DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 223 Cell Biology and Genetics

1. Different types of cytoskeletal filaments found in eukaryotic cells

Cytoskeleton is found underlying the cell membrane within the cytoplasm and it

provides scaffolding for membrane proteins to anchor to and forms organelles that

extend from the cell. It is consists of complex network of fiber that extending through

the cytoplasm of eukaryotic organisms which can interact with hundreds of other

proteins structures and can perform multitude functions (refer Figure 1.1, Figure 1.2

and Figure 1.3).

Figure 1.1: A fluorescence micrograph of cytoskeleton that extends throughout the

cell. Green colored represents microtubules and orange for microfilaments. However, intermediate filaments not evident here. In the nucleus, blue color is the DNA. (Source: Biology, Benjamin Cummings Pub., 6th Ed.)

Figure 1.2: The structural differences of cytoskeleton filaments. Microtubule in a form

of hollow tubule (left); intermediate filaments in a form of twisted fiber (centre); and microfilaments in a form of spiral filament (right). Each type has it unique structures and arrangement. (Source: Biology, McGraw Hill, 3rd Edition)

Figure 1.3: The different protein composition in each of the cytoskeleton filaments allows them to play different functions respectively. Microtubules composed of tubulin (left); intermediate filaments composed of different proteins e.g keratin and lamin (centre); and microfilaments made up of protein actin (right). (Source: Biology, McGraw

Hill, 3rd Edition).

DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 223 Cell Biology and Genetics

The table below shows the comparisons of the three major components of cytoskeleton:

Components Microtubules

Intermediate Filaments

Microfilaments Characteristics

Distribution All eukaryotic cells All plant cells, but not all animal cells

All eukaryotic cells

Structure Long, hollow, cylindrical tubule

Twisted, rope-like fiber

Long, thin, spiral filament

Diameter + 25 nm + 10 nm + 7nm

Protein Properties alpha & beta tubulin

Composed of 5 types of proteins including lamina, keratin & other form of inter- mediate filaments

2 interwined chains composed of globular actin monomer called G-actin

Major Group of Associated Protein

MAPs (microtubule associate proteins)

Plakins (also referred to as cytolinkers)

F-actin (actin binding protein molecules made up of polymerized actin)

Motor Protein Kinesins, Dynein No motor protein (no polarity)

Myosins

Enzyme Activity GTPase - ATPase

Location

In cilia & flagella as motile appendages (e.g human sperms, Chlamydomonas, & Paramecium). Also found in centrosomes & centrioles (region near the nucleus).

Keratins found as major components of epithelial cells like hair, nails, skin, kidney and intestine. Lamin A, B & C located within the nuclear lamina of cells.

Highly concentrated at plasma membrane that known as actin bundle (actin stress fiber span)

Function

Undergo dynamics instability (growing & shortening) in many cellular activities; sorting chromosomes during mitosis/cell division; promote cell shape & organization

As tension bearing that maintain cell shape & rigidity (mechanical strength); provide anchorage point for nuclear pore; line the inner nuclear membrane

Play key role in cell shape & strength; allow intra- cellular of cargo; provide cell motility (amoeboid movement); perform muscle contraction; cytokinesis in cell (aid cell division); short distance transport

Special Features

Have polar structures with positive & negative end. Grow only at positive end. Shorten at both ends. Short live.

The protein binds to each other to form twisted filaments. More permanent and durable. Not assembled and disassembled as other cytoskeleton filaments.

Readily shorten and lengthen at anytime in cell. Have positive & negative end. Grow at positive end by actin monomers. Side of actin filaments often anchor to other protein near plasma membrane.

DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 223 Cell Biology and Genetics

2. Illustration of the complete structure of the well known Fluid Mosaic Model

of a cell membrane

A cell membrane is a thin film that partitioning the cell into functional and

segregated compartments. Like all biological membranes, it exhibits selective

permeability which allows certain substances to pass through it and separates life

from the non-life on its exterior. The “fluid mosaic membrane model” that was

proposed in 1972 by S. Jonathan Singer and Garth Nicolson consistent with the

observation that membranes are composed of two primary components, namely

phospholipids bilayer that project from both side of the membrane and globular

proteins embedded in or loosely attach to membrane surface. Carbohydrate

molecules also found attach to the membrane lipids and proteins (refer Figure 2.1).

Figure 2.1: A drawing of the refined structure of Fluid Mosaic Membrane Model

(Reference: Biology, McGraw Hill, 3rd Edition)

DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 223 Cell Biology and Genetics

Phospholipids

A phospholipid is a lipid formed by combining a glycerol molecule with two fatty acids

and a phosphate group. It is bilayered and known as the basic framework to the cell

membrane structure. Phospholipid bilayer consist of amphipatic molecules, having the

polar hydrophilic (water attracting) heads made up of the phosphate group and the

non-polar hydrophobic (water dreading) tails, a hydrocarbon chain. The polar ‘heads’

are oriented on one side toward the outside of the cell and into the fluid cytoplasm on

the other side and are hydrophilic, while the non-polar ‘tails’ (also known as fatty acid

tails) face and attract each other in membrane interior, and are repelled by water

(Figure 2.2). The external of lipid bilayer is called extracellular leaflet while the internal

of the lipid bilayer is called cytosolic leaflet (intercellular leaflet). The membrane’s lipid

bilayer is known as fluid because it allows proteins to move around within it.

Figure 2.2: The cross section of phospholipid bilayer

(Source: Biology, Benjamin Cummings Pub., 6th Ed.)

Membrane proteins

Proteins and cellular membrane associated as integral membrane proteins (IMP) and

peripheral membrane proteins (PMP), as shown in Figure 2.3. The IMP (intrinsic

proteins) cannot be released from membrane unless the membrane dissolved into

organic solvent (or detergent) that can disrupt the integrity of the protein. Two ways

association of IMP named transmembrane proteins and lipid anchorage proteins.

Transmembrane proteins have one or more regions inserted into hydrophobic interior

of phospholipid bilayer, where each segment folded into alpha helix structure and

stable because the non-polar amino acids interact favorably with the hydrophobic fatty

acyl tails of the lipid membrane. Lipid anchorage protein associates with membrane

because it has lipid molecules that covalently bound to amino acid side chain within

the protein to keep it firmly attached. Meanwhile, the PMP (extrinsic protein) is the

Polar head (hydrophilic)

Polar head

(hydrophilic)

Non-polar tails (hydrophobic)

Extracellular leaflet

Cytosolic leaflet

DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 223 Cell Biology and Genetics

third way association of protein with membrane. It does not interact with hydrophobic

interior of the phospholipid bilayer, instead non-covalently bound to regions of IMP,

that project out from the membrane to polar head group of phospholipids. PMP

typically bound to membrane by hydrogen and ionic bonds.

Figure 2.3: Three ways of association of membrane proteins with cell membrane.

(Source: Biology, McGraw Hill, 3rd Edition)

Carbohydrates

On the external surface (extracellular environment) of the membrane, carbohydrate

that have the covalent bound with proteins make up glycoprotein, while carbohydrate

that have covalent bound with lipids make up glycolipid (Figure 2.4). Both

combinations are often functioning as cell identity markers which allow one cell to

identify the others, from the same tissue types or different tissue types. The

glycoproteins and glycolipids are individually unique, thus enable our immune system

to use them to identify foreign invaders (antigen) including bacteria or viruses so that

they can be destroyed.

Figure 2.4: The general structure of carbohydrate, glycoprotein and glycolipid on a

plasma membrane. (Source: Michael J. Gregory, Human Biology 100)

Extracellular environment

Transmembrane alpha helix

Transmembrane protein

Peripheral membrane protein

Lipid

Lipid anchorage protein

Cytosol

DEBBRA MARCEL JP/8544/13 ASSIGNMENT 1

JIB 223 Cell Biology and Genetics

Cholesterol

Cholesterol can be found within the tails regions of the lipid bilayer (Figure 2.5). It can

only be found in the plasma membranes and organelle membranes of animal cells.

The cholesterol molecules are wedged between phospholipid molecules, and helps

stabilize the fluidity of cell membrane at different temperature. At moderate

temperature, the cholesterol makes the membrane less fluid by reducing phospholipid

movement. However, at low temperatures, it hinders the solidification by disrupting the

regular packing of phospholipids. Therefore, cholesterol function as “fluidity buffer” for

the membrane by resisting changes in membrane fluidity that can caused by changes

in temperature.

Figure 2.5: The arrangement of cholesterol between lipid molecules of a lipid bilayer

within an animal cell. (Source: Biology, Benjamin Cummings Pub., 6th Ed.)

References:

1. Brooker, R. J., Widmaier, E. P., Graham, L. E., and Stiling, P.D. (2014).

Biology. Third Edition. McGraw Hill International Education. p. 1263.

2. Campbell, N. A., and Reece, J. B. (2008). Biology. Sixth Edition. San

Francisco (CA): Benjamin Cummings. p. 1247.

3. Michael J. Gregory (Ph.D). General Biology 1: Bio 101. Human Biology 100.

(http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20100/b

io%20100%20lectures/membranes/membrane.htm, last accessed Oct. 25,

2014).