Cell Membranes

Components

• Lipids (L) – 25-50% (includes glycolipids)

• Proteins (P) – 50-75% (includes glycoproteins)

The plasma membrane: roles attributed to membrane components (L=lipids; P=proteins)

• A cell’s plasma membrane provides a barrier to large, polar or ionic solutes– L

• The membrane has passageways through which specific ions flow, nutrients enter and wastes leave – P.

• The plasma membrane is equipped with sensors, to detect and initiate responses to the environment –P

• The membrane exterior surface bears markers that reflect the cell’s function and identity – P

• The membrane has pumps that, by doing work on solutes, maintain the osmotic pressure and ionic composition of the internal environment – P.

• The membrane has specialized areas that stabilize the cell’s shape and relationship with its neighbors – L,P.

• The extensions of the plasma membrane function in motility as they cover cilia, flagella and pseudopodia that allow many cells to move themselves or the environment L, P.

Membranes in the animal cell include:

• 1. plasma membrane• 2. the double membrane of the nucleus• 3. the endoplasmic reticulum and Golgi

apparatus• 4. The membranes of organelles like lysosomes

and peroxisomes• 5. the membranes of the mitochondrion• 6. others…

Q: Where do the cell’s various membranes come from?

A: All membranes come from pre-existing membranes, i.e., they are made by

expansion of membranes that are already present, either by synthesis of new

phospholipid or by transfer of membrane from another part of the cell. Most

membrane synthesis is associated with the smooth endoplasmic reticulum (ER) and

delivery in vesicles from the Golgi apparatus.

Synthesis of membrane phospholipids

Synthesis is accomplished by enzymes present in the cytosol side of the smooth ER membrane:

In the first steps, the enzymes activate a fatty acid by attaching a molecule of Coenzyme A (CoA), unite the activated fatty acid with a glycerol-3-phosphate, and then add a second activated fatty acid.

Smooth ER cytoplasm

A phosphatase then removes the phosphate group from the glycerol backbone, leaving a diacylglycerol.

In the last step, a phosphorylated amine (phosphoethanolamine in this example) is activated by cytosine triphosphate and then is attached to the 3rd carbon of the glycerol backbone.

What is this coenzyme A thing,anyway?

Hey, this is ADP!

This is pantothenic acid – Vitamin B5

But…the components and the synthetic enzymes are present only in the cytosol and inner membrane leaflet…Do you see a problem?

The solution? • a protein class known as flippases allows new membrane

lipids to move to the outer leaflet of the plasma lipid bilayer

or inner layer of the organelle membranes.

From Flippin' lipidsMarcus R Clark Nature Immunology12, 373–375(2011)

(a) Comparison of the functions of flippases, floppases and scramblases in the plasma membrane. Flippases (left) use ATP to move the aminophospholipids PS and, to a lesser extent, phosphatidylethanolamine (PE), from the outer leaflet to the inner leaflet of the PM against a concentration gradient. Floppases (middle) use ATP to transport substrates such as phosphatidylcholine (PC), sphingolipid (SL) and cholesterol against concentration gradients in the opposite direction. Scramblases (right) are ATP independent and less substrate specific and facilitate the movement of lipids along concentration gradients.

(b) (b) Some PS functions in cells. When cells undergo apoptosis (left), the activation of scramblases allows the rapid appearance of PS on the outer leaflet of the plasma membrane, where it provides an 'eat me' signal. On the PM inner leaflet (middle), PS helps organize lipid rafts and can serve to recruit members of the protein kinase C (PKC) family through their C2 domain, as well as signaling molecules containing hydrophobic side chains, such as Ras, Rho and Src. PS can induce local membrane curvature (right) and recruit specific effector complexes that facilitate receptor endocytosis, vesicle formation and endocytic trafficking.

Detailed caption for the previous slide

How do these differences in membrane lipid composition arise?

The lipids and proteins needed in different membrane domains are targeted to those sites after their synthesis, by incorporation into labeled vesicles. In these epithelial cells , there are apical and basolateral membrane domains that are characterized by specific proteins.

surfaces.

* Note that in epithelial cells tight junctions both join adjacent cells and restrict the movement of the different kinds of membrane components between the apical (red) and basal (green) membrane domains within the same cell

Tight junctions are sites where proteins extending from adjacent cells link the cells to form a sheet

that is relatively impenetrable

Looking at the membrane: Membrane proteins – the mosaic part of the membrane

• Method of visualization: Freeze-fracture microscopy

Actual fractured membranes: the frozen membrane fractures along its path of least resistance, which is the nonpolar interior. The membrane leaflets are

coated with a deposit of gold or platinum, often followed by carbon. The biological material is digested off and the delicate replica is viewed with the

electron microscope.

View of an erythrocyte: the outer surface is smoother than the view revealed by fracturing off half of the bilayer to reveal the inner surface

or inner leaflet and proteins that extend between the two bilayers.

An Almost-up-to-date Membrane Cartoon

2. Transmembrane alpha helices: hydrophobic regions of the protein

1. Proteins anchored by attachment to lipids

3. Beta sheets (hydrophobic regions) forming a beta barrel, found in bacteria, mitochondria and chloroplasts

Structural features of membrane-protein interaction

Hydropathy plots use a protein’s amino acid sequence to predict whether a protein will be an intrinsic membrane protein and which parts of the protein will be the intramembrane domains. For an intramembrane domain, there must be a run of about 20 hydrophobic amino acids in the sequence.

Proteins can also associate with the membrane by electrostatic attraction to phospholipids, integral membrane proteins or surface sugars.

Electrostatic attraction holds annexin (a Ca++-binding protein important in membrane fusion reactions) in place on the membrane surface.

Rafts in the membrane. • Rafts result from preferential association of special lipids, such as

sphingomyelin; these are semisolid regions that allow concentration of certain proteins or attachment points for the internal skeleton

How do we know about rafting and other factors that affect dynamics of proteins in membranes? Here, a fluorescent bead visible with a light

microscope is coupled to a membrane protein, allowing it to be tracked over time and to identify conditions that restrict movement.

Useful features of membranes: 1. enzymes

• Many enzymatic reactions take place on the membrane surface – we will focus on mitochondrial reactions, but there are many more.

Useful features of the membranes: 2. receptors

• Receptors allow the cell to detect cues from the environment. (cues can be paracrines, hormones or neurotransmitters, the level of CO2 or glucose, the presence of a new type of cell next door…)

• Receptors set in motion the chain of events that coordinate cell’s responses to its environment. (responses can be quick or prolonged: opening a K+ channel, changing the level of an enzyme, turning on a set of genes…)

Useful features of membranes: 3. ionic gradients

The fact that pumps in the cell’s membranes can separate different concentrations of ions and other substances across the membrane (cytosol vs. extracellular, intraorganelle vs. cytosol) can be used in the following processes:

Processes driven by ionic gradients

• Uptake/efflux of nutrients, salts, metabolites• Osmotic regulation (water follows salt)• ATP synthesis in mitochondria and in prokaryote

cell membranes• drug and toxin efflux, pH regulation• Signal transduction (Ca++ entry, action

potentials)• H+-driven flagellar rotation (bacterial flagella).

Formulate definitions of the following terms

• Gradient

• Diffusion

• Permeability

• Passive versus active transport

• Primary active transport versus secondary active transport

Movement through the membrane: Passive = down a free energy gradient

1. Simple Passive diffusion: small, uncharged molecules pass through the phospholipid structure by “dissolving” in it, so no gradient for this category of molecules is maintained across the membrane.

2. Facilitated (passive) diffusion:

a) channel proteins form open pores through the membrane, selecting what can pass through on the basis of size and charge.

b) Passive carrier proteins bind to the molecule to be transported and undergo a conformational change to deliver it to the other side of the membrane. They simply facilitate downhill movement.

Water channels, or aquaporins

• Aquaporins belong to an ancient family that has been conserved from bacteria to humans. The two halves of the protein arose by gene duplication. The 4 subunits provide 4 separate water channels. The presence of these channels in red blood cells explains why they swell and shrink so rapidly when exposed to hypotonic (diluted) or hypertonic (concentrated) solutions.

The glucose transporter (GLUT1) increases the rate that glucose travels down its concentration

gradient: structure of the 12 transmembrane helices and model illustrating its operation

An example of an ion channel at work • Ion channels are designed to allow the passage of specific ions

on the basis of size and charge. Those that are open constitutively allow a specific type of ion to pass through the membrane freely; however, most ion channels are “gated”. An example of a free-passage or “leak” channel is the one for potassium ions that is open in the “resting” membrane of a nerve cell such as the squid axon, illustrated below:

Active transport = movement of solute against a free energy gradient

• 1. Primary active transport: the carrier protein is also an ATPase – ATP provides the energy to concentrate solute against a chemical (or electrochemical) gradient.

• 2. Secondary active transport: The carrier draws on one solute’s transmembrane energy gradient to move another solute against its transmembrane gradient.

Active transport driven by ATP hydrolysis• The ABC transporters

are the largest family of membrane transporters, characterized by their ATP binding cassettes. The chloride transporter that is defective in cystic fibrosis belongs to this family, as does the multidrug resistance gene product, MDR. MDR normally pumps out toxins, but its expression can be greatly increased by selection pressure in cancer cells during chemotherapy.

HHowow MDR

How natural selection defeats chemotherapy

Channels that apply energy to push ions against their concentration gradient are called pumps

• Active transport driven by ATP: the Na+-K+ pump maintains a critical difference: the concentration of K+ is higher in the cytoplasm and the concentration of Na+ is around 10x higher outside than in the cytoplasm.

The Na+/K+ pump

Model of the Na+K+ pump: Goal is to throw out 3 Na+’s and get 2 K+’s – cost is 1 ATP

In some cells, secondary active transport of glucose is driven by the Na+ gradient (which is

maintained by the Na+/K+ pump) The electrochemical gradient of the Na+ ions is the force that drives the movement of glucose up its concentration gradient.

In mammals, this SGLT1 transporter is found in kidney tubules, the intestine, and cerebral capillaries.

Conclusions

• Membranes create the possibility for the cell to regulate its internal environment. The membrane’s phospholipids form a barrier and at the same time position the proteins to mediate the cell’s interactions across the barrier. The following functions are regulated by interactions at the cell’s membrane: ion concentrations (and therefore water), accumulation of molecules to fuel the cell’s demands for growth, repair and energy, release of toxins inadvertently taken in or produced, regulation of gene activity and decisions about cell division (based on molecular cues sent by neighbors and detected by membrane receptors), and a host of specific functions characteristic of the different membranes and different kinds of cells…