Chapter 4
A Tour of the Cell
SC 101
General Biology
Essential Biology
CM Lamberty
The Microscopic World of Cells
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Cells are marvels of complexity
Trillions of cell in human body
Many specialized types
Main tool for exploration--Microscope
Microscopes as Windows on the World
of Cells
• Light Microscope (LM)
– Visible light projected through specimen
– Lenses enlarge the image and project to eye
– View living cells
• Magnification
– Increase in size, depends on lens
• Resolving Power
– Clarity of magnified image
• Electron Microscope (EM) beam of electrons
– Scanning Electron Microscope (SEM)
• View cell surfaces
– Transmission Electron Microscope (TEM)
• View internal structures
Microscopes as Windows on the World
of Cells
Microscopes as Windows on the World
of Cells
The 2 Major Categories of Cells
The countless cells on earth fall into two categories:
• Prokaryotic cells — Bacteria and Archaea
• Eukaryotic cells — plants, fungi, and animals
All cells have several basic features.
• They are all bound by a thin plasma membrane.
• All cells have DNA and ribosomes, tiny structures that
build proteins
• All cells have cytosol (aka cytoplasm)
The 2 Major Categories of Cells
Prokaryotic and eukaryotic cells have important
differences.
Prokaryotic cells are older than eukaryotic cells.
• Prokaryotes appeared about 3.5 billion years ago.
• Eukaryotes appeared about 2.1 billion years ago.
Prokaryotes
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Are smaller than eukaryotic cells
Lack internal structures surrounded by membranes
Lack a nucleus, DNA is contained in a nucleoid
Have a rigid cell wall
Idealized Prokaryotic Cell
The 2 Major Categories of Cells
Eukaryotes
• Only eukaryotic cells have organelles, membranebound structures that perform specific functions.
• The most important organelle is the nucleus, which
houses most of a eukaryotic cell’s DNA.
Idealized Eukaryotic Cell
Overview of Eukaryotic Cells
• Eukaryotic cells are fundamentally similar.
• The region between the nucleus and plasma
membrane is the cytoplasm.
• The cytoplasm consists of various organelles
suspended in fluid.
• Unlike animal cells, plant cells have
• Protective cell walls
• Chloroplasts, which convert light energy to the
chemical energy of food
Membrane Structure
• Separates living cell from nonliving
surroundings
• Regulates traffic of chemicals in and out of cell
• Key to how it works is the structure
Plasma Membrane: Lipids & Proteins
• Phospholipids
– Related to dietary fats
– Only 2 fatty acid tails not
3
• hydrophobic
– Phosphate group in 3rd
position
• Charged, hydrophilic
• Phopholipid bilayer
– 2-layered membrane
– Proteins embedded in
bilayer
• Regulate traffic
Plasma Membrane: Lipids & Proteins
• Fluid Mosaic
– Not static (fluid)
– Diverse proteins (mosaic)
– Phospholipids and proteins free to drift about in the
plane of the membrane
• Illness can result if membrane is compromised
– Superbugs: staphylococcus aureus
– MRSA
– Flesh eating disease!!
Cell Surfaces
• Plant cells have rigid cell wall surrounding plasma
membrane
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Made of cellulose
Protect the cells
Maintain cell shapes
Keep cells from absorbing too much water
• Cells connected via channels through cell walls
– Join cytoplasm of each cell to neighbor
– Allow water and small molecules to move between
cells
Cell Surfaces
• Animal cells lack cell wall
– Extracellular matrix
• Sticky coating to hold cells together
• Protects and supports cells
• Cells junctions
– Connect cells together
– Allow cells in tissue to function in coordinated way
Genetic Control of Cell
• Nucleus chief of the cell
– Genes store information necessary to produce
proteins
– Proteins do most of the work of the cell
– Contains “MOST” of the cells DNA
Structure and Function of Nucleus
• Nuclear Envelope
– Double membrane that surrounds nucleus
– Similar in structure to plasma membrane
– Pores allow transfer of materials- pore complex passes
through the double membrane
– Inner portion is covered in a nuclear lamina
• Nucleolus
– Prominent structure
– Here a type of RNA called rRNA is made by DNA
instructions
• Chromatin
– Fibers formed from long DNA and associated
proteins
• Chromosome
– One chromatin fiber
The nucleus
DNA, chromatin and chromosomes
Ribosomes
• Responsible for protein synthesis
• In eukaryotic cells, ribosomes are made in nucleus and
by RNA called ribosomal RNA (rRNA) and then
transported into cytoplasm where small and large
subunits join
– Suspended in fluid making proteins that remain in
fluid
– Attached to outside of endoplasmic reticulum, making
proteins incorporated into membranes or secreted by
cell
– Free ribosomes function in cytosol, ie for glysolysis
How DNA Directs Protein Production
• DNA programs protein production in
cytoplasm via mRNA
• mRNA exits through pores in nuclear
envelope, travels to cytoplasm, and binds to
ribosomes
• As ribosomes move along mRNA, genetic
message translated into protein with specific
amino acid sequence.
How DNA Directs Protein Production
DNA
RNA
Protein
The Endomembrane System
• Cytoplasm of eukaryotic cells partitioned by
organelle membranes
• Some are connected
– Directly by membranes (which are not always
identical)
– Indirectly by transfer of membrane segments or tiny
vesicles
• Together form endomembrane system
– Includes nuclear envelope, endoplasmic reticulum,
Golgi apparatus, lysosomes and vacuoles
Endoplasmic Reticulum (ER)
• Main functioning facility in cell
• Continuous with Nuclear Envelope
• Rough ER
– Ribosomes stud the surface
– Produce membrane and secretory proteins (i.e. salivary
glands)
– Products transferred via transport vesicles
• Smooth ER
– Lacks ribosomes on surface
– Synthesis of lipids (steroids)
– Helps liver detoxify drugs
Endoplasmic Reticulum (ER)
The Golgi Apparatus
• Refinery, warehouse and shipping center
• Products made in ER reach Golgi in transport
vehicles
• Receiving dock and shipping dock
• Modifications by enzymes as products move from
receiving to shipping
– Phosphate groups added as tags for different
destinations
– CIS face is the receiving side
– TRANS face is the transporting and delivery side
The Golgi Apparatus
Lysosomes
• Sac of digestive enzymes (animal cells)
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Proteins
Polysaccharides
Fats
Nucleic acids
• Develop from vesicles budding from Golgi
• Food vacuoles
– fuse with lysosomes, exposing food to enzymes for digestion
– Small molecules from digestion leave the lysosome and
nourish the cell.
• Breakdown damaged organelles
• Sculpturing feature
– Digest webbing between fingers and toes
Lysosomes
Vacuoles
• Sacs that bud from ER, Golgi or plasma
membranes
• Variety of size and function
– Contractile vacuoles of protists pump out excess
water in the cell.
– Central vacuoles of plants
• Store nutrients
• Absorb water
• May contain pigments or poisons
Vacuoles
Review of Endomembrane System
Chloroplasts and Mitochondria
• Energy Conversion
• Cellular power stations
Chloroplasts
• Photosynthetic cells of plants and algae
• 3 compartments
– Space between membranes that surround
chloroplast
– Stroma: thick fluid
– Network of disks and tubes
• Grana: interconnected stacks of disks
• Solar power pack
Chloroplasts
Mitochondria
• Site of cellular respiration
– Harvest Energy from food and converts to ATP
• Found in all eukaryotic cells
• Structure
– Enveloped by 2 membranes filled with matrix
– Inner membrane has several infoldings (cristae)
• Contain DNA that encodes their own protein
Mitochondria
Endosymbiont Theory
• Theory states that an early ancestor of eukaryotic
cells engulfed an oxygen using non
photosynthetic prokaryotic cell. Eventually, the
engulfed cell formed a relationship with the host
it was enclosed, becoming an endosymbiont.
Eventually these 2 merged into a single cell which
was a eukaryote with mitochondria.
• At least one of theses mitochondrion
endosymbiont cells also took in a prokaryotic cell
containing chloroplasts leading to plant life
Endosymbiont Theory Supports
• Unlike endomembranous organelles with one
membrane, chloroplasts and mitochondria
have double membranes surrounding them
• Both contain ribosomes and their own DNA
attached to inner membranes
• Bothe are somewhat independent and grow
and reproduce within the cell.
Peroxisomes
• Detoxify harmful substances that enter the
cell.
• Seen especially in kidney and liver
• Contain peroxidase and catalase (some
chemical reactions in the body create
hydrogen peroxide which is toxic to cells.
Peroxidase breaks this down.
The Cytoskeleton
• Network of fibers extending throughout
cytoplasm
• Skeleton and muscles
– Support and movement
Maintaining Cell Shape
• Series of fibers
– Microtubules
• Straight hollow tubes composed of proteins
• Guide movement of chromosomes when cells divide
– Intermediate filaments and Microfilaments
• Both thinner and solid
• Give shape much like keratin in dead skin
– ACTIN filaments- smallest fibers
• Anchorage and reinforcement for organelles
• Dynamic cytoskeleton (amoeboid movements)
Cytoskeleton
• Microtubules
– Made of tubulin- globular protein
• DIMER- molecule with 2 subunits alpha and beta
– Supports cell shape and tracks for organelles.
– Animal cells have a CENTROSOME- near nucleus
that acts as an organizing center for microtubules.
• CENTRIOLES are located within the centrosome and
have 9 sets of triplet microtubules arranged in a ring
ACTIN
• Actin filaments are smaller than intermediate
fibers and work with other proteins to
contract muscle tissue.
• In plants the interaction of actin and myosin
cause a cytoplasmic streaming- circular flow of
cytosol that helps speed distribution of
materials in the cell
Maintaining Cell Shape
Cilia and Flagella
• Mobile appendages
• Aid in movement
• Flagella
– Generally occur singly
– Propel cell
– Undulating whiplike motion
• Cilia
– Shorter and more numerous than flagella
– Promote movement by back and forth motion
– Some function to move fluid over tissue surfaces
Cilia and Flagella
Extracellular Matrix (ECM)
• Animals have an elaborate ECM made of glycoproteins (aka
proteoglycans). Most abundant proteoglycan is COLLAGEN.
• Collagen makes up 40% of protein found in humans.
Collagen is secreted from the matrix
• Some cells attach to ECM by proteins called INTEGRINS.
ECM integrins transmit signals to the ECM and cytoskeleton
and integrate changes occurring outside and inside the cell.
– Current research suggests that these ECM integrins can regulate
cell behavior and may even influence the nucleus of the cell.