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Signal Transduction
requires a ligand (signal molecule) and a receptor protein to which the ligand binds
Four basic ways of communication
direct contact, paracrine, endocrine, and synaptic signaling
Direct contact
molecules on the surface of one cell are recognized by receptors on the adjacent cell
Paracrine Signaling
A signal molecule (hormone) is released by a cell affecting neighboring cells
Endocrine signaling
a hormone released to travel to affect cells throughout the body
What happens when a ligand binds a receptor?
different cells have the same (glucagon) or different (epinephrine) response
Synaptic signaling
nerves release a signal (neurotransmitter) that bind to receptors on nearby cells
A cell response to a signal
may be adding (kinase) or removing (phosphatase) phosphates to an enzyme
What does protein phosphorylation mostly use?
ATP on OH groups of serine, threonine or tyrosine residues
Receptors can be
cell surface on the outside of the cell membrane or intracellular
What do cell receptors include? (3)
G protein-coupled (GPCR), tyrosine kinase (RTK), guanylyl cyclase
Enzymatic membrane receptors
are enzymes activated by a ligand. Almost all are protein kinase
Nonpolar steroid hormones
cross freely through the membrane and bind intracellular receptors in the cytoplasm
What is the result of binding the hormone to the receptor?
causes complex to shift to the nucleus to regulate gene expression
Steroid receptor has
hormone-binding domain, DNA-binding domain, and coactivator domain
Inactive steroid receptors have
an inhibitor in the DNA domain. Steroid binding displaces inhibitor
What varies greatly due to coactivators?
cell response, such as regulation, to a lipid-soluble signal
Intracellular receptors
can act as enzymes. NO catalyzes the synthesis of cGMP thoroughly through guanylyl cyclase
cGMP (cyclic guanosine monophosphate)
is an intracellular messenger that relaxes smooth muscle
What does the RTK influence?
cell cycle, migration, metabolism, and proliferation, and can induce cancer of altered
What do RTKs have?
extracellular binding domain and intracellular kinase domain
What happens when insulin binds to an RTK?
dimerization and autophosphorylation occur in the kinase domain
What does the kinase domain phosphorylate?
insulin response protein that promotes glycogen synthesis
MAP (mitogen-activated protein) kinase
are a series of kinases that phosphorylate each other
What are MAP kinases?
are cytoplasmic, stimulate cell division and are activated by kinase cascade
What can MAP kinase amplify?
the signal because kinases at each step can affect multiple substrates
Scaffold proteins
organize cascade components into a protein complex for optimal function
What do scaffold proteins provide?
efficiency but reduce amplification as each kinase affects one line only
RAS proteins
are small G proteins linking RTK and MAP with kinase cascade. Mutate in many human tumors
Inactive and active RAS
RAS is active bound to GTP, and inactive bound to GDP. Activated RAS activates the first kinase in the MAP cascade
GPCR
is the largest type of receptors that act by coupling to a G protein
How are all G proteins active and inactive?
Active when bound to GTP and inactive when bound to GDP
What does G protein provide?
link between receptor and effector proteins, usually enzymes G protein activates
How are effector proteins activated?
by G proteins produce a second messenger
Two common effector proteins
adenylyl cyclase and phospholipase C
What does adenylyl cyclase produce?
cAMP that activates protein kinase A (PKA) which phosphorylates proteins
What does phospholipase cleave?
PIP2 into IP3 and DAG, both are secondary messengers. I = inositol
What does IP3 bind to?
Gated Ca ion channel receptors on ER releasing Ca+2 that binds to calmodulin
What causes cellular response? (4)
Calmodulin, a cytoplasmic protein, binds kinase, ion channeles, etc.
What can different receptors produce?
the same 2nd messenger
What causes glucose to be released?
glucagon and epinephrine
Same signaling moleculeā¦
can produce different effects
9 isoforms of epinephrineā¦
causes different G proteins, leading to different signal transduction pathways
GPCRs and RTKs
can activate the same pathways. Both activate MAP kinase and phospholipase C.
Photosynthesis Combines
CO2 + H2O + light to make carbohydrates
Oxygenic Photosynthesis (using O2) is carried out by
cyanobacteria, 7 groups of algae, and all land plants (in chloroplast)
Photosynthesis has two stages
light-dependent reactions and carbon fixation reactions
Where does light-dependent reaction occur?
Thylakoid membrane of chloroplast
Grana
are stacks of flattened sacs of thylakoid membrane
What are thylakoid membranes surrounded by?
semiliquid called stroma
Where does the carbon fixation reaction (Calvin Cycle) take place?
In the stroma
Pigment absorb photons
which are inversely proportional to wavelength
The visible spectrum colors
400nm (blue, high energy) and 740nm (red, low energy)
What does absorption of the spectrum do?
gives range and efficiency of molecules to absorb photons
What are two general pigments present in green plants?
chlorophyll (a and b) and carotenoids
What happens only to the chlorophyll-a in the reaction center?
it absorbs red and blue light and converts it to chemical energy
Where are pigments present?
in the antenna complex surrounding chlorophyll-a reaction center
What are the differences between the chlorophylls (a and b)?
a: has CH3 group; b: aldehyde CHO
What do accessory pigments do?
absorbs light in different regions
Porphyrin Ring
Mg in the center of an unsaturated ring structure in chlorophylls
What happens in the porphyrin rings?
photons excite the electrons and the electrons are shuttled away from the ring
What do carotenoids have?
have chains of alternating double bonds and also serve as antioxidants
Light that is captured by 2 photosystems (PSI, PSII) is composed of
antenna complex + reaction center
Pigments in antenna
gather photons and feed the reaction center, the process is complementary
2P680 chlorophyll-a in PSII
absorb 2 photons and excite 2 electrons that move to the plastoquinone (PQ)
Photosystems
Two complexes (PSI, PSII) capturing light energy.
Antenna Complex
Pigments gathering photons for the reaction center.
P680
Chlorophyll-a in PSII absorbing photons to excite electrons.
Plastoquinone (PQ)
Electron carrier transferring electrons from PSII.
b6f Complex
Proton pump in thylakoid membrane passing electrons to PC.
Photosystem I
Complex accepting electrons to produce NADPH.
Cyclic Photophosphorylation
Process generating ATP by skipping PSI.
Calvin Cycle
Three-step process converting CO2 to glucose.
RUBISCO
Enzyme catalyzing CO2 fixation in Calvin cycle.
C3 Photosynthesis
Calvin cycle named for three-carbon intermediate.
C4 Plants
Plants adding CO2 to pyruvate to form oxaloacetate.
Oxaloacetate
Intermediate converted to malate in C4 pathway.
CAM Plants
Plants fixing CO2 at night to conserve water.
Aerobic Respiration
Uses oxygen as final electron acceptor.
Fermentation
Uses organic molecules as final electron acceptor.
Pyruvate Oxidation
Process producing acetyl-CoA from pyruvate.
Citric Acid Cycle
Produces 6CO2, 4ATP, 10NADH, 2FADH2 from glucose.
Electron Transport Chain (ETC)
Series of carriers transferring electrons to O2.
Chemiosmosis
Proton gradient driving ATP synthesis via ATP synthase.
ATP Synthase
Enzyme converting ADP to ATP using proton flow.
Allosteric Regulation
Control of enzyme activity by molecule binding.
Anaerobic Respiration
Oxidation using inorganic molecules instead of O2.
Deamination
Removal of amino group from amino acids.
Fat Catabolism
Breakdown of fats into fatty acids and glycerol.
Beta-Oxidation
Process breaking down fatty acids into acetyl-CoA.
Glucose
Product of photosynthesis used in cellular respiration.
Stroma
Site of Calvin cycle, closes to conserve water.
Sucrose
Transport form of glucose for starch synthesis.
Note 
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