Cell Membrane Overview

Cell Membrane and Phospholipid Introduction

Cell membranes protect us from the outside world, compartmentalize organelles and reactions, etc.

• Cell membranes are largely made of phospholipids- which themselves contain a glycerol backbone, phospholipid head, and 2 fatty acid tails

  • the phosphate head is polar and hydrophilic

  • the fatty acid tails are nonpolar and hydrophobic

• In aqueous solutions, the phospholipids spontaneously form membrane bilayers with hydrophobic interiors

  • the formation of a phospholipid bilayer is primarily driven by an increase in entropy of the surrounding water molecules

  • lipids cause water to arranged in an ordered, unfavorable cage-like structure (called a Clathrate cage)

    • forcing lipids into a bilayer reduces this effect, thus increasing entropy

The cell membrane is semi-permeable. What can pass?

• small nonpolar molecules (gases like oxygen and carbon dioxide) go through fairly quickly via passive diffusion

• small polar molecules (ethanol and water) can also go through but much more slowly

• large nonpolar molecules (such as benzene) also go through the membrane, but also slowly

• large polar molecules, such as glucose, CANNOT pass through the membrane

• charged molecules (Cl-, Na+, many ions and amino acids) also CANNOT pass through the membrane

Phospholipids often bond with another head group via a phosphodiester bond:

• phophotidyl serine

• phosphotidyl ethanolamine

• diphosphotidyl glycerol ( aka cardiolipin)

• phosphotidyl choline

• phosphotidyl inositol

The fatty acid ‘R’ group is made of long fatty acids, which can sometimes have double bonds

• if cis double bonds, they’ll have a kink, which affects cell membranes

Cell Membrane Proteins and the Fluid Mosaic Model

The main component of the cell membrane is phospholipids, but it can have other molecules incorporated in the the cellular membrane as well.

• Cholesterol: maintains membrane fluidity at low temps and membrane rigidity at high temps

• Proteins: carry out membrane processes. Two main types:

  1. integral/transmembrane proteins: crosses the whole membrane, strongly bonded (hydrophobically) with phospholipids and must be removed by a detergent. Also contain polar and nonpolar regions (charged parts on the outside in aqueous environments)

  2. peripheral proteins: noncovalently bonded to outside of the membrane. Can come and go as its needed

  3. lipid bound proteins: found within the interior of the bilayer. Rare because they cannot access either side of the membrane and thus don’t play a role in membrane performance

• Channel proteins: one of the main types of integral proteins that have a channel/hole that allows things (such as ions) to pass through

  • can also pump things out

  • generally don’t require energy, or ATP and thus go down a concentration gradient

  • ex: aquaporin protein, which selectively allows water through

• Carrier protein: will protect the substance so it can carry the molecule in

  • can go against the concentration gradient if needed (which uses ATP)

  • glucose typically enters the cell through facilitated diffusion via a carrier protein

• Glycoprotein: membrane proteins that are also bound to carbohydrates (glycoproteins); these play a big role in cell-cell communication and signaling with/recognizing other cells

  • carbohydrates are always on the outside of the cell for this reason

  • there also can be glycolipids, carbohydrates covalently bonded to a lipid that anchors it

Fluid mosaic model (1972): the cell is made of many different components that are relatively fluid, the different ‘pieces’ can undergo rapid lateral diffusion around their layer

Cell Membrane Fluidity

Temperature

• as temperature decreases, fluidity decreases (phospholipids will cluster together and not have energy to move around- called crystallized state at very low temps)

  • membrane is rigid and may break

• as temperature increases, fluidity increases (more space between phospholipids)

  • membrane won’t hold shape

Cholesterol: sort of a buffer, allows the membrane to maintain a certain level of fluidity

• at low temps, cholesterol helps maintain fluidity by creating space between phospholipids

• at high tempts, cholesterol can help maintain integrity (b/c phospholipids want to get closer to cholesterol molecules)

Unsaturated or saturated fatty acids in the phospholipids:

• saturated fatty acid chains pack tighter than unsaturated, and thus will have lower fluidity than the kinked unsaturated fatty acid chains

Membrane Dynamics

How do phospholipids move in a cell membrane?

• Uncatalyzed movement:

  • Transbilayer, or ‘flipflop’ diffusion: when a phospholipid goes from the inner leaflet to outer leaflet, or vice versa. This is a very slow and uncommon

  • Lateral diffusion: phospholipids move all around their leaflet. Very fast and common

• Catalyzed movement:

  • Flippase protein: catalyzes the movement of a phospholipid from OUTTER leaflet to the INNER, using ATP. Fast compared to transbilayer diffusion

  • Floppase protein: catalyzes the movement of a phospholipid from INNER leaflet to OUTER leaflet, using ATP

  • Scramblase: catalyzes the simultaneous movement of one phospholipid from inner to outer leaflet and the second phospholipid from the outer to the inner leaflet. DOES NOT REQUIRE ATP

What happens when there is a problem with the cell membrane’s ability to uptake/export important molecules or communicate?

• There are many diseases associated with problems in the ability of the phospholipid bilayer to perform these functions. One of these is Alzheimer’s disease, characterized by brain shrinkage and memory loss. One idea explaining why Alzheimer’s disease occurs is the forming of plaque sticking to the phospholipid bilayer of the brain neurons. These plaques block communication between the brain neurons, eventually leading to neuron death and in turn causing the symptoms of Alzheimer’s such as poor short-term memory

Cell-Cell Interaction