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Option 9.9 Biochemistry: 8. The structure of
chloroplasts
Extract from Biology Stage 6 Syllabus (Amended October
2002) © Board of Studies, NSW.
[Edit 18 June 09]
Prior learning: Stage 4, Models, Theories
and Laws 4.8.1(a) and (b), Structures and Systems 4.8.2 (c),
Interactions 4.10 (c).
Recall statements in Preliminary course: Preliminary module 8.2 (subsection 2) module 8.3 (subsection 4).
Background: Photosynthesis occurs inside
chloroplasts. Chloroplasts are distinctive organelles found in
cells. Chloroplasts contain DNA and have internal and external
membranes. The internal membrane is greatly folded. The
development of the electron microscope in increasingly
sophisticated forms has allowed the visualisation of
chloroplast structure.
gather and process information from secondary sources, including electron micrographs to:
- draw and label the structure of a chloroplast
- compare the size, shape and distribution of chloroplasts in a named alga, terrestrial angiosperm and aquatic angiosperm
-
Gather information from a range of
resources such as CD ROM or journals with diagrams,
electron micrographs or other representations of
chloroplasts. Use the Internet to find an electron
micrograph and search for other information for the
chloroplasts of an alga, a flowering plant on land and in
water. Draw and label a chloroplast showing
- the thylakoid membrane and lumen or space
- the grana
- the stroma
- the inner and outer chloroplast membrane
- lamellae joining different grana throughout the stroma
- the thylakoid membrane where the photosynthetic units are found. (Photosynthetic unit = antenna complex + reaction centre)
- Process the information by assessing the relative importance of the information gathered. To do this draw a table as a comparison between the size, shape and distribution of chloroplasts in named examples of alga, terrestrial angiosperm and aquatic angiosperm. One is modelled below.
As a starting point try the sites below.
Chlorophyta 
University of Hawaii, Botany
Faculty, USA.
Scroll down to see several electron micrographs of chloroplasts and parts of chloroplasts.
Chloroplasts University of NSW. Scroll down until you see the diagram of a chloroplast. There are links to electron micrographs.
identify the average size of a chloroplast and describe the range of sizes observed across species
- The average size of a chloroplast is 3μ wide, 6μ long
- The table below compares the size, shape and distribution of chloroplasts in the named organisms.
Chloroplasts Alga e.g. Terrestrial angiosperm Aquatic angiosperm Example Spirogyra Privet, Grevillea Elodea Size 6μ long 3μ wide, 6μ long 6μ long Shape Spiral Disc Disc Distribution
Length of cells, mobile running the length of the cell Concentrated in the palisade mesophyll ad spongy mesophyll. A few in the upper and lower epidermis. Edges of cells, large air spaces surrounding the cells
describe the structure of a chloroplast as seen under a transmission electron microscope
- In photosynthetic eucaryotic cells, the photosynthetic membranes are found in the chloroplasts. The chloroplast has three membranes. The outer and inner membranes are similar in composition. The inner membrane is not convoluted. A third membrane, called the thylakoid membrane, is folded to make lamellae that are in turn stacked to make the grana. The colourless part of the chloroplast is called the stroma. Lamellae join different grana throughout the stroma.
describe the thylakoids as flattened, hollow discs with chlorophyll dissolved in the lipid layers of the membrane
- The thylakoids are a collection of interconnected flattened discs that form the grana. Chlorophyll is located within the thylakoid membrane.
describe the stroma as matrix lying within the inner membrane containing DNA, ribosomes, lipid droplets and starch granules
- The matrix of the chloroplasts is called the stroma. It is electron dense and contains circular DNA and ribosomes.
- The stroma also contains lipid droplets and starch granules. The starch granules are formed from the products of photosynthesis.
- In higher plants, the chloroplast genome codes for twenty chloroplast proteins, several proteins that are part of photosystems I and II, subunits of ATP synthase and other proteins used in the chloroplast. The genes also code for about 40 proteins with unknown functions. The similarities between chloroplast and bacteria genomes indicate that chloroplasts arose from photosynthetic bacteria as the endosymbiotic hypothesis suggests. The chloroplast genome has remained unchanged for at least several hundred million years.
gather and process information from secondary sources to identify and explain the location of the sites of light absorption and the site of the Calvin cycle
Useful information
- The light dependent reaction occurs in the thylakoid membrane where the photosynthetic units are found. The chlorophyll and accessory pigments that capture light energy (antenna complex and reaction centres) are localised within the thylakoid membrane.
- The electron transport chain that connects photosystem II and photosystem I is also embedded in the thylakoid membrane. Photosystem II splits water and feeds electrons into the electron transport chain. The electrons are activated and used by photosystem I to make NADPH for use in the Calvin Cycle.
- During this process hydrogen ions are transferred from the stroma side of the thylakoid membrane to the lumen (or inside) of the thylakoid. The protons are then used for the synthesis of ATP using the chemiosmotic theory. ATP synthase is situated in the thylakoid membrane and uses the electrochemical gradient provided by the flow of protons back to the stroma to make ATP from ADP and Pi. ATP synthesis does not occur if no membranes are present.
- The light independent reaction or Calvin Cycle occurs in the stroma. The thylakoid membranes are “sided”. The ATP and NADPH produced by the light dependent reactions are produced on the outside of the thylakoid; they are released into the stroma.
- The Calvin Cycle uses 18 ATP available in the matrix of the stroma, 12 to phosphorylate 3-phosphoglycerate and six to regenerate the 5-carbon acceptor. The 12 NADPH released to the stroma side of the thylakoid membrane supplies H+ ions to replace the phosphate group provided by ATP and form glyceraldehyde phosphate.
- Use scientific journals, CD ROMs or the Internet to gather information about the site of the Calvin cycle. The information you collected for Biochemistry 9.9:6, Using isotopes to prove carbon dioxide is used in the light independent reaction may be useful here. Summarise and collate the information that you have gathered.
- Process the accuracy of the information as you gather it. Try to use Internet sites that are linked to educational institutions such as Universities rather than personal homepages.
Here is a site to start with
Calvin cycle J Kimball, USA.
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