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Lecture

lecture 6 for BGYA01

by OC4

Department
Biological Sciences
Course Code
BIOA01H3
Professor
Clare Hasenkampf

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Lecture 6 BGYA01 September 27, 2007
All right so far we have talked about the macromolecules that make up the cell, and we
have lloked at the process by which cells make their DNA, RNA and proteins.
The carbohydrates and lipids are made in biochemical pathways that are controlled by the
enzymes.
Now I would like to look at how the proteins , carbohydrates and lipids are organized into
cells and cellular structures, and where the DNA and RNA are found and organized.
THE MAJOR GROUPS OF ORGANISMS
Scientists have studied organisms, living and extinct, and have placed them into groups of
related organisms. The kinds of properties they consider when classifying organisms are:
- the structure of the cells
- the organization of cells into an organism
- the methods of acquiring their building materials and energy
- the method of reproduction.
Current classification schemes group the living and extinct species into three domains.
The three domains are Bacteria, Archaea, and Eukarya. Figure 1.11 page 12.
The tree diagram of figure 1.11 illustrates the idea that the first type of organism diverged into
two types of organisms. One type gave rise to the domain known as Bacteria, and the second
type was the ancestor of both the Archaea and the Eukarya.
Available evidence suggests that the Archaea and Eukarya share a common ancestor, not shared
with the Bacteria. At a later point in time this second type of organism diverged into the separate
domains of Archaea and Eukarya.
It is estimated that organisms within a domain have been evolving separately from organisms in
the other domains for at least a billion years.
Lets look at some key characteristics of the three domains. (Well discuss these characteristics
in more detail later).
Bacteria
(1) Have cells that lack double membrane bound organelles
(2) Are single celled organisms
(3) Have either no cell walls OR a cell wall containing peptidoglycan
(4) Most have a single circular double stranded DNA as its hereditary material
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Archaea
(1) Have cells that LACK double membrane bound organelles
(2) Are single celled organisms
(3) Have some members with cell walls but the walls do NOT contain peptidoglycan
(4) Most have a single circular double stranded DNA as its hereditary material
Eukarya
(1) Have cells that DO have double membrane bound organelles
(2) Have some single celled members, and many multi-cellular members.
(3) Have some members with cell walls, but the walls do NOT contain peptidoglycan
(4) All have several linear double stranded DNAs as its hereditary material
The two most ancient domains,
Bacteria are both in the important category we call prokaryotes.
All of the other branches of the Tree of Life are called eukaryotes.
The distinction between prokaryote or eukaryote is based on the degree of structural organization
within the cell.
The prokaryotes (both Bacteria and the Archea) are single celled organisms WITHOUT internal
membrane-bound, compartments.
They have relatively simple, internal cell structure.
Most prokaryotes have one circular, double stranded, DNA molecule as their hereditary material.
All eukaryotes have internal membrane-bound, compartments, and a true nucleus. All
eukaryotes have several, linear DNA stranded molecules as their hereditary material.
The differences between pro- and eukaryotes will be more clear to you later as we study cell
structure.
Clicker questions
An organism as a circular DNA molecule, and a cell wall with no peptidoglycan. It is a member
of
a) Eukarya
b) Bacteria
c) Archaea ** is the correct answer
d) Gram negative bacteria
e) Gram positive bacteria
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Now I want to talk about Cell Structure.
Cells are highly organized structures, that are filled with energy molecules, and are composed of
energetically expensive to make DNA , RNA , carbohydrates and lipids.
Many biologically important molecules like ions, sugars, and free amino acids are soluble in
water and will move by diffusion from an area of high concentration to an area of low
concentration.
Water is abundant in cells, and cells typically find themselves in an aqueous environment. We
don’t think of ourselves as being surrounded by water, but most of the cells within our bodies are
indeed surrounded by water.
So the cell has a challenge to meet. How does it keep its internal compartments, which contain
water with many important molecules in that water, separate from the external watery
environment?
How does the cell keep all of its energetically expensive-to-make molecules for itself?
The answer is cells have a barrier around it.
Earlier in the course we looked at phospholipids. Figure 3.20, page 56. You learned that
phospholipids when exposed to an aqueous (water) spontaneously form a bilayer.
This happens so that the water loving part of the phospholipids can be near water, and so that the
water hating part of the molecules can be away from water.
Cells use phospholipid bilayers to form a boundary around the cell.
Why do cells need boundaries?
All cells must carry out many functions in order to grow and divide. To do this, cells must keep
their internal environment different from the outside environment.
Therefore one of the essential structural features that ALL cells have is a plasma
membrane that surrounds the cell.
Cells can maintain separate compartments of hydrophilic molecules by surrounding the
compartment with a phospholipid barrier. We call these organized phospholipid bilayers
membranes.
The plasma membrane is a phospholipid bilayer which has special proteins embedded in it,
separates the cell from its surrounding and regulates traffic of molecules into and out of the cell.
Figure 5.9, page 98.
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