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Plant cells share many characteristics with animal cells. Plant cells use cellular respiration for energy, as reviewed earlier. They also have a plasma membrane, cytoplasm, a nucleus, and a nucleolus which is surrounded by a nuclear membrane, mitochondria, rough and smooth endoplasmic reticula, Golgi bodies, peroxisomes, microtubules, and other structures. However, plant cells have some unique structures and carry out some unique, and vital, processes.
The most distinguishing characteristic of plant cells is their rigid cell wall that surrounds the inner plasma membrane. A plant cell wall is made from cellulose, which is made from glucose. Thin chains of these glucose molecules are joined as microfibrils, which give cellulose its crystal-like properties. The microfibrils are wound together into threads, which themselves are wound into cables. Cellulose is the most abundant substance in plants, but lignin is a close second.
Lignin, a glucose-based polymer, adds further rigidity and strength to the cell wall. Lignin is the major material in wood.
Plants need strong, rigid cell walls for two important reasons. First, the cell walls help support the plant by, for example, keeping its stem upright. Second, water drawn up into the plant via the roots is under pressure, and strong cell walls are required to maintain cell shape and prevent cell explosion during water intake.
Vacuoles are, in some ways, similar to vesicles in animal cells. However, plant cell vacuoles are usually far larger than vesicles (taking up 90 percent of the space in some plant cells). Vacuoles are membrane-bound sacs filled with liquid called cell sap, which is mostly water. In some plants, the cells’ vacuoles contain the pigments that give the plant its color, such as the red pigment in cherries.
Example of a typical plant cell
Chloroplasts are plant cell structures that contain chlorophyll and carotenoid pigments, and are the sites where photosynthesis occurs. Chloroplasts are disk-shaped plastids, components of plant cells bounded by a two-layer membrane. The base substance inside a chloroplast is called the stroma. Within the stroma is an intricate and complex system of flat, sac-like membranes called thylakoids. A large stack of thylakoids is called a granum (plural, grana). Chlorophyll and carotenoid pigments are found in the membranes of the thylakoids. In most plants, these pigments derive from green chlorophyll.
Chloroplasts are the energy dynamos of plant cells
What are the two main substances that give rigidity to the cell wall?
The correct answer is B. Cellulose forms microfibril chains that, along with lignin, add rigidity to plant cell walls. Microtubules are cytoskeletal structures in animal cells. Thylakoids are pigment-carrying parts of chloroplasts. Grana are stacks of thylakoids.
The grana that occur in chloroplasts are primarily made up of
The correct answer is C. Grana are the stacks of pigment-rich thylakoids that occur in the chloroplasts. A chloroplast is itself a plastid, a type of organelle, but its constituents are not plastids. The stroma is the liquid that fills the area around the grana in the chloroplasts. Carotenoid pigments may be incorporated into the thylakoids that make up the grana, but the grana are not made up of these pigments.
What are the two main substances that give rigidity to the cell wall?
The correct answer is B. Cellulose forms microfibril chains that, along with lignin, add rigidity to plant cell walls. Microtubules are cytoskeletal structures in animal cells. Thylakoids are pigment-carrying parts of chloroplasts. Grana are stacks of thylakoids.
Plants are autotrophs because they make their own food via the process of photosynthesis. Photosynthesisis a complex chemical process in which the energy in sunlight is used to convert carbon dioxide and water into a form of glucose a plant can use as food, with oxygen as a byproduct. Photosynthesis takes place in the chloroplasts, particularly in the thylakoids.
Of course, plants take in water through their roots, and the water is drawn upward to every cell in the plant. In most plants, the underside of the leaf contains small, pore-like openings called stomates that take in air.
This is the chemical formula for photosynthesis:
CO2 + H2O → light energy → (CH2O) + O2
CH2O is the building block of carbohydrates and when synthesized, can be used as food by the plant.
Different plant pigments absorb different wavelengths, or colors, of light. Chlorophyll, which gives most plants their green color, absorbs wavelengths of light in the violet-blue-red range of the spectrum. Other plant pigments, including carotenoids and phycobilins, absorb other spectra of light.
A photosystem includes chlorophyll and other pigments that are embedded in thylakoids. One chlorophyll a molecule per photosystem absorbs one photon of light. The energy then boosts one of the chlorophyll a electrons to a higher level. It moves to an acceptor molecule, and an electron flow is initiated. The chlorophyll a molecule is thus oxidized and has a positive charge. In Photosystem I, the activated chlorophyll a molecule is designated P 700 (P for pigment, 700 for the optimum wavelength absorbed). In Photosystem II, chlorophyll a is P 680. Both photosystems acquire energy from sunlight, each using a slightly different wavelength of light and terminal electron acceptor in the pathway.
P 680 traps a photon, and its excited electron is transferred to an acceptor molecule. The P 680 is then able to replenish its electrons from a water molecule that it splits in a process known as photolysis. The electrons cascade along an electron transport chain, forming ATP in the process. The term for this process is photophosphorylation.
In Photosystem I, electrons from P 700 are passed downhill until they reach the coenzyme NADP, which is reduced to form NADPH 2. NADPH 2 provides energy for the synthesis of fuel for the cell.
Thus, in the light reaction, electrons flow continuously from water to Photosystem II, and from there back to Photosystem I and to NADP.
Despite its name, the dark reaction often takes place in the presence of light, though light is not required. In this second stage of photosynthesis, carbon from carbon dioxide is chemically reduced. The process occurs in the stroma, and this carbon fixation process is referred to as the Calvin cycle after the scientist Melvin Calvin, who first described it.
The Calvin cycle begins and ends with a five-carbon sugar that has two phosphate groups attached to it (ribulose 1,5-biphosphate, or RuBP). Carbon dioxide is attached to the RuBP, which then splits to form two molecules of PGA (3-phosphoglycerate). The cycle must repeat six times for the final product glyceraldehyde 3-phosphate to result.
Calvin cycle:
6 CO2 + 12 NADPH2 + 18 ATP → 1 glucose + 12 NADP + 18 ADP + 18Pi + 6 H2O
In the electron transport chain, electron carriers (NADH and FADH) transfer electrons to lower energy levels, with the energy given up by the electrons being used to create ATP from ADP. The electron transport chain begins with the glucose molecule’s carbon atoms thoroughly oxidized. Some of the energy produced in this process was used to create ATP from ADP. But a good deal of energy remains in the electrons that were removed from the carbon-carbon and carbon-hydrogen bonds, which were transferred to the electron carriers NAD and FAD, now NADH and FADH2. These high-energy electrons are then transferred in a downward process to ever-lower energy levels, bottoming out with oxygen. At each step, an electron carrier moves the electrons along to the next lower energy level. The energy the electrons give up on their downward path is used to re-create ATP from ADP. The process that produces ATP from the energy released as the electrons are moved to each lower level is called oxidative phosphorylation.
Electron transport system
The main carriers in the electron transport chain are known as cytochromes, which are made up of protein and a porphyrin ring. At each step, a different cytochrome, designed specifically to carry an electron at a particular energy level, carries the electron to the next lower level, ending with low-energy oxygen. At the end of the downward slide, the electrons link up with oxygen and then combine with hydrogen ions to yield water.
At each lower step in the chain, the energy released by a pair of electrons is sufficient to transform, or phosphorylate, one ADP to one ATP molecule. Once created, an ATP molecule is moved across the mitochondrial membrane. Simultaneously, an ADP molecule moves into the mitochondrion to begin its phosphorylation into ATP. You can see that the transformation of ADP into ATP, and vice versa, is a perpetual cycle in which a cell creates and uses the fuel that keeps it going.
In summary, glycolysis and cellular respiration yield 38 molecules of ATP to energize the cell.
In the designation P680, the subscript number represents
The correct answer is B. The subscript number of the pigment indicates the wavelength of light it best absorbs. The subscript number does not indicate the number of atoms in the molecule of pigment. The number of glucose molecules produced in the Calvin cycle varies and is not dependent on the type of pigment. The number of carbon dioxide molecules fixed during photosynthesis depends on certain conditions and is not indicated by the subscript wavelength number designating the pigment.
ATP is created in the electron transfer chain process because
The correct answer is D. In the C4 cycle, carbon molecules are reduced and the carbon dioxide that results is used in the Calvin cycle instead of being released, so this is a more efficient use of carbon in photosynthesis. All cell functions require the energy of ATP, which has nothing to do with the double processing of the more efficient C4 cycle. B is not correct because PEP is an intermediate material made during C4 photosynthesis and does not explain why this type of photosynthesis uses carbon more efficiently. The C4 cycle does not use fewer carbon dioxide molecules; it just uses the ones it has completely instead of losing them.
The substance that provides the necessary acetyl group in the Krebs cycle is
The correct answer is C. In photolysis, a pigment traps an excited electron, creating energy to cleave a water molecule into its components hydrogen and oxygen. A photon is not itself transferred among molecules, but the photon-activated electron is. B is not correct because carbon fixation is a process separate from photolysis, which involves the splitting of water molecules. Photolysis does not involve the oxidation and release of carbon dioxide; reoxidation of carbon dioxide occurs in photorespiration.