Unit One - Bacterial Cell Structure and Function

I. Membranes

Cytoplasmic membrane in proks involved in transport and selective permeability, but also in a variety of other critical cell functions - cell division, sporulation, e- transport, ATP formation, DNA replication

A. Chemical Components

Proteins + lipids

1. Lipids

asymmetric (polar & nonpolar ends = amphipathic)
charged polar group = 3 C glycerol + 2 fatty acid chains

Most are charged groups are phosphates --> phospholipids

no sterols - but hopanoids

2. Proteins

Peripheral - loosely connected, soluble
Integral - tightly connected, insoluble, amphipathic

can diffuse laterally
some span membrane
some have carbos attached
many integral proteins are transport proteins

B. Physical Arrangement

Bilayer
Stabilization

hydrophobic interactions & H bonding btwn lipids

H bonding btwn proteins and lipids

Ionic interactions btwn neg charges on proteins and divalent cations (Mg²⁺, Ca²⁺)

Hopanoids

*Be able to draw and label a typical bacterial cytoplasmic membrane

Q. How do either hydrophobic or hydrophilic molecules cross a membrane that is primarily hydrophobic?

Q. What determines the upper size limit beyond which materials cannot be transported?

3. Review of early theories

a. sieve and pore - attempt to explain size discrimination

homogeneous and full of holes

(sieve - all same size, pore - different sizes)

transport molecule fits thru holes

explained selective permeability & size, but not charge

b. mosaic - slight improvement

hydrophobic & hydrophilic areas

transported molecule goes thru compatible area

c. reversible chemical combination

transport involved an interaction btwn a component in membrane & transported molecule, in a reversible way
didn't specify component or nature of interaction

d. permease - portion still in use today

transport resulted from reversible interaction of transported material w/ a specific carrier protein (permease) that allowed transport of the material from one area of the membrane to another

* concept of involvement of carrier proteins remains central

e. molecular reorientation

above
+ interaction btwn transported material & membrane altered membrane structure to allow transport

*f. fluid mosaic

mosaic because definite pattern
fluid because lipids move freely

C. Functions

1. Retains the cytoplasm - osmotic barrier

2. Selective barrier

3. Transport

Basic types of transport systems

a. Passive Diffusion = simple diffusion

no transport protein
nonspecific
no energy required
no conc. gradient made
molecules capable of passive diffusion must be soluble in lipids & aqueous

b. Facilitated Diffusion

transport protein

specific

no energy required

no conc. gradient made

not widely used in bacteria

glycerol is transported this way by some bacteria

animation

c. Active Transport

transport proteins are molecular pumps that pump their substrates against a conc. gradient
significant accumulation occurs in the

cytoplasm

some sugars, most aa and organic acids, SO₄, PO₄, K⁺
transport proteins may be highly specific and carry only 1 molecule or may be more general and carry 1 class of molecules
fuel comes in two forms, ATP or proton motive force.

animation

d. Group Translocation

transport protein
specific
requires energy
molecule is chemically modified
no conc. of transported substance (since it's changed into a diff. substance)

animation

fuel comes from phosphoenol pyruvate (PEP)

sugars (glc, mannose, fructose) are phosphorylated by the phosphotransferase system

Types of Transport Proteins

uniporters - move subst. from one side of the membrane to the other
symporters - carry 2 subst. across in same direction

animation

antiporters - 1 subst. in 1 direction, 2nd subst. in opposite direction

animation

A Summary of Bacterial Transport Systems

Property	Passive Diffusion	Active Diffusion	Active Transport	Group Translocation
Carrier mediated	-	+	+	+
Specificity	-	+	+	+
Conc. against a gradient	-	-	+	NA
Energy expended	-	-	+	+
Solute modified	-	-	-	+

+ = yes

- = no

4. Generation of Energy - Proton Motive Force (PMF)

Passage of e- during from 1 carrier to another during e- transport, protons are pumped from inside to outside - outside becomes + charged, inside becomes - charged - proton gradient energizes the cell like charging a battery. This charge is called the proton motive force (PMF) - can be used to do work directly or can be channeled into an ATPase and convert ADP --> ATP. Photosynthetic cells also have a membrane system. Here light excites e- and the e- are again passed down through a series of e- carriers, a PMF is generated and ATP is synthesized. All the photosynthetic machinery is situated in the membrane.

5. Synthesis Membranes also contain specialized enzymes that carry out many biosynthetic functions. These functions include:

a. Membrane synthesis

b. Cell wall assembly

Secretion of many proteins