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Cells: The Living Unit
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Cells – general • 4 aspects of cell theory –
– A cell is the basic structural & functional unit of life
• Cell properties directly relate to the properties of life
– The activity of an organism depends on the activities of individual and collective cells
– Principle of complementarity –• Activities of cells are dictated by specific structures of
cells
– Continuity (the smooth working) of life is based on a cellular level
• Cells come from pre-existing cells
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Cells – general cont. • Characteristics of cells –
– Cells vary greatly in their size, shape, and function
– All cells are composed primarily of carbon, hydrogen, nitrogen, and oxygen (CHON)
– All cells have the same basic parts & some common functions
– All general cells contain a plasma membrane, cytoplasm, and nucleus
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Cell diversity
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Plasma membrane structure
• Plasma membrane –– Defines the extent of the cell – Separates bodies major fluid components
• Intracellular fluid within cells• Extracellular fluid outside cells
• The plasma membrane is composed of a double layer of phosopholipids embedded with smaller amounts of cholesterol and glycolipids and proteins
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Plasma membrane structure cont.
• Fluid mosaic model =– Membrane structure composed of a double
layer (bilayer) of lipid molecules with protein molecules dispersed in it.
– Proteins which “float” within the bilayer form an ever changing mosaic pattern
• Composed of…– Double layer of phospholipids (lying tail to tail
with their polar heads exposed to the water inside and outside the cell) embedded with cholesterol, proteins, and glycolipids
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Plasma membrane structure cont.
• Surfaces of the plasma membrane –– Outside surfaces of the cell contain more lipids
• Contains glycolipids (sugar lipid)• Helps to make the outer surface more polar
– Inside contains integral proteins, phospholipids, & cytoskeleton
• Phospholipids –– Modified lipid containing phosphorus– 2 components –
• Hydrophilic –– Polar “head” – Has a charge– Water loving
• Hydrophobic – – Nonpolar “tail”– Does not have a charge– Water hating
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P.M.
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Plasma membrane structure cont.
• Plasma membrane proteins –– Integral proteins are firmly inserted into the plasma
membrane• Have both hydrophobic and hydrophilic regions • Some protrude from only one side of the plasma
membrane – Usually involved in receptors for hormones or chemical
messengers or act as chemical relay messengers • Transmembrane proteins span the entire plasma
membrane – Mainly involved in transport– Channel proteins –
» Have a pore in which water soluble molecules pass – Carrier proteins –
» Bind to substances to move them through the membrane
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P.M
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Plasma membrane structure cont.
– Peripheral proteins are not embedded in the plasma membrane, but attach to integral proteins or to phospholipids
• Can be removed w/o disturbing the plasma membrane • Contain filaments that helps support the membrane on
the cytoplasmic side • Some involved in changing the cells shape (during
division)
– The glycocalyx is the fuzzy, sticky, carbohydrate-rich area surrounding the cell
• “sugar covering”• Provides a highly specific biological markers
– Recognize self from non-self
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P.M.
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Specialization of the plasma membrane
• Microvilli – – “shaggy hairs” – Fingerlike extensions of the plasma membrane
that increase the surface area of the cell – Most often found in cells with an absorptive
function – Kidneys and intestines – Contains actin –
• Contractile protein that helps stiffen the microvilli to keep them erect
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Specialization of the plasma membrane cont.
• Membrane junctions –– Many cells are close together and in
tight communities • Glycoproteins act as the cells “adhesive” • Wavy contours of cells edges allow cells to
stick together in a tongue and grove fashion • Special membrane junctions are formed
– Tight Junctions– Desmosomes– Gap Junctions
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Specialization of the plasma membrane cont.
– Tight junctions• Integral proteins on
adjacent cells fuse together to form impermeable junctions
• Prevents molecules from passing through the extracellular space between cells
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Specialization of the plasma membrane cont.
• Membrane junctions cont. -– Desmosomes –
• Anchoring junctions • Scattered like rivets to prevent the cells separation • Held together by linker proteins that extend from the
plaque on the cytoplasmic face• Linker proteins span from one cell to the other &
attach themselves to the other cells plaque• Reduces the chances of ripping the cell when tension
is applied
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Specialization of the plasma membrane cont.
• Membrane junctions cont. –– Gap junctions –
• Communication junction • Allows chemical substances to pass between
adjacent cells • Connected by connexons – hollow tubules
that allow the cells to communicate because their cytoplasm's are so close
• Allows small molecules to pass between two cells
• In electrically charged tissues
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Gap junctions
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P.M. Functions – general
• Cells are “bathed” in interstitial fluid –– Contains important substances cells need to
survive– Cells extract nutrients to remain healthy – Where the cells eject their waste
• The plasma membrane is a selectively permeable barrier, regulating how substances pass into and out of the cell – Allows some substances to pass while excluding
others
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P.M. Functions cont. • Solute –
– Substance being dissolved within a solution– Ex. Sugar, salt, calcium, etc.
• Solvent – – Substance in which solutes are dissolved– “vat” in which substances are dissolved – Typically water
• Concentration gradient – – The difference in solute concentration inside
and outside the cell
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Passive processes • Passive processes –
– Does not require energy (ATP)– Move substances down (or with) a
concentration gradient• Substances move from areas of higher concentration
to areas of lower concentration – Types –
• Diffusion• Simple diffusion • Facilitated diffusion• Osmosis • Filtration
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Passive processes
• Diffusion –– The tendency of molecules/ions to scatter
evenly throughout the environment – Substances move directly through the
plasma membrane if its…• Lipid soluble• Small enough to pass through membrane
channels• Assisted by carrier proteins
– Goal of diffusion – to reach equilibrium – where molecules are moving equally in all directions – there is no net movement – molecules are balanced on each side
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Passive processes cont.
• Diffusion cont –– Move from areas of higher concentration to
areas of lower concentration – move down the concentration gradient
• Molecules move very quickly & erratically • Bounce off each other • They don’t like to be close they naturally move to
areas where numbers are lower (move from high to low)
• The greater the concentration difference the faster the diffusion process
• Movement occurs due to the kinetic energy of the molecules
– The smaller the molecule the faster the diffusion– Warmer the temp the faster the diffusion
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Diffusion
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Passive processes cont.
• Diffusion cont. –– Simple diffusion –
• Unassisted diffusion of lipid-soluble or very small particles
– Substances that are nonpolar– Oxygen, carbon dioxide, alcohol, fat-soluble vit
• Down the concentration gradient
– Facilitated diffusion - • Substances are moved through the plasma membrane
by binding to protein carriers or by membrane channels • Transport proteins change shape to allow substances
(glucose & simple sugars) through• Transported down the concentration gradient
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Passive processes
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Passive processes cont. • Osmosis –
– Diffusion of water – Water is polar, but it is a small molecule that
can slip through the plasma membrane as the membrane changes shape
– Occurs until solute concentrations are balanced – Movement occurs due to solute concentrations
• Water moves when solute concentrations differ • Requires less energy to move water than a solute • Osmolarity – the total number of all solute particles
w/in a solution • Osmotic pressure – the cells ability to resist further
(net) water movement – when solute concentrations are equal
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Osmosis
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Passive processes cont• Osmosis cont. –
– Tonicity –• A solutions ability to change the tone or shape of
cells by altering their internal water volume • The number of nonpenetrating solutes
– Isotonic –• Solutions with the same concentration of
nonpenetrating solutes on either side of the cell or the total solute concentration
• Solutes are equal inside and outside the cell • Cells keep their “normal” shape because there is no
net movement of water • Body fluids = isotonic
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Passive process cont.• Osmosis cont. –
– Hypertonic –• Solutions with high concentrations of solutes • Cells loose water trying to equalize the solutes• Loosing water causes them to shrink or crenate
– Hypotonic –• Solutions that contain fewer solutes• The solution bathing the cell is more dilute than the
cell itself • Cells take on water as it tries to equalize the lower
number of solutes on the inside of the cell • Cells take on so much water they eventually burst or
lyse • Rehydrating – sports drinks, cola, and apple juice are
hypotonic increasing the amount of water cells absorb, causing rehydration
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Tonicities
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Passive processes cont.
• Filtration –– A pressure-driven process that forces water
and solutes through a membrane or capillary wall by fluid or hydrostatic pressure
– Passive processes – Involves a pressure gradient that pushes the
solute fluid (filtrate) from higher-pressure areas to lower-pressure areas
– Not a selective process – Anything that is small enough will be pushed,
by pressure, through the plasma membrane
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Active processes
• Cells use energy, ATP, to move substances against their concentration gradient
• Active transport – – Uses solute pumps to move substances against
their concentration gradient – Energy is needed either directly or indirectly – Provide movement for substances who can’t
pass by diffusion – Ions such as Na+ and K+ need a protein
transport (solute pump) to move across the membrane against their concentration (moving them uphill)
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Active processes cont.
• Active transport cont. –– Primary active transport –
• Energy provided directly by the hydrolysis of ATP• Causes the transport protein to change shape which
pumps the bound solute across the membrane • Sodium potassium pump • K+ higher inside the cell• Na+ higher outside the cell • They leak across the plasma membrane along their
concentration gradient – the pump is needed to keep K+ higher inside and Na+ higher outside
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Active processes cont.
• Active transport cont. –– Secondary active transport –
• The driving forces of primary transport can indirectly drive the transport of other solutes
• As sodium is transported it drags other solutes with it – they are cotransported (sugars and amino acids)
• Even though solutes are cotransported which is passive – they would not be able to be transported w/o the energy required movement of sodium
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Active transport
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Active transport cont.
• Active transport cont –– Vesicular transport –
• Means by which large particles (macromolecules) & fluids are transport across the plasma membrane
• Energized by ATP • Two types –
– Endocytosis – Exocytosis
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Active transport cont.
• Vesicular transport –– Exocytosis –
• “out of the cell”• Used to move substances from inside the
cell to the extracellular environment • Hormone secretion, neurotransmitting,
ejection of waste, mucus secretion • Substance to be secreted is enclosed in a
membranous sac called a vesicle
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Exocytosis
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Active transport cont.
• Endocytosis – – “into the cell”– Move substances into the cell using protein-
coated vesicles• Clathrin-coated• Non-clathrin-coated (Caveolae or coatomer)
– Moved into the cell by extensions of the plasma membrane
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Active transport cont.
• Endocytosis cont –– 3 Types of Clathrin-coated vesicles
• Phagocytosis• Pinocytosis• Receptor-mediated endocytosis
– Phagocytosis –• “cell eating”
– Cytoplasmic extensions = pseudopods – protrude from cell and cover/contain large/solid material
• Formed vesicle = phagosome – Fuses with a lysosome to digest the contents
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Active transport cont.
• Endocytosis cont –– Pinocytosis (fluid phase endocytosis)
• Infolding of the plasma membrane• Contains extracellular fluid w/ dissolved
molecules• Allows the cell to “sample” the contents of
the extracellular fluid– Important for cells that absorb nutrients
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Active transport cont. • Receptor-mediated endocytosis –
– Most common method for specific uptake– Enzymes, insulin, hormones, and iron– Flu viruses utilize this method to attack our
cells– Receptors = membrane proteins that only
bind with certain substances – Create a vesicle = coated pit– Contents dissolved/utilized within the cell
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Enodcytosis
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Active transport cont.
• Non-clathrin-coated vesicles– Caveolae – inpocketings of plasma
membrane• Caveolin proteins• Capture specific molecules (folate, tetanus toxin)• Close association with lipid rafts, important for
cell signaling
– Coatomer (COP1 and COP2) proteins• Vesicular trafficking
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Membrane potentials
• A membrane potential is a voltage across the cell membrane that occurs due to a separation of oppositely charged particles.
• In a resting stage a cell exhibits a resting membrane potential – the cell is polarized – the inside of the cell is more negative than the outside – Charge only exists at the membrane– Charges within in the cell are neutral
• Determined by…– Concentration gradient of potassium (K+) and sodium
(Na+)– More K+ within cells– More Na+ in the extracellular fluid – K+ diffuses out of the cell – cells is impermeable to Na+– Now more positive charges are outside the cell giving it
its negative charge
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Cellular environmental interactions
• Membrane receptors –– Diverse groups of integral proteins and
glycoproteins that serve as binding sites – Function in:
• Contact signaling –– Touching of cells– Method for recognizing other cells– Bacteria and viruses utilize this method
• Electrical signaling –– Responding to the changes in voltage– Neural and muscle tissue
• Chemical signaling –– Job of most membrane receptors – Nervous & endocrine systems
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Developmental aspects of cells
• Aging –– Due to wear-and-tear – Accumulation of free radicals – May be a result of autoimmune responses &
progressive weakening of the immune system• Apoptosis –
– Programmed cell death – Cell suicide – Cancer = cells fail to go through apoptosis – Strokes & heart attacks increase the rate of
apoptosis
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• The rest of the material is not necessary for the test.
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Components of the cell • The cytoplasm is the cellular material between
the cell membrane and the nucleus, and is the site of most cellular activity.
• Major elements of the cytoplasm –– Cytosol –
• Fluid in which the other cytoplasmic elements are suspended
• Largely water w/ salts, proteins, sugars & other solutes – Cytoplasmic organelles –
• Major components/workings of the cell – Cytoplasmic inclusions –
• Not a functional unit• Chemical substances • Ex: lipid droplet in a fat cell
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Components of the cell
• Organelles –– Mitochondria –
• Sausage-shaped organelle • Powerhouse of the cell• Produce ATP• The more mitochondria – the more energy a
particular cell needs • Large quantities in liver and muscle cells
– Ribosome – • Consist of proteins & ribosomal RNA• Site of protein synthesis
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Components of the cell cont. • Endoplasmic reticulum –
– Continuous with the nuclear membrane – “network within the cell”– Two types –
• Rough endoplasmic reticulum –– Contains ribosomes making it appear rough – Manufactures all proteins secreted from the cell– Produces components of the membranes – Liver & secretory cells
• Smooth endoplasmic reticulum –– Continuation of the RER – Lipid & cholesterol synthesis – Synthesis of steroid-based hormones (sex hormones) – Absorbtion, synthesis & transort of fats– Detoxification of drugs– Breakdown of glycogen to form free glucose
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Components cont. • Golgi apparatus –
– Moves & directs cellular proteins– Modifies, concentrates, & packages proteins– Proteins bud off in vesicles from the ER & bind
with the golgi in order to be modified – Golgi vessicles are created allowing proteins to
be transferred to the plasma membrane in order to be excreted into the extracellular space
• Lysosomes –– Involved in digestion & phagocytosis (bacteria,
viruses, & toxins)– Degrading nonfunctional/worn-out organelles– Break down nonuseful tissues
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Components cont. • Peroxisomes –
– Digestion of alcohol • Cytoskeleton –
– Cells skeleton – Supports cell structure & generates cell movements
• Centrosomes & centrioles –– Organizes mircrotubules – Arranged at right angles to each other– Organize mitotic spindles – Form base of cilia and flagella
• Cellular extensions –– Cilia –
• Occur in large numbers• Move together in order to move substances • Propels other substances
– Flagella –• Single• Whip like motion • Sperm• Propels itself
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Nucleus • Control center of the cell• Nuclear envelope –
– Binds the nucleus – Double layer membrane – Continuous with rough ER
• Nuclear pores – – Regulates entry and exit of large particles
• Nucleoli – – Site of ribosome production– Contain DNA
• Chromatin – – DNA (genetic materials) & histone proteins
• Nucleosomes –– Eight histone proteins & DNA molecule
• Chromosomes – – Condensed chromatin – Utilized when cells are going to divide