Homeostasis is the ability of organisms and cells to create and maintain a balanced internal environment when the external environment is different.
The cell membrane surrounds every cell and is what allows certain things in and out of a cell to maintain homeostasis.
It is made up of a phospholipid bilayer. Proteins are found in these like protein channels and protein carriers. Both passive transport and active transport use these proteins.
The phospholipid bilayer is semi-permeable, which means that it allows some but not all things to enter.
Things that can get into the cell are food, water (osmosis), and oxygen (diffusion).
Things that can exit the cell are waste, carbon dioxide (diffusion), water (osmosis, and proteins.
Osmosis is the diffusion of water through the cell membrane.
In osmosis, water flows toward the higher concentration solute. Water flows from high to low, which is the concentration gradient.
A solvent is a substance (like water) that dissolves another substance.
A solute is a substance (like salt) that is dissolved by the solvent.
Passive Transport is a form of transport which allows movement of molecules from a high to low concentration gradient. This requires no energy because the molecules move with the concentration gradient.
- Simple diffusion is when the molecules pass through the cell membrane without help from the transport proteins.
- Facilitated diffusion is the diffusion of solutes with help from the transport proteins.
Active Transport is a form of transport which allows movement of molecules from a low to high concentration gradient. This requires energy because the molecules move against the concentration gradient.
- Active transport uses ATP, which is a form of energy used by the cell, to help move the solutes through the cell membrane.
A cell’s reaction to different environments:
- If a cell swells, the concentration of the solute is greater inside than outside the cell. Hypotonic, think “hippo”.
- If a cell does not change, the concentration of the solute is equal inside and outside the cell. Isotonic.
- If a cell shrinks, the concentration of the solute is smaller inside than outside the cell. Hypertonic, think sugar rush.
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