Membranes

Question 1

Why is it important for cells to have membranes? Why is it especially important that these membranes be semipermeable?

Answer 1

They act as a protective barrier, separating the cell’s internal environment from the external environment. Cause they only let certain materials come in & come out.

Question 2

What does it mean to say that phospholipids and some proteins are amphipathic? Why is this important for cell membranes?

Answer 2

Phospholipid: Hydrophilic “head” that contains glycerol & negatively charged phosphate group. Hydrocarbon “tail” that is Non-polar and hydrophobic. Water molecules cannot form hydrogen bonds with the tail.

Proteins: The polar & charged Amino acids are hydrophilic. The Non-polar Amino acids are hydrophobic.

Important so both can work together as the proteins are integrated into the membrane.

Question 3

When placed in water, what two main structures may form spontaneously? Why? How are these two structures different from each other?

Answer 3

1. Micelles. Tiny spherical aggregates.Form from free fatty acids.
2. Lipid bilayers. Created when lipid molecules align in paired sheets.
   The reason is b/c no input of energy is required.

Question 4

What are liposomes?

Answer 4

Artificial membrane-bound vesicles.

Question 5

Which types of molecules can pass through a lipid bilayer easily? Which cannot pass through easily? (memorize slide 23)

Answer 5

Small or nonpolar molecules move across phospholipid bilayers quickly. Ex. Oxygen, carbon dioxide, & Nitrogen

Small, uncharged polar- molecules: Water & glycerol

Charged or large polar substances cross slowly, if at all. Ex. Glucose & sucrose.

Small ions: Potassium ion (+), Chlorine(-) ion, Sodium ion (+)

Question 6

How can we make proteins amphipathic?

Answer 6

By integrating it into the cell membrane.

Question 7

Describe the fluid mosaic model. Why is it important that cell membranes have this structure and dynamic?

Answer 7

It describes the cell membrane. A tapestry of several types of molecules (phospholipids, cholesterols, and proteins)
They are constantly moving.
This movement helps the cell membrane maintain its role as a barrier between the inside and outside of the cell environments.

Question 8

What do evolutionary scientists believe protocells (precursors to the first cells) may have looked like? What biomolecule was likely on the inside?

Answer 8

Simple vesicle-like structures that harbor nucleic acids
Possible intermediates in evolution of cell

Question 9

What is a concentration gradient? In passive transport, do solutes flow with or against the concentration gradient?

Answer 9

A physical space in which there is a range of concentrations of a single substance. Exists when the concentration of a solute or ion is different on two sides of a membrane.

In passive transports the solute flows c/ concentration gradient until equilibrium is reached (concentration is the same on both sides).

Question 10

What is diffusion? What is osmosis? How are they different from one another? What is facilitated diffusion and how is it different from “regular” diffusion?

Answer 10

Solutes move from high to low concentration (i.e. with their concentration gradient) with no additional energy required. This is passive transport.

Osmosis is the movement of solvent (WATER) across a semipermeable from high to low solvent concentration.

Materials diffuse across the plasma membrane with the help of membrane proteins in facilitated diffusion.

Question 11

When and why do solutes “stop” diffusing across the membrane?

Answer 11

when the concentration of the solute becomes equal on both sides of the membrane. The concentration gradient is eliminated.

Question 12

What happens when you place a cell in a hypotonic/isotonic/hypertonic solution? Why? (Be able to recognize these solutions or cells placed in these solutions on the exam)

Answer 12

Hypotonic solutions have lower solute concentrations than the inside of a cell. Cell is swelled up.

Isotonic solutions have the same solute concentration as the inside of a cell. Cell is normal.

Hypertonic solutions have greater solute concentrations than the inside of a cell. Cell has shrunk.

Question 12

What is active transport and which direction to solutes move during active transport?

Answer 12

Solutes move against their concentration gradient (from low to high concentration) - that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa). Requires Energy (usually ATP).

Question 13

What are three main types of membrane transport proteins?

Answer 13

1. Channel Proteins- Open or close to allow molecules to flow in or out. Use passive transport.
2. Carrier proteins- Change shape to facilitate the movement of molecules through the membrane. Can be passive or active transport.
3. Protein pumps- Use energy to pump molecules across a membrane. Use active transport.

Question 14

CHANNNEL PROTEINS
Whether the protein facilitates passive or active transport, or both
What types of molecules the protein generally transports
How the protein works and how it operates during transport

Answer 14

Channel proteins are selective
Each channel protein permits only particular type of ion or small molecule to pass through it. Use passive transport.
Gated channels: Open or close in response to signal
Movement of substances through channel proteins is passive (diffusion)
Aquaporins. Allow ions to pass through.

Question 15

CARRIER PROTIEN
Whether the protein facilitates passive or active transport, or both
What types of molecules the protein generally transports
How the protein works and how it operates during transport

Answer 15

Move substances via structural changes.
Carrier proteins selectively pick up solute on one side of membrane and drop it on the other side.
Transport can be passive or active. BOTH
Best studied carrier protein is glucose.

Question 16

PROTEIN PUMPS
Whether the protein facilitates passive or active transport, or both
What types of molecules the protein generally transports
How the protein works and how it operates during transport

Answer 16

move substances against their concentration gradient, carry out active transport
Requires energy
EX: Sodium-Potassium pump
Uses ATP - a nucleic acid used for energy
Transports Na+ (3) and K+ (2) ions against their concentration gradient

Question 17

What are aquaporins? What type of transport protein are these?

Answer 17

a type of protein channel that permit water to cross plasma membrane

Question 18

What are phospholipids?

Answer 18

Consist of glycerol linked to phosphate group and two hydrocarbon chains. The primary role is to form cell membranes.

Question 19

What are the four main components of a phospholipid?

Answer 19

1. Polar- functional group
2. Charged phosphate group
3. Hydrocarbon backbone: glycerol
4. 2 Fatty Acid Chains

All these components arrange themselves into a bilayer, with a hydrophobic inside and water soluble outside.

Question 20

How do triglycerides and phospholipids differ from each other?

Answer 20

They differ in the Presence or Absence of a Hydrophilic Region

Question 21

What is equilibrium?

Answer 21

Solutes will move with their concentration gradient until concentration is the same on both sides.

Question 21

What are vesicles?

Answer 21

They can form from phospholipids in a lab. Have small bubble-like structures surrounded by phospholipids.
Ex: liposomes

Question 22

What is tonicity?

Answer 22

It describes the osmotic pressure due to differences in solute concentration across a semi-permeable membrane.
3 types:
1. Hypotonic
2. Hypertonic
3. Isotonic

Question 23

What are the principle components of the plasma membrane?

Answer 23

1. lipids (phospholipids and cholesterol [composed of four fused carbon rings])
2. Proteins
3. carbohydrates (that are attached to some of the lipids and some of the proteins)

Question 24

Where are the carbohydrates found on the membrane?

Answer 24

Carbohydrates are present only on the exterior surface of the plasma membrane and are attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids. Carbohydrates form specialized sites on the cell surface that allow cells to recognize each other.

Question 25

What are the types of proteins involved in the membrane?

Answer 25

1. Integral Proteins. Are integrated completely into the membrane structure. Their hydrophobic regions interact with the hydrophobic region of the phospholipid bilayer. Same c/hydrophilic region. with the hydrophobic region of the protein adjacent to the tails of the phospholipids and the hydrophilic region or regions of the protein protruding from the membrane and in contact with the cytosol or extracellular fluid.
2. Peripheral Proteins. On the exterior and interior surfaces of membranes, attached either to integral proteins or to phospholipids. May serve as enzymes, structural attachments for the fibers of the cytoskeleton, or as part of the cell’s recognition sites. A.K.A “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens.

Question 26

What factors affect diffusion?

Answer 26

1. Extent of the concentration gradient
2. Mass of the molecules diffusing
3. Temp.
4. Solvent Density
5. Solubility
6. Surface area and thickness of the plasma membrane
7. Distance Travelled

Question 27

What happens to a red blood cell when water enters excessively?

Answer 27

A red blood cell will burst, or lyse, when it swells beyond the plasma membrane’s capability to expand.

Question 28

What happens when excessive water leaves the red blood cell?

Answer 28

The cell shrinks, or crenates. This affects concentration of the solutes left in the cell, making the cytosol denser & interfering with diffusion within the cell. The cell’s ability to function will be compromised and may also result in the death of the cell.

Question 29

What happens to a plant cell if the environment has higher water concentration.

Answer 29

the cytoplasm in plants is always slightly hypertonic to the cellular environment, and water will always enter a cell if water is available. This inflow of water produces turgor pressure, which stiffens the cell walls of the plant. Turgor pressure supports plant.

Question 30

What happens to a plant cell if the environment has a higher hypertonic solution?

Answer 30

The extracellular fluid will become hypertonic, causing water to leave the cell. In this condition, the cell does not shrink because the cell wall is not flexible. However, the cell membrane detaches from the wall and constricts the cytoplasm. This is called plasmolysis. Plants lose turgor pressure in this condition and wilt.

Question 31

What is the electrochemical gradient?

Answer 31

The combined gradient of concentration and electrical charge that affects an ion. The interior of living cells is electrically negative, and at the same time, cells have higher concentrations of potassium (K+) and lower concentrations of sodium (Na+) than does the extracellular fluid. So in a living cell, the concentration & electrical gradient of Na+ tends to drive it into the cell/inward to the negatively charged interior. The electrical gradient of K+ (potassium), a positive ion, also tends to drive it into the cell, but the concentration gradient of K+ tends to drive K+ out of the cell.

Question 32

What are the 3 different types of Carrier Proteins for Active Transport?

Answer 32

1. Uniporter carries one specific ion or molecule
2. Symporter carries two different ions or molecules, both in the same direction.
3. Antiporter carries two different ions or molecules, but in different directions.

All of these transporters can also transport small, uncharged organic molecules like glucose. Are also found in facilitated diffusion, but they do not require ATP to work in that process.

Question 33

How does this electrogenic pump (a pump that creates a charge imbalance, creating an electrical imbalance across the membrane and contributing to the membrane potential) function as a primary active transport?

Answer 33

One of the most important pumps in animals cells is the sodium-potassium pump (Na+-K+ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells. The sodium-potassium pump moves K+ into the cell while moving Na+ out at the same time, at a ratio of three Na+ for every two K+ ions moved in. The Na+-K+ ATPase exists in two forms, depending on its orientation to the interior or exterior of the cell and its affinity for either sodium or potassium ions. The

Question 34

What are the 6 steps in the process of the sodium- potassium pump?

Answer 34

1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
2. ATP is hydrolyzed by the protein carrier and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

Question 35

What is endocytosis?

Answer 35

A type of active transport that moves particles, such as large molecules, parts of cells, and even whole cells, into a cell. There are different variations of endocytosis ( Phagocytosis, pinocytosis, & receptor-mediated), but all share a common characteristic: The plasma membrane of the cell invaginates, forming a pocket around the target particle. The pocket pinches off, resulting in the particle being contained in a newly created intracellular vesicle formed from the plasma membrane.

Question 36

What is Phagocytosis?

Answer 36

Phagocytosis (the condition of “cell eating”) is the process by which large particles, such as cells or relatively large particles, are taken in by a cell. In preparation for phagocytosis, a portion of the inward-facing surface of the plasma membrane becomes coated with clathrin (a protein), which stabilizes this section of the membrane. The coated portion of the membrane then extends from the body of the cell and surrounds the particle, eventually enclosing it. Once the vesicle containing the particle is enclosed within the cell, the clathrin disengages from the membrane and the vesicle merges with a lysosome for the breakdown of the material in the newly formed compartment (endosome). When accessible nutrients from the degradation of the vesicular contents have been extracted, the newly formed endosome merges with the plasma membrane and releases its contents into the extracellular fluid. The endosomal membrane again becomes part of the plasma membrane.

Question 37

What is pinocytosis?

Answer 37

Means “cell drinking”. A process that takes in molecules, including water, which the cell needs from the extracellular fluid. Results in a much smaller vesicle than does phagocytosis, & the vesicle does not need to merge with a lysosome.

Question 38

What is potocytosis?

Answer 38

A variation of pinocytosis that uses caveolin (coating protein), on the cytoplasmic side of the plasma membrane, which performs a similar function to clathrin. The cavities in the plasma membrane that form the vacuoles have membrane receptors and lipid rafts in addition to caveolin. The vacuoles or vesicles formed in caveolae (singular caveola) are smaller than those in pinocytosis. Potocytosis is used to bring small molecules into the cell and to transport these molecules through the cell for their release on the other side of the cell, a process called transcytosis.

Question 38

What is receptor-mediated endocytosis?

Answer 38

Employs receptor proteins in the plasma membrane that have a specific binding affinity for certain substances

Question 39

What is excytosis?

Answer 39

Its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space.