About how thick is the plasma membrane?
About 8 nm
What is selective permeability?
A property of biological membranes whereby it allows some substances to cross it more easily than others
What are the most important ingredients of membranes?
Lipids and proteins mostly; some carbohydrates
What are the most abundant lipids in the membrane?
Phospholipids
Are phosopholipids hydrophilic, hydrophobic, or both?
Both – this is called being amphipathic. They have a hydrophilic region and a hydrophobic region
How are phospholipids and proteins arranged in the membranes of cells?
- They are arranged in the fluid mosaic model.
- The membrane is a fluid structure with a mosaic of various proteins embedded in or attached to a phospholipid bilayer.
Envision:
- Fibers of extra-cellular matrix
- Glycoproteins
- Carbohydrates
- Glycolipids
- Integral proteins
- Membrane proteins
- Cholesterol
- Microfilaments of cytoskeleton
What primarily holds together membranes?
Mostly hydrophobic interactions. Much weaker than covalent bonds.
How and how often do the proteins and lipids in the bilayer shift around?
Laterally – very often, 10^7 times per second
Vertical flip-flop - rarely, once per month
What causes kinks in the tails of bilayer lipids, and what effect do those kinks have? Why?
Kinks are caused by double bonds that create unsaturated hydrocarbons. The kinks lower the solidification temperature of the membrane because the layers cannot pack together as closely as saturated tails.
What does cholesterol do to the membrane?
Cholesterol is a “fluidity buffer” that makes it resist changes in membrane fluidity caused by temprature.
Describe evolution and its impact on cell membrane lipid composition.
Adaptations to environments: Fish living in cold environments evolved bilayers with lots of unsaturated lipid tails; bacteria in super hot geysers developed bilayers with lipids that prevented excessive fluidity.
What are the two major populations of membrane proteins?
- Integral proteins
- Peripheral proteins
Describe integral proteins.
Integral proteins penetrate the hydrophobic interier.
Most are transmembrane proteins, spanning the whole membrane.
Others go only part way into the hydrophobic interior.
Describe peripheral proteins.
Peripheral proteins are not embedded in the bilayer; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins.
What are the six major functions performed by proteins of the plasma membrane?
- Transport
- Enzymatic activity
- Signal Transduction
- Cell-cell recognition
- Intercellular joining
- Attachment to the cytoskeleton and extracellular matrix
What membrane macromolecule is most crucial to cell-cell recognition?
Membrane carbohydrates are the most important.
Describe the structure of membrane carbohydrates. How long are they?
They are usually short, branched chains of fewer than 15 sugar units. They might be bonded covalently to lipids (–> glycolipids) or proteins (–> glycoproteins)
Are membranes sided? If so, how does that sidedness arise?
The asymmetrical arrangement of proteins/lipids/carbs in the membrane is determined as the membrane is being built by the ER and Golgi apparatus
Describe the steps of membrane synthesis
- Membrane proteins and lipids are synthesized in the ER.
- In Golgi, glycoproteins’ carbs are modified, and lipids get carbs, becoming glycolipids
- Glycoproteins, glycolipids, and secretory proteins are transported in vesicles to the membrane
- Vesicles fuse with the membrane and release the secretory proteins (exocytosis)
What moves easily across the plasma membrane in both directions?
small molecules, ions, nonpolar molecules (because they are hydrophobic)
Polar molecules have trouble because they are hydrophilic, Charged molecules/atoms also have trouble because they’ll have a surrounding shell of water.
How can some specific ions and a varity of polar molecules pass through the cell membrane? How does it work?
transport proteins (aka channel proteins) that span the membrane.
They act as a sort of tunnel through the membrane
What transports water (polar…) across the membrane?
transport proteins called aquaporins
What are carrier proteins?
Carrier proteins are a type of transport protein that hold onto their passengers and change shape enough to send them across.
