3.3 - Cytoplasmic Membrane Systems

What organelles compose the Endomembrane System?

The ER, Golgi complex, endosomes, lysosomes, and vacuoles

The Endomembrane System

• functions as a coordinated unit

• Distinct compartments bounded by membrane barriers

• Contain specialized proteins for particular activities

Transport through the endomembrane system

• Materials packaged in small, membrane-bounded transport vesicles:

• Bud from a donor membrane compartment.

• Move via motor proteins on microtubules and microfilaments of the cytoskeleton.

• Fuse with the membrane of the acceptor compartment

Pathways through the cytoplasm

• Biosynthetic pathway

• Constitutive secretion

• Regulated secretion

Biosynthetic pathway

Proteins are synthesized in the ER, modified at the Golgi complex, and transported to various destinations.

• Also referred to as the secretory pathway – proteins discharged from the cell.

Constitutive secretion

Materials are transported in secretory vesicles and discharged in a continual manner.

Regulated secretion

Materials are stored in vesicles and discharged in response to a stimulus.

• ex. endocrine cells that release hormones, insulin released from cells in the pancreas, enzymes important for digestion that come from pancreas

• the right things need to get to the right places

• involved sorting signals (encoded in the amino acid sequence of proteins or attached oligosaccharides that are attached to proteins)

Which structure is NOT part of the endomembrane system?

a) Nuclear envelope

b) Chloroplast

c) Golgi apparatus

d) Endoplasmic reticulum

b) Chloroplast

A protein is transported in a secretory vesicle and discharges into the extracellular space in a continuous fashion. What type of secretion is this?

a) Regulated

b) Biosynthetic

c) Constitutive

d) Unregulated

c) Constitutive

The Endoplasmic Reticulum

• Network of membranes that penetrates much of the cytoplasm

• Highly dynamic structure; two compartments that share some proteins and activities (rough ER and smooth ER)

• constantly undergoing continual turnover and reorganization

Smooth ER (SER)

• Lacks ribosomes

• Membranes are highly curved and tubular (form interconnecting system of pipelines)

• Continuous with the RER (lipids and proteins are capable of diffusing from one type of ER into the other)

SER functions include

• Steroid hormone synthesis in endocrine cells of the gonad and adrenal cortex.

• Detoxification of organic compounds in the liver via oxygenases, including the cytochrome P450 family.

• Sequestering calcium ions for regulated release

• see this a lot in the smooth ER of skeletal muscle and cardiac muscle

• calcium is REALLY important for muscle contraction

Rough ER (RER)

• defined by the presence of ribosomes bound to the cytosolic surface

• Flattened sacs (cisternae) connected by helicoidal membranes

• Continuous with the outer membrane of the nuclear envelope.

• RER is the starting point of the biosynthetic pathway

• Site of synthesis for proteins

RER organization in cell

• Organelles situated in a way that produces distinct polarity in the cell

• RER at basal side; Golgi complex central; secretory granules at apical side

• Apical side is site of discharge

The smooth endoplasmic reticulum is responsible for all of

the following EXCEPT:

a) Protein modifications

b) Sequestration (storage) of calcium ions

c) Synthesis of steroid hormones

d) Detoxification in the liver

a) Protein modifications

Free ribosomes

not attached to the RER; primarily synthesize proteins for use within the cell

Bound ribosomes

attached to cytosolic surface of the RER membranes; primarily synthesize proteins for export

Synthesis of secretory, lysosomal, or plant vacuolar proteins in RER (Part 1)

• The ER membrane provides a large surface area for ribosomes to attach, and the lumen gives a specialized local environment that favors protein processing.

• Signal sequence at N-terminus (6–15 hydrophobic amino acids) directs the emerging polypeptide and ribosome to the ER membrane.

• Signal sequence is recognized by a signal recognition particle (SRP) – binds to the SRP receptor on the ER membrane.

Synthesis of secretory, lysosomal, or plant vacuolar proteins in RER (Part 2)

• The polypeptide moves into the lumen through the translocon in the ER membrane

• Movement of proteins into ER involves G proteins

• GTP hydrolysis triggers the release of the signal sequence by the SRP

• The signal peptide is removed by signal peptidase and carbohydrates are added

by oligosaccharyltransferase.

• Both enzymes are integral membrane proteins associated with the translocon.

Translocon

a protein that forms a pore in the ER membrane