Cell Biology- Final Exam

Describe cleavage furrow formation in animal cells.

• cleavage furrow is formed by a belt-like bundle of actin filaments called the contractile ring

• contractile ring forms just beneath the plasma membrane (aka in the cortex) during anaphase

• cleavage furrow → cytokinesis (cytoplasm division)

• as cleavage progresses, the ring of actin tightens around the cytoplasm eventually pinching the cell in 2

• entire contractile ring is dismantled shortly after cytokinesis is complete

During cleavage furrow formation in animal cells a ring of actin tightens around the cytoplasm and pinches the cell in 2- how is the actin ring tightened?

• tightening of the actin ring involves interactions between actin filaments and myosin II

• movement of myosin along the actin causes the contraction and tightening of actin filaments

Describe cell plate formation.

• plant cell cytokinesis

• microtubule associated

  • vesicles travel on microtubules to midline in cell division

• during late anaphase a group of small vesicles derived from the golgi complex align themselves across the equatorial region of the spindle

  • vesicles contain polysaccharides and glycoproteins required for cell wall formation

• vesicles contain polysaccharides and glycoproteins required for cell wall formation

• vesicles are guided to the spindle equator by an array of microtubules and associated motor proteins

  • microtubule motor proteins: dynein, kinesin, myosin

  • vesicles fuse together to produce a cell plate which represents the cell wall in the process of formation

• vesicles fuse together to produce a cell plate which represents the cell wall in the process of formation

• plant cell cytokinesis DOES NOT INVOLVE ACTIN

What role does microtubules play in the formation of the cell plate.

vesicles travel on microtubules to midline in cell division

What cytoskeletal filaments are needed for cytokinesis in animal cells and plant cells?

• animal cells: actin and microtubules

• plant cells: ONLY microtubules

How do cells move? What are the steps of movement?

• cells move by protruding a portion of the cytoplasm

• steps in movement:

  • 1. protrusion

  • 2. adhesion

  • 3. contraction

What types of cells need to move?

amoeba, cells of immune system, embryonic cells, macrophages, fibroblasts during development

Do cilia and flagella use actin, microtubules, or both?

only microtubules

Describe the first step of cell locomotion.

• 1. protrusion- front of cell pushes out an extension called a lamellipodium

  • lamellipodium filled with actin filaments that have (+) end facing towards plasma membrane

    • formation of lamellipodium:

      • actin assembly is required for protrusion (assembly pushes on leading edge of cell membrane to form protrusion)

      • myosin I moves along actin filaments causing them to slide past one another

Describe the second step of cell locomotion.

• 2. adhesion

  • cells must adhere to substrate in order to move

  • adhesion is mediated by integrin protein

  • integrin: transmembrane adhesion protein

    • bound to an intracellular bundle of actin filaments

    • bound to extracellular matrix

    • focal adhesion

Describe the third step in cell locomotion.

• 3. contraction

  • back of cell contracts to move the front of the cell forward

  • maybe myosin based since myosin II has been localized to the rear end of the cell

  • myosin II in rear of cell interacts with actin filaments to contract the filaments (propels cell forward)

Describe the cell motility of disease-causing organisms and the role of actin in them.

• disease causing microorganisms can use the cells normal cell adhesion and cell motility systems to enter a cell

• gram (+) bacteria - Lysteria monocytogenes

• infects cells by binding to a cell adhesion molecule on the cell surface and enters the cell (phagocytosis) [not destroyed by the lysosome]

• once inside the cell, the bacterium moves in the cytoplasm where it can divide rapidly

• the bacterium moves inside the cell by producing a “comet tail” of polymerized actin [using the cytoplasmic g-actin of the host cell]

Describe microtubule associated motor proteins.

• motor proteins: kinesin, dynein, myosin I

• kinesin: (+) end directed, movement mediated by ATP hydrolysis

  • ex: movement of chromosomes during mitosis; intracellular vesicle transport

• dynein: (-) end directed, movement by ATP hydrolysis

  • ex: movement of chromosomes during mitosis, movement of sperm flagella

• myosin I: (+) end directed, movement by ATP hydrolysis

• functions: form mitotic spindle, chromosome segregation in mitosis, intracellular transport, anchor organelles

What is the role of microtubules in cell divison?

• form mitotic spindle

• microtubules radiate from centrosome [(-) embedded in centrosome]

  • centrosome: forms poles of mitotic spindle

  • centromere

  • kinetochore

• microtubule types: astral, polar, kinetochore

How do chromosomes move in cell division?

• microtubules can change length while attached to the kinetochore

  • tubulin subunits add to the (+) end

  • tubulin subunits can be removed at the kinetochore or at the pole

Describe microtubule length change in cell divison.

• microtubule length change:

  • microtubule lengthens as subunits are added to the (+) end

  • lengthens from the pole to capture the kinetochore

  • microtubule lengthens to find a target (chromosome)

Describe the role microtubule motor proteins play in cell divison.

• microtubule motors

  • dynein located at kinetochore is trying to move along the filaments towards the (-) end

  • chromosomes move toward pole

  • tubulin subunits lost where filaments connect to kinetochore

  • kinesin is located at poles and is trying to move toward (+) end

  • subunits removed at pole region (overall shortening of microtubule)

  • combo of dynein and kinesin moves chromosomes during anaphase

  • role of katanin:

    • microtubule associated protein

    • severing protein promotes removal of tubulin

    • localized to pole region

Describe role of microtubules in the metaphase stage of cell divison.

• balance between forces exerted by kinesin at poles and dynein at kinetochore

• no overall loss of subunits results in chromosome alignment

• process not well understood

Describe loss of function experiments associated with the cell cytoskeleton.

• knock-out or loss of function (kinesin/dynein)

• knock-out or block protein function

• look at position of the chromosomes

• look for a chromosome shift or absence of chromatid segregation

• stain chromosomes following loss of function (ex: DAPI)

• stain microtubules using immunofluorescence and look for shortening or abnormalities

What are some cell cycle controls?

• p53

• transcription factors

• cell cycle checkpoints

• apoptosis

What is p53? What is its function?

• p53: tumor suppressor gene that encodes a transcription factor

• functions:

  • activate DNA repair enzymes

  • can halt a cell at G1/s (regulation point- halts growth and DNA replication)

  • activate apoptosis (programmed cell death)

What role does the p53 gene play in HPV and cervical cancer?

• HPV: human papilloma virus

• upon infection, HPV produces a viral protein called E6

• E6 can bind to p53 protein and inactivate it

What role does transcription factors play in the cell cycle? What would a mutation in transcription factor do?

• needed to activate transcription

• bind to enhancer sequences and other DNA regulatory sequences to loosen interaction of DNA with histones and allow RNA polymerase to bind

• mutation in the transcription factor can:

  • prevent activation of gene expression (DNA repair enzymes or proteins involved in cell cycle checkpoints)

What is the purpose of cell cycle checkpoints?

• allow for cells to progress through cell cycle stage without errors

• genes encode for cell cycle checkpoint proteins

• oncogene is a mutated form of ones of these genes

What is the role of apoptosis in the cell cycle?

• aka programmed cell death

• biochemical events lead to changes in cell morphology and cell death

• changes in cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation

• dependent on family of proteins called caspases

• caspases:

  • cleave nuclear lamins causing the break down of the nuclear envelope cleave inactive forms of DNase to create an active form

    • DNase: degrades DNA

How do elephants and other large mammals crush cancer?

• have more cellular DNA (more mutations)

• numerous rounds of cell division (more chances for DNA errors)

• yet large mammals can survive up to 60 years in the wild

• elephants don’t develop cancer:

  • evolved to have 40 copies of the p53 gene (humans have 2 copies)

  • cells with damaged DNA are required or apoptosis is triggered

What are anti-mitotic drugs? What are some examples?

• anti-mitotic drugs: used to treat cancer (don’t target only cancer cells)

  • Vinblastine: binds to tubulin and prevents microtubule formation

    • tubulin is a dimer (α and β tubulin)

      • α and β subunits join to form protofilaments

      • protofilaments form wall of microtubule

  • Taxol: binds to microtubule and prevents disassembly

What is taxol and how does it stop cell divison or mitosis?

• taxol: binds to microtubule and prevents disassembly

• the microtubules can’t disassemble during anaphase to search for chromosomes

  • chromosome movement during cell division requires disassembly of microtubules

  • if microtubules cannot disassemble, chromosomes can’t segregate anaphase

• taxol-treated cells divide abnormally into more than 2 cells

• cells have jumbled chromosomes and are destroyed by apoptosis

What is metastasis?

• migration of cancer cells

• mechanism is unknown (actin mechanism)

• cells break cell adhesion

What role does the cellular cytoskeleton play in neurodegenerative diseases (such as alzheimers and parkinsons)?

• cause a change in the shape of the axon of neurons

• axon microtubules are stable- do not undergo dynamic instability

  • Tau protein (MAP): binds to and stabilizes microtubules

    • abundant in neurons of CNS (also found in other cells)

• microtubules collapse into a constricted structure (not well understood)

• abundant abnormal aggregates of cytoskeletal proteins in neurons and glial cells of CNS (ex: tau protein)

• tau protein becomes phosphorylated

• phosphorylated tau forms aggregates (clumps)

  • causes constriction of microtubule bundles

• phosphorylated tau can leave an affected nonfunctioning neuron and attach to a healthy neuron

What is the function of microtubules (with associated proteins)?

• guide intracellular transport

• anchor intracellular organelles

• form mitotic spindle

• function in chromosome segregation during cell divison

Define centrosome, centromere, and kinetochore in terms of mitosis.

• centrosome: poles

• centromere: holds chromosomes/chromatids together and connects chromosome to spindle fibers via the kinetochore

• kinetochore: complex of proteins and RNA molecules attached to the centromere

What is intracellular transport?

• proteins made in the cytoplasm must be transported to different compartments in the cell

• proteins carried in vesicles

• vesicles attach to cytoskeletal filaments (ex: microtubules, actin)

• movement of vesicles is mediated by motor proteins

Regarding intracellular transport, how do proteins find their correct target?

signal sequence: stretch of amino acids that specifies a destination

Describe the structure of the nucleus and the nuclear pore.

• nuclear envelope: double membrane, contains nuclear pore complexes

• nuclear pore structure:

  • octagonal structure of repeating subunits that forms a channel through the membrane

  • repeating subunits: nucleoporins

  • fibrils extend into the cytoplasm and nucleus: nuclear cage

How do small and large molecules transport across the nuclear envelope?

• small molecules move through the aqueous channel by diffusion (ex: water, small ions)

• large molecules:

  • move through the channel by active transport (require energy)

  • proteins to be imported require a nuclear localization signal

    • nuclear localization signal: stretch of amino acids in the protein sequence that directs the proteins to the nucleus

  • ex: histones, proteins, ribosomes, DNA replication enzymes, proteins needed for transcription, mRNA

Provide an example of a molecule that would be exported from the nucleus.

mRNA, ribosomes (produced by the nucleolus)

Describe the mechanism of transport for large molecules being imported into the nucleus.

• active transport

• nuclear import receptors:

  • soluble cytoplasmic proteins (importins)

  • soluble protein receptors found in cell cytoplasm

  • recognize the nuclear localization signal on the protein to be transported

  • recognize the nucleoporin structure (FG repeats)

    • FG repeats: short stretches of phenylalanine and glycine

      • importin proteins bind to FG repeats

      • in nucleoporin- proteins that form nuclear pore

• guide the transport of proteins through the pore (post-translational)

• how does transport occur?:

  • transport is powered by the hydrolysis of GTP: GTP binds to the import receptor and pore

  • hydrolysis of GTP causes a conformational change in pore structure

Describe the export of large molecules out of the nucleus.

• same process that is used for import of molecules

  • nuclear export signals are located on molecules to be exported

  • nuclear export receptors: soluble proteins

  • export receptors bind to the export signal and the nucleoporin and guide the cargo through the pore to the cytoplasm

  • export is powered by the hydrolysis of GTP

Describe protein synthesis in different parts of the cell.

• populations of ribosomes in:

  • 1. cytoplasm: structural proteins (ex: actin and tubulin), enzymes needed for metabolism

  • 2. associated with ER: soluble proteins that are destined for secretion (ex: hormones), resident ER proteins

  • 3. mitochondria and chloroplasts: synthesize proteins needed to carry out their own function

Where are proteins destined for the ER synthesized?

proteins destined for the ER are initially synthesized on cystolic ribosomes

Describe protein transport to the ER.

• ER signal sequence directs the protein to the ER

  • ER signal sequence: stretch of amino acids within the protein sequence

• signal recognition particle (SRP) recognizes and binds to the signal sequence

  • SRP:

    • composed of 6 polypeptides and an RNA molecule

    • binds to signal sequence on protein being synthesized

• binding of the SRP temporarily halts translation of the protein

• SRP/ ribosome complex is transported to the ER membrane

• SRP binds to a receptor on the ER membrane

• SRP is released from the complex and translation resumes

• as the protein is translated, it is threaded through a pore in the ER membrane (requires the hydrolysis of GTP)

  • co-translational import

What is the main difference between nuclear protein transport and protein transport into the ER?

• nuclear transport: post-translational

• ER transport: SRP, co-translational

What types of proteins get transported to the ER?

1. soluble/ secretory proteins

2. single pass trans-membrane ER proteins

3. multi pass trans-membrane ER proteins

Describe transport of soluble/secretory proteins to the ER.

• ER signal sequence acts as a start transport sequence

• ER signal sequence is cleaved off by the signal peptidase enzyme

• protein is released into the endoplasmic reticulum

  • co-translational transport

Describe transmembrane proteins in the ER and the two types.

• transmembrane proteins:

  • span width of membrane

  • contain a domain that faces outside ER and a domain that faces ER lumen

• single pass transmembrane proteins:

  • cross membrane one time

• multi pass proteins:

  • cross membrane more than once

Describe transport of single-pass transmembrane proteins into the ER

• ER protein: signal sequence initiates translocation (as described for soluble proteins)

• signal sequence found at amino terminal end of protein or in middle of sequence

• stop transfer signal:

  • anchors the protein in the membrane

  • middle of protein sequence

Describe transport if multi-pass transmembrane proteins to the ER.

• crosses the membrane more than once

• protein signal sequence initiates translocation (as previously described)

• protein signal sequence is located in the middle of the polypeptide sequence (portein signal sequence serves as the start transfer sequence)

  • ALWAYS in the middle whereas for single pass the signal sequence can be at the end or the middle

• protein is threaded through the membrane until the stop transfer sequence

Why do proteins need to be folded in the ER?

ER soluble proteins ae passed through the membrane unfolded and must refold inside the lumen of the ER

What are binding proteins (BIP)?

• specific to ER (resident ER protein)

• class of proteins that bind to the incoming soluble proteins and aid in protein folding (ER)

• chaperone protein

How do chaperone proteins work?

• found in ER and cytoplasm

• process is not well understood

• some chaperone bind to proteins to be folded and protect the protein from degradation by forming a large aggregate with the protein

• once the protein is properly folded, the chaperone releases

• other chaperones: heat shock proteins (Hsp60, Hsp70)

• disruption of the folding process can affect cell development and growth

• abnormalities in chaperone proteins can cause: photoreceptor degeneration in the eye, CNS abnormalities, male infertility

What are Hsp chaperone proteins?

• heat shock proteins

• produced or activated in response to an increase in temperature

• help prevent protein denaturation or protein unfolding

What are the two methods of protein degradation?

1. degradation by the lysosome

2. ubiquitin proteosome pathway (UPP)

What is the ubiquitin proteosome pathway?

• method of protein degradation for incorrectly folded proteins/ short lived proteins

• two steps:

  • 1. tagging of the substrate protein by the covalent attachment of multiple ubiquitin molecules

    • ubiquitin: small protein (76 amino acids in size)

    • role of enzymes (E1, E2, E3)

      • E1: ubiquitin activating enzyme

      • E2: ubiquitin conjugating enzyme

      • E3: transfers ubiquitin from E2 to protein to be degraded

      • protein destined for degradation must be tagged with multiple ubiquitin’s

    • poly ubiquitin protein vs. mono ubiquitin protein

  • 2. subsequent degradation of the tagged protein by the proteasome complex

    • large proteolytic complex that breaks down the tagged portein

    • proteins targeted by this system are short-lived proteins

    • regulatory proteins (ex: cyclin = cell cycle regulator)

    • cyclin protein combines with CDK protein (cyclin dependent kinase) to regulate movement through cell cycle

• quicker than lysosome degradation

Similarities between protein import in mitochondria and chloroplasts?

• proteins destined for mitochondria and chloroplasts are synthesized in the cytoplasm on cystolic ribosomes

• contain a signal sequence that specifies their destination

• transported into the organelles post-translationally

• signal sequence is cleaved by peptidases following import

What are the functions of cell membranes?

• form a cell boundary- confine biochemical reactions

• sense external signals

• protection

• transport

• cell shape

• cell signalling

Describe the structure of the cell membrane.

• composed of lipids and proteins

• described by the fluid mosaic model:

  • membrane lipids are arranged in bilayer

  • proteins are embedded in the bilayer

  • fluidity:

    • lipid molecules able to move and behave more like a liquid than a solid

    • lipids can diffuse laterally in the membrane

      • cell membrane is a dynamic structure

      • molecules in cell membrane held together by noncovalent interactions

What are the reasons for fluidity in the cell membrane?

• 1. phospholipid tails are short: increases fluidity

  • because short tails reduce the tendency of the lipids to react with one another

• 2. phospholipid tails contain double bonds: increases fluidity

  • because double bonds create kinks in fatty acid tail and presence of kinks prevents packing together of phospholipids

What are the three types of cell membrane lipids?

• phospholipids

• glycolipids

• cholesterol

Describe phospholipids.

• membrane lipid

• polar head (hydrophilic) and nonpolar tails (hydrophobic)

• ex: phosphotidylcholine:

  • head group: glycerol, choline, phosphate group

  • tail region: fatty acid tails

• amphipathic: contain hydrophilic and hydrophobic regions

• most abundant lipid in the membrane

• phospholipids can move laterally in the membrane

Based on what we have discussed in lecture, briefly describe two
different cellular mechanisms or processes that when defective can lead to
the appearance and accumulation of unfolded or misfolded proteins
within cells

• chaperone proteins: mutated or missing chaperone proteins could lead
  to misfolded proteins or unfolded proteins.

• lysosome: non-functioning organelle could lead to the accumulation of
  misfolded or unfolded proteins in a cell.

• Ubiquitin Proteosome Pathway: pathway does not function correctly or
  at all and could lead to the accumulation of misfolded or unfolded
  proteins in a cell.

Describe glycolipids.

• membrane lipid

• lipids with a carbohydrate side chain (sugar side chain is exposed at the cell surface)

• function: protection of cell from harsh conditions, cell recognition and cell signaling

• prominent in membranes of nerve cells (brain) and epithelial cells that line the intestine

What is an example of a bacterial infection interacting with glycolipids?

• vibrio cholerae produces cholera toxin (enterotoxin)

  • V. cholerae- bacteria occurs in both marine and freshwater habitats

  • pathogenic to humans

  • the toxin acts on the mucosal epithelium of digestive tract

  • causes a sudden onset of massive diarrhea causing the individual to lose gallons of protein-free fluid and associated electrolytes, bicarbonates, and ions

  • enterotoxin binds to glycolipids on the surface of the intestinal cells

  • binding activates the adenylate cyclase enzyme in the cells converting the enzyme into a pump which extracts water and electrolytes from blood and tissues and pumps it into the lumen of the intestine (dehydration)

Describe cholesterol.

• membrane lipid

• found in some membranes

• steroid: ring structure

• ring interacts with tails of phospholipids

• presence of cholesterol decreases the fluidity of the membrane

What are the types of membrane proteins?

• integral

• peripheral

• lipid-anchored

Describe integral membrane proteins.

• contain one or more hydrophobic regions embedded in the lipid bilayer

• most integral membrane proteins are transmembrane proteins

• ex: transmembrane protein

  • membrane protein that extends through the lipid bilayer

  • single pass and multi-pass proteins

  • multi-subunit proteins- composed of several polypeptide chains

Describe peripheral membrane proteins.

• found on the periphery of the membrane attached to phospholipid head groups or other adjacent proteins

• attach to the membrane by electrostatic interactions and H bonds

How would you purify transmembrane proteins and peripheral proteins?

• 1. transmembrane proteins: use detergent to break membrane

• 2. peripheral proteins (attached to membrane with weak interactions): can shift pH to break interactions

Describe lipid anchored membrane proteins.

• peripheral protein

• covalently bound to lipid molecules that are embedded in the lipid bilayer

  • linked to the outer membrane surface

    • attached by GPI anchor

  • linked to the inner membrane surface

    • attached by a fatty acid group

Describe the fluid property of membrane proteins.

• fluidity

  • proteins can rotate in the membrane

  • proteins can diffuse laterally in the membrane

How was membrane protein fluidity discovered?

• two experiments:

• 1. mouse/human cell fusion

  • mixing of mouse cell membrane proteins and human cell membrane proteins after cell fusion

  • membrane proteins diffuse over time, can be seen by mixing of cells

• 2. FRAP (fluorescent redistribution after photo-bleaching)

  • label a membrane protein with a fluorescent antibody

  • bleach the fluorescent molecules in a small area using a high intensity laser beam

  • fluorescence intensity recovers as the bleached molecules diffuse away and unbleached molecules diffuse into the irradiated area

What are factors that might prevent the movement of membrane proteins within the membrane?

• proteins are linked to extracellular matrix, membrane components, cellular junctions (cell-cell interaction)

• proteins can be attached to phospholipids, cholesterol, other proteins

• cell junctions limit membrane protein mobility- proteins can migrate up to the junction but not past it

• cells within a tissue layer have two surfaces:

  • apical surface

  • basal surface- attaches cells to ecm

What are the types of cell junctions?

tight junctions, gap junctions, anchoring junctions

Describe tight junctions.

• bind cells in a tissue layer

• prevent the “leaking” of material between cells

Describe gap junctions.

• tunnels that connect cells

• allow for exchange of materials between cells

• allow for easy exchange of materials between cells- especially important in cell signaling and exchange of cell signal products between cells

• opening and closing of gap junctions is a regulated process (controlled by levels of intracellular calcium)

  • high levels of intracellular calcium = closes cell junctions

  • PIP2 pathway (release calcium into the cell)

• tested by labeling molecules with tag and looked to see if calcium molecules moved between cells

Describe anchoring junctions.

• connect cells to each other and to extracellular matrix

• can be connected to the cytoskeleton

• focal adhesions

Difference between isolating peripheral and integral proteins?

• 1. peripheral proteins

  • extracted from the membrane by changes in pH or ionic strength

• 2. integral proteins

  • removed by detergents which disrupt the hydrophobic interactions between lipids in the bilayer

    • detergent: Triton X, NP-40

What are the steps of separating isolated proteins?

1. SDS-PAGE

2. Protein detection

What is an SDS-PAGE?

• separating isolated proteins

• SDS-PAGE: SDS polyacrylamide gel electrophoresis

  • SDS (sodium dodecyl sulfate)

    • denatures proteins and coats them with a negative charge

• electrophoresis separates proteins based on size

• polyacrylamide gel: PAGE

  • negatively charged proteins migrate in the gel toward the (+) electrode

  • migration rate is dependent on size of the protein

How would you detect proteins once you separate them?

• Western blot

• ELISA (enzyme linked immunosorbent assay)

  • indirect ELISA → HRP

  • microtiter plates

  • apply antigen to the surface of the plate

  • add antibody specific for antigen (use 1 or 2 antibodies)

    • antibody contains enzyme tag (HRP)

  • add substrate (peroxidase)

  • monitor color change

Describe cell membrane transport.

• membrane transport is affected by size and polarity of the molecule

• small nonpolar molecules can diffuse rapidly across the lipid bilayer

  • ex: oxygen, carbon dioxide

• small uncharged, polar molecules such as water and urea will slowly diffuse across the lipid bilayer

  • ex: water (osmosis)

• lipid bilayers are impermeable to charged molecules

• the charge on the molecule and the molecule’s degree of hydration prevent it from entering the bilayer

  • ex: Na⁺, Ca²⁺, K⁺

• membrane transport is mediated by membrane porteins

• two main classes of membrane transport proteins: carrier proteins and channel proteins

Describe carrier proteins.

• bind to the molecule to be transported

• undergo a series of conformational changes to transfer the bound molecule across the membrane

• carrier mediated transport:

  • 1. uniport: transport of a single molecule (ex: Ca²⁺ transport [ER membrane])

  • 2. symport: transport of 2 molecules (coupled transport), occurs in a single direction (ex: glucose, Na⁺)

  • 3. antiport: transport of 2 molecules (coupled), occurs in opposite directions (ex: Na⁺/K⁺ pump)

Describe passive transport.

• 1. passive transport: occurs by diffusion

  • two types of diffusion:

    • simple:

      • concentration gradient drives transport

      • diffusion occurs from an area of high concentration to an area of low concentration

    • facilitated:

      • diffusion of molecules through the use of carriers of channels in the protein

What are two methods of driving membrane transport?

1. passive transport

2. active transport

Describe the methods of active transport.

• 1. coupled carriers

  • free energy released during the movement of one molecule down an electrochemical gradient is used as the force to pump the other molecule against the electrochemical gradient

  • energy stores in the electrochemical gradient of one molecule is used to drive the movement of the other molecule

    • ex: sodium and glucose

• 2. ATP driven pump

  • uses hydrolysis of ATP to pump molecules across the membrane

  • ex: Na+/K+ pump found in the plasma membrane of most animal cells

    • Na+ is pumped out of the cell against its electrochemical gradient

    • K+ is pumped into the cell

    • hydrolysis of ATP powers transport

• 3. light driven pump

  • found mainly in bacteria (halophilic bacteria)

  • couple transport against an electrochemical gradient to an input of light energy

  • carrier proteins are light-gated

  • absorption of light causes conformational change in the carrier protein

Describe channel proteins.

• form aqueous pores that extend across the lipid bilayer

• when pores open, molecules can pass

• transport occurs quickly

• can be channel protein or a gated-channel protein

• gated channel protein requires stimuli to open the channel

What are the three stimuli that can open membrane channels?

• 1. voltage change across the membrane: dependent on membrane potential

  • voltage gated channels

  • ex: muscle contraction and nerve function

• 2. mechanical stress: connected to cell cytoskeleton

  • mechanically gated channels

  • many of the channels have cytoplasmic extensions that link the channel to the cytoskeleton

• 3. ligand-gated: connected to cell surface receptors

  • binding of a ligand opens the channel

  • ligand can be an extracellular mediator (ex: neurotransmitter)

  • ligand can be an intracellular mediator (ex: nucleotide-ATP)

1. glucose is pumped into the cell by a sodium-powers glucose symport (coupled transport that occurs in the apical domain of the cell)

2. the sodium gradient driving the glucose symport is maintained by a sodium pump which keeps the internal concentration of sodium low

3. glucose passes out of the cell (down its concentration gradient) by passive transport mediated by a glucose carrier protein (basal domain of the cell) = facilitated diffusion

ESSAY QUESTION: POSSIBLE TOPICS:
COLOCALIZATION STAINING
ORGANELLE/PROTEIN ISOLATION AND IDENTIFICATION
CELL FUNCTION EXPERIMENTS (SHOW THE IMPORTANCE OF
A MOLECULE IN A SPECIFIC CELL FUNCTION)
EXOCYTOSIS (PROTEIN PRODUCTION TO SECRETION)
ISOLATION AND IDENTIFICATION OF MEMBRANE PROTEINS