unit 4 biology

homeostasis

homeostasis

- the state of relatively stable internal conditions
- organisms detect and respond to a stimulus
- body uses feedback loops to maintain

set points

- values for various physiological conditions that the body tries to maintain
- has a normal range for which it can fluctuate
- ex: body temp set point= 98.6; normal range= 97-99

stimulus

variable that will cause a response (actions)

receptor/sensor

- sensory organs that detect a stimulus
- info sent to brain (organs)

effector

muscle or gland that will respond (organs)

response

changes (decreases or increases) the effect of the stimulus (actions)

negative feedback

- returns/reduces the effect of the stimulus
- most common
- ex: sweat, blood sugar, breathing rate, body temp

negative feedback example

body temp regulation
- stimulus: heat
- receptor: temp receptors in skin
- effector: sweat glands
- response: sweat

positive feedback

- increases the effect of a stimulus
- ex: child labor, blood clotting, fruit ripening, lactation

positive feedback example

childbirth
- stimulus: baby pushes on cervix
- receptor: nerve cells in cervix send signal to brain
- effector: pituitary gland releases oxytocin
- response: oxytocin stimulates contraction
repeats

homeostatic imbalances examples

genetic disorders, drug/alcohol abuse, intolerable conditions

disease

- when the body is unable to maintain homeostasis
- cancer: body can't regulate cell growth
- diabetes: body can't regulate blood glucose levels

cell communication

- occurs through signal transduction pathways
- vital to cell function, survival, and maintaining homeostasis
- responsible for growth and development of multicellular organisms
- communication methods: direct contact, local signaling, long distance signaling

direct contact

- communication through cell junctions
- signaling substances and other material dissolved in the cytoplasm can pass freely between adjacent cells
- animal cells= gap junctions
- plant cells= plasmodesmata
- ex: immune cells (antigen presenting cells communicate to T cells)

local regulators

- a secreting cell will release chemical messages (local regulators/ligands) that travel a short distance through the extracellular fluid
- chemical messages will cause a response in a target cell
- paracrine and synaptic signaling

paracine signaling

secretory cells release local regulators (ex: growth factors, the start of mitosis) via exocytosis to an adjacent cell

synaptic signaling

- occurs in animal nervous system
- neurons secrete neurotransmitters
- diffuse across synaptic cleft (space between nerve and target cell)

long distance signaling

- animals and plants use hormones
- plants release hormones that travel in the plant vascular tissue (xylem, pholem) or through the air to reach target tissues
- animals use endocrine signaling: specialized cells release hormones into circulatory system where they reach target cells
- ex: insulin released by the pancreas into bloodstream where it circulates through body and binds to target cells (animals)

cell signaling

1. reception: ligand binds to receptor
2. transduction: signal is converted
3. response: a cell process is altered

reception

- the detection and receiving of a ligand by a receptor in the target cell
- when ligand binds to receptor, receptor activates via a conformational change. this allows receptors to interact with other cellular molecules, initiating the transduction signal
- receptors can be in plasma membrane or intracellular

receptor

- macromolecule that binds to a signal molecule (ligand)
- all have an area that interacts with the ligand and an area that transmits a signal to another protein
- binding between ligand and receptor is highly specific

plasma membrane receptors

- most common type of receptor involved in signal pathways
- binds to polar, large, water-soluble ligands
- ex: G protein coupled receptors, ligand-gated ion channels

intracellular receptors

- found in cytoplasm or nucleus of target cell
- binds to ligands that can pass through the plasma membrane
- ex: hydrophobic molecules (steroid and thyroid hormones, gases like nitric oxide)

transduction

- the conversion of an extracellular signal to an intracellular signal that will bring about a cellular response
- requires a sequence of changes in a series of molecules known as a signal transduction pathway
- the signal transduction pathway regulates protein activity through phosphorylation and dephosphorylation
- signal is amplified

kinase

- turns signal on
- phosphorylates
- enzyme protein

phosphatase

- turns signal off
- dephosphorylates
- enzyme protein

second messengers

- small, nonprotein molecules and ions help relay the message and amplify the response
- ex: cyclic AMP

response

- the final molecule in the signaling pathway converts the signal to a response that will alter a cellular process
- ex: protein that can alter membrane permeability, enzyme that will change a metabolic process, protein that turns genes on/off

signal transduction pathways

- can influence how a cell responds to its environment
- can result in changes in gene expression, cell function, phenotypes, or cell death
- mutations to receptor proteins or to any component of the signaling pathway will result in a change to the transduction of the signal

cell membrane receptors

- G protein coupled receptors
- ion channels (ligand gated)

G protein coupled receptors

- largest category of cell surface receptors
- important in animal sensory systems
- binds to a G protein that can bind to GTP
- GPCR, enzyme, and G protein inactive until ligand binds to GPCR on extracellular side
- ligand binding causes cytoplasmic side to change shape, allowing G protein to bind to GPCR and activating the GPCR and G protein, GDP becomes GTP
- part of activated G protein can bind to enzyme, activating enzyme and amplifying signal and leads to a cellular response

GTP

energy molecule that can phosphorylate like ATP

ion channels (ligand gated)

- located in plasma membrane
- important in nervous system
- receptors that act as a "gate" for ions
- when a ligand binds to the receptor, the "gate" opens/closes allowing the diffusion of specific ions
- initiates a series of events that lead to a cellular response

cell cycle

the life of a cell from its formation until it divides

cell division

allows for the reproduction of cells, growth of cells, and tissue repair

what cells do before division

organize and package their DNA

nucleosomes

formed when DNA associates with and wraps around proteins (histones)

chromatin

- strings of nucleosomes
- in a non-condensed form when cell is not actively dividing

chromosome

- formed when chromatin condenses after DNA replication
- densely packed to allow for easier division
- too elongated -> risks not being replicated and duplicated correctly
- each has a duplicated copy since DNA is replicated

sister chromatids

copies of chromosomes joint together

centromere

the region on each sister chromatid where they are most closely attached

kinetochore

proteins attached to the centromere that link each sister chromatid to the mitotic spindle

genome

all of a cell's genetic information (DNA)

prokaryotes

singular, circular DNA

eukaryotes

- one or more linear chromosomes
- has a specific number of chromosomes
- ex: humans have 46 chromosomes

homologous chromosomes

two chromosomes (one from each parent) that are the same length, have the same centromere position, and carry genes controlling the same characteristics

somatic cells

- body cells
- diploid
- divide by mitosis
- ex: humans -> 2n=46

gametes

- reproductive cells (sex, sperm)
- haploid
- divide by meiosis
- ex: humans -> n=23

diploid

- 2n
- two sets of chromosomes (one from each parent)

haploid

- n
- one set of chromosomes

cell cycle

- consists of alternating phases of interphase and mitosis
- G1 -> S -> G2 (interphase) -> mitosis -> cytokinesis

interphase

- longest portion of the cell cycle (90%)
- G1 "first gap" phase
- S "synthesis" phase
- G2 "second gap" phase

G1 "first gap" phase

cell grows and carries out normal functions

S "synthesis" phase

DNA replication and chromosome duplication occurs

G2 "second gap" phase

final growth and preparation for mitosis

M phase

- mitosis: nucleus divides
- cytokinesis: cytoplasm divides
- mitosis results in 2 identical diploid daughter cells

phases of mitosis

1. prophase
2. prometaphase
3. metaphase
4. anaphase
5. telophase and cytokinesis

prophase

- chromatin condenses
- nucleoli disappear
- duplicated chromosomes appear as sister chromatids
- mitotic spindle begins to form
- centrosomes move away from each other

prometaphase

- nuclear envelope fragments
- microtubules enter nuclear area and some attach to kinetochores (center)

metaphase

- centrosomes are at opposite poles
- chromosomes line up at the metaphase plate
- microtubules are attached to each kinetochore

anaphase

- sister chromatids separate and move to opposite ends of the cell due to the microtubules shortening
- cell elongates

telophase and cytokinesis

- two daughter nuclei form
- nucleoli reappear
- chromosomes become less condensed
- cytokinesis occurs

cytokinesis

- animals: cleavage furrow appears due to a contractile ring of actin filaments
- plants: vesicles produced by the golgi travel to the middle of the cell and form a cell plate

tracking chromosomes

- parent cell: 2n=2; 2 chromosomes
- daughter cells: 2n=2; 2 chromosomes
- after S phase: 2 chromosomes, 4 chromatids
- anaphase: 4 chromosomes
- humans: 46 chromosomes, 46 chromatids

checkpoints

- control points that regulate the cell cycle
- cells receive stop/go signals
- G1, G2, M

G1 checkpoint

- checks for cell size, growth factors, and DNA damage
- most important checkpoint
- GO: cell completes whole cell cycle
- STOP: cell enters a non-dividing state (G0)

G0 phase

- some cells (muscle/nerve) stay here forever
- some cells can go back into the cell cycle

G2 checkpoint

- checks for completion of DNA replication and DNA damage
- GO: cell proceeds to mitosis
- STOP: cell cycle stops, cell attempts to repair damage. if damage cannot be repaired, the cell will undergo apoptosis

apoptosis

programmed cell death

M (spindle) checkpoint

- checks for microtubule attachment to chromosomes at the kinetochores at metaphase
- GO: cell proceeds to anaphase and completes mitosis
- STOP: cell will pause mitosis to allow for spindles to finish attaching to chromosomes

cyclins and cyclin-dependent kinases

regulation of the cell cycle involves an internal control system

cyclin

- protein
- concentration varies
- synthesized and degraded at specific stages of the cell cycle

cyclin-dependent kinases

- enzymes
- concentration remains constant
- active only when its specific cyclin is present
- for full activation, many CDKS require phosphorylation by a cyclin-activating kinase
- each has a specific regulatory effect
- active CDK complexes phosphorylate target proteins, helping regulating key events in the cell cycle

growth factors

- hormones released by cells that stimulate cell growth
- signal transduction pathway is initiated
- CDKs are activated leading to progression through the cell cycle

contact (or density) inhibition

- cell surface receptors recognize contact with other cells
- initiates signal transduction pathway that stops the cell cycle in G1 phase

anchorage dependence

cell rely on attachment to other cells or the extracellular matrix to divide

DNA mutations

how normal cells become cancerous

normal cells

- follow checkpoints
- divide 20-50 times in culture
- significant errors -> apoptosis

cancer cells

- don't follow checkpoints
- divide infinitely in culture
- "immortal"
- escape apoptosis and continue to divide with errors

tumor

- mass of tissue formed by abnormal cells
- can be caused by uncontrollable growth of cancer cells
- types: benign and malignant

benign tumor

- cells abnormal but not cancerous yet
- cells remain at tumor site and unable to spread throughout body

malignant tumor

mass of cancerous cells that lose their anchorage dependency and can leave the tumor site

metastasis

when cells separate from the tumor and spread elsewhere in the body

cancer prevention

- don't smoke: nicotine causes mutations at high rates
- eat healthy and drink water: fatty foods and dehydration can affect cell function
- protect skin from sun: sun damages skin cells and can cause mutations