthy)

Using the concept of osmosis, explain why water is sprayed over produce in a grocery store. How might this change the appearance of the produce, and why would this change be desirable?

The water makes the large central vacuoles of the cells swell up and exert more turgor pressure, therefore making the plants look plumper and more vibrant.

Suppose you made a lettuce salad in the afternoon, added salt and other seasonings, and then put the salad in the refrigerator. When you took the salad out of the refrigerator for dinner, the lettuce looked wilted and some water was in the bottom of the bowl. Use the principles of osmosis to explain what happened.

The water was drawn out of the plant cells through osmosis. Because the salt concentration was higher on the outside than inside, water moved to balance out the concentration. This causes the lettuce to lose turgor pressure and wilt.

In extreme cases, it is possible to die from drinking too much water. The consumption of several liters of water in a short amount of time can lead to brain edema (swelling) and death. Explain the effect of ingesting an extremely large amount of water at the level of the brain cells, including the role of osmosis in this process.

When someone drinks too much water, the blood becomes hypotonic (less solute inside than outside) compared to the fluid inside cells. Therefore, the water would move to the area with less water and more solutes (inside the cells) through osmosis. The water would move into the brain cells, which would swell up as they take in the excess water.

Paramecium contains a specialized organelle called a contractile vacuole that helps maintain osmotic balance. Predict how this organelle might help the organism survive given that it is constantly immersed in a hypotonic solution.

Paramecium is in a solution that is hypotonic compared to the cell, which means the cell has more solute and less water. Therefore, since there is less water inside, osmosis happens and water moves inside of the cell. The organism needs the contractile vacuole to constantly eject the excess water so it can survive.

Red blood cells (and other animal cells) placed in a distilled water solution usually swell up and burst. What prevented the red onion cells from swelling up and bursting when they were placed in the distilled water?

They are plant cells, so they have cell walls.

If a shipwrecked crew drinks sea water, they will probably die. Why?

Seawater is hypertonic compared to body fluids (a high concentration of solute [salt] and a low concentration of water). Thus, the water, through osmosis, would leave the inside of the cell, leading to crenated cells (they would shrivel up with the lack of water).

If a bowl of fresh strawberries is sprinkled with sugar, a few minutes later the berries will be covered with juice. Why?

The outside is now hypertonic compared to the inside, meaning there is a higher solute concentration and lower water concentration outside the cell compared to the inside. Osmosis dictates that the water would move from an area of high concentration to low concentration; thus, the solution would move from inside to outside, forming the juice seen on the outside of the berries.

Review Sheet

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Why is a cell membrane a phospholipid bilayer instead of a monolayer? Why does it assume that configuration?

Phospholipids are amphipathic, meaning that one side is hydrophilic (the head) and one is hydrophobic (the tails). It assumes this configuration when coming into contact with water because the heads are attracted to the water, while the tail is repelled.

If you repeated the potato core lab using cores that were half the volume of the ones you used, what do you think would happen to your graphs?

Smaller objects would have a higher surface-area-to-volume ratio. With more surface area relative to their size, the cores would reach equilibrium faster. As such, the x-intercept should remain the same, but the slope should be steeper.