ch.3 Flashcards
Cell
structural and functional unit of life
Organismal functions depend on individual and collective cell functions
Biochemical activities of cells dictated by their shapes or forms, and specific subcellular structures
Continuity of life has cellular basis
Cell Diversity
Over 200 different types of human cells
Types differ in size, shape, subcellular components, and functions
Generalized Cell
All cells have some common structures and functions
Human cells have three basic parts
Plasma membrane—flexible outer boundary
Cytoplasm—intracellular fluid containing organelles
Nucleus—control center
Plasma Membran
Lipid bilayer and proteins in constantly changing fluid mosaic
Plays dynamic role in cellular activity
Separates intracellular fluid (ICF) from extracellular fluid (ECF)
Membrane Lipids
75% phospholipids (lipid bilayer)
Phosphate heads: polar and hydrophilic
Fatty acid tails: nonpolar and hydrophobic
5% glycolipids
Lipids with polar sugar groups on outer membrane surface
20% cholesterol, ncreases membrane stability
Membrane Proteins
- Allow communication with environment
- ½ mass of plasma membrane
- Most specialized membrane functions
- Some float freely
- Some tethered to intracellular structures
Two Types of Membrane Proteins
Integral proteins; peripheral proteins or
Membrane Proteins
• Integral proteins
– Firmly inserted into membrane (most are transmembrane)
– Have hydrophobic and hydrophilic regions
• Can interact with lipid tails and water
– Function as transport proteins (channels and carriers), enzymes, or receptors
Membrane Proteins
Peripheral proteins
Loosely attached to integral proteins
Include filaments on intracellular surface for membrane support
Function as enzymes; motor proteins for shape change during cell division and muscle contraction; cell-to-cell connections
Six Functions of Membrane Proteins
Transport
Receptors for signal transduction
Attachment to cytoskeleton and extracellular matrix
Six Functions of Membrane Proteins
Enzymatic activity, Intercellular joining, Cell-cell recognition, 20% of outer membrane surface,Contain phospholipids, sphingolipids, and cholesterol, More stable; less fluid than rest of membrane
The Glycocalyx
Sugar covering” at cell surface
– Lipids and proteins with attached carbohydrates (sugar groups)
• Every cell type has different pattern of sugars
– Specific biological markers for cell to cell recognition
– Allows immune system to recognize “self” and “non self”
– Cancerous cells change it continuously
Cell Junctions
• Some cells "free" – e.g., blood cells, sperm cells • Some bound into communities – Three ways cells are bound: • Tight junctions • Desmosomes • Gap junctions
Tight Junctions
• Adjacent integral proteins fuse to form impermeable junction encircling cell
– Prevent fluids and most molecules from moving between cells
• Where might these be useful in body?
Cell Junctions: Desmosomes
• “Rivets” or “spot-welds” that anchor cells together at plaques (thickenings on plasma membrane)
– Linker proteins between cells connect plaques
– Keratin filaments extend through cytosol to opposite plaque giving stability to cell
– Reduces possibility of tearing
Gap Junctions
• Transmembrane proteins form pores (connexons) that allow small molecules to pass from cell to cell
– For spread of ions, simple sugars, and other small molecules between cardiac or smooth muscle cells
Plasma Membrane
Cells surrounded by interstitial fluid (IF)
– Contains thousands of substances, e.g., amino acids, sugars, fatty acids, vitamins, hormones, salts, waste products
• Plasma membrane allows cell to
– Obtain from IF exactly what it needs, exactly when it is needed
– Keep out what it does not need
• Plasma membranes selectively permeable
– Some molecules pass through easily; some do not
• Two ways substances cross membrane
– Passive processes
– Active processes
Passive processe
– No cellular energy (ATP) required
– Substance moves down its concentration gradient
two types: diffusion and filtration
Active processes
– Energy (ATP) required
– Occurs only in living cell membranes
Diffusion
• Simple diffusion
• Carrier- and channel-mediated facilitated diffusion
• Osmosis
• Collisions cause molecules to move down or with their concentration gradient
– Difference in concentration between two areas
• Speed influenced by molecule size and temperature
Passive Processes
• Molecule will passively diffuse through membrane if
– It is lipid soluble, or
– Small enough to pass through membrane channels, or
– Assisted by carrier molecule
Filtration
Usually across capillary walls
Simple Diffusion
• Nonpolar lipid-soluble (hydrophobic) substances diffuse directly through phospholipid bilayer
– E.g., oxygen, carbon dioxide, fat-soluble vitamins
Facilitated Diffusion
• Certain lipophobic molecules (e.g., glucose, amino acids, and ions) transported passively by
– Binding to protein carriers
– Moving through water-filled channels
Carrier-Mediated Facilitated Diffusion
• Transmembrane integral proteins are carriers
• Transport specific polar molecules (e.g., sugars and amino acids) too large for channels
• Binding of substrate causes shape change in carrier then passage across membrane
• Limited by number of carriers present
– Carriers saturated when all engaged
Channel-Mediated Facilitated Diffusion
• Aqueous channels formed by transmembrane proteins
• Selectively transport ions or water
• Two types:
– Leakage channels
• Always open
– Gated channels
• Controlled by chemical or electrical signals
Importance of Osmosis
- Osmosis causes cells to swell and shrink
* Change in cell volume disrupts cell function, especially in neurons
Tonicity
Ability of solution to alter cell’s water volume
– Isotonic: Solution with same non-penetrating solute concentration as cytosol
– Hypertonic: Solution with higher non-penetrating solute concentration than cytosol
– Hypotonic: Solution with lower non-penetrating solute concentration than cytosol
Active transport
Active Transport: Two Types
• Primary active transport
– Required energy directly from ATP hydrolysis
• Secondary active transport
– Required energy indirectly from ionic gradients created by primary active transport
Primary Active Transport
• Energy from hydrolysis of ATP causes shape change in transport protein that "pumps" solutes (ions) across membrane. • Sodium-potassium pump – Most well-studied – Carrier (pump) called Na+-K+ ATPase – Located in all plasma membranes
Secondary Active Transport
• Depends on ion gradient created by primary active transport
• Energy stored in ionic gradients used indirectly to drive transport of other solute
• Cotransport—always transports more than one substance at a time
– Symport system: Substances transported in same direction
– Antiport system: Substances transported in opposite directions
Vesicular Transport
• Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles
• Requires cellular energy
• Functions:
– Exocytosis—transport out of cell
– Endocytosis—transport into cell
• Phagocytosis, pinocytosis, receptor-mediated endocytosis
– Transcytosis—transport into, across, and then out of cell
– Vesicular trafficking—transport from one area or organelle in cell to another
Roles of Cell Adhesion Molecules
- Thousands on approximately every cell in body
- Anchor to extracellular matrix or each other
- Assist in movement of cells past one another
- Attract WBCs to injured or infected areas
- Stimulate synthesis or degradation of adhesive membrane junctions
- Transmit intracellular signals to direct cell migration, proliferation, and specialization
Roles of Plasma Membrane Receptors
• Contact signaling—touching and recognition of cells; e.g., in normal development and immunity
• Chemical signaling—interaction between receptors and ligands (neurotransmitters, hormones, and paracrines) to alter activity of cell proteins (e.g., enzymes or chemically gated ion channels)
– Same ligand can cause different cell responses
– Response determined by what receptor linked to inside cell
Chemical Signaling
• Ligand binding to receptor structural change protein alteration
– Catalytic receptor proteins become activated enzymes
– Chemically gated channel-linked receptors open and close ion gates changes in excitability
– G protein–linked receptors activate G protein, affecting an ion channel or enzyme, or causing release of internal second messenger, such as cyclic AMP
Cytoplasm
• Located between plasma membrane and nucleus
– Composed of
• Cytosol
– Water with solutes (protein, salts, sugars, etc.)
• Organelles
– Metabolic machinery of cell; each with specialized function; either membranous or nonmembranous
• Inclusions
– Vary with cell type; e.g., glycogen granules, pigments, lipid droplets, vacuoles, crystals
Cytoplasmic Organelles
• Membranous – Mitochondria – Peroxisomes – Lysosomes – Endoplasmic reticulum – Golgi apparatus • Nonmembranous – Cytoskeleton – Centrioles – Ribosomes
Mitochondria
• Double-membrane structure with inner shelflike cristae
• Provide most of cell’s ATP via aerobic cellular respiration
– Requires oxygen
• Contain their own DNA, RNA, ribosomes
• Similar to bacteria; capable of cell division called fission
Ribosomes
- Granules containing protein and rRNA
- Site of protein synthesis
- Free ribosomes synthesize soluble proteins that function in cytosol or other organelles
- Membrane-bound ribosomes (forming rough ER) synthesize proteins to be incorporated into membranes, lysosomes, or exported from cell
Endoplasmic Reticulum (ER)
• Interconnected tubes and parallel membranes enclosing cisterns • Continuous with outer nuclear membrane • Two varieties: – Rough ER – Smooth ER
Rough ER
- External surface studded with ribosomes
- Manufactures all secreted proteins
- Synthesizes membrane integral proteins and phospholipids
- Assembled proteins move to ER interior, enclosed in vesicle, go to Golgi apparatus
Smooth ER
• Network of tubules continuous with rough ER
• Its enzymes (integral proteins) function
• Its enzymes (integral proteins) function in
– Lipid metabolism; cholesterol and steroid-based hormone synthesis; making lipids of lipoproteins
– Absorption, synthesis, and transport of fats
– Detoxification of drugs, some pesticides, carcinogenic chemicals
– Converting glycogen to free glucose
– Storage and release of calcium
Golgi Apparatus
• Stacked and flattened membranous sacs
• Modifies, concentrates, and packages proteins and lipids from rough ER
• Transport vessels from ER fuse with convex cis face; proteins modified, tagged for delivery, sorted, packaged in vesicles
• Three types of vesicles bud from concave trans face
– Secretory vesicles (granules)
• To trans face; release export proteins by exocytosis
– Vesicles of lipids and transmembrane proteins for plasma membrane or organelles
– Lysosomes containing digestive enzymes; remain in cell
Peroxisomes
• Membranous sacs containing powerful oxidases and catalases
• Detoxify harmful or toxic substances
• Catalysis and synthesis of fatty acids
• Neutralize dangerous free radicals (highly reactive chemicals with unpaired electrons)
– Oxidases convert to H2O2 (also toxic)
– Catalases convert H2O2 to water and oxygen
Lysosomes
• Spherical membranous bags containing digestive enzymes (acid hydrolases)
– “Safe” sites for intracellular digestion
• Digest ingested bacteria, viruses, and toxins
• Degrade nonfunctional organelles
• Metabolic functions, e.g., break down and release glycogen
• Destroy cells in injured or nonuseful tissue (autolysis)
• Break down bone to release Ca2+
Endomembrane System
• Overall function
– Produce, degrade, store, and export biological molecules
– Degrade potentially harmful substances
• Includes ER, golgi apparatus, secretory vesicles, lysosomes, nuclear and plasma membranes
Cytoskeleton
• Elaborate series of rods throughout cytosol; proteins link rods to other cell structures – Three types • Microfilaments • Intermediate filaments • Microtubules
Microfilaments
• Thinnest of cytoskeletal elements
• Dynamic strands of protein actin
• Each cell-unique arrangement of strands
• Dense web attached to cytoplasmic side of plasma membrane-terminal web
– Gives strength, compression resistance
• Involved in cell motility, change in shape, endocytosis and exocytosis
Intermediate Filaments
- Tough, insoluble, ropelike protein fibers
- Composed of tetramer fibrils
- Resist pulling forces on cell; attach to desmosomes
- E.g., neurofilaments in nerve cells; keratin filaments in epithelial cells
Microtubules
- Largest of cytoskeletal elements; dynamic hollow tubes; most radiate from centrosome
- Composed of protein subunits called tubulins
- Determine overall shape of cell and distribution of organelles
- Mitochondria, lysosomes, secretory vesicles attach to microtubules; moved throughout cell by motor proteins
Motor Proteins
- Protein complexes that function in motility (e.g., movement of organelles and contraction)
- Powered by ATP
Centrosome and Centrioles
• “Cell center” near nucleus
• Generates microtubules; organizes mitotic spindle
• Contains paired centrioles
– Organelles; small tubes formed by microtubules
• Centrioles form basis of cilia and flagella
Cellular Extensions
• Cilia and flagella
– Whiplike, motile extensions on surfaces of certain cells
– Contain microtubules and motor molecules
– Cilia move substances across cell surfaces
– Longer flagella propel whole cells (tail of sperm)
Cilia and Flagella
• Centrioles forming base called basal bodies
• Cilia movements alternate between power stroke and recovery stroke current at cell surface
• Primary cilia
– Single, nonmotile projection on most cells
– Probe environment for molecules receptors can recognize; coordinate intracellular pathways
Cellular Extensions
• Microvilli
– Minute, fingerlike extensions of plasma membrane
– Increase surface area for absorption
– Core of actin filaments for stiffening
Nucleus
- Largest organelle; genetic library with blueprints for nearly all cellular proteins
- Responds to signals; dictates kinds and amounts of proteins synthesized
- Most cells uninucleate; skeletal muscle cells, bone destruction cells, and some liver cells are multinucleate; red blood cells are anucleate
- Three regions/structures
The Nuclear Envelope
- Double-membrane barrier; encloses nucleoplasm
- Outer layer continuous with rough ER and bears ribosomes
- Inner lining (nuclear lamina) maintains shape of nucleus; scaffold to organize DNA
- Pores allow substances to pass; nuclear pore complex line pores; regulates transport of large molecules into and out of nucleus
Nucleoli
• Dark-staining spherical bodies within nucleus
• Involved in rRNA synthesis and ribosome subunit assembly
• Associated with nucleolar organizer regions
– Contains DNA coding for rRNA
• Usually one or two per cell
Chromatin
- Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%)
- Arranged in fundamental units called nucleosomes
- Histones pack long DNA molecules; involved in gene regulation
- Condense into barlike bodies called chromosomes when cell starts to divid
Cell Cycle
• Defines changes from formation of cell until it reproduces
• Includes:
– Interphase
• Cell grows and carries out functions
– Cell division (mitotic phase)
• Divides into two cells
Interphase
• Period from cell formation to cell division
• Nuclear material called chromatin
• Three subphases:
– G1 (gap 1)—vigorous growth and metabolism
• Cells that permanently cease dividing said to be in G0 phase
– S (synthetic)—DNA replication occurs
– G2 (gap 2)—preparation for division
DNA Replication
• Prior to division cell makes copy of DNA
• DNA helices separated into replication bubbles with replication forks at each end
– Each strand acts as template for complementary strand
• DNA polymerase begins adding nucleotides at RNA primer
• DNA polymerase continues from primer
– Synthesizes one leading, one lagging strand
• DNA polymerase only works in one direction
– Leading strand synthesized continuously
– Lagging strand synthesized discontinuously into segments
– DNA ligase splices short segments of discontinuous strand together
DNA Replication
• End result: two identical DNA molecules formed from original
Cell Division
• Meiosis - cell division producing gametes
• Mitotic cell division - produces clones
– Essential for body growth and tissue repair
– Occurs continuously in some cells
• Skin; intestinal lining
– None in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle
• Repairs with fibrous tissue
Events Of Cell Division
• Mitosis—division of nucleus
– Four stages ensure each cell receives copy of replicated DNA
• Prophase
• Metaphase
• Anaphase
• Telophase
– Cytokinesis—division of cytoplasm-by cleavage furrow
Protein Synthesis
• DNA is master blueprint for protein synthesis
• Gene - segment of DNA with blueprint for one polypeptide
• Triplets (three sequential DNA nitrogen bases) form genetic library
– Bases in DNA are A, G, T, and C
– Each triplet specifies coding for number, kind, and order of amino acids in polypeptide
Role of RNA
– DNA decoding mechanism and messenger – Three types–all formed on DNA in nucleus • Messenger RNA (mRNA); ribosomal RNA (rRNA); transfer RNA (tRNA) • RNA differs from DNA – Uracil is substituted for thymine Roles of the Three Main Types of RNA • Messenger RNA (mRNA) – Carries instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm Roles of the Three Main Types of RNA • Ribosomal RNA (rRNA) – Structural component of ribosomes that, along with tRNA, helps translate message from mRNA Roles of the Three Main Types of RNA • Transfer RNAs (tRNAs) – Bind to amino acids and pair with bases of codons of mRNA at ribosome to begin process of protein synthesis Protein Synthesis • Occurs in two steps – Transcription • DNA information coded in mRNA – Translation • mRNA decoded to assemble polypeptides
Role of Rough ER in Protein Synthesis
• mRNA–ribosome complex directed to rough ER by signal-recognition particle (SRP)
• Forming protein enters ER
• Sugar groups may be added to protein, and its shape may be altered
• Protein enclosed in vesicle for transport to Golgi apparatus
Summary: From DNA to Proteins
• Complementary base pairing directs transfer of genetic information in DNA into amino acid sequence of protein
– DNA triplets mRNA codons
– Complementary base pairing of mRNA codons with tRNA anticodons ensures correct amino acid sequence
– Anticodon sequence identical to DNA sequence except uracil substituted for thymine
Extracellular Materials
• Body fluids-interstitial fluid, blood plasma, and cerebrospinal fluid
• Cellular secretions-intestinal and gastric fluids, saliva, mucus, and serous fluids
• Extracellular matrix–most abundant extracellular material
– Jellylike mesh of proteins and polysaccharides secreted by cells; acts as “glue” to hold cells together
Developmental Aspects of Cells
• All cells of body contain same DNA but cells not identical
• Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and others off
• Development of specific and distinctive features in cells called cell differentiation
Apoptosis and Modified Rates of Cell Division
• During development more cells than needed produced (e.g., in nervous system)
• Eliminated later by programmed cell death (apoptosis)
– Mitochondrial membranes leak chemicals that activate caspases DNA, cytoskeleton degradation cell death
– Dead cell shrinks and is phagocytized
Apoptosis and Modified Rates of Cell Division
• Organs well formed and functional before birth
• Cell division in adults to replace short-lived cells and repair wounds
• Hyperplasia increases cell numbers when needed
• Atrophy (decreased size) results from loss of stimulation or use
Theories of Cell Aging
• Wear and tear theory-Little chemical insults and free radicals have cumulative effects
• Mitochondrial theory of aging–free radicals in mitochondria diminish energy production
• Immune system disorders-autoimmune responses; progressive weakening of immune response; C-reactive protein of acute inflammation causes cell aging
Theories of Cell Aging
• Most widely accepted theory
– Genetic theory-cessation of mitosis and cell aging programmed into genes
• Telomeres (strings of nucleotides protecting ends of chromosomes) may determine number of times a cell can divide
• Telomerase lengthens telomeres
– Found in germ cells; ~ absent in adult cells
