Notes -The Cell
Cell: the smallest entity that retains the properties of life.
Cytoplasm: cellular material outside the nucleus and inside the plasma membrane; contains cytosol, organelles, and inclusions
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Cytosol: semitransparent fluid that suspends other elements; composed of water, nutrients, and dissolved substances
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Organelles: metabolic machinery
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Inclusions: chemical substances that may or may not be present ex fat droplets, melanin, mucus, etc.
Organelles: an internal membrane bound sac or compartment that serves one or more specialized functions inside the cells. Found in eukaryotic cells
1.
nucleus: localizes the cell’s DNA which contains the instructions for building proteins; surrounded by a double-membrane system with two lipid layers – nuclear envelope ; nucleolus contains a dense cluster of RNA and proteins that will be assembled into subunits of ribosomes; chromatin – cell’s collection of DNA together with all the proteins associated with it. Each chromosome is one DNA molecule and its associated proteins, regardless of whether it is threadlike or condensed form, doesn’t always look the same during the life of a cell
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ribosomes: assembles polypeptide chains;
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endoplasmic reticulum : routing and modifying the newly formed polypeptide chains; also synthesizing lipids; starts at the nucleus and curve through the nucleus rough – polypeptide chains are assembled on the ribosomes; smooth – main site of lipid synthesis.
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golgi body : modifies polypeptide chains into mature proteins; sorting and shipping proteins and lipids for secretion or for use inside the cell.
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vesicles : transports or stores variety of substances and structures within the cell; other functions
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mitochondria: double membrane system that creates two distinct compartments in the mitochondria; used for ATP formation; produces many ATP molecules in highly efficient fashion.
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cytoskeleton: imparts shapes and internal organization to cell; moves the cell and its internal structures; microtubules and microfilaments
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lysosome: type of vesicle that is used for intracellular digestion, speeds the breakdown of complex carbohydrates, proteins, nucleic acids, and some lipids.
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peroxisomes : break down fatty acids and amino acids.
10. flagella, cilia and pseudopods : specialized organisms for cell movement
Cell Membrane:
Phospholipid bilayer – boundary that bars the free passage of water-soluble substances into and out of the cell.; diverse proteins embedded in the phospholipid bilayer – serve as passive channels for water-soluble substances. phosphate containing head (hydrophilic) and a fatty acid
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tail (hydrophobic); transport electron or pump substances across the cell membrane, receptors for hormones and other cell signaling molecules
Fluid mosaic model of cell membrane : cell membranes are a mixed composition – a mosaic - , the membrane is fluid owing to the motions and interactions of its component parts.
Membrane proteins
1.
transport proteins: allow water soluble substances to move through their interior, which spans the bilayer. - globular
2.
Receptor proteins: bind extracellular substances such as hormones that trigger changes in cell activities. - globular
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Recognition proteins: at the cell surface are molecular fingerprints - globular
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adhesion - fibrous microvilli
– tiny fingerlike projections that greatly increase the cell’s surface area for absorption
Membrane junctions
 tight junctions : adjacent plasma membranes fuse together tightly like a zipper; impermeable; leakproof; prevent substances from passing through the space between cells ex. In small intesting to prevent digestive enzymes from getting into the blood stream
 desmosomes : anchoring junctions that prevent cells subjected to mechanical stress from pulling apart ex. Skin cells
 gap junctions : allows communication between cells ex. Embryonic cells and heart cells
Transport through the cell membrane passive mechanisms:
1. diffusion
– process by which molecules become scattered or are spread spontaneously from regions of high concentration to regions of low concentration to achieve a state of uniform distribution of molecules (equilibrium); random motion
2. facilitated diffusion – a carrier protein molecule is used to transfer molecules from an area of high concentration to an area of low concentration, ex. glucose molecule combines with a specific carrier protein molecule at the surface of the cell membrane, this compound is soluble in lipid and diffuses to the other side, the glucose is released and the carrier molecule returns to the original side to pick up another glucose molecule; insulin (hormone) promotes facilitated diffusion of glucose through the membranes of certain cells; rate of facilitated diffusion is limited by the number of carrier molecules
3. osmosis – diffusion of water molecules from a region of higher concentraton to a region of lower concentration; the membrane involved is selectively permeable (allows water to pass through readily but not glucose molecules); the amount of pressure needed to stops osmosis is osmotic pressure ; potential pressure due to the presence of nondiffusable solute particles in the solution; water tends to diffuse toward the region of greater osmotic pressure hypertonic
– solution with a greater number of solute particles outside the cell
(higher osmotic pressure) water will diffuse from the cell and it will begin to shrink
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hypotonic – solution in which the number of solute particle is greater inside the cell, water will enter the cell and is will swell, if RBC are placed into distilled water they will hemolyze isotonic – solution that contains the same concentration of solute particles as a cell, if RBC are placed into a 0.9 % NaCl solution (saline) the cells will remain unchanged
4. filtration – molecules are forced through membranes by hydrostatic pressure (blood pressure) that is greater on one side of the membrane than on the other; hydrostatic pressure is created by the weight of the water due to gravity
Active mechanisms
1. Active transport – movement of substances from areas of low concentration to areas of high concentration; involves specific carrier molecules found in cell membranes; proteins that have binding sites; sugars and amino acids and ions are carried across by active means
2. endocytosis: molecules or other particles that are too large to enter by diffusion or active transport enter by means of a vesicle formed by a portion of the cell membrane pinocytosis – process by which cells take in tiny droplets of liquid from their surroundings phagocytosis – process by which the cells take in a solid particle
Cell Reproduction
Mitosis – two phases 1). Division of the nuclear parts 2). Division of the cytoplasm
The Cell Cycle is the sequence of growth, DNA replication, growth and cell division that all cells go through. Beginning after cytokinesis, the daughter cells are quite small and low on ATP. They acquire ATP and increase in size during the G1 phase of Interphase. Most cells are observed in
Interphase, the longest part of the cell cycle. After acquiring sufficient size and ATP, the cells then undergo DNA Synthesis (replication of the original DNA molecules, making identical copies, one "new molecule" eventually destined for each new cell) which occurs during the S phase. Since the formation of new DNA is an energy draining process, the cell undergoes a second growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is used in cell division (in this case mitosis).
Mitosis
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Prophase is the first stage of mitosis proper. Chromatin condenses (remember that chromatin/DNA replicate during Interphase), the nuclear envelope dissolves, centrioles (if present) divide and migrate, kinetochores and kinetochore fibers form, and the spindle forms
Metaphase follows Prophase. The chromosomes (which at this point consist of chromatids held together by a centromere) migrate to the equator of the spindle, where the spindles attach to the kinetochore fibers.
Anaphase begins with the separation of the centromeres, and the pulling of chromosomes (we call them chromosomes after the centromeres are separated) to opposite poles of the spindle.
Telophase is when the chromosomes reach the poles of their respective spindles, the nuclear envelope reforms, chromosomes uncoil into chromatin form, and the nucleolus (which had disappeared during Prophase) reform. Where there was one cell there are now two smaller cells each with exactly the same genetic information. These cells may then develop into different adult forms via the processes of development.
Cytokinesis is the process of splitting the daughter cells apart. Whereas mitosis is the division of the nucleus, cytokinesis is the splitting of the cytoplasm and allocation of the golgi, plastids and cytoplasm into each new cell.
Cell differentiation – process by which cells develop different characteristics in structure and function; this involves the expression of DNA information (protein synthesis)
Protein synthesis
DNA – master blueprint for protein synthesis; gene – DNA segment that carries the information for building one protein or polypeptide chain
Messenger RNA – carries the message containing instructions for protein synthesis from the
DNA in the nucleus to the ribosome in the cytoplasm
Ribosomal RNA – helps form the ribosomes where proteins are built
Transfer RNA – transfers amino acids to the ribosome; about 20 different amino acids, 20 different transfer RNA
Transcription: involves the transfer of information from the DNA’s base sequence into the complementary base sequence of mRNA
Translation: the language of nucleic acids is translated into the language of proteins (amino acid sequence)
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