Unit 2 - Cell Structure and Function

Question 1

What are three differences between prokaryotic and eukaryotic cells?

Answer 1

Prokaryotic cells have no nucleus, instead, the DNA (single circular chromosome) is concentrated in a region called the nucleoid. Eukaryotic cells have a membrane-bound nucleus that contains the DNA (linear chromosomes)

Prokaryotic cells have no membrane bound organelles in the cytosol while eukaryotic cells have many.

Prokaryotic cells are on average much smaller and less complex than eukaryotic cells

Question 2

What features do prokaryotic and eukaryotic cells share?

Answer 2

They both contain a plasma membrane, ribosomes (not membrane bound), and a cytosol

Question 3

What organelles are found only in plant cells?

Answer 3

Central vacuole
Chloroplasts
Cell wall
Plasmodesmata

Question 4

What organelles are found only in animal cells?

Answer 4

Lysosomes
Centrioles
Flagella

Question 5

What is the plasma membrane made up of? Describe the structures/functions of the components.

Answer 5

1) Phospholipids
Provide a hydrophobic barrier that separate the cell from its liquid environment – result in the membrane’s selective permeability

2) Embedded proteins
Serve as transport channels that allow materials to cross the hydrophobic interior of the phospholipid bilayer or as receptors that signaling molecules (ligands) bind to

3) Carbohydrates attached to proteins or lipids on the surface (glycoproteins and glycolipids)
Crucial in cell-cell recognition which is used for sorting cells into tissues in animal embryo and rejecting foreign cells by the immune system

Question 6

Nucleus

Answer 6

Function: stores the cell’s DNA or genetic materials; contains genes that code for mRNA and in turn proteins

Structure: surrounded by a nuclear membrane with nuclear pores that regulate what can enter or leave the nucleus. In the center is a region called the nucleolus which synthesizes rRNA and combines them with proteins to form ribosomal subunits

Question 7

Ribosomes

Answer 7

Function: synthesize proteins. Free ribosomes float in the cytosol and make proteins that stay there while bound ribosomes (attached to the rough ER) make proteins that are incorporated into membranes or are secreted (exported) from the cell

Structure: made up of rRNA and proteins; has a large subunit and a small subunit

Question 8

Endoplasmic Reticulum

Answer 8

Function: carries out protein synthesis on membrane bound ribosomes and plays a role in intracellular transport

Structure: network of membranes and sacs with an internal area called the cisternal space

Question 9

Golgi Apparatus

Answer 9

Function: receive, chemically modify, package, and ship newly synthesized proteins from the ER

Structure: consists of flattened sacs of membranes arranged in stacks; has a cis face which receives vesicles and a trans face which ships vesicles (they bud off)

The Golgi Apparatus is extensive in cells specialized for secretion

Question 10

Lysosomes

Answer 10

Function: digest macromolecules, break down cell junk, and conduct apoptosis through hydrolysis

Structure: membrane bound sacs filled with hydrolytic/digestive enzymes; has an acidic environment that these enzymes work best in

If a lysosome breaks open or leaks, the enzymes won’t be active in the neutral pH of the cell and may digest important cell components — demonstrates importance of cell compartmentalization

Question 11

Central Vacuole

Answer 11

Function: storage of water + organic materials

Structure: found only in plant cells; large clear bubble in the center of the cell

Question 12

Mitochondria

Answer 12

Function: sites of cellular respiration

Structure: consist of an inner and outer membrane. The inner membrane is highly convoluted, forming folds called cristae that increase the surface area where metabolic reactions (that form ATP) can be performed and thus maximize productivity of cellular respiration. Many reactions take place in the inside of the mitochondria, the matrix (which contains mitochondrial DNA, enzymes, and ribosomes)

Question 13

Chloroplasts

Answer 13

Function: sites of photosynthesis in plants + photosynthetic algae

Structure: have a double outer membrane. Thylakoids are membranous sacs stacked into grana. The fluid outside the thylakoids is the stroma which contains chloroplast DNA, enzymes, and ribosomes

Question 14

What is the endosymbiont theory and what are three pieces of evidence that support it?

Answer 14

This theory proposes that mitochondria and chloroplasts share an evolutionary origin, they are both descended from prokaryotic cells once engulfed by ancestors of eukaryotic cells. So membrane bound organelles evolved from previously free living prokaryotic cells via endosymbiosis.

Evidence

1) Both organelles have a double membrane structure
2) Both organelles have their own ribosomes and circular DNA molecules
3) Both organelles can grow and reproduce independently within the cell

Question 15

Peroxisomes

Answer 15

Function: detoxify alcohol and break down fatty acids to be sent to the mitochondria for fuel by transferring hydrogen from compounds to oxygen, producing hydrogen peroxide (H2O2)

Structure: single membrane bound compartments containing enzymes

Question 16

Why is compartmentalization important for the cell? Give examples

Answer 16

Cells must remain compartmentalized so that organelles can do their job properly without affecting the other parts of the cell.

For example, in lysosomes, the digestive enzymes must be kept within the membrane so they don’t accidentally digest other important components in the cell and so that they also work best in the acidic environment found inside.

Peroxisomes also demonstrate compartmentalization because the enzymes that produce hydrogen peroxide and those that dispose of this toxic compound remain separate from other cellular components that could be damaged

Question 17

Cell Wall

Answer 17

Function: protects the plant and helps maintain its shape

Structure: outside the plasma membrane; the main component is cellulose

Question 18

What is the fluid mosaic model?

Answer 18

The fluid mosaic model is used to describe the phospholipid bilayer. Fluid means the components are free to move around. Mosaic means the variety of proteins embedded in the membrane

Question 19

What are the two types of proteins on the membrane?

Answer 19

1) Integral proteins
Penetrate the hydrophobic interior of the phospholipid bilayer. Most are transmembrane proteins which span the membrane

2) Peripheral proteins
Loosely bound to the surface of the membrane

Question 20

What is selective permeability and how does the plasma membrane exhibit this characteristic?

Answer 20

Selective permeability means that the membrane allows some materials to cross it more easily than others

Small, nonpolar molecules (hydrocarbons, CO2, oxygen) are hydrophobic and can cross the hydrophobic interior easily

Ions and polar molecules (water, glucose) are hydrophilic and require transport proteins to pass through. The proteins allow the molecules to avoid contact with the hydrophobic interior

Question 21

Aquaporins

Answer 21

Channel proteins that facilitate the passage of water molecules across the plasma membrane

Question 22

What is diffusion and how does it work?

Answer 22

Diffusion is the movement of molecules from an area of high concentration to low concentration, diffusing down their concentration gradient

Question 23

Osmosis

Answer 23

Diffusion of water across a selectively permeable membrane

Question 24

Isotonic

Answer 24

Concentrations of the cell and the surroundings are the same

No net movement of water across the plasma membrane (water crosses at the same rate in both directions)

Animal cell - normal conditions
Plant cell - becomes flaccid and limp

Question 25

Hypertonic

Answer 25

Surroundings have a higher solute concentration and lower water concentration than the cell

Water moves out of the cell to the surrounding environment.

Animal cell- loses water and shrivels
Plant cell - loses water and shrivels; experiences plasmolysis which is when the plasma membrane pulls away from the plant cell wall at multiple places, causing death

Question 26

Hypotonic

Answer 26

The surroundings have a lower solute concentration and a higher water concentration than the cell

Water enters the cell from the surrounding environment

Animal cell - swell with water and lyse (burst)
Plant cell - turgid, firm, healthy

Question 27

Osmoregulation

Answer 27

An organism’s control of their internal solute concentrations and water potential

Question 28

What is facilitated diffusion and how does it work?

Answer 28

Facilitated diffusion is the PASSIVE transport of hydrophilic substances such as polar molecules and ions across the plasma membrane with the help of transport proteins

Channel proteins provide a hydrophilic channel through which molecules can pass

Carrier proteins bind to the molecules and carry them through the membrane

Question 29

Active transport

Answer 29

Molecules move against their concentration gradient, from an area of low to high concentration; requires energy in the form of ATP

Question 30

What is the sodium potassium pump and how does it work?

Answer 30

This pump is a form of active transport because it requires ATP energy to work.

The transmembrane protein pumps sodium out of the cell and potassium into the cell

The inside of the cell is negatively charged compared to the outside due to unequal distribution of ions. This difference in electric charge across a membrane is called the membrane potential and is measured in voltage. Positive ions are attracted to the negative charges inside the cell and negative ions are repelled

Diffusion is driven by a chemical force (ion’s concentration gradient) AND an electrical force (voltage gradient/membrane potential). These two forces combine to form the ion’s electrochemical gradient

Question 31

What is cotransport and how does it work?

Answer 31

An ATP pump that transports a solute indirectly drives the active transport of another

When H+ ions are pumped across a membrane using ATP, they store potential energy that is then released when it moves back across the membrane by diffusion. As they move back, they use the released energy to bring another compounds (such as sucrose) against its concentration gradient

Question 32

Vesicular transport (passive or active?)

Answer 32

A form of ACTIVE transport that uses vesicles to bring substances in/out of the cell

EX: Exocytosis and endocytosis

Question 33

Exocytosis

Answer 33

Vesicles from the inside of the cell fuse with the plasma membrane to expel substances out of the cell

Question 34

Definition of endocytosis

Answer 34

The cell forms new vesicles from the plasma membrane that carry new substances into the cell

Question 35

Identify and describe the three types of endocytosis.

Answer 35

1) Phagocytosis
Cellular eating, takes in solute particles. The vesicles formed are lined by a layer of coat protein

2) Pinocytosis
Cellular drinking, takes in liquid particles. The vesicles are also lined by coat protein

3) Receptor mediated endocytosis
Specialized type of pinocytosis that allows the cell to take in bulk quantities of specific substances. Solutes bind to receptor proteins on the plasma membrane and are taken into vesicles lined by coat protein. After the ingested material is freed from the vesicles, the emptied receptors are recycled to the membrane

Question 36

Draw a flowchart of the types of transport

Answer 36

Transport
| ||
Passive Active
| | | || ||
Diffusion Osmosis Facilitated Vesicular Ion pumps
|| ||
Endocytosis Exocytosis
|| || ||
Pinocytosis Phagocytosis Receptor-mediated

Question 37

What are the three stages of cell communication?

Answer 37

1) Reception - the target cell detects a signal molecule coming from outside
2) Transduction - a series of steps that converts the signal to a form that can bring about a specific cellular response (thru a signal transduction pathway passed on by relay proteins)
3) Response - signal triggers a response in the target cell (enzyme activity, activation of a gene, etc)

Question 38

What is the purpose of the conformational change induced by the binding of a ligand to its specific receptor?

Answer 38

It allows a message to be transmitted from the outside of the cell to the inside, marking the initial transduction of the signal

Question 39

Describe the pathway of a ligand gated ion channel.

Answer 39

The gate starts off closed. When a specific ligand binds to the receptor, the gate opens and allows specific ions to flow through the channel. This can rapidly change the concentration of that ion inside the cell, generating a response. When the ligand dissociates from the receptor, the gate closes and ions no longer enter the cell

Question 40

Describe the pathway of an intracellular receptor.

Answer 40

These receptors are found inside the cell so the signal molecule must be hydrophobic in order to cross the plasma membrane (EX: steroid hormones).

The hormone passes into the cell and bonds to the receptor in the cytoplasm, activating it. The hormone-receptor complex enters the nucleus and binds to specific genes, acting as a transcription factor.

Question 41

How can a cell signal be amplified in transduction?

Answer 41

Signal transduction pathways often involve a phosphorylation cascade. At each step, protein kinases phosphorylate (transfer a phosphate) and thus activate proteins at the next level (conformational change). This multi step pathway allows the signal to be amplified to many different proteins in the cell, allowing for a large cellular response

Question 42

Protein phosphatases

Answer 42

Enzymes that remove phosphate groups and inactivate proteins that were activated by kinases. Thus, the signal transduction pathway can be turned on by kinases and OFF by phosphatases (when the initial signal is no longer present)

Question 43

Second messengers

Answer 43

small, non protein molecules/ions that can initiate a phosphorylation cascade when activated (function in the cytosol)

EX: cyclic AMP or cAMP

Question 44

How is a cell response different in the cytoplasm vs nucleus?

Answer 44

In the nucleus, the final activated molecule in a signal pathway can serve as a transcription factor that induces the synthesis of a protein via gene regulation.

In the cytoplasm, signal pathways regulate the activity of proteins rather than their synthesis. The final molecule may affect the activity of enzymes

Question 45

Why is apoptosis essential for the normal functioning of multicellular organisms?

Answer 45

Development - normal morphogenesis of hands and feet for human embryo

Damage - protects neighboring cells from damage by destroying a dying cell so it doesn’t leak out its digestive enzymes and other harmful components to the extracellular environment.

Apoptosis is triggered by autocrine signals that activate a cascade of suicide proteins in the cell

Question 46

Describe the pathway of a G protein (three stages of cell signaling pathways)

Answer 46

Reception - Ligand binds to a G protein coupled receptor.

Transduction - This causes a conformational change in the receptor that enables it to bind to the inactive G protein. GTP displaces the GDP which activates the G protein. G protein leaves the receptor and binds to an enzyme called adenylyl cyclase. This enzyme is activated and converts ATP to cAMP, which is a second messenger. cAMP initiates a phosphorylation cascade with protein kinases.

Response - This leads to a response in the cell. The ligand is released once the response is complete, which stops the signaling pathway.

Question 47

What is water potential and how does it affect the direction of water flow?

Answer 47

Water flows from an area of high water potential (high water concentration) to an area of low water potential (low water concentration).

Question 48

How do plants exert pressure potential? How does it affect water potential?

Answer 48

Plants have a rigid cell wall that exerts pressure when water moves into the cell. This increases the water potential and limits the gain of water. Thus, hypotonic solutions are ideal for a plant to be turgid and healthy.

Question 49

What factors affect water potential?

Answer 49

Water potential is affected by solute potential and pressure potential

High solute potential = low water potential

Adding a solute lowers the water potential of a solution, causing it to be less likely to leave and more likely to flow into it.

High pressure potential = high water potential

Question 50

Dialysis

Answer 50

Diffusion of solutes across a selectively permeable membrane

Question 51

Identify and describe the three main types of signaling.

Answer 51

1) Paracrine (local) signaling
Uses messenger molecules named local regulators (EX: growth factors) to generate a response in target cells in the vicinity

2) Endocrine (long distance) signaling
Uses hormones that travel via the bloodstream and reach distant target cells that can recognize and respond to them

3. Autocrine signaling
   Secrete signaling molecules to oneself (kind of local signaling)

Question 52

Identify and describe the three types of local signaling.

Answer 52

1) Direct contact
Cell to cell recognition with glycoproteins or signaling molecules being sent thru connected cell junctions

2) Paracrine signaling
A cell secretes molecules of a local regulator to nearby cells

3) Synaptic signaling
Occurs in the nervous system; an electrical signal moves along a nerve cell which triggers the release of neurotransmitter molecules that carry the signal. These molecules diffuse across the synapse and cause a response in the target cell

Question 53

Smooth ER

Answer 53

Smooth ER is responsible for the synthesis of lipids, detoxification of alcohol and drugs, storage of calcium ions, the metabolism of carbohydrates.

Question 54

Rough ER

Answer 54

Rough ER is responsible for compartmentalizing the cell by housing the synthesis of proteins that are secreted from the cell.

Synthesized polypeptide chains travel across the ER membrane into the cisternal space and along the way, they reach their conformation and coil into proteins. Then the proteins are packaged into transport vesicles that bud off the ER and move toward the Golgi apparatus

Question 55

Why is the surface area to volume ratio of a cell important?

Answer 55

Surface area dictates the cell’s ability to obtain necessary resources (thermal energy, chemicals, food) and eliminate waste products since these materials are exchanged through the plasma membrane.

Volume dictates the amount of resources the cell needs and the amount of waste it produces.

High surface area to volume ratio results in the most efficient exchange of materials with the environment. This is present in smaller cells because volume increases much faster than surface area

Question 56

Passive Transport

Answer 56

Does not require an input of energy

Plays a primary role in the import of materials and the export of wastes (exhibited by non-polar molecules like hydrocarbons, CO2, and oxygen)

Question 57

In what direction does water move?

Answer 57

From areas of high water potential/low osmolarity/low solute concentration to areas of low water potential/high osmolarity/high solute concentration

Question 58

Describe similarities and/or differences in compartmentalization between prokaryotic and eukaryotic cells

Answer 58

Membrane bound organelles evolved from once free living prokaryotic cells via endosymbiosis

Prokaryotic cells generally lack internal membrane bound organelles but have internal regions with specialize structures and functions

Eukaryotic cells maintain internal membranes that compartmentalize the cell into specialized regions

Question 59

Which was engulfed first, mitochondria or chloroplasts?

Answer 59

Mitochondria because all eukaryotic cells have a mitochondria but not all have a chloroplast. This means the mitochondria was engulfed first then a later eukaryotic cell engulfed the chloroplast on a different lineage

Question 60

How can you increase surface area without increasing volume? Give examples of specific adaptations or organelles.

Answer 60

Adaptations like villi/microvilli in the intestines are extensions of the membrane that increase the surface area. Cristae are folds of the mitochondria that also increase the surface area for cellular respiration to take place.

Question 61

Contractile vacuole

Answer 61

Responsible for osmoregulation in freshwater protists

Question 62

Food vacuole

Answer 62

Forms from phagocytosis; fuses with a lysosome after being made in order to facilitate digestion

Question 63

How does the difference in ligands lead to different cellular responses?

Answer 63

If a ligand is nonpolar like a steroid hormone, it will be able to pass through the hydrophobic interior of the plasma membrane and bind to a receptor protein inside the cell in the cytosol. This forms the hormone-receptor complex that enters the nucleus to serve as a transcription factor that stimulates gene transcription and thus protein synthesis as well. The speed of this response is slow because the complex has to interact with DNA.

If a ligand is polar like an amino acid hormone, it binds to a receptor protein on the surface of the plasma membrane and the response will be altering enzyme activity rather than its synthesis. The speed of this response is rapid because the enzymes being activated are already present in the cell

Question 64

Describe one potential change that will reduce the binding of an active ligand to its receptor.

Answer 64

A mutation in the receptor gene that causes a substitution of a charged amino acid for a nonpolar amino acid in the ligand binding site of the receptor. This will change the structure, binding affinity, and thus function

Question 65

Why do only some cells exhibit a response to the release of a hormone?

Answer 65

Cell signaling depends on the ability to detect a signal molecule. Not all cells have receptors for the hormone. Only cells with such receptors are capable of responding