Lecture 8

Cell structure III

LIF101

28-1-2014Tuesday Dr. Jonaki Sen

In eukaryotic cells mitochondria and chloroplasts are organelles that convert energy to forms that cells can use for work.

The Mitochondria and Chloroplast change energy from one form to the another

Mitochondria are the sites of cellular respiration that generates energy from sugars, fats and other fuels with the help of oxygen.

Chloroplasts found only in plants, algae and fungi are the sites of photosynthesis. They convert solar energy to chemical energy by using it to drive the synthesis of organic componds such as sugars from carbon dioxide and water.

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Mitochondria: Chemical energy conversionMitochondria are 1 to 10m long, present in all eukaryotic cells and their numbers range from one to hundreds or even thousands per cell. The number of mitochondria per cell is related directly to the cells metabolic activity. Motile or contractile cells have many more mitochondria.

The mitochondrion is enclosed by two membranes each has a unique collection of proteins.

The outer membrane is smooth but the inner membrane is convoluted with infoldings called cristae. The inner membrane divides the mitochondrial space into two: The intermembrane space and the mitochondrial matrix. The matrix contains many different enzymes, ribosomes and mitochondrial DNA.

Many enzymes are present in the matrix and many are embedded in the inner membrane, the highly convoluted nature of the cristae is responsible for increasing the surface area and enhancing the productivity of cellular respiration.

Mitochondria are responsible for the production of ATP. Many ATP molecules are formed when organic compounds are broken down into CO2 and H2O in the mitochondrion.

During aerobic respiration, hydrogen ions accumulate between the two membranes

The buildup causes the ions to flow across the inner mitochondrial membrane, through membrane transport proteins that drive the formation of ATP

Theory of endosymbiosis: Mitochondria evolved from aerobic bacteria that took up permanent residence inside a host cell

1) Resemble bacteria in size, form, and biochemistry2) Have their own DNA, which is similar to bacterial DNA3) Divide independently of the cell, and have their own ribosomes

Chloroplast: Capture of light energyThe chloroplast is a specialized member of a family of closely related plant organelles called plastids.Amyloplast: is a colorless plastid storing starch in roots.Chromoplast: have pigments that give fruits and flowers their orange or yellow color.

These organelles convert the energy from sunlight to the chemical enrgy of ATP, which is used to make sugars and other organic compounds. 2 to 5 m in size they are found in leaves and other green parts of plants and fungi.

The chloroplast is also bound by a double membrane with a very thin intermebrane space. Inside the chloroplast is another system of interconnected sacs called thylakoids. Each stack of sacs is called granum and the fluid outside the thylakoid is called the stroma containing the chloroplast DNA, ribosomes and many enzymes.

The first stage of photosynthesis starts and ends at a thylakoid membrane. Many light trapping pigments, enzymes and other proteins carry out the reactions. They work together to absorb light energy and “store” it in the form of ATP. Then in the stroma this energy is used to make starch and other organic compounds from CO2 and H2O .

Central vacuole: • Present in mature plants enclosed by a membrane called tonoplast. • The central vacuole forms by fusion of many smaller vacuoles from the ER and golgi. • The tonoplast is selective for certain solutes thus the contents of the central vacuole

called the cell sap differs from the surrounding cytoplasm. • It can expand to occupy 50 to 90% of the cells interior.

The central vacuole is repository for the plant cell. It stores the following:

1) Important organic compounds e.g. proteins stored in the vacuoles of the seeds.

2) It stores inorganic ions e.g. potassium and chloride

3) Pigments stored in the central vacuole help attract pollinating insects

4) Some vacuoles may store poisonous compounds or unpalaltable ones to help protect the plant from predators.

The CytoskeletonThe cytoskeleton is a system of interconnected fibers threads and lattices in the fluid part of the cytoplasm of eukaryotic cells.

• The function of the cytoskeleton is to give mechanical support to the cell and maintain its shape. The cytoskeleton provides anchorage for many organelles and even cytosolic enzyme molecules.

• The cytoskeleton is also involved in several types of cell motility (movement) encompassing both changes in cell location and more limited movements of parts of the cell.

• The most recent addition to the list of possible cytoskeletal functions is the regulation of biochemical activities in the cell. Evidence suggests that the cytoskeleton can transmit mechanical forces exerted by extracellular molecules via surface proteins of the cell to its interior—and even into the nucleus

Components of the CytoskeletonThere are three main types of fibers that make up the cytoskeleton

Microtubules Microfilaments Intermediate filaments

• Maintenance of cell shape (compression-resisting “girders”)• Cell motility (as in cilia or flagella)• Chromosome movements in cell division• Organelle movements

• Maintenance of cell shape (tension-bearing elements)• Changes in cell shapeMuscle contraction• Cytoplasmic streaming• Cell motility (as in pseudopodia)• Cell division (cleavage furrow formation)

• Maintenance of cell shape (tension-bearing elements)• Anchorage of nucleus and certain other organelles• Formation of nuclear lamina

MicrotubulesThey are hollow rods measuring about 25 nm in diameter and from 200 nm to 25 μm in length.

The wall of the hollow tube is constructed from a globular protein called tubulin.

Each tubulin molecule is a dimer consisting of two slightly different polypeptide subunits, α-tubulin and β-tubulin.

A microtubule grows in length by adding tubulin dimers to its ends. Microtubules can be disassembled and their tubulin used to build microtubules elsewhere in the cell.

Microtubules guide secretory vesicles from the Golgi apparatus to the plasma membrane.

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Microtubules are also responsible for the separation of chromosomes during cell division.

Microtubules shape and support the cell and also serve as tracks along which organelles equipped with motor proteins can move.

Microfilaments (Actin Filaments)Microfilaments are solid rods about 7 nm in diameter. They are also called actin filaments, because they are built from molecules of actin, a globular protein. A microfilament is a twisted double chain of actin subunits. The structural role of microfilaments in the cytoskeleton is to bear tension (pulling forces).

The ability of microfilaments to form a three-dimensional network just inside the plasma membrane helps support the cell s shape. ′

This network gives the cortex (outer cytoplasmic layer) of a cell the semisolid consistency of a gel, in contrast with the more fluid (sol) state of the interior cytoplasm

In animal cells specialized for transporting materials across the plasma membrane, such as intestinal cells, bundles of microfilaments make up the core of microvilli, the delicate projections that increase the cell surface area.

In a muscle cell thousands of actin filaments are arranged parallel to one another along the length of a muscle cell, interdigitated with thicker filaments made of a protein called myosin. Myosin acts as a motor protein by means of projections (arms) that “walk” along the actin filaments. Contraction of the muscle cell results from the actin and myosin filaments sliding past one another in this way, shortening the cell.

Localized contraction brought about by actin and myosin also plays a role in amoeboid movement, in which a cell, such as an amoeba crawls along a surface by extending and flowing into cellular extensions called psudopodia (false feet). Pseudopodia extend and contract through the reversible assembly of actin subunits into microfilaments and of microfilaments into networks that convert cytoplasm from sol to gel. Filaments near the cell s ′trailing end interact with myosin, causing contraction. Like squeezing on a toothpaste tube, this contraction forces the interior fluid into the pseudopodium, where the actin network has been weakened. The pseudopodium extends until the actin reassembles into a network. Amoebas are not the only cells that move by crawling; so do many cells in the animal body, including some white blood cells.

In plant cells, both actin-myosin interactions and sol-gel transformations brought about by actin may be involved in cytoplasmic streaming, a circular flow of cytoplasm within cells.

This movement, which is especially common in large plant cells, speeds the distribution of materials within the cell.

Chloroplasts can move to a new location within the cell depending on the changing angle of the sun.

Intermediate filamentsIntermediate filaments are named for their diameter, which, at 8–12 nm, is larger than the diameter of microfilaments but smaller than that of microtubules (see Table 6.1, above). Specialized for bearing tension intermediate filaments are a diverse class of cytoskeletal elements. Each type is constructed from a different molecular subunit belonging to a family of proteins whose members include the keratins.

Intermediate filaments are more permanent fixtures of cells than are microfilaments and microtubules, which are often disassembled and reassembled in various parts of a cell.

Intermediate filaments are especially important in reinforcing the shape of a cell and fixing the position of certain organelles.

For example, the nucleus commonly sits within a cage made of intermediate filaments, fixed in location by branches of the filaments that extend into the cytoplasm.

Other intermediate filaments make up the nuclear lamina that lines the interior of the nuclear envelope.

In eukaryotes, a specialized arrangement of microtubules is responsible for the beating of flagella (singular, flagellum) and cilia (singular, cilium), locomotor appendages that protrude from some cells. Many unicellular eukaryotic organisms are propelled through water by cilia or flagella, and the sperm of animals, algae, and some plants have flagella. When cilia or flagella extend from cells that are held in place as part of a tissue layer, they can move fluid over the surface of the tissue. For example, the ciliated lining of the windpipe sweeps mucus containing trapped debris out of the lungs.

Flagella and Cilia

Cilia usually occur in large numbers on the cell surface. They are about 0.25 μm in diameter and about 2–20 μm in length.

Flagella are the same diameter but longer than cilia, measuring 10–200 μm in length. Also, flagella are usually limited to just one or a few per cell.

Flagella and cilia differ in their beating patterns.

A flagellum has an undulating motion that generates force in the same direction as the flagellum s axis. ′

In contrast, cilia work more like oars, with alternating power and recovery strokes generating force in a direction perpendicular to the cilium s axis. ′

A cilium or flagellum has a core of microtubules sheathed in an extension of the plasma membrane. Nine doublets of microtubules, the members of each pair sharing part of their walls, are arranged in a ring. In the center of the ring are two single microtubules. these are motor proteins.

This arrangement, referred to as the “9 + 2” pattern, is found in nearly all eukaryotic flagella and cilia. Flexible “wagon wheels” of cross-linking proteins, evenly spaced along the length of the cilium or flagellum, connect the outer doublets to each other (the wheel rim) and to the two central microtubules (the wheel spokes). Each outer doublet also has pairs of side-arms spaced along its length and reaching toward the neighboring doublet; these are motor proteins.

Each motor protein extending from one microtubule doublet to the next is a large protein called dyenin. These dynein arms are responsible for the bending movements of cilia and flagella. A dynein arm performs a complex cycle of movements caused by changes in the conformation of the protein, with ATP providing the energy for these changes

The dynein arms of one doublet attach to an adjacent doublet and pull so that the doublets slide past each other in opposite directions. The arms then release from the other doublet and reattach a little farther along its length. Without any restraints on the movement of the microtubule doublets, one doublet would continue to “walk” along and slide past the surface of the other, elongating the cilium or flagellum rather than bending it. In cilia and flagella, the microtubule doublets seem to be held in place by the cross-linking proteins just inside the outer doublets and by the radial spokes and other structural elements. Thus, neighboring doublets cannot slide past each other very far. Instead, the forces exerted by the dynein arms cause the doublets to curve, bending the cilium or flagellum