Cells : Life's Basic Units

SC 135 Cells [Notes]

	I. Unifying themes that relate to all cells
	   A. All cells share basic FUNCTIONS performed by physical
	      STRUCTURES. The universality of these functions and
		 structures is a strong argument for the unity of life.
	   B. Each cell's specialization depends on its shape, size,
		 and organelles.
	   C. The life of a cell depends on the arrangement of its
		 parts and their integrated activities.
	   D. Linnaeus was one of the first to recognize that 
		 patterns of structure and development indicated rela-
		 tionships between groups of organisms.
	   E. The development of the electron microscope and the
		 ultracentrifuge enabled biologists and chemists to see
		 patterns within the structure and chemistry of cells 		
                                 and thus to develop generalizations regarding the
		 relationships between and among cells and organisms.
	   F. Chemists discovered that the chemistry of living 
		 organisms did not differ in principle from the chemi-
		 stry of the inorganic world. Biochemistry was thus a
		 complex and fascinating branch of chemistry as applied
		 to living systems.

    II. Fundamental features of cellular organization
	   A. Compartmentalization - a property which allows the
		 "effective" size of an object to increase without
		 unduly affecting the surface area to volume relation-
		 ship (S2V3)
		1. allows for increased surface area by subdividing
		   sections of the cell
		2. limits certain cell constituents to certain parts of
		   cell (destructive hydrolytic enzymes)
		3. makes it possible to concentrate substances in 
		   specific regions where specialized biochemical 
		   activities can occur
	   B. Self Assembly - spontaneous arrangement of molecules
		 into specific structures; requires no additional 
		 energy input and is often the result of chemical 
		 bonding; e.g. phospholipids into bilayers in mem-
		 branes; proteins into tubules and filaments
	   C. Repitition - cells from pre-existing cells; thus
		 patterns of cellular organization are repeated from
		 one generation to another
	   D. An Inherent Genetic Program - embodied in cell's DNA
		 each gene represents a segment of a DNA strand:
		 DNA--->RNA--->polypeptides (proteins) is usual 
		 sequence; The program of an organism represents a plan
		 for action - a blueprint which provides a set of 
		 instructions to be used under specific circumstances,
		 in a sequence that may be triggered by the operation
		 of the program itself

	   E. Redundancy - represented by multiple copies of mole-
		 cules' (e.g. enzymes); multiple copies of the genetic
		 information; All body cells of the organism contain
		 identical genetic information. Given the correct 
		 chemical signals and environmental conditions, the 
		 cell is capable of differentiation in several possible

   III. Microscopes were developed in 16th and 17th century 
	   Europe. Robert Hooke discovered cells in cork. Other
	   scientists concluded that all living things are made of
	   cells and that living cells arise only from preexisting
	   cells. Non scientists like Leeuwenhoek made important

    IV. The most fundamental differences in the living world are
	   differences between prokaryotic and eukaryotic cells.
	   A. The Kingdom Monera includes prokaryotes.
		1. Prokaryotes lack a membrane-bound nucleus; the DNA
		   is a naked strand in a region called the nucleoid.
		2. Prokaryotes do not have other membrane-bound
	   B. The other four kingdoms, Protista, Fungi, Plantae, and
		 Animalia, are eukaryotic.
		1. Like prokaryotes, eukaryotic cells are surrounded by
		2. Eukaryotic cells have internal membranes that
		   surround the nucleus and other internal structures.
		3. Eukaryotes have a highly organized CYTOPLASM with an
		   internal latticework, the CYTOSKELETON, that contri-
		   butes to structure and movement within the cell.
		4. In multicellular eukaryotes, cells may specialize
		   for given tasks.
		5. Common features of plant and animal cells include a
		   a. Plant cells may be surrounded by a CELL WALL and
			 may be largely filled by a storage organelle, 
			 the VACUOLE.
		   b. Plant cells may contain CHLOROPLASTS, which 
			 carry out photosynthesis.
		   c. Cellular specialization leads to specialization
			 of tissues, organs, and organisms.

	V. Cells vary in size and number in organisms
	   A. Most cells are small. Prokaryotes are the smallest
		 cells. The largest animal cells include ostrich eggs
		 and giraffe nerve cells.
	   B. The surface-to-volume ratio (S/V) determines a cell's
		 ability to exchange materials with its environment. A
		 cell's shape affects it S/V.

    VI. All cells must perform certain tasks.
	   A. The cytoplasm must be separated from its environment
		 so appropriate internal conditions can be maintained.
	   B. The cell must take in raw materials and expel wastes 
		 through the barrier that separates it from its
	   C. It must take in energy and convert it to a form useful
		 for powering the cellular machinery.
	   D. It must synthesize molecules and cell parts for repair
		 growth and replacement (reproduction).
	   E. It must coordinate and regulate its activities.

   VII. All eukaryotic cells have certain structures.
	   A. The PLASMA OR CELL MEMBRANE separates the cell from
		 its environment and controls the movement of
           substances into and out of the cell.
		1. The membrane is a LIPID BILAYER composed of PHOSPHO-
		   LIPID molecules with hydrophilic heads and hydro-
		   phobic tails. It is SEMIPERMEABLE.
		2. The membrane includes proteins that stabilize the
		   lipid bilayer and acts as gates, pumps, markers, or
		   signal receptors.
		3. The membrane may fold inward to import materials
		   (endocytosis) or outward to expel materials from
		   the cell (exocytosis).
	   B. The NUCLEUS is the largest organelle and contains the
		 genetic material, DNA.
		1. The genetic information passes from DNA to RNA to 
		   proteins, which carry out the work of the cell.
		2. The NUCLEOPLASM of the nucleus is surrounded by a
		   double-layer membrane, the NUCLEAR ENVELOPE OR 
		   MEMBRANE, which is perforated by pores. Each pore is
		   a cluster of proteins that form a channel.
		3. RIBOSOMAL RNA is formed in the dark-stained regions
		   known as NUCLEOLI. After being exported to the cyto-
		   plasm, this RNA forms RIBOSOMES, which build protein
		   according to the genetic blueprints encoded in
		   messenger RNA.
	   C. The cytoskeleton is a 3-dimensional latticework com-
		 MEDIATE FILAMENTS that maintain the cell's shape and
		 move materials within the cell.
	   D. A system of INTERNAL MEMBRANES is involved in the
	      manufacture, storage, transport, and export of pro-
		 teins and raw materials.
		1. The ENDOPLASMIC RETICULUM (ER) is a series of mem-
		   brane channels that may be studded on the outside 
		   with ribosomes (rough ER) for protein synthesis or
		   without ribosomes (smooth ER) and involved in the
		   synthesis of non-proteins such as lipids.
		2. At the ends of the ER channels, membrane sacs 
		   (vesicles) pinch off and carry the products to their
		   destination, which may be another membrane system,
		   the GOLGI COMPLEX, which modifies and packages
		   proteins, lipids, and other substances and exports
		   most of them from the cell.
		3. Lysosomes contain digestive enzymes that break down
		   ingested food or, if broken open, digest the cellu-	
		   lar components.
	   E. MITOCHONDRIA provide chemical fuel for cellular 
		 processes by converting the energy in carbon-contain-
		 ing molecules into the energy of ATP molecules. This
		 process is called AEROBIC RESPIRATION.
	   F. Plastids harvest solar energy and produce and store
		1. CHLOROPLASTS are the organelles of photosynthesis,
		   converting solar energy into chemical energy in the
		   form of carbon compounds. Chloroplasts contain
		   CHLOROPHYLL and other light-absorbing pigments
		   embedded in membranous sacs called THYLAKOIDS.
		2. CHROMOPLASTS are plastids that store yellow, red,
		   or orange energy-trapping pigments and give color
		   to fruits and flowers.
	   G. Many plant cells have a CENTRAL VACUOLE that contains
		 water and various storage products.
		1. This reduces the volume of cytoplasm and thus
		   increases the cell's S/V ratio.
		2. The pressure of the water in the vacuole keeps the
		   cell inflated. A plant wilts when the vacuole pres-
		   sure drops because of drought.
	   H. Many protists that live in fresh water have a CONTRAC-
		 TILE VACUOLE that collects and pumps out water that
		 could otherwise build up and burst the cell.
	   I. Organelles of movement include CILIA, FLAGELLA, and
		 microfilaments and microtubules.
		1. Cilia and flagella have the same internal structure,
		   with 9 pairs of microtubules in a circle surrounding
		   2 inner microtubules.
		2. Using ATP energy, the microtubules slide past one
		   another, causing the structure to bend.
	   J. The plasma membranes of most cells are surrounded by
		 cell coverings that protect the delicate membrane.
		1. An EXTRACELLULAR MATRIX, a meshwork of secreted
		   molecules, protects many cells that live within
		   multicellular organisms.
		2. CELL WALLS are made largely of cellulose and
		   surround plant cells.
		3. Virtually all animal cells secrete a meshwork of
		   molecules that surrounds them. These molecules are
		   mostly fibrous proteins. COLLAGEN is the most common
		   of the fibrous proteins.

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