Download Cell Function – General Membrane Transport

Cell Function – General
Membrane Transport
What does the cell do = cell physiology:
cell life must be maintained within a narrow range of
conditions (requirements for life)
1. Membrane Transport
movement of materials into and out of each cell
the “external” environment changes much more
drastically than internal environment
2. Secretion
eg. gland cells, mucus, hormones
!a cell’s survival depends on its ability to maintain
this difference
3. Membrane Potential
all cells hold an electrical charge across their cell
membranes;
some cells, eg nerve and muscle cells, can alter that
charge to send impulses or to contract
1st line of defense in protecting the cell is cell
membrane
interface between living and nonliving
4. Cell Cycle & Cell Division
DNA replication
mitosis
meiosis
ALL living organisms possess a cell membrane
The cell membrane is semipermeable
(=selectively permeable)
5. Metabolism
enzymes
synthesis and decomposition
ATP and energy use
metabolic pathways
major energy pathways
protein synthesis
! some small molecules cross by passive
diffusion
! some substances require expenditure of
energy
6. Cellular Interactions
also called cell signaling; how they “talk” to each other
hormones, neurotransmitters, white blood cells, etc
must interact with or trigger responses in cells
! some require extra “helpers” to get across
7. Cellular Control: Genes &
Chromosomes
Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
passive
active
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Two general kinds of movement
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resembles ordinary diffusion
works with a gradient
occurs spontaneously
does not require extra ATP
move lipid insoluble things across membrane
– they need help to cross
- specific for certain solutes
works against gradient
requires ATP
doesn’t usually occur outside living cells
accelerates diffusion through membrane
PASSIVE MOVEMENTS
uses specific carrier molecule (=carrier
protein)
1. Simple Diffusion
movement of a substance down a concentration
gradient
may be through channels or pores created by
these proteins
main way small things cross membranes and
move within cells and fluid compartments
does not require ATP (energy)
eg. absorb glucose & amino acids in intestine and kidneys
materials can diffuse through air, liquids, and
solids
3. Osmosis
diffusion of water, across a membrane, down a
water concentration gradient
eg. O2 , CO2 , small molecules, lipid soluble
molecules
nonpolar, lipid soluble
some small polar molecules
hypertonic = sol has more solutes than another
(eg. human cell is hypertonic to distilled water)
cell does not have to expend any energy
hypotonic = sol has fewer solutes than another
define: solute and solvent
isotonic
2. Facilitated Diffusion
4. Filtration
movement of a solute across a membrane down a
concentration gradient using a carrier protein
Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
= solutions have equal concentrations
of solutes
movement of water and solutes through a
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membrane down a pressure gradient
controlling cell volume
generating body heat
excitability of muscle and nerve cells
hydrostatic pressure = blood pressure
~ half of calories you “burn” every day are used just to operate
your Na/K pumps
unidirectional (no equilibrium is reached)
eg. kidney filtration to form urine
2. Vesicular Transport (=bulk transport)
eg. materials leaving blood in capillary beds
movement of large particles, macromolecules, and
cells across membrane inside bubble-like vesicles
ACTIVE MOVEMENT
requires ATP
1. Active Transport (=solute pumping)
several kinds of vesicular transport:
movement of a solute across a membrane up a
concentration gradient using ATP and a protein
carrier
a. Exocytosis
moves substances out of cell
secretory cells, gland cells
requires energy (ATP) and protein carriers
eg. how golgi bodies secrete things
very specific
b. Endocytosis
moves substances into cell
cell membrane forms vesicle around material and
pinches off to form vacuole
moves specific solutes against (up) concentration
gradient
=physiological pump
kinds of endocytosis
helps maintain concentration gradients
1. phagocytosis
“cell eating”
esp macrophages
fuses with lysosome for digestion
able to move things in or out quickly
counteracts diffusion
eg. Na/K pump
! roles in
2. pinocytosis
“cell drinking”
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Secretion
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Membrane Potential
a specific “application” of membrane transport
“application” of membrane transport
many body cells function in secretion
all cells are polarized
! separation of charge
!correlated with number of golgi bodies
more “+” charges on outside
more “-“ charges on inside
either as individual gland cells or as components of
multicellular glands
determined mainly by
differing concentrations of Na+, K+, Cl-,
& proteins-
glandular secretions can be through a tube or
passageway = exocrine glands
this separation of charge creates a voltage difference
(=potential) across the membrane
or directly into blood capillaries = endocrine
glands
eg. as in a battery
examples of materials secreted:
varies between -20mv to –200mv
1. sweat
exists across all cell membranes
! temperature homeostasis
this difference on nerve and muscle cells
= resting potential
2. oils and fats
! scents, lubrication, protection, waterproofing
! because they can alter it
3. mucus
! protection, trap pathogens
4. collagen and various fibers
! important components of connective tissues
5. digestive enzymes
6. hormones
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The Cell Cycle & Cell Division
eg. humans: ~100 trillion
! repair or replace damaged cells
Cell Cycle
cells lining intestine must be replaced ~ every 3
days
the cell cycle is the life cycle of a cell:
from its formation until it divides or dies
red blood cells are replaced every 3 months
! each day ~50 Billion body cells die and are replaced
each new cell must receive:
cell cycle typically involves a period of interphase
alternating with a period of cell division
!a complete set of instructions (chromosomes, genes, DNA)
!and a basic assortment of cellular structures to continue this
metabolism
Intephase
two basic types of cell division:
during most of a cell’s “life” (cycle) it performs its
specialized functions and activities
Mitosis
we’ve defined what some of those “normal” activities
are
= “normal” metabolism
makes identical copies of cells
! parent cell is genetically identical to the two
daughter cells produced (clone)
Cell Division
at one or more points during its life it must reproduce
to make copies of itself
almost every cell in your body is essentially a
clone of the original fertilize egg
only difference is which genes are “switched on”
! determines what it has specialized into
! for growth (># of cells)
all life begins as a single cell
Meiosis
in complex organisms that single cell grows and
divides repeatedly to form a complex
multicellular organism
the formation of egg and sperm require a different
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Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
kind of cell division in which the daughter cells
contain only a single set (23) of chromosomes
only sex cells (eggs and sperm) are formed by
Meiosis
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Mitosis
Interphase
the nondividing stage of a cells life cycle = interphase
normal metabolism characteristic of the cell occurs
here
as the cell approaches time to divide:
it begins to duplicate all the organelles and
materials the new cells will need to get started
it also must duplicate the genetic instructions
(chromosomes) that will be needed
the chromosomes are replicated during interphase
! this process in not visible to us
DNA Replication
1. DNA in each chromosome unzips
2. an enzyme, DNA polymerase, collects the proper nucleotides
from its surroundings and uses each strand as a template
to build 2 new complementary strands
replication progresses at a rapid pace:
eg. eucaryotes ! 200 nucleotides/sec
occasionally errors are made
another group of enzymes “edits” and corrects those
errors
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Stages of Mitosis
cytokinesis doesn’t always occur during mitosis
after DNA is replicated, the cell begins the division
process that becomes visible (under a microscope and
special stains) as a series of distinct stages or phases
called Mitosis
eg coenocytic striated muscle cells, no cytokinesis)
The end result of mitosis:
1 parent cell produces two daughter cells
mitosis in most human cells typically takes ~ 1-2 hours
each new daughter cell has the
1. Prophase
same #
nuclear membrane disappears
nucleoli disappear
chromosomes begin to appear
centrioles split and begins to form spindle
same kinds of chromosomes
as the original parent cell
2. Metaphase
Variations in the Cell Cycle
spindle is fully formed
replicated chromosomes (2 chromatids) line up along the center
of cell (equatorial plane)
3. Anaphase
many cells are dividing about every 30 minutes
b. in adults cell cycles vary considerably between
different organs and tissues:
4. Telophase
the chromosomes reach opposite sides of cell
begin to disappear (no longer visible)
the nucleus reforms
the spindle disappears
telophase is end of nuclear division; overlaps with…
cytokinesis ! division of the cytoplasm
= the cell splits (pinches) into two separate cells
lifespan of some human cells:
inner gut epithelium
skin
red blood cells
bone
intercostals muscle cells
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! 15.9 yrs
c. some cells divide only irregularly or as needed in
adult life
!
!
!
!
!
5 days
2 weeks
120 days
10 yrs
15.1 yrs
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eg. embryo has webbing between digits
this normally goes away as cells between digits die
eg. causes shrinkage of uterus after pregnancy ends
eg. immune cells can trigger cancer cells to commit suicide
by apoptosis
eg. liver & kidney cells only divide if repair is needed
d.
the length of a the cell cycle varies greatly from
one cell to another
a. during development, most cell cycles are relatively
short
centromere splits and chromatids (each half of the replicated
chromosome) separate and begins to migrate toward the
opposite side of the cell (now each chromatid
= chromosome)
gut cells (not lining)
and
some cells stop dividing shortly after birth
! divide little all our adult lives
eg. muscle cells, nerve cells
an inability to stop cycling at a rapid rate is a
characteristic of cancer cells
Cell Death (Apoptosis)
Apoptosis = programmed cell death
normal death of cells that have completed their function
programmed cell death plays a crucial role in growth
and development
may also play a role in maintaining normal cell lines by
killing off unneeded cells
billions of cells die every hour by apoptosis
! don’t trigger inflammation since cell contents
never escape; cell is engulfed by WBC’s
eg. embryo produces ~ twice as many neurons as we need
those that make connections survive, those that don’t
die
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Cell Metabolism
decomposition of organic polymers occurs through
Hydrolysis
metabolism refers to all the 1000’s of chemical
reactions that are occurring within a cell
Metabolism = Anabolism + Catabolism
there are two basic kinds of reactions:
most of these reactions require a specific enzyme in
order to occur
Anabolic Reactions & Catabolic Reactions
Characteristics of Enzymes:
1. Anabolic Reactions (Anabolism)
1. all enzymes are proteins
refer to all the synthesis reactions occurring in
the cell
2. each reaction has a specific enzyme that catalyzes
it
!synthesis reactions make bonds
3. enzymes (and all proteins) are very sensitive to
environmental conditions
!synthesis reactions require or store
energy
synthesis of organic polymers occurs by dehydration
synthesis
each enzyme operates under a narrow range of
conditions
2. Catabolic Reactions (Catabolism)
refer to all the decomposition reactions occurring
in the cell
!decomposition reactions generally break
bonds
5. enzymes are very efficient:
eg. a single enzyme molecule can cause
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300-400 reactions/ second
6. because they are used over and over and are very
efficient, enzymes are needed in only very
low amounts
7. some enzymes require coenzymes or cofactors
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Compartmentalization of cells helps to organize
enzyme activities and makes them more efficient
eg. aerobic respiration
! mitochondria
eg. lipid synthesis
! endoplasmic reticulum
ATP & Energy Use
coenzymes
= essential organic compounds many are called vitamins
that cannot be made by body
ATP is the immediate source of energy for cells
eg. NAD, FAD, NADP, etc
the energy released by catabolism of glucose is
absorbed by ATP
or
cofactors
= inorganic metal or ion required for proper configuration of
active site
ATP then transfers this energy to various cell processes
that require it
1. Active Transport and Bulk Transport
eg. Fe, Zn, Mg, etc
moving things in and out of cell against concentration
gradients
Metabolic Pathways
2. Bioelectricity
Metabolism in most cells is a collection of groups of
enzymes working together
this produces the ability to produce and conduct a
nerve impulse along a neuron
3. Movement
many of the reactions occurring in cells occur in a
sequential, stepwise fashion
contraction of muscle cells requires large amounts of ATP
also beating of cilia and flagella and the amoeboid movement
of WBC’s
= metabolic pathways
Most chemical reactions and entire metabolic pathways
that occur in cells are reversible:
4. Anabolic Reactions (biosynthesis)
the formation of new organic molecules requires ATP since
new bonds are made
ATP is a special energy transfer molecule
same enzyme may catalyze reaction in either direction
Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
4. enzymes are not used up in reactions
they can be used over and over again
E + Substrate ! ES Complex ! E + Product
!decomposition reactions generally release
energy
Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
since the shape of the enzyme is due mainly to
weak hydrogen bonds
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it can easily absorb energy from the breakdown of
organic molecules and release energy for
various cellular processes
! its bonds easily store and release energy
Cellular Interactions
the 100 trillion cells in the body must be able to
communicate and interact with each other to
coordinate their activities
!need to be able to “talk” to each other
ATP exists mainly in two forms:
high energy ATP
lower energy ADP
Cell Receptor Molecules
=adenosine triphosphate:
A~P~P~P
a diverse group of chemicals, mainly proteins, on cell’s
surface act as receptors or binding sites
high energy bonds
each receptor reacts to a specific chemical
between the phosphorus atoms are “high energy”
bonds
specific chemicals bind to receptors on or in cell to
cause change in cell function
ATP absorbs energy by forming these bonds
!
!
!
!
!
ATP releases energy by breaking these bonds
! release 4-6 x’s more energy than that released
when “ordinary” bonds are broken
cell only responds to a chemical if it has the correct
receptor protein
! target cell
ATP
ADP
ATP
releases
energy
cause cell to change metabolism
cause cells to secrete a product
cause a cell to divide or prevent it from dividing
cause a nerve cell to fire an impulse
cause a muscle cell to contract
ATP
stores
energy
different cells respond in different ways to same
chemical
AMP
eg. ACh
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! stim skeletal muscle cells
! inhibits heart muscle cells
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Cellular Control:
Genes & Chromosomes
Many different kinds of chemicals can be used for
signaling:
the cell is a bag of controlled chemical reactions
eg. neurotransmitters
eg. hormones
eg. tissue hormones
these reactions are controlled by the DNA of the
chromosomes in the nucleus of the cell
on these chromosomes are the genetic instructions
that control everything the cell does
each instruction = gene
(human cells contain ~22,000 different genes)
each gene is a unit of inheritance
each chromosome contains many genes
(~500-600 genes/chromosome)
almost all reactions occurring in a cell require a
specific enzyme to occur
all enzymes are proteins
therefore, if you control protein synthesis
you can regulate all metabolism
a typical cell is continually producing 100’s of different
kinds of proteins, many of them are enzymes
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as an enzyme is needed the gene(s) is (are) activated
and the required protein (enzyme) is made
Two problems with protein synthesis:
what is the “code” contained on the mRNA molecule
1. DNA is in chromosome in nucleus and
proteins are made at ribosome
Triplet Code
! how does info move from nucleus to ribosomes?
4 nucleotides must code for 20 different amino acids:
2. proteins consist of a series of 20 different kinds
of amino acids
nucleic acids are a series of only 4 kinds of
nucleotides
! how do 4 different nucleotides code for 20 different
amino acids
if 1 nucleotide codes for 1 amino acid
! could only get 4 different AA’s
if 2 nucleotides codes for 1 amino acid
! could get no more than 16 different AA’s
if 3 nucleotides code for 1 amino acids
! can get 64 different combinations
more than enough for the 20 AA’s
plus some extra for punctuation
protein synthesis is a 2 step process:
transcription
translation
the code is nearly universal (bacteria to humans)
! all life is interrelated
involves a second group of nucleic acids = RNA
translation is the process by which the genetic code on
the RNA is converted to a specific sequence of
amino acids at the ribosome
Transcription
RNA copies the code on the gene in the DNA molecule
each strand of RNA may be used to make several
copies of the same protein
based on complementary bonds
a sequence of nucleotides on a DNA (=gene) acts as a
template for synthesis of RNA strand
eventually mRNA is degraded into constituent
nucleotide
Translation
Microscopy & Cells: Cell Function; Ziser Lecture Notes, 2005
RNA moves out to ribosomes and directs the
construction of a specific protein using other RNAs
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