5. What kinds of things can pass through the phospholipid bilayer (e.g. nutrient

Question

5. What kinds of things can pass through the phospholipid bilayer (e.g. nutrients, waste, water, electrolytes, proteins, macromolecules, bacteria, cells...)? B. MEMBRANE TRANSPORT 1. Define solution, solute and solvent. Solution Solute Solvent: 2. What is diffusion? 3. List the factors that affect the rate (speed) of diffusion through a membrane. 4. There are two properties of any given molecule that determine its movement across cell membranes: the size of the molecule and its solubility in lipids. Large non lipid-soluble molecules cannot cross directly, whereas small lipid-soluble molecules can cross directly through the bilayer. Based on this knowledge, predict which of the following things will be able to pass directly through the phospholipid bilayer. Explain your reasoning in the justification column. Justification Substance Pass YES/NO CO2 NaCI albumin (protein) Water 02 glucose steroid hormone 5. How could something that cannot get through the phospholipid bilayer manage to get across the plasma membrane? 98

Explanation / Answer

5. This is a very abstract question. All of the substances listed can pass through the phospholipid bilayer. The bilayer may be permeable or impermeable to a substance (e.g. oxygen and water, respectively) but if the cell needs it, the cell devises mechanisms that can overcome this barrier. Similarly (and yet very differently), pathogens and certain cells can also make their way across the bilayer through some intelligent mechanisms.

However, strictly speaking, only small and non-polar substances can pass through the phospholipid bilayer.

B.

1. A solution is a homogeneous mixture of two or more substances; the substance present in lower amount is termed the solute and deemed to be dissolved in the substance present in higher amount, termed the solvent.

2. The net passive movement of molecules down their concentration gradient is called diffusion.

3. Rate of diffusion depends on temperature, viscosity of medium, and molecular mass of particle.

4.

5. Such molecules pass into the cells through channels, exchangers, and transporters or through endocytosis (phagocytosis, pinocytosis, etc.) and exocytosis.

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