Lect. 7 Membrane Structure - Function

Question 1

Membranes provide:

Answer 1

the structural basis for metabolic order

without membranes: no homeostasis, no metabolism

each contains specific proteins and precise enzymatic solutions and can carry out particular functions

Question 2

Membranes form:

Answer 2

most of the eukaryotic cell’s organelles and therefore partition the cell into compartments

ex of a cellular membrane: plasma membrane

Question 3

The plasma membrane forms:

Answer 3

a boundary between the cell and its surroundings.

Question 4

The plasma membrane controls:

Answer 4

the traffic of molecules in and out of the cell and is selectively permeable.

Question 5

The plasma membrane takes up:

Answer 5

substances needed by the cell and helps with the disposal of waste.

ex: Ions such as Na+ & K+ must travel across using a sodium/potassium pump (maintains resting potential of nerve cell)

Question 6

All membranes of the cell, including the plasma membrane, are:

Answer 6

selectively permeable.

It allows some substances to cross more easily than others and block the passage of some substances completely.

Question 7

The type of membrane transport used depends on:

Answer 7

the type of molecule looking to cross.

simple diffusion
membrane transport: carrier protein, channel protein

Question 8

Describe the Fluid Mosaic Model

Answer 8

• cell membrane is composed of a phospholipid bilayer
• both phospholipids and proteins move laterally but rarely vertically (if at all)
• embedded and surface proteins interspersed throughout the bilayer

Question 9

Membrane Fluidity

What happened when a human cell was fused with a mouse cell in an experiment?

Answer 9

The respective membrane proteins were eventually randomly distributed in the fused cell.

membrane proteins are not fixed

Question 10

Membrane Fluidity

The fluid state of the membrane depends on:

Answer 10

the lipid components

Same principle that explains why butter is solid at room temperature whereas vegetable oil is liquid (saturated vs unsaturated)

Question 11

Membrane Fluidity

In a living cell, at 37°, the bilayer remains as ____ as ____ at room temp.

Answer 11

In a living cell, at 37°, the bilayer remains as fluid as salad oil at room temp.

Question 12

Membrane Fluidity

At lower temperatures, the motion of the fatty acids chains:

Answer 12

slows.

Question 13

Membrane Fluidity

At a critical point, the membrane becomes a ____ and transport across the membrane ____.

Answer 13

At a critical point, the membrane becomes a solid gel and transport across the membrane ceases.

Question 14

Membrane Fluidity

Many organisms have a way of maintaining optimal membrane fluidity: to compensate for temperature changes, they:

Answer 14

alter the FA content of their membrane lipids.

Question 15

Membrane Fluidity - Altering Fatty Acid Saturation

ex: When the temperature goes down, the proportion of unsaturated FA ____ and the membrane stays fluid.

Answer 15

When the temperature goes down, the proportion of unsaturated FA goes up and the membrane stays fluid.

Question 16

Membrane Fluidity - Altering Fatty Acid Saturation

A more fluid membrane has fatty acids with:

Answer 16

unsaturated hydrocarbon tails with kinks.

kink: =bond

Question 17

Membrane Fluidity - Altering Fatty Acid Saturation

A more viscous membrane has fatty acids with:

Answer 17

saturated hydrocarbon tails.

Question 18

Membrane Fluidity - FA Chain Length

The longer the FA chains, the ____ fluid the membrane is likely to be.

FA Chain Length can vary

Answer 18

The longer the FA chains, the less fluid the membrane is likely to be.

The number of potential vdW association b/w chains increases.

Question 19

Membrane Fluidity - Cholesterol

Cholesterol acts as a:

Answer 19

Fluidity Buffer

Question 20

Membrane Fluidity - Cholesterol

At low temps, cholesterol molecules act as:

Answer 20

spacers between the hydrocarbon chains of the FA which restricts vdW interactions that promote solidifying.

Question 21

Membrane Fluidity - Cholesterol

At high temperatures, cholesterol:

Answer 21

prevents the membrane from becoming unstable.

Question 22

The mosaic aspect of the membrane is due to:

Answer 22

the protein molecules that are associated with it.

Question 23

2 major classes of membrane proteins:

Answer 23

1. integral membrane proteins
2. peripheral proteins

the difference b/w the 2 is defined by how they associate with the phospholipid bilayer

Question 24

Describe integral proteins

Answer 24

• they are firmly bound to the membrane, penetrating the core of the lipid bilayer
• they are amphipathic

Amphipathic: the hydrophilic region extends out of the cell and/or into the cytoplasm; the hydrophobic regions interact with the FA tails.

Question 25

Some integral proteins do not extend all the way through the membrane, while others are ____ integral proteins.

Answer 25

Some integral proteins do not extend all the way through the membrane, while others are **transmembrane** integral proteins.

Question 26

Some transmembrane proteins span the membrane just once; others do so many times (up to ____ times.)

Answer 26

up to 24 times

Question 27

The most common transmembrane structure:

Answer 27

α-helix

Question 28

Describe peripheral proteins

Answer 28

- they are not embedded in the lipid bilayer - they are located on the inner or outer surface of the membrane

Question 29

How are the proteins of the membrane oriented?

Answer 29

They are asymmetrically oriented. When the 2 sides of the membrane are compared, more proteins are firmly attached to one side than the other.

Question 30

The asymmetry of the bilayer is produced by:

Answer 30

the highly specific way each protein is inserted into the bilayer. Each side of the membrane has a specific characteristic.

Question 31

Membrane proteins that will become part of the *inner* surface of the plasma membrane are made by:

Answer 31

free ribosomes in the cytosol.

Question 32

Membrane proteins that will be associated with the *outer* surface of the cell are manufactured by:

Answer 32

bound ribosomes on the rough ER.

Question 33

Outer surface membrane proteins are for the most part:

Answer 33

glycoproteins.

Question 34

Describe how outer surface membrane glycoproteins are made

Answer 34

1. sugars are added to certain parts of the proteins in the lumen of the ER 2. transport vesicle brings glycoprotein to outer surface 3. carb portion of the protein extends from the surface of the cell

Question 35

Membrane Protein Functions

Answer 35

1. attach the membrane to the cytoskeleton and to the external fibres of the ECM 2. provide identification tags to cells 3. form junctions b/w cells 4. play a role in transport across membrane 5. can act as enzymes w/in the membrane 6. are involved in signal transduction

Question 36

Particular proteins that aid in keeping the membrane in place are ____ linked to the cytoskeleton and to fibres of the adjacent ECM.

Answer 36

Particular proteins that aid in keeping the membrane in place are **non-covalently** linked to the cytoskeleton and to fibres of the adjacent ECM.

Question 37

The ECM consists of a gel-like substance made of:

Answer 37

carbohydrates and fibrous proteins.

Question 38

The most prominent carb in the ECM is:

Answer 38

collagen, which forms tough fibres.

Question 39

Certain glycoproteins in the ECM, called fibronections, help:

Answer 39

- organize the matrix - aid cells in attaching to the matrix by binding to proteins that extend from the plasma membrane (integrins)

Question 40

Integrins are internal transmembrane proteins that:

Answer 40

- serve as membrane receptors for the ECM: they activate cell-signaling pathways that communicate info to the cell from ECM - play a role in cell movement and in organizing the cytoskeleton, so that cells assume a definite shape - anchor ECM to the microfilaments of the internal cytoskeleton

Question 41

ex: Cancer cells can metastasize because:

Answer 41

they have lost the requirement to be anchored to the ECM

Question 42

# Providing identification tags to cells The outer portion of the plasma membrane contains:

Answer 42

glycoproteins and glycolipids. ## Footnote These molecules vary from species to species, from individual to individual w/in a species, and from cell to cell w/in an individual

Question 43

# Providing identification tags to cells Membrane proteins:

Answer 43

- allows the immune system to recognize and reject foreign cells - enables cells to sort themselves into tissues and organs during embryogenesis

Question 44

Autoimmune diseases occur when:

Answer 44

antibodies produced by the immune system fail to distinguish what is "self" from what is foreign, attacking the body's own tissues.

Question 45

One of the most common autoimmune diseases is:

Answer 45

rheumatoid arthritis, which attacks the synovial membrane of joints ## Footnote synovial fluid (secretes by synovial membrane) lubricates joints and nourishes articular cartilage

Question 46

Cells in close contact with each other develop:

Answer 46

intercellular junctions

Question 47

Animal cells are connected to each other by several types of junctions:

Answer 47

1. anchoring junctions (desmosomes) 2. tight junctions 3. gaps or communication junctions

Question 48

Plant cells are connected by:

Answer 48

plasmodesmata

Question 49

Each desmosome consists of:

Answer 49

a pair of button-like discs associated with the cytosolic side of the plasma membrane of adjacent cells and the intercellular protein filaments that connect them.

Question 50

The proteins that form tight junctions hold the cells together in actual physical contact which forms:

Answer 50

a sheet of tissue ## Footnote Tight junctions b/w cells are so tight that substances cannot leak b/w them.

Question 51

Ex: The sheet of tissue that lines the digestive tract and capillaries in the brain are formed by:

Answer 51

tight junctions between cells. - The contents of the digestive tract are prevented from leaking into surrounding tissue. - The brain-blood barrier prevents many substances in the blood from passing into the brain

Question 52

Gap/Communicating junctions bridge space between cells like desmosomes, but the space they span is ____.

Answer 52

Gap/Communicating junctions bridge space between cells like desmosomes, but the space they span is **narrower**.

Question 53

Gap/Communicating junctions contain channels connecting:

Answer 53

the cytoplasm of adjacent cells, thus allowing for communication. ## Footnote Membrane proteins from adjacent cells line up to form a channel.

Question 54

Gap junctions allow for rapid ____ and ____ communication between cells.

Answer 54

Gap junctions allow for rapid chemical and electrical communication between cells. ## Footnote ex: cells in the pancreas are linked by gap junctions If one group of cells receives a signal to secrete insulin, the signal is passed on to other pancreatic cells, ensuring a coordinated response

Question 55

ex: Heart muscle cells are linked by ____ junctions that permit the flow of ions to synchronize contractions.

Answer 55

Heart muscle cells are linked by **gap** junctions that permit the flow of ions to synchronize contractions.

Question 56

Plant cells do not need ____ since they have a cell wall for strength.

Answer 56

Plant cells do not need **desmosomes** since they have a cell wall for strength. But they do require connections through cell walls that are functionally equivalent to gap junctions. ## Footnote Plants can dilate the channels. H2O, small molecules and ions can pass through these channels so that plant cells share H2O, nutrients, and chemical messages.