Organisms are made of cells, the simplest are unicellular organisms, otherwise multi cellular.

A plasma membrane surrounds the cell and separates it from its external environment. It is a lipid bilayer. Each layer is a sheet of hydrophilic heads and hydrophobic tails. The tails are set towards each other, so the heads are pointing outside and inside, to the watery environment.

The plasma membrane lets water through, but not charged molecules and large molecules.

Proteins can move within the membrane.

cells need energy (food or light).

cells make waste which must be exported.

cells have DNA which contain the genetic information

cells reproduce.

cells produce proteins that carry out functions.


A single cell organism that has no organelles. Examples are bacteria and archae.


Cyto means cell.

The cytoskeleton is made of proteins.

Functions of the cytoskeleton

It provides structural support

Helps with movement

Helps transport substances within the cell

It has three main components:


Long cylinders of about 25nm diameter

Mitotic spindle?

Cilia, outside of the cell

Flagella, outside of the cell

Formed by a dimer of α-tubulin and β-tubulin, repeated in a sheet, and then rolled up into a hollow cylinder.

One end is anchored to the microtubule organising centre (MTOC).

The other end is dynamic and can grow / shrink in length by adding or removing the tubilin dimer.

There are two types of MTOC, centrosome and basal body.

Centrosome is an organelle near the nucleus. There are two centrioles which are like rods. Centrioles are made of 9x3 microtubules. Again this is a hollow cylinder. It is used during mitosis.

Basal body is where the flagella or cilia anchors to the cell.

Sperm cells move with their flagella.

Cilia helps you cough up phlegm from the respiratory tract. There are multiple on a cell.

A cross section of flagella or cilia will show a 9 pairs + 2 microtubules in the centre. Nexin keeps the microtubules in place, dynein helps with movement.

Nerve cells are very long, microtubules run along the nerve cell, inside the axon. Kinesin and dynein (both motor proteins), help shuttle cargo from the nucleus to the synaptic terminal via the microtubles. They shuttle molecules like synaptic vesicles, proteins, lipids and organelles

Can also transport backwards.

Microtubules at Khan Academy.

Ron Vale (UCSF) Part 1: Introduction to Motor Proteins at youtube.com

Intermediated filaments

10nm diameter. Structural support.. e.g. springs of a mattress, or resist mechanical stress.

Made of different type of protein polymers, which are then twisted together. Permanent as they don't grow in size.


Long cylinders of about 7nm diameter.

Gross (overall) cell movement. Helps with cell division. Pseudopod amoebas use microfilaments to capture their food.

Microfilaments are made of actin polymers that are twisted together to form a filament.

They are considered dynamic as they can change length via actin polymerisation of depolymerisation.

Introduction to cytoskeleton at Khan Academy.

Signal transduction

Signal transduction occurs when an extracellular (external to the cell) signalling molecule binds to a receptor on the cell's surface. There is a chain of events which triggers a response in the cell.