Signal Transduction and Photosynthesis Overview

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 molecule…

can produce different effects

9 isoforms of epinephrine…

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.