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上海交通大学生命科学与技术学院School of Life Science and Biotechnology, SJTU

Lecture 3-2 Microbial cell biology

Chapter 4 inBROCK BIOLOGY OF MICROORGANISMS

College of Life Science and Biotechnology,Shanghai Jiao Tong Universityhttp://micro.sjtu.edu.cn

Shanghai Jiao Tong University


Major Bacterial Cell Morphologies• Coccus (plural, cocci)球菌• Rod杆菌• Spirillum (plural, spirilla)螺菌• Spirochetes螺旋体• Appendaged bacteria带附器的细菌• Filamentous bacteria丝状细菌

3.4 Cell Morphology and the Significance of Being Small



The size of microbial cells

Prokaryotes vary dramatically in both size and volume from cells smaller than 0.2 μm in diameter to those more than 50 μm in diameter.Most very large prokaryotes are either sulfur chemolithotrophs or cyanobacteria.多数大型的原核生物为化能无机营养生物或蓝细菌



Significance of being small (1): metabolism

Nutrients and waste products pass more readily into and out of a small cell, thus accelerating cellular metabolism and growth. 营养物和代谢废物更易进出小型细胞,从而加速代谢和生长.

• Reason: surface-to-volume (S/V) ratio

Relative to cell volume, small cells contain more surface area than do large cells: higher surface-to-volume (S/V) ratio高比表面积



Significance of being small (2): evolution

Per unit of available resources, small cells will typically develop larger populations

⇒larger cell populations mean more cell divisions⇒more mutations accumulate from spontaneous

errors in DNA replication⇒mutations are ‘row material’ that drives

evolutionary change



II. Cell Membrances and Cell Walls

3.5 Cytoplasmic membranes: Structure3.6 Cytoplasmic membranes: Function3.7 Membrane transport systems3.8 Cell Wall3.9 The Outer Membrance of Gram-negative Bacteria


3.5 Cytoplasmic membranes: Structure

Question:How cells collect and uptake nutrients from the environment?

p.66 section 4.5



Cytoplasmic membrane细胞膜

a thin structure that completely surrounds the cell.• Physical barrier separating inside of a cell from its

outside environment. 是细胞的物理屏障• Highly selective permeability barrier enabling a cell

to concentrate specific metabolites and excrete waste materials. 是细胞吸收营养物或排出废物的选择透性屏障



3.5.1 Chemical Composition of MembranesPhospholipid bilayer-磷酯双分子层

• 8nm thick• Occurs in Bacteria and Eukarya, not in Archeae.• Structure: Glycerol backbone attached with fatty

acids and phosphate-containing groups by ester linkage 结构: 甘油骨架, 以酯键连接脂肪酸与磷酸.

enlarged schematic view of a single unit membrane.

electron micrograph of photosynthetic membranestacks derived from thecytoplamic membrane in a phototrophic bacterium

The cytoplasmic membrane.


3.5.2 Membrane proteins: Fluid mosaics

流动镶嵌

Out

In

The matrix of the unit membrane is composed ofphospholipids, with the hydrophobic groups directed inward and the hydrophilic groups toward the outside, where they associate with water.

Diagram of the structure of the cytoplasmic membrane

Fluid mosaics (1) 流动镶嵌

Out

In

Embedded in the matrix are proteins that have considerable hydrophobic character in the region that traverses the fatty acid bilayer. Hydrophilic proteins and other charged substances, such as metal ions, may be attached to the hydrophilic surfaces.

Diagram of the structure of the cytoplasmic membrane





Although appearing somewhat rigid, the cytoplasmic membrane is actually quite fluid;

• phospholipid and protein molecules have significant freedom to move about the membrane surface. Thus, membranes can be thought of as fluid mosaics.细胞膜虽具有一定的刚性,其中的磷脂分子与蛋白质分子仍具有流动性



3.5.3 Membrane strengthening Agents: Sterols and Hopanoids 固醇与藿烷类化合物

The eukayotes have sterols in their membranes.真核细胞膜上有固醇.Sterols are absent from the membranes of virtually all prokaryotes (except methanotrophic bacteria and the mycoplasmas) 原核生物细胞膜中无固醇(除嗜甲烷细菌及支原体之外) Depending on the cell type, sterols can make up from 5 to 25% of the total lipids of eukaryotic membranes. 固醇可达到真核细胞膜总脂量的5-25%.



Sterols and Hopanoids: Functions

Sterols serves to stabilize its structure and make it less flexible. (Sterols are rigid, planar molecules, whereas fatty acids are flexible.) 固醇: 稳定膜的结构,减少细胞膜柔性.Hopanoids are molecules similar to sterols, they present in the membranes of many Bacteria• may play a role similar to that of sterols in eukaryotic

cells. 藿烷类化合物的作用与固醇可能相似



3.5.4 Differences of membrane structure : Achaeal Membranes

The lipids in Bacteria and Eukarya: fatty acidslinked with glycerol molecules by ester linkages.细菌/真核: 酯键连接脂肪酸与甘油Lipids from Archaea: have ether linkagesbetween glycerol and their hydrophobic side chains. 古生菌: 醚键连接疏水侧链与甘油In addition, archaea lipids lack fatty acids and instead have side chains composed of repeating units of the five-carbon hydrocarbon isoprene.古生菌:异戊二烯代替脂肪酸侧链

Chemical bonds in lipids.(a) The esterlinkage as found in the lipids of Bacteria andEukarya.

(b) The etherlinkage of lipids fromArchaea.

(c) Isoprene异戊二烯, the parent structure of the hydrophobic side chains (R) ofarchaeal lipids. By contrast, in lipids of Bacteria and Eukarya, R are fatty acids.

Glycerol diethers and diglycerol tetraethers are the major classes of lipids present in Archaea. In the tetraether molecule the phytanyl (composed of 4 linked isoprenes) side chains from each glycerol molecule are covalently bonded together. 甘油二醚与二甘油四醚是古生菌的主要脂类. 四醚中的植烷(由四个异戊二烯组成)与甘油共价连接

Phytanyl植烷is composed of 4 linked isoprenes

Membrane structure in Archaea: Lipid bilayer and lipidmonolayer

Diether yields bilayer, while Tetraether yields a lipid monolayerinstead of a lipid bilayer. Monolayer membrane is quite resistant to peeling apart.甘油二醚形成脂双层, 而二甘油四醚导致脂单层而非脂双层,脂单层结构难被剥离Monolayer: Widespread among hyperthermophilic Achaea脂单层广泛存在于嗜热古生菌中



Differences of membrane structure between Achaeaand Bacteria

Phylogenetic groups

Hydrophobic side chain

Linkagebetweenglycerol andside chain

Membranemorphology

Membranestrengthening agent

Achaea Isoprene ether Bilayer or monolayer

Bacteria Fatty acids ester Bilayer

Eukarya Fatty acids ester Bilayer sterols

hopanoid



3.6 Cytoplasmic Membrane: Functions

Permeability barrier半透膜• Prevent passive leakage

Protein anchor蛋白质锚点• Lipoproteins• Transporters• Enzymes

Energy conservation产能• Proton motive force for ATP generation产生ATP的质子动势

Necessity for Transport Proteins

Simple diffusion is not enough for cell functions简单扩散• Simple diffusion can not occur

against gradient• Nutrient concentration out of cell

is too low

Carrier-mediated transportis necessary载体运输• Uptake of nutrients can occur

against concentration gradient• the rate thus can be constant and

controllable • Number and type of carriers are

regulated by cell

Relationship between uptake rate and external concentration in diffusion and transport.


上海交通大学生命科学与技术学院School of Life Science and Biotechnology, SJTU3.7 Membrane transport systems膜运输系统

There are at least three classes of membrane-transporting systems:三种膜运输系统

• Simple transport简单运输• Group translocation基团转运• ABC system (ATP-binding cassette) ABC系统

简单运输: involving only one membrane-spanning component跨膜成份

基团转运: involving a periplasmic-binding component plus a membrane-spanning component周质蛋白与跨膜蛋白

ABC系统: involving a series of proteins that cooperate to mediate the transport event



3.7.1 Structure of membrane-spanning transporters 穿膜运输蛋白

In prokaryotes, membrane-spanning transporters typically contain 12 alpha helices that align with each other in a circle to form a channel through the membrane. 原核生物的穿膜运输蛋白由12个α螺旋形成穿膜的通道

Three individual transporters, each showing a different type of transport event. The cotransported molecule is shown in yellow.有三种运输蛋白, 协同运输分子为黄色

Uniporters are proteins that simply transport a molecule in a unidirectional fashion across the membrane单向运输蛋白


Antiporters are proteins that transport a substance across the membrane in one direction while a second substance in the opposite direction反向运输蛋白


Symporters are proteins that transport a substance along with another substance, frequently a proton (H+)同向运输蛋白



3.7.2 Symporter: Lac permease乳糖透酶Rely on proton motive force for uptake• One proton needed for each lactose taken into cell每吸收一分子乳糖消耗一个质子

• Does not modify substance (lactose) during transportation运输过程中不修饰底物



3.7.3 Group translocation基团转运

A transport process in which the transported substance is chemically altered during passage across the membrane.底物在运输过程中被化学修饰Example: Phosphotransferase system for sugars uptake磷酸转移酶系统

Mechanism of the phosphotransferase system.

For glucose uptake, the system consists of 5 proteins. Sequential phosphate transfer occurs from PEP to Enzyme IIc, the latter actually transports (and phosphorylates) the sugar.The system in E. coli contains 24 proteins, at least 4 necessary for one given sugar

Energy source: phosphoenolpyruvate磷酸烯醇式丙酮酸



3.7.4 ABC system

Three proteins per system• Periplasmic binding protein周质蛋白• Membrane spanning transporter跨膜运输蛋白

• ATP-hydrolyzing protein (ATP hydrolysis provides energy) ATP水解蛋白

Example: maltose transport system in E. coli. 麦芽糖运输系统

ATP-Binding Cassette (ABC-type) transporter functions:The periplasmic binding protein has high affinity for substrate, it is mobile in Gram negative and anchored in Gram positive.周质蛋白The membrane-spanning protein is the transport channel,膜运输蛋白The cytoplasmic ATP-hydrolyzing protein supplies the energy for the transport event. ATP水解蛋白



Summarization: Differences between transporting systems

Transporting system

Components

Energy source Substrate affinity

Modification of substrate

Simple transporter

One protein

Proton motive force

low No

Group translocation

4 proteins Phosphoenolpyruvate (PEP)

low Yes (Phosphorylation)

ABC system 3 proteins ATP high No

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