Chloroplasts & Photosynthesis Flashcards
Describe the outer covering of a chloroplast.
The outer covering consists of an envelope composed of two membranes separated by a narrow space. The outer membrane contains porins, and though they are large, exhibit some selectivity. In contrast, the inner membrane is highly impermeable; substances moving through this membrane do so only with the aid of a variety of transporters.
Define thylakoids
Thykaloids are flattened membranous sacs into which the internal membrane of a chloroplast is organized.
What are the grana?
Grana are orderly stacks into which the thylakoids are arranged.
What is the stroma?
The stroma is the space outside the thylakoid and within the chloroplast envelope. It contains the enzymes responsible for carbohydrate synthesis.
Describe the protein and lipid content of thylakoid membranes.
Thylakoid membranes have a high protein content and are unusual in having relatively little phospholipid. Instead, they have a high percentage of galactose-containing glycolipids. Fatty acids of these lipids contain several double bonds, which makes the lipid bilayer of the thylakoid membranes highly fluid, facilitating lateral diffusion of protein complexes through the membrane during photosynthesis.
What was Van Niel’s hypothesis?
Van Niel wanted to overturn the belief that during photosynthesis, carbon dioxide was being split into its two atomic components. He proposed instead that it was water that was split. This framed photosynthesis as the reverse of cellular respiration.
How can photosynthesis be seen as the reverse of cellular respiration?
Whereas respiration in mitochondria reduces oxygen to water, photosynthesis in chloroplasts oxidizes water to oxygen. The former process releases energy, so the latter process must require energy.
What happens in the first stage of photosynthesis, the light-dependent reactions?
During this stage, energy from sunlight is absorbed and stored as chemical energy in ATP and NADPH. ATP is the primary source of chemical energy, and NADPH is the primary source of reducing power.
What happens in the second stage of photosynthesis, the light-independent reactions?
Carbohydrates are synthesized from carbon dioxide using the energy stored in the ATP and NADPH molecules produced in the light-dependent reactions.
Define pigments.
Pigments are compounds that appear colored because they only absorb light of particular wavelengths within the visible spectrum. For example, leaves are green because their chloroplasts contain large quantities of the pigment chlorophyll, which absorbs most strongly in the blue and red, leaving the intermediate green wavelengths to be reflected to our eyes.
Describe the structure of chlorophyll.
Each molecule of chlorophyll consists of two parts:
1. a porphyrin ring that functions in light absorption
2. a hydrophobic phytol chain that keeps the chlorophyll embedded in the photosynthetic membrane.
How does the porphyrin ring of chlorophyll work to absorb light?
The porphyrin ring contains an atom of magnesium. The alternating single and double bonds along the edge of the porphyrin ring delocalize electrons, which forms a cloud around the ring. Conjugated systems such as these are excellent absorbers of light.
What effects does a conjugated system have on a chlorophyll molecule’s ability to absorb light?
When the molecule absorbs light, the energy causes a redistribution of the electron density of the molecule, which in turn favors the loss of an electron to an acceptor. The conjugated bond system also broadens the absorption peaks, enabling individual molecules to absorb energy of a range of wavelengths.
Define absorption spectrum.
An absorption spectrum is a plot of the intensity of light absorbed relative to its wavelength.
Define carotenoids and explain their function.
Carotenoids are like chlorophyll in that they are a type of light-absorbing photosynthetic pigment with conjugated bonds. They act as secondary light collectors during photosynthesis and draw excess energy away from excited chlorophyll molecules and dissipate it as heat.
What would happen if the excess energy in a plant cell were not absorbed by carotenoids?
The energy would be transferred to oxygen, producing an ultrareactive form of the molecule called singlet oxygen that can destroy biological molecules and cause cell death.
Define action spectrum and contrast it with an absorption spectrum.
An action spectrum is a plot of the relative rate (or efficiency) of photosynthesis produced by light of various wavelengths. Unlike an absorption spectrum which simply measures the wavelengths of light that are absorbed by particular pigments, an action spectrum identifies the wavelengths that are effective in bringing about a given physiologic response.
Describe the transfer of excitation energy and the structure needed for this to occur.
Several hundred chlorophyll molecules act together as one photosynthetic unit in which only the one reaction-center chlorophyll actually transfers electrons to an electron acceptor. Energy is transferred randomly through a network of pigment molecules that absorb light of increasingly longer wavelengths until the energy reaches the reaction-center chlorophyll, which transfers an excited electron to a primary acceptor.
