2 - Plasma membrane

Question 1

Cell envelope

Answer 1

Outermost part of the prokaryotic cell (outer membrane, cell wall and plasma membrane)

Question 2

What colour does gram positive stain

Answer 2

Purple (two ps) / monoderms

Question 3

What colour does gram negative stain

Answer 3

Pink (one p) / diderms

Question 4

Functions of cell envelope

Answer 4

• provides rigidity and structure
• prevents osmotic lysis
• encloses the cell contents
• includes receptors for responding
  to external conditions
• includes motility and attachment structures
• critical to evolution

Question 5

Phospholipid bilayer

Answer 5

• Have polar (hydrophilic) and non polar (hydrophobic) end
• Outer surfaces are hydrophilic, interior is hydrophobic

Question 6

Bacterial lipids

Answer 6

• Bacterial membranes differ from
  eukaryotic membranes in lacking sterols
  such as cholesterol
• Have sterol like molecules called hopanoids

Question 7

Hopanoids

Answer 7

• Help stabilise membrane, increase bacterial survival under stress
• Most abundant biological molecule

Question 8

Peripheral proteins

Answer 8

Loosely connected to membrane, easy to remove, soluble in water

Question 9

Integral proteins

Answer 9

Not easily extracted, insoluble, amphipathic (hydrophilic and hydrophobic ends)

Question 10

Function of integral proteins

Answer 10

• Transport
• Energy production
• Exposed regions allow cell to interact with environment (via carbohydrate side chains)

Question 11

Fluid mosaic model

Answer 11

• Proteins float in lipid bilayer, free to move laterally
• Membrane lipids homogeneously distributed

Question 12

Why are aspects of fluid mosaic model now being questioned

Answer 12

Due to existence of functional membrane microdomains (FFM)

Question 13

Flotillins

Answer 13

Function to assemble large protein complexes invloved in specific cellular processes (e.g. protein secretion, cell wall metabolism)

Question 14

Membrane protein synthesis

Answer 14

• Begins in the cytoplasm
• A peptide leader sequence binds to the membrane at sites determined by specific docking proteins
• The protein is threaded into the membrane

Question 15

Functions of plasma membrane

Answer 15

• Selectively permeable osmotic barrier
• Controls movement of chemicals to and from the cell
• The site of many important cellular processes

Question 16

Selectively permeable osmotic barrier

Answer 16

Retains cell contents and prevents leakage of important cell chemicals

Question 17

Controls movement of chemicals to and from the cell

Answer 17

• Has specific transport systems for nutrient uptake, waste excretion, and secretion of enzymes

Question 18

The site of many important cellular processes

Answer 18

• Assembly and synthesis of cell membrane lipids
• Assembly and secretion of extracellular proteins
• Energy generation

Question 19

Plasma membrane transport

Answer 19

• Some chemicals diffuse into and out of the cell (no energy required)
• Some chemicals must be transported (energy required)
• Transport occurs against a concentration gradient

Question 20

Five transport processes in bacterial cells (2 types of diffusion, 3 types of active transport)

Answer 20

• Passive diffusion
• Facilitated diffusion
• Active transport (Primary, Secondary, Group translocation)

Question 21

Passive diffusion

Answer 21

• Molecules move from a region of higher concentration to one of lower concentration (down concentration gradient)
• Rate depends on size of concentration gradient between inside and outside cell
• Only small, neutral or weakly charged molecules can diffuse across PM

Question 22

Channel proteins

Answer 22

Form pores in membranes through which substances can pass

Question 23

Carrier proteins (permeases)

Answer 23

Carry substances across membranes (more substrate specific)

Question 24

Facilitated diffusion

Answer 24

• Diffusion across the plasma membrane is aided by a channel protein or carrier proteins
• ATP is not required
• Continues until inside = outside
• Can become saturated (reaches plateau)
• Rate of diffusion depends on size of concentration gradient
• Rate increases much more rapidly than passive diffusion

Question 25

Carrier mediated facilitated diffusion

Answer 25

Solute binding permease triggers conformational change and transport

Question 26

Active transport

Answer 26

- Transport of non-diffusible chemicals against a concentration gradient with input of energy - Typically involves specific integral membrane permeases and substrate binding proteins - Driven by ATP or proton motive force

Question 27

Primary active transport

Answer 27

- Mediated by carriers such as ABC transporters - Use ATP move substances against a concentration gradient without modifying them - Uniporters (move a single molecule across membrane)

Question 28

ABC transporters

Answer 28

ATP Binding Cassette

Question 29

Steps of primary active transport

Answer 29

1. Solute-binding protein binds substrate and attaches to the transporter 2. ATP hydrolysis releases energy at the transporter complex 3. Substrate is transported through pore

Question 30

Secondary active transport

Answer 30

- Couples the potential energy of ion gradients to move substances against a concentration gradient without modifying them - Cotransporters (moves two substances simultaneously)

Question 31

Symporters

Answer 31

Move both substances in the same direction

Question 32

Antiporters

Answer 32

Move two substances in opposite directions

Question 33

Example of antiport permease transport

Answer 33

- Na+ transport in E.coli - Helps E. coli grow under quite high salt conditions

Question 34

Group translocation

Answer 34

- Molecule is chemically modified as it is transported into cell - Phosphorylated by a sugar phosphotransferase system (PTS) - Involves several enzymes transferring phosphate to the sugar (phosphorelay system) - Only found in prokaryotes

Question 35

What is the phosphate donor in group translocation

Answer 35

- Phosphoenolpyruvate (PEP) - A high energy molecule that can be used to make ATP, but here its energy is used to fuel transport

Question 36

Benefits of phosphorelay system

Answer 36

- Phosphorylated sugar cannot diffuse back out of the cell - Group translocation allows nutrient uptake under conditions where the membrane is deenergised and ion coupled sugar transport is failing.