Cell Membranes: Protein Functions and Types of Transport

Cell Membranes: Protein Functions and Types of Transport – Brandon Skenandore

  1. Membrane Proteins

Functions –

  1. Transporter molecule – transport something across the membrane
  2. Channels (open)
  3. Carriers (bind to the object and help carry it through)
  4. Enzymes – binds a molecule and changes it into something else
  5. Cell surface receptors – message comes to a cell and its message needs to be signaled inwards and to the inside (like a hormone)
  6. Identity molecules – If there is a sugar attached, expresses some form of identity
  7. Cell adhesion – cells sticking to one another. Some of these proteins are involved in sticking to another one and holding two cells together
  8. Attachment to the cytoskeleton – membrane and proliferal protein holding stuff together

An inner part of the lipids is hydrophobic. Inner protein is hydrophobic. Chemical part of the reaction


  1. Membrane transport

Lots of things that have to move, endless things that have to move. Methods of movement:

  1. Diffusion – movement of molecules from and area of high concentration to an area of low concentration (drop of dye in water)

Example: High [O2] goes from lung cells to capillary cells. Capillary cells have high [CO2]. Oxygen does “simple diffusion”.

No energy input. Movement is always from high [ ] (concentration) to low [ ]. No corner/channel protein. Just goes through

  1. Osmosis – diffusion of H2O molecules through a differentially permeable membrane. Beaker of pure H2O, molecule. Tiny pores (channels) called aquaporin. Little tiny pores that allow the water to diffuse through membranes. Lots of kinds discovered. Human examples would be a capillary. Some solutes in the capillary and some solutes nearby. They are often at the same concentration. They can be called “isosmotic” to each other. Isotonic is the same thing. Identical to isosmotic. Does water move in bulk? No. Water concentration is the same on both sides.
    Another situation would be in the Boise River w/ a cell. Water would move in, from higher to smaller molecules. Contractile vacuoles pump H2O back to the water concentration. River: Hypoosmotic. Organism: Hyperosmotic.
    Pressure in a plant cell is called “Turgor pressure”
  2. Diffusion using a protein channel.
    For ions (little charged sodiums, potassiums, chlorines, etc.) – have specific channels made for them, let something through. Fig. 5.10 in book. No energy use, only from a High [ ] to a low [ ].
  3. Facilitated diffusion (“help out”).
    A carrier protein is used, helps out. No energy use. Still only from High [ ] to Low [ ]. There is a carrier which helps out (fig 5.10b) How sugar gets into our red blood cells. Glucose is transported with a facilitated system.

All of the processes so far have been passive, now we have one that is:

  1. Active transport –
    Takes energy (usually ATP). Uses a protein carrier, can move molecules from low [ ] and accumulate them up to a high [ ]. Our cells require certain concentration (higher concentration of potassium on one side of a cell than the other side). Best example of this is the “Na/K pump”. In a typical cell, they use 1/3 or more of their energy on this pump (large amount of energy) Fig 5.13. In this diagram, using a carrier that has the ability to pump sedum out and pump potassium in. Takes 3 sodiums and with use of some ATP, dumps them to the outside. Then attaches a couple of potassiums and pulls them in. Sodium is pumped out, potassium  is pumped in. Way that our metabolic processes are set up.
    Energy input, carrier, [low] to [high]
  2. Coupled Transport (couple of things come out together) -Active transport, but a little more complex. based on need for ATP. Involves getting something from the membrane, bet the glucose in. Sodium comes back in, along with a glucose that sneaks in. (fig. 5.14). Different protein that lets in sugar. Dependent on active transport protein


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