PEROXISOMES
Peroxisomes are membrane-bound vesicles that are smaller than lysosomes. Peroxisomes contain enzymes that break down fatty acids and amino acids. Hydrogen peroxide which breaks down hydrogen peroxide to water and oxygen. Cells that are active in detoxification, such as liver and kidney cells, have many peroxisomes.
MIHONDRIA
Mitochondria usually are small, rod-shaped structures. In living cells, time lapse photomicrography shows that mitochondria constantly change shape from spherical to rod-shaped or even to long, threadlike structures. Mitochondria are the major sites of ATP production, which is the major energy source for most endergonic chemical reactions within the cell. Each mitochondrion has an inner and outer membrane separated by an intermembranous space. The outer membrane has a smooth contour, but the inner membrane has numerous infoldings called cristae that project like shelves into the interior of the mitochondria.
A complex series of mitochondrial enzymes forms two major enzyme systems that are responsible for oxidative metabolism and most ATP synthesis. The enzymes of the citric acid (or Krebs) cycle are found in the matrix, which is the substance located in the space formed by the inner membrane. The enzymes of the electron transport chain are embedded within the inner membrane. Cells with a greater energy requirement have more mitochondria with more cristae than cells with lower energy requirements. Within the cytoplasm of a given cell, the mitochondria are more numerous in areas in which ATP is used.
Increases in the number of mitochondria result from the division of preexisting mitochondria. When muscles enlarge as a result of exercise, the number of mitochondria within the muscle cells increases to provide the additional ATP required for muscle contraction.
The information for making some mitochondrial proteins is stored in DNA contained within the mitochondria themselves, and those proteins are synthesized on ribosomes within the mitochondria. The structure of many other mitochondrial proteins is determined by nuclear DNA, however, and these proteins are synthesized on ribosomes within the cytoplasm and then transported into the mitochondria. Both the mitochondrial DNA and mitochondrial ribosomes are very different from those within the nucleus and cytoplasm of the cell. In addition, unlike nuclear DNA, mitochondrial DNA does not have associated proteins.
CENTRIOLES AND SPINDLE FIBERS
The centrosome is a special area of the cytoplasm close to the nucleus that contains two centrioles. Each centriole is a small, cylindrical organelle about 0.3 – 0.5 um in length and 0.15 um in diameter, and the two centrioles are usually found perpendicular to each other within the centrosome. The wall of the centriole is made up of nine evenly spaced, side by side units, or triplets. Each unit consists of three microtubules located side by side and joined together.
The centrosome is the center of microtubule formation. Microtubules appear to have some control over the distribution of actin and intermediate filaments. Through its control of microtubule formation, the centrosome is closely involved in determining cell shape and movement. The microtubules extending from the centrosomes are constantly growing and shrinking.
Before there is cell division, the two centrioles double in number, the centrosome divides into two, and one centrosome, containing two centrioles, moves to each end of the cell. Spindle fibers extend out in all directions from the centrosome. These microtubules grow and shrink even more rapidly than those of nondividing cells. If a spindle fiber comes in contact with a kinetochore, the fiber attaches itself to the kinetochore and stops growing or shrinking. Eventually spindle fibers from each centromere attach to the kinetochores of all the chromosomes. Then the chromosomes are pulled apart and moved by the microtubules toward the two centrosomes during cell division.
CILIA AND FLAGELLA
Cilia are appendages that come from the surface of cells and are capable of movement. They are usually found on only one surface of a given cell and vary in number from one to thousands per cell. Cilia are cylindrical in shape, about 10 um in length and 0.2 um in diameter, and the shaft of each cilium is covered by the plasma membrane. Two centrally located microtubules and nine peripheral pairs of fused microtubules extend from the base to the tip of each cilium. Movement of the microtubules past each other, a process that requires energy from ATP, is responsible for movement of the cilia. A basal body is located in the cytoplasm at the base of the cilium. There are many cilia on surface cells that line the respiratory tract and the female reproductive tract. In these regions cilia move with a power stroke in one direction and a recovery stroke in the other direction. Their motion moves materials over the surface of the cells.
Flagella have a similar structure like cilia but are longer, and there is usually only one per cell. Whereas, cilia moves small particles across the cell surface, flagella moves the cell.
MICROVILLI
Microvilli are cylindrically shaped extensions of the plasma membrane about 0.5-1 um in length and 90 nm in diameter. Many microvilli are on each cell increasing the cell surface area. Microvilli are only one tenth to one twentieth the size of cilia. Microvilli does not move, and they are supported with actin filaments, not microtubules. They are found in the intestine, kidney, and other areas in which absorption is an important function. In some locations of the body, microvilli are highly modified to work as sensory receptors.
SUMMARY OF CELL PARTS
CELL PARTS STRUCTURE FUNCTION
Plasma Membrane Lipid bilayer composed of phospholipids and Outer boundary of cells that controls entry
cholesterol with proteins that extend across and exit of substances; receptor
or are buried in either surface of the lipid molecules function in intercellular
bilayer communication; marker molecules enable cells to recognize one another
Nucleus
Nuclear envelope Double membrane around the nucleus; the Separates nucleus from cytoplasm and
outer membrane is continuous with the controls movement of materials into and
endoplasmic reticulum; nuclear pores go out of the nucleus
through the nuclear envelope
Chromatin Thin strands of DNA, histones, and DNA controls protein synthesis and the
other proteins; condenses to form chemical reactions of the cell; DNA is
chromosomes during cell division the genetic or hereditary material
Nucleolus One to four dense bodies making up of Large and small ribosomal subunits are made
ribosomal RNA and proteins here
Cytoplasm: Cytosol
Fluid Part Water with dissolved ions and molecules; Contains enzymes that start
colloid with suspended proteins decomposition and synthesis reactions; ATP is produced in glycolysis reactions
Cytoskeleton
Microtubules Hollow tubes composed of the protein Support the cytoplasm and form centrioles,
tubulin; 25 nm in diameter spindle fibers, cilia, and flagella; responsible for cell movements
Actin filaments Small fibrils of the protein actin; 8 nm in Support the cytoplasm and form centrioles,
diameter microvilli, responsible for cell movement
SUMMARY OF CELL PARTS
CELL PARTS STRUCTURE FUNCTION
Intermediate filaments Protein fibers; 10 nm in diameter Support the cytoplasm
Cytoplasmic inclusions Groups of molecules made or taken in Function depends on the molecules; energy
by the cell; may be surrounded storage, oxygen transport, skin color,
by a membrane and others
Cytoplasm: Organelles
Ribosome Ribosomal RNA and proteins form large and Site of protein synthesis
small subunits; attached to endoplasmic
reticulum or free
Rough endoplasmic reticulum Membranous tubules and flattened sacs with Protein synthesis and transport to Golgi
attached ribosomes apparatus
Smooth endoplasmic reticulum Membranous tubules and flattened sacs with Makes lipids and carbohydrates;
attached ribosomes makes harmful chemical; stores calcium
Golgi apparatus Flattened membrane sacs stacked on each other Modification, packaging, and distribution of proteins and lipids for secretion or internal use
Secretory vesicle Membrane-bound sac pinched off Golgi Carries proteins and lipids to cell surface
apparatus for secretion
Lysosome Membrane-bound vesicle pinched off Golgi Contains digestive enzymes
apparatus
Peroxisome Membrane-bound vesicle One site of lipid and amino acid breakdown and breaks down hydrogen peroxide
Mitochondria Round, rod-shaped, or threadlike Major site of ATP production when oxygen
structures; surrounded by double membrane; is available
inner membrane forms cristae
SUMMARY OF CELL PARTS
CELL PARTS STRUCTURE FUNCTION
Centrioles Pair of cylindrical organelles in the centrosome Centers for microtubule formation;
consisting of triplets of parallel microtubules determine cell polarity during cell division; form the basal bodies of cilia and flagella
Spindle fibers Microtubules extending from the centrosome to Assist in the separation of chromosomes
chromosomes and other parts of the cell during cell division
Cilia Extensions of the plasma membrane containing Move materials over the surface of cells
doublets of parallel microtubules
Flagellum Extensions of the plasma membrane containing In humans, responsible for movement of
doublets of parallel microtubules spermatozoa
Microvilli Extension of the plasma membrane containing Increase surface area of the plasma
microfilaments membrane for absorption and secretion; modified to form sensory receptors
CELL METABOLISM
Cell metabolism is all the decomposition and synthesis reactions in the cell. The breakdown of food molecules such as carbohydrates, lipids, and proteins releases energy that is used to synthesize ATP. Each ATP molecule has a portion of the energy stored from the chemical bonds of the food molecules. The ATP molecules are small energy packets that are used to drive other chemical reactions or processes such as active transport.
ATP production takes place in cytosol and in mitochondria through lots of chemical reactions. Food molecules transfer energy to ATP. If a cell was to receive all the energy from food molecules, it would literally burn up.
To show ATP production from food molecules: the breakdown of sugar glucose. For example: sugar from a candy bar. Once glucose is put into a cell, lots of reactions takes place inside the cytosol. These chemical reactions, glycolysis, change the glucose to pyruvic acid. Pyruvic acid can go into different biochemical pathways, if oxygen is available.
Aerobic respiration happens when oxygen is available. Pyruvic acid molecules enter mitochondria through chemical reactions called citric acid cycle and the electron transport chain, which are then changed to carbon dioxide and water. Energy stored in each glucose molecule can produce 36-38 ATP molecules through aerobic respiration.
Anaerobic respiration happens without oxygen and includes the change of pyruvic acid to lactic acid. There is a production of two ATP molecules for each glucose molecule used. Anaerobic respiration doesn’t produce as much ATP as aerobic respiration. But it does allow cells to work for a short time when oxygen is too