cell biology

1. 1
CHLOROPLAST
• members of plastid
family
• present in all living plant
cells
• all plastids contain
genome
• plastids compose of 2
membrane layers
• Chloroplast has 3
membrane layers

2. 2
Plastids
• 3 types – pigments inside
–Chloroplast – contain chlorophylls
–Chromoplast – other pigments than chlorophylls
–Leucoplast – colorless plastid, containing
• Protein – aleuroplast
• Starch – amyloplast
• Oil – oleoplast

3. 3
3
CHLOROPLAST STRUCTURE
• Outer membrane – intermembrane space
• Inner membrane – stroma (liquid part)
• Thylakoid membrane – thylakoid space

4. 4
STROMA
• colourless fluid inside
inner membrane
• analogous to the
mitochondrial matrix
• contains many metabolic
enzymes, CO2 fixation
• contains a special set of
ribosomes, RNAs, and
DNA
• H+ for electron transport
and ADP, Pi source

5. 5
THYLAKOIDS
• Membrane that form a set
of flattened disc like sacs
• Different protein functions:
– electron-transport chains
– photosynthetic light-
capturing systems
– ATP synthase
• thylakoid space – lumen
connects to other thylakoid
form a space

6. 6
Chloroplast functions
• Photosynthesis
• Biosyntheses:
–fatty acids and a number of amino acids
–reduction of nitrite (NO2
-) to ammonia
(NH3) for
•amino acids
•nucleotides.

7. 7
Photosynthesis
Electrons play a primary role in photosynthesis
In eukaryotes, photosynthesis takes place in
chloroplasts
Autotrophs use energy to make their own organic
molecules from CO2 and inorganic sources such as
water
Photoautotrophs use light as an energy source
Heterotrophs consume or decompose organic
molecules

8. 8
Two stages of photosynthesis:
Light-dependent reactions – energy from sunlight is
converted into chemical energy to replenish ATP and
NADPH
Light-independent reactions (Calvin cycle) – excess
energy is stored by building high-energy sugar molecules
to be used when sunlight is not available

9. 9
The two stages of photosynthesis are
linked together and occur at the same
time.

10. 10
Light-Dependent Reactions
Two main processes:
Light absorption
LIGHT behaves as if it were composed of "units" or
"packets" of energy that travel in waves. These packets are
photons.
Synthesis of ATP and NADPH

11. 11
Synthesis of ATP and NADPH Summary
 Energy from sunlight is captured in excited electrons and used to
synthesize more useful ATP and NADPH
 Electron transfer systems are used to extract energy from excited
electrons
 Some of the energy is used to create a gradient of H+ across the
thylakoid membranes that provides energy for ATP synthesis
 Electrons are ultimately passed along to reduce NADP+ to NADPH

12. 12
Water Splitting
Water splitting complex provides a source of
electrons to be excited during light-dependent
reactions
2 H2O → 4 H+ + 4 e– + O2
Located close to photosystem II

13. 13
Light-Independent Reactions
Light-independent reactions (Calvin cycle) store
some of the energy captured from sunlight in the
form of high-energy molecules such as sugar
When sunlight is not available, stored carbohydrates
are broken down by aerobic respiration in the
mitochondria to replenish ATP and NADH

14. 14
Calvin Cycle
During the Calvin cycle, CO2 is reduced and
converted into organic substances
NADPH provides electrons and hydrogen
ATP provides additional energy
Carbon fixation involves capturing CO2 molecules
with the key enzyme rubisco (RuBP
carboxylase/oxygenase)

15. 15
Rubisco
As rubisco provides the source of organic molecules
for most of the world’s organisms
100 billion tons of CO2 are converted into
carbohydrates each year
Represents 50% or more of the total protein in
leaves

16. 16
Calvin Cycle
Each turn of the Calvin cycle captures one CO2 molecule
Three turns of the Calvin cycle are needed to capture
the three carbons in one G3P molecule
Six turns are needed to make a six-carbon sugar such as
glucose
6 CO2 + 12 NADPH + 18 ATP
↓
C6H12O6 + 12 NADP+ + 18 ADP + 18 Pi

17. 17
Calvin Cycle Summary
 Carbon fixation – CO2 added
to RuBP to produce two 3PGA
molecules
 Reduction – NADPH and ATP
used to convert 3PGA into
G3P, a higher energy molecule
used to build sugars
 Regeneration – remaining
G3P molecules are used to
recreate the starting material
RuBP
Fig. 9-13a, p. 192

18. 18
Photosynthetic Products
Surplus G3P formed in the Calvin cycle can be the
starting material for many organic molecules
Carbohydrates, lipids, proteins, nucleic acids
Sucrose (disaccharide) is used to circulate
photosynthetic products from cell to cell in plants
Starch is used for longer storage of energy

19. 19
What is ribosome?
• Ribosome - protein synthesizer
consisting of two subunits
• Basically the protein factory.
Subunits each have role in
making of proteins.
• Larger one, “50S”, is upper
picture. Smaller is “30S”
(They look the same size here
because of space restrictions.)

20. 20
• 50S (left) and 30S. This time you can see them from different
angles, through different style of picture
• Related to their respective sizes. Numbers actually measures of
how quickly each subunit sinks to the bottom of a container of
liquid when spun in a centrifuge
• One subunit smaller than other, but both are larger than average
protein

21. 21
Protein synthesis
• Process starts from DNA
through “transcription”
• “Translation” is where
ribosome comes in.
Translation occurs when
protein formed from code
on mRNA
• Ribosome carries out the
translation of the
nucleotide triplets

22. 22
Initiation: The first phase of translation
• Translation begins when
mRNA attaches to the
30S
• tRNA comes and binds to
mRNA where nucleotide
code matches
• This triggers 50S binding
to 30S. 50S is where all
tRNAs will bind. Now we
move on to elongation

23. 23
Elongation: The second phase
• Two binding sites on 50S:
A site and P site, which
aid in continuing
translation
• First tRNA connected at A
site. Now moves to P site
as another tRNA
approaches
• Second tRNA binds to A
site

24. 24
Elongation (continued)
• Peptide bond forms
between amino acids of
tRNAs (methionine and
proline)
• First tRNA now detached
from its amino acid, and
it leaves ribosome.
Second tRNA still has
proline and methionine
attached

25. 25
Elongation (continued)
• The tRNA left now
moves to P site.
Ribosome ready to
accept another tRNA
and continue process
• Each tRNA adds another
amino acid to growing
peptide chain (thus
“elongation”)
• Eventually process has

26. 26
End of translation
• Ribosome was moving
along nucleotide triplets
one by one
• Ribosome reaches “stop
codon,” peptide chain
finished. Last tRNA
leaves ribosome,
leaving behind
completed peptide
chain

27. 27
End of translation (continued)
• Ribosome separates
from mRNA
• Ribosome subunits
also separate, and
will remain this way
until another mRNA
comes along to
restart the process

28. 28
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THE END