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ORGANIZATION OF THE CELL
Chapter 4
The cell is the basic unit of life. It is the smallest self-sufficient unit of living material.
CELL THEORY
Matthias Schleiden, a German botanist, in 1838 stated that all plants are made of cells.
Theodor Schwann, a German zoologist, made the same pronouncement about animals in 1839.
Rudolph Virchow observed cells dividing and giving rise to new cells in 1855.
August Weismann pointed out 1880 that the ancestry of all cells can be trace back to ancient
times.
1. Cells are the basic unit of structure and function of all living things.
2. All cells come from preexisting cells.
CELL ORGANIZATION
All cells have a similar organization:
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semipermeable plasma membrane that surrounds the cell
internal structures called organelles.
DNA that contains the genetic material..
The micrometer is the unit normally used to measure cells.
1m = 1 millionth of a meter (10-6) or 1 thousandths of a millimeter (0.001 ml).
The nanometer is used to measure cellular organelles.
1nm = 1 billionth of a meter (10-9) or 1 thousandths of a m.
Starting with the meter, the ml, m and nm are 0.001 of the previous unit.
As cells become larger, the volume increases at a greater rate than the surface area.
Above a critical size, the number of molecules needed by the cell could not be transported into
the cell fast enough to sustain its needs.
Once inside the cell molecules must be transported to their place of utilization.
Microvilli increase the surface area of the cell.
Cells divide in order to maintain an optimal ratio of surface to volume.
Sizes and shape of cells are relate to the functions they perform.
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MICROSCOPES
Light or compound microscope.
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Visible light passes through a specimen and lenses.
The lenses refract light and the image is enlarged.
Good for magnifying objects up to 1000x.
Resolution or resolving power is the ability to distinguish fine detail.
-Minimum distance between two points that can be seen separately.
Fluorescent microscopes are used to detect the location of certain molecules in the cell.
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Fluorescent stains absorb light of certain wavelength and release light of another
wavelength.
The emitted light allows to observe the location of the molecules and the structures to which
they are bound.
Computing imaging methods have improved the resolution of structures labeled by
fluorescent dyes.
Electron microscopes.
Energized electron beam is focused by electromagnets.
1. Transmission electron microscope (TEM).
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Magnification 250,000x or more.
The electron beam passes through the specimen and is projected on a fluorescent
screen or photographic plate.
2. Scanning electron microscopes (SEM).
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Magnification 3,000 to 10,000x.
The specimen is coated with metal.
The electron beam strikes the metal and dislodges electrons from the metal coat.
The intensity of these electrons varies with the contour of the surface and gives a threedimensional figure of the surface.
Cell fractionation is methods of purifying organelles.
Cells are broken apart and the mixture is centrifuged into a pellet and the supernatant.
In differential centrifugation the supernatant is spun at successively higher speeds in order to
separate the components on the basis of their different sizes and densities.
CELL TYPES
There are two types of cells, prokaryotic and eukaryotic.
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Prokaryotic cells are simpler than eukaryotic cells.
Prokaryotic cells are considered to be more primitive than eukaryotic cell.
Eukaryotic cells have highly organized membrane-bounded organelles.
Nucleoplasm is the term used for the material inside the nuclear membrane.
Cytoplasm refers to the part outside the nucleus of the cell.
Organelles are suspended on the cytosol of the cytoplasm.
Plant cells differ from animal cells in that they have cell wall, plastids, vacuoles, and usually lack
centrioles. Plant cell division occurs by the formation of a cell plate during the telophase of
mitosis. Animal cells divide by pinching in the dividing cell into two daughter cells.
Membranes
Divide the cells into compartments that allow cells to conduct specialized activities. They never
have free ends and always contain an internal space.
Functions of organelle membranes.
1. Molecules are concentrated in these spaces and many metabolic reactions occur here.
2. Membranes allow the storage of energy. As molecules move from areas of higher
concentration to areas of lower concentration, the cell converts potential energy into
chemical energy.
3. Membranes serve as work surfaces for enzymes that are arranged in sequence to carry
out reactions that take several steps.
A network of internal membranes forms the endomembrane system of the cell.
Small membrane-bounded sacs called vesicles transport materials between organelles.
EUKARYOTIC CELLS
PROKARYOTIC CELLS
Have a nuclear membrane
Have two to hundreds of chromosomes.
DNA is double helix.
Have membrane-bounded organelles.
Large ribosomes (80S).
Sexual reproduction by fusion.
Asexual reprooduction by mitosis.
Lack a nuclear membrane.
Have a single chromosome and plasmids.
DNA in a single strand.
Lack membrane-bounded organelles.
Small ribosomes (70S).
Sexual reporduction unknown.
Asexual reproduction by fission.
EUKARYOTIC CELL STRUCTURES AND THEIR FUNCTIONS
Cell nucleus
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Large structure surrounded by double membrane; contains nucleolus and chromosomes.
Nuclear envelope is made of a double membrane: two concentric membrane.
Nuclear pores allow the passage of material in and out of the nucleus.
Controls the functions of the cell.
Nucleolus
Granular body within nucleus; consists of RNA and protein.
Site of ribosomal RNA synthesis and ribosome subunit assembly.
It adjoins chromatin containing instructions for making rRNA.
Chromosomes.
Composed of a complex of DNA and protein known as chromatin; become visible as
rod-like structures when the cell divides.
Information in DNA is transcribed in RNA synthesis; specifics cellular proteins.
Contain genes (units of hereditary information that govern structures and activity of cell).
Plasma membrane
Forms the boundary of living cells.
Regulates the movement of materials in and out of the cell.
Communicates with other cells.
Endoplasmic Reticulum (ER).
Network of internal membranes extending through the cytoplasm.
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Smooth ER lacks ribosomes on the outer surface. It is the site of lipid (fatty acids,
steroids, phospholipids) biosynthesis and drug detoxification.
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Rough ER has ribosomes embedded on its surface. It manufactures proteins destined
to secretion and making of membrane..
Ribosomes.
Granules composed of RNA and protein; some attached to ER, some free in cytosol.
Synthesis of polypeptides.
Golgi complex.
Stacks of flattened membrane sacs.
Modifies proteins made in the ER and packages them into vesicles to be sent to other parts of
the cell.
Finished products may be excreted, incorporated into the plasma membrane or lysosomes, etc.
Lysosomes.
Membranous sacs found in most eukaryotic cells. Their presence in plant cells is debatable.
Digestive enzymes are stored here and isolated from the rest of the cell.
Fuse with food vacuoles for the digestion of engulfed particles.
Digest infecting bacteria and damaged organelles.
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Apoptosis: programmed cell death.
Vacuoles.
Large membranous sacs found in plant, fungi and alga cells.
Single membrane called tonoplast.
Variety of sizes and functions.
Storage of food, pigments, salts and wastes.
Regulates the amount of water in the cell.
Involved in regulating plant cell growth.
Peroxisome.
Roughly spherical with a crystalline core.
Single membrane sacs involved in metabolic reactions where H is transferred to H2O2.
Break down fatty acids into smaller molecules.
In the liver, they detoxify alcohol.
H2O2 is a poison and it is broken down into water by enzymes.
Glyoxysomes.
A type of peroxisomes found in plant seeds.
Convert fatty acids into sugars to provide energy to the emerging seedling.
Mitochondrion.
Sacs consisting of two membranes; inner membrane is folded to form crista and encloses
matrix.
Site of most reactions of cellular respiration.
In mammals, 5-10 circular DNA molecules.
Proplastids.
All plastids develop from proplastids, which are small, pale green or colorless organelles of
the size of a mitochodrion.
Proplastids are formed from the division of proplastids or mature plastids.
Chloroplast.
A type of plastid.
Double membrane structure enclosing internal thylakoid membranes.
Chloroplasts contain chlorophyll in thylakoid membranes.
Thylakoids are arranged into stacks called grana.
Fluid outside the thylakoids is called the stroma.
Chlorophyll captures light energy.
ATP and other energy-rich compounds are formed and then used to convert C02 to glucose.
Chromoplasts.
A type of plastid.
Found in plants cells.
Contain pigments that give flowers and fruits their characteristic color.
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Leucoplasts.
A type of plastid.
 Amyloplasts synthesize starch.
 Elaioplasts synthesize plant oils.
CYTOSKELETON OF THE CELL
Microtubules.
Hollow tubes made of subunits of the protein tubulin. Reversible assembling.
Provide structural support to the cell.
Provide anchorage to organelles and allow organelles to move within the cytoplasm.
Guide the movement of chromosomes when the cell divides.
Important in the movement of cilia and flagella.
Components of centrioles and basal bodies.
Microtubule associated proteins (MAP) of two kinds have been identified:
1. Fibrous MAPs cross-link microtubules so they form bundles that help to form the shape
of cells.
2. Motor MAPs use energy (ATP) to produce movement and transport of organelles within
the cell.
3. Some motor proteins: kinesin and dynein move organelles along a static microtubules
but in different direction; dynactin appears to bind to both microtubule and dynein.
Microfilaments.
Solid, helical rods of the protein actin. Reversible assembling.
Pinching in of mother cell during animal cell mitosis.
Involved in the changing of shape of cells and their movement, e.g. amoeboid movements.
Interact with other filaments to make cells contract, e.g. muscle cells.
Extend or reduce microvilli on the surface of the cell.
Intermediate filaments.
Fibers made of polypeptides.
Abundant in parts of the cell subject to mechanical stress; for strengthening the cell.
Form the nuclear lamina inside the nuclear envelope; involved in disassembling and
reassembling of the nuclear membrane during mitosis.
Fixes the nucleus in a more or less permanent position.
Probably irreversible assembling of polypeptides.
Centrosomes.
Clouds of cytoplasmic material that in animal cells contains the centrioles.
Also known as the microtubule-organizing center (MTOC).
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Centrosomes present in both animal and plant cells.
Centrioles.
Found in the centrosome of animal cells; at right angles to each other.
Nine sets of three microtubules forming a hollow tube; 9 x 3 structures.
Function is unknown.
Absent in plant cells.
Cilia.
Relatively short projections extending from the surface of the cell.
Usually many present on the surface.
Covered by plasma membrane.
Made of two central and nine peripheral microtubules (9 + 2 structures).
Movement of some single-celled organisms.
Use to move materials on the surface of some tissues, e.g. in animals, lining of internal ducts of
the body.
Flagella.
Long projections, usually one or few.
Covered by plasma membrane.
Made of two central and nine peripheral microtubules (9 + 2 structures).
Movement of some single-celled organisms and gametes.
EXTRACELLULAR MATRIX (ECM).
Most eukaryotic cells are surrounded by a glycocalix, or cell coat, formed by polysaccharide
side chains of proteins (glycoproteins) and lipids (glycolipids) that are part of the plasma
membrane.
Molecules of the ECM allow cells to recognize one another, make contact and form
associations. Other molecules contribute to the mechanical strength of tissues.
Animal cells have a glycocalyx gel made of carbohydrates and fibrous proteins.
Collagen, a glycoprotein, is the main structural protein of the ECM.
Fibronectin proteins are bound to receptor proteins called integrins that are part of the plasma
membrane.
Fibronectins bind to collagen.
Integrins cross the plasma membrane and bind to the cell cytoskeleton, making integrins
capable of transmitting changes in the ECM to the cytoskeleton.
Changes in the ECM may influence activity of genes in the nucleus by triggering a combination
of mechanical and chemical pathways.
Most bacteria, fungi and plant cells are surrounded by a cell wall.
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Multiple layers of the polysaccharide cellulose make plant cell walls.
Other polysaccharides are also involved in making the cell wall.
The middle lamella is a layer of polysaccharides called pectins located in between the cell
walls of adjacent cells.
It serves to glue cells together.
Young plant cells secrete a flexible primary cell wall that is hardened when the cell stops
growing by the addition of certain substances into the primary cell wall.
Some plant cells add several layers of strong secondary cell wall between the plasma
membrane and the primary cell wall
Wood consists mostly of secondary cell wall.