The Cell Membrane Overview The cell membrane separates
The Cell Membrane
Overview § The cell membrane separates a living cell from its nonliving surroundings u thin barrier = 8 nm thick (n=nano=10 -9) § Controls traffic in & out of the cell u u selectively permeable allows some substances to cross more easily than others- “choosy” § Made of phospholipids, proteins , cholesterol, and carbohydrates.
Phospholipids § Tails are made of fatty acids Hydrophobic § “Water fearing” Heads are made of phosphate u Hydrophilic § “Water loving” Arranged as a bilayer Phosphate u § § Fatty acid Inside cell Why do the phospholipids arrange themselves like this? Outside cell
Cholesterol also makes up the cell membrane structure. It is between the tails of the phospholipids. Fluid outside the cell Phospholipids Cholesterol Cytoplasm
Fluid Mosaic Model § In 1972, S. J. Singer & G. Nicolson proposed § § that membrane proteins are inserted into the phospholipid bilayer The composition of the cell membrane is called the fluid mosaic model because the phospholipid bilayer and the embedded proteins can move laterally in the membrane like a “fluid” to let compounds into and out of the cell. The cell membrane has the consistency of vegetable oil because of the cholesterol and fatty acid tails.
§ What is the natural movement of molecules into and out of the cell?
Simple Diffusion § Movement from HIGH to LOW concentration “passive transport” u no energy needed (ATP)- besides natural kinetic energy u diffusion
Simple diffusion through phospholipid bilayer § What molecules can get through directly? fats & other lipids (hydrophobic) u tiny molecules like O 2 and CO 2 u Sometimes H 2 O can sneak through u inside cell NH 3 lipid salt § What molecules can NOT get through directly? u u outside cell sugar aa H 2 O u Sometimes H 2 O Large molecules § Salts § Sugars Ions (particles with a charge)
How do the non-fat compounds get in/out? § Membrane becomes selectively-permeable with protein channels formed by integral proteins. u This is called facilitated diffusion high low facilitate = to help “The Bodyguard”
Facilitated Diffusion no energy needed (ATP) u still allowing molecules to move from high to low concentration u Integral proteins are specific to what they transport because of their shape u inside cell NH 3 salt H 2 O aa sugar outside cell
Osmosis § Water is very important to cell function § Diffusion of water from high concentration of water to low concentration of water is osmosis u across a selectively-permeable membrane
§ The direction of osmosis is determined by comparing solute concentrations on each side of the membrane. What is a solute? u A dissolved substance u Hypertonic solution - more solute, less water than another solution u Hypotonic solution - less solute, more water than another solution u Isotonic solution - equal solute, equal water with another solution water hypotonic hypertonic net movement of water
For each cell, label the solutions as hypertonic, hypotonic or isotonic and draw an arrow to show the direction of water movement.
Managing water balance § Cell survival depends on balancing water uptake & loss freshwater balanced saltwater
Managing water balance § Another example: u Plant cells § When in a hypotonic (freshwater) environment they are constantly taking up water. § Do they burst? w No! w Plant cells have a cell wall that prevents bursting. w They build up turgor pressure which makes the plant stand tall.
Managing water balance § Another example: u Plant cells § How do they deal with a hypertonic environment? w. They lose water causing the cell membrane to pull away from the cell wall. This is called plasmolysis.
Getting through cell membrane § Passive Transport u diffusion with the concentration gradient § high low u Simple diffusion § diffusion of hydrophobic molecules, small molecules and sometimes water u Facilitated diffusion § diffusion of large molecules, ions and sometimes water § through a protein channel w high low concentration gradient § Active transport u diffusion against the concentration gradient § low high u u uses a protein pump requires ATP
Transport summary simple diffusion facilitated diffusion active transport ATP
Active Transport § Why is active transport necessary? u The Na+/K+ pump is the way that our nervous system works. By pumping ions up their concentration gradients, electricity is generated. conformational change ATP
Na+/K+ Pump u The steps of the pump are as follows: 1. 3 Na+ ions inside the cell bind to the carrier protein (pump). 2. A phosphate from ATP attaches to the pump and the pump changes shape. 3. The 3 Na+ ions are dumped outside the cell. 4. 2 K+ ions outside the cell bind to the pump. 5. The phosphate detaches from the pump and the pump goes back to its original shape.
How about large molecules and fluids? ALSO- ACTIVE TRANSPORT! u Endocytosis. The taking in of large molecules (too big to pass through the membrane) or a large amount of a fluid by “engulfing”. To engulf means that the cell membrane pinches in and surrounds what is being ingested. The molecules are then digested in a pouch called a vesicle. § phagocytosis = Ingesting very large molecules and sometimes whole cells § pinocytosis = Ingesting large amounts of a fluid
u Exocytosis- Active Transport § The opposite of endocytosis. The vesicle fuses with the cell membrane and it then opens up to release the contents.
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