BIOL10002

amyloids

amyloids

if a protein changes the amino acid sequence, making hydrophobic regions that were inside are now outside, it will form flat sheets that act like fibrous proteins

homeostasis

physical and chemical composition must be maintained within a narrow range of physiological conditions that support survival and function

genes

specific segments of DNA that encode information about structure and function

genome

entire set of DNA instructions

ex. 23 pairs of chromosomes in humans

phylogenetic trees

portray the evolutionary histories of the different groups of organisms

able to find the closest living relatives

evidence comes from fossils, structures, metabolic processes, behavior, and molecular analyses of genomes

heat of vaporization

amount of heat needed to turn 1g of a liquid into a vapor

specific heat

amount of heat energy required to raise the temperature of 1 gram of a substance by 1 degree Celsius

dehydration reaction

every polymer formed from monomer, a water molecule is released

hydrolysis

breakdown of polymer to monomer

denatured protein

protein structured that is disrupted

heated slowly and moderately—disrupt weak interactions, causing secondary and tertiary structure to break down

larger volume, many shapes

all information needed to specify the unique shape of a protein is contained in its primary structure

some are reversible, some are irreversible (ex. when bond to the wrong molecule/egg)

chaperones

protein that protect the 3D structures of other proteins

bind to target protein when they just form or undergo denaturation

cage-like structure that allows denatured protein to enter and fold to its appropriate shape when the lid is on, and releases when it is in its correct shape

tumor makes chaperones to stabilize proteins essential for cancer growth

carbohydrate

4 biochemical role: store energy, transport energy, extracellular support (cellulose for cell wall), carbon skeleton

glycosidic bond form between each monomer

glucose

monosaccharides

blood sugar

breaking of glucose convert stored energy to chemical-bond energy and produce carbon dioxide

straight/ring formed (ring is more stable in water)

alpha and beta glucose where beta glucose has OH—group above the ring when attached to carbon 1

3 catabolic processes harvest energy in chemical bonds of glucose: glycolysis, cellular respiration, and fermentation

oligosaccharide

3-20 monomer forms a polymer

different human blood groups get their specificities from oligosaccharides chains

covalently bonded to proteins and lipids

starch

polysaccharide with alpha-glycosidic bond

energy storage for plants

binds with water

when there is no water, hydrogen bonds form between unbranched polysaccharides chains, which then combines to become starch grains

readily degraded—easily broken down to supply glucose

unbranched or moderately branched

glycogen

water insoluble

highly branched

alpha glycosidic bond

energy storage for animals

1000 glucose molecules would exert 1000 times the osmotic pressure of a single glycogen molecule, causing water to enter cells where glucose is stored, which needs a lot of energy to expel excess water from their cells

cellulose

structural support of cell wall

most abundant organic compound

beta glycosidic bond

amino sugar

form parts of glycoprotein in RER—keeping tissues together

amino group substitute hydroxyl group in carbohydrate

glycoprotein

one or more oligosaccharides chains covalently bonded to a protein

function in cell recognition and cell adhesion

proteoglycan

a class of glycoprotein that has more carbohydrates attached and longer carbohydrate chains

make up major parts of extracellular matrix

function in cell recognition and cell adhesion

found in sponge with 2 kinds of carbohydrate—one binds with membrane components to keep proteoglycan attached to cell, one help with specific recognition and adhesion

lipids

not polymer because individual lipid molecules are not covalently bonded

insoluble in water

hydrophobic—non-polar bonds

ex. phospholipid, triglyceride

triglyceride

simple lipid

contain 1 glycerol (small molecule with three hydroxyl groups) and 3 fatty acids (long non-polar hydrocarbon chain and an acidic polar carboxyl group, saturated/unsaturated)

ester linkage

form through 3 condensation reaction—carboxyl group of fatty acid binds to hydroxyl group of glycerol—ester linkage and release water molecule

ex. fats, oils

fatty acid

saturated—single bond, straight (ex. animal fat)

unsaturated—one or more double bond, kinks (determine fluidity and melting point) (ex. plants)

excellent storehouses for chemical-bond energy

amphipatic

a substance is part hydrophobic and part hydrophilic

ex. lipid (hydrophilic head and hydrophobic tail)

phospholipid

2 phospholipid and 1 phosphate bound to glycerol

ester linkage

lipoprotein

phospholipid form a single-layered spherical with hydrophilic exterior and hydrophobic interior

bind to cholesterol and transport it in the blood to various tissues in the body

nucleotide

3 components: nitrogenous base, pentose sugar, 1-3 phosphate group

nucleotide that make up nucleic acid is monophosphate

oligonucleotide has 20 nucleotide and include RNA molecules (primer, regulate expression of gene) and synthetic DNA molecules (analyze other longer nucleotide sequences)

polynucleotide are much longer and include DNA and some RNA

DNA: adenine with thymine, cytosine with guanine (hydrogen bond)

RNA: adenine with uracil, cytosine with guanine (hydrogen bond)

ex. ATP, GTP, cAMP

pyrimidine

6 member single ring structure

ex. cytosine, uracil, thymine

purine

double ring structure

ex. adenine, guanine

transesterification

formation of nucleic acid through the combination of monomers between third—OH on the sugar and the first phosphate of the incoming nucleoside triphosphate, forming phosphodiester bond

ATP (adenosine triphosphate)

nucleotide

agent of energy transfer

energy source

drive endergonic reactions

convert into building block for nucleic acids

in eukaryotic cell, ATP is produced in mitochondria

in prokaryotic cell, ATP is produced on the cell membrane

use ATP to capture and transfer free energy

ATP synthesis occur through cellular respiration

formation and hydrolysis of ATP is energy-coupling cycle, because ADP picks up energy from exergonic reactions to become ATP, which then donates energy to endergonic reactions

energy is retained in P—O bonds of ATP in phosphate group

GTP (guanosine triphosphate)

energy source, especially in protein synthesis, and transfer information from environment to cells

cAMP (cyclic adenosine monophosphate)

actions of hormones and transmission of information by nervous system

cell theory

cells are fundamental units of life

all living organisms are composed of cells

all cells come from preexisting cells

modern cells evolved from a common ancestor

implication: life is continuous (all cells come from a single cell which marks the origin of life)

surface-area-to-volume ratio

high ratio means diffusion is faster and more effective

high surface area but low ratio means it takes too long to transport materials across cells, so cells divide to increase ratio

some cells change shape through infoldings to maintain high ratio

resolution

minimum distance 2 objects can be apart and still be seen as 2 objects

ex. human eye is 0.2 mm

electron microscope has better resolution, but only see dead cells because it is investigated under vacuum

light microscope has resolution of 0.2 micrometer on living cells

cell membrane

phospholipid bilayer with proteins on them

maintain homeostasis

semi-permeable barrier

communicate with other cells and receive signals from environment

contribute and support cell shape

cytosol

fluid part of cytoplasm

cytoplasm

everything in the cell

not static—under constant motion

proteins move around and collide with other molecules

ribosome

sites of protein synthesis

on rough ER (endoplasmic reticulum), in cytoplasm, mitochondria, and chloroplasts

cell structure rather than organelle because it lacks membrane

consist of ribosomal RNA (rRNA)

contain >50 different protein molecules

hydrophobic interaction

prokaryotic cells have ribosomes unattached in the cytoplasm

cell wall

support cell and determine shape

most bacteria’s cell wall contains peptidoglycan (amino sugars linked to short peptides at regular interval), and some as an outer membrane outside of peptidoglycan layer (polysaccharide-rich phospholipid membrane) or capsule (some protect bacteria from white blood cell and some help attach to other cells)

fractionation

allows cell organelles and other cytoplasmic structures to be separated from each other and examined using chemical methods

begins with destruction of cell membrane to allow cytoplasmic components to flow out into a test tube

biochemical analyzes are then done on the isolated organelle

microscopy and cell fractionation gives a complete picture of composition and function of each organelle and structure

nucleus

storage of hereditary information in sequence of nucleotides in DNA

site of DNA replication

site where gene transcription is turned on or off

liquid content of nucleus and insoluble molecules is nucleoplasm

nuclear envelop enclose nucleus, which has thousands of nuclear pores (a protein channel that allows some molecules to pass and some not, also regulates its nucleus’s information-processing functions)

chromatin

fibrous complex that is a combination of DNA molecule with protein

during cell division, chromatin is tightly compacts and condensed so that chromosomes are visible

attached to nuclear lamina (protein formed by polymerization of lamin into intermediate filaments), which maintains shape of nucleus by attachment of chromatin and nuclear envelop

chromosomes

long, thin threads of chromatin

chromosomes near each other might swap DNA regions, causing abnormal cell reproduction and cancer

chromatids

held together by cohesin (protein complex)

at mitosis, cohesin is replaced by centromere

at the end of G2, condensins coats the DNA molecules to make them more compact

endomembrane system

interconnected system of membrane-enclosed compartments

include cell membrane, nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes

vesicles transfer substances between various components of the endomembrane system

static—constant motion

Endoplasmic reticulum (ER)

lumen is the space inside ER (10% interior volume) and when protein enter lumen, it undergoes several changes and folding into their tertiary structure

folds increase surface area, much greater than that of cell membrane

rough ER (RER): ribosomes attached and transport proteins to other locations in the cell

-most membrane-bound proteins are made in the RER

-cells involve in protein secretion contains a lot of RER (ex. antibodies for immune system, cells of pancreas/intestine)

smooth ER (SER) lacks ribosome but it is a continuity with portions of RER that is responsible for chemical modification of small molecules taken in the cell that may be toxic by making them more polar, which makes it more water-soluble and easily removed

-site for glycogen degradation in animal cells

-site for lipid and steroids synthesis and some polysaccharides

-store CA2+ which can trigger numerous cell responses

lysosomes

organelles that contain enzyme that are capable to break down all types of biological polymers

carbohydrate groups help to address the right protein to lysosome

primary lysosome are formed from Golgi apparatus, which contain digestive enzymes and sites where macromolecules hydrolyzed into polymers

-interior is more acidic (pH 5) than external cytoplasm (pH 7.2)

-breakdown food, other cells, or foreign objects through phagocytosis (cell membrane forms a pocket that engulf inward to enclose the material and forms a vesicle with the material called phagosome)

secondary lysosome forms when phagosome fuses with primary lysosome, where digestion occur

-waste are used to provide energy and raw materials for cellular processes

-second lysosome fuses with cell membrane and releases undigested contents into the environment through exocytosis

some cells are continually broken down and replaced through autophagy, which can engulf entire organelles

failure of lysosome to digest specific cellular components can lead to lysosomal storage disease (ex. Tay-Sachs disease when ganglioside (lipid) is not broken down and accumulates in brain cells)

plant cells don’t have lysosome, so vacuole hydrolyze macromolecules

Golgi apparatus

consist of cisternae (flattened membranous sacs) and small membrane-enclosed vesicles

receives protein-containing vesicles from RER

modifies, concentrates, packages, and sorts protein before they are sent

add carbohydrates to modify

some polysaccharides for plant cell wall are synthesized

cuts certain precursor proteins to smaller functional fragments (ex. 241-amino acid polypeptide produce by pituitary gland)

cis face lies nearest to nucleus, where protein-containing vesicles fuse in and release their cargo into lumen

trans face lies closest to cell membrane, where vesicles exits to go to cell membrane or lysosome

medial face lies between cis and trans

each face contain different enzymes and perform different functions

mitochondria

transform chemical-bond energy into ATP

reproduce and divide independently of the central nucleus

cells that are active in movement and growth have the most mitochondria

2 membrane where inner membrane folds inward, leading to large surface area and give rise to cristae

inner membrane has many large protein complexes that participate in cellular respiration

mitochondrial matrix is the space within inner membrane that contains ribosomes and DNA used to make proteins needed for cellular respiration

chloroplasts

plastids that divide independently

contains green pigment chlorophyll

site of photosynthesis

2 membrane

folding forms lamellae , which is on the surface of thylakoid

stack of thylakoid lipids are granum

thylakoid lipids have 10% phospholipid and the rest is galactose-substituted diglycerides and sulfolipids (most abundant lipids)

-membrane contain chlorophyll and other pigments that harvest light energy for photosynthesis

grana is the site where light reactions occur

stroma is the fluid in chloroplast, which contains ribosomes and DNA

peroxisomes

accumulate toxic peroxides (H2O2) that are byproduct of some biochemical reactions

membrane enclosed—without mixing other parts of the cell

glyoxysomes

found only in plants, especially young plants

membrane enclosed

stored lipids are converted into carbohydrates for plant growth

vacuoles

mostly in plants, fungi, protists

from ER or Golgi apparatus

take up 90% of cell volume and grows as cell grows

water enter vacuoles from cytosol, making it swell, but not in mature plants due to the cell wall

contain some pigments to attract animals that assist pollination or seed dispersal

vacuoles of seeds contain molecules that hydrolyze stored proteins into monomers (used for food for plant growth in germination)

store toxic molecules and waste products for defense and survival

cytoskeleton

support cell and maintains its shape

holds and moves cell organelles and other particles in position within the cell

movement of cytoplasm—cytoplasmic streaming

interacts with extracellular structures, helping cell anchor in place

3 components: microfilaments, intermediate filaments, microtubules

microfilaments

7nm in diameter

help entire cell or cell parts move

maintain cell shape—tension-bearing

changes in cell shape

polymerization of monomer actin (protein)—reversible

form long, double-helical chains

cytoplasmic streaming

pinch when dividing animal cell—form cleavage furrow

formation of pseudopodia (false feet)—extensions that enable some cells to move

muscle contraction

intermediate filament

6 molecular classes that share same general structure

form from fibrous protein keratin

8-12 nm

anchor nucleus and some other organelles

maintain shape—tension-bearing

formation of nuclear lamina

microtubules

long, hollow, unbranched cylinder (25nm)

wall consists of 13 columns of tubulin molecules

protein tubulin (alpha and beta)—reversible

form rigid internal skeleton for some cells

allow motor protein to move organelles

maintain shape—girder

capable to change length rapidly—adapt to new cell purposes

framework for formation of cellulose and help orient cellulose fibers of cell wall

2 types of motor proteins that move chromosomes during cell division: kinesin (positive end that allow movement of vesicles inside neurons) and dynein (negative end that allow microtubule doublets to slide past one another in cilia and flagella)

cilia and flagella contains microtubules (9 fused and 2 unfused), which nexin holds them together and forces from dynein causes it to bend rather than move

cilia

0.25 micrometer

hair-like structure covering the cell

move fluid

movement is through sliding of microtubules doublets past one another, driven by dynein

flagella

100-200 micrometer

singly or in pairs

tail like structure

cell wall

contain cellulose fiber with other complex polysaccharides and proteins

provide rigid support but still flexible to bend

barrier to infection

cell wall on leaves have porous, but not for vascular system

plasmodesmata

20-40 nm

plant cell wall channels

allows diffusion of water, ions, small molecules, RNA, proteins

extracellular matrix

composed of collagen (fibrous protein), proteoglycans (matrix of glycoproteins), and a third group of protein that link collagen and proteoglycans matrix

hold cells together in tissue

contribute to physical property of tissue

filter materials—important in kidney

orient cell movement during embryonic development and during tissue repair

chemical signaling between extracellular matrix and cytoplasm

theory of endosymbiosis

some organelles arose by one cell ingesting another cell, giving rise to a symbiotic relationship

ingested cell loses its autonomy and some function

gene of ingested cell were transferred to host’s DNA

ex. mitochondria, plastids (ex. chloroplast), nucleus

nucleus—starts from membrane of bacterial cell folding around nucleoid, then nucleoid became nuclear envelope

primary endosymbiosis of a purple bacteria—mitochondria

primary endosymbiosis of a photosynthetic cyanobacteria

primary endosymbiosis: cyanobacterium/purple bacterium have 2 membrane, not one. cyanobacterium/purple bacterium enters host eukaryote via endocytosis and gets surrounded by a food/phagocytic vacuole. instead of being eaten, it stays within the cell. over time, the outer membrane of vacuole breakdown leaving an endosymbiont with its original 2 membranes. it transfer its genetic information to the nucleus

secondary endosymbiosis: a eukaryote eats another eukaryotic cell that has a chloroplast through phagocytosis. vacuole membrane breakdown over time, leaving a triple-membrane chloroplast. eaten cell’s genetic material is transferred to pirate’s nucleus. it will have 3 sources of genomes in the nucleus: mitochondrial, chloroplastic, nuclear (ex. chloroplast of brown algae)

cholesterol

20-50% of animal cell membrane

locate inside membrane

hydroxyl group of cholesterol interact with polar end of phospholipid

important for membrane unity and fluidity

not hazard to health

cholesterol with long chained, saturated fatty acid are pack tightly, so it is less fluid

integral membrane proteins

membrane protein

hydrophilic—amino acids on R group is polar

hydrophobic—other amino acids on the R group is nonpolar

cross through membrane or embedded in bilayer

ex. transmembrane protein cross through

peripheral membrane protein

polar region interact with exposed integral membrane protein’s polar head

non-covalently bond to either membrane surface

locate on one side

anchored membrane protein

covalently bind to hydrophobic lipid group

insert into phospholipid

homotypic

binding of cells in a tissue

same molecule binding with each other

keep skin cells together in a sheet of cells

heterotypic

binding between different molecules on different cells

ex. sperm and egg

glycolipid

carbohydrate covalently bonded to lipid

serve as recognition signal

ex. carbohydrates changes when cells become cancerous, which allows white blood cells to target cancer cells for destruction

cell junctions

additional membrane structure that connect 2 cells

seal intercellular space

reinforce attachments to one another

communication

3 types: tight junction, desmosomes, gap junctions

tight junction

prevent substances from moving through the space between cells

ex. cells lining the bladder have tight junction so urine cannot leak out

maintain a cell by restricting migration of membrane proteins

-ex. endocytosis

desmosomes

hold neighboring cells firmly together

materials move in the extracellular matrix

stabilize tissues that receive physical stress

ex. skin

gap junctions

channel that run between membrane pores in adjacent cells

allow substance to pass between cells

ex. rapid spread of electric current in the heart so heart muscle cells beat

integrin

transmembrane protein

maintain cell structure via interaction with cytoskeleton

noncovalently binds to extracellular matrix—reversible

passive transport

no require input of chemical-bond energy

energy comes from concentration gradient

ex. simple diffusion (phospholipid bilayer), facilitated diffusion (channel or carrier protein)

active transport

driven by chemical-bond energy

often through ATP, produced in mitochondria

directional

3 kinds of membrane protein that carry out active transport: uniporter (move one in one direction, ex. calcium-binding protein), symporter (move 2 in same direction), antiporter (move 2 in opposite direction, integral membrane glycoprotein, ex. sodium—potassium pump Na+ in, K+out)

coupled transporter: symporters and antiporters

2 basic type of active transport: primary and secondary

primary active transport

direct hydrolysis of ATP—form ADP, phosphate group, and energy

ex. sodium—potassium pump

second active transport (co-transport)

doesn’t use ATP

supply energy by an ion concentration gradient established by primary active transport

ex. sodium—potassium pump establish concentration gradient that provide energy for secondary active transport of glucose

simple diffusion

more lipid-soluble, more rapid diffusion across membrane

hydrophobic

electrically charged/polar doesn’t pass through (ex. amino acid, sugar, ions) because they form hydrogen bonds with water

osmosis

movement of water molecules across the membrane

passive

isotonic: equal solute concentraiton

hypotonic: lower concentration than others

hypertonic: higher concentration than others

water moves from hypotonic to hypertonic

cell wall limits intake of water, so water build up turgor pressure, enlarge vacuoles/plant cells, which is why plants are upright when hydrated

facilitated diffusion

passive transport

aid by channel proteins (integral membrane proteins) or carrier proteins (binds substances and speed up diffusion)

ion channels

channel proteins for particular ions

gated channel: open when stimulus (ligand) causes a change in 3D shape

other channels open in response to physical stimuli, such as sound waves, voltage

aquaporins

protein channels that allow water to cross

cellular plumbing system

water molecules move through the channel, excluding ions, which maintain electrical properties of cells

glucose transporter

binds to glucose, which changes the 3D shape of protein and release glucose to the other side of the membrane

have strong concentration gradient that favor glucose entry

cells that require large amounts of energy have high concentration to maximize diffusion rate, such as muscle cells and human brain

endocytosis

brings small molecules, macromolecules, large particles, and small cells

only in eukaryotes

3 types: phagocytosis, pinocytosis, receptor-mediated endocytosis

phagocytosis

engulf large particles or entire cell

unicellular use phagocytosis to feed

white blood cells use phagocytosis to defend the body by engulfing foreign cells and substances

food vacuoles/phagosomes—lysosome for digestion

relatively nonspecific

pinocytosis

form smaller vesicles

bring fluids and dissolved substance into the cell

relatively nonspecific

receptor-mediated endocytosis

molecules at the cell surface recognize and trigger uptake of specific material

capture specific macromolecules

depends on receptor proteins (integral membrane proteins) that bind to specific region called coated pits

when ligand binds to receptor proteins, coated pits invaginates and forms a coated-vesicles and clathrin (protein) strengthen and stabilize the vesicles to travel away to cytoplasm

ex. cholesterol: LDL receptors on the cell surface binds to LDL, and the clathrin-coated pits engulf and become a vesicle. vesicle is fused with a lysosome, in which LDL is digested and cholesterol are available for cells to use. in healthy individuals, liver takes up unused LDLs for recycling

exocytosis

process of packaging materials in vesicles and secretes from a cell

release vesicle contents into the environment

2 ways: vesicle fuses with the cell membrane and release the vesicle content or vesicle touches the cell membrane and a pore forms, releasing the vesicle contents

cell division of prokaryotes

cell division signal (inside or outside of the cell) is usually environmental conditions and nutrient concentration

-nutrient, pH, and temperature has to be optimal

DNA replication starts at the ori and ends at the ter

-most prokaryotes have one main chromosome

DNA segregation is when each daughter cell receives a copy of every chromosome. once replicated, ori regions move toward opposite ends of the cell. protein bind to the DNA sequences, which hydrolyze ATP

cytokinesis is when cytoplasm divides to form 2 new cells with cell membrane/cell wall

results in reproduction of entire single-celled organism, and they are identical

ex. binary fission

cell division of eukaryotes

cell division signal for single-celled is appropriate environmental conditions, while for multicellular is suitable internal environments that relate to function but it doesn’t mean it is necessary to occur

DNA replication contain more chromosomes and starts at numerous origins of replication

DNA segregation—mitosis, which involves spindle (special cytoskeletal structure composed of microtubules)—produce identical cells

cytokinesis—plant cells form cell plates (multiples vesicles that contain pectin, cellulose, and hemicellulose) while animal cells form cleavage furrow that pinch off

cell (division) cycle

4 phases: G1, S, G2, M

M phase include mitosis and cytokinesis

cell spends most of the time in interphase, which includes G1, S, G2

G1 phase: single, unreplicated DNA molecule with associated proteins

S phase (DNA replication): chromosome is duplicated, which consists of 2 sister chromatids (remained join until mitosis)

G2 phase: cells get prepared for mitosis

G0 (resting phase), where there is no cell division, but sometimes cells can reenter G1 under certain environmental conditions (growth factor)

mitosis

spindle in mitosis is formed by microtubule structure

centrosome determines orientation of spindle and it consist of a pair of centrioles

in prophase, chromatids are visible and kinetochores (protein) develop in the centromere region

prophase and prometaphase: microtubule link the poles and chromosomes to make up spindle

-polar microtubule form framework of spindle and run from one pole to another, while kinetochore microtubule attach kinetochores on chromosomes

prometaphase: nuclear envelope breaks, spindle is formed

metaphase: chromosomes line up at the equator

anaphase: separation of chromatids, which activates anaphase-promoting complex (APC) when cohesin is hydrolyzed by separase

telophase: spindle disappears, nuclear envelope forms

cytokinesis: contractile ring (composed of microfilaments of actin and myosin) pinches the cell for animal cell, while membranous vesicles from Golgi apparatus fuse to form cell plate for plant cell

CDK (cyclin-dependent kinases)

enzyme that controls the transition from one phase to another, which is the restriction point

CDK is always present, but inactive, until cyclins binds (protein) to it

binding of cyclin changes the shape of CDK and expose its active site to substrates. a protein substrate and ATP binds to CDK. protein substrate is phosphorylated, which regulates cell cycle.

G1/S cyclin—CDK catalyze phosphorylation of retinoblastoma protein (RB), which is an inhibitor at the restriction point. cyclin—CD changes 3D structure of RB, so the resulting product is ADP and RB-P. this inactivates the RB protein, and the cell cycle proceed

ex. if DNA is damaged by radiation during G1, p21 (protein) binds to G1/S—CDK, preventing cyclin binding, which keeps it inactive and cell cycle pauses until DNA is repaired, in which p21 breaks down and cyclin binds to CDK. if DNA is unable to repair, the cell undergoes cell death

growth factors

external chemical signal that brings cells from G0 to G1

ex. platelets produce platelet-derived growth factor that help heal the wound

ex. interleukins are growth factors for white blood cell

ex. erythropoietin are growth factor for red blood cell

phosphorylation

addition of a phosphate group

ex. reaction from ADP to ATP

cell fusion experiment

reveal existence of internal signals that control the transitions between stages of the cell cycle