Weeks 4 & 5 Flashcards
1
Q
Cytoskeleton -
A
network of filaments extending throughout the cytoplasm
2
Q
Cytoskeleton is composed of 3 types of filaments:
A
- Microfilaments: actin filaments, the thinnest components
- Intermediate filaments: filaments with middle-range diameters, composed by different types of proteins
- Microtubules: tubulin filaments, the thickest of the three components of the cytoskeleton
3
Q
Microtubules structure -
A
hollow tubes; walls consist of 13 columns of tubulin molecules
4
Q
Microtubules diameter
A
25 nm w/ 15-nm lumen
5
Q
Microtubules protein subunits:
A
tubulin
6
Q
Microtubules main fns (4):
A
- maintenance of cell shape
- cell motility
- mitotic spindle formation => chromosome mvmnt in cell division
- organelle mvmnts
7
Q
Microtubules can increase or decrease in size by
A
addition or removal of monomers
8
Q
Microtubules consist of
A
α & β tubulin dimers => form 13 protofilaments
each dimer has 2 GTP bound:
(+) end: fast polymerisation (addition of monomers)
(-) end: slow polymerisation
9
Q
The continuous polymerisation or depolymerisation of microtubule is controlled by:
A
GTP hydrolysis:
-GTP attached to β-tubulin hydrolyzed to GDP during tubulin polymerisation
-The GTP bound to α-tubulin does not hydrolyze during tubulin polymerisation (has structural role)
also by Cytosolic calcium concentration: [Ca+2] > 0.5 mM => depolymerisation
10
Q
Drugs that affect microtubule stability/formation:
A
Αnti-mitotic drugs: inhibit the mitotic spindle formation, for ex:
Colchicine, anti-inflammatory: binds to tubulin monomers => inhibits microtubule polymerisation = stopes mitotic spindle formation (acts in profase)
Τaxol, anti-cancer: binds to tubulin monomers => stabilises microtubules by inhibiting their depolymerisation during mitotic (acts in anaphase)
11
Q
Microtubule polymerisation begins at the
A
ΜΤOC (Microtubule organizing centers) of the cells
12
Q
Microtubule organizing centers (MTOC) (4), how microtubules are oriented:
A
- Centrosome: in most non-dividing cells
- Βasal body: in flaggelated and ciliated cells
- Polar body: in some fungi (part of the nuclear envelope)
- Chromosomal kinetochores of the mitotic spindle: in dividing cells (during metaphase)
Microtubule orientation:
- Τhe (-) end is oriented towards the cell center (MTOC)
- The (+) end is oriented towards cell periphery
13
Q
Centrosome structure
A
has 2 centrioles (centriole pair), each consists of 9 triplets of microtubules (9+0 arrangement), at right angles to one another
14
Q
Pericentriolar material (cloud)
A
space around centrosome, contains γ-tubulin, which:
- facilitates the nucleation of the α/β tubulin dimers by binding to the (-) end of microtubules
- induces their nucleation (polymerisation) by forming rings into which the microtubule assemble and elongate
fn: microtubule nucleation (initiation of polymerisation)
15
Q
Microtubules: role in motility
A
- used as “monorails” for the mvmnt of cellular cargo (vesicles, organelles and chromosomes)
- from the cell centre to the periphery and vice versa
- interact w/ motor proteins to produce motility
16
Q
Motor proteins in cytosol (2) and fn
A
fn: transport cellular cargo toward opposite ends of microtubules
Dynein: involved in transport from periphery to the cell center (retrograde to microtubule; from + to – end)
Kinesin: involved in transport from the cell center to the periphery (anterograde to microtubule; from – to + end)
17
Q
Cilia and flagella: what are they and microtutubles arrangement
A
– permanent locomotor appendages of some eukaryotic cells
– contain specialized arrangements of microtubules: 9 pairs around 2 single central ones = 9+2 arrangement
18
Q
Flagella
A
• Typically a single flagellum per cell (in eukaryotes)
• Flagella motility pattern: snakelike motion
• Example: sperm cells
19
Q
Cilia
A
• Typically a lot of cilia per cell
• Ciliary motility pattern: back-and-forth motion
• Example: trachea cells, protists, fallopian tubes
20
Q
Αxoneme -
A
The central strand of a cilium or flagellum
axoneme is surrounded by the PM
21
Q
Axonemal proteins:
A
- Dynein: motor protein responsible for motility (diff from cytosolic: bigger, more ATP): bending mvmnt of cilia & flagella
- Nexin: connects microtubule doublets (pairs) between them
22
Q
Basal body:
A
protein structure found at the base of a eukaryotic cilium or flagellum. Consists of 9 triplets of microtubules (like centrioles => 9+0 arrangement).
23
Q
Why movement of cilia & flagella diff?
A
due to the diff in length, essentially mvmnt is the same
24
Q
Motor proteins & their fns (4):
A
- cytosolic kinesin: vesicle and organelle transport from the cell centre to the periphery (anterograde to microtubule; from – to + end) [Karry Kargo trucK]
- cytosolic dynein: vesicle and organelle transport from periphery to the cell centre (retrograde to microtubule; from + to – end) [Dive Down Dynein the centre of the cell]
- axonemal dynein: on axonemal microtubules; causes movement of cilia/flagella.
- spindle kinesin: mitotic spindle assembly and chromosome segregation during cell division
25
All the motor proteins have ATPase activity
=> ATP hydrolysis => produce energy used for motility
26
Subcellular structures composed of microtubules:
cilia: 9+2 (9 doublets + 2 central microtubules)
flagella: 9+2 (-II-)
centriole: 9+0 (9 triplets + 0 central microtubules)
basal bosy: 9+0 (-II-)
centrosome: 2 centrioles places @ right angles to each other
27
Microfilaments structure:
2 intertwined strands of actin
28
Microfilaments diameter
7 nm
29
Microfilaments’ protein subunit
Actin
30
Microfilaments’ main fns (5):
- maintenance of cell shape (ex: *microvili core of intestinal epithelial cells*)
- changes in cell shape (*formation of pseudopodia: filaments in direction of cell mvmnt polymerize and depolymerize in opposite direction*)
- muscle contraction (*actin-myosin contractile system*)
- cell motility (*pseudopodia + cytoplasmic streaming in plant & large fungal cells*)
- cell division
31
Microfilament polymerisation
- Energy provided by ATP hydrolysis
- Filamentous F-actin is assembled from globular G-actin subunits containing bound ATP
- growth at + end, dissociation of actin-ATP at - end
32
Intermediate Filaments’ Functions (2) and characteristics (2):
– Support cell shape: provide tissue w/ resistance to mech stress
– Fix organelles in place: participate in cell junction formation (ex: epithelial cell desmosomes by kadherin)
– More permanent than other filaments
– Composed of different protein family categories (e.g. keratins)
33
Intermediate filament types, where found (6):
- **Keratin**: in epithelial cells
- **Desmin**: in muscle cells
- Vimentin: in mesenchymal cells
- Neurofilaments: in neurons
- GFAP (glial fibrillary acidic proteins): in neuroglia (glia)
- **Lamins**: in nuclear envelope
34
Keratins:
• Found in epithelial and epidermal cells
• In epithelial cell desmosomes => cytokeratins
• Major component of hair and nails, in intestinal epithelium, squamous epidermal epithelium
35
Desmin
• In muscle cells
• **Connects myofibrils and Ζ-disks of the sarcomeres to each other**
*Sarcomere*: basic unit of contraction of striated muscle tissue= the area between the two Z-disks.
*Z-disk*: a thin, dark disk that transversely bisects a striated muscle fiber.
*Vimentin* (another IF) also participates in Z-disk structure organisation in muscle cells
36
Glia fibrils
- in neuroglia
• Neuroglia (glia): non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the nervous system.
• Glia fibrils: found in neuroglia (e.g. astrocytes)
- GFAP= Glial Fibrillary Acidic Protein => polymerised to form glia fibrils
- Role in astrocytic projection formation => CNS morphology
37
Lamins
lamin filaments found in the inner site of the nuclear envelope: provide structural support => make up nuclear envelope
38
Clinical correlations: cytoskeletal disorders
Chediak-Higashi syndrome
Kartagener's syndrome
39
Chediak-Higashi syndrome:
microtubule-based lysosomal mobility inhereted defect (lysosomal trafficking deffect)
=> reduced fusion of phagosomes and lysosomes during phagocytosis
=> recurrent infections (inability to destroy microorganisms by phagocytosis)
40
Kartagener's syndrome:
- immotile cilia/flagella due to axonemal dynein arm inherited defect.
- Results in male and female infertility (immotile sperm), sinusitis (bacteria and particles not pushed out)
41
Extracellular structures:
- cell walls of plant cells
- the extracellular matrix (ECM)
- intracellular junctions
42
ECM of animal cells:
- covers animal cells
- consists of: glycoproteins and proteoglycans
43
Glycoproteins -
glycosylated proteins (proteins with attached carbohydrate residues) *e.g. collagen, fibronectin, laminin*
44
Proteoglycans -
proteinated carbohydrates
(carbohydrates with attached protein residues)
45
ECM functions
- support
- adhesion
- movement
- regulation of gene expression
46
**ECM components**
Major proteins and glycoproteins (5):
- Collagen: major ECM glycoprotein (12 types)
- Fibronectin: ECM glycoprotein that connects to plasma membrane proteins (integrins) and to other ECM components (e.g. collagen) => connects plasma membrane with extracellular molecules
- Laminin: basement membrane glycoprotein
- Entactin: basement membrane glycoprotein
- Εlastin: connective tissue protein
47
**ECM components**
Proteoglycans
composed of proteins + glucosaminoglycans (GAGs)
48
Integrins
- **transmembrane** proteins that bind to several ECM components
- CAMs - Cell adhesion molecules: Cell surface transmembrane proteins that bind to the different ECM components
- Heterodimers made of one α and one β subunit
49
Inregrins’ extra- & intracellular domain:
- Extracellular domain: binds to the ECM glycoproteins (e.g. fibronectin) via a specific tripeptide sequence (Arg-Gly-Asp= RGD sequence)
- Intracellular domain: binds to cytoskeletal filaments (microfilaments or intermediate filaments)
50
Integrins Function:
link ECM components to cytoskeletal components inside the cell
=> Activation of cell-signalling pathways
=> signal transduction
=> cell survival/proliferation
51
Collagen: what, produced by, structure
- Major ECM glycoprotein and most abundant protein in the human body.
- 12 different collagen types (I-IV most common)
- produced by **fibroblasts, epithelial cells**
- Structure: 3 helical chains (triple helix); Repetitive motif Gly-X-Υ (X,Υ= proline, hydroxyproline, or hydroxy-lysine)
52
Basement membrane (basal lamina):
specialized ECM type that separates epithelium/mesothelium/endothelium from underlying connective tissue
53
Impt collagen types:
Type I (most common)
- skin, tendon, organs, bone
- associated syndrome: **Reduced production in osteogenesis imperfecta (OI) type I**.
Type II
- cartilage
Type III
- skin, blood vessels, uterus, fetal tissue, etc
- associated syndrome: **deficient in vascular type Ehlers-Danlos syndrome (osteoarthritis)**
Type IV
- basement membrane
- associated syndrome: **Defective in Alport syndrome - glomerulonephritis**
54
**osteogenesis imperfecta (OI) type I** is associated w/
reduced production of collagen type I
55
**vascular type Ehlers-Danlos syndrome (osteoarthritis)** is associated w/
Collagen type III is deficient
56
**Alport syndrome (glomerulonephritis)** is associated w/
Defective collagen type IV
57
Fibronectin and fn
- Major ECM glycoprotein
- Function: cell attachment to ECM components
- Fibronectin has domains for binding integrins (cell surface receptors) and other ECM glycoproteins/proteoglycans (e.g. collagen, heparin, etc).
=> It mediates cell adhesion to the ECM
58
Εlastin
- stretchy protein in connective tissue
59
Elastin Function -
allows many tissues (e.g. blood vessels, lungs, ligaments, vocal cords) to resume their shape after stretching or contracting
(Major arterial extracellular component => confers elasticity)
60
Εlastic fibers structure:
consist of glycoproteins (e.g. fibrillin) connected with cross-linked elastin
61
Elastin Clinical correlations:
- Marfan syndrome: connective tissue disorder caused by a defect in fibrillin, a glycoprotein that forms a sheath around elastin (in elastic fibers)
- Wrinkles of aging are due to reduced collagen and elastin production
62
Laminin
- Located mainly in basement membrane, synthesized by adjacent epithelial cells
- Binds to basement membrane components (collagen ΙV, heparin, etc) and cell surface receptors (e.g. integrins)
- **Fn**: connects epithelial cells with the ECM (mainly basement membrane)
63
Entactin
- ECM glycoprotein
- Binds to basement membrane components (laminin, collagen IV and proteoglycans) only (does not bind to integrins)
- **Fn**: Assists assembly of basement membrane components (ex: Links laminin with collagen in the basement membrane)
64
ECM proteins and glycoproteins (summary)
**Collagen**: Most abundant glycoprotein. Organizes and strengthens extracellular matrix. Binds to the cell (via integrin)
**Fibronectin**: Connects cells (via integrins) to ECM components (e.g. collagen)
**Laminin**: Connects cells (via integrins) to basement membrane components
**Entactin**: Connects basement membrane components between them
**Elastin**: Major component of elastic fibers together with **fibrillin**
65
Proteoglycan composition:
core protein (5-10%) + GAG (90-95%)
ex: aggrecan
66
Glucosaminoglycans (GAGs):
Polysaccharide composed from a repetitive disaccharide unit (70-200 units)
Disaccharide unit composed of: N-acetyl-glucosamine or N-acetyl-galactosamine + glucuronic acid or iduronic acid
67
4 GAG groups (types):
1. Hyaluronic acid (hyaluronan), **isn’t linked** to core protein
2. Chondroitin sulfate and dermatan sulfate, **linked** to core protein
3. Heparin and heparan sulfate, **linked** to core protein
4. Keratan sulfate, **linked** to core protein
68
Common proteoglycans:
- aggrecan (aggrecan aggregate = aggrecan + hyaluronic acid, found in cartilages)
- perlecan
69
Intercellular Junctions
what neighbouring cells in tissues,
organs, or organ systems often adhere, interact, and communicate through
70
Intercellular Junctions fn:
help coordinate the behavior of all cells in a tissue
71
Types of intercellular junctions:
**Animal cells**:
– Tight junctions
– Desmosomes
– Gap junctions
**Plant cells**
– Plasmodesmata (connect neighbouring plant cells through plant cell walls)
- Channels (communicating junctions) - allow molecule exchange
72
Types of intercellular junctions in animals (3):
*connect, but NO molecule exchange:*
1. Tight junctions: prevent leakage of fluid across a layer of cells (occluding junctions)
2. Desmosomes: fasten cells together into sheets (anchoring junctions); attach muscle cells to each other in a muscle
*ALLOW molecule exchange:*
3. Gap junctions: serve as channels allowing ions and small molecules across cells => facilitate communication between cells in tissues (communicating junctions)
73
Some ‘muscle tears’ involve the rupture of
desmosomes
74
plasma membrane -
boundary that separates the living cell from its non-living surroundings
75
Properties of plasma membrane (3):
1. Fluidity - constant movement of the PM components
2. Mosaicism - presence of many different molecules
3. Selective permeability - PM allows some substances to cross it more easily than others
76
Liposomes
- are formed by phospholipids in aqueous environment
- bilayer spherical structures
- used for efficient delivery of certain drugs/compounds to the cells
77
Membrane lipids (make up lipid bilayer):
• Phospholipids: the major membrane lipid type
• Glycolipids
• Sterols
78
2 types of phospholipids:
- Phosphoglycerides: basis - glycerol + 2 fatty acids + phosphate + organic molecule (Phosphatidyl-choline, Phosphatidyl-ethanolamine, Phosphatidyl-serine, Phosphatidyl-inositol)
- Phosphosphingolipids: basis - sphingosine + 1
fatty acid + phosphate + organic molecule (sphingomyelin (only in animal cell membrane))
79
Glycolipids and glycosphingolipids structure
- Glycolipids: sugar(s) + lipids (glycosylated lipids)
- Glycosphingolipids: sphingosine + 1 fatty acid + sugar residue(s)
80
Common membrane glycolipids
Glycosphyngolipids:
Cerebrosides: a monosaccharide
Gangliosides: oligosaccharide residue
81
Sterols: what & where?
Sterols: steroid alcohols (steroids): on animal cell membrane
Phytosterols: in plant cell membranes
Ergosterol: in fungal and protozoal cell membranes
82
Membrane lipids of animal cells (summary)
Membrane lipids:
1. Phospholipids
1a. Glycerophospholipids (glycerol + 2FA + PO4 + alcohol) = Phosphoglycerides
1b. Phosphosphingolipids (sphingosine + 1FA + PO4 + choline) = Phosphosphingolipids - sphingomyelin
2. Glycolipids - Glycosphingolipids (sphingosine + 1FA + mono- (cerebroside) / oligosaccharide (ganglioside))
3. Cholestreol (steroid compound)
83
Phosphatidyl-choline structure
Alcohol: Glycerol
FA: 2
Phosphate: yes
Organic molecule: Choline
Sugar molecule: no
84
Phosphatidyl-serine structure
Alcohol: Glycerol
FA: 2
Phosphate: yes
Organic molecule: Serine
Sugar molecule: no
85
Phosphatidyl-ethanolamine structure
Alcohol: Glycerol
FA: 2
Phosphate: yes
Organic molecule: Ethanol-amine
Sugar molecule: no
86
Phosphatidyl-inositol structure
Alcohol: Glycerol
FA: 2
Phosphate: yes
Organic molecule: Inositol
Sugar molecule: no
87
Sphingomyelin structure
Alcohol: Sphingosine
FA: 1
Phosphate: yes
Organic molecule: Choline
Sugar molecule: no
88
Cerebrosides structure
Alcohol: Sphingosine
FA: 1
Phosphate: no
Organic molecule: no
Sugar molecule: monosaccharide
89
Gangliosides structure
Alcohol: Sphingosine
FA: 1
Phosphate: no
Organic molecule: no
Sugar molecule: oligosaccharide
90
The Fluidity of Membranes: role of phospholipids
The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane:
- **Unsaturated** hydrocarbon tails with kinks => **higher** fluidity (fluid PM)
- **Saturated** hydrocarbon tails => **lower** fluidity (viscous PM)
91
The Fluidity of Membranes: role of cholesterol
has different effects on membrane fluidity at different temperatures:
- At warm temperatures (37°C), cholesterol restrains movement of phospholipids => reduces fluidity
- At cool temperatures, it maintains fluidity by preventing tight packing
92
Membrane protein categories:
(a) Integral:
- transmembrane: completely span the membrane
- Lipid-bound: attached to a membrane lipid
(b) Peripheral: Loosely bound to the surface of the membrane on internal or external side
93
Integral transmembrane proteins
- span the cell membrane 1 or more times
- Penetrate the hydrophobic core of the lipid bilayer
- Their hydrophobic region contains non-polar amino acids
94
Integral transmembrane proteins, 2 types of secondary structure:
- α-helical structure: e.g. *growth factor receptors (EGFR)*, *insulin*, *membrane immunoglobulins (Ig)*
- β-pleated sheet structure (β-barrel): e.g. *bacterial porin*
95
EGFR:
Epidermal Growth Factor Receptor
- Overexpressed in many cancers (ex: breast cancer)
- Single-pass transmembrane protein with α-helical structure
96
Integral lipid-bound proteins, how attached and their fn
- Attached to the plasma membrane through a covalent bond with a lipid molecule
- Directly attached to the lipids at the internal side of the plasma membrane
- Indirectly attached to phosphatidyl-inositol at the external site of the plasma membrane through an oligosaccharide chain
- Fn: hydrolases, receptors
97
Peripheral proteins and fn
- interact with the polar surfaces of the membrane or with proteins embedded in the membrane
- Internal membrane proteins **fn**: connection with the cytoskeleton
ex: *erythrocyte spectrin*
98
Six major functions of membrane proteins
1. Transport (transmembrane proteins; either hydrophilic channel across the membrane that is selective for a particular solute or shuttle a substance from one side to the other by changing shape)
2. Enzymatic activity
3. Signal transduction.
4. Cell-cell recognition (some glycoproteins serve as identification tags that are specifically recognized by other cells)
5. Intercellular joining (gap junctions, tight junctions)
6. Attachment to the cytoskeleton and ECM
99
Membrane carbohydrates: where & fns
where: on the external side of PM
fn: Cell-cell recognition - cell’s ability to distinguish one type of neighbouring cell from another
100
3 types of membrane-associated carbohydrates (glycocalyx):
- Glycoproteins: carbohydrates covalently bonded to proteins (content: protein > carbohydrates); where: membrane and ECM
- Glycolipids: carbohydrates covalently bonded to lipids; where: membrane only
- Proteoglycans: proteins covalently linked to carbohydrates (content: carbohydrates > protein); where: ECM only
101
Glycocalyx -
carbohydrate cover on the external side of the cell membrane protecting the cell surface from mechanical/chemical damage
ex: *the human blood cell types A, B, AB and O reflect variation in the RBC surface carbohydrates*
102
What molecules can pass through the membrane rapidly?
and ex
Hydrophobic (non-polar) molecules: are lipid-soluble => can pass through the membrane rapidly
ex: CO2, O2, hydrocarbons
103
What molecules can pass through the membrane?
and ex
Hydrophilic (polar) molecules: not lipid-soluble
ex: sugars, ions - charged
104
Transport Proteins
- Allow passage of hydrophilic substances across the membrane
- Most are extremely specific for the substance they are transporting
105
2 types of Transport Proteins:
1. **Channel proteins**: transport proteins that have a hydrophilic channel through which certain molecules or ions pass
ex:
- *Aquaporins*: special transport proteins for water
- *Ion channels*: transport proteins for ions
2. **Carrier proteins**: transport proteins that bind to molecules and change shape to shuttle them across
ex: *glucose transporters GLUT*
106
Types of transport of molecules through the PM:
1. Active transport: transport of a substance across a membrane that requires energy investment
2. Passive transport: transport of a substance across a membrane with no energy investment, b/c it favours dynamic equilibrium
107
Passive transport results in:
equalization of the concentration of a substance in the internal and external membrane region (equilibrium)
108
2 types of passive transport processes:
1. Diffusion: movement of solute molecules across the PM down their concentration gradient (from high solute to low solute)
2. Osmosis: movement of solvent (water) molecules across the PM against the solute
concentration gradient (from low solute to high solute); occurs when diffusion can’t
109
facilitated diffusion -
- Larger molecules and ions require to be transferred by transport proteins (e.g. ion channels, carrier proteins), **w/out E**
- Movement of molecules is always down their concentration gradient (from high solute concentration to low solute concentration)
110
dynamic equilibrium -
as many molecules cross one way as cross in the other direction (equal molecule distribution)
111
Channel proteins -
- Channels that allow a specific molecule or ion to cross the membrane
ex: water channels (aquaporins), ion channels (gated)
112
Carrier proteins -
- Bind to the solute and undergo a change in shape that translocates the solute-binding site across the membrane => shuttle molecules across
ex: GLUT
113
Cystic fibrosis - transporter protein disorder:
mutation in chloride ion channel protein => viscous secretions in respiratory tract => pulmonary infections
114
Cystinuria (kidney disease) - transporter protein disorder
mutations in a renal membrane carrier protein => prevention of cysteine reabsorption into the blood => concentrates in urine => kidney stone formation (crystals)
115
Osmosis
- movement of water across a semipermeable membrane
- affected by the concentration gradient of dissolved substances
- occurs when the molecules/ions of a solute cannot pass through the PM (semipermeable)
116
hypotonic
area of lower solute/higher water concentration
(water moves from here)
117
hypertonic
area with higher solute/lower water concentration (water moves to here)
118
isotonic solution -
result of osmosis: substance concentrations of the 2 areas become equal (equilibrium is reached)
119
Tonicity: what, depends on, affects more which cells
- the ability of a solution to cause a cell to gain or lose water
- depends on the concentration of solutes that cannot penetrate the membrane
- has a great impact on cells without walls, b/c cell walls protect cells against osmotic pressure
120
Cells w/out cell walls in Hypotonic solution
Lysed
121
Cells w/out cell walls in isotonic solution
Normal
122
Cells w/out cell walls in Hypertonic solution
Shriveled, shrink (water out)
123
Cells w/ cell walls in Hypotonic solution
Turgid (normal)
124
Cells w/ cell walls in Isotonic solution
Flaccid
125
Cells w/ cell walls in Hypertonic solution
Plasmolysis
126
Osmosis in animal vs plant cells
**Hypertonic solution**
- Animal cells: Shrivelled (cells lose water and shrink) => lethal upon prolonged exposure
- Plant cells: Plasmolysis (lethal upon prolonged exposure)
**Isotonic solution**
- Animal cells: Optimum (normal) state
- Plant cells: Flaccid (not rigid enough)
**Hypotonic solution**
- Animal cells: Lysis (lethal immediately), b/c cells absorb water and burst
-Plant cells: Turgid (rigid) => Optimum (normal) state
127
Active Transport
– Moves substances against their concentration gradient (from low concentration to high concentration) => allows cells to maintain concentration gradients that differ from their surroundings
– Requires energy, usually in the form of ATP
– performed by specific membrane
proteins (ion pumps, for ex: sodium-potassium (Na+/K+) pump)
128
Structure of ATP:
adenine + ribose + 3 Phosphate groups
129
Membrane potential -
- voltage difference across a membrane, which is created by differences in the distribution of positive and negative ions
- cytoplasm is negatively (-) charged compared to the outside
- Μembrane potential acts like a battery and favors:
- passive transport of cations **(+) into** the cell
- passive transport of anions **(-) out** of the cell
130
how Na+/K+ pump contributes to the creation and maintenance of the membrane potential
– transports 3 Na+ out and 2 K+ in = net transfer of one (+) charge out
131
Electrochemical gradient -
combination of two forces driving the diffusion of an ion:
– a chemical force = the ion’s concentration gradient
– an electrical force = the effect of the membrane potential on the ion’s movement
132
Electrogenic pumps -
transport proteins that generate voltage across a membrane => create membrane potential (Na+/K+ pump in animals) => E source
133
Cotransport -
– Coupled transport of substances by a membrane protein (cotransporter)
– Active transport driven by indirect spending of E
– The concentration gradient of one substance indirectly drives the active transport of another substance
ex: *passive transport of Η+* to the inside of the cell by diffusion coupled with *active transport of sucrose*
134
Bulk transport of large macromolecules across the PM occurs by
- exocytosis & endocytosis - transport of large macromolecules across the membrane using transport vesicles
- Active transport processes: vesicle formation requires E
135
Exocytosis
- Transport of macromolecules packaged in vesicles from the inside of the cell to the outside via fusion of the transport vesicles with the plasma membrane
ex: pancreatic cells produce insulin and secrete it to the extracellular fluid by exocytosis
136
Endocytosis -
- Transport of macromolecules from the outside of the cell to the inside via formation of transport vesicles as a projection/extension of the plasma membrane to the inside of the cell (new vesicles form from PM)
137
Three types of endocytosis
- Pinocytosis: the intake of liquid or soluble material by the cell
- Phagocytosis: the intake of solid/insoluble material by the cell or ingestion of whole cells (e.g. microorganisms)
- Receptor-mediated endocytosis: the intake of specific molecules selected by a receptor
138
Phagocytosis
- cell engulfs a solid particle (macromolecule or microorganism) in a vacuole (phagosome/food vacuole) => it then fuses with a lysosome to digest the particle (phagolysosome formation)
ex: macrophages - Specialized immune cells that are able to engulf microorganisms: Cell membrane receptors recognize the microorganism => Pseudopodia are formed around the microorganism and enclose him into a vesicle (phagosome) => destruction by lysosomes
139
Receptor-mediated endocytosis
- Special type of endocytosis
- Binding of ligands to receptors triggers vesicle formation
ex: cholesterol uptake by hepatocytes
