Download 3 - Cell Structure and Function

10/9/2016
Ch. 3: Cell Structure and Function
• Topics
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Cell theory
Comparing prokaryotic and eukaryotic cells
Eukaryotic cells
Cell membrane
Passive transport
Active transport
Cell theory and cells
• Unified cell theory:
– All living things are composed of
at least one cell
– A cell is the smallest (basic) unit of
life
– New cells come from existing cells
• Cells
– Are small – thus
they have a
relatively high
surface area
(supply) to volume
(demand) ratio
• Note the approximate sizes of different types of cells
– Over 200 types
make up a human
• Cells form tissues,
which form organs,
which form organ
systems, which
form organisms
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Cells: prokaryotic and eukaryotic
• Prokaryotic cells (“pro-”
= before, “karyo-” =
nucleus)
• Common to all cells:
– Bacteria domain
– Archaea domain
• Eukaryotic cells
(“eu-” = true)
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– Plasma (cell) membrane (to
separate and protect)
– Cytoplasm
– DNA (genetic material)
– Ribosomes (for protein
synthesis)
Animal cells
Plant cells
Fungi
Protists
Note: microorganisms
(microbes) are too small
to be seen without a
microscope, and include
bacteria, archaea, some
eukaryotes, and viruses
See Table 3.1 for a
detailed comparison of
prokaryotic and eukaryotic cells
Prokaryotic cells
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No nucleus; DNA is in
– A chromosome located in the nucleoid
(the central part of the cell)
– Plasmid(s)
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Bacteria (shown here) have a cell wall
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Glycocalyx
– Shape, protection, dehydration prevention
– External polysaccharides or glycoproteins
that form either a capsule (shown here) or
slime layer, respectively
– Attachment, protection
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Projections include
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Pili – exchange genetic material
Fimbriae (not shown here) – attachment
Flagellum – locomotion
Axial filament – locomotion
Endospore (dormant state) formation
– Occurs during rough environmental times
• E.g. lack of nutrients
– Preserves genetic material
– Resistant to high temperatures, UV
radiation, chemicals, enzymes
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Eukaryotic cells – major components
• Membrane-bound
nucleus for DNA
• Organelles
– Membrane-bound sacs
with specific functions
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This figure shows (a) a typical animal cell
and (b) a typical plant cell.
Plasma (cell) membrane
• A phospholipid bilayer with embedded proteins and other molecules
• Separates cytoplasm from environment
• More details on cell membranes coming later in the chapter…
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Cytoplasm
• Contents of a cell
between the
membrane and the
nucleus
• Includes
– Organelles
– Watery, gel-like
cytosol
– Cytoskeleton
– Chemicals such
as
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Ions
Proteins
Fatty acids
Nucleic acids
Amino acids
Glucose
Cytoskeleton
• Functions to
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Maintain cell shape
Anchor organelles
Cell movement
Movement of organelles,
vesicles, and
chromosomes within the
cell
• Is a network of protein
fibers, including
– Microfilaments (actin
filaments)
– Intermediate filaments
– Microtubules
• Centrosome
– Center of organization of
microtubules
– Composed of 2 centrioles
– Helps move chromosomes
during cell division
This cell is preparing
to divide
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Flagella
and cilia
• Are made of
microtubules
• Flagella
(singular = flagellum)
– Propel cell
• Cilia
– Propel cell
OR
– Propel substance along
a cell’s surface
• Note: not all cells have
flagella or cilia
Endomembrane system
• Includes
– Nuclear envelope
– Endoplasmic reticulum
(ER)
– Golgi apparatus
– Lysosomes
– Vesicles
– Plasma (cell) membrane
• All of these membranes
are phospholipid
bilayer-based
• Function
– Produce, modify,
package, and transport
lipids and proteins
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Nucleus
• Contains DNA
– DNA can exist as
• Chromatin (most of the time) – unwound
• Chromosomes (during cell division) – condensed
• Directs synthesis of protein and ribosomes
• Nucleolus – formation of RNA
• Nuclear envelope
– Membrane that separates the nucleoplasm from
cytoplasm
– Nuclear pores – passage of RNA, ions, etc.
micro.magnet.fsu.edu/cells/nucleus/chromatin.html
Endoplasmic reticulum (ER)
• Interconnected
membranous tubules
• Rough ER
– Has ribosomes
attached
– Modifies proteins
made by ribosomes
• Smooth ER
– Lacks ribosomes
– Carbohydrate
synthesis
– Lipid synthesis
– Detoxification
– Storage of calcium
ions
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Vesicles and vacuoles
• Membranous sacs
– Vesicles tend to be smaller and can fuse with the plasma
membrane and other
membranous organelles
– Vacuoles tend to be larger
and can’t
• Functions
– Transport
– Storage
Golgi apparatus
• A.k.a. Golgi
complex
• Series of flattened
membranous sacs
that
– Modify,
– Sort,
– Tag,
– Package,
– And distribute
lipids and proteins
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Lysosomes and peroxisomes
• Are both membranous
sacs
• Lysosomes
– Low internal pH
– Contain digestive
enzymes (lysozymes)
that break down…
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Proteins
Lipids
Carbohydrates
Bacteria
Worn-out organelles
And more!
• Peroxisomes
– Contain oxidative enzymes
that break down…
• Fatty acids
• Amino acids
• Alcohol (in liver)
Ribosomes
• Protein synthesis
• Fixed
– Attached to rough ER
OR
• Free
– Floating in cytoplasm
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Mitochondria
Singular = mitochondrion
Membranous
Make adenosine triphosphate (ATP), the main
form of energy used by cells
– Via cellular respiration (aerobic)
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• This requires O2
Have their own DNA (mtDNA) and ribosomes
– The size of mitochondria (and chloroplasts in plant
cells), along with their bacteria-like DNA and
ribosomes, supports the theory of endosymbiosis…
• Ingested (but not destroyed) aerobic bacteria eventually
became mitochondria, and ingested photosynthetic
bacteria eventually became chloroplasts
Animal vs. plant cells
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Chloroplast
Animal cells have centrosomes/
centrioles and lysosomes (plant cells
don’t)
Plant cells have a cell wall, chloroplasts,
and a large central vacuole (animal cells
don’t)
More about plant cells
– Cell wall
• Found in plants, fungi, and
protists
• Protects, supports, and provides
shape
– Chloroplasts
• Found in plants and algae
• Contain green pigment called
chlorophyll
• Photosynthesis (CO2 + H2O +
light → glucose + O2)
• In addition to mitochondria,
further evidence for
endosymbiotic theory
– Central vacuole
• Contains fluid, providing turgor
pressure to support cell wall
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Direct contact between cells in multicellular
organisms for attachment and communication
Plasmodesmata (a) in plant cells – transport of
signaling chemicals and nutrients
In animal cells
Intercellular
junctions
– Tight junctions (b) – watertight seals
– Desmosomes (c) – hold cells together
– Gap junctions (d) –
transport of
signaling
chemicals
and
nutrients
Plasma (cell) membrane
• Fluid mosaic model – it’s made up of many separate components
that can freely flow and change position while maintaining the
integrity of the membrane; the components include
– Lipids
• Bilayer of phospholipids – separates cytoplasm from extracellular fluid (ECF)
– Polar/hydrophilic heads interact with watery fluids, nonpolar/hydrophobic tails interact with
each other
• Cholesterol (in animal cells) – regulates fluidity/stability based on temperature
• Glycolipids – attachment and recognition/identification (ID)
– Proteins – provide many different functions, including
• Transport: carriers, channels, or pumps
• Enzymes
• Structural
support
• Receptors
for neurotransmitters and
hormones
• Attachment
• Recognition/ID
(glycoproteins)
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Transport across cell membranes
• Cell membranes are selectively
permeable (allow some substances
through but not others)
• Types of transport
– Passive transport (diffusion)
• Does not require spending ATP energy
• Due to natural movement of substances down
their concentration gradients (see next slide)
• Includes
– Simple diffusion
– Facilitated transport (facilitated diffusion)
– Osmosis
– Active transport
• Requires spending ATP energy
• Can move substances against their
concentration gradients
• Includes
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Primary active transport
Secondary active transport
Endocytosis
Exocytosis
Concentration gradients
• Are differences in concentrations of substances (e.g. solutes such
as molecules or ions) between two areas
• Substances naturally move down their concentration gradients; i.e.,
from areas of higher to areas of lower concentration, until
equilibrium is reached
• This movement is due to random molecular motion
• This movement is called diffusion
Solution terminology
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Solute(s) = substance(s) (molecules
or ions) that are dissolved in a…
Solvent = the liquid/fluid that the
solutes are dissolved in
Solution = solvent + solute(s)
Example: sugar water is a solution
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Solute is sucrose (table sugar)
Solvent is water
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Rate of diffusion (flux)
• Is influenced by…
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Degree of the concentration difference (∆C)
Solubility of substance
Mass of molecules diffusing
Temperature
Density of solvent
Distance
• For living systems, across membranes,
essentially…
Rate of diffusion (flux) = P x ∆C
where P = membrane permeability of the substance
(influenced by solubility, mass, and the presence of
protein carriers or channels for that substance)
Simple diffusion
• Small, lipid-soluble
(nonpolar/hydrophobic)
molecules move from
high to low concentration
directly through
phospholipid bilayer
• E.g. O2, CO2, alcohol, fatsoluble vitamins A, D, E
and K
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Facilitated transport
(facilitated diffusion)
• Ions and water-soluble
(polar/hydrophilic)
molecules move from
high to low concentration
through protein carrier or
channel
• E.g. ions, glucose, amino
acids
More on passive transport (diffusion)
• Again, solubility is important…
– This picture is misleading because it shows the
simple diffusion of what appears to be a
monosaccharide such as glucose
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Osmosis: the diffusion of water
• Water moves through a semipermeable membrane (permeable to
water, not permeable to solutes) from where it (water) is more
concentrated to where it is less concentrated…
– I.e., water moves down its own concentration gradient
– I.e., water moves toward where there are more solutes
• In living cells, osmosis is aided by membrane proteins called
aquaporins (water channels)
• Osmolarity
– Refers to the
total concentration of all
solutes in a
solution
– We can relate
the osmolarities
of living cells and
extracellular fluid
using the concept
of tonicity…
Tonicity
• Refers to the osmolarity of a solution compared to the osmolarity of
the cytoplasm of a cell in that solution
• So the prefixes (hyper-, iso-, or hypo-) refer to concentration of
solutes in the solution compared to the concentration of solutes in
the cell’s cytoplasm
– Hypertonic solutions have more solutes than cytoplasm
– Isotonic solutions have the same amount of solutes as cytoplasm
– Hypotonic solutions have less solutes than cytoplasm
cells crenate (shrink)
no net change
cells swell/rupture
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Active transport processes
• Require the expenditure of cellular energy (ATP)
• Substances can be moved against their
concentration gradients; i.e., from areas of lower
concentration toward areas of higher
concentration
• Ions can be pumped through the membrane via
carrier proteins that use ATP
– Thus these carrier proteins are often referred to as
pumps
• Larger molecules, cell particles, and bacteria can
be moved through the membrane via vesicular
transport
– Endocytosis (in) or exocytosis (out)
– These processes also use ATP
Primary active transport
• ATP is used at the site of transport
• An important example: the sodium-potassium pump
– For every ATP spent,
• 3 Na+ are pumped out
• 2 K+ are pumped in
– Maintains concentration and electric charge differences to help
establish a membrane potential (more on this soon)
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Secondary active transport
• First, ATP is used via primary active transport to establish a
concentration gradient for one substance (e.g. the Na+ gradient
established by the sodium-potassium pump)
• Then, that gradient is used at a different site to transport/pump a
different molecule through the membrane
• Glucose and amino acids enter cells this way
Membrane potential
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Is a separation of electric
charge across a membrane
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The inside of the cell is negative
relative to the outside, due to
– K+ leaks out faster than Na+ leaks in
– Unequal ion pumping by sodiumpotassium pump
– Negative charges of proteins
Na+ ion
K+ ion
+++++++++++++++++++++++++++++++++++++++++++++
K+ leak
channel
Na+/K+ pump
- - - - - - - - - - - - - - - - - - - - - - - - -+ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Na leak channel
Protein
(negatively
charged)
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As we’ve seen, there are
concentration gradients across
membranes
– These are also referred to as
chemical gradients
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Thanks to the membrane
potential, there are also electrical
gradients across membranes
Electrochemical
gradients
– The separation of ions results in
charged sides of membranes
– The inside of the cell is relatively
negative (-), and the outside of
the cell is relatively positive (+)
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Combining them:
electrochemical gradients
E.g. due to sodium-potassium
pump (not shown here), there is
a higher concentration of Na+
outside the cell and a higher
concentration of K+ inside the cell
The electrochemical gradient for
Na+ is very strong inward, but
comparatively the
electrochemical gradient for K+ is
not as strong outward
Vesicular transport
• Is a type of active transport; i.e., requires the expenditure of
ATP
• Involves vesicles
• Includes
– Endocytosis (in)
• Phagocytosis
• Pinocytosis
• Receptor-mediated endocytosis
• Exocytosis (out)
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3 types of endocytosis
• Phagocytosis (“cell eating”): large particles, cells (e.g.
bacteria)
• Pinocytosis (“cell drinking”): extracellular fluid and
solutes
• Receptor mediated endocytosis: specific substances
(e.g. LDL cholesterol)
Exocytosis
• Ejection or secretion
of material out of cell;
some examples
include
– Sweat
– Hormones
– Saliva
– Mucus
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