Membrane (cell bio)

Question 1

CELL MEMBRANE

No biological membranes = No ?!
* regulates the movement of material into and out of the cell
* facilitates ? signaling between cells
* defines the boundaries of ? and separates complex ? reactions
 ? differing functions

cell membranes: good fences make good neighbours

Answer 1

CELL MEMBRANE

No biological membranes = No life!
* regulates the movement of material into and out of the cell
* facilitates electrical signaling between cells
* defines the boundaries of organelles and separates complex chemical reactions
 multiple differing functions

cell membranes: good fences make good neighbors

Question 2

CELL MEMBRANE

Plasma membrane, cytoplasmic membrane

Define the ? boundaries of a cell

Are ? to allow for growth and movement, ? and ? to polar solutes

Regulate the ? across boundary

In eukaryotic cells: ? to segregate processes and compartments

Are central to both biological ? and cell-to-cell ?

Can break and re-seal for ? or ?

Serve as attachment surface for “ * ? * “ and extracellular structures

Answer 2

CELL MEMBRANE

Plasma membrane, cytoplasmic membrane

Defines the external boundaries of a cell

Are flexible to allow for growth and movement, self-sealing and selectively permeable to polar solutes

Regulate the molecular traffic across boundary

In eukaryotic cells: compartmentalization to segregate processes and compartments

Are central to both biological * energy and cell-to-cell communication*

Can break and re-seal for * fusion or fission *

Serve as attachment surface for cytoskeleton and extracellular structures

Question 3

THE EUKARYOTE CELL: A “Membranous”
Unit

Cellular compartmentalization:
? separation within the cell

1. Plasma membrane
2. Nuclear membrane (inner and outer)
3. ER membrane
4. Golgi apparatus membrane
5. Mitochondrial membrane (inner and outer)
6. Peroxisomes and Lysosomes

(ETC (oxidative phosphorylation) and TCA occur in which part of mitochondria?)

sER: lipids are sort by enterocytes (** ? ** process)

rER: post-transitional modification of

GA: also some modifications of ?

Answer 3

THE EUKARYOTE CELL: A “Membranous”
Unit

Cellular compartmentalization:
functional separation within the cell

1. Plasma membrane
2. Nuclear membrane (inner and outer)
3. ER membrane
4. Golgi apparatus membrane
5. Mitochondrial membrane (inner and outer)
6. Peroxisomes and Lysosomes

(ETC (oxidative phosphorylation) and TCA occur in which part of mitochondria?)

sER: lipids are sort by enterocytes (** re-esterifaction ** process)

rER: post-transitional modification of

GA: also some modifications of proteins

Question 4

CELL MEMBRANE -COMPARTMENTALIZATION

1. Separation of ? processes
   * i.e. anabolic and catabolic processes can be held in ? compartments avoiding competition for the ? or
   interference in the ?
   Example: Fatty acid synthesis in ? and fatty acid oxidation in ?
2. Separation of similar reactions serving different ?
   * i.e. similar reactions for different purposes and must therefore be held in ? compartments
   Example: fatty acid oxidation
   * in mitochondria for ? production
   * in peroxisomes for ? production
3. ? of different reactions which are involved in the same pathway (energy efficiency)
   Example: TCA and electron transport chain are the central point of ? metabolism in cells and are located in the mitochondria

Answer 4

CELL MEMBRANE -COMPARTMENTALIZATION

1. Separation of antiparallel processes
   * i.e. anabolic and catabolic processes can be held in separate compartments avoiding competition for the substrate or
   interference in the rxn
   Example: Fatty acid synthesis in cytosol and fatty acid oxidation in mitochondria
2. Separation of similar reactions serving different purposes
   * i.e. similar reactions for different purposes and must therefore be held in independent compartments
   Example: fatty acid oxidation
   * in mitochondria for energy production
   * in peroxisomes for heat production
3. coordination of different reactions which are involved in the same pathway (energy efficiency)
   Example: TCA and electron transport chain are the central point of energy metabolism in cells and are located in the mitochondria

Question 5

CELL MEMBRANE - PROPERTIES

• ? the cellular interior from the exterior
• ? information with the environment in a controlled manner
• Membranes are extremely ?
• Membranes get information from the cell about ? status
• Membranes are ?, allow ? transport of molecules into and out of the cell
• Membranes contain ?, ?, ? and other proteins

Answer 5

CELL MEMBRANE - PROPERTIES

• separates the cellular interior from the exterior
• exchanges information with the environment in a controlled manner
• Membranes are extremely dynamic
• Membranes get information from the cell about metabolic status
• Membranes are selective barriers , allow selective transport of molecules into and out of the cell
• Membranes contain receptors, transporters, ENZYMES and other proteins

Question 6

Membrane Fusion and Fission

Central to ? cellular processes involving organelles and the plasma membrane.

Membrane Fusion:
2 separate ? merge to become one.
*Example: Transport vesicles from the ER fusing with ?

Membrane Fission:
Involves splitting of a membrane into 2 parts
* Example: formation of ? by ER/Golgi apparatus to transport ? and ? to other organelles and to cell ?

 Both processes involve ? without loss of continuity.

Answer 6

Membrane Fusion and Fission

Central to many cellular processes involving organelles and the plasma membrane.

Membrane Fusion:
2 separate lipid bilayers merge to become one.
*Example:Transport vesicles from the ER fusing with Golgi membranes

Membrane Fission:
Involves splitting of a membrane into 2 parts
* Example: formation of vesicles by ER/Golgi apparatus to transport lipids and proteins to other organelles and to cell membrane

 Both processes involve membrane organization without loss of continuity.

Question 7

CELL MEMBRANE - COMPONENTS

*Lipids - phospholipids, ?, glycolipids
about ?% of the ? of most animal cell membranes

*? (transmembrane, peripheral)

*? (glycan groups)

?

? (Ca2+, Mg2+)

Answer 7

CELL MEMBRANE - COMPONENTS

*Lipids - phospholipids, sterol, glycolipids
about 50% of the ? of most animal cell membranes

*proteins (transmembrane, peripheral)

*carbohydrates (glycan groups)

water

divalent cations (Ca2+, Mg2+)

Question 8

CELL MEMBRANE

Proteins that are part of or interact with biological membranes

• Most ? are transmembrane and mediate many functions such as ? and ? (enzymes)
• Some transmembrane proteins serve as ? connecting the cytoskeleton through the lipid bilayer to either the ? or to an ? cell
• Others serve as ? to detect and transduce signals

Numerous different proteins are necessary for proper cell ? and ?
-> 30% of human ? proteins are membrane proteins

-> Membrane proteins are ** ? ** of over ?% of all modern ? drugs

Answer 8

CELL MEMBRANE

Proteins that are part of or interact with biological membranes

• Most membrane protiens are transmembrane and mediate many functions such as transporting and catalysing of reactions (enzymes)
• Some transmembrane proteins serve as “structural links” connecting the cytoskeleton through the lipid bilayer to either the extracellular links or to an adjacent cell
• Others serve as receptors to detect and transduce signals

Numerous different proteins are necessary for proper cell function and interaction
-> 30% of human genome’s proteins are membrane proteins

-> Membrane proteins are ** TARGETS ** of over 50% of all modern medicinal drugs

Question 9

CELL MEMBRANE - LIPIDS

The lipid bilayer is a relatively ? barrier to most ?-soluble (polar) molecules

Lipid molecules make up about ?% of the mass of most animal cell membranes.

• Three main lipids:
   ? (phosphoglycerides and sphingolipids)
   ? (cholesterol in eukaryotic cells, not in prokaryotes!!!)
   ?

All lipid molecules in cell membranes are ?:
 one hydrophilic (? end)
 one hydrophobic (? end)

Answer 9

CELL MEMBRANE - LIPIDS

The lipid bilayer is a relatively impermeable barrier to most water-soluble (polar) molecules

Lipid molecules make up about 50% of the mass of most animal cell membranes.

• Three main lipids:
   phospholipids (phosphoglycerides and sphingolipids)
   sterol (cholesterol in eukaryotic cells, not in prokaryotes!!!)
   glycolipids

All lipid molecules in cell membranes are amphiphatic:
 one hydrophilic (polar end)
 one hydrophobic (? end)

Question 10

CELL MEMBRANE

Universal basis for membrane structure:

 The bilayer structure results from special properties of ? molecules that cause their ? assemblage into bilayers.

 Common general structure: very ? film (average 5nm thick) of ? and ? molecules, held together mainly by covalent or noncova? interactions

 Easily seen by ? microscopy.
Specialized techniques are necessary (x-ray diffraction, freeze- fracture) to study the ? of its organization.

Fluid, ? structures with most their molecules able to move in the plane of the membrane -> ? ? MODEL

Answer 10

CELL MEMBRANE

Universal basis for membrane structure:

 The bilayer structure results from special properties of lipid molecules that cause their spontaneous assemblage into bilayers.

 Common general structure: very thin film (average 5nm thick) of lipid and proteins molecules, held together mainly by noncovalent interactions

 Easily seen by electron microscopy.
Specialized techniques are necessary (x-ray diffraction, freeze- fracture) to study the ? of its organization.

Fluid, dynamic structures with most their molecules able to move in the plane of the membrane -> FLUID MOSAIC MODEL

Question 11

CELL MEMBRANE

A red blood cell’s membrane–Thin layer

Viewed in cross-section:

All cell membranes share a characteristic ** ? ** appearance: plasma membrane appears as three-layer structure, 5-8 ?thick

Trilaminar image consists of two ?-dense layers separated by a ? dense central region.

Answer 11

CELL MEMBRANE

A red blood cell’s membrane–Thin layer

Viewed in cross-section:

All cell membranes share a characteristic ** TRI-LAMINAR ** appearance: plasma membrane appears as a three-layer structure, 5-8 nm thick

Trilaminar image consists of two electron-dense layers separated by a less dense central region.

Question 12

CELL MEMBRANE–
Prokaryotes vs Eukaryotes

Prokaryotic plasma membranes are often composed:
 one main type of ?
 contain no ? (with some rare exceptions)
(Their mechanical stability is steadied by an overlying cell wall)

Most eukaryotic cells are more varied, composed:
 mixtures of different ?
 large amounts of ?

Answer 12

CELL MEMBRANE–
Prokaryotes vs Eukaryotes

Prokaryotic plasma membranes are often composed:
 one main type of phospholipid (PL)
 contain no cholesterol (with some rare exceptions)
(Their mechanical stability is steadied by an overlying cell wall)

Most eukaryotic cells are more varied, composed:
 mixtures of different PL
 large amounts of cholestrol

(plants: phytosterol; animal cell: cholestrol)

Question 13

Micelles and bilayers

Shape and amphipathic nature of lipid molecules:
 causes their formation into bilayers ? in ? environments

RECAP:
* Hydrophilic molecules: dissolve readily in water due to their ? groups, or ? groups, which form either favourable ? interactions, or ? bonds, with water molecules
* Hydrophobic molecules are insoluble in water due to most or all of their atoms being uncharged and nonpolar and therefore unable to form ? interactions with water molecules

Lipid molecules assemble with their hydrophobic tails in the interior and hydrophilic heads outside to water

 Can do this in two ways:
1. Spherical ?
2. Bimolecular sheets- ?

Answer 13

Micelles and bilayers

Shape and amphipathic nature of lipid molecules:
 causes their formation into bilayers spontaneoously in aqeuous environments

RECAP:
* Hydrophilic molecules: dissolve readily in water due to their charged groups, or uncharged polar groups, which form either favourable electrostatic interactions, or H bonds, with water molecules
* Hydrophobic molecules are insoluble in water due to most or all of their atoms being uncharged and nonpolar and therefore unable to form energitically favorable interactions with water molecules

Lipid molecules assemble with their hydrophobic tails in the interior and hydrophilic heads outside to water

 Can do this in two ways:
1. Spherical *micelles
2. Bimolecular sheets- bilayers

Question 14

CELL MEMBRANE

Lipids spontaneously form micelles or bilayers in an ? environment

• Cone-shaped amphipathic molecules (* ? *) form micelles,
• Cylinder-shaped molecules (* ? *) form bilayers

The spontaneous closure of a phospholipid bilayer to form a sealed compartment is energetically most favorable.
Why?

Closed structure is ? as it avoids the exposure of the ? hydrocarbon ? to water (which would be energetically ?).
This provides the bilayer‘s * ? *: all free edges are avoided by closing in on itself.

Answer 14

CELL MEMBRANE

Lipids spontaneously form micelles or bilayers in an aqueous environment

• Cone-shaped amphipathic molecules (*i.e. FATTY ACIDS *) form micelles,
• Cylinder-shaped molecules (* PLs *) form bilayers

Why?
Closed structure is stable as it avoids the exposure of the hydrophobic hydrocarbon tails to water (which would be energetically unfavourable).

This provides the bilayer‘s self-healing property: all free edges are avoided by closing in on itself.

Question 15

CELL MEMBRANE
Techniques for visualizing cells:

Electron microscopy (EM)
1. Scanning electron microscope (SEM): directly produces an image
of the ?-dimensional structure of the ? of a specimen

2. Transmission electron microscope (TEM): a ? of electrons is transmitted through a specimen to form an ?, capturing fine ?
3. Freeze-fracture and freeze-? electron microscopy: provide ? of surfaces ? the cell

Answer 15

CELL MEMBRANE
Techniques for visualizing cells:

Electron microscopy (EM)
1. Scanning electron microscope (SEM): directly produces an image
of the 3-dimensional structure of the surface of a specimen

2. Transmission electron microscope (TEM): a beam of electrons is transmitted through a specimen to form an image, capturing fine detail
3. Freeze-fracture and freeze-etch electron microscopy: provide views of surfaces inside the cell

pic: In the TEM image the cells’ cytoplasm is rich in filaments of the protein cytokeratin (dark brown).

Question 16

THE ?-FRACTURE TECHNIQUE:

A special method to study the cell ?

Views of the ? organization of membranes are possible, expanding our understanding of the cell ?

It physically ? (?) a frozen biological sample

Structural detail exposed by the fracture plane is then coated with a ? layer and visualized by ?

1. Rapidly freezing in liquid ?
2. fracture with a ?
3. deposition of ? at an angle of ? degree
4. deposition of ? at an angle of ?
5. washing ? films
6. collection of ? films on a TEM grid

Answer 16

THE FREEZE-FRACTURE TECHNIQUE:

A special method to study the cell membrane

Views of the internal organization of membranes are possible, expanding our understanding of the cell membrane

It physically breaks apart (fracturing) a frozen biological sample

Structural detail exposed by the fracture plane is then coated with a metal layer and visualized by TEM

1. Rapidly freezing in liquid propane
2. fracture with a glass knife
3. deposition of platinum at an angle of 45 degree
4. deposition of carbon at an angle of 90
5. washing replica films
6. collection of replica films on a TEM grid

Question 17

CELL MEMBRANE

• Polar head group and two hydrophobic hydrocarbon tails (? ?)
• Tails can differ in ? (normally: 14-24 C)
• One tail usually contains one or more ?-double bonds (unsaturated), while other does not (?)
• Double bond creates small ? in tail
• Differences in ? and ? of fatty acid tails affect the ability of phospholipid molecules to pack together, and so guarantee the ?

what attaches hydrocarbon tail to phosphate? and what is attached to phosphate?

Answer 17

CELL MEMBRANE

• Polar head group and two hydrophobic hydrocarbon tails (FAs)
• Tails can differ in length (normally: 14-24 C)
• One tail usually contains one or more cis-double bonds (unsaturated), while other does not (saturated)
• Double bond creates small kink in tail
• Differences in length and saturation of fatty acid tails affect the ability of phospholipid molecules to pack together, and so guarantee the membrane fluidity.

what attaches hydrocarbon tail to phosphate? = GLYCEROL and what is attached to phosphate? = choline

Question 18

CELL MEMBRANE -
PHOSPHOLIPIDS

Four major ? predominate in the plasma membrane of many ? cells (make up more than half lipid mass in most membranes):

1. Phosphatidyl?
2. Phosphatidyl?
3. Phosphatidyl?*
4. ?

*phosphatidylserine carries a net + or - charge, other phospholipids are electrically neutral at physiologic pH, carrying one positive and one negative charge.

IMP!!!
Other phospholipids, such as phosphatidylinositol, are present only in ? quantities, but are very important ? (e.g. cell ?)
The bilayer is ? with unequal distribution of phospholipids, other lipids, and membrane proteins between the inner and the outer layers.

Answer 18

CELL MEMBRANE -
PHOSPHOLIPIDS

Four major PLs predominate in the plasma membrane of many mammalian cells (make up more than half lipid mass in most membranes):

1. Phosphatidylcholine
2. Phosphatidylethanolaminie
3. Phosphatidylserine*
4. sphingomyelin

*phosphatidylserine carries a net NEGATIVE charge, other phospholipids are electrically neutral at physiologic pH, carrying one positive and one negative charge.

IMP!!!
Other phospholipids, such as phosphatidylinositol, are present only in small quantities but are very important functionally (e.g. cell signalling)
The bilayer is asymmetric with unequal distribution of phospholipids, other lipids, and membrane proteins between the inner and the outer layers.