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Cell Membrane
How the Fluid Mosaic Model Controls Transport
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The cell membrane is a thin, flexible boundary that controls what enters and leaves a cell. It is not a solid wall, but a fluid structure made mostly of phospholipids and proteins. This matters because cells must take in nutrients, remove wastes, send signals, and maintain stable internal conditions. The fluid mosaic model explains how the membrane can be both protective and dynamic at the same time.
In this model, phospholipids form a bilayer with water-loving heads facing outward and water-fearing tails facing inward. Proteins float within or on the bilayer and act as channels, carriers, receptors, enzymes, and anchors. Cholesterol helps adjust membrane fluidity, while carbohydrate chains help cells recognize and communicate with each other. Transport across the membrane depends on molecule size, polarity, charge, concentration gradients, and the presence of specific membrane proteins.
Key Facts
- Phospholipid bilayer: hydrophilic heads face water, hydrophobic tails face inward away from water.
- Fluid mosaic model: lipids and many proteins move laterally within the membrane.
- Simple diffusion moves small nonpolar molecules from high to low concentration without energy.
- Osmosis is diffusion of water across a selectively permeable membrane.
- Facilitated diffusion uses channel or carrier proteins and does not require ATP.
- Active transport moves substances against a gradient and requires energy, often ATP.
Vocabulary
- Phospholipid
- A membrane molecule with a hydrophilic phosphate head and hydrophobic fatty acid tails.
- Selective permeability
- The property of a membrane that allows some substances to pass through more easily than others.
- Transport protein
- A membrane protein that helps specific ions or molecules cross the cell membrane.
- Concentration gradient
- A difference in the amount of a substance between two regions.
- Cholesterol
- A lipid in animal cell membranes that helps stabilize the bilayer and regulate fluidity.
Common Mistakes to Avoid
- Thinking the membrane is rigid like a shell. This is wrong because phospholipids and many proteins can move sideways, making the membrane flexible and dynamic.
- Assuming all molecules can diffuse through the bilayer. This is wrong because large, polar, or charged particles usually need transport proteins.
- Confusing facilitated diffusion with active transport. Facilitated diffusion moves substances down their gradient without ATP, while active transport moves substances against a gradient using energy.
- Forgetting that water movement depends on solute concentration. Water moves by osmosis toward the side with higher solute concentration if the solute cannot cross easily.
Practice Questions
- 1 Oxygen concentration is 12 units outside a cell and 3 units inside. In which direction will oxygen diffuse through the membrane, and does this require ATP?
- 2 A cell has 0.2 M solute inside and is placed in a 0.8 M solute solution. If the solute cannot cross the membrane, predict the direction of net water movement.
- 3 Explain why sodium ions cannot easily pass directly through the phospholipid bilayer, but carbon dioxide can.