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Chapter 6: A Tour of the Cell - Coggle Diagram
Chapter 6: A Tour of the Cell
Biologists use microscopes and biochemistry to study cells
Microscopy
Light microscope
Microscope utilizing light passing through specimen and glass lenses that refract the light to magnify the image
Offers advantages in studying living cells
Fluorescent labeling increases detail of cellular structures/molecules
Confocal and deconvolution microscopy have produced sharper, 3-D images of tissues/ cells
Super-resolution microscopy
Group of new techniques allowing researching to break resolution barrier and distinguish subcellular structures
Magnification
Ratio of an object's image to its real size
Contrast
The difference between light and dark areas of an image
Resolution
The measure of clarity of an image, or the minimum distance two objects can maintain distinguished forms
Most important tool of cytology
The study of cell structure
Organelles
Membrane enclosed structures within eukaryotic cells
Electron microscope
Microscope that focuses a beam of electrons through or onto a specimen's surface
Kills cells and can introduce artifacts to specimen
introduced in 1950s to effectively study organelles
Scanning electron microscope (SEM)
Electron beam excites surface electrons which are translated into an electronic signal sent to a video screen; produces three-dimensional image
Used for detailed study of topography of specimen
Transmission electron microscope (TEM)
Electron beam aimed through small section of specimen which has been stained with heavy metals that attach to cellular structures and enhance electron density of certain parts of cell
Used to study internal structure of cell
Cryo-electron microscopy (cryo-EM)
Allows specimens to be viewed at low temperature, limiting use of preservatives and allowing visualization of cellular structures
Typically uses electromagnets as lenses instead of glass lenses
Cell Fractionation
Technique for studying cell structure/function by separating cell into major organelles and other subcellular units
Centrifuge
Equipment that enables differential centrifugation, spinning test tubes holding mixtures of cells at differing speeds
Force from speed causes pellet, or subset of cell components, to settle at bottom of tube
Higher speeds = pellet of higher components
Lower speeds = pellet of larger components
Eukaryotic cells have internal membranes that compartmentalize their functions
Prokaryotic and Eukaryotic cells
All cells have:
Cytosol
Jelly-like semifluid substance in which subcellular components are suspended
Chromosomes
Carry genes in the form of DNA
Plasma membrane (cell membrane)
Selective barrier
Ribosomes
Tiny complexes that make proteins according to genetic instructions
Location of DNA
P
Concentrated in nucleoid, an area that is not membrane enclosed
E
Inside of the nucleus, bounded by a double membrane
Interior of cell; cytoplasm
P
Some contain regions surrounded by proteins homing specific reactions
No membrane-bound organelles
E
Region between nucleus and plasma membrane
Suspends variety of organelles with specialized forms/functions
Size
P
Much smaller
Typically, in bacteria 1-5 μm
E
Much larger
Typically, 10-100 μm in diameter
Metabolic requirements pose upper and lower limits on cell size
Panoramic view of the eukaryotic cell
Extensive and elaborate internal membranes called organelles
Local environments that support incompatible processes via metabolic functions
Plasma and organelle membranes participate directly in cell's metabolism due to enzyme formation
Biological membranes
Double layer of phospholipids and other lipids
Embedded with diverse/unique proteins and lipids depending on cells function
The eukaryotic cell's genetic instructions are housed in the nucleus and carried out by the ribosomes
Nucleus: Information central
Contains majority of genetic information
Averages 5 μm in diameter
Enclosed by nuclear envelope, separating contents from cytoplasm
Double membrane of two lipid bilayers
Perforated by pore structures
Proteins called pore complexes line these pores and regulate entry/exit of proteins, RNAs, and macromolecule complexes
Nuclear side of envelope at pores is lined by nuclear lamina
Netlike array of protein filaments that maintain shape of nucleus by supporting envelope
Evidence of nuclear matrix; framework of fibers extending throughout interior
Chromosomes
Structures that carry genetic information called DNA
Each chromosome contained a long DNA molecule associated with many proteins
Complex of DNA and proteins is called chromatin
When not dividing, chromosomes cannot be distinguished; appears as a diffuse mass
As cell prepares for division, chromosomes form loops and coils
Each eukaryotic species contains characteristic number of chromosomes
Ex: Humans - 46
Nucleolus
Where rRNA is synthesized from genes in DNA
Proteins imported from cytoplasm are assembled with rRNA into subunits of ribosomes, which can exit into cytoplasm to assemble ribosomes
Ribosomes: Protein factories
Cellular components that carry out protein synthesis
Not organelles because they are not membrane bound
Bound
Attached to outside of endoplasmic reticulum or nuclear envelope
Most proteins made here insert into membranes, for packaging within certain organelles like lysosomes or for export
Free
Suspended in the cytosol
Most proteins made here function within cytosol, such as enzymes that catalyze sugar breakdown
The endomembrane system regulated protein traffic and performs metabolic functions
Endomembrane system
Carries out variety of tasks such as protein synthesis, protein transport, movement of lipids, and detoxification of poisons
Nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles/vacuoles, plasma membrane
The endoplasmic reticulum: Biosynthetic factory
Accounts for over half of membrane in most eukaryotic cells
Consists of membranous tubules and sacs: cisternae
Membrane separates continuous compartment of ER lumen/cisternal space from cytosol
Continuous with the nuclear envelope - space between two membranes of envelope is continuous with lumen of ER
Rough ER
Studded with ribosomes
Transport vesicles bud off region of this ER called transitional ER to travel to GA, etc.
Ribosomes here often create secretory proteins
Glycoproteins
Proteins covalently bonded to carbohydrates, which are attached to proteins in ER lumen by enzymes built into ER membrane
Released within membranes of vesicles = transport vesicles
Membrane factory
Grows membrane proteins from ribosomes are inserted into ER membrane
Makes membrane phospholipids
Smooth ER
Not studded with ribosomes
Enzymes here help synthesize lipids; cells that secrete sex hormones (such as within testes/ovaries) are rich in smooth ER
Has enzymes that help detoxify poisons/drugs, particularly within liver cells. Hydroxyl groups are added to drug molecules making them easier to flush
Some drugs can induce proliferation of smooth ER, increasing tolerance
Stores calcium ions
Can stimulate muscle contraction by pumping calcium from lumen back into cytosol
The Golgi Apparatus: Shipping and receiving center
ER products are modified, stored, and sent off
Cis face: receiving
Trans face: sending
Consists of flattened membranous sacs called cisternae, the membranes within separating internal space from cytosol
Transport vesicles move material from ER to Golgi apparatus
Manufactures some macromolecules
Secretes many polysaccharides
Nonprotein products secrete from trans face inside vesicles to eventually fuse with cell membrane
Adds molecular identification tags like phosphate onto products for proper transport
Lysosomes: Digestive compartments
Membranous sac of hydrolytic enzymes used to digest macromolecules
Enzymes work best in acidic environments
Phagocytosis
A lysosome fuses with a food vacuole whose enzymes digest the food
Autophagy
Recycles cell's organic material from cytoplasm with hydrolytic enzymes
Tay-Sachs disease
Lysosomes do not produce functioning lipid-digesting hydrolytic enzymes, causing accumulation of lipids in brain
Vacuoles: Diverse maintenance compartments
Large vesicles derived from ER and Golgi apparatus
Food vacuoles
Formed by phagocytosis
Contractile vacuoles
Pump excess water out of cell
Can carry out enzymatic hydrolysis
Can hold reserves of both important organic compounds and poisonous ones - or pigments
Large central vacuole
Found in plants
Contains solution called cell sap which main storage for inorganic ions
Plant cells enlarge as vacuole absorbs water, enabling cell growth with minimal cytoplasmic investment
Mitochondria and chloroplasts change energy from one form to another
Mitochondria
Sites of cellular respiration
Metabolic process generating ATP from oxygen and nutrients
Chloroplasts
Site of photosynthesis
Process that synthesizes organic compounds with sunlight from CO2 and water (solar to chemical)
The evolutionary origins of mitochondria and chloroplasts
Endosymbiont theory
States that early ancestor of eukaryotic cells engulfed certain prokaryotic cell, which eventually became an endosymbiont and then later a mitochondrion within a single organism
Both are autonomous organelles surrounded by two membranes and contain ribosomes and DNA molecules
Mitochondria: Chemical energy conversion
Two phospholipid bilayer membranes with cristae-riddled internal one, which divides two internal compartments
Intermembrane space
Narrow region between inner and outer membranes
Mitochondrial matrix
Enclosed by inner membrane
Contains different enzymes, ribosomes, and DNA
Chloroplasts: Capture of light energy
Lens-shaped organelles found in leaves and other green organs of plants/algae
Two membranes with narrow intermembrane space
Contains membranous system of flattened sacs called thylakoids
Stack of thylakoids: granum
Fluid outside thylakoids is called stroma
Contains DNA and ribosomes and enzymes
Compartmental organization of intermembrane space, stroma, and thylakoid space allow for photosynthesis
Specialized member of family of closely related plant organelles called plastids
Peroxisomes: oxidation
Specialized metabolic compartment with one membrane
Contain enzymes that remove hydrogen atoms from substrates and transfer them to oxygen, producing hydrogen peroxide as by-product
Contains enzyme to turn toxic hydrogen peroxide into water
Can be used to break down fatty acids, detoxify alcohol, etc.
The cytoskeleton is a network of fibers that organizes structures and activities in the cell
Cytoskeleton
Network of fibers extending throughout the cytoplasm
Roles of the cytoskeleton: Support and motility
Gives mechanical support to cell and maintains shape; important because animal cells lack walls
Stabilized by balance between opposing forces, exerted by its elements
Cell motility
Requires cytoskeleton interaction with motor proteins
Ex. Cytoskeletal elements and motor proteins working w/ plasma membrane molecules to allow cells to move along external fibers
Describes changes in cell location and movement of cell parts
Components of the cytoskeleton
Microfilaments
Made of molecules of actin; twisted double chain of actin subunits
Present in all eukaryotic cells; can form structural networks
Instead of providing compression-resistant structure, they bear tension to help support cell shape
GIves cortex (outer cytoplasmic layer of cell) semisolid consistency
Important for cell motility
Actin filaments made of myosin interact to initiate muscle contraction
Cell crawls along surface via extensions called pseudopodia
Cytoplasmic streaming
Circular flow of cytoplasm within plant cells contributed by actin-protein interactions
Intermediate filaments
Only found in cells of some animals
Diverse class of element for bearing tension
Inconsistent with diameter and composition, but are permanent fixtures within cells
Ex. outer layer of skin ample with keratin filaments
Microtubules
Hollow rods made of globular proteins called tubulins; each tubulin protein is called a dimer, meaning molecule of two components. Dimers grow via added dimers
One end can accumulate/release dimers at higher rate, thus growing and shrinking often
Shape/support cell; serve as tracks for organelles w/ motor proteins to follow
Grow out of centrosome in animals, located near nucleus - microtubules act as girders.
Pair of centrioles inside centrosome which arrange microtubules in ring formation
Flagella and cilia
Can act as motor propellers for unicellular protists, or within sperm of animals/plants/algae
Can move fluid over surface of tissue when arranged in sheets
Cellular extensions containing microtubules
May act as signal-receiving antenna
All have similar structure of group of microtubules sheathed in extension of plasma membrane
Extracellular components and connections between cells help coordinate cellular activities
Cell walls of plants
Extracellular structure of plants
Thicker than cell membranes
Protects plant cell, maintains shape, and prevents excessive water uptake
Enzyme cellulose synthase synthesizes and secretes microfibrils from polysaccharide cellulose into the extracellular space to embed in matrix
Primary cell wall
Thin and flexible wall secreted by young plant cell
Middle lamella
A thin layer of sticky polysaccharides called pectins sandwiched in-between primary cell walls to keep cells together
Secondary cell wall
Added between membrane and primary cell wall for additional structure, support, and protection in some plants
The extracellular matrix (ECM) of animal cells
Made of glycoproteins and other carbohydrate-containing molecules
Fibronectin
Bind to cell-surface receptor proteins called integrins
ECM can regulate cell behavior via integrin communication
Ex. migrating cells within an embryo
Most abundantly made of collagen
Collagen fibers embedded in network made of proteoglycans, or molecules composed of multiple carbohydrate chains attached to one core protein
Cell junctions
Plasmodesmata in plant cells
Channels that connect cells
Filled with cytosol; cells share same internal environment
Adjacent cell plasma membranes line the channels, thus are continuous
Water and small solutes pass freely; large macromolecules move along cytoskeletal fibers
Tight junctions, desmosomes, gap junctions
Tight junctions
Forms continuous seal around cell to prevent fluid leakage
Desmosomes
Fasten cells together in strong sheets
Gap junctions
Provide cytoplasmic communication channels between cells
A cell is greater than the sum of its parts
No cell components work alone
The scale of molecular machinery in cells can provide context for the collaborative nature of cellular components