Slide 1
A cell membrane separates while allowing exchange
Every cell needs a boundary separating its interior from its surroundings. The plasma membrane controls exchange while allowing communication and transport.
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The membrane is different from a cell wall. A wall can provide extra mechanical support outside the membrane, but it does not replace the membrane’s selective control over entry and exit.
Slide 2
Lipids arrange heads outward and tails inward
Many membrane lipids have water-compatible heads and water-avoiding tails. In water, they arrange with tails sheltered inside a bilayer.
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This arrangement forms a barrier to many charged and water-soluble substances. Small nonpolar molecules cross more readily, so permeability depends on molecular properties rather than a universal rule that small things always pass.
Slide 3
Channels and carriers provide selective routes
Channels and carriers help selected substances cross the membrane. Their structure allows the cell to distinguish among molecules or ions.
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A channel provides a pathway that may open and close, while a carrier changes shape during transport. Both can be selective, but their mechanisms and maximum transport rates differ.
Slide 4
Concentration and charge differences can store potential
Cells maintain differences in concentration and electrical charge across membranes. Moving substances against those gradients requires an energy source.
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For an ion, concentration and voltage both influence movement. A low concentration on one side does not by itself determine direction if an opposing electrical difference is strong enough.
Slide 5
The membrane stays organized while its parts move
Membranes are organized but flexible structures. Lipids and many proteins can move within the layer rather than forming an immobile shell.
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Cells can also move larger cargo in membrane-bound vesicles. Budding and fusion change membrane organization while keeping enclosed compartments separated from the surrounding fluid.
Sources
- Cell Membrane Plasma Membrane
National Human Genome Research Institute
- Principles of Membrane Transport
NCBI Bookshelf