Review Of Unit One, Cells

I.B.

Biology – Unit 1, Cells The Cell theory: 1. The cell is the basic unit of life 2. All living things are composed of cells 3. Cells can only come from other (pre-existing) cells Contributors to cell theory Jansen

Time Aprox. 1590

Contribution

Hooke

1663

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Van Leeuwenhoek

1675

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 Oken

1805

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Brown

1833

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Schleiden

1838

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Schwann

1839

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Virchow

1855

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Pasteur

1860

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between and cells:

Developed 1st compound microscope Named the cell Examined Cork cells under microscopes Ground lenses to make powerful microscopes (aprox. 200X) Observed blood cells, and bacteria, and protozoa. “All living things originate from and are made of cells” Discovered and named the nucleus Studied plant cells All plant cells had a nucleus Studied animal cells All animal cells had a nucleus “All cells come from preexisting cells” Did experiments to disprove spontaneous generation Differences plant cells animal

Plant Cells  Have  Have  Have  Have  Have

a cell wall chloroplasts no flagellum a larger central vacuole no centrioles

Animal Cells  Have no cell walls  Have no chloroplasts  Can have a flagellum  Smaller vacuole, if present  Have centrioles

Differences between prokaryotic and eukaryotic cells:

Prokaryotic  Have pili  No nucleus  DNA is in cytoplasm and it is “naked”  Reproduce using binary fission (Pg. 215 figure 12.10)  No formal organelles (e.g. mitochondria, E.R., etc…)  Have 70S ribosomes

Eukaryotic  Have no pili  Have a nucleus  DNA is in nucleus and associated with proteins  Divide using mitosis/meiosis  Have membrane bound organelles  Have 80S ribosomes

Phospholipids: A phospholipid is an amphipatic molecule, meaning that is has both a hydrophilic region (dissolves in water) and a hydrophobic region (does not dissolve in water). At the surface of a cell, phospholipids are arranged in a bilayer, or double layer. The hydrophilic heads of the molecules are on the outside of the bilayer, in contact with the aqueous solutions inside and outside of the cell. The hydrophobic tails point toward the interior of the membrane, away from the water. The phospholipid bilayer forms a boundary between the cell and its external environment.

Cell Membrane: The fluid mosaic model How do proteins fit into the cell membrane?

Unlike

proteins dissolved in the cytosol, membrane proteins are not very soluble in water . Membrane proteins have hydrophilic and hydrophobic regions ; they are amphipathic, as are their phospholipid partners in membranes. If proteins were layered on the surface of the membrane, their hydrophobic parts would be in an aqueous environment. Proteins are placed in a location compatible with their amphipathic character. Membrane proteins are dispersed and individually inserted into the phospholipid bilayer, with only their hydrophilic regions protruding far enough from the bilayer to be exposed to water. This molecular arrangement would maximize contact of hydrophilic regions of proteins and phospholipids with water while providing their hydrophobic parts with a non aqueous environment. According to this model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids ; hence the term fluid mosaic model. What is meant by selectively permeable? Selectively permeable means that the cell membrane can select what goes in and out, such as water or ions. For example the hydrophobic core of the membrane impedes the transport of ions and polar molecules, which are hydrophilic. Cell membranes also use transport proteins to allow certain molecules to pass through it. Each transport protein is specific for the substances it translocates (moves), allowing only a certain substance of closely related substances to cross the membrane. For example, glucose carried in blood to the human liver enters liver cells rapidly through specific transport proteins in the plasma membrane. How is the Membrane a fluid mosaic model? Fluid = can flow/has movement Mosaic = Made of different components The cell membrane has a hydrophobic as well as a hydrophilic region because it’s a phospholipid bilayer (made up of phospholipids). The cell     

membrane contains proteins Transmembrane proteins (span the bilayer) Integral proteins –only inside the membrane Peripheral proteins Glyco proteins (act as a receptor/landing sites Cholesterol inside the bilayer (cholesterols makes the bilayer fluid)

How things go in and out of the cell 1)Passive transport

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movement of molecules without spending energy movement DOWN concentration gradient

a) Simple Diffusion  movement of molecules down concentration gradient through cell membrane ex: 02 and other small molecules b) Osmosis  diffusion of water through a membrane. Water moves from where there is more (dilute solution) to where there is less water (high concentration) Osmotic Conditions: • Isotonic solution: same concentration • Hypertonic solution: solution is more concentrated • Hypotonic solution : solution that is less concentrated c) Facilitated diffusion: Specialized transport proteins allow particles to diffuse Proteins are highly selective, based on size, shape, and charge. Carrier proteins (ferry) :  Protein that “carries” substances across Channel proteins (tunnel):  Tunnel for particles  Usually charged particles 2)Active transport: Active transport is when transport proteins can move solutes against their concentration gradients, across the plasma membrane from the side where they are less concentrated to the side where they are more concentrated. This transport is “uphill” and therefore requires work. To pump a molecule across a membrane against its gradient, the cell must expand its own metabolic energy ; therefore, this type of membrane traffic is called active transport. Active transport is a major factor in the ability of a cell to maintain internal concentration of small molecules that differ from concentrations in its environment. For example, compared to its surroundings, an animal cell has a much higher concentration of potassium ions and a much lower concentration of sodium ions. The plasma membrane helps maintain these steep gradients by pumping sodium out of the cell, and potassium into the cell. One example of this happening is the sodium-potassium pump, which exchanges sodium for potassium across the plasma membrane of animal cells. The membrane potential, the voltage across a membrane, acts like a battery, an energy source that affects the traffic of all charged substances across the membrane. Because the inside of the cell is negative compared to the outside, the membrane potential favours the passive transport of cations into the cell, and anions out of the cell,. Thus, not one (as in simple diffusion) but two forces drive the diffusion of ions across the membrane: a chemical force (the ion’s concentration gradient) and an electrical force (the effect of the membrane potential on the membrane potential on the ion’s movement). This combination of forces acting on an ion is called the electrochemical gradient. An ion does not simply diffuse down its concentration gradient, but diffuses down its electrochemical gradient. For example, the concentration of sodium ions (Na+) inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels that facilitate Na+ diffusion will open. Sodium ions then “fall” down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of cations to the negative side of the membrane, resulting in concentration of Na+ building up on one side of the membrane. Some membrane proteins that actively transport ions contribute to the membrane potential. For example, the aforementioned sodium-potassium pump. The pump does not actually translocate Na and K one for one, but actually pumps three sodium ions for every two potassium ions it pumps into the cell. With each crank of the pump, there is a net transfer of one positive charge from the cytoplasm to the extracellular fluid, a process that stores energy in the form of voltage. Large molecules, such as proteins, generally cross the membrane by a different process than their smaller compatriots. They become transported by either exocytosis or endocytosis. (For the purpose of this question, we will not explore exocytosis). In endocytotsis, the cell takes in macromolecules and particulate matter by forming new vesicles from the plasma membrane. A small area of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell. Phagocytosis, and pinocytosis are both types of endocytosis. In phagocytosis, a cell engulfs a particle by wrapping pseudopodia (a cellular extension of amoeboid cells used in moving and feeding.) around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lyosome containing hydrolytic enzymes. In pinocytosis, the cells “gulps” droplets of extracellular fluid in tiny vesicles. Because any and all solutes dissolved in the droplet are taken into the cell, pinocytosis is unspecific in the substances it transports. As can be seen, active transport is very important towards cell function. Exocytosis: See figure 8.7 on page 135 Exocytosis is when the cell secretes macromolecules by the fusion of vesicles with the plasma membrane (exportation). A transport vesicle budded from the golgi apparatus is moved by the cytoskeleton to the plasme membrane. When the vesicle membrane and the plama membrane come into contact, the lipid molecules of the two bilayers rearrange themselves so that the two membranes fuse. The contents then spill to the outside of the cell.

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EX. Certain cells in the pancreas manufacture the hormone insulin and secrete it into the blood cells by way of exocytosis.

Endocytosis: See figure 8.18 on page 144 Endocytosis is when the cells take in macromolecules and particulate matter by forming new vesicles from the plasma membrane, basically the reverse of exocytosis. A small area of the plasma membrane sinks inward to form a pocket. As the pocket deepens, it pinches in, forming a vesicle containing ,material that had been outside of the cell. There are three types of endocytosis: 1. Phagocytosis 2. Pinocytosis 3. Receptor mediated endocytosis *Number 1 and 2 were explained in active transport, number three we do not need to know Some Quick definitions : Diffusion: is the spontaneous tendency of a substance to move down its concentration gradient from a more concentrated area, to a less concentrated area. Solute: A substance that is dissolved in a solution Solvent: The dissolving agent of a solution. Water is the most versatile solvent known. Solution: A homogeneous, liquid mixture of two or more substances. Osmosis: The diffusion of water across a selectively permeable membrane. Hypertonic: A solution with a greater solute concentration than another, hypotonic solution. Hypotonic: A solution with a lesser solute concentration than another, hypertonic. Isotonic: Solution of equal solute concentration. Osmotic pressure: A measure of the tendency of a solution to take up water when separated from pure water by a selectively permeable membrane. Flaccid: Limp; walled cells are flaccid in isotonic surroundings, where there is no tendency for water to enter. Turgid: Firm; walled cells become turgid Cytolysis: cell lysis; occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell. Lysis: The disintegration or rupture of the cell membrane, resulting in the release of cell contents or the subsequent death of the cell.