Cell and organelles

What is a cytoskeleton of a cell?

It consists of a network of fibres that extend across the cytoplasm of a cell, The cytoskeleton is the intracellular filament system that controls the shape of the cell and allows the cell to organize its internal components and to move. It also provides trafficking routes for intracellular transport.

Describe the structure of the cytoskeleton

highly dynamic structure

What techniques can we use to visualise the cytoskeleton

Use antibodies that recognise specific proteins

Using a fluorescent protein

List the three major components of cytoskeleton

Intermediate filaments 

Microtubules 

Actin filaments

What is the purpose of intermediate filaments?

Intermediate filaments provide tensile strength and lack the structural polarity characteristic of microtubules and actin filaments.

Which cells are abundant in intermediate filaments?

Cells that require a lot of strength (animal cells: Nerve cell axons, muscles etc..)

Why are intermediate filaments present in cells that require a lot of strength

Intermediate filaments have great tensile strength, enabling cells to resist stretching and mechanical stress

Are the toughest and most durable of the three cytoskeletal filament systems

Describe the location of intermediate filaments within the cell

Typically form networks throughout the cytoplasm, providing mechanical support

Often anchored to the plasma membrane at cell-cell junctions called desmosomes, helping neighbouring cells withstand mechanical stress

Describe the structure of the intermediate filaments 

They are made up of a-helical monomers twisted together to form a coiled-coil dimer.

These dimers are staggered to form a tetromere. 8 tetromeres join together to form a lateral association, which when multiple join together form the intermediate filament

Give two take aways from the intermediate filaments structure

Take away 1: Intermediate filaments have NO polarity

Take away 2: Intermediate filaments assemble without ATP and GTP hydrolysis

What are keratins?

They represent a class of intermediate filaments

What are microtubules' purpose?

Maintain shape (e.g. axons)

Maintain cell polarity (e.g. during movement)

Transport and positioning of membrane enclosed organelles within the cell

Guiding intracellular transport of various cytosolic macromolecules

Form the mitotic spindle

Regulate cellulose deposition in plant cell walls

Describe the structure of microtubules

Microtubules are hollow tubes with structurally distinct ends

Tubes formed from dimers of two globular proteins: α and β tubulin

Microtubules have a definite structural polarity with a designated plus (+) and minus (-) end

Tubulin dimers are generally added more rapidly to the plus end than to the minus end

The polarity is crucial for microtubule assembly and function

What do microtubules display?

Dynamic instability

Describe characteristics of dynamic instability of microtubules

Dynamic instability: switching back and forth between polymerization and depolymerization

This allows microtubules to undergo rapid remodelling and is crucial for their function

Plus ends can be stabilized by binding to specific proteins (e.g. capping proteins)

Selective stabilization of microtubules can polarize a cell

Where is GTP found

Free tubulin dimers contain one GTP molecule (red)

tightly bound to β-tubulin, which hydrolyzes the GTP

to GDP (dark green) shortly after the dimer is added

to a growing microtubule

What does microtubule activity and function depend on?

Variety of accessory proteins that bind to them: microtubule associated proteins.

What can microtubule associated proteins do?

Stabilize microtubules against disassembly

Link microtubules to other cell components

Act as motor proteins that actively transport organelles, vesicles, and other macromolecules along

microtubules: kinesins move towards (+), dyneins toward (-)

What are kinesins and dyneins

Kinesins and dyneins are dimers

Are microtubules formed by tubulin monomers?

No dimers

What are actin filaments?

are thin and flexible, and allow animal cells to adopt a variety of shapes and can have

a range of functions

Give characteristics of actin filaments

Actin filaments are thin and flexible

Actin and Tubulin polymerize by similar mechanisms

Many proteins bind to actin and modify its properties

Cell crawling depends on cortical actin

Extracellular signals can alter the arrangement of actin

Filaments

Actin associated with myosin forms contractile

structures

Describe the actin filament structure

Subunit of each actin filament is an actin monomer

A cleft provides a binding site for ATP or ADP

Actin filament is a two stranded helix

Actin and Tubulin polymerize by similar mechanisms

Free actin monomers carry a tightly bound ATP

This is quickly hydrolysed to ADP soon after the actin

monomer is incorporated into the filament

This reduces the strength of binding between

monomers, decreasing the stability of the polymer

Describe treadmilling of actin filament

Occurs when ATP-actin adds to the plus end of an actin filament at the same rate as ADP-actin is lost from the minus end

Describe actin-binding proteins: Motor proteins

All actin-dependent motor proteins belong to the myosin family

They bind to and hydrolyse ATP, which provides the energy for their movement along actin filaments

Myosin-I is the simplest myosin

The tail varies among the different types of myosin-

I and determines what type of cargo the myosin

drags along

What does muscle contraction rely on?

actin and myosin filaments

Myosin-II molecules can associate with one another to form myosin filaments

• Coiled-coiled tails of myosin-II molecules associate with one another to form a bipolar myosin filament

• The bare region in the middle consists of tails only

Give the purpose of the nucleus 

Contains genome;

site of DNA replication and RNA synthesis

Give the purpose of the rough endoplasmic reticulum

Synthesis of proteins destined

for secretion, insertion into the

plasma membrane, or residence

in the endomembrane system

Give the function of the golgi body

Modification, sorting

and packaging of

proteins and lipids for

either secretion or

delivery to another

organelle

Give the function of the mitochondria 

ATP synthesis by

oxidative

phosphorylation

Give the function of the endosome

Sorting of

endocytosed

material

Give the function of the lysosome

Intracellular

degradation

Give the function of cytosol

Site of many

metabolic reactions

and protein

biosynthesis

Give the function of peroxisome

Oxidative

breakdown of

toxic molecules

Give the function of free ribosomes 

Translate

proteins in the

cytosol

How do proteins synthesized in the cytoplasm reach their destination organelle (how do they "know" where to go)?

Proteins contain specific signal sequences that will direct them to specific organelles

Describe how proteins contain specific signal sequences that direct them to particular cellular compartments.

These signals are often short amino acid sequences, although their structure and location vary.

Some targeting signals are removed after

transport, while others remain part of the mature protein.

Both necessary and sufficient to direct a protein to a particular destination

Describe what targeting signals do

Targeting signals direct newly

synthesised proteins to specific

cellular compartments or into the

secretory pathway.

Describe exocytosis 

the process of vesicles fusing with the plasma membrane,

releasing or “secreting” their contents

Describe endocytosis

Describe protein coats in endocytosis (Clathrin-coated)

Clathrin-coated vesicles transport selected cargo molecules 

Surface receptors ensure specificity of transport

Each vesicle starts off as a clathrin-coated pit; clathrin molecules assemble into a basketlike network on the cytosolic surface of the membrane

When budding from the plasma membrane this process is also called clathrin-mediated endocytosis (CME)

Does the budding of clathrin-coated vesicles cost energy?

Yes

What are the three forms of endocytosis

Phagocytosis - engulf particles

Pinocytosis  - engulfs liquids 

Receptor mediated endocytosis - receptors will bind to specific cargo

Describe phagocytosis

A form of endocytosis in which cells engulf large particles, such as bacteria or cellular debris.

Used for feeding in some unicellular organisms and for defence (and scavenging dead and damaged cells) in animals

Performed by specialised cells including white blood cells such as neutrophils and macrophages

Recognition of a particle triggers actin-dependent membrane extensions (pseudopods) that surround and engulf it

The resulting phagosome matures and can fuse with lysosomes, where the contents are degraded

Describe pinocytosis

Is performed by most cells and occurs continuously

This process of cellular drinking is not molecule specific

Relatively small quantities of extracellular fluid, containing salts, water, amino acids etc. is taken up

Whatever happens to be around the invaginating region will end up being engulfed into the cell

Describe receptor mediated endocytosis (RME)

Cells need to take up cholesterol to make new membrane

It is extremely water insoluble

Cholesterol bound to Low-density lipoproteins (LDL) 

LDL-cholesterol bind to receptors → RME

Naked vesicle fuses with the endosome

Acidic environment triggers dissociation of LDL with its receptor

What is the pH of lysosomes?

pH 5 - Acidic

How is the pH of the lysosomes kept low?

The presence of many protons, by using a proton pump

Following exocytosis glycosylated domains of a protein can end up where?

Outside the cell

Describe protein targeting Specific signal sequences direct proteins to the appropriate

cellular compartments.

Describe Vesicle transport

Transport vesicles move proteins (and lipids) between

compartments. Exocytosis releases material outside the cell, while endocytosis brings material into the cell.

Describe Vesicle formation and fusion

Coat and adaptor proteins help select cargo

and form vesicles, while Rab proteins, tethering factors and SNAREs ensure

correct targeting and membrane fusion