Unit 4: Cell Transport

Cell Membrane:

• All cells have a cell membrane

• Selectively-permeable/semi-permeable (Chooses what enters and exits through the cell)

• Reasons things can’t go directly through the membrane:

  • Size, Polarity, etc.

• Functions:

  • Controls what enters and exits the cell and maintains homeostasis

  • Provides protection and support for the cell

  • The receptors' proteins sit on it. This includes pumps and such

• Made primarily of a phospholipid bilayer

  • Has two layers, each made of a hydrophilic head and hydrophobic tail

  • Phospholipid head:

    • Hydrophilic

    • Faces outward

    • Made of glycerol and a phosphate group

  • Phospholipid tail:

    • Hydrophobic

    • Faces inward

    • Made of 2 fatty acid tails

• Called a Fluid-Mosaic Model

  • Fluid = Pliable/easily moved

  • Mosaic = Made of many molecules, such as transport proteins, cholesterol strengtheners, carbohydrate identifier chains, etc.

• Other Molecules in the Membrane:

  • Cholesterol: A type of steroid(lipid), changes fluidity of the membrane; stiffens membrane in higher temperatures, prevents membrane from freezing in lower temperatures. Hydrophobic, so located on the inside of the membrane around the tails.

  • Proteins:

    • Recognition Proteins(Glycoprotein): Proteins with carbohydrates attached that help identify the cells(“Name tags”). Also called marker proteins. On the surface of the cell membrane.

    • Receptor Proteins: Proteins that receive chemical signals from other cells, i.e nervous system cells. Also on the surface of the membrane.

    • Channel Proteins: Proteins that let certain substances in and out of cells. Aquaporins let water in and out of cells; Ion Channels let certain ions in and out of cells; Glucose Transporters(GLUT Transporter) move glucose in and out of the cell membrane through facilitated diffusion(“Tunnel”).

    • Protein Pump: Uses energy(ATP) to push substances in and out of the cell. Primary active transport.

    • Integral Proteins: Proteins that cross both layers of the phospholipid bilayer, i.e channel proteins, protein pumps, etc.

    • Peripheral Proteins: Proteins that are only on one side of the phospholipid bilayer, i.e some receptor proteins. Proteins used for identification and communication purposes.

• How molecules enter the membrane:

  • Oxygen: Oxygen is constantly used in the cell to begin cellular respiration, so oxygen levels in the cell are always lower than they are outside of the cell. Because of this, the oxygen outside the cell is pushed into the cell through diffusion. Oxygen passes directly through the lipid bilayer of the cell membrane.

  • Carbon Dioxide: Carbon Dioxide is produced as a byproduct of cellular respiration, so there’s always an excess amount of carbon dioxide in the cell. Because of this, it will naturally diffuse and leave the cell through the lipid bilayer of the cell membrane.

  • Glucose: Glucose is transported into the cell through the GLUT Transporter through facilitated diffusion. Once it enters the cell, an enzyme binds to it, thus making it unable to exit the cell. After the glucose is binded, then more glucose will enter the cell due to diffusion.

  • Potassium: Potassium moves through diffusion in the ion channels into the cell. 1000+ ions can move through the channel per second.

  • Sodium: The cell requires a low concentration of potassium outside the cell and a low concentration of sodium inside the cell. To do this, it employs the use of sodium-potassium pumps. Sodium-potassium pumps use primary active transport to export sodium atoms and import potassium atoms. It pulls two potassium ions per three sodium atoms it expels.

  • Water: Water can enter the cell through both the lipid bilayer and pores in the membrane called aquaporins.

  • Enzymes: Enzymes are exported out of the cell through exocytosis. A lipid bilayer sphere approaches the cell membrane, then combines with the membrane, and releases them into the membrane, where it then gets released outside the cell. Enzymes, hormones, antibodies, bacteria, viruses, and more can also be imported into the cell through endocytosis, which is the same process as exocytosis except from the outside. Both are types of active transport, as they both require energy.

Cell Transport:

• There are two main types of cell transport: active transport and passive transport.

• Passive transport does not require energy/ATP. Molecules move from a high concentration to a low concentration, moving along the concentration gradient.

• Types of passive transport:

  • Diffusion: Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration. When talking about cell transport, diffusion will be directly across the semipermeable membrane. This type of transport will eventually stop when it reaches equilibrium, given no other action occurs. Examples include small nonpolar molecules, such as oxygen(O2), carbon dioxide(CO2), etc, as well as small polar molecules, such as water(H20).

  • Facilitated Diffusion: Facilitated diffusion is the movement of molecules from higher concentration to lower concentration through a protein channel. These larger molecules require protein channels due to the fact that they can’t fit through the membrane. Examples include large polar molecules, such as glucose, as well as charged molecules, such as H+ ions.

  • Osmosis: Osmosis is the movement of water molecules from a region of higher concentration of water molecules to a region of lower concentration of water molecules(Diffusion but just for water). Solutes are generally the ones that move. The question may specify this though safe to ask the teacher if the answer is unclear.

Solutions:

• Solution: Solute + Solvent = Solution

• Solute is the material being dissolved in the solvent.

• Concentration refers to the amount of solute; a solution is concentrated if there is a lot of solute, and it is diluted if it does not have a lot of solute.

• Types of Solution:

*If the percentage of solute in the solution is greater than the percentage of solute in the cell, then solute will move into the cell, and water will move out of the cell. This will cause the cell to shrivel. This solution would be called Hypertonic.

*If the percentage of solute in the solution is less than the percentage of solute in the cell, then solute will move out of the cell, and water will move into the cell. This will cause the cell to expand. This solution would be called Hypotonic.

*If the percentage of solute in the solution is equal to the percentage of solute in the cell, then solute and water will move in and out of the cell at the same rate, meaning that the net change will be zero. The cell will stay the same. This solution would be called Isotonic.

• Hypotonic: Hypotonic solutions have less solute and more water in comparison to the subject. This leads to the subject, in this case the cell, releasing solute and absorbing more water, making the cell swell up.

• Isotonic: Isotonic solutions have equal amounts of solute and water in comparison to the subject. This is the ideal solution, and means that an equal amount of water and solute will enter and exit the subject, in this case the cell. This allows the cell to maintain homeostasis.

• Hypertonic: Hypertonic solutions have more solute and less water in comparison to the subject. This leads to the subject, in this case the cell, absorbing solute from the solution and releasing water due to diffusion, leading to a cell shriveling up.

• Represented as solute/solvent.

U-Tube:

• Under the assumption that the solute can’t move through the cell membrane, but the water can, water will diffuse from the right side of the U-Tube to the left side of the U-Tube, making the water level on the left rise and the water level on the right lower. This is because the left is hypertonic, and the right is hypotonic, meaning that the concentration was lower in the right side causing water to move from right to left.

• A: Water level rises

• B: Hypertonic Solution

• C: Solutes

• D: Semi-permeable membrane

• E: Hypotonic Solution

• F: Water level lowers

Osmosis in Cells:

• Turgor Pressure: Turgor Pressure is the amount of pressure a plant’s fluid exerts on its cell wall; low turgor pressure will cause the plant to wilt, while a high turgor pressure allows it to stay upright and healthy(High TP = rigid). Plants with high turgor pressure have filled, enlarged vacuoles, while those with low turgor pressure have shrunk vacuoles. The cell wall prevents the cell from bursting even if the vacuole swells.

• Plasmolysis: Plasmolysis occurs when there isn’t enough turgor pressure, making the cell membrane detach from the cell wall, shrink, and the overall plant wilts.

• Plants are healthiest in hypotonic environments, where water is constantly flowing into the cell through osmosis.

• Cytolysis: The bursting of a cell when it fills with too much water. Can happen to any cell that doesn’t have a cell wall.

• Paramecium, a protist, has the ability to pump out water from its contractile vacuoles, thus allowing us to infer they live in a hypotonic environment(If there’s a need to pump out water, that means there's excess water already moving in). Note that this is an example of active transport.

• Osmometer: Measures osmotic pressure exerted by liquid passing through a semipermeable membrane.

Active Transport:

• Requires energy in the form of ATP(Adenosine Triphosphate)

• Molecules move from a low concentration to high concentration, the opposite of passive transport. This is against the concentration gradient. NOTE: although true for most cases, some molecules will still need to have active transport even though they are going from high to low. Molecules with polarity. It was a trick question last year. :)

• Types of Active Transport:

  • Molecular Transport: Transport of smaller molecules, i.e protein pumps

  • Bulk Transport: Transport of larger molecules, i.e endocytosis and exocytosis

• A popular example of active transport is sodium potassium pumps.

• Endocytosis: The process by which cells absorb materials from outside the cell by engulfing the material with its membrane. They surround the material with their cell membrane, then trap it in a vesicle.

  • There are two types of Endocytosis: Pinocytosis and Phagocytosis.

  • Pinocytosis is the movement of liquids into the cell.

  • Phagocytosis is the movement of solids into the cell.

• Exocytosis: A cellular process where cells eject waste products or chemical transmitters.

Experiment/Lab Conclusions:

• Egg Osmosis Experiment:

  • Experiment Description:

  • Procedure:

  • Conclusion:

• Potato Lab:

  • Experiment Description:

  • Procedure:

  • Conclusion: If I remember correctly it was based on turgor pressure and osmosis. More salt outside, water goes outside, reducing turgor pressure.