1.3 & 1.4 (Topic 1)

Question 1

Draw a simplified diagram of the structure of the phospholipid, including a phosphate-glycerol head and two fatty acid tails.

Answer 1

Head = phosphate and glycerol, Tails = fatty acids

Question 2

Define hydrophilic and hydrophobic.

Answer 2

Hydrophilic: Polar and/or charged molecules (or regions of molecules) to which water can attract because they are “Water loving” substances.

Hydrophobic: Nonpolar molecules (or regions of molecules) to which water will not attract because they are “Water fearing” substances.

Question 3

Define amphipathic and outline the amphipathic properties of phospholipids.

Answer 3

A molecule that contains both hydrophilic and hydrophobic regions. → i.e. a phospholipid
Amphipathic means there are both hydrophilic and hydrophobic regions in a single molecule. Phospholipids have a hydrophilic head region and hydrophobic tails.

Question 4

Explain why phospholipids form bilayers in water, with reference to hydrophilic phosphate heads and two hydrophobic hydrocarbon tails.

Answer 4

There is water both outside and inside the cell. Phospholipids will arrange themselves in a bilayer so that the hydrophilic head associates with water and the hydrophobic tails face each other, away from the water.

Question 5

State the primary function of the cell membrane.

Answer 5

It’s semi-permeable and controls the movement of substances into and out of the cell.

Question 6

Contrast the structure of integral and peripheral proteins.

Answer 6

Peripheral proteins sit on one side of the surface of the cell membrane.

Integral proteins are embedded in the hydrophobic middle of the bilayer. Some integral proteins are “transmembrane” meaning they span both sides of the bilayer.

Question 7

List at least four functions (with example) of membrane bound proteins.

Answer 7

1. Receptor proteins receive extracellular signals.
2. Transport proteins move ions and molecules across the bilayer.
3. Enzymes catalyze reactions.
4. Adhesion proteins anchor the cell to other cells.
5. Recognition proteins identify the cell type in a multicellular organism.

Question 8

Contrast the two types of transport proteins: pumps and channels

Answer 8

Channel proteins are used for passive transport of molecules as they move across the bilayer from higher to lower concentration.
Pump proteins are used for active transport of molecules as they move across the bilayer from lower to higher concentration.

Question 9

Identify the structure of cholesterol in molecular diagrams.

Answer 9

Cholesterol is a lipid that can be distinguished by its characteristic four-ring structure.

Question 10

Describe the structural placement of cholesterol within the cell membrane.

Answer 10

Cholesterol fits between phospholipids in the cell membrane, with its hydroxyl (-OH) group by the heads and the hydrophobic rings by the fatty acid tails.

Question 11

Describe the function of cholesterol molecules in the cell membrane. Then, outline how temperature affects cell membrane fluidity.

Answer 11

Cholesterol acts as a regulator of membrane fluidity. At high temperatures if stabilizes the membrane and reduces melting. At low temperatures is prevents stiffening of the membrane.
Membrane fluidity influences how permeable the structure is to solutes.
Too fluid (higher temps) = too permeable
Too stiff (lower temps) = not permeable enough

Question 12

Describe the observations and conclusions drawn by Davson and Danielli in discovering the structure of cell membranes.

Answer 12

Davson and Danielli proposed the “protein-lipid sandwich” model of the cell membrane, in which a phospholipid bilayer was embedded between two layers of proteins.

Question 13

Drawing of the fluid mosaic model

Answer 13

Phospholipid bilayer shown with heads facing in opposite directions
Phospholipids with labelled hydrophilic/phosphate head and hydrophobic/hydrocarbon tail
Peripheral protein, shown as globular structure at the surface of the membrane
Integral protein shown as embedded globular structure
Glycoprotein shown as embedded globular structure with protruding carbohydrate (shown as a branching, antenna-like structure)
Channel protein shown with a pore passing through it
Cholesterol shown in between adjacent phospholipids

Question 14

Describe conclusions about cell membrane structure drawn from electron micrograph images of the cell membrane.

Answer 14

Cells are rapidly frozen and then fractured. Fracture occurs along lines of weakness, including between the two layers of phospholipids. Freeze-etched cell membranes provided evidence that the membrane was a bilayer with embedded proteins.

Question 15

Describe conclusions about cell membrane structure drawn from cell fusion experiments.

Answer 15

Cell fusion experiments showed that protein molecules can move from place to place within the cell membrane; there is fluidity.

Question 16

Describe conclusions about cell membrane structure drawn from improvements in techniques for determining the structure of membrane proteins.

Answer 16

• Improvements in tools and techniques allows scientists to extract membrane proteins and determine their chemical and physical properties.
• The membrane proteins were found to be varied in shape and size. Additionally, some proteins had hydrophobic regions.
• These findings did not match the model proposed by Davson and Danielli, in which proteins were uniform in shape and hydrophilic.

Question 17

Compare the Davson-Danielli model of membrane structure with the Singer-Nicolson model.

Answer 17

• Singer and Nicolson proposed a membrane model that incorporated evidence about membrane proteins that did not comply with the Davson-Danielli model.
• Rather than having proteins on the surface of the phospholipids, Singer-Nicolson proposed a model in which proteins were embedded within and through the membrane, called the fluid mosaic model.

Question 18

Describe simple diffusion.

Answer 18

Net movement of molecules from areas of higher concentration to areas of lower concentration, without the input of energy (passive).

Question 19

Explain two examples of simple diffusion of molecules into and out of cells.

Answer 19

1. Gas exchange by diffusion in lung alveoli cells.

2. Gas exchange by diffusion through eye cornea cells.

Question 20

Outline factors that regulate the rate of diffusion.

Answer 20

Concentration of the diffusing molecule:
Increase concentration gradient, increase diffusion rate
Temperature:
Increase temperature, increase diffusion rate
Pressure:
Increase pressure, increase diffusion rate

Question 21

Describe facilitated diffusion.

Answer 21

Movement of molecules from higher to lower concentration through a transport protein without the input of energy.

Question 22

Describe one example of facilitated diffusion through a protein channel.

Answer 22

The CFTR protein is a channel protein that controls the flow of H2O and Cl- ions into and out of cells inside the lungs. When the CFTR protein is working correctly, ions freely flow in and out of the cells. However, when the CFTR protein is malfunctioning, these ions cannot flow out of the cell due to a blocked channel. This causes Cystic Fibrosis, characterized by the buildup of thick mucus in the lungs.

Question 23

Define osmosis.

Answer 23

The movement of water by diffusion across a membrane.

Question 24

Predict the direction of water movement based upon differences in solute concentration.

Answer 24

Water moves from hypotonic solutions into hypertonic solutions.

Question 25

Compare active transport and passive transport.

Answer 25

Passive Transport:
Does not require energy input
Molecules move from high to low concentration, “with” the gradient.

Active Transport:
Requires energy input
Molecules move from low to high concentration, “against” the gradient.

Question 26

Explain one example of active transport of molecules into and out of cells through protein pumps.

Answer 26

Pumps are proteins that actively transport other molecules using ATP as an energy source.
For example, the proton pump is used in photosynthesis and respiration.

Question 27

Describe the fluid properties of the cell membrane and vesicles.

Answer 27

Fluidity refers to the viscous flow of phospholipids in the cell membrane and organelles of the endomembrane system (including vesicles).
Fluidity is affected by:
-fatty acid length
-fatty acid saturation
-presence of cholesterol

Question 28

Explain vesicle formation via endocytosis.

Answer 28

In endocytosis, the cell activity transports molecules into the cell by engulfing them into vesicles formed from the cell membrane.

Question 29

Outline two examples of materials brought into the cell via endocytosis.

Answer 29

• White blood cells can engulf bacteria when fighting infection.
• Single celled organisms like amoeba can engulf bacteria as a food source.

Question 30

Explain release of materials from cells via exocytosis.

Answer 30

A secretory vesicle moves towards the cell membrane, fuses with the membrane and releases its contents into the extracellular space.

Question 31

Outline two examples of materials released from a cell via exocytosis.

Answer 31

• Secretion of neurotransmitter at synaptic terminus.

- Secretion of digestive juices from exocrine glands.

Question 32

List two reasons for vesicle movement.

Answer 32

• Transport vesicles can move molecules between locations inside the cell (e.g. proteins from the ER to the Golgi).
• Secretory vesicles can move molecules from inside the cell to outside of the cells (e.g. to secrete a protein hormone).

Question 33

Describe how organelles of the endomembrane system function together to produce and secrete proteins

Answer 33

1. In the nucleus, transcription of DNA, creating mRNA.
2. Translation of mRNA at a ribosome on the Rough ER, creating a protein.
3. Packaging of the protein into a transport vesicle.
4. Transport of the protein inside the vesicle to the Golgi.
5. Modification of the protein within the Golgi.
6. Packaging of the protein into a secretory vesicle.
7. Secretion of the protein when the vesicle fuses with the cell membrane during exocytosis.

Question 34

Outline how phospholipids and membrane bound proteins are synthesized and transported to the cell membrane.

Answer 34

• Phospholipids are synthesized at the ER. The phospholipids become part of the ER membrane.
• When a transport vesicle buds off the ER, the newly made phospholipid will be part of the vesicle. There may also be proteins (made at a ribosome on the ER) then embed in the vesicle.
• As the vesicle moves through the cell towards the Golgi and then towards the cell membrane, the new phospholipid and protein are also transported.
• When the vesicle fuses with the cell membrane, the new phospholipid and protein will become part of the cell membrane.

Question 35

Describe the structure of the sodium-potassium pump.

Answer 35

The sodium-potassium pump is an integral membrane protein. It had binding sites for three sodium ions, two potassium ions and an inorganic phosphate group (which comes from ATP).

Question 36

Describe the role of the sodium-potassium pump in maintaining neuronal resting potential.

Answer 36

The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell. The rest of the ion movement is a net negative charge in the cell, called the resting potential.

Question 37

Outline the six steps of sodium-potassium pump action.

Answer 37

1. Three sodium ions bind with the protein pump inside the cell.
2. The pump protein is phosphorylated by ATP and changes shape.
3. By changing shape, the three sodium ions are released out of the cell.
4. At that point, two potassium ions from outside the cell bind to the protein pump.
5. The inorganic phosphate (which came from the ATP) is released from the pump, restoring the original shape of the protein.
6. The potassium ions are then released into the cell, and the process repeats.

Question 38

Describe the structure of the potassium channel.

Answer 38

• The potassium channel is an integral membrane protein that facilitates the diffusion of potassium ions out of the cell.
• The channel has a “ball and chain” gate mechanism that will only open the channel for potassium movement when a specific cell voltage is reached.

Question 39

Explain the specificity of the potassium channel.

Answer 39

Potassium channels are designed to allow the flow of potassium ions across the membrane, but to block the flow of other ions–in particular, sodium ions.

Question 40

Explain what happens to cells when placed in solutions of the same osmolarity, higher osmolarity and lower osmolarity.

Answer 40

• Same: Isotonic solutions are solutions that have the same osmolarity. Water moves into and out of the cell equally, resulting in no NET movement of water.
• Higher: Hypertonic solutions are solutions that have more solutes than the cell. Water will move out of the cell and as a result the cell will shrivel (animal) or plasmolyze (plant).
• Lower: Hypotonic solutions are solutions that have fewer solutes than the cell. Water will move into the cell. Animal cells will swell and may burst. Plant cells will become turgid with a vacuole full of water and pressure on the cell wall.

Question 41

Outline the use of normal saline in medical procedures.

Answer 41

• Normal saline is a solution of water and salt ions that is isotonic to human blood. It is used as an eye wash, to flush wounds and intravenously to rehydrate patients. During organ transplant, while out of a body the organs are bathed in normal saline.
• Because the solution is isotonic to body cells, the cells will not shrink or swell when exposed to the saline solution.