Topic 10 - Membrane Structure

Question 1

what are membranes?

Answer 1

-lipid bilayers with embedded proteins that act as channels, transporters, sensors and receptors for the cell
-has various properties

Question 2

what is the role of channels and transporters within the cell?

Answer 2

-exchange of materials

Question 3

what is the role of sensors and receptors within the cell?

Answer 3

-receiving info about environment changes to allow the cell to form a response

Question 4

what are the properties of a membrane?

Answer 4

-grows with the cell
-deforms without tearing
-self-healing (can reseal itself if it breaks very quickly since the exposure of the inside of the membrane is very unfavourable)

Question 5

where are membranes present in bacteria and eukaryotes?

Answer 5

-bacteria = plasma membrane
-eukaryotes = plasma membrane and internal membranes (organelles)

Question 6

how do the plasma membrane and organelle membranes differ?

Answer 6

-within their proteins embedded

Question 7

what is the most abundant lipid within membranes? what is its structure?

Answer 7

-phospholipid
-phosphate containing head and a fatty acid hydrocarbon hydrophobic tail (amphipathic)
-ex: phosphatidylcholine which is abundant in plants and animals. has a small choline molecule attached to the phosphate

Question 8

why is it important for phospholipids to be amphipathic?

Answer 8

-drives bilayer assembly in aqueous environments (spontaneously folds into sealed compartments)
-forces fatty acid tails to face towards each other, not in touch with water
-forces hydrophilic head groups to face away from each other, in touch with water/aqueous environments

Question 9

what are steroids?

Answer 9

-amphipathic molecules embedded in the membrane
-ex: cholesterol

Question 10

what is a glycolipid?

Answer 10

-lipid with a sugar molecule attached to the hydrophilic head

Question 11

what does it mean to say the membrane has fluidity?

Answer 11

-molecules move constantly
-they change places with each other and have rapid rotation (500 revolutions per second)
-mostly change within their side/plane of the membrane, flip flops are rare (energetically unfavourable and would need a protein assist)

Question 12

what does it mean to say the membrane is flexible?

Answer 12

-bends and moves with the cell as needed

Question 13

what affects membrane fluidity and flexibility?

Answer 13

-saturation of the hydrocarbon tails
-fatty acid tails are often 14-24 carbon atoms long anf can be saturated or unsaturated which affects the closeness of packing within the membrane
-close = less fluidity (less space to move)

Question 14

what is a saturated fatty acid tail?

Answer 14

-longer fatty acids with fewer double bonds
-solid at room temp
-can withstand higher temperatures

Question 15

what is a unsaturated fatty acid tail?

Answer 15

-shorter fatty acids with more double bonds
-liquid at room temp
-cannot withstand high temperatures

Question 16

how do organisms that live in variable environments keep their fatty acid tails? saturated or unsaturated?

Answer 16

-many organisms have the ability to adjust their fatty acid tails within their membranes
-can flip between saturated and unsaturated depending on the need
-ex: bacteria and yeast

Question 17

what role does cholesterol play in membrane fluidity?

Answer 17

-modulates membrane fluidity in animal cells
-fills spaces (left by double bond kinks) in between neighbouring phospholipids
-stiffens the bilayer by making it less flexible and therefore less permeable
-makes up 20% of the lipids in the plasma membrane by weight

Question 18

what does membrane fluidity allow proteins to do?

Answer 18

-allows for proteins to diffuse rapidly in the bilayer and interact with each other (important in cell signalling)

Question 19

where does membrane assembly begin?

Answer 19

-begins in the endoplasmic reticulum
-phospholipids are assembled by enzymes bound to the smooth ER (substrate for the enzymes are fatty acids, glycerol, and phosphate groups)

Question 20

where are newly made phospholipids inserted within the cell?

Answer 20

-inserted into the cytosolic side of the ER membrane
-some will go to other areas within the cell (golgi or plasma membrane)

Question 21

how do newly made phospholipids get redistributed equally among both sides of the membrane?

Answer 21

-enzyme called scramblase
-will randomly flip phospholipids to the opposite side of the membrane

Question 22

what is membrane asymmetry? what facilitates it?

Answer 22

arises in the golgi apparatus (where flippase is)
-means that different phospholipids can be found on opposite sides of the membrane (not consistent or even)
-flippase is a family of enzymes that will transfer SPECIFIC (difference from scramblase) phospholipids to opposite sides of the membrane using energy from ATP hydrolysis

Question 23

how is membrane asymmetry maintained?

Answer 23

-maintained with the budding and fusing of various membranes within the cell (from one organelle to another or from the plasma membrane to organelle or vice versa)
-conserves the orientation of lipids and of membrane bound proteins (glycolipids always found only on the non-cytosolic side of a membrane)

Question 24

what membranes are glycolipids commonly found in? where do they acquire the sugar for their head group?

Answer 24

-most commonly found on the plasma membrane, but they can also be in the membrane of other organelles where they face the lumen (non-cytosolic)
-sugar is acquired from the golgi apparatus
-glycolipids often introduce lopsidedness to increase asymmetry in the membrane

Question 25

what is considered the non-cytosolic side of a membrane in respect to both the plasma membrane and the membrane of an organelle?

Answer 25

-exterior of the cell or the interior lumen of an organelle

Question 26

what is the purpose of membrane proteins? how much of the membrane do they make up?

Answer 26

-to carry out most of the membrane functions -make up about 50% of the plasma membrane MASS (in terms of quantity, there are many more lipids) -rest of the membranes mass is lipids and a small bit of carbohydrates

Question 27

what are the 2 broad kinds of membrane proteins?

Answer 27

-integral membrane proteins -peripheral membrane proteins

Question 28

what are the types of integral membrane proteins?

Answer 28

-transmembrane proteins -cytosolic proteins -lipid-linked proteins -protein-attached proteins

Question 29

what are the characteristics of transmembrane proteins?

Answer 29

-span the entire membrane, have part of their mass on either side (in contact with both aqueous environments) -amphipathic (hydrophobic regions exist crossing the membrane, hydrophilic regions are exposed to the aqueous environments)

Question 30

what are the characteristics of cytosolic proteins?

Answer 30

-only associated with the cytosolic side of the membrane (monolayer associated) through an amphipathic alpha helix on the proteins surface

Question 31

what are the characteristics of lipid linked proteins?

Answer 31

-attached to the membrane by covalent bonds with lipids -present on one side of the membrane or the other, not embedded within directly

Question 32

what are the characteristics of protein attached proteins?

Answer 32

-bound to the membrane by interactions with other membrane proteins -present on one side of the membrane or the other

Question 33

can membrane proteins fall into 2 categories?

Answer 33

-yes! -lipid linked proteins can be considered cytosolic proteins as well

Question 34

how do polypeptides cross the membrane?

Answer 34

-through alpha helices or beta barrels

Question 35

how do polypeptides cross the membrane through alpha helices?

Answer 35

-backbone is polar, but since there is an absence of water, the backbone forms H-bonds with itself, creating the alpha helix -hydrophobic side chains will stick out to interact with the hydrophobic tails

Question 36

what is the difference between single pass and multi-pass transmembrane proteins?

Answer 36

-single pass proteins are often signal receptors (one alpha helix) -multi-pass proteins are made up of a series of amphipathic protein regions formed by multiple alpha helices (has channels that form aqueous pores)

Question 37

what is a beta barrel?

Answer 37

-cylindrical formation of rolled beta sheets -forms a hydrophilic aqueous channel with hydrophobic regions in contact with the fatty acid tails

Question 38

what is an example of a transmembrane protein that crosses the membrane as Beta sheets?

Answer 38

-porin proteins (only in bacteria and mitochondrial membranes) -large water filled pores that allow for the passage of small nutrients, metabolites, and ions across the membrane without allowing larger molecules to cross -has less specificity which is why they are not present within a eukaryotic plasma membrane (not a problem in mitochondrial membranes because the eukaryotic plasma membrane has already been so specific)

Question 39

how are membrane proteins studied?

Answer 39

-through biochemical tests within aqueous solutions -membrane proteins are built for environments that are partially aqueous and partially fatty -use an amphipathic molecule to satisfy environmental conditions of the proteins -use detergents which are single tailed and act to solubilize the membrane by disrupting hydrophobic interactions -detergents will aggregate to form micelles and eventually attach themselves to both the membrane proteins and the membrane lipids -the protein and detergent aggregates can be seperated from the lipid detergent aggregates

Question 40

how is the plasma membrane reinforced?

Answer 40

-plants (cellulose), bacteria (peptidoglycan), yeast (chitin), and fungi (chitin) have rigid cell walls made of various materials -animal cells use a cell cortex

Question 41

what is the structure of the cell cortex? what does this structure allow for?

Answer 41

-meshwork of filamentous proteins that are attached to the underside of the cell membrane -allows for the cell to selectively take up materials from the environment, change shape, and move

Question 42

how do membrane proteins move within the plasma membrane?

Answer 42

-able to diffuse freely within, but often move from high to low concentrations so that there is an even spread throughout -however, sometimes an even spread is not always wanted -proteins will instead move based off of membrane domains

Question 43

what is an example of an animal cell's cell cortex and the proteins involved?

Answer 43

-red blood cells -has a lattice formed by spectrin proteins -spectrin is a dimeric protein that forms a long thin flexible rod (100nm long) -the lattice provides support and maintains the biconcave shape of the cell -intracellular attachment proteins are also present and act to link spectrin to transmembrane proteins so they can better associate with the membrane

Question 44

what structures are proteins tethered to to prevent movement?

Answer 44

-extracellular matrix -adjacent cells -cell cortex

Question 44

what are membrane domains?

Answer 44

-functionally specialized regions in the lipid bilayer where protein movement is restricted -proteins are tethered to structures inside and outside the cell -barriers are also formed to restrict movement

Question 45

what are the barriers formed within membranes to restrict membrane protein movement?

Answer 45

-barriers formed along the line where a cell is sealed to adjacent cells by tight junctions (proteins cannot get past this junction) -ex: epithelial cells within the intestine -transporters of nutrients stay on the apical side -transporters for stuff leaving stay on the basal side

Question 46

what are most membrane proteins? what is the structure of these proteins?

Answer 46

-glycoproteins -proteins with a single oligosaccharide attached (sugar is on the noncytosolic side) -can also have proteoglycans which are proteins with one or more polysaccharide chains (larger than oligosaccharides)

Question 47

what is the glycocalyx?

Answer 47

-all carbohydrates located on the outside of the plasma membrane (glycoproteins, proteoglycans, glycolipids) -protects the cell surface from mechanical stress -attraction to water molecules makes a slimy surface which allows for cells to slide and decrease abrasion

Question 47

what is the structure of carbohydrates?

Answer 47

-linear or branched arrangements -variety of covalent linkages (glycosidic bonds) which creates diversity (1-4, 1-6, 3-6, etc)

Question 48

what is the function of carbohydrates?

Answer 48

-cell-to-cell recognition -adhesion (attach to other cells/tissues)

Question 49

what is lectin?

Answer 49

-transmembrane protein that binds to oligosaccharide side chains -attached to cells lining the bloodstream so that neutrophils within the blood can bind and enter sites of infection (cell-cell recognition)