General Biology 1
Mitochondria and Chloroplasts
Adenosine Triphosphate (ATP)
It is the major energy currency of the
cell that provides the energy for
most of the energy-consuming activities
of the cell. The ATP regulates many
biochemical pathways.
Mechanism:
When the third phosphate group of ATP
is removed by hydrolysis, a substantial
amount of
free energy is released.
ATP + H2O → ADP + Pi where ADP is
adenosine diphosphate and Pi is inorganic
phosphate.
Energy release in Hydrolysis (Source: (n.d.). Retrieved from
http://scienceaid.co.uk/biology/biochemistry/atp.html)
Illustration
Chemical Energy and ATP (Source: (n.d.). Retrieved from
http://winklebiology.weebly.com/chemical- energyatp.html)
Synthesis of ATP
• ADP + Pi → ATP + H2O
• requires energy: 7.3 kcal/mole
• occurs in the cytosol by glycolysis
• occurs in mitochondria by cellular
respiration
• occurs in chloroplasts by photosynthesis
Synthesis - the production of a substance by combining
simpler substances through a chemical process.
Consumption of ATP
• active transport of molecules
and ions
ATP powers most energy- • conduction of nerve impulses
consuming activities of cells, such • maintenance of cell volume by
as: osmosis
• anabolic (synthesis) reactions, • addition of phosphate groups
such as: (phosphorylation) to different
• joining transfer RNAs to amino proteins (e.g., to alter their
acids for assembly into proteins activity in cell
• synthesis of nucleoside signaling)
triphosphates for assembly into • muscle contraction
DNA and RNA • beating of cilia and flagella
• synthesis of polysaccharides (including sperm)
• synthesis of fats • bioluminescence
Extracellular ATP
In mammals, ATP also functions outside of cells.
ATP is released in the following examples:
• from damaged cells to elicit inflammation and pain
• from the carotid body to signal a shortage of oxygen
in the blood
• from taste receptor cells to trigger action potentials
in the sensory nerves leading back to the brain
• from the stretched wall of the urinary bladder to
signal when the bladder needs emptying
In eukaryotic cells, the mitochondria and chloroplasts are the organelles that
convert energy to other forms which cells can use for their functions.
Mitochondria
(singular, mitochondrion)
mitochondria are the sites of cellular
respiration, the metabolic process
that uses oxygen to drive the
generation of ATP by extracting
energy from sugars, fats, and other
fuels.
Mitochondia
Retrieved from https://en.m.wikipedia.org/wiki/intermembrane
_space
The mitochondria are oval-shaped organelles
found in most eukaryotic cells. They are
considered to be the ‘powerhouses’ of the cell.
As the site of cellular respiration, mitochondria
serve to transform molecules such as glucose
into an energy molecule known as adenosine
triphosphate (ATP).
ATP fuels cellular processes by breaking its
high-energy chemical bonds. Mitochondria are
most plentiful in cells that require significant
amounts of energy to function, such as liver and
muscle cells.
The mitochondria has two membranes that are
similar in composition to the cell membrane:
Outer membrane
is a selectively permeable
membrane that surrounds the
mitochondria. It is the site of
attachment for the respiratory
assembly of the electron transport
chain and ATP Synthase. It has
integral proteins and pores for
transporting molecules just like the
cell membrane
Inner membrane
folds inward (called cristae) to
increase surfaces for cellular
metabolism. It
contains ribosomes and the DNA of
the mitochondria. The inner
membrane creates two enclosed
spaces within the mitochondria:
• intermembrane space between the
outer membrane and the inner
membrane; and
• matrix that is enclosed within the
inner membrane.
During electron
transport, the participating
protein complexes push
protons from the matrix out
to the intermembrane
space. This creates a
concentration gradient of
protons that another
protein complex, called ATP
synthase, uses to power
synthesis of the energy
carrier molecule ATP.
Chloroplasts
Chloroplasts, which are found in plants
and algae, are the sites of
photosynthesis. This process converts
solar energy to chemical energy by
absorbing sunlight and using it to drive
the synthesis of organic compounds such
as sugars from carbon dioxide and water.
The word chloroplast is derived from the
Greek word chloros which means ‘green’ and
plastes which means ‘the one who forms’. The
chloroplasts are cellular organelles of green
plants and some eukaryotic organisms. These
organelles conduct photosynthesis. They
absorb sunlight and convert it into sugar
molecules. They also produce free energy
stored in the form of ATP and NADPH through
photosynthesis.
CHLOROPLAST
Retrieved from https://en.m.wikipedia.org/wiki/Chloroplast
Structure of the Chloroplast
• Outer membrane—This is a semi-porous
membrane and is permeable to small
molecules and ions which diffuse easily. The
outer membrane is not permeable to larger
proteins.
• Intermembrane Space—This is usually a thin
intermembrane space about 10-20
nanometers and is present between the outer
and the inner membrane of the chloroplast.
• Inner membrane—The inner membrane of the
chloroplast forms a border to the stroma. It
regulates passage of materials in and out of the
chloroplast. In addition to the regulation activity,
fatty acids, lipids and carotenoids are synthesized
in the inner chloroplast membrane.
• Stroma—This is an alkaline, aqueous fluid that
is protein-rich and is present within the inner
membrane of the chloroplast. It is the space
outside the thylakoid space. The chloroplast DNA,
chloroplast ribosomes, thylakoid system, starch
granules, and other proteins are found floating
around the stroma.
Thylakoid System
The thylakoid system is suspended in the
stroma. It is a collection of membranous sacks
called thylakoids. Thylakoids are small sacks
that are interconnected. The membranes of
these thylakoids are the sites for the light
reactions of the photosynthesis to take place.
The chlorophyll is found in the thylakoids. The
thylakoids are arranged in stacks known as
grana. Each granum contains around 10-20
thylakoids.
Thylakoid System
The word thylakoid is derived from the Greek
word thylakos which means 'sack'.
Important protein complexes which carry out the
light reaction of photosynthesis are embedded in the
membranes of the thylakoids.
The Photosystem I and the Photosystem II are
complexes that harvest light with chlorophyll and
carotenoids. They absorb the light energy and use it
to energize the electrons.
The molecules present in the thylakoid
membrane use the electrons that are energized to
pump hydrogen ions into the thylakoid space. This
decreases the pH and causes it to become acidic in
nature. A large protein complex known as the ATP
synthase controls the concentration gradient of the
hydrogen ions in the thylakoid space to generate
ATP energy. The hydrogen ions flow back into the
stroma.
Thylakoids are of two types: granal thylakoids
and stromal thylakoids. Granal thylakoids are
arranged in the grana. These circular discs that are
about 300-600 nanometers in diameter. The stromal
thylakoids are in contact with the stroma and are in
the form of helicoid sheets.
The granal thylakoids contain only Photosystem II
protein complex. This allows them to stack tightly
and form many granal layers with granal membrane.
This structure increases stability and surface area for
the capture of light.
The Photosystem I and ATP synthase protein
complexes are present in the stroma. These protein
complexes act as spacers between the sheets of
stromal thylakoids
