MODULE 10
COURSE INTENDED LEARNING OUTCOMES
At the end of the lesson, the students are expected to:
1.
Explain the evolving concept of life based on emerging pieces of evidence and discuss the
unifying themes in the study of life.
Week 10: Introduction to Life Science
CONCEPT OF LIFE
What is life?
Before we can address this question, we must first consider what qualifies something as
“living.” What is life? This is a difficult question to answer, largely because life itself is not a simple
concept. If you try to write a definition of “life,” you will find that it is not an easy task, because of the
loose manner in which the term is used. Imagine a situation in which two astronauts encounter a
large, amorphous blob on the surface of a planet. How would they determine whether it is alive?
Movement. One of the first things the astronauts might do is observe the blob to see if it moves. Most
animals move about, but movement from one place to another in itself is not diagnostic of life. Most
plants and even some animals do not move about, while numerous non-living objects, such as clouds,
do move.
Sensitivity. The astronauts might prod the blob to see if it responds. Almost all living things respond
to stimuli. Plants grow toward light, and animals retreat from fire. Not all stimuli produce responses,
however. Imagine kicking a redwood tree or singing to a hibernating bear. This criterion, although
superior to the first, is still inadequate to define life.
Death. The astronauts might attempt to kill the blob. All living things die, while inanimate objects do
not. Death is not easily distinguished from disorder, however; a car that breaks down has not died
because it was never alive. Death is simply the loss of life, so this is a circular definition at best. Unless
one can detect life, death is a meaningless concept, and hence a very inadequate criterion for defining
life.
Complexity. Finally, the astronauts might cut up the blob, to see if it is complexly organized. All living
things are complex. Even the simplest bacteria contain a bewildering array of molecules, organized
into many complex structures. However a computer is also complex, but not alive. Complexity is a
necessary criterion of life, but it is not sufficient in itself to identify living things because many
complex things are not alive.
Fundamental Properties of Life
All known organisms share certain general properties. To a large degree, these properties
define what we mean by life. The following fundamental properties are shared by all organisms on
earth.
Cellular organization. All organisms consist of one or more cells—complex, organized assemblages
of molecules enclosed within membranes.
Sensitivity. All organisms respond to stimuli—though not always to the same stimuli in the same
ways.
Growth. All living things assimilate energy and use it to grow, a process called metabolism. Plants,
algae, and some bacteria use sunlight to create carbon bonds from CO2 and H2O through
photosynthesis. This transfer of the energy in covalent bonds is essential to all life on earth.
Development. Multicellular organisms undergo systematic gene-directed changes as they grow and
mature.
Reproduction. All living things reproduce, passing on traits from one generation to the next. Although
some organisms live for a very long time, no organism lives forever, as far as we know. Because all
organisms die, ongoing life is impossible without reproduction.
Regulation. All organisms have regulatory mechanisms that coordinate internal processes.
Homeostasis. All living things maintain relatively constant internal conditions, different from their
environment.
ORIGIN OF THE FIRST LIFE FORM
Theories about the Origin of Life
The question of how life originated is not easy to answer because it is impossible to go back
in time and observe life’s beginnings; nor are there any witnesses. There is testimony in the rocks of
the earth, but it is not easily read, and often it is silent on issues crying out for answers. There are, in
principle, at least three possibilities:
1. Special creation. Life forms may have been put on earth by supernatural or divine
forces.
2. Extraterrestrial origin. Life may not have originated on earth at all; instead, life may
have infected earth from some other planet.
3. Spontaneous origin. Life may have evolved from inanimate matter, as associations
among molecules became more and more complex.
Special Creation.
The theory of special creation, that a divine God created life is at the core of most major
religions. The oldest hypothesis about life’s origins, it is also the most widely accepted. Far more
Americans, for example, believe that God created life on earth than believe in the other two
hypotheses. Many take a more extreme position, accepting the biblical account of life’s creation as
factually correct. This viewpoint forms the basis for the very unscientific “scientific creationism”.
Extraterrestrial Origin.
The theory of panspermia proposes that meteors or cosmic dust may have carried significant
amounts of complex organic molecules to earth, kicking off the evolution of life. Hundreds of
thousands of meteorites and comets are known to have slammed into the early earth, and recent
findings suggest that at least some may have carried organic materials. Nor is life on other planets
ruled out. For example, the discovery of liquid water under the surface of Jupiter’s ice-shrouded
moon Europa and suggestions of fossils in rocks from Mars lend some credence to this idea. The
hypothesis that an early source of carbonaceous material is extraterrestrial is testable, although it
has not yet been proven. Indeed, NASA is planning to land on Europa, drill through the surface, and
send a probe down to see if there is life.
Spontaneous Origin.
Most scientists tentatively accept the theory of spontaneous origin, that life evolved from
inanimate matter. In this view, the force leading to life was selection. As changes in molecules
increased their stability and caused them to persist longer, these molecules could initiate more and
more complex associations, culminating in the evolution of cells.
EVOLUTION (UNIFYING THEMES IN THE STUDY OF LIFE)
One of the most important theories in biology is evolution. Ever since its formulation in the
mid-1800s by two English naturalists, Charles Darwin and Alfred Russel Wallace, the theory of
evolution has been supported by fossil finds, geological studies, radioactive dating of rocks, genetics,
molecular biology, biochemistry, and breeding experiments.
Evolution is the unifying theory that explains the origin of diverse forms of life as a result of
changes in their genetic make-up. The theory of evolution states modern organisms descended, with
modification, from pre-existing life-forms. In the word of biologist Theodosius Dobzhansky, “Nothing
in biology makes sense, except in the light of evolution.” Why don’t snakes have legs? Why are there
dinosaur fossils but no living dinosaurs? Why are monkeys so like us, not only in appearance, but
also in the structure of their genes and proteins? The answers to those questions, and thousands
more, lie in the process of evolution. Evolution is so vital to our understanding and appreciations of
biology that we must review its important principle before going further.
BIOENERGETICS
CELL
Unicellular organisms are capable of independent existence and they can perform the
essential functions of life. Anything less than a complete cell does not ensure independent living.
Hence, cell is called the fundamental structural and functional unit of life.
DISCOVERY OF THE CELL
The invention of the microscope help scientists to study what a living organisms composed
of. Even today the study of cells reveals more detail, and its secrets, which are in fact the secrets of
life itself, are revealed with ever increasing clarity.
Robert Hooke an English scientist was the first to observed cell and in doing so he named
them cells. He examined a slice of cork in a primitive microscope and he saw tiny boxes, which he
thoughts looked like a room and led to him calling them cell.
However what Hooke actually saw was the dead cell walls of plant cells (cork) as it appeared
under the microscope.
THE CELL THEORY
The cell theory was first proposed by Matthias Schleiden (1838) and Theodore Schwann
(1839). Rudolf Virchow (1855) later added the concept of formation of cells; to this theory. The cell
theory is as follows:
a. All living things are made of cells
b. It is the smallest living unit structure and function of all organisms.
c. All cells arise from pre-existing cells.
TYPE OF CELL
Living things vary in terms of the number of cells they have. Some living things are
multicellular. Others are unicellular. Two types of cells compose living things. In the case of bacteria
and cyanobacteria have prokaryotic cells. These cell lack distinct nuclei and only have few organelles
that are not membrane-bound. In contrast, eukaryotic cells have distinct nuclei and contained several
membrane-bound organelles. Animals, plants, protists and fungi have eukaryotic cell. (See the
illustration below for the comparison of the two types of cells)
PROKARYOTES
vs.
EUKARYOTES
Comparison of a prokaryotic and eukaryotic cell
PARTS AND FUNCTIONS OF A EUKARYOTIC CELL
The structures that make up a Eukaryotic cell are determined by the specific functions carried
out by the cell. Thus, there is no typical eukaryotic cell. Nevertheless, eukaryotic cells generally have
three main components: A cell membrane, a nucleus, and a variety of other organelles.
THE CELL MEMBRANE (PLASMA MEMBRANE)
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The cell membrane is a complex barrier
separating every cell from its external
environment.
It is “Selectively Permeable"- which
means it regulates what passes into and
out of the cell.
The cell membrane functions like a gate,
controlling which molecules can enter
and leave the cell.
Carrier proteins in or on the membrane are
specific, only allowing a small group of very similar molecules through. For instance, αglucose is able to enter; but β – glucose is not. Many molecules cannot cross at all.
The cell membrane is a fluid mosaic of proteins floating in a phospholipid bilayer.
The rest of the cell membrane is mostly composed of phospholipid molecules. They have only
two fatty acid ‘tails’ as one has been replaced by a phosphate group (making the ‘head’. The
head is charged and so polar; the tails are not charged and so are non-polar. Thus the two
ends of the phospholipid molecule have different properties in water. The phosphate head is
hydrophilic and so the head will orient itself so that it is as close as possible to water
molecules.
THE NUCLEUS
 It is a membrane bound structure that contains the
cell's hereditary information and controls the cell's
growth and reproduction.
 It is the command center of a eukaryotic cell and is
commonly the most prominent organelle in a cell.
 The nucleus is surrounded by a double membrane
called the nuclear envelope, which has many nuclear
pores through which mRNA, and proteins can pass.
These pores make it look like a golf ball.
 Most nuclei contain at least one nucleolus (plural,
nucleoli). The nucleoli are where ribosomes are
synthesized. (see fig. above for the illustration).
THE CYTOPLASM
Everything within the cell membrane which is not the nucleus is
known as the cytoplasm.
Cytosol is the jelly-like mixture in which the other organelles are
suspended.
Organelles carry out specific functions within the cell. In Eukaryotic cells, most organelles are
surrounded by a membrane , but in prokaryotic cells
there are no membrane-bound organelles.
The Different Organelles and Their Functions
ORGANELLES
1. Cell wall
2.
3.
4.
5.
mitochondrion
vacuole
Golgi Apparatus
lysosomes
6. centrioles
7. endoplasmic reticulum
8. chloroplastids
9. nuclear membrane
10. Nucleoplasm
11. Ribosomes
12. Cytoskeleton
13. Microbodies
FUNCTION
Provides mechanical support and maintains cell shape in
plant cell. It prevents water loss in plants and protect from
over expansion by too much water.(Animals have no cell
wall)
Provides energy for the cell in the form of ATP
Stores water, food and waste for the cells
Sorts, packages and secretes cellular products
The “suicide bag”. They digest excess or worn out
organelles, food particles, and engulfed viruses or bacteria.
Formation of the spindle fiber during cell division
Translocation of materials within the cell and in and out of
the nucleus
Gives green color of plants
Separates the nuclear contents from the contents of
cytoplasm
Synthesis of RNA and production of ribosomes
They use the RNA synthesized by the nucleolus in making
specific amino acid.
The cytoskeleton is responsible for cell shape, motility of
the cell as a whole, and motility of organelles within a cell
They contain enzymes that are essential in neutralizing
toxic materials that are product of cellular metabolism
PHOTOSYNTHESIS
Life on Earth is solar powered. The chloroplasts in plants and other photosynthetic organisms
capture light energy from the sun and convert it to chemical energy that is stored in sugar and other
organic molecules. This conversion process is called photosynthesis.
Photosynthesis nourishes almost the entire living world directly or indirectly. An organism
acquires the organic compounds it uses for energy and carbon skeletons by one of two major modes:
autotrophic nutrition or heterotrophic nutrition.