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.
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
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 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
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:
Signalling cell ============> Target cell