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Module: BIOM - 1006

Lecturer: Dr Sherwood

Date: 11/10/16

Nucleus and Endoplasmic Reticulum
o

The membranes found enveloping cells and organelles are composed primarily of a number of different
lipids, forming a phospholipid bilayer
 It also contains other molecules such as:
 Cholesterol
 Glycolipids/proteins
 Integral and peripheral proteins
 The membrane is modelled as the Fluid Mosaic Model due to its complexity and ability to change
shape

o

One phospholipid is composed of a phosphate-choline, hydrophilic head, one saturated fatty acid,
hydrophobic chain and one unsaturated fatty acid, hydrophobic chain bonded together with a glycerol
molecule
 The opposite water preferences in each
component means that a phospholipid is
amphipathic
 This means it is both hydrophilic and
hydrophobic in the same molecule
 Because of this, the phospholipid
bilayer automatically forms a dome
shape, shielding the hydrophobic
tails from the water, and encasing the
cells organelles inside
 The diagram to the right shows one
phospholipid molecules chemical make-up:

o

The proteins in a cell membrane are designed to carry out specific functions and are highly variable in type:
 They can either be associated with the inside or outside of the membrane, or can span the whole
length of the membrane
 They can have any of the following functions:
 Signal relaying between the cells internal and external environments
 The transportation of ions across the membrane
 For example, the sodium ion and potassium ion channels in a nerve axon
 They can be membrane enzymes such as oxidoreductase
 They can also be cell adhesion molecules that allow cells to identify and interact
 For example, the CD4+ glycoprotein on TH cells

o

The nucleus is the most prominent organelle in the cell and composes 5-10% of the cell volume
 The nucleus contains the cell’s DNA and some mRNA, as well as chromatin (DNA & Proteins)
 The cell also contains a nucleolus
...
These
pores allow mRNA and ribosomes to escape into the
endoplasmic reticulum to be transported across the cell
 The pores allow direct communication between
the nucleus and the cytosol
 The right image is an electron micrograph of a nucleus:

o

The endoplasmic reticulum is a membrane bound maze of interconnecting networks that spans
throughout the cell
 This is continuous with the outer membrane of the nucleus
 The inner space that proteins etc
...
This makes molecules such as enzymes
 However, the ribosomes are not continuously attached
...
Until then, they float
in the cytosol
o Proteins that are destined for the cytosol, nucleus or mitochondria are
made from ribosomes that are free in the cytosol
o All other proteins are made when the ribosomes are attached to the ER
 Smooth (sER)
 The sER does not have ribosomes and is used in the production of lipids to make
molecules such as steroids
 Below is a comparison of rER (top) and sER (bottom)



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