Factors That Affect The Permeability of The Cell Surface Membrane
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of membrane structure and the role of temperature and solvents on lipid behavior. You can test your knowledge on these below.
What is the structure of the cell membrane according to the fluid mosaic model?
The cell membrane consists of a phospholipid bilayer with embedded proteins, cholesterol, glycoproteins, and glycolipids. The structure is fluid, due to lateral movement of phospholipids, and mosaic due to the proteins embedded within the phospholipid bilayer.
How do small, non-polar molecules cross the membrane?
They diffuse directly through the phospholipid bilayer by simple diffusion because they are lipid-soluble.
What is the role of cholesterol in the cell membrane?
Cholesterol regulates membrane fluidity by preventing it from becoming too rigid at low temperatures or too fluid at high temperatures.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains factors that affect the permeability of the cell surface membrane or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Factors Affecting Cell Membrane Permeability
Cell membranes control the movement of substances into and out of cells. Their permeability can be affected by various factors, including temperature, solvent concentration, and pH.
Temperature
- Low temperatures: Phospholipids have less kinetic energy, so the membrane becomes less fluid and more rigid.
- Moderate temperatures: The membrane remains fluid, allowing molecules to pass through at a normal rate.
- High temperatures:
- Phospholipids gain kinetic energy, making the membrane more fluid, increasing permeability.
- Proteins in the membrane denature, disrupting membrane structure and increasing permeability.
- The cell surface membrane is damaged, increasing permeability.
Solvent Concentration
Organic solvents (e.g., ethanol) dissolve phospholipids, increasing permeability.
Higher ethanol concentration dissolves the phospholipid bilayer more, causing damage to the membrane and leakage of substances (e.g., beetroot pigment in practical experiments).
pH
Extreme pH levels can denature membrane proteins, affecting permeability.
Changes in pH alter hydrogen and ionic bonds within membrane proteins (such as channel and carrier proteins), disrupting their structure.
Investigating Membrane Permeability
- A common practical investigation involves using beetroot cells to measure how much pigment leaks when temperature or ethanol concentration changes.
- The colorimeter measures absorbance, with higher absorbance indicating greater permeability and more pigment leakage out of the beetroot cells.
Key Terms
- Phospholipid bilayer: Double layer of phospholipids that forms the membrane, with hydrophobic tails facing inward and hydrophilic heads facing outward.
- Cholesterol: A lipid that regulates membrane fluidity and stability.
- Denaturation: Structural breakdown of proteins as their tertiary structure changes due to extreme heat or pH.
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Exam Tips
When discussing how temperature effects permeability, always mention both phospholipids and proteins.
If asked about solvent concentration, explain how phospholipids dissolve, increasing permeability.
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A student investigated the effect of temperature on membrane permeability using beetroot cells. The student measured the absorbance of light through solutions containing beetroot pigment at different temperatures.
Explain how increasing temperature affects membrane permeability and how this will affect the leakage of pigment from beetroot cells. (3 marks)
- As temperature increases, phospholipids in the bilayer gain kinetic energy, making the membrane more fluid and increasing permeability.
- At high temperatures, membrane proteins denature, causing gaps in the membrane.
- This allows more pigment to leak from beetroot cells, leading to higher absorbance readings.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!