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The Nature of Life
Theoretical physicist Stephen Hawking:
“The laws of science do not distinguish between the past and the future.
In order to survive, human beings have to consume food which is an
ordered form of energy, and convert it into heat, which is a disordered
form of energy… The progress of the human race in understanding the
universe has established a small corner of order in an increasingly
disordered universe.”
 This principle of physics called entropy, or randomness, appears to be the driving force of all
life in our universe.
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Chapter 4 Pages 4-2 & 4-3
The Nature of Life
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Defining life appears simple if you compare a fish and a rock. From a scientific point of view,
it’s not quite so cut-and-dried. Often life and nonlife share the same elements: matter,
carbon atoms and energy reactions.
Energy reactions found in living systems also exist outside of life.
For example: fire results when a reaction releases chemical
energy within substances. Living systems use energy similarly
– by releasing chemical energy for life processes.
All life uses energy. Therefore it is possible to define “life” based
on the characteristics living systems have apart from nonliving
systems with respect to energy use.
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Chapter 4 Page 4-3
The Nature of Life
Elements Essential for Life
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Matter and Energy
 Life requires matter and energy to exist. All living organisms are composed of
about 13 of 118 known elements from the periodic table.
 Carbon, hydrogen, oxygen, and nitrogen account for 99% of the mass. Nine other
elements account for the remaining 1%. These elements, in combinations, account for
all biological chemicals.
Chapter 4 Pages 4-3 & 4-4
The Nature of Life
 Scientists recognize more than 1.6 million different species, as many
as 30 million may exist.
 Despite this huge number, all organisms
organize matter into biological chemicals
and into cells.
 A cell is the smallest whole structure
that can be defined as a living system.
 Organisms can consist of a single cell
or billions of codependent cells.
 All life organizes matter into cells.
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Matter and Energy (continued)
 Energy is defined as the capacity to do work.
 Energy is necessary for life because living systems use it to accomplish the
processes of life: reproduction, growth, movement, eating, etc.
 Organisms need energy to help break down complex molecules into simple molecules.
They need more energy to build distinct complex molecules from simple molecules.
 Organisms cannot create
energy – but can use it to
perform useful work. Living
systems must acquire
energy from outside sources.
Chapter 4 Pages 4-4 & 4-5
The Nature of Life
 The first law of thermodynamics states that energy can be transferred from one
system to another in many forms. However, it cannot be created nor destroyed.
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Entropy
Chapter 4 Pages 4-4 & 4-6
The Nature of Life
 The second law of thermodynamics states that disorder increases with time and
eventually all energy and matter will be distributed evenly.
 Entropy is the measure of how much unavailable energy exists in a system due to even
distribution. High entropy = low organization and low energy potential.
 Living systems use energy to create order and to gather and store potential
energy. The increased order is local and temporary, and requires more energy to
create than it retains. Here, matter
exists in a low-entropy
(organized) state.
 Example: About 85% of the
energy required to organize
protein into complex muscle
tissue is ultimately lost as heat
in creating the tissue.
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 Terrestrial and most marine organisms get their energy directly or indirectly
from the sun.
 Autotrophy is the process of self-feeding by creating energy-rich compounds
called carbohydrates.
 Autotrophs obtain energy from the sun or chemical processes.
 They do this by converting the energy from sunlight and inorganic compounds into
carbohydrates. Plants are autotrophs.
 Many organisms, including virtually all animals, cannot produce their own
carbohydrates. These organisms get their energy and matter by consuming
other organisms. This is called heterotrophy.
Chapter 4 Page 4-7
How Matter and Energy Enter Living Systems
Autotrophy and Heterotrophy
 Heterotrophs are organisms that rely on other organisms for sources of energy.
 We are heterotrophs. Humans rely on photosynthesizing plants, bacteria, and other
micro-organisms for life.
 This is one reason why the health of the natural environment is a crucial issue.
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Respiration
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Whether an organism is an autotroph or a heterotroph, it must convert carbohydrates into
usable energy.
Organisms use oxygen to engage in cellular respiration.
Respiration is the process of releasing energy from carbohydrates to perform the functions of life.
(This is not the same as breathing.)

The chemical process
for respiration is:
Chapter 4 Pages 4-7 & 4-8
How Matter and Energy Enter Living Systems
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 Because they create energy-rich compounds, autotrophs are also known as
primary producers.
 Primary producers combine energy from sunlight with inorganic materials to
form energy-right organic compounds.
 Conduit through which the biosphere gets almost all its energy.
 Organisms with chlorophyll are the majority of primary producers.
 Chlorophyll allows for the collection of sunlight.
 The process of using light energy to create carbohydrates from inorganic
compounds is called photosynthesis.
 Because carbon dioxide and water have more oxygen than is needed, the process also
releases oxygen.
 Without photosynthesis we would not have the oxygen we need to breathe.
Chapter 4 Page 4-9
How Matter and Energy Enter Living Systems
Photosynthesis
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 Note that even organisms with chlorophyll respire. If you
look at photosynthesis you can see it is a complementary
process to respiration.
 The chemical process for photosynthesis is:
 Aerobic respiration meaning respiration that uses oxygen.
 Anaerobic respiration releases energy through chemical
reactions that do not require oxygen.
Chapter 4 Pages 4-9 & 4-10
How Matter and Energy Enter Living Systems
Photosynthesis (continued)
 Anaerobic respiration is not as efficient as aerobic respiration.
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Chemosynthesis
 It is similar to photosynthesis because it
produces carbohydrates.
 Chemosynthesis differs from photosynthesis; it
does not use sunlight as an energy source, it uses
chemical energy within inorganic compounds.
 Chemosynthetic organisms are primary producers.
 Fixation is the process of converting, or fixing,
an inorganic compound into an organic compound.
Chapter 4 Pages 4-11 & 4-12
How Matter and Energy Enter Living Systems
 Chemosynthesis is the process of using chemicals to create energy-rich
organic compounds.
 In 1977, there was an important discovery of a major
biological community in the deep ocean relying on chemosynthesis.
 These communities use chemical energy from minerals in the hot spring water coming
from the hydrothermal vents.
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Marine Biomass
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The main “products” of primary production
are carbohydrates.
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Biomass is the mass of living tissue. The
biomass at a given time is called the
standing crop.
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Chapter 4 Pages 4-13 to 4-15
The Ocean’s Primary Productivity
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Scientists measure primary productivity in
terms of the carbon fixed (bound) into
organic materials.
Example: The average standing crop in the
oceans is 1-2 billion metric tons. On land, the
average standing crop is 600 to 1,000 billion
metric tons.
Comparing primary productivity of the seas
to that of the land, the land’s primary
production is slightly higher.
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How is it possible that the total primary
production from marine ecosystems is only a
bit less than that of terrestrial ecosystems? –
marine ecosystems cycle their energy and
nutrients much more rapidly.
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Plankton
Chapter 4 Pages 4-15 & 4-16
The Ocean’s Primary Productivity
 The term “plankton” does not describe a kind of organism, but a group of
organisms with a common lifestyle and habitat. Plankton include autotrophs,
heterotrophs, predators and grazers.
 Plankton drift/swim weakly at the mercy of water motion.
 Plankton are not a species, but include many species.
 Most are very small, some, like the jellyfish, grow several
meters long.
 Some start life as planktonic larvae and then become
nektonic organisms that swim or attach themselves
to the bottom as benthic organisms.
 Meroplankton live part of their lives as plankton.
 Holoplankton remain plankton all their life.
 Phytoplankton are primary producers responsible
for more than 92% of marine production.
 Zooplankton are primary and secondary consumers
of other plankton.
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Plankton (continued)
 Four most important kinds of phytoplankton:
 1. Diatoms are the most dominant and efficient
photosynthesizers known.
 2. Dinoflagellates are characterized by one or two whip-like
flagella which they use to move in water.
 Most are autotrophs. They are the most significant primary producers
in coral reefs. They are also the principal organisms
responsible for plankton blooms.
Chapter 4 Pages 4-17 & 4-18
The Ocean’s Primary Productivity
 They convert more than 50% of the light energy they
absorb into carbohydrate chemical energy. They have a rigid cell
wall made of silica called a frustule which admits light. This is an
ideal cell material for a photosynthesizer.
 3. Coccolithophores are single-cell autotrophs characterized
by shells of calcium carbonate.
 They live in bright shallow water.
 4. Silicoflagellates are micro-organisms with internal support
structures made of silica and have one or more flagella.
 They are structurally and chemically more primitive than diatoms.
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Plankton (continued)
Chapter 4 Pages 4-18
The Ocean’s Primary Productivity
 Understanding the role of picoplankton has
changed the way marine biologists think
about tropical region productivity.
 Picoplankton are extremely tiny plankton.
 May account for up to 79% of the photosynthesis
in tropical waters.
 Many are cyanophytes, which are bacteria
with chlorophyll.
 Can also be called cyanobacteria or
blue-green algae.
 Their role in primary productivity is to be food
for heterotrophic bacteria.
 They may also play a significant role in producing
oxygen and taking up carbon dioxide.
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Limits on Marine Primary Productivity
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Limiting factors are physiological or biological necessities that restrict survival. Too much
or too little of a limiting factor will reduce population.
Limiting factors in the ocean include:
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Tropical waters have low productivity.
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Chapter 4 Pages 4-19 to 4-21
The Ocean’s Primary Productivity
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Warm upper water act to trap nutrients in
the cold layers that are too deep for
photosynthesizing autotrophs.
The Arctic and Antarctic have little
temperature difference allowing nutrients to cycle to shallower water.
Temperate regions, coastal areas, have more primary productivity due to more nutrients
from rain runoff.
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Inorganic nutrients such as nitrogen
and phosphorus compounds.
Sunlight due to season, depth, or
water clarity.
Shallow water keeps them from sinking too deep.
Areas of highest productivity are in the Antarctic Convergence Zone and near shore
temperate regions due to nutrient availability.
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Limits on Marine Primary Productivity (continued)
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Light is an important limiting factor.
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Depth is a limiting factor too.
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Chapter 4 Page 4-21
The Ocean’s Primary Productivity
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The amount of daylight affects photosynthesis and primary productivity.
For example, the Antarctic Convergence Zone
has optimum nutrients available, seasonal
sunlight limits its productivity.
Depth affects photosynthesis and primary
productivity. Suspended particles and the
light’s angle limit how much light penetrates
water. Even in very clear water, very little
photosynthesis takes place below 100 meters
(328 feet).
Too much light can be bad too. Photoinhibition takes place when too much light
overwhelms an autotroph. It cannot photosynthesize when water is too shallow.
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Limits on Marine Primary Productivity (continued)
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Different phytoplankton species have different
optimal depths.
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Autotrophs produce carbohydrates and oxygen,
but they also respire.
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Chapter 4 Page 4-22
The Ocean’s Primary Productivity
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As light conditions change, the advantage shifts
from species to species.
They use carbohydrates and some oxygen for
respiration. The less light, the less photosynthesis
and the less carbohydrates are produced.
At some point, the amount of carbohydrates
produced exactly equals the amount required
by the autotrophs for respiration.
The point of zero net primary production is called
the compensation depth.
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This is the depth at which about 1% of the surface
light penetrates.
If phytoplankton remain below compensation
depth, they will die within a few days.
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Trophic Relationships
 The hierarchy of what-eats-what can be illustrated with a trophic pyramid.
 It is a representation of how energy transfers as they consume each other.
 Primary consumers, the first level of heterotrophs, eat the primary
producers. Most of these are herbivores (animals that eat plants).
In the ocean, zooplankton are primary consumers.
 Secondary consumers, eat primary consumers.
Chapter 4 Pages 4-24 to 4-26
Energy Flow Through the Biosphere
 Primary producers, mainly photosynthesizers, make
up the base. Most of these are plants. In the ocean,
phytoplankton are primary producers.
 Each level eats the level below and has significantly
less biomass (living matter) than the level it eats.
Energy Loss Through Trophic Levels
 Only about 10% of the energy transfers from one level to the next, so each level
is about a tenth of the size of the level underneath. 90% of the energy is lost
to entropy.
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Food Webs and Decomposition
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A food web is a way to illustrate different levels of consumers and energy flow. In real life,
organisms consume across levels, not just below. The food web better represents the flow
of energy through consumption in nature.
Decomposers break down organic material into
inorganic form. They take out the very last usable
energy from organic matter to sustain themselves.
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Chapter 4 Pages 4-27 & 4-28
Energy Flow Through the Biosphere
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Decomposers are primarily
bacteria and fungi, their job is
to convert dead organisms
into the compounds primary
producers use.
Bacteria are the most important
decomposers.
Decomposition is important
because it completes the
materials cycle.
Within systems, energy flows and
matter cycles.
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