Question to anyone who has studied biology.
Given that cells can shrink and expand due to osmotic changes, what happens to the cell membrane ?
Is it elastic like a balloon ?
Does it pucker up during shrinkage and then possibly split during expansion ?
Large Zap reward for good answers.
The cellular membrane plays a crucial role during osmotic changes that lead to cell shrinkage or expansion. Here's what happens in both scenarios:
### **Cell Shrinkage (Crenation)**
- **Osmotic Pressure**: When a cell is placed in a hypertonic solution (a solution with a higher solute concentration than the cell's cytoplasm), water moves out of the cell through osmosis, attempting to equalize the concentration of solutes on both sides of the membrane.
- **Membrane Behavior**:
- **Increased Surface Area**: As water leaves the cell, the volume inside decreases, but the total amount of membrane material remains the same, leading to an increase in the surface area-to-volume ratio.
- **Ruffling or Folding**: The excess membrane doesn't just disappear; it often folds or ruffles, causing the cell to take on a shriveled or crenated appearance. This is particularly visible in red blood cells under a microscope, where they go from a biconcave shape to looking like a spiny ball.
- **Membrane Integrity**: If the osmotic pressure is too high, the membrane might not fold gracefully, and in extreme conditions, it could lead to membrane stress or even rupture, although this is more common in plant cells due to the presence of a cell wall.
### **Cell Expansion (Swelling or Lysis)**
- **Osmotic Pressure**: When a cell is immersed in a hypotonic solution (a solution with a lower solute concentration than the cell's cytoplasm), water moves into the cell to dilute the cytoplasm's higher solute concentration.
- **Membrane Behavior**:
- **Stretching**: The influx of water increases the cell's volume, stretching the membrane. The membrane is somewhat elastic and can expand to accommodate the incoming water, but only up to a certain point.
- **Increased Tension**: As the cell swells, tension in the membrane increases. The phospholipid bilayer can stretch, but this stretch is limited by the constraints of the proteins embedded in the membrane and the overall structure of the cytoskeleton.
- **Lysis**: If the osmotic pressure continues to build without counteraction (like in animal cells without a cell wall), the membrane can reach a point of failure, leading to cell lysis or bursting. This is why red blood cells placed in pure water will eventually lyse.
- **Regulatory Mechanisms**: Many cells have regulatory systems to counteract osmotic swelling, like ion channels that release solutes, reducing osmotic pressure, or mechanisms to increase membrane area through vesicle fusion.
### Key Points:
- The cell membrane's ability to respond to osmotic changes is due to its fluid and dynamic nature, allowing for temporary changes in shape and size.
- The response to osmotic changes helps in maintaining cell volume homeostasis, which is vital for cell function and survival.
- In plant cells, the presence of a cell wall prevents the cell from bursting by exerting counter pressure (turgor pressure), although severe dehydration or overhydration can still cause damage to the cell.
These processes illustrate the membrane's role as a semi-permeable barrier and its importance in maintaining the cell's internal environment in the face of external osmotic challenges.
It's why they use a saline solution in IV's. Cells must do a lot of work to keep osmotic balance. There is some play. But they will rupture. You can put erythrocytes in distilled water and watch them pop.
Thank you very much.
I had never seen that so I searched and found a video of it.
BUT I have watched it repeatedly and it appears that the red cells gradually actually shrink and then fade away. This is the opposite of bursting !
Compare their sizes at 0;06 and0;58 in the video.
🤯
I'm not actually sure what's going on in that video. It could be that the cells on the top are sinking or floating out of focus of the microscope.
You can shrink cells under a microscope too. By putting them in salt water. This will cause them to lose water and shrink.
This is better seen with plant cells, like onion cells because they have a cell wall around the membrane that keeps the cells in place and easy to view.
In the video below first the cells are exposed to salt water. Then distilled water
Cell membranes are inelastic. If a cell is flacid (not full) the membrane will wrinkle like your skin if you are in the bath for a long time. When the cell is full, or turgid, the cell membrane is stretched tight.
It is possible for cell membranes to burst. One such scenario is when it is infected with a virus. The virus uses the cell mechanics to replicate until there is no space left and then the cell bursts. Another scenario, although less likely in the body, is if the cell is in a highly saline solution.
