Lesson 7 – Cell Membranes
and the Cytoskeleton
Hank – in da club
Crash Course Biology - Cell membranes
Cell membranes
On your desks draw out the structure of
the plasma membrane of a eukaryotic cell.
Cell Membrane
From now on we will refer to the exterior cell
membrane as the plasma membrane as it is more
descriptive of its nature.
The plasma membrane
separates the
contents of the cell
from its environment.
Membranes within
the cell are also used
to create
compartments
Functions of Membranes
There are 6 main functions of membranes in
cells:
1. Providing selectively permeable barriers
2. Compartmentalisation
3. Localising reactions in the cell
4. Transport of solutes
5. Signal Transduction
6. Cell-Cell Recognition
Providing Selectively Permeable Membranes
Membranes within the cell as well as the plasma
membrane only allowing some molecules to pass
through.
Compartmentalisation
Membranes help to divide up areas within eukaryotic
cells. Membranes are used for the nuclear envelope,
endoplasmic reticulum, Golgi apparatus, mitochondria,
and chloroplasts.
Localising Reactions within a Cell
One of the reasons for compartmentalism is to organise
the chemical reactions in a cell within certain areas.
Example: Light reactions of photosynthesis
You do not need to know
the details of the
chemistry of the light
reactions but you do
need to understand that
these reactions are
associated with enzymes
and carrier located in
the membranes of the
chloroplast.
Transport of Solutes
Membranes are able to transport solutes across the
membrane. Sometimes this is done against a
concentration gradient.
Signal Transduction
There are receptors found on the surface of the cell
membrane that can recognise and respond to the
presence of specific molecules.
Cell-Cell Recognition
Molecules on the surface of the cell are unique to the
cell and can be used for cell-cell recognition.
Membrane Structure
In your jotters or on your desks draw the cell membrane.
Use your monogram to produce a fully labelled diagram on the plasma
membrane. Include all detail from the monograph in your poster.
Function of Membrane Proteins
Membrane proteins can be classified based on their function or
their arrangement in the membrane.
Based on Arrangement in the Membrane
1. Intrinsic (also known as embedded)
a) Transmembrane – these go right through the membrane
b) One side only – these are only embedded on one side of the
membrane
2. Extrinsic (also known as peripheral)
These are only loosely attached to the surface of the membrane a
non-covalent association with other proteins.
Extrinsic Proteins
Function of Membrane Proteins
Classification based on function
There are 6 main functions of membrane proteins
1.
2.
3.
4.
5.
6.
Transport (more on this!)
Enzymes
Receptors
For Cell Adhesion
Cell-Cell Recognition
As part of the Membrane Linked
Cytoskeleton
Transport Proteins
Create a second poster using the diagram on page 46 of
the monograph (Fig 2.5.2) to detail all of the functions of
membrane proteins.
The Cytoskeleton
In eukaryotes there is an internal structure consisting of actin filaments,
intermediate filaments and microtubules that support the internal structure
of the cell.
Cytoskeleton
Microtubules
Microtubules are hollow tubes made up of the protein
tubulin. Heterodimers (one α-tubulin and one β-tubulin
per dimer) are arranged as a set of 13 protofilaments to
make a microtubule.
You will recall the importance of microtubules are important as
they make up the spindle fibre network crucial in cell division.
They are also responsible for moving components around in the cell.
Actin Filaments
Actin filaments are made up of the protein actin. They are
arranged as 2 strands of protein molecules twisted together.
The diameter is about 7nm.
They are important in cell movements and are mostly
concentrated inside the cell membrane.
Intermediate Filaments
Intermediate filaments are about 10nm in diameter and are made
of fibrous proteins twisted together. They are very stable and
provide mechanical strength to the cell. They are anchored to the
cell membrane to provide support.
How it Works
Microtubules
can grow
longer through
polymerisation
or can get
shorter
through depolymerisation.
This is how the
cell structure
can be
dynamic.
Centrosomes (centrioles)
The centrosomes
(also called the
microtubule
organiser) controls
the polymerisation
and
depolymerisation
of microtubules.