Cells: The Living Unit
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
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
Cell diversity
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
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
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
P.M.
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
P.M
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
P.M.
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
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
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
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
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
Gap junctions
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
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
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
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
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
Diffusion
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
Passive processes
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
Osmosis
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
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
Tonicities
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 lowerpressure areas
– Not a selective process
– Anything that is small enough will be pushed, by
pressure, through the plasma membrane
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)
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
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
Active transport
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
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
Exocytosis
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
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
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
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
Enodcytosis
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
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
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
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
• The rest of the material is not necessary for
the test.
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
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
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
–
–
–
–
–
–
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
• Smooth endoplasmic reticulum –
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
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
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