1.4: Membrane Transport

*Describe simple diffusion.*



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

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

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



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

1. Gas exchange by diffusion in lung alveoli cells.

2. Gas exchange by diffusion through eye cornea cells.

*Outline factors that regulate the rate of diffusion.*



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

*Concentration of the diffusing molecule*
Increase concentration gradient, increase diffusion rate

*Temperature*
Increase temperature, increase diffusion rate

*Pressure*
Increase pressure, increase diffusion rate

*Describe facilitated diffusion.*



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

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

*Describe one example of facilitated diffusion through a protein channel.*



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

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, as shown in Panel 1, ions freely flow in and out of the cells. However, when the CFTR protein is malfunctioning as in Panel 2, 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.

*Define osmosis.*



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

The movement of water by diffusion across a membrane.

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



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

Water moves from hypotonic solutions into hypertonic solutions.

*Compare active transport and passive transport.*



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

*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.

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



Understanding: Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

*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.

*Describe the fluid properties of the cell membrane and vesicles.*



Understanding: The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

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

*Explain vesicle formation via endocytosis.*



Understanding: The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

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

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



Understanding: The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

White blood cells can engulf bacteria when fighting infection.

Single celled organism like amoeba can engulf bacteria as a food source.

*Explain release of materials from cells via exocytosis.*



Understanding: The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

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

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



Understanding: The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

Secretion of neurotransmitter at synaptic terminus.

Secretion of digestive juices from exocrine glands.

*List two reasons for vesicle movement.*



Understanding: Vesicles move materials within cells.

*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).

*Describe how organelles of the endomembrane system function together to produce and secrete proteins (rough ER, smooth ER, Golgi and vesicles).*



Understanding: Vesicles move materials within cells.

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.

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



Understanding: Vesicles move materials within cells.

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) than 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.

*Describe the structure of the sodium-potassium pump.*



Application: Structure and function of the sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

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).

*Outline the steps of sodium-potassium pump action.*



Application: Structure and function of the sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

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.

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



Application: Structure and function of the sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

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*.

*Describe the structure of the potassium channel.*



Application: Structure and function of the sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

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.

*Explain the specificity of the potassium channel.*



Application: Structure and function of the sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

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.

*Describe the action of the "voltage gate" of the potassium channel.*



Application: Structure and function of the sodium-potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

When a neuron is firing, the voltage of the cell changes. The potassium channel will only open when the voltage of the cell has reached its peak (of about 30mv).

*Explain what happens to cells when placed in solutions of the same osmolarity.*



Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

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.

*Explain what happens to cells when placed in solutions of higher osmolarity.*



Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

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).

*Explain what happens to cells when placed in solutions of lower osmolarity.*



Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

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.

*Outline the use of normal saline in medical procedures.​*



Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent osmosis.

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.

*Define osmolarity.*



Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

The concentration of solutes in a solution.

*Define isotonic.*



Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

The osmolarity of two solutions is the same.

*Define hypotonic.*



Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

A solution with a lower osmolarity (fewer solutes) compared to another solution.

*Define hypertonic.*



Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

A solution with a higher osmolarity (more solutes) compared to another solution.

*Determine osmolarity of a sample given changes in mass when placed in solutions of various tonicities.​*



Skill: Estimation of osmolarity in tissues by bathing samples in hypotonic and hypertonic solutions.

The osmolarity of a sample is the point at which there is no net movement between the sample and the solution in which it is placed.

Samples will gain mass when placed in a hypotonic solution (as water moves into the sample). Samples will lose mass when placed in a hypertonic solution (as water moves out of the sample). There will be zero change in mass when the sample is placed in an isotonic solution.

*Define quantitative.*



Nature of Science: Experimental design- accurate quantitative measurement in osmosis experiments are essential.

Data that is in the form of a *number* obtained in a count or measurement.

*Define qualitative.*



Nature of Science: Experimental design- accurate quantitative measurement in osmosis experiments are essential.

Data that is descriptive or subjective.

*Explain the need for repeated measurements (multiple trials) in experimental design.*



Nature of Science: Experimental design- accurate quantitative measurement in osmosis experiments are essential.

Multiple trials allows one to see if the results of each measurement show consistency. Consistent findings reinforce the strength of the conclusion.