1.6 Exocytosis and Endocytosis

1.Exocytosis:

- from TGN to PM, - proteins, lipids, polysaccharides, glycoproteins, proteoglycans

Endocytosis:

- retrives excess membrane for recycling through the formation of PM membrane infolding.

- turn over PM and cell wall molecules

- remove activated receptors from the cell surface

- In animals, plays major role in the uptake of nutrients but little evidence suggests such a role in plants

1) In plants , turgor pressure affects membrane events associated with exocytosis and membrane recycling.

Fig 1-29. Cross section images of secretory vesicles.

2) Turgor pressure also affects endocytosis and membrane recycling

3) The membrane compartments associated with endocytosis can be identified by following the uptake of tracer molecules.

(A) Clathrin-coated vesicles budding from a PM(B) higher magnification TEM of clathrin-coated vesicles.

The endocytic pathway

Plasma membrane/extracellular space

Clathrin-coated pits and vesicles

Non-coated vesicles

Partially coated reticulum (likely extention of the TGN)

Multivesicular bodies, organelles (not to be destined for degradation)

Vacuoles

Multivesicular body in the cytoplasm of a tobacco cell

1.7. Vacuoles- Fluid filled compartments encompassed by a membrane called the tonoplast- Conspicuous organelles of most plant cells- Numerous small vacuoles in apical meristem cells- combine into one or a few larger vacuoles as the cell matures and expands

Mesophyll leaf cell Root meristem cell

View of spongy mesophyll cells in a bean leaf

Vacuoles store a large variety of molecules: inorganic ions, organic acids, sugars, enzymes, store proteins and many types of secondary metabolites.

- solutes -> water -> turgor pressure -> cell enlargment

Many hydrolytic enzymes found in vacuoles, suggesting a role in turnover of cellular constituents

(like lysosomes in animal cells)

1) Plants use vacuoles to produce large cells cheaply

- To maintain the turgor pressure of, solutes must be actively transported into the growing vacuole.

- Electrochemical gradient is produced by two proton pumps (V-type H+-ATPase and H+-pyrophosphatase (H+-PPase)

- The movements of water across the tonoplast is mediated by aquarin channels

2) Plant vacuoles are multifunctional compartments

a. Storage: inorganic ions, organic acids, sugars, enzymes, store proteins and many types of secondary metabolites

b. Digestion: acid hydrolases (protease, nuclease, glycosidase, and lipases) are found in. recycling (turnover and retrieval of nutrients)

c. pH and ionic homeostasis: reseiviers of protons and ions (calcium) regulates cytosolic pH, the activity of enzymes, the assembly of cytoskeletal structures and membrane fusion

d. Defense against microbial pathogens and hebivores: - phenolic compounds, alkaloids, cyanogenic glycosides and protease inhibitor to insect and herbivores - cell degrading enzymes;chitinase and glucanase, defense molecules to fungi and bacteria - latexes, to insect and fungi herbivores

e. Sequstration of toxic compounds: heavy metals toxic metabolites (oxalate)

f. Pigmentation: antocyanin pigments – attract pollinator and seed dispersers- screen out UV and visible light preventing photooxidative damage

3) Many plant cells contains two different vacuole systems

a. Neutral protein vacuoles (V1)

b. Acidic, lytic vacuoles (V2)

4) Vacuoles may be the only membrane compartments that can be created de novo

2) the central vacuole appears to arise from smooth ER tubes

these tubes assemble in to a cage-like structure

cleared of organells

then autophagocytosed into the forming vacuole

or displaced as the vacuole tube fuse and inflate.

1) provacuoles arise from ER domain

in which H+-ATPases, -TIP and stoage proteins accumulate

After their separation from ER, provacuoles inflate

they become full-fledged vacuoles

1.8 The nucleus

- Contains most of the cell’s genetic information

- Serves as the center of regulatory activity

Nucleus of root tip cell

NE: nuclear envelopesNP: nuclear pores

1) The nuclear envelope is a dynamic structure with many functions

2) Nuclear pore complexes function both as molecular sieves and as active transporters

RNA

n Protein & r Protein

Basic process of transport in Nuclear Pore Complex

1. Permit free diffusion of small molecules through 9 nm diameter channel2. A larger regulated central channel functions in the active transport of proteins and RNA molecules

3) The nucleolus, a prominent organelle in the interphase nucleus, is the ribosome factory of the cell

- Not membrane bounded - Specialized regions of the nucleous - A product of active ribosomal genes- >100 different proteins and nucleic acids- Transcibing rDNA- Processing rRNA transcript- assembling rRNA- import rProteins into ribonucleoprotein

4) During mitosis, the nuclear envelope disassembles into vesicles that participate in the formation of new envelopes around the daughter nuclei.

1.9 Peroxysome

The roles of peroxysome

-Participate in lipid mobilization in germinating fat–storing seeds

-Play a key role in photorespiration in leaves of C3 plants

-Invoved in the conversion of recently fixed N2 into nitrogen-rich organic compounds.

: called “peroxysome” because they generate and destroy hydrogen peroxide (H2O2)

Contains catalase , glycolate oxidase urate oxydase, enzyme for oxydation

the presence of catalase (stained by diaminobezimidine)

Unspecialized peroxysome

http://www.plantphysiol.org/content/vol132/issue4/images/large/pp0837992003.jpeg

1) The toxic H2O2 produced by peroxysomal oxidases is destoryed in situ by catalase

2H2O2 O2 + 2H2O

Catalatic reaction

RH2 + O2 R + H2O2

Peroxydatic reaction

R’H2 +H2O2 R’ + 2H2O

Production of H2O2

2) Leaf peroxysomes participate with chloroplasts and mitochondria in the glycolate pathway (photorespiration)

2) Glyoxysomes are specialized peroxysomes that assist in breaking down fatty acid during the germination of fat storing seeds.

Produce H2O2

4) In some leguminous root nodules, peroxysomes play an essential role in the conversion of recently fixed N2 into ureides for nitorgen export.

Ureides

Urate oxydase produce H2O2

New peroxysome arise by

1) division of preexisting peroxysomes 2) import peroxysomal proteins from cytosol