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3.7.1 Define cell respiration.
Cell respiration is the controlled release of energy from organic compounds in cells to
form ATP.
All living things need energy to stay alive. The energy is used to power all the
activities for life including digestion, protein synthesis, and active transport.
A cell’s energy sources are sugars and other substances taken from nutrients,
which can be broken down to release the energy that is stored in the bonds.
Aerobic cell respiration:
glucose + oxygen  carbon dioxide + water + energy
Cellular respiration is the gradual breakdown of nutrient
molecules such as glucose and fatty acids in a series of reactions
that release energy in the form of ATP.
The first stage in cellular respiration is glycolysis. Glucose that is present in the cytoplasm
of a cell is broken down by a series of enzymes, to produce 2 molecules of a simpler
compound- pyruvate
As this occurs there is a net production of 2 ATP molecules
2 pyruvate + ATP
3.7.2: State that, in cell respiration, glucose in the cytoplasm is broken down by
glycolysis into pyruvate, with a small yield of ATP.
IB Question: In the cytoplasm of the cell, glucose is broken down into pyruvate in a process
called glycolysis. State one product of glycolysis. [1]
ATP / NADH + H+ / 2 NADH / reduced NAD [1]
Aerobic Respiration
The next stage of cell respiration depends on whether there is
oxygen present or not.
Aerobic respiration- with oxygen
Anaerobic respiration- without oxygen
Aerobic respiration is the most effective way of producing ATP.
Aerobic respiration is carried out by cells in the mitochondria and it
produces a large amount of ATP.
Pyruvate molecules produced by glycolysis enter the mitochondria
and are broken down, or oxidized, in a series of reactions that release
CO2 and H2O and produce ATP
Krebs Cycle
The two pyruvate molecules from glycolysis first lose carbon dioxide and become 2
molecules of acetyl CoA
Acetyl CoA then enters the Krebs Cycle and is modified further releasing carbon dioxide.
Products of the cycle react with oxygen and result in the release of large amounts of
Glucose+ O2
CO2 + H2O + 38 ATP net result of 36ATP
3.7.3 Explain that, during anaerobic cell respiration, pyruvate can be converted in the cytoplasm into
lactate, or ethanol and carbon dioxide, with no further yield of ATP
Anaerobic Respiration
Anaerobic respiration occurs in the cytoplasm of cells.
In animal cells the pyruvate produced by glycolysis is converted to lactate, which is
a waste product of cells.
When exercising rigorously the cardiovascular system is unable to supply the
muscles with enough oxygen, resulting in cramps and muscle soreness.
In other organisms, such as yeast, anaerobic respiration is known as
fermentation and produces a different outcome.
The pyruvate molecules are converted to enthanol (alcohol) and carbon
No further ATP is produced in anaerobic respiration.
IB Question: List two end products of aerobic cell respiration. [2]
aerobic respiration;
Carbon dioxide , water, ATP
IB QUESTION: Distinguish between the process of anaerobic respiration in yeast and humans.
yeast: pyruvate to ethanol and carbon dioxide;
humans: pyruvate to lactic acid; [2]
Award [1 max] if products are appropriately linked to organisms without the mention of
3.7.4 Explain that, during aerobic cell respiration, pyruvate can be broken
down in the mitochondrion into carbon dioxide and water with a large yield of ATP.
IB Question: Using a table, compare aerobic and anaerobic respiration in a eukaryotic cell. [5]
Award [1] for each correct row, up to [5 max].
Aerobic respiration Anaerobic respiration
occurs in mitochondria occurs in cytoplasm;
requires 2 O occurs without 2 O ;
both produce pyruvate from glucose (glycolysis);
uses fatty acids/lipids/amino acids doesn’t use fatty acids;
(Krebs cycle) produces CO2 and H2 O (fermentation) produces ethanol / CO2
(in yeast) ;
(Krebs cycle) produces CO2 and H 2O (fermentation) produces lactate in
animals (humans) ;
NADH produced in both;
large amount of ATP (36 per glucose
molecule) produced
small amount of ATP (2 per glucose
molecule) produced; [5 max]
IB Question: Explain the process of aerobic cell respiration. [8]
cell respiration produces energy;
controlled release of energy;
by breakdown of organic molecules/glucose;
energy from them is used to make ATP;
aerobic respiration is in mitochondria;
requires oxygen;
pyruvate is produced by glycolysis / glucose broken down;
pyruvate is broken down in the mitochondria;
into carbon dioxide and water;
large production of ATP;
per molecule/mass of glucose;
much higher production of ATP than in anaerobic respiration; [8 max]
IB Question: Compare anaerobic cellular respiration and aerobic cellular respiration. [5]
Direct comparisons must be made to achieve a mark.
anaerobic in the absence of oxygen whereas aerobic in the presence of oxygen;
both may produce 2 CO ;
both produce ATP;
aerobic releases considerably more ATP per glucose molecule than anaerobic;
anaerobic/fermentation in plants produces alcohol / anaerobic in animals produces
lactic acid neither produced in aerobic respiration;
glucose can be the substrate for both;
glucose can be the substrate for both;
anaerobic entirely in cytoplasm whereas aerobic requires mitochondria/specialized
region of membrane;
glucose is broken down into pyruvate in the cytoplasm in both; [5 max]
3.8: Photosynthesis
carbon dioxide + water  glucose + oxygen
The sun is the source of energy for almost all life on Earth
3.8.1 State that photosynthesis involves the conversion of light energy into chemical energy.
Light energy from the Sun is captured by plants and other photosynthetic
organisms, and converted into stored chemical energy.
The energy is stored in molecules such as glucose, which provide a source of food
for organisms that cannot use light energy directly
Visible light is composed of a spectrum of colors, which can be separated using
a prism.
A prism bends rays of light and separates the colors because each one has a
slightly different wavelength, and is bent at a different angle.
Most important regions of visible light are red and blue for photosynthesis
3.8.2 State that light from the Sun is composed of a range of wavelengths (colours).
The color of any object is determined by the wavelength of the light it reflects
back into your eyes.
A blue shirt appears blue because it reflects blue light, which our eyes can
perceive, but it absorbs other wavelengths that fall on it and we don’t see
those colors.
A black shirt will absorb all wavelengths of light, while a white one will reflect
them all.
Most plants have green leaves. They do not absorb the green part of
the spectrum, green light is reflected making the leaf appear green.
The green color is due to the chloroplasts, which contain a green pigment called
Chlorophyll is unable to absorb green light, which it reflects, but it absorbs other
wavelengths well. Red and blue light are absorbed particularly well and provide the
energy needed for photosynthesis.
3.8.4: Outline the differences in absorption of red, blue and green light by
3.8.3 State that chlorophyll is the main photosynthetic pigment.
The Chemistry of Photosynthesis
• Photosynthesis is a complex series of reactions
catalyzed by a number of different enzymes.
• Photosynthesis can be split into 2 stages: Lightdependent reactions and Light- independent
Light-dependent reactions
• The first stage is called light-dependent reactions because
light is essential for them to occur.
• Chlorophyll absorbs light energy and this energy is used to
produce ATP.
• Energy is used to split water molecules into hydrogen and
oxygen is a process called photolysis.
Hydrogen ions,
electrons, and oxygen
are released. Oxygen
is released as a waste
product (good for
The ATP, hydrogen
ions and electrons are
used in the light
Light-independent reactions
During the light independent reactions carbon dioxide (CO2), taken in
from the air, is combined with hydrogen and ATP to form a range of
organic molecules for the plant.
The conversion of inorganic carbon dioxide to organic molecules such
as glucose is called carbon fixation.
ATP provides the energy for the process.
Summary of Photosynthesis
Measuring the Rate of Photosynthesis
The equation shows that when photosynthesis occurs, carbon dioxide is
used and oxygen is released.
The mass of the plant (its biomass) will also increase as glucose is used to
produce other plant materials.
3.8.7 Explain that the rate of photosynthesis can be measured directly by the production of oxygen
or the uptake of carbon dioxide, or indirectly by an increase in biomass.
Aquatic plants release bubbles of oxygen as they photosynthesize and if the
volume of these bubbles is measured for a period of time, the rate of
photosynthesis can be determined directly.
Aquatic plants also remove carbon dioxide from the environment,
causing the pH of the water to rise. Carbon dioxide dissolves in water
to a form a weak acid so as it is removed, the pH will go up.
Another way to determine the rate of photosynthesis is to monitor
the change in pH of the surrounding water over time.
Indirect Method
A thirds method of measuring the rate of photosynthesis in plants is to
determine their biomass at different times.
Samples of the plants can be collected and measured at different times
and the rate of increase in their biomass calculated to determine the
rate of photosynthesis.
Limits of Photosynthesis
The rate of photosynthesis depends on factors in the environment.
On a warm, sunny
afternoon, photosynthesis
will be more rapid than on
a cold, cloudy morning.
More oxygen will be
produced and more carbon
dioxide used.
Limiting Factors
Photosynthesis cannot increase beyond certain limits
Light, temperature, and carbon dioxide are limiting factors involved in
Effect of light intensity on rate of photosynthesis
An increase in light intensity, when all other variables are the same, will
produce an increased rate of photosynthesis. However, at a certain light
intensity enzymes will be working at maximum rate, limited by
temperature and availability of CO2
3.8.8: Outline the effects of temperature, light intensity and carbon dioxide
concentration on the rate of photosynthesis.
Effect of Temperature on rate of photosynthesis
Increasing temperature also increases the rate of photosynthesis as the
frequency and energy of molecular collisions increases. Photosynthesis
has an optimum temperature above it the rate decreases sharply as
enzymes are denatured, or the plant wilts and is unable to take in CO2
Effect of carbon dioxide concentration on rate of photosynthesis
An increase in the concentration of CO2 causes the rate of photosynthesis
to increase, as CO2 is a vital raw material for the process. AT very high
concentrations, the rate will plateau as other factors such as light and
temperature limit the rate of the reaction.
The effects of temperature, light, and CO2 concentrations are well
known to farmers who grow crops in greenhouses.
Commercial producers of cucumbers and tomatoes keep their
greenhouses warm and well lit. They may also introduce CO2 to
boost the photosynthesis to maximum rate, thereby increasing crop
production and profits.
IB Question: i. State the principal conversion of energy that occurs in photosynthesis.
ii. State the molecule necessary for this conversion of energy.
(i) light (energy) to chemical (energy)/ATP/glucose/NADPH/sugar; [1]
(ii) chlorophyll [1]
IB Question: Outline the difference in absorption of red, blue and green light by chlorophyll for
the process of photosynthesis.
red and blue light is absorbed and green light is reflected / blue light is absorbed
the most and green light is absorbed the least [1]
IB QUESTION: Outline how light energy is used and how organic molecules are made in
chlorophyll is the (main) photosynthetic pigment;
absorbs (mainly) red and blue light;
green light is reflected;
light energy absorbed is converted into chemical energy;
ATP produced;
water split;
to form oxygen and hydrogen;
ATP and hydrogen used to fix carbon dioxide to make organic molecules; [6 max]
IB Question: Explain how the rate of photosynthesis can be measured. [5]
rate can be measured by the disappearance of raw materials / 2 CO (in solution);
rate of change of 2 CO can be measured (indirectly) by pH change;
rate can be measured by the appearance of products/ 2 O /starch;
rate can be measured by measuring rate of change of biomass;
description of apparatus to measure the rate of photosynthesis / annotated diagram;
explanation of expected experimental outcome: e.g. increased photosynthesis in
an aquatic plant – more 2 O bubbles counted per unit time; [5 max]
IB Question: Explain how photosynthesis can be measured both directly and indirectly. [3]
(a) directly:
by production of oxygen / measuring the
volume/number of bubbles of gas produced;
by measuring the uptake of carbon dioxide;
changes in pH;
Do not accept “production of gas”.
by measuring the increase in biomass;
by measuring the production of glucose / starch / other suitable molecule; [3 max]
Award [2 max] if only indirect measurements or direct measurements are addressed.
Apply ECF if direct and indirect is confused for more than one method.
IB Question: Outline how three different environmental conditions can affect the rate of
photosynthesis in plants. [6]
light: [2 max]
rate increases with increasing light;
it reaches maximum then plateaus;
as all chloroplast molecules are working at optimal pace;
temperature: [2 max]
rate increases with increasing temperature;
to a maximum/optimum temperature;
but then falls off rapidly;
as enzymes are denatured above the optimal temperature;
carbon dioxide: [2 max]
rate increases with increasing carbon dioxide level;
it reaches maximum then plateaus;
as photosynthesis operating at optimal level; [6 max]
Award any of the above points if clearly drawn in a diagram.
IB Question: Outline the effects of temperature, light intensity and carbon dioxide concentration
on the rate of photosynthesis. [6]
Award credit for the following points if annotated on sketched graphs.
all three variables can be classified as limiting factors;
as temperature increases, photosynthetic rate increases;
maximum rate of photosynthesis at optimum temperature;
at (very) high temperatures photosynthesis slows/stops;
as light intensity increases, photosynthetic rate increases;
at high light intensity, photosynthesis stops increasing;
minimum light intensity necessary for photosynthesis to occur;
(because concentration is rarely high) 2 CO is the main rate-limiting variable;
as concentration of 2 CO rises, photosynthetic rate rises (up to a certain point); [6 max]