Section E Membrane

E1 Membrane lipid

E2 Membrane protein

E3 Membrane transport: small

molecules

E4 Membrane transport: macro- molecules

A cell body (plasma and alveolar membranes tightly apposed)

A cilium

mitochondrion

A digestive vacuole

The endoplasmic reticulum

A secretory vesicle

Electron micrograph of the membranes of two adjacent cells

25 nm

1. Biomembranes are organized, sheetlike assemblies consisting mainly of proteins and lipids

1.1 The functions carried out by membranes are diverse and indispensable for life.

1.1.1 Biomembranes form boundaries around the cell and aroud distinct subcellular compartments.

1.1.2 Biomembranes are highly selective permeability barriers with specific protein channels and pumps regulating the molecular and ionic compositions of the intracellular medium (transport across membranes).

1.1.3 Biomembranes control the flow of information between cells and their environment through specific receptors on the plasma membrane (signal transduction).

1.1.4 Eukaryotic cells have an extensive internal membrane system dividing cells into various compartments (forming different organelles).

1.1.5 The two most important energy conver

sion processes, photosynthesis (occurring in the in

ner membranes of chloroplasts) and oxidative ph

osphorylation (ocurring in the inner membranes

of mitochondria) are carried out by membrane sy

stems.

1.1.6 Certain biosynthesis (e.g., synthesis of l

ipids and some proteins) occur on biomembranes.

1.2 Membranes consist mainly of lipids and protei

ns with mass ratio ranges between 1:4 to 4:1 (mem

branes with different functions have different prot

eins, some lipids and proteins have covalently link

ed carbohydrates).

1.2.1 In membranes the three major classes o

f lipids are the glycerophospholipids ( 甘油磷脂） , th

e sphingolipids （鞘磷脂） and the sterols （固醇） .

(1)The glycerophospholipids ( 甘油磷脂） have a glycerol backbone that is attached to two fatty acid hydrocarbon chains and a phosphorylated head group.

These include ： phosphatidate ( 磷脂酸）phosphatidylcholine （磷脂酰胆碱） phosphatidylethanolamine （磷脂酰胆胺） phosphatidylglycerol （磷脂酰甘油） phosphatidylinositol （磷脂酰肌醇） diphosphatidylglycerl （二磷脂酰甘油）

H

glycerol

(2) The sphingolipids （鞘氨醇磷脂类） are

base on sphingosine （鞘氨醇） to which a

single fatty acid chain is attached and eithe

r a phosphorylated head group (sphingom

yelin) or one or more sugar sesidues.

Ceramide

神经酰氨

(3) The major sterol ( 固醇） in animal plasma membranes is cholesterol （胆固醇） , while the structurally related stigmasterol ( 豆甾醇） and –sitosterol （谷甾醇） are found in plants.

1.2.2 All membrane lipids are amphipathic t

hat form bilayer structures spontaneously in aqu

eous media.

1.2.3 Membranes contain specific proteins

(e.g., pumps, channels, receptors, energy transdu

cers, and enzymes) to mediate their distinctive fu

nctions.

1.2.4 Membranes are cooperative noncovalen

t assemblies.

1.2.5 Membranes are always asymmetric wit

h two different faces.

1.2.6 Membranes are fluid two-dimensional s

tructures (the fluid mosaic model) with oriented p

roteins and lipids (which form bilayer structures).

1.3 Proteins attach to membranes in different way

s.

1.3.1 Some proteins, called integral proteins,

span the lipid bilayer.

1.3.2 Some proteins, called peripheral proteins, are bound to membranes loosely and reversibly.

1.3.3 Sugar residues are found only on the extracellular side of the plasma membrane attached either to lipids to form glycolipids or to proteins to form glycoproteins.

Fluid mosaic model

2. Proteins facilitate ions and solutes to move across the hydrophobic membranes in various ways.(E3 Membrane transport: small molecules)

2.1 Simple diffusion of ions and polar molecules in living organisms is impeded by selectively permeable biomembranes.

2.1.1 Only relatively nonpolar molecules (like O2, N2) cross biomembranes by simple diffusion (i.e., move from higher concentration area to lower one until they become evenly distributed).

2.1.2 Water, though polar, diffuses rapidly across biomembranes by mechanisms not fully understood. High concentration (55M) may be the reason.

2.2 Most ions and polar solutes move across biomembranes by carrier-mediated transport.

2.2.1 Carriers are usually proteins (also called pumps (active), transporters or permeases （透 ( 性 ) 酶） (passive)).

2.2.2 Carrier proteins are similar to enzymes, lowering the activation energy of simple diffusion process, by providing an alternative hydrophilic transmembrane pathway.

2.3 In active transport, solutes move against the c

oncentration gradient (uphill) resulting in the acc

umulation of a solute on one side of a membrane.

2.3.1 Active transport occurs only when an e

nergy source (exergonic process) is coupled.

2.3.2 In primary active transport, energy is pr

ovided directly by the hydrolysis of ATP (as with th

e Na+-K+ ATPase on vertebrate plasma membrane

s), by electrons flowing down an electron transport

system (as with H+ pumping out of the mitochondri

a inner membranes), or by absorption of sunlight

(as with the light-driven H+ pumping of bacteriorh

odopsin in halobacterium).

Directly coupled to an energy source or a che

mical reaction.

2.3.3 In secondary active transport, ion gradients across the membrane are used to drive the concentration uptake of other ions or metabolites.

2.3.4 Many cells contain secondary transport systems that couple the spontaneous, downhill flow of H+ or Na+ to the simultaneous uphill pumping of another ion, sugars, or amino acids.

2.4 Na+-K+ ATPase is responsible to maintain the Na+ and K+ gradients across animal cell plasma membranes.

2.4.1 Most animal cells have a high concentration of K+ (145 mM) and a low one of Na+ (5 mM) relative the external medium (K+, 5mM; Na+, 150 mM?). (fig.)

2.4.2 The Na+-K+ gradient in animal cells controls cell volume, renders nerve and muscle cells electrically excitable, and drives the active transport of sugars and amino acids.

3. Membrane fusion is central to many biological processes (E4 Membrane transport: macromolecules)

Exocytosis

Endocytosis

Phagocytosis

Pinocytosis

Receptor-mediated endocytosis

Clathrin-coated pits and vesicles