9/17/14 The Cells !  The early studies with microscopes already established that cells are quite small. !  It also led to the cell theory, established in the 1800’s, which states that: !  All living things are composed of cells and !  All cells come from previous cells The Cells !  So what was the going philosophy about life in general before the cell theory was established ( prior to the studies done with microscopes ) ? !  Many theories existed and the most popular, Spontaneous Genera-on (first proposed by Aristotle (384-­‐322 BCE)), sSpulated that all life arose abruptly from inanimate maTer (meat yields maggots, roUng wood generates mushrooms,…). Jean BapSst Van Helmont (1580-­‐1640) promoSng the idea with his suggesSon that mice come from dirty rags and roUng grains 1 9/17/14 The Cells !  The first person to challenge the idea of Spontaneous GeneraSon was Francesco Redi (1620-­‐1690). Redi performed a now famous 'meat/maggot' experiment. The Cells •  It took the work of Louis Pasteur to completely disprove the idea that non-­‐living things can produce living organisms. 2 9/17/14 The Cells !  Pasteur proposed the germ theory and those germs is what we now commonly call the bacteria !  Remember that Bacteria and Archaea are prokaryo5c cells (cells without a real nucleus) !  All other forms of life are eukaryo5c cells (cells with a true nucleus) The Cells •  Both prokaryoSc and eukaryoSc cells have a plasma membrane that separates intracellular environment (cytoplasm) from the extracellular medium •  The cells should be large enough to house DNA, proteins, and structures needed to survive and reproduce, but •  Need to remain small enough to allow for a surface-­‐to-­‐
volume raSo that will allow adequate exchange with the environment. 3 9/17/14 The Cells •  As an example : •  A red blood cell has a diameter of ~ 8 micrometers and a volume of ~ 100 micrometer3 •  Other human cells have volumes close to 1000 micrometer3 •  1 micrometer3 = 1 liter/1015 Plasma Membrane "  The cell membrane (plasma membrane) surrounds all living cells and controls the movement of molecules into and out of the cell. "  It is thus a gate keeper capable of determining what can be passed back and forth into the cell "  But some components/molecules can pass through a cell membrane with no problem depending on some basic physical laws. Cell boundary Cell Membrane Human cheek cells 4 9/17/14 Plasma Membrane "  This trait of the cell membrane is called being selec5ve permeability or being semi-­‐permeable "  The structure of the membrane with its component molecules is responsible for this characterisSc "  Cell Membranes are made of lipids, proteins, and some carbohydrates, but the most abundant molecule that forms the basic backbone of the membrane are the phospholipids Plasma Membrane Phosphate
group
Hydrophilic head
Symbol
Remember from Chapter 2 that phospholipids are
modified tri-glycerides.
A glycerol molecule with two long fatty acids and a
charged (phosphate-nitrogen) group on the other end.
This thus creates a water-hating tail portion (the
hydrophobic tails) with water-loving head (the
charged, hydrophillic region) on top.
Hydrophobic tails
5 9/17/14 Plasma Membrane Phosphate
group
Hydrophilic head
Symbol
Hydrophobic tails
If you would mix millions of these molecules in
water, they would not organize themselves into a
mono-layer of phospholipids since the hydrophobic
tails don’t like to be exposed to water.
Two mono-layers will thus combine to form a bi-layer,
with the hydrophobic tails facing each other, excluding
as much water as they can !
The hydrophillic heads are the ones in contact with
the water.
Plasma Membrane #  All biological membranes (such as the plasma membrane) are formed like a two-­‐layer sheet of these molecules called a phospholipid bilayer –  Hydrophilic heads face outward, and hydrophobic tails point inward –  Thus, hydrophilic heads are exposed to water, while hydrophobic tails are shielded from water #  The fact that they are mostly faTy molecules makes them very fluidy and flexible (like oil). 6 9/17/14 Plasma Membrane #  Most of the funcSonality that can be controlled in the cell membranes comes from proteins embedded within this sea of lipid molecules. #  Some proteins span across, some are loosely aTached. But all proteins have a disSnct funcSon. Plasma Membrane Hydrophilic
heads
Outside cell
Hydrophobic
region of
protein
Hydrophobic
tails
Inside cell
Proteins
Hydrophilic
region of
protein
7 9/17/14 Plasma Membrane Outside cell
Hydrophobic
tails
Inside cell
Since most things dissolved in water are hydrophillic, the hydrophobic nature of
the plasma membrane keeps them from entering (or leaving the cell).
The plasma membrane is thus a safety barrier that maintains the cell’s integrity.
The proteins in the membrane will carry out the function of communicating
between inside and outside the cell.
Plasma Membrane For example, ions, carbohydrates
(such as glucose) and amino
acids are mostly hydrophillic and
dissolve easily in water. BUT, the
plasma membrane is mostly
hydrophobic. These substances
can thus not enter the cell unless
specific membrane proteins help
them get across.
Fatty molecules however can
easily penetrate cells since fat
easily dissolves in oily things
( such as the membrane).
8 9/17/14 The Cells •  A basic organizaSon of a “generic cell” is as follows Cell membrane Cytoplasm : the interior fluid of the cell GeneSc material (DNA) Ribosomes : structures where protein is made 9 9/17/14 The Prokaryote Cells !  The basic organizaSon of a prokaryote is not much different from this drawing. !  Prokaryotes are characterized by !  No special membrane structures inside the cell ( no internal organelles) !  DNA is coiled into a nucleoid structure, not surrounded by a membrane and freely floaSng within the cytoplasm !  Prokaryotes have an outer, rigid cell wall that provides shape protects their structural integrity. !  Prokaryotes are mostly unicellular and about 10x smaller than eukaryoSc cells (size is 10 micrometer or smaller) The Prokaryote Cells 10 9/17/14 The Eukaryote Cells Pro-­‐karyote means “before having a true nucleus” : thus, no membraneous structure around the nucleus ( and no other organelles). It is referred to as the nucleoid region Eu-­‐karyote actually translates as “having a true nucleus” : this refers to the fact that the nucleus is an organelle, an internal membraneous structure where the chromosomes (DNA) are located. The Eukaryote Cells !  Thus what makes eukaryotes different from prokaryotes is the presence of internal membraneous organelles !  These organelles funcSon as internal, compartmentalized areas, where specific funcSons are executed. !  This allowed cells to be more efficient in metabolism and resulted into larger cells with cooperaSvity ( yielding the mulScellular organisms). 11 9/17/14 The Eukaryotic Organelles Four basic life processes in eukaryoSc cells depend upon organelles and other cellular structures . These can someSmes be compared with a well running business operaSon. 1)  ExecuSve control and execuSon of the blue print •  Starts with the DNA in nucleus and directed towards the ribosomes for protein manufacturing 2)  Manufacturing, distribuSon and re-­‐organizaSon •  Involves endoplasmic reSculum, Golgi apparatus, lysosomes, vacuoles, peroxisomes 3)  Energy plant to keep things running •  Mitochondria ( and chloroplasts in plants) 4)  Structural support, movement, communicaSon •  Cytoskeleton, cell membrane, chemicals,… NUCLEUS:
Nuclear envelope
Smooth endoplasmic
reticulum
Chromosomes
Nucleolus
Rough
endoplasmic
reticulum
Lysosome
Centriole
Ribosomes
Peroxisome
Golgi
apparatus
CYTOSKELETON:
Microtubule
Intermediate
filament
Microfilament
Plasma membrane
Mitochondrion
12 9/17/14 The Nucleus: genetic control center !  The nucleus controls the cell’s acSviSes and is responsible for inheritance •  Inside is a complex of proteins and DNA called chroma5n, which makes up the cell’s chromosomes •  DNA is copied within the nucleus prior to cell division !  The nuclear envelope is a double membrane with pores that allow material to flow in and out of the nucleus •  It is aTached to a network of cellular membranes called the endoplasmic reSculum The Nucleus: genetic control center Two membranes of nuclear envelope Nucleus Nucleolus Chroma5n Pore Endoplasmic re5culum Ribosomes 13 9/17/14 Endoplasmic Reticulum !  There are two kinds of endoplasmic reSculum—smooth and rough: both are contained by a membrane. !  In Rough ER, the outer surface of the membranes are lined with ribosomes !  Smooth ER lacks aTached ribosomes Nuclear envelope Smooth ER Ribosomes Rough ER 14 9/17/14 Endoplasmic Reticulum !  The funcSon of Rough ER is : !  To make proteins (with the help of the ribosomes) desSned for secreSon !  Once proteins are synthesized, they are transported in vesicles to other parts of the endomembrane system !  To make addiSonal membrane for itself !  Smooth ER is involved in a variety of diverse metabolic processes !  For example, enzymes produced by the smooth ER are involved in the synthesis of lipids, oils, phospholipids, and steroids Ribosomes !  Ribosomes are not surrounded by membranes. They are crucial in the cell’s protein synthesis !  Ribosomes are synthesized within the nucleus !  Cells that must synthesize large amounts of protein have a large number of ribosomes ( like the rough ER) !  Ribosomes usually occur with the rough ER but can also occur freely, suspended in the cytoplasm 15 9/17/14 Rough ER !  So what happens at the Rough ER ? 1)  mRNA, produced by nucleus and containing the geneSc informaSon how to make a specific protein, is directed towards the ribosomes of the RER 2)  The ribosomes start making the polypepSde which is threaded via a cavity in the membrane into the inside of the RER 3)  Once the complete polypepSde is inside, it folds into its 3-­‐dimensional shape ; some sugar chains may be added to form a glyco-­‐protein 4)  When ready, it becomes isolated in a vesicle 5)  Vesicle pinches off and is routed towards the Golgi apparatus for more processing. Transport vesicle buds off 4 Ribosome Secretory protein inside trans-­‐ port vesicle 3 Sugar chain 1 Polypep5de 2 Glycoprotein Rough ER 16