S294: Book 2: Chapter 2: Cell membranes

What are peripheral proteins?

Peripheral proteins are not associated with the hydrophobic interior of the lipid bilayer; instead, they either associate with the membrane indirectly via non-covalent interactions with other membrane proteins (5, 6) or directly via interactions with the hydrophilic head groups of the lipids (7).

What do polar and non-polar mean and how does this affect their relationship to water?

Polar is hydrophilic (water loving) Non-polar is hydrophobic (water hating)

Learning Outcomes: 2.1 Describe the structure and characteristics of biological membranes, including the fluid-mosaic model and the more recent concept of lipid rafts. 2.5 Explain how the chemical environment within the lipid bilayer dictates the structure of membrane proteins. 2.7 Explain the factors determining water movement across membranes. 2.8 Explain how ion gradients across the cell membrane result in the membrane potential. 2.9 Explain what is meant by passive diffusion, facilitated transport and active transport. 2.10 Outline how different substances are transported within a cell via the secretory pathway and how extracellular substances are taken up by the cell by endocytosis.

2.1

2.3 Relate the physical properties of biological membranes to their chemical composition and their biological functions.

2.3: The functions of membranes depend on their physical and chemical properties; so to explore the structure-function relationship of membranes more closely, it is necessary to consider the properties of their molecular components.

Why do you think flip-flop of membrane lipids occurs much less often than lateral diffusion?

Flip-flop requires the passage of the hydrophillic polar head group of the lipid through the non-polar hydrophobic interior of the bilayer. So this would be energetically unfavourable.

2.4 Describe, using examples, the different types of membrane lipids and membrane proteins.

2.4: There are three different type of lipids: Phospholipids, Glycolipids and Chlolesterol 1) Phospholipids: PC - phosphatidylcholine PS - phosphatidylserine PE - phosphatidylethanolamine PI - phosphatidylinositol In phospholipids, the hydrophilic head group is composed of a negatively charged phosphate group (-PO4−) 2) Glycolipids: In glycolipids, the hydrophilic head group is a sugar residue (galactose or glucose Both phospholipids and glycolipids come in two varieties: those that contain glycerol (glycerophospholipids and glyceroglycolipids; collectively called glycerolipids) and those that contain sphingosine (sphingophospholipids and sphingoglycolipids; collectively called sphingolipids) 3) Chlolesterol: Is structurally very different from the phospholipids and glycolipids, though it too is amphipathic. Cholesterol is a sterol which is a type of steroid, a group of molecules that are distinguished by a common four-ring chemical structure Proteins: There are three kinds of membrane protein which are categorised depending on how they associate with the lipid bilayer. 1) Integral Proteins Integral Proteins have some portion that interacts with the hydrophobic core of the lipid bilayer. - transmembrane proteins are lipid proteins that span the lipid bilayer. - single pass transmembrane proteins pass only once throughout eh membranes in Glycophorin C - multipass transmembrane proteins - the polypeptide chain loops and crosses the membrane several times. such as in the protein: bacteriorhodopsin - with some proteins only one part is embedded and anchored in the bilayer - by a hydrophobic alpha segment usually at the cystolic side - 2) Peripheral proteins Peripheral proteins are not associated with the hydrophobic interior of the lipid bilayer; instead, they either associate with the membrane indirectly via non-covalent interactions with other membrane proteins (5, 6) or directly via interactions with the hydrophilic head groups of the lipids (7). 3) Lipid Linked membrane proteins Lipid linked membrane proteins are anchored in the bilayer by means of a lipid linkage at the cytosolic surface (8) or the extracellular surface (9). Functions of membrane proteins include: 1) transport of specific molecules and ions into or out of the cell or organelle (Section 2.8) 2) receptors for chemical signals from outside the cell or organelle (to be discussed in Chapter 4) 3) structural links between the cytoskeleton and the extracellular matrix (Sections 2.3.3 and 2.3.5) 4) intercellular recognition and adhesion (Section 2.3.5) 5) catalysis - the membranes of organelles are characterised by distinctive enzyme components.

What is the fluid mosaic model of the cell membrane?

A model of membrane structure, which visualises the membrane as a fluid lipid bilayer with membrane proteins embedded in, spanning or attached to this bilayer. Key to this model is the idea that the lipids and proteins of the membrane are highly mobile in the plane of the membrane.

What are integral proteins?

Integral proteins are amphipathic having both hydrophobic and hydrophilic portions. The hydrophobic part of an integral protein interacts with the hydrophobic core of the lipid bilayer. A polypeptide chain can travel across the bilayer. If it loops and travels across only once it is call a single pass transmembrane protein and if it loops and crosses more than once it is called a multi-pass transmembrane protein. Transmembrane proteins span across the lipid bilayer whereas some integral proteins only associate on one side, being embedded in the membrane.

(a) Briefly describe how integral membrane proteins associate with the lipid bilayer. Using a simple outline shape to represent the protein, draw a diagram to illustrate your answer. (4 marks)

Integral proteins are amphipathic having both hydrophobic and hydrophilic portions. The hydrophobic part of an integral protein interacts with the hydrophobic core of the lipid bilayer. A polypeptide chain can travel across the bilayer. If it loops and travels across only once it is call a single pass transmembrane protein and if it loops and crosses more than once it is called a multi-pass transmembrane protein. Transmembrane proteins span across the lipid bilayer whereas some integral proteins only associate on one side, being embedded in the membrane. Depending on the protein the transmembrane region is either an alpha helix, a bundle of alpha helices or a Beta barrel.

What is the function of the cell membrane?

It regulates the passage of solutes into and out of the cell.

What are lipid proteins?

Lipid-linked membrane proteins are anchored in the bilayer by means of a lipid linkage at the cytosolic surface or the extracellular surface.

What is a liposome?

Liposomes are spherical fluid filled bilayer structures that form spontaneously in an aqueous environment from two tailed amphipathic lipids

(b) How do non-covalent interactions help to stabilise the structure of the integral membrane protein and its interactions with the membrane?

The interior of the membrane is non-polar hydrophobic. The polar portions of the protein associate with the polar head (hydrophilic) groups (outer layer) of the lipids via hydrogen bonding and ionic interactions, or are buried deep within the protein. There are so many of these hydrogen bonds that they make up for the relative weakness of the bond.