Membrane Transport (1.4) Flashcards
Passive Transport
Passive transport does not use energy as molecules are moving from a high to a low concentration i.e down the concentration gradient. Examples of passive transport are diffusion, facilitated diffusion and osmosis.
Active Transport
Active transport does use energy as molecules are moving from a low to a high concentration i.e against the concentration gradient. Examples of active transport are ion pumps and phagocytosis.
Diffusion
Diffusion is the passive transport of particles from an area of high concentration to one of low concentration. It does not use energy and occurs as a result of random particle motion.
- Membranes are semi permeable so most substances cannot diffuse across because of their size or polarity.
- Steeper concentration gradient=faster rate of diffusion
- It is mostly gases (oxygen, carbon dioxide), minerals and water molecules that move in and out of cells by diffusion.
Factors that affect the rate of diffusion are;
1) The size of the molecule
2) The concentration gradient
3) The temperature
Facilitated Diffusion
Facilitated diffusion is the passive transport of molecules/ions across a cell membrane through specific transmembrane integral proteins.
Polar molecules (glucose, amino acids and large ions dissolved in water) cannot diffuse through the membrane because of the hydrophobic lipid tails of the phospholipids in the bilayer. So all polar molecules are transported by integral proteins. Small, non-polar molecules like oxygen and carbon dioxide can diffuse easily through the membrane un-facilitated.
Examples of facilitated diffusion (short answer)
- Sodium ions, chloride ions, amino acids, glucose molecules and nucleosides.
Ion Channels (specifically potassium ion channels)
Channel proteins are also used in a different type of facilitated diffusion and open a channel which allows for the rapid transport of smaller molecules/ions eg. sodium, potassium, calcium and chlorine into the cell.
The potassium ion channel is used to move K+ ions across the membrane. The channel works with the sodium potassium pump in the axons of neurons.
Osmosis
Osmosis is the passive transport of water molecules from an area of high water concentration to an area of low water concentration.
A cell placed in a high salt solution (low water concentration) will lose water and shrink, become flaccid. This is because the cell is hypotonic and the solution is hypertonic and in osmosis water moves to the hypertonic side. If it is a plant cell the cell membrane will pull away from the cell wall, and in extreme cases cannot recover. This is called plasmolysis.
A cell placed in a low salt solution (high water concentration) will gain water and swell, become turgid. This is because the cell is hypertonic and the solution is hypotonic and in osmosis water travels to the hypertonic side. If it is a plant cell the water will be constrained by the cell wall. Animal/protist cells may burst.
If a cell is the same concentration as the solution then the cell will remained unchanged. This is because the solution is isotonic to the cell.
Osmolarity
A measure of the concentration of the solutes in a solution. Water has an osmolarity of zero.
Protein Pumps (Active Transport)
Protein pumps transport specific substances in a specific direction. The substance binds to a protein and uses ATP energy to move across the membrane.
Uniport (one substance in one direction)
Symport (two different substances in the same direction)
Antiport (two different substances in different directions)
What are vesicles?
Vesicles are small vacuoles storing protein like hormones/enzymes which form off of the endoplasmic reticulum or golgi bodies and travel to the cell membrane.
Exocytosis (secretion)
In exocytosis, vesicles fuse with the cell membrane and expel their contents. Examples of exocytosis are contractile vacuoles in paramecium.
Endocytosis
In endocytosis the vesicle is entering the cell from the outside. Part of the cell membrane is pulled inward, the material is engulfed and a vesicle is pinched off.
Phagocytosis is when solid material is engulfed. For example white blood cells engulfing bacteria.
Pinocytosis is when fluid material is engulfed. For example amoeba engulfing fluid with pseudopods.
