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Chapter 3

Key Processes of Exchange

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Essential nutrients

Synthesis, growth, and maintenance


Specific requirements vary



3.1 Life on Earth is based on carbon

Made up of complex carbon molecules

Plant and animal material

Ultimate source is atmospheric carbon dioxide

Photosynthesis - green plants

Assimilation - transform carbon and other essential nutrients

Respiration - chemical energy generated by breakdown of carbon compounds in living cells

Primary producers - energy form sunlight

Secondary producers - energy from consuming plant and animal tissue


3.2 Photosynthesis converts carbon dioxide and hydrogen into carbohydrates

CO 2 + H 2 O + Sunlight --- > C 6 H 12 O 6 + O 2


C 3 Photosynthesis

Chlorophyll acts as an antenna - absorbs energy from light

split water molecule

O 2 released

plants use H 2 to transform CO 2 into simple sugars

sugars make more complex carbon compounds

carbohydrates and proteins

build leaves, stems, roots, flowers, seeds



Chemical compounds that speed up or catalyze chemical reactions.


ribulose biphosphate carboxylase - oxygenase

most abundant enzyme on Earth

N 2 rich farm fertilizers encourage production of ( chlorophyll + rubisco )


3.3 Photosynthesis involves key changes

Terrestrial plants

Stomata duel role

1. CO 2 by diffusion

2. Transportation

diffusion = equilibrium

ppm = parts per million

high concentration to low concentration

transpiration = loss of water

humidity = low to high concentration

drier air lower humidity


To carry out photosynthesis, the plant must open its stomata; but when it does, it will lose water, which it must replace. If water is scarce, the plant must balance the opening and closing of the stomata, taking up enough CO 2 while minimizing the loss of water.

This balancing act affects the characteristics of plants and of ecosystems.


Submerged plants lack stomata

Dissolved CO 2 and bicarbonates HCO 3--

diffuse directly across the outer cell walls of the plants

water loss does not exist

salinity ?

3.4 Plants of warm, dry environments use a different photosynthetic path:

C 4 see figure 3.2 on page 23

plant anatomy differs from C 3 plants

C 3 Mesophyll cells

C 4 Vascular bundles

sheath cells


two types of cells

mesophyll cells

bundle sheath cells


First stage of C 4

Mesophyll cells


do have PEP ( phosphoenopyruvate carboxylase )

fixes CO 2 into 4 carbon acids

malate and asparate

goes to bundle sheaths ( fig. 3.3 )

these enzymes breakdown acids to form CO 2

reverse process in mesophyll cells

bundle sheaths CO 2 = sugars by RUBISCO


Advantages of C 4

Effective use of CO 2

Concentration 6 times greater than of outside air

Max rate of photosynthesis is generally greater in C 4 plants


Second stage of C 4

Uptake of CO 2 and loss of water through stomata

Water - use efficiency

C 4 plants exhibit greater water use efficiency

fix more carbon

Advantage in hot, dry climates

water limited

Higher price: C 4 pathway - higher energy expenditure = need to produce extra enzymes, PEP.


3.5 Desert plants modify photosynthesis to save water

Heat high, water scarce

CAM = Crassulacean Acid Metabolism

similar to C 4 pathway

mesophyll and bundle sheath occur at same time

open stomata at night

take in CO 2

convert to malate using PEP

conserve water

day time

temperature low, humidity high

closes stomata

reconverts malate into CO 2 - sugar

allows continuation of photosynthesis


3.6 Respiration generates energy from carbon compounds

Chemical energy in form of ATP

( adenosine triphosphate )

generally by cellular respiration


breakdown of

simple sugars and carbohydrates yielding CO 2, water, ATP

aeorbic = requires O 2 = oxygenation

stored as ATP

released as heat

The cell uses energy released by respiration to make new ATP to drive all metabolic processes in plants and animals


3.7 Decomposition returns CO 2 to the atmosphere

Focus on Ecology 3.1 " Studying Decomposition " page 25.

Define decomposition: breakdown of complex organic substances into simpler ones

releases heat and converts organic compounds into CO 2 and inorganic nutrients


Complex processes

Rainwater leaches out soluble compounds from dead organic matter

Detritivores fragment it

Bacteria and fungi break it down

Animals eat bacteria and fungi

Organisms excrete organic and inorganic compounds


All consumers function as a decomposer

Digest food

metabolize food - change it

release it, - feces


True decomposers accomplish most of decomposition






dead leaves and animals

oxidize organic compounds

release energy

release CO 2

degrade into simpler products


Bacteria and fungi

Converts organic material into inorganic material

dead organic matter now available to primary producers and microbes



Feed on bacteria and fungi

When decompose

nutrients return to soil

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