Chapter 1.

Membrane Structure and Membrane Organelles

Fig.1-1. (A) A diagramatic representation of mesophyll leaf cell. (B) Thin section TEM through a meristematic root tip cell.

► The function of membranes is to serve as a barrier to the diffusion of most water soluble molecules.

1. delimit compartments (different chemical composition, optimized for particular activity)

2. serve as scaffolding for certain proteins

► The function of proteins in membrane

1. transporting molecules 2. transmitting signals 3. processing lipid 4. assemble glycoproteins and polysaccarides 5. provide mechanical links between cyt. and cell wall

1-1. Common properties and inheritance of cell membranes

- A bilayer of polar lipid and proteins

- Asymmetrical structure ( one side inside, the other side external)

1) Cell membrane possess common structural and functional properties

2) All basic types of cell membranes are inherited. (cell must maintain the integrity to survive)

The rules of membrane inheritance

1. Daughter cells inherit a complete set of membranes from their mother

2. Each potential mother cell maintains a complete set of membrane

3. New membranes can be produced only by growth and fission of existing membranes: they cannot arise de novo.

Table 1.1 Membrane types found in plant cells

1.2 The fluid-mosaic membrane model

1) The amphipathic nature of membrane lipids

Fig. 1-3. plant membrane lipids

Fig.1-4. Sterols found in plant plasma membranes

Fig. 1-5. Organization of amphipathic lipid molecules in a bilayer

Fatty acid tail:14-24 C atomsDouble bond: one or more

2) Phospholipid moves rapidly in the plane of the membrane but very slowly from one side of the bilayer to the other

Fig. 1-7. Mobility of phospholipid molecules in a lipid bilayer

Fig.1-8. Mechanism of action of a “flippase” a phopholipid translocator.

3) Cells optimize the fluidity of their membrane by controlling lipid composition

Low temperature: - shortening carbon chain length- increasing the number of double bonds- increasing the size or charge of the head groups- increasing the sterols (disrupting gelling of phospholipid)

High temperature: - increasing carbon chain length- decreasing the number of double bonds- decreasing the size or charge of the head groups- increasing the sterols (interfering with the flexing motions)

Table 1.2. The effects of fatty acid chain length and double bonds on the Tm of some phospholipids.

4) Membrane proteins associate with lipid bilayers in many different ways

Lipid modification of proteins (Fatty acid-linked proteins)

Myristoylation (C14)

Palmitoylation (C16)

Prenylation (C15 and C20)

GPI anchored p

5) The fluid-mosaic membrane model predicts structural and dynamic properties of cell membranes

- Lipid + peripheral & integral proteins (originally) + lipid anchored proteins + membrane protein-cytoskeletal interactions (recently).

- Membrane fluidity involves the movement not only of lipid molecules but also of the different proteins.

- The importance of protein movement in membranesa) the transfer of substrates in mitochondria and chloroplast.

b) the assembly of multi-protein membrane complexes.c) the signaling pathways

1.3. Plasma membrane► Plasma Membrane forms the outermost boundary of the living cell and functions as an active interface between the cell and its environment

► Plasma Membrane

a) controls the transporter of molecules into and out of cell

b) transmits signals from the environment to the cell interior

c) participates in the synthesis and assembly of cell walls.

d) provides physical links btwn the cytoskeleton & the extracellular matrix

Plasmodesmata

1) Lipid composition of plasma membrane is highly variable.

- The composition of PM of plants cells; Lipids: proteins: carbohydrates= 40: 40 : 20

-The ratio of lipid classes varies remarkably (Table 1.3.)

-The composition of lipids is important in membrane fluidity and protein activity

- The most common sterols of plant PM: campesterol, sitosterol, stigamsterol

- Cholesterol is a minor component in plant species.

2) Cold acclimation leads to characteristic changes in PM lipid composition.

- After cold acclimation, plants can withstand freezing (-40℃),

- Without cold acclimation, plants can be injured by a freeze (-3℃).

- The most ciritical alterations during cold acclimation are changes in lipid composition in PM

a. increase of phospholipid b. decrease of glucocerebrosidesc. increase of diunsaturated phospholipidd. shortening of fatty acid chaine. increase the size of head group

i. increase sterols

3) PM proteins serve a variety of functions

a) controls the transporter of molecules into and out of cell

b) transmits signals from the environment to the cell interior

c) participates in the synthesis and assembly of cell walls.

d) provides physical links btwn the cytoskeleton & the extracellular matrix

4) The electrochemical gradient produced by H+-ATPase drives many other transporter systems

The functions of H+ pumping in PM

1. acidify cell walls alkalizes the cytosol affecting cell growth and expansion affecting other cellular activities

2. produces an electrochemical gradient across PM. used by secondary active transport

5) Some PM receptors have been identified or cloned.

- By physiological approaches: Receptors that bind hormones, oligosaccharides, peptides, toxin

- By genetic approaches: receptor protein genes

- By computer analysis: mammalian receptor homologues

6) Several classes of plasma membrane proteins mediate interactions with the cell wall

The principle functions of ER

1) Synthesizing, processing, and sorting proteins

2) Adding N-linked glycans

3) Synthesizing a diverse array of lipid molecules

4) Anchoring sites for actin filaments bundles

5) regulating cytoplasmic Ca2+ concentration

1.4. Endoplasmic reticulum

Plant ER system

Rough ER

Rough ER

Smoth ER

Actin filament (AF) attached to cortical ER.

Oil Body in ER

Protein body in ER

Ex. Proamines (alcohol soluble storage protein)

Cf) globulin proteins (water soluble storage protein)-storage vacuole

Transport vesicles budding from ER cisterna in the vicinity of a Golgi stack

1.5 Golgi apparatus

The function of Golgi apparatus

1) receiving newly synthesized proteins and lipids from the ER

2) directing them to cell surface or vacuoles

3) assembling complex polysaccharides of cell wall matrix

4) synthesizing and processing the O-linked oligosaccharide side chains of membrane

5) processing the N-linked oligosaccharide side chains of membrane

6) producing glycolipids for plasma membrane and tonoplast

Freeze fractured Golgi stack of a pea root tip cell

Tobacco BY2 cells expressing a mannosidase 1-GFP.

A dividing Golgi stack in a root cap cell of pea.

The functions of glycosylation (N-linked, O-linked)

1.Protect proteins against proteolysis

2.Specify PM-cell wall interaction

3.Prevent premature activation of lectins

4.Contribute to protein folding or assembly of multiful protein complex