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BSC 2010C: Principles of Biology I: Membranes

This library guide supports the FSCJ course BSC 2010C Principles of Biology.




Keeping It Together: Cell Membranes (32:22)
With all the activities inside and outside the cell, how does the cell membrane hold it all together? This program closely examines the structure and function of cell membranes, including compartmentalization, intercellular interaction, regulation of the movement of materials, and serving as a location for biochemical activities. Using a combination of narration, film footage, and engaging graphics, the program covers the various ways in which materials can cross the cell membrane, such as diffusion, active and passive transport, osmosis, and endo- and exocytosis. The effects of osmosis—plasmolysis and turgor in plants—are also presented. (33 minutes)

Open Educational Resource



Plasma membranes

Plasma membranes surround cells, (plant cells also have a cell wall). Membranes are selectively permeable (only allow certain molecules through) and are roughly 10 nm thick.

Membranes are made of phospholipid and protein, arranged in a fluid mosaic. There are two layers of phospholipid, with large protein molecules floating in this oily layer (most proteins go all the way across the membrane). "Fluid" means that the membrane is liquid. "Mosaic" refers to the proteins scattered around like mosaic tiles on a floor. Diagram

Unsaturated fatty acids make the membrane fluid.
Saturated fatty acids make the membrane viscous (so it does not change shape easily).

Cholesterol is added to some membranes as an antifreeze; organisms living in cold climates can survive temperatures below freezing because their membranes stay fluid.

Proteins in the membrane have many functions including : transport, enzymes, receptors for hormones and joining cells together.

Carbohydrates (oligosaccharides) on the outside of the membrane let the immune system identify the cell.
These carbohydrates are important in organ transplants: the donor organ has to match (as closely as possible) the recipient.

Movement of Chemicals

Passive transport : the cell uses no energy. Passive transport includes diffusion and osmosis.

Diffusion : the movement of molecules from high concentration to low concentration ( liquid or gas ).
Cells get a lot of materials like glucose and oxygen by diffusion.

Osmosis : the diffusion of water through a selectively permeable membrane.

  • Isotonic solution : has the same water concentration as the cell, eg blood
    Cells in an isotonic solution stay the same size.

    • Hypotonic solution : has more water than inside the cell, eg freshwater
      Cells in a hypotonic solution swell (plants, animals) and may burst (animals). Diagram

      • Hypertonic solution : has less water than inside the cell, eg seawater
        Cells in hypertonic solution shrink (dehydrate). Membranes and water

      Active transport : requires energy ( ATP ).

      There are three types: pumps, exocytosis and endocytosis.
      • Pumps : membrane proteins that transport chemicals
        example : sodium/potassium pump. Pump animation.
        sodium is pumped out of the cell, potassium is pumped in.

        • Endocytosis : large molecules are pulled into the cell
          a) Phagocytosis : solids brought in
          e.g. white blood cells eat bacteria. Movie (Click on "510K Time-lapse Movie")

          b) Pinocytosis : liquids brought in
          e.g. cells around blood vessels

          • Exocytosis: large molecules are pushed out of the cell.
            example : tears, milk
          Last edited September 2014, by David Byres.