Physiology of CellsPhysiology of Cells

Chap 4 version 2.0Chap 4 version 2.0

Movement of SubstancesMovement of Substances

membrane transport can be either membrane transport can be either active or passiveactive or passive

Passive requires no energy Passive requires no energy expenditure. Particles move using expenditure. Particles move using energy that they already have.energy that they already have.

In active transport the particles are In active transport the particles are “pulled” across a membrane. This “pulled” across a membrane. This requires energy by the cell.requires energy by the cell.

Passive Transport Passive Transport

1) Diffusion – particles will collide with 1) Diffusion – particles will collide with each other and eventually even out within each other and eventually even out within a solution.a solution.

Move from an area of high concentration Move from an area of high concentration to an area of low concentration down a to an area of low concentration down a “concentration gradient”. “concentration gradient”.

Will eventually produce an equilibrium in Will eventually produce an equilibrium in which both solutions have equal which both solutions have equal concentrations.concentrations.

Selectively Permeable Selectively Permeable Sometimes molecules diffuse directly Sometimes molecules diffuse directly

through the phospholipid bilayer and into the through the phospholipid bilayer and into the cell. But elements such as Sodium (Na+) and cell. But elements such as Sodium (Na+) and glucose cannot pass through a membrane glucose cannot pass through a membrane this way. Instead they need specific protein this way. Instead they need specific protein tunnels or pores. In such cases the cell is tunnels or pores. In such cases the cell is said to be permeable to the specific element said to be permeable to the specific element ie permeable to sodium. ie permeable to sodium.

The living membranes can be permeable to The living membranes can be permeable to some molecules and not to others. The some molecules and not to others. The membrane is said to be membrane is said to be selectively selectively permeablepermeable. Even in these cases the energy . Even in these cases the energy for transport comes from the random for transport comes from the random collisions of the molecules and not from the collisions of the molecules and not from the cell itself. cell itself.

Passive Transport - Passive Transport -

2) Dialysis – Diffusion of small particles but 2) Dialysis – Diffusion of small particles but not larger ones due to a semi-permeable not larger ones due to a semi-permeable membranemembrane

3) Osmosis – a special case of diffusion. 3) Osmosis – a special case of diffusion. The diffusion of The diffusion of water water through a through a selectively permeable membrane. selectively permeable membrane.

Water is allowed to equilibrate on both Water is allowed to equilibrate on both sides of the membrane but the sides of the membrane but the impermeant particles cannot. impermeant particles cannot.

One solution loses volume while the other One solution loses volume while the other solution gains volume. solution gains volume.

Osmotic PressureOsmotic Pressure Cells are closed containers, therefore a change in Cells are closed containers, therefore a change in

volume causes a change in pressure. volume causes a change in pressure. This is called osmotic pressure. Osmotic pressure This is called osmotic pressure. Osmotic pressure

develops in the solution that originally has the develops in the solution that originally has the higher concentration of impermeant particles. higher concentration of impermeant particles.

Because homeostasis is necessary to maintain Because homeostasis is necessary to maintain the healthy functioning of human cells, the the healthy functioning of human cells, the intracellular and extracellular fluids must have intracellular and extracellular fluids must have roughly the same osmotic pressure. If not, the cell roughly the same osmotic pressure. If not, the cell can either balloon out and rupture or become can either balloon out and rupture or become dehydrated and shrivel. (Hypotonic or Hypertonic dehydrated and shrivel. (Hypotonic or Hypertonic solutions) Water always osmoses from the solutions) Water always osmoses from the hypotonic solution to the hypertonic solution.hypotonic solution to the hypertonic solution.

Passive DiffusionPassive Diffusion

4) Facilitated Diffusion – particles are 4) Facilitated Diffusion – particles are attracted to a binding site on special attracted to a binding site on special proteins on the cell surface. Once proteins on the cell surface. Once bound, the protein changes shape bound, the protein changes shape and the particle is released inside the and the particle is released inside the cell. This is faster than the typical cell. This is faster than the typical type of diffusion. Also known as type of diffusion. Also known as “carrier mediated passive diffusion”.“carrier mediated passive diffusion”.

Passive DiffusionPassive Diffusion 5) Filtration – The passing of water and 5) Filtration – The passing of water and

permeable particles through a membrane permeable particles through a membrane by the force of hydrostatic pressure (the by the force of hydrostatic pressure (the force, or weight, of a fluid pushing against a force, or weight, of a fluid pushing against a surface. Eg. Drip coffee maker.surface. Eg. Drip coffee maker.

In the body this occurs most frequently in In the body this occurs most frequently in capillaries (tiny blood vessels). The capillaries (tiny blood vessels). The pressure created by the contraction of the pressure created by the contraction of the heart pushes water out of the blood into heart pushes water out of the blood into the interstitial (between cells) spaces. The the interstitial (between cells) spaces. The red blood cells and other blood proteins are red blood cells and other blood proteins are filtered out and stay in the blood stream. filtered out and stay in the blood stream. Filtration is also the first step used by the Filtration is also the first step used by the kidney in urine production. kidney in urine production.

Active TransportActive Transport 1) 1) Active DiffusionActive Diffusion - Energy is used to move - Energy is used to move

particles “up hill” from an area of low particles “up hill” from an area of low concentration to an area of high concentration to an area of high concentration. concentration.

Allows cells to move certain ions or other Allows cells to move certain ions or other water-soluble particles to specific areas. Eg. water-soluble particles to specific areas. Eg. Na+/K+ pumpNa+/K+ pump3Na+ out / 2K+ in3Na+ out / 2K+ in

Cystic FibrosisCystic Fibrosis – no chloride pumps in – no chloride pumps in membrane therefore secretions are salty and membrane therefore secretions are salty and thick. Impaired breathing and digestion. thick. Impaired breathing and digestion.

Active TransportActive Transport

2) Endocytosis – The cell has protein-2) Endocytosis – The cell has protein-binding sites on its surface. Once a binding sites on its surface. Once a particle has been bound, the cells particle has been bound, the cells exoskeleton pulls the sites inward exoskeleton pulls the sites inward and the cell membrane slowly and the cell membrane slowly engulfs the particles. Phagocytosis = engulfs the particles. Phagocytosis = engulfing a solid. Pinocytosis = engulfing a solid. Pinocytosis = engulfing a liquid.engulfing a liquid.

Active TransportActive Transport

3) Exocytosis – large molecules within 3) Exocytosis – large molecules within the cell that are too large to pass the cell that are too large to pass through the cell membrane are through the cell membrane are enclosed in a membranous vesicle. The enclosed in a membranous vesicle. The vesicles are pulled out to the cell vesicles are pulled out to the cell membrane by the exoskeleton. membrane by the exoskeleton. Vesicles then fuse with the cell Vesicles then fuse with the cell membrane and the contents are membrane and the contents are released outside of the cell. released outside of the cell.

EnzymesEnzymes

functional proteins functional proteins Most chemical reactions in the body do not Most chemical reactions in the body do not

just happen on their own at normal just happen on their own at normal temperatures. Needs a catalyst, which will temperatures. Needs a catalyst, which will lower the activation energy (the energy lower the activation energy (the energy required to “get the ball rolling”. required to “get the ball rolling”.

Catalysts participate in chemical reactions Catalysts participate in chemical reactions but are not themselves changed by the but are not themselves changed by the reaction. This is the roll of enzymes in the reaction. This is the roll of enzymes in the cell. cell.

EnzymesEnzymes

Most enzymes are either tertiary or Most enzymes are either tertiary or quaternary proteins (complex shapes) and quaternary proteins (complex shapes) and contain a non-protein part called a cofactor.contain a non-protein part called a cofactor.

Inorganic ions or vitamins may make up part Inorganic ions or vitamins may make up part of a cofactor. If the cofactor is an organic of a cofactor. If the cofactor is an organic non-protein molecule is it called a non-protein molecule is it called a co-co-enzymeenzyme. .

This is how some vitamins and minerals are This is how some vitamins and minerals are so vital to our health. They enable enzymes so vital to our health. They enable enzymes to do their jobs. to do their jobs.

EnzymesEnzymes

The enzyme has a “active site” which The enzyme has a “active site” which fits onto a molecule involved in a fits onto a molecule involved in a chemical reaction like a chemical reaction like a lock and lock and keykey. Once the key is in the lock the . Once the key is in the lock the enzyme changes shape and can enzyme changes shape and can either unite or split the molecule. either unite or split the molecule.

EnzymesEnzymes An entire metabolic pathway can be turned An entire metabolic pathway can be turned

on or off by the activation or inactivation of on or off by the activation or inactivation of any one of the enzymes that catalyze any one of the enzymes that catalyze reactions in that particular pathway.reactions in that particular pathway.

Enzymes also have agents that can activate Enzymes also have agents that can activate

or inactivate them by changing their shape. or inactivate them by changing their shape. These are called allosteric effectors. These are called allosteric effectors. Basically it controls the enzymes function. It Basically it controls the enzymes function. It is the gas pedal that controls the rate at is the gas pedal that controls the rate at which a reaction occurs. which a reaction occurs.

EnzymesEnzymes

The end product of a reaction can act The end product of a reaction can act as an allosteric effector as well as as an allosteric effector as well as produce a negative feedback loop. produce a negative feedback loop. Basically when enough of a substance Basically when enough of a substance has been made, it automatically slows has been made, it automatically slows down or stops more from being made. down or stops more from being made. Cofactors, temperature and pH all Cofactors, temperature and pH all have allosteric effects on enzymes.have allosteric effects on enzymes.

EnzymesEnzymes A A pro-enzymepro-enzyme is an inactive precursor to is an inactive precursor to

an enzyme. Eg trypsinogen is a pro-an enzyme. Eg trypsinogen is a pro-enzyme to trypsin. Trypsin is a powerful enzyme to trypsin. Trypsin is a powerful digestive agent. It is stored in the digestive agent. It is stored in the pancreas as trypsinogen (pro-enzyme). In pancreas as trypsinogen (pro-enzyme). In this way is does not harm the pancreas. this way is does not harm the pancreas.

Once it is released into the small intestine Once it is released into the small intestine an activating agent also known as a an activating agent also known as a “kinase” (eg.enterokinase) activates it and “kinase” (eg.enterokinase) activates it and changes its shape into trypsin where it can changes its shape into trypsin where it can then digest proteins that have been then digest proteins that have been consumed. consumed.

CatabolismCatabolism There are many different catabolic There are many different catabolic

pathways that operate inside of pathways that operate inside of human cells.human cells.

Cellular Respiration- Cellular Respiration- The conversion The conversion within the cell of nutrients (such as sugar within the cell of nutrients (such as sugar molecules) into chemical energy in the molecules) into chemical energy in the form of ATP, by reacting the food with form of ATP, by reacting the food with oxygen (O2) until the food has completely oxygen (O2) until the food has completely been degraded into CO2 and H2O.been degraded into CO2 and H2O.

CatabolismCatabolism

There are 3 Main PathwaysThere are 3 Main Pathways

1.1. GlycolysisGlycolysis

2.2. Citric Acid CycleCitric Acid Cycle

3.3. Electron Transport ChainElectron Transport Chain

GlycolysisGlycolysis

Occurs in the cytoplasm of cells Occurs in the cytoplasm of cells outside of any particular organelle.outside of any particular organelle.

Does not require any Oxygen it is Does not require any Oxygen it is said to be said to be Anaerobic.Anaerobic.

Breaking down of Glucose to release Breaking down of Glucose to release ATPATP and and Pyruvic Acid Pyruvic Acid

GlycolysisGlycolysis Pyruvic AcidPyruvic Acid

Pathway where glucose is broken apart into Pathway where glucose is broken apart into

2 pyruvic acid molecules to give a small 2 pyruvic acid molecules to give a small amount of energy. amount of energy.

In order to create maximum ATP the body In order to create maximum ATP the body must use oxygen. must use oxygen.

If oxygen is unavailable it will continue If oxygen is unavailable it will continue along the anaerobic pathway to be broken along the anaerobic pathway to be broken down into down into Lactic AcidLactic Acid and shipped to Liver and shipped to Liver Cells to be convert back into Pyruvic AcidCells to be convert back into Pyruvic Acid

Citric Acid CycleCitric Acid Cycle

Also named Kreb’s Cycle after Sir Also named Kreb’s Cycle after Sir Hans Krebs who discovered it in the Hans Krebs who discovered it in the early 1900’s. early 1900’s.

Takes place in the Takes place in the MitochondriaMitochondria

Citric (Kreb’s) CycleCitric (Kreb’s) Cycle

Aerobic (oxygen using) – If oxygen is Aerobic (oxygen using) – If oxygen is present the Pyruvate is converted to present the Pyruvate is converted to Acetyl CoA and is sent to the citric Acetyl CoA and is sent to the citric (Krebs) cycle. (Krebs) cycle.

There it undergoes a series of chemical There it undergoes a series of chemical reactions each one requiring a specific reactions each one requiring a specific enzyme. These enzymes are located in enzyme. These enzymes are located in the inner chamber of the Mitochondrion. the inner chamber of the Mitochondrion.

Citric (Krebs) CycleCitric (Krebs) Cycle The citric cycle produces some ATP The citric cycle produces some ATP

directly but mostly the energy that is directly but mostly the energy that is released is stored in the form of released is stored in the form of energized electrons .energized electrons .

Electrons get attached to the co-Electrons get attached to the co-enzyme FAD or NAD to become FADH enzyme FAD or NAD to become FADH and NADH, which get sent to the and NADH, which get sent to the Electron Transport System.Electron Transport System.

Citric Acid CycleCitric Acid Cycle The big picture: Glycolysis breaks 1 The big picture: Glycolysis breaks 1

glucose into 2 pyruvate, producing 2 glucose into 2 pyruvate, producing 2 ATP.ATP.

Pyruvate is used to make acetyl-CoA, Pyruvate is used to make acetyl-CoA, the starting product for the citric acid the starting product for the citric acid cycle. Each turn of the cycle oxidizes cycle. Each turn of the cycle oxidizes 1 pyruvate, so it takes 2 turns to 1 pyruvate, so it takes 2 turns to completely oxidize 1 glucose.completely oxidize 1 glucose. ﾊ ﾊ

Two turns produce 8 NADH, 2 FADH2, Two turns produce 8 NADH, 2 FADH2, and 2 ATP.and 2 ATP.

Electron Transport ChainElectron Transport Chain(inside mitochondria)(inside mitochondria)

With the end products of the Citric Acid With the end products of the Citric Acid Cycle: NADH and FADH2.Cycle: NADH and FADH2.

FADH and NADH2 release the ionized FADH and NADH2 release the ionized electrons onto one side of a membrane. electrons onto one side of a membrane.

As the electrons move across this As the electrons move across this membrane they drive a kind of “water membrane they drive a kind of “water wheel” and are ultimately attached to an wheel” and are ultimately attached to an oxygen molecule. This is why it is aerobic oxygen molecule. This is why it is aerobic and needs oxygen. and needs oxygen.

Electron Transport ChainElectron Transport Chain

The energy created by the water wheel is The energy created by the water wheel is used to pumpused to pumpprotons (H+) into the space between protons (H+) into the space between mitochondrial membranes.mitochondrial membranes.

These protons flow back into the inner These protons flow back into the inner chamber through carrier molecules in the chamber through carrier molecules in the cristae, and their energy of movement is cristae, and their energy of movement is transferred to ATPtransferred to ATP

Electron Transport ChainElectron Transport Chain

34 ATP molecules are produced during the 34 ATP molecules are produced during the citric cycle and in the Electron Transport citric cycle and in the Electron Transport system. system.

Add this to the 2 that were created in Add this to the 2 that were created in glycolysis and you get 36 ATP from one glycolysis and you get 36 ATP from one molecule of glucose.molecule of glucose.

AnabolismAnabolism

The “building pathways in cellsThe “building pathways in cells Protein SynthesisProtein Synthesis

Structural vs. FunctionalStructural vs. Functional

DNADNA

50 yrs ago Watson and Crick 50 yrs ago Watson and Crick announced they had solved the announced they had solved the puzzle of DNA’s Molecular Structure.puzzle of DNA’s Molecular Structure.

Received the Nobel Prize for their Received the Nobel Prize for their work 9 years laterwork 9 years later

DNADNA

Deoxyribonucleic Deoxyribonucleic Acid- Acid- DNADNA Giant among other Giant among other

moleculesmolecules The importance of The importance of

its shape and its shape and function surpass function surpass any other molecule any other molecule in the worldin the world

0102030

405060708090

1stQtr

3rdQtr

EastWestNorth

DNADNA

ShapeShape An extremely An extremely

long narrow long narrow ladderladder

Now see it Now see it twisting around twisting around on its axison its axis

Double HelixDouble Helix

DNADNA

DNADNA

DNA Structure-DNA Structure- Polymer- Polymer- Large molecule made up of Large molecule made up of

many smaller molecules, “Nucleotides,” many smaller molecules, “Nucleotides,” joined together in a sequencejoined together in a sequence

Nucleotides- Nucleotides- formed by combining formed by combining phosphoric acid, a sugar, and phosphoric acid, a sugar, and nitrogenous basenitrogenous base

DNADNA

Nucleotides- Nucleotides- There are 4 different types There are 4 different types Differ in their Nitrogenous Base Differ in their Nitrogenous Base

componentcomponent Either contain a Either contain a Adenine or Guanine Adenine or Guanine Or contain a Or contain a Cytosine or ThymineCytosine or Thymine

DNA StructureDNA Structure

DNA is a Spiral Staircase who’s steps DNA is a Spiral Staircase who’s steps are formed by are formed by Obligate Base-Obligate Base-Pairing Pairing of Nucleotides. of Nucleotides. Adenine must bind w/ ThymineAdenine must bind w/ Thymine Guanine must bind w/ CytosineGuanine must bind w/ Cytosine

Base-Pairing plays a key role in DNA Base-Pairing plays a key role in DNA Replication and Protein SynthesisReplication and Protein Synthesis

DNA StructureDNA Structure

Although the kinds of base pairs in a Although the kinds of base pairs in a DNA strand are the same, the order DNA strand are the same, the order of the pairings is not.of the pairings is not.

It is the sequence of the base pairs It is the sequence of the base pairs that determines all our hereditary that determines all our hereditary traitstraits

DNA StructureDNA Structure

Genes- Genes- They are segments of DNAThey are segments of DNA One gene contains 1000 pairs of One gene contains 1000 pairs of

NucleotidesNucleotides There are some 30,000 genes There are some 30,000 genes

constituting a cell’s DNA that determine constituting a cell’s DNA that determine what a cell looks like and what it doeswhat a cell looks like and what it does

TranscriptionTranscription

RNA- RNA- differs from DNA in that it differs from DNA in that it contains ribose instead of contains ribose instead of deoxyribose, and contains deoxyribose, and contains Uracil Uracil instead of instead of ThymineThymine

Complementary Pairing- Complementary Pairing- Uracil, Uracil, instead of Thymine, will bind to instead of Thymine, will bind to adenine to make RNAadenine to make RNA

Protein Synthesis - Protein Synthesis - TranscriptionTranscription

Now that we know what the cook Now that we know what the cook book looks like how do we bake?book looks like how do we bake?

Transcription- Transcription- a portion of the DNA a portion of the DNA is “transcribed,” into a usable form is “transcribed,” into a usable form called messenger RNA (mRNA).called messenger RNA (mRNA).

TranscriptionTranscription

Transcription – occurs in the nucleus. Transcription – occurs in the nucleus. The DNA molecule “unzips” RNA The DNA molecule “unzips” RNA temporarily attaches to the exposed temporarily attaches to the exposed bases and makes a complementary bases and makes a complementary copy of the piece of DNA. Messenger copy of the piece of DNA. Messenger RNA (mRNA). The mRNA then moves RNA (mRNA). The mRNA then moves out of the nucleus by way of special out of the nucleus by way of special pores.pores.

transcriptiontranscription

TranslationTranslation

The next step of protein synthesis is The next step of protein synthesis is to take the edited piece of mRNA and to take the edited piece of mRNA and translate that information into translate that information into ProteinsProteins

The cell can interpret the mRNA and The cell can interpret the mRNA and form a specific sequence of amino form a specific sequence of amino acids acids

TranslationTranslationIn the cytoplasm mRNA attaches to In the cytoplasm mRNA attaches to ribosomes that can read the code ribosomes that can read the code “rRNA.”“rRNA.”

The code is read in 3 nucleotide segments The code is read in 3 nucleotide segments known as “Codons”known as “Codons”

Transfer RNA (tRNA) go out and find specific Transfer RNA (tRNA) go out and find specific amino acids that are randomly floating amino acids that are randomly floating around the cytoplasm and bring them back around the cytoplasm and bring them back to the ribosome which acts as a kind of to the ribosome which acts as a kind of holding place for the mRNA and the tRNA. holding place for the mRNA and the tRNA.

TranslationTranslation

The tRNA knows which amino acids to get The tRNA knows which amino acids to get by reading the mRNA triplet sequences of by reading the mRNA triplet sequences of base pairs (like words). base pairs (like words).

The amino acids are joined together at the The amino acids are joined together at the ribosome by peptide bonds. Eventually an ribosome by peptide bonds. Eventually an amino acid chain is assembled to form a amino acid chain is assembled to form a protein. protein.

DNA ReplicationDNA Replication

As cells become larger it triggers the As cells become larger it triggers the formation of complete new set of DNA to be formation of complete new set of DNA to be produced. produced.

This prepares the cell for reproductionThis prepares the cell for reproduction The strands of DNA uncoil except for the endsThe strands of DNA uncoil except for the ends The Intracellular Fluid contains nucleotides The Intracellular Fluid contains nucleotides

that attach to the correct places along the that attach to the correct places along the “Old strand.”“Old strand.”

Each Half of the strand has a new strand Each Half of the strand has a new strand attached to it. attached to it.

Dna repDna rep

Cell ReproductionCell Reproduction

Cells and simple organisms reproduce Cells and simple organisms reproduce by splitting themselvesby splitting themselves

One parent cell becomes two One parent cell becomes two daughter cells “asexual” reproduction.daughter cells “asexual” reproduction.

Each daughter cell must possess all Each daughter cell must possess all the resources necessary for survival. the resources necessary for survival.

Each cell must have a complete set of Each cell must have a complete set of DNA needed to run a cell properly.DNA needed to run a cell properly.

MitosisMitosis

Mitosis – splitting of a cell into 2 identical Mitosis – splitting of a cell into 2 identical daughter cells. All the cells of the body, daughter cells. All the cells of the body, except the sex cells, reproduce this way.except the sex cells, reproduce this way.

There are 46 chromosomes in the human There are 46 chromosomes in the human nucleus but are in homologous pairsnucleus but are in homologous pairs

Diploid - 22 pairs and 1 pair of sex Diploid - 22 pairs and 1 pair of sex chromosomes. chromosomes.

MitosisMitosis Prophase- Prophase-

beginning of cell beginning of cell division. division. Cell starts to pinch Cell starts to pinch

itself in half and itself in half and nuclear envelopes nuclear envelopes falls apartfalls apart

DNA molecules coil DNA molecules coil tightly to form tightly to form ChromosomesChromosomes

Centrioles move Centrioles move to opposite ends to opposite ends of the cellof the cell

MitosisMitosis

Metaphase-Metaphase-Position changing Position changing phasephase

Chromosomes are Chromosomes are lined up at the lined up at the equatorequator

MitosisMitosis Anaphase-Anaphase- “apart “apart

phase.”phase.” Each chromosome Each chromosome

is pulled apart to is pulled apart to form a single DNA form a single DNA molecule and is molecule and is pulled towards the pulled towards the nearest polenearest pole

MitosisMitosis

Telophase- “end Telophase- “end phase.” phase.”

Cell rebuilds Cell rebuilds nuclear envelope nuclear envelope DNA elongates DNA elongates back to chromatidsback to chromatids

Spindle fibers Spindle fibers disappeardisappear

MeiosisMeiosis

Meiosis – splitting of a sex cell so that only Meiosis – splitting of a sex cell so that only ½ of the 46 chromosomes are left in the ½ of the 46 chromosomes are left in the cell. 22 single chromosomes plus one sex cell. 22 single chromosomes plus one sex chromosome.chromosome.

Process that allows for Sexual Process that allows for Sexual ReproductionReproduction

During the process of Meiosis the Number During the process of Meiosis the Number of chromosomes is reduced from 46 to 23of chromosomes is reduced from 46 to 23

MeiosisMeiosis The 23 chromosomes that result from The 23 chromosomes that result from

meiosis is known as the haploid number meiosis is known as the haploid number of chromosomes. This is a sperm cell of chromosomes. This is a sperm cell and an ovum. (XY = male, XX = and an ovum. (XY = male, XX = female). female).

Meiosis 1 – number of chromosomes is Meiosis 1 – number of chromosomes is halved but the chromatid pairs remain halved but the chromatid pairs remain together. together.

Meiosis 2 – chromatids split apart. Cell Meiosis 2 – chromatids split apart. Cell ends up with 23 chromosomesends up with 23 chromosomes