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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.

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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