SCIE1106 Final: SCIE1106 EXAM NOTES: Overview of Basic Concepts
TOPIC ONE: Review of Basic Concepts
Cells
Cells can be defined as small membrane-bound units filled with an aqueous
solution of chemicals. They comprise all living things, whether they are single-
celled, or multicellular organisms. Cells are able to grow, divide, respond to
stimuli, convert energy and are all highly specialized. All cells, regardless of
whether they belong to single-celled or multicellular organisms are comprised of
the same sorts of molecules (proteins, nucleic acids, carbohydrates, lipids), all
carry out the same basic chemistry and store the same genetic material as DNA.
The genetic mechanisms of cells are also the same, with genetic material being
replicated and passed onto the next generation by cell division. Finally, all cells
contain the same set of 20 amino acids in their proteins.
Structure of Prokaryotic Cells:
Prokaryotes are known as simple cells, being only a few micrometers long,
comprised of one cell without a nucleus. Prokaryotes are often covered by a
tough protective wall enclosing a plasma membrane, with essentially no
membrane-bound organelles inside. Ribosomes and circular DNA are found
floating free in the cytosol. Despite their simple structure, prokaryotes are the
most diverse group of cells, successfully inhabiting many different environments.
They are able to reproduce quickly and exhibit many different growth forms –
either as spherical or rod-shaped, or chains or clusters. Prokaryotes are able to
harvest energy in one of three ways:
• Organotrophic prokaryotes use any organic molecule as an energy
source
• Phototrophic prokaryotes are able to use light as an energy source
• Lithotrophic prokaryotes use nitrogenous, sulfurous or any ion
containing compounds as energy sources
There are two main groups of bacteria, which are as different
to each other as either is from eukaryotes
• Eubacteria are known as true bacteria in the way that
they are found in environments familiar to us
• Archeabacteria are mainly found in hostile
environments, but can also be found in more familiar
ones
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Structure of Eukaryotic Cells:
Eukaryotic cells can be differentiated from prokaryotic cells by the presence of a
nucleus. Eukaryotic cells can be unicellular (such as protists) or multicellular
(such as animals, plants and funghi). Membranes cover the surface of eukaryotic
cells, as well as the organelles within them. Cytoskeletons, made up of
microtubules, actin filaments and intermediate filaments also make up a
eukaryotic cell.
Membranes
The main function of membranes is to compartmentalize cells. This can either
mean to separate cells from their environment, to separate organelles from each
other and from the cytosol, or both. Double membranes surround organelles
such as the nucleus, the mitochondria and in plant cells, the chloroplasts.
The Plasma Membrane
The main functions of the plasma membrane are for cell signaling, the transport
of solutes and for cell growth and motility. The plasma membrane is made up of
an asymmetrical bilayer of molecules phospholipids. A phospholipid is made up
of two components:
• The hydrophilic, phosphate head, and
• The hydrophobic, fatty acid tail
The bilayer is arranged so that the hydrophilic phosphate heads face outside
(either in the intercellular fluid or cytosol) while the hydrophobic fatty acid tails
face each other on the inside. Irregularly interspersed within the phospholipid
bilayer are protein molecules, that are either integral (embedded in the layer)
or peripheral (attached loosely to the bilayer). The plasma membrane is fluid in
the way that both proteins and phospholipids can move laterally. In order to
control movement into or out of the cell, the plasma membrane is selectively
permeable. Only small, hydrophobic, uncharged molecules (such as gases or
water) can cross freely. Larger, charged, polar molecules (e.g. amino acids,
glucose) must move through protein channels in order to enter the cell.
Carbohydrate groups are also integral on the plasma membrane, found on the
external (noncytosolic) side. Carbohydrate groups can appear as glycolypids
(attached to lipids) or glycoproteins (attached to proteins) and play roles in
cell-to-cell communication and adhesion.
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The Nucleus
The nucleus is an organelle surrounded by a double membrane, called the
nuclear envelope, which is continuous with the endoplasmic reticulum. The
membrane of the nucleus is interrupted by nuclear pores, which allow the
passage of selected molecules between the cytosol and the nucleus. The nucleus
typically contains a nucleolus, or nucleolar region, which is a dense site of RNA
synthesis. The nucleus contains most cellular DNA, which is stored in the form of
• Heterochromatin: comprised of DNA and proteins and is highly
condensed, even at interphase
• Euchromatin: comprised also of DNA and proteins, but is not condensed
until mitosis
Ribosomes
Ribosomes are found in prokaryotic and eukaryotic cells,
however ribosomes in eukaryotic cells are larger than those
found in prokaryotic cells. Ribosomes are mainly the sites of
protein synthesis. Ribosomes are either cytosolic (found free
or attached to the endoplasmic reticulum) or in mitochondria
and chloroplasts.
Mitochondria
The mitochondria are the sites of cellular respiration and
major energy (ATP) production. They are also comprised
of a double membrane:
• The smooth outer membrane which is
permeable to ions and small molecules, and
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Document Summary
Cells can be defined as small membrane-bound units filled with an aqueous solution of chemicals. They comprise all living things, whether they are single- celled, or multicellular organisms. Cells are able to grow, divide, respond to stimuli, convert energy and are all highly specialized. The genetic mechanisms of cells are also the same, with genetic material being replicated and passed onto the next generation by cell division. Finally, all cells contain the same set of 20 amino acids in their proteins. Prokaryotes are known as (cid:494)simple cells,(cid:495) being only a few micrometers long, They are able to reproduce quickly and exhibit many different growth forms comprised of one cell without a nucleus. Prokaryotes are often covered by a tough protective wall enclosing a plasma membrane, with essentially no membrane-bound organelles inside. Ribosomes and circular dna are found floating free in the cytosol.
