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3-1
New Unit: Cells
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Some History…
• 1665- Robert Hooke looks at a thin slice of cork under a microscope.
• 1674- Anton van Leeuwenhoek observes small living organisms in pond water
• 1839-Mathias Schleidan concludes that all plants are made up of cells
• 1839-Theodor Schwann concludes that all animals are made up of living cells
• 1855- Rudolph Virchow proposes that all cells come from existing cells
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Cell Theory
• All living things are composed of cells
• Cells are the basic units of structures and function in living things
• New cells are produced from old cells
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Cell Characteristics
• What characteristics do we use to define a cell?
• All cells have two things in common
• 1. The are surrounded by a barrier called a cell membrane
• 2. At some point in their lives the contain genetic material.
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Prokaryotes vs. Eukaryotes
• Cells can be split up in two broad categories, depending on whether they contain a nucleus
• Prokaryotic cells have genetic material that is not contained in the nucleus
• Eukaryotic cells contain a nucleus in which their genetic material is separated from the rest of the cell.
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Prokaryote
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Eukaryote
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Functions of the Cell
• Basic unit of life
• Protection and support
• Movement
• Communication
• Cell metabolism and energy release
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Cell Characteristics
• Plasma Membrane– Outer cell boundary
• Cytoplasm – Cytosol– Cytoskeleton
• Organelles– Specialized structures that perform specific
functions
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Cell Membrane
• ALL cells are surrounded by a thin flexible barrier called the cell membrane
• Many cells have an additional barrier called the cell wall
• The cell membrane regulates what goes in and out of the cell
• The cell membrane is called a lipid bilayer
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Cytoplasm• Cellular material outside
nucleus but inside plasma membrane
• Cytosol: Fluid portion• Cytoskeleton: Supports
the cell– Microtubules
– Microfilaments
– Intermediate filaments
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Organelles
• Small specialized structures for particular functions
• Most have membranes that separates interior of organelles from cytoplasm
• Related to specific structure and function of the cell
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Nucleus
• DNA dispersed throughout• Consists of :
– Nuclear envelope: Separates nucleus from cytoplasm and regulates movement of materials in and out
– Chromatin: Condenses to form chromosomes during cell division
– Nucleolus: Assembly site of large and small ribosomal units
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Centrioles• In specialized zone
near nucleus:• Each unit consists of
microtubules• Before cell division,
centrioles divide, move to ends of cell and become spindle fibers
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Ribosomes
• Sites of protein synthesis
• Composed of a large and small subunit
• Types– Free
– Attached to endoplasmic reticulum
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Endoplasmic Reticulum
• Types– Rough
• Attached ribosomes
• Proteins produced and modified
– Smooth• Not attached ribosomes
• Store enzymes, Manufacture lipids
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Golgi Apparatus
• Modification, packaging, distribution of proteins and lipids for secretion or internal use
• Flattened membrane sacs stacked on each other
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Function of Golgi Apparatus
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Mitochondria
• Provide energy for cell• Major site of ATP
synthesis• Membranes
– Cristae: Infoldings of inner membrane
– Matrix: Substance located in space formed by inner membrane
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Lysosomes
-Small organelles filled with enzymes-Involved in digestion or breakdown of lipids carbohydrates, and proteins.-Also involved in breaking down old organelles, and removing“junk” of the cell.
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Chloroplasts
• Capture energy from sunlight and convert it into chemical energy during photosynthesis
• Found in plants and other organisms.
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Vacuoles• Store materials such as
water, salts, proteins, and carbohydrates.
• Plants use vacuoles as support for heavy structures
• Contracting vacuoles are found in some animals and single celled organisms to pump out excess water.
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Cilia• Appendages
projecting from cell surfaces
• Capable of movement• Moves materials over
the cell surface
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Flagella
• Similar to cilia but longer
• Usually only one exists per cell
• Move the cell itself in wavelike fashion
• Example: Sperm cell
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Microvilli
• Extension of plasma membrane
• Increase the cell surface
• Normally many on each cell
• One tenth to one twentieth size of cilia
• Do not move
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Organelle DNA
• Some organelles have their own DNA
• These include the mitochondria, ribosomes, and chloroplasts
• What implications do you think this has?
• Evolutionary?
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Endosymbiosis
• Proposed by Lynn Margulis in 1970.
• Suggests that mitochondria and chloroplasts were once free living organisms and were taken into other cells as endosymbiotes.
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Cell Membrane
Passive Transport and Active Transport
Inheritance
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Lipid Bilayer
• Two layers of lipids = bilayer
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Lipid Bilayer
• Hydrophobic = water fearing
• Hydrophilic = water loving
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Lipid Bilayer
• Also contains proteins and carbohydrates
• Proteins = channels and pumps
• Carbs = identification
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Lipid Bilayer
• In order to survive, every cell must take in and eliminate substances
• This means the cell membrane must allow certain substances to penetrate it.
• It is called selectively permeable
Functions of the Cell Membrane
• Isolates the cytoplasm from the external environment
• Regulates the flow of materials into and out of the cell
• Communicates with other cells
The Fluid Mosaic Model
• Currently accepted model of the cell membrane
• Phospholipid bilayer– Hydrophilic “head” –
exposed to the outside
– Hydrophobic “tail” – hides inside
• Membrane proteins are randomly dispersed in phospholipid bilayer
Fluidity of the Membrane
• The lipids and proteins can drift throughout the membrane
• Membrane is NOT stiff/rigid
• Cholesterol makes the membrane stronger by limiting the movement of phospholipids
Membrane as a Mosaic
• Lipid bilayer has membrane proteins “stuck” in it
• Integral proteins– Go through the
membrane (both sides)
• Peripheral proteins– attached to the
surface of the membrane
Diffusion
Movement of molecules from high to low concentration until equilibrium is reached.
Passive Transport= no energy required
What substances may diffuse across membrane? Nonpolar (non-charged) molecules; small polar molecules
Diffusion
• Each substance diffuses down its OWN concentration gradient and is unaffected by concentration gradients of other substances
Diffusion
• Does all movement stop once equilibrium is reached??– NO!!
– Equal rates in all directions
Osmosis• Def’n:
– The passive transport of water across a selectively permeable membrane
– Hyper-, hypo-, iso- tonic• RELATIVE TERMS!!
• Always referring to solute concentration
• Water moves from areas of lower concentration of solutes (hypotonic) to areas of higher solute concentration (hypertonic)
Osmosis in Plant and Animal Cells
• Animal Cells:– Plasmolysis
• Occurs when a cell is in a hypertonic solution
– Water goes from cell into solution
– Cytolysis• Occurs when a cell is in a
hypotonic solution– Water goes from solution
into cell
• Plant Cells:– Turgid– Flaccid
Facilitated Diffusion
• Def’n:– The diffusion of large molecules across the cell
membrane using transport proteins– Glucose; ions
• Does NOT require an input of energy– Solute is still moving down its concentration
gradient
Facilitated Diffusion
• Transport proteins are specific for their solutes
• Transport proteins can become saturated
• Some are gated channels:– Chemical or electrical
stimulus causes them to open
Example
• Which direction will sucrose move?
• Which direction will glucose move?
• Which direction will fructose move?
Active Transport
• Def’n:– The pumping of solutes against their gradients– Requires an input of energy by the cell– Used so cells can “stockpile” extra supplies
Electrogenic Pumps
• Voltage across membranes = stored energy that can be used for cellular work
• Sodium-Potassium Pump:– 3 Na+ OUT of the cell for every 2 K+ pumped in– Net transfer of one positive charge from
cytoplasm to ECF
• Very important for transferring signals between nerve cells
Sodium-Potassium Pump
Cotransport
• Substance that has been pumped across a membrane can do work as it “leaks” back by diffusion
• Another substance “hitches a ride”
Endocytosis & Exocytosis
• Def’n:– The movement of large molecules
(polysaccharides, proteins, etc.) across the membrane
– Endocytosis = cell takes in macromolecules– Exocytosis = cell secretes macromolecules
Endocytosis
• Cell takes in macromolecules by forming vesicles made from the plasma membrane
• Phagocytosis = “cell eating”– Large molecules
• Pinocytosis = “cell drinking”– Small molecules & liquids
• Receptor-mediated endocytosis = seeks out specific molecules
Endocytosis
08-19a-PhagocytosisAnim.mov08-19b-PinocytosisAnim.mov
Exocytosis
• The cell secretes macromolecules by the fusion of vesicles with the plasma membrane
• Used to release hormones, chemical signals, etc.
08-19-ExocytosisAnim.mov
Signal Transduction Pathway
• We’ll discuss this later…
• But for now:– The cell’s plasma membrane is an important
player in a cell’s ability to sense and respond to environmental change