Download Cells and Cell Membranes

Cells and Cell Membranes
Chapter 6-7: The Cell: The Basic Unit of Life
Cell Theory
1. All organisms are made up of cells.
2. The cell is the basic living unit of organization for all
organisms.
3. All cells come from pre-existing cells.
Biological Diversity & Unity
• Underlying the diversity of life is a striking unity.
DNA is universal genetic language.
Cells are the basic units of structure &
function.
• Lowest level of structure capable of
performing all activities of life.
Activities of Life
• Most everything you think of a whole organism needing to do, must be done at the cellular level…
o reproduction
o growth & development
o energy utilization
o response to the environment
o homeostasis
How do We Study Cells?
• Microscopes opened up the world of cells
o Robert Hooke (1665) was the 1st cytologist.
• Microscopes
o Light microscopes
• 0.28m resolution
• ~size of a bacterium
• light passes through specimen
• can be used to study live cells
o Electron microscope
• 2.0nm resolution
• 100 times > light microscope
• reveals organelles
• but can only be used on dead cells
o Transmission electron microscopes (TEM)
• used mainly to study internal
structure of cells
• aims an electron beam through thin
section of specimen
o Scanning electron microscopes (SEM)
• studying surface structures
• sample surface covered with thin
film of gold
• beam excites electrons on surface
• great depth of field = an image that
seems 3-D.
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How do We Study Cells (Continued)
• Isolating Organelles
o Cell fractionation
• Separate organelles from cell
• Variable density of organelles
• ultracentrifuge can spin up to 1 million X gravity (1,000,000g)
Cell Characteristics
• All cells:
o Surrounded by a plasma membrane
o Have Cytosol
• Semi-fluid substance within the membrane
• Cytoplasm = cytosol + organelles
o Contain chromosomes which have genes in the form of DNA
o Have ribosomes
• Tiny “organelles” that make proteins using instructions contained in genes.
Types of Cells
• Prokaryotic vs. Eukaryotic cells
o Location of Chromosomes
• Prokaryotic cell
o DNA in nucleoid region, without a membrane separating it from rest of cell.
• Eukaryotic cell
o Chromosomes in nucleus, membrane enclosed organelle.
Eukaryotic Cells
• Eukaryotic cells are more complex than prokaryotic cells.
o Within cytoplasm is a variety of membrane-bounded organelles.
o Specialized structures in form & function.
• Eukaryotic cells are generally bigger than prokaryotic cells
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What Limits Cell Size?
• Surface to volume ratio.
o As cell gets bigger its volume increases faster than its surface area.
Smaller objects have greater ratio of surface area to volume.
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Metabolic requirements set upper limit.
o In large cell, cannot move material in & out of cell fast enough to support life.
How to get bigger?
o Become multi-cellular (cell divides)
Cell Membrane
o Exchange organelle
o Plasma membrane functions as selective barrier
o Allows passage of O2, nutrients & wastes
Organelles & Internal membranes
o Eukaryotic cell
o Internal membranes
Partition cell into compartments
Create different local environments
Compartmentalize functions
Membranes for different compartments are specialized for their function.
• Different structures for specific functions.
• Unique combination of lipids & proteins.
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Chapter 6 The Cell: Nucleus, Ribosomes
Nucleus
• Function
o Contains eukaryotic cell’s genetic library.
Most genes in nucleus.
Some genes located in mitochondria & chloroplasts.
Nucleus Structure
• Separated from cytoplasm by a double
membrane, nuclear envelope
• Double membrane is fused in spots
forming pores
o Allows large macromolecules &
particles to pass through.
• Within nucleus, DNA organized into
fibrous material, chromatin
o In normal cell appears as diffuse
mass
• When cell prepares to divide, chromatin
fibers coil up as separate structures,
chromosomes.
• Densely stained region = nucleolus
o Responsible for production of
ribosomal subunits from rRNA &
proteins
Pass through nuclear pores to cytoplasm & combine to form ribosomes.
Ribosomes
• Function
o Protein production
• Structure
o Ribosomes contain rRNA & protein
o Composed of 2 subunits that combine to carry out
protein synthesis
• Types of Ribosomes
o Free ribosomes
Suspended in cytosol
Synthesize proteins that function within cytosol
o Bound ribosomes
Attached to outside of endoplasmic reticulum
Synthesize proteins for export or for membranes
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Chapter 6 The Cell: Endomembrane System:
(Endoplasmic Reticulum, Golgi Apparatus, Lysosomes, Peroxisomes, Vacuoles & Vesicles)
Overview
• Play key role in synthesis (&
hydrolysis) of macromolecules
in cell.
• Various “players” modify
macromolecules for various
functions.
Endoplasmic Reticulum
• Function
o Manufactures
membranes & performs
many bio-synthesis
functions
• Structure
o Membrane connected to
nuclear envelope &
extends throughout cell.
o Accounts for 50%
membranes in
eukaryotic cell
Rough ER = bound ribosomes
Smooth ER = no ribosomes
• Smooth ER function
o Factory processing operations
Many metabolic processes
• synthesis & hydrolysis
Enzymes of smooth ER…
• Synthesize lipids, oils, phospholipids, steroids & sex hormones
• Hydrolysis (breakdown) of glycogen (in liver) into glucose
• Detoxify drugs & poisons (in liver)
o ex. alcohol & barbiturates
• Rough ER Function
o Produce proteins for export out of cell.
Protein secreting cells.
Packaged into transport vesicles for export.
Membrane Factory
• Synthesize membrane phospholipids.
o Build new membrane.
o As ER membrane expands, bud off & transfer to other parts of cell that need membranes.
• Synthesize membrane proteins.
o Membrane bound proteins synthesized directly into membrane.
o Processing to make glycoproteins.
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Golgi Apparatus
• Function
o Finishes, sorts, & ships cell
products
• “Shipping & receiving
department”
o Center of manufacturing,
warehousing, sorting &
shipping.
o Extensive in cells specialized
for secretion.
• Structure
o Flattened membranous sacs =
cisternae.
• Look like stack of pita
bread.
• 2 sides = 2 functions
o Cis = receives material by
fusing with
o Vesicles = “receiving” trans
buds off vesicles that travel to
other sites = “shipping”
(transport).
Golgi Processing
• During path from cis to trans, products from ER are modified into final form.
• Tags, sorts, & packages materials into transport vesicles.
o Golgi = “UPS headquarters”
o Transport vesicles = “UPS trucks”
Delivering packages that have been tagged with their own barcodes.
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Lysosomes
• Structure
o Membrane-bounded sac of hydrolytic enzymes that digests macromolecules
o Enzymes & membrane of lysosomes are synthesized by rough ER & transferred to the
Golgi.
• Function
o A little “stomach” for the cell.
lyso– = breaking things
apart
–some = body
o Also the “clean up crew” of the
cell.
• Cellular Digestion
o Lysosomes fuse with food
vacuoles.
o Polymers are digested into
monomers.
Pass to cytosol to become
nutrients of cell.
• Lysosomal Enzymes
o Lysosomal enzymes work best at
pH 5. The lysosome creates a
custom pH. How?
Proteins in lysosomal
membrane pump H+ ions
from the cytosol into lysosome
o Why would it do this?
Because enzymes are proteins they are very sensitive to pH (Denaturation!)
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o Why evolve digestive enzymes which function at pH different from cytosol?
Digestive enzymes won’t function well if leak into cytosol = don’t want to digest
yourself!
What if a lysomome digestive enzyme doesn’t function?
o Don’t digest a biomolecule
Instead biomolecule collects in lysosomes and they fill up with undigested material.
Lysosomes grow larger & larger eventually disrupting cell & organ function.
“Lysosomal storage diseases” are usually fatal
o Tay-Sachs Disease
Lipids build up in brain cells and the child dies before age 5.
Apoptosis = Cell Death
o Critical role in programmed destruction of cells in multicellular organisms.
Auto-destruct mechanism
• “cell suicide”
Some cells have to die in an organized fashion, especially during development
• Ex: development of space between your fingers during embryonic
development.
• Ex: if cell grows improperly this self-destruct mechanism is triggered to
remove damaged cell.
o Cancer over-rides this to enable tumor
growth.
Peroxisomes
o Other digestive enzyme sacs
In both animals & plants
Breakdown fatty acids to sugars
• Easier to transport & use as energy source
Detoxify cell
• Detoxifies alcohol & other poisons
Produce peroxide (H2O2)
• Must breakdown H2O2 → H2O
Vacuoles & Vesicles
o Function
Little “transfer ships”
• Food vacuoles
o Phagocytosis, fuse
with lysosomes.
• Contractile vacuoles
o In freshwater
protists, pump
excess H2O out of cell.
• Central vacuoles
o In many mature plant cells.
Vacuoles in Plants
o Functions
Storage
• Stockpiling proteins or inorganic ions
• Depositing metabolic byproducts
• Storing pigments
• Storing defensive compounds against herbivores
• Selective membrane
o Control what comes in or goes out.
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Chapter 6 The Cell: Mitochondria & Chloroplasts
Overview
• Mitochondria & chloroplasts are the organelles that convert
energy to forms that cells can use for work
o Mitochondria: from glucose to ATP
o Chloroplasts: from sunlight to ATP &
carbohydrates
ATP = active energy
Carbohydrates = stored energy
Important to see the similarities
• Transform energy
o Generate ATP
• Double membranes = 2 membranes
• Semi-autonomous organelles
o Move, change shape, divide
• Internal ribosomes, DNA & enzymes
Mitochondria
• Function
o Cellular respiration
o Generate ATP
From breakdown of sugars, fats &
other fuels
In the presence of oxygen
• Break down larger molecules into smaller to generate energy = catabolism
• Generate energy in presence of O2 = aerobic respiration
• Structure
o 2 Membranes
Smooth outer membrane
Highly folded inner membrane
• The Cristae
Increase surface area for membrane bound enzymes that synthesize ATP.
o Fluid-filled space between 2 membranes
o Internal fluid-filled space
Mitochondrial matrix
DNA, ribosomes & enzymes
• Dividing Mitochondria
o What else divides like mitochondria?
Bacteria: What does this tell us about the evolution of eukaryotes?
• Almost all eukaryotic cells have mitochondria.
o There may be 1 very large mitochondrion or 100s to 1000s of individual mitochondria.
o Number of mitochondria is correlated with aerobic metabolic activity.
More activity = more energy needed = more mitochondria.
o What cells would have a lot of mitochondria?
Muscle and Nerve Cells
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Chloroplasts
• Chloroplasts are plant organelles.
o Class of plant structures = plastids
Amyloplasts
• Store starch in roots &
tubers.
Chromoplasts
• Store pigments for
fruits & flowers.
Chloroplasts
• Store chlorophyll &
function in
photosynthesis.
• In leaves, other green structures of plants & in eukaryotic algae.
• Structure
o 2 Membranes
Outer membrane
Inner membrane
o Internal fluid-filled space = stroma
DNA, ribosomes & enzymes
Thylakoids = membranous sacs where ATP is made.
Grana = stacks of thylakoids.
o Why internal sac membranes?
Increase surface area for membrane-bound enzymes that synthesize ATP.
• Function
o Photosynthesis
o Generate ATP & synthesize sugars.
Transform solar energy into chemical energy.
Produce sugars from CO2 & H2O
o Semi-autonomous
Moving, changing shape & dividing.
Can reproduce by pinching in two.
o What else divides like chloroplasts?
Bacteria: Crazy? I think not.
Mitochondria & Chloroplasts are Different from Other Organelles
• These organelles not part of endomembrane system.
• Grow & reproduce
o Semi-autonomous organelles
• Proteins come primarily from free ribosomes in cytosol & a few from their own ribosomes.
• Own circular chromosome.
o Directs synthesis of proteins produced by own internal ribosomes.
• What else has a circular chromosome not bound by a nucleus?
o BACTERIA. Wow. That’s awesome.
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Chapter 6 The Cell: Cytoskeleton
Cytoskeleton
• Function
o Structural support
Maintains shape of cell
Provides anchorage for organelles
o Motility
Cell locomotion
Cilia, flagella, etc.
o Regulation
Organizes structures & activities of cell
• Structure
o Network of fibers extending throughout cytoplasm
o 3 main protein fibers
Microtubules
Microfilaments
Intermediate filaments
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Evolutionary Perspective
o Proteins that make up the fibers are very similar in all living things.
From bacteria to humans
• tubulin (all cells)
• actin (eukaryote cells)
o Means that they are both ancient and essential for life.
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Microtubules
• Structure
o Thickest fibers
o Hollow rods about 25nm in diameter.
o Constructed of protein, tubulin.
o Grow or shrink as more tubulin molecules are added or removed.
• Function
o Structural support & cell movement.
Move chromosomes during cell division
• centrioles
Tracks that guide motor proteins carrying organelles to their destination.
• motor proteins: myosin & dynein
Motility
• cilia
• flagella
o Centrioles
Cell Division
In animal cells, pair of centrioles organize microtubules guiding chromosomes in
cell division.
o Cilia & Flagella
Extensions of eukaryotic cytoskeleton
Cilia = numerous & short (hair-like)
Flagella = 1-2 cell & longer (whip-like)
• Move unicellular & small multicellular
organisms by propelling water past them
• Cilia sweep mucus & debris from lungs
• Flagellum of sperm cells
Structure
• Remember 9+2!
• 9 pairs of microtubules around 2 single
microtubules in center
• Bending of cilia & flagella is driven by
motor protein
o Dynein
Microfilaments (Actin Filaments)
• Structure
o Thinnest class of fibers
o Solid rods of protein, actin
o Twisted double chain of actin subunits
o About 7nm in diameter
• Function
o 3-D network inside cell membrane
o In muscle cells, actin filaments interact with myosin filaments to create muscle contraction
• Dynamic process
o Actin filaments constantly form & dissolve making the cytoplasm liquid or stiff during
movement
Movement of Amoeba
Cytoplasmic streaming in plant cells
• Speeds distribution of materials
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Intermediate Filaments
• Structure
o Specialized for bearing tension.
o Built from keratin proteins.
Same protein as hair.
o Intermediate in size 8-12nm.
• Function
o Hold “things” in place inside cell.
o More permanent fixtures of cytoskeleton.
o Reinforce cell shape & fix organelle location.
Nucleus is held in place by a network of intermediate filaments.
Summary
• Microtubules
o Thickest
o Cell structure & cell motility
o Tubulin
• Microfilaments
o Thinnest
o Internal movements within cell
o Actin, myosin
• Intermediate filaments
o Intermediate
o More permanent fixtures
o Keratin
Chapter 6 The Cell: Cell Junctions
Plant Cell Wall
• Structure
o Cellulose
o Primary cell wall
o Secondary cell wall
o Middle lamella = sticky polysaccharides
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Intercellular Junctions
• Plant Cells
o Plasmodesmata
Channels allowing cytosol to pass between cells.
Animal Cell Surface
• Extracellular Matrix
o Collagen fibers in network of glycoproteins.
Support
Adhesion
Movement
Regulation
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Intercellular Junctions
• Animal Cells
o Tight Junctions
Membranes of adjacent cells fused forming barrier between cells.
Forces material through cell membrane.
o Gap Junctions
Communicating junctions.
Allow cytoplasmic movement between adjacent cells.
o Desmosomes
Anchoring junctions.
Fasten cells together in strong sheets.
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Chapter 7: Cell Membrane Structure and Function
Diffusion
• 2nd Law of Thermodynamics governs biological systems.
o Universe tends towards disorder.
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Diffusion
o Movement from high → low concentration
o Diffusion of 2 Solutes
Each substance diffuses down its own concentration gradient, independent of
concentration gradients of other substances.
o Move from HIGH to LOW concentration.
“passive transport”
No energy needed
Cell (Plasma) Membrane
• Cells need an inside & an outside…
o Separate cell from its environment.
o Cell membrane is the boundary.
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Lipids of Cell Membrane
• Membrane is made of phospholipids
o Phospholipid bilayer
Semi-permeable Membrane
• Need to allow passage through the
membrane.
• But need to control what gets in or out.
• Membrane needs to be semi-permeable.
Phospholipid Bilayer
• What molecules can get through directly?
o Fats & other lipids can slip directly through the phospholipid cell membrane.
Permeable Cell Membrane
• Need to allow more material through
o Membrane needs to be permeable to…
• all materials a cell needs to bring in
• all waste a cell needs excrete out
• all products a cell needs to export out
• “Holes”, or Channels, in cell membrane allow material in & out.
o Diffusion through a channel means movement from high to low concentration.
• But the cell still needs control
o Membrane needs to be semi-permeable
o Specific channels allow specific material in & out
• Protein Channels
o Proteins are mixed molecules
• Hydrophobic amino acids
• Stick in the lipid
membrane
• Anchors the protein in
membrane
• Hydrophilic amino acids
• Stick out in the watery
fluid in & around cell
• Specialized “receptor” for
specific molecules
Facilitated Diffusion
• Globular proteins act as doors in membrane.
o Channels to move specific molecules
through cell membrane
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Active Transport
• Globular proteins act as ferry for specific molecules
o Shape change transports solute from one side
of membrane to other → protein “pump”
o “costs” energy
Getting Through Cell Membrane
• Passive Transport
o Diffusion of hydrophobic (lipids) molecules
o High → low concentration gradient
o No Energy Required
• Facilitated Transport
o Diffusion of hydrophilic molecules
o Through a protein channel
o High → low concentration gradient
o No energy required.
• Active Transport
o Diffusion against concentration gradient
o Low → high
o Uses a protein pump
o Requires ATP
Gated Channels
• Some channel proteins open only in presence of stimulus (signal).
o Stimulus usually different from transported molecule.
• ex: ion-gated channels when neurotransmitters bind to a specific gated channels on
a neuron, these channels open = allows Na+ ions to enter nerve cell.
• ex: voltage-gated channels change in electrical charge across nerve cell membrane
opens Na+ & K+ channels.
Active Transport
• Cells may need molecules to move against concentration situation
o need to pump against concentration
o protein pump
o requires energy
• ATP
Transport Summary
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How about large molecules?
• Moving large molecules into & out of cell
o Through vesicles &
vacuoles
o Endocytosis
• phagocytosis =
“cellular eating”
• pinocytosis =
“cellular
drinking”
• receptormediated
endocytosis
o Exocytosis
Osmosis
• Diffusion of water
• Water is very important, so we
talk about water separately
• Diffusion of water from high concentration of
water to low concentration of water.
o Across a semi-permeable membrane
Concentration of Water
• Direction of osmosis is determined by
comparing total solute concentrations.
o Hypertonic - more solute, less water
o Hypotonic - less solute, more water
o Isotonic - equal solute, equal water
Managing Water Balance
• Cell survival depends on balancing water
uptake & loss.
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Managing Water Balance
• Isotonic
o Animal cell immersed in isotonic solution
• Blood cells in blood
• No net movement of water across plasma membrane
• Water flows across membrane, at same rate in both directions
• Volume of cell is stable
• Hypotonic
o Animal cell in hypotonic solution will gain water, swell & burst.
• Paramecium vs. pond water
• Paramecium is hypertonic
• H2O continually enters cell
• To solve problem, specialized organelle, contractile vacuole.
• Pumps H2O out of cell = ATP
o Plant Cell
• Turgid
• Hypertonic
o Animal cell in hypertonic solution will loose water, shrivel & probably die.
• Salt water organisms are hypotonic compared to their environment.
• They have to take up water & pump out salt.
o Plant cells
• Plasmolysis = wilt
Aquaporins
• Water moves rapidly into & out of cells provides evidence that there were water protein channels.
Fluid Mosaic Model
• In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the
phospholipid bilayer.
• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
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Membrane Proteins
• Proteins determine most of membrane’s specific functions.
o Cell membrane & organelle membranes each have unique collections of proteins.
• Membrane proteins:
o Peripheral proteins = loosely bound to surface of membrane.
o Integral proteins = penetrate into lipid bilayer, often completely spanning the membrane =
transmembrane protein.
Membrane Carbohydrates
• Play a key role in cell-cell recognition.
o Ability of a cell to distinguish neighboring cells from another.
o Important in organ & tissue development.
o Basis for rejection of foreign cells by immune system.
Membranes Provide a Variety of Cell Functions
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