Anatomy Chapter 3.2

cell membrane

selective barrier controlling which substances enter and leave the cell

• includes both passive and active mechanisms

passive

requires no energy

active

requires cellular energy in the form of ATP

diffusion

movement of atoms, molecules, and ions from an area of high concentration to an area of low concentration

• atoms, ions, and molecules diffuse until diffusional equilibrium is met

• occurs because of constant and random motion of molecules → molecules collide and bounce off in opposite directions

diffusional equilibrium doesn’t normally occur in organisms

diffusion depends on

1. if the cell membrane is permeable to that substance

2. concentration gradient exists

physiological steady state (diffusion)

concentrations are unequal but stable

facilitated diffusion

movement follows concentration gradient with the help of protein carriers or channels

• used for substances that cannot diffuse across the selectively permeable membrane, since they are not lipid-soluble

• no energy required

  • ex: glucose

osmosis

movement of water across a selectively permeable membrane from region of higher water concentration to region of lower water concentration

• often called “diffusion of water”

water moves into region containing higher impermeant

aquaporins (osmosis)

water channels between phospholipids of cell membrane

osmotic pressure

ability of osmosis to generate enough pressure to lift a volume of water

• osmotic pressure increases as the concentration of impermeant solutes increases in a solution

• water moves toward solutions with higher osmotic pressure → water moves by osmosis (toward the regions with the trapped solute)

tonicity

ability of a solution outside the cell to alter water volume inside cell

• water wants to move towrd the area with higher concentration

isotonic solution

same osmotic pressure

• cells in an isotonic solution have no net gain or loss of water

hypertonic solution

higher osmotic pressure

• cells loser water; results in shrinking

hypotonic solution

lower osmotic pressure

• cells gain water; results in swelling or bursting

filtration

molecules forced through membranes by exerting pressure

• hydrostatic pressure: weight of water due to gravity

  • ex: when blood plasma leaves capillaries, water and small solutes are filtered, but large plasma proteins and blood cells are not

active transport

carrier molecules have specific binding sites for specific particles → bind and energy released from ATP → molecule (or channel) changes shape and drives particles to the other side

• often called pumps, ex: Na/K pumps

against a concentration gradient

• requires the use of ATP

secondary active transport

does not require energy

• some particles rely on Na/K pumps to get through cell membranes → pumps change the concentration gradient so other substances can get through

endocytosis

movement of a substance into the cell inside a vesicle

• substances too large to enter by other methods can enter this way

3 types: pinocytosis, phagocytosis, receptor-mediated endocytosis

pinocytosis

cell drinking

• cells take in tiny droplets of liquid with solutes (vitamins, antibodies) from surroundings

phagocytosis

cell eating

• cells takes in solids

receptor mediated endocytosis

membrane engulfs specific substances, which have to bound to receptor proteins on the membrane

exocytosis

release of substances/particles from cell

• vesicles containing particles fuse with cell membrane and release contents

transcytosis

combination of receptor-mediated endocytosis and exocytosis that moves particles through a cell layer

• quickly transports substance from one end of cell to the other

the cell cycle

series of changes cells

• interphase: growth of cell, maintenance of normal functions

• mitosis: division of nucleus

• cytokinesis: division of cytoplasm

interphase: intermission

waiting for action but an active period

• cell grows and maintains functions but not actively dividing

begins once new cells are created in the cycle and ends when cell is prepared to divide

• phase where many contents of the cell are duplicated (ribosomes, lysosomes, mitochondria)

G1 phase (interphase): Growth

cell grows and develops

• restriction checkpoint marks the end of this phase

S phase (interphase): Synthesis

DNA copied → two sister chromatids → two copies of chromosomes (one for each daughter cell)

G2 phase: Gap

enzymes and proteins needed for cell division produced

• additional organelles are made → daughter cells

• 2nd checkpoint

mitosis

occurs in somatic cells (non sex cells)

• division of the nucleus via karyokinesis

• broken into phases (prophase, metaphase, anaphase, telophase)

prophase: prepare

chromatin condenses into chromosomes

• centrioles move to opposite sides of cytoplasm

• nuclear envelope and nucleolus disperse

• microtubules assemble and associate with centrioles and the two sister chromatids making up each chromosome

metaphase: middle

nucleus is gone

• chromosomes migrate to the middle of the cell

• fibers attach to each centromere

chromosome moves along fibers until aligned - midway between centrioles

anaphase: apart

centromeres of chromatids separate

• chromatids are now individual chromosomes

• chromosomes move in opposite directions

• fibers shorten and pull chromosomes toward centrioles

cell will elongate and a slight furrow in cytoplasm develops → first signs of cytokinesis

telophase: twins

final stage of mitosis

• chromosomes approach centrioles → elongate and uncoil back to chromatin

• nuclear envelope forms around chromatin and new nuclei are visible

• microtubules disassemble

more pronounced pinching (furrowing) of the cytoplasm into two separate bodies, but still only one cell

cytokinesis: cell movement, complete

contraction of a ring of microfilaments pinches off and forms two cells from one → two new nuclei and about half of the organelles into each

control of cell division

frequency of cell division is strictly regulated, and varies by cell type:

• skin cells, intestinal cells, and blood-forming cells divide often and continually

• neurons divide a specific number of times, and then cease

size, hormones and growth factors, contact inhibition

chromosome tips (telomeres)

shorten with each mitosis and provide a mitotic clock

• mitotic clock: states that cells can divide and die at any point in the cell cycle

kinases and cyclins

fluctuating levels of certain proteins in cell control cell cycle

size

cells divide to provide a more favorable surface area to volume relationship

• the larger the cell, the more nutrients it needs

• volume of the cell increases faster than surface area

• cell will divide → smaller cells

hormones and growth factors

external controls for cell division

contact inhibition

healthy cells stop dividing when they become crowded

• balance: too few mitoses and there is no growth, but too many leads to abnormal growth → tumors

tumors

2 types

• benign

• malignant

benign

remains in local area, often enlarges

malignant

invasive, cancerous, can spread or metastasize → cancer is latin for “the crab”

oncogenes

abnormal forms of genes that control cell cycle, but are overexpressed → increase the cell division rate

tumor suppressor genes

normally limit mitosis, but if inactivated/removed, cannot regulate mitosis

differentiation

guides cells specialization by activating and suppressing functions of many genes → essential for growth and repair

stem cells

undifferentiated cell that can divide to yield two daughter stem cells or a stem cell and progenitor cell

• self-renewal: can give rise to at lease one other stem cell

progenitor cells

daughter of a stem cell whose own daughter cells are restricted to follow specific lineages

totipotent

ability of a cell to differentiate to any type of cell (only a fertilized egg)

pluripotent

cell able to differentiate to yield specialized cell types → follow one pathway

apoptosis

programmed cell death: very controlled

• sculpts organs from tissues that naturally overgrow

• protective

necrosis

cell death from damage