Cell Membranes and Transport Systems: Diffusion, Osmosis, Channels, and Carrier Proteins, Slides of Zoology

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Cells as Units of Life

Chapter 3

Membrane Function

 Membranes surround the outside of the cell and the organelles inside it.  The plasma membrane acts as a selective gatekeeper.  A substance may cross the membrane:  By diffusion  By a mediated transport system  By endocytosis

Diffusion & Osmosis

 Cell membranes are selectively permeable – water can pass through, but not most solutes.  Gases (oxygen & carbon dioxide), urea, lipid soluble solutes can cross the membrane.

Diffusion & Osmosis

 Osmosis - if there is a membrane between two solutions with unequal concentration of solutes that can not cross the membrane, water will flow toward the side with less water / more solute until the two sides have equal concentrations.

Diffusion & Osmosis

 Animals utilize osmosis to control internal fluid and solute levels.  The blood of marine fishes has 1/3 the salt content of the water. They are hypoosmotic to seawater.  Freshwater fishes have blood that is saltier than the water. They are hyperosmotic to the water.  If the solute concentrations were the same, the two solutions would be isoosmotic.

Diffusion Through Channels

 Charged substances, like water and dissolved ions, can’t simply diffuse across the cell membrane.  They pass through channels created by transmembrane proteins.  Some channels always open.  Some are gated channels.

Carrier Mediated Transport

 Sugars & amino acids must be able to enter cells and waste products must be able to leave.  These molecules cross the membrane with the help of transporter proteins.  Transporter proteins are specific.  Facilitated diffusion  Active transport

Facilitated Diffusion

 In facilitated diffusion , the transporter protein binds to the substrate molecule on one side of the plasma membrane then changes shape to release it on the other side.  Takes place in the direction of the concentration gradient.

Endocytosis

 Endocytosis is the ingestion of material by cells.  Phagocytosis – cell eating – method of feeding by single- celled organisms.  Pinocytosis – small molecules or ions are enclosed in vesicles called caveolae.  Receptor-mediated endocytosis – method of bringing large molecules into a cell with the help of the protein clathrin.

Exocytosis

 Exocytosis - membranes of a vesicle inside the cell can fuse with the plasma membrane to discharge the contents of the vesicle outside the cell.  Transcytosis – a substance may be picked up on one side of the cell, transported completely across the cell and discharged on the other side.