Cells & Their Environment

8.1 -
 8.2 - Cell Transport

Cells maintain homeostasis (keeping a stable internal environment despite a changing external environment) by moving materials into and out of the cell.  This is one of the major roles of the cell membrane.

The cell membrane uses two basic types of transport:
  - Active Transport requires the cell to spend energy to move
     things in and out.
  - Passive Transport does not require the cell to spend energy.

Passive Transport
There are several different types of passive transport.
  1. Diffusion occurs when particles spread out.  In more
      scientific terms, we'd say that particles move from an area
      of higher concentration to an area of lower
      concentration
.  This difference in concentration over a
      distance is called the concentration gradient.
      a. Small, nonpolar molecules can diffuse across the cell
          membrane.
  2. Facilitated Diffusion is similar to diffusion, but involves using
      using proteins to "help" molecules pass through the membrane.
      a. Channel Proteins allow certain molecules/ions to pass
          through into the cell.  (Also called "pores").
      b. Carrier Proteins change shape when certain molecules
          bind to them.  The change in shape moves the molecule
          inside the cell, where it is released.
  3. Osmosis is the diffusion of water molecules.  However, water
      molecules are polar and cannot pass directly through the cell
      membrane.  Instead, they pass through special channel
      proteins called water channels.

Whether water moves into or out of a cell depends on the concentration of the cell in relationship to its environment.
 - A hypertonic solution has a higher concentration than the cell's
    cytoplasm.  Water will move out of the cell to try to dilute the
    environment.  The cell may shrivel up and die.
 - An isotonic solution has the same concentration as the cell's
    cytoplasm.  Water will move freely in and out of the cell,
    maintaining a balance.
 - A hypotonic solution has a lower concentration than the cell's
    cytoplasm.  Water will move into the cell to try to dilute it.  The
    cell may swell up and burst.

Active Transport
Active transport is generally broken into two types:
  1. Pumps are proteins that actively pump ions/molecules against
      their concentration gradients.  (In other words, they create
      areas of higher and lower concentration - rather than allowing
      substances to spread evenly until reaching equilibrium.)
      a. The Sodium-Potassium Pump works by pumping 3
        sodium ions out of the cell and 2 potassium ions in.  ATP is
        used to power the pump, causing the protein to change shape
        to move the ions in and out.
  2. Endocytosis/Exocytosis has to do with the cell membrane
      either forming or absorbing vessicles.
      a. In endocytosis, the cell membrane pinches in to form a
        vessicle inside the cell.
      b. In exocytosis, a vessicle inside the cell merges with the cell
        membrane.  This causes its contents to be dumped out of the
        cell.

 8.3 - Cell Communication
The cells in a multicellular organism - like humans - need to coordinate their activities.  So, they need to be able to communicate with each other.  Cells communicate by sending chemical signals that carry information to other cells.

The pathway for cell communication is:

   

        Signal

Signalling cell ============> Target cell


A signal is anything that directs, guides, or warns.  Cells make and send signal molecules.  Some signals can come from outside the body.  Light and temperature are two examples of environmental signals that cause the body to react.

Sending Signals
 - Cells can communicate directly with neighboring cells through
    the contact of their cell membranes.
 - Communication over longer distances are done by way of...
    1. Hormones
        - Hormones are signal molecules that are made in one part of
           the body and spread throughout the body in the blood
           stream.
        - Hormones only affect their specific target cells.
    2. Nerve Cells
        - Nerve cells transmit messages that have both chemical and
           electrical components.
        - Nerve signals are directed to specific locations - they are
          not spread throughout the body like hormones are.

Receiving Signals
Target cells do not respond to every signal that hits them.  They recognize and respond to only a few signals.  This is because their cell membranes contain receptor proteins - proteins that only bind to certain signal molecules, and which cause the cell to respond.

Receptor proteins have specific binding sites that only allow certain signal molecules to fit in them.  This means that a receptor protein only binds to signals that match the shape of its binding site.  (Some cells have receptor proteins that detect and respond to light.)

When a signal molecule binds to a receptor protein, the receptor protein changes shape - this relays information to the target cell.  This information can cause the target cell to respond in one of three basic ways...
 1. Change in Permeability - transport (channel) proteins may
     open or close, changing the cell's permeability to certain
     substances.
 2. Activation of Enzymes - enzymes may be activated to drive
     chemical reactions inside the cell.
 3. Formation of a Second Messenger - a second messenger
     molecule is formed inside the cell.  This second messenger can
     cause changes elsewhere in the cell.