Cell Structure and Function (Exam 2)

An animal pancreatic cell that exports digestive enzymes would have a lot of_____

ER and Golgi

An animal testis cell that exports lipid-soluble signals (testosterone), would be packed with ________.

smooth ER

Plant leaf cell that manufactures ATP and sugar would be packed with _______.

chloroplasts

Plant root cell that stores starch would be packed with _______.

vacuoles

Nuclear envelope

surrounds the nucleus

double membrane

studded with pore-like openings

continuous with the ER

Nuclear lamina

intermediate filament network on the inside of the nucleus

linked to the nuclear envelope

Nucleolus

rRNA synthesis

Assembly of ribosome subunits

distinct region in the nucleus

Nuclear pores

regulate traffic in and out of the nucleus

highly selective

Outbound nuclear traffic

ex) RNAs exit nucleus into cytoplasm

Inbound traffic into the nucleus

• Nucleoside triphosphates

• Proteins responsible for copying DNA & synthesizing RNA

• Proteins needed for assembling ribosomes

Free Ribosomes

ribosomes located in the cytosol

synthesize proteins mostly for use within the cell

Bound Ribosomes

ribosomes bound to organelles

synthesize proteins for export (secretion) from the cell or for use in lysosomes

Protein sorting

the process by which proteins end up where they need to be to perform their function

Signal sequences

directs a protein’s localization

Do all proteins have signal sequences

NO

Where will a protein with no signal sequence end up?

cytosol

Where will a protein with an animo-terminal signal go?

chloroplast, mitochondria

Where will a protein with an internal signal go?

nucleus

nuclear localization sequence

allows the proteins destined for the nucleus to enter the nucleus

Proteins made on the RER enter the endomembrane system and go to these destinations:

1. INSIDE an endomembrane organelle

2. secreted from the cell

3. as a transmembrane protein

Endomembrane system: Structure

a set of interconnected membranes within a cell

connected physically and by vesicles

Endomembrane system: Function

Produce, modify, package, and deliver proteins to the correct locations

What membranes compose the endomembrane system:

nucleus, ER, Golgi, Lysosome, Peroxisome, and plasma membrane

The Signal Hypothesis

proteins bound for the endomembrane system have a “zip code” that directs the growing polypeptide to enter the ER

“zip code” on proteins

20-amino acid long signal sequence

Signal Hypothesis steps

1. Signal sequence is synthesized by ribosome

2. Signal sequence binds to signal recognition particle (SRP)

3. Signal recognition particle binds to SRP receptor in ER membrane

4. Protein synthesis continues. Protein enters ER. SRP is released.

5. Protein synthesis is complete. Signal sequence is removed.

How to proteins enter the endomembrane system?

The signal hypothesis

What happens when the protein reaches the Golgi?

They are sorted into vesicles and delivered

What does each protein that comes out of the Golgi contain?

A molecular tag that places it in a particular type of transport vesicle. The transport vesicle also has a tag that allows it to be transported to the correct destination

What side of the Golgi do proteins enter?

cis face

What side of the Golgi do proteins exit/ bud off?

trans face

How are proteins intended for secretion from the cell made and processed?

1. Protein enters ER while being synthesized by a ribosome

2. Protein exits ER, travels to the cis face of Golgi apparatus

3. Protein enters Golgi and is processed as the cisternum moves toward the trans face

4. Protein exits Golgi at trans face in a transport vesicle and moves to plasma membrane

5. Protein is secreted from cell

What types of tags do proteins bound for lysosomes or RER have?

carbohydrate tags

How are proteins targeted to the membrane

Transmembrane protein targeting

Transmembrane Protein targeting

1. Polypeptide containing a signal-anchor sequence are inserted into the RER membrane

2. When a signal-anchor sequence is encountered, the RER channel releases the protein into the membrane

3. The signal anchor sequence is hydrophobic and diffuses laterally in the lipid bilayer

4. The ribosome disassociates from the channel and translation continues

5. When translation is complete, the carboxyl end of the protein is in the cytosolic side of the RER and the amino end is in the lumen of the RER

6. These transmembrane proteins can stay in the membrane of the ER or end up in the plasma membrane. They can serve as channels, pumps, receptors, or enzymes