lecture 5: cell membranes and cell walls

(14-28)

permeability barrier

permeability barrier

prevents leakage and functions as a gateway for transport of nutrients into and out of the cell

protein anchor

site of many proteins involved in transport, bioenergetics, and chemotaxis

energy conservation

site of generation and use of the proton motive force

besides permeability, what other functions does the cytoplasmic membrane have

• selective transport

• cell signaling

• cell communication

• structural support and shape

• energy production

• compartmentalization

• protection

list two reasons why a cell cannot depend on diffusion as a means of getting nutrients into the cell

1. slow and inefficient over long distances

2. inability to move against a concentration gradient

why is physical damage to the cytoplasmic membrane a more critical problem for the cell than damage to some other cell component

• damage to the membrane can result in loss of essential molecules and uncontrolled entry of harmful substances, leading to cellular imbalance

• the membrane helps maintain the cell’s shape and protects internal structures from external stress. damage can lead to lysis (bursting) or collapse

membrane transport systems

• simple transport

• group translocation

• ABC system

simple transport

uses transport proteins, driven by the energy in the proton motive force (ion gradients)

group translocation

chemical modification of the transported substance driven by phosphoenolpyruvate (PEP)

ABC system

moves various molecules across the cell membrane, periplasmic binding proteins are involved and energy comes from ATP

simple transporter (uniporter)

one that goes out to in

simple transporter (antiporter)

two that go in opposite directions

simple transporter (symporter)

two that both go from out to in

Translocases

• secretion of macromolecules

• insertion into the cytoplasmic membrane

importance of protein secretion

enzymatic functions needed on the outside of the cell

which transport system is best suited for the transport of nutrients present in the environment in extremely low amounts, and why

ABC system because:

• substrate-binding proteins captures nutrients with high affinity

• use ATP to transport molecules regardless of external concentration

• binding proteins are highly specific and will only capture the right nutrient making the process efficient in competitive environments where nutrients are scarce

how are proteins exported from the cell

through enzymes called translocases (SecYEG)