Module 3 Unit 1

Question 1

What do all cells have in common?

Answer 1

• • all bounded by a selective membrane (phospholipid bilayer)
• • all have cytosol
• • all contain chromosomes
• • all have ribosomes

Question 2

What is the difference between eukaryotic and prokaryotic cells?

Answer 2

• • In eukaryotes, most of the DNA is in an organelle called the nucleus
• • In a prokaryotic cell, the DNA is concentrated in a region that is not membrane-enclosed, called the nucleoid
• • within the cytoplasm of a eukaryotic cell is a variety of organelles that are absent in almost all prokaryotic cells
• • Eukaryotic cells are generally much larger and diverse than prokaryotic cells
• • prokaryotic cells are almost always single-celled organisms
• -Some prokaryotes may also have a capsule surrounding the cell wall which protects it from phagocytosis, helps to hold water
• • prokaryotes have short fimbriae that help attach to other cells or substrates and longer projections called flagella that have a white-like motion to propel it

Question 3

What type of cell is bacteria and archaea?

Answer 3

– prokaryotic cells

Question 4

How does cell size affect its function?

Answer 4

• • Cells must be large enough to house DNA, proteins, and structures needed to survive and reproduce, but small enough to allow exchange with the environment.
• • The logistics of carrying out cellular metabolism sets limits on cell size
• • As a cell increases in size, its surface area grows proportionately less than its volume (cell volume increases at a faster rate)

Question 5

What is the smallest cell?

Answer 5

• • the smallest cells known are bacteria called mycoplasmas, which have diameters between 0.1 and 1.0 μm
    1) mollicutes
    2) genus mycoplasma
    3) M pneumoniae

Question 6

What are mollicutes?

Answer 6

• • Absence of cell wall
• • Plasma membrane limiting boundary
• • Parasitic/Commensal

Question 7

What is the genus mycoplasma?

Answer 7

• • Small size (150-250nm)
• • no DNA homology with known bacteria.
• • They have low guanine, cytosine content.
• • They exhibit no reversion to walled forms

Question 8

What is M pneumoniae?

Answer 8

– the smallest free-living organism capable of self-replication (smaller than some viruses)

Question 9

How do scientists differentiate between different types of bacteria?

Answer 9

• • Scientists use Gram stain to classify bacteria by cell wall composition
• • Differentiates bacteria by detecting peptidoglycan.
• • Gram‐positive bacteria have simple walls with a large amount of peptidoglycan that can absorb gram stains
• • Gram‐negative bacteria have an outer membrane and less peptidoglycan so it does not hold on to the stain very well

Question 10

What are the four basic functions of the organelles in eukaryote?

Answer 10

• • The nucleus and ribosomes are involved in the genetic control of the cell.
• • The endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes are involved in the manufacture, distribution, and breakdown of molecules.
• • Mitochondria in all cells and chloroplasts in plant cells are involved in energy processing.
• • Structural support, movement, and communication between cells are functions of the cytoskeleton, plasma membrane, and cell wall

Question 11

What is the function of the nucleus?

Answer 11

• • contains most of the cell’s DNA
• • controls the cell’s activities by directing protein synthesis (by making messenger RNA (mRNA)).
• • It is surrounded by the nuclear envelope; a double membrane (each a lipid bilayer) that is perforated by pores (pores regulate the passage of proteins and RNA into and out of the nucleus)
• • lined by the nuclear lamina, a netlike array of protein filaments that maintains the shape of the nucleus by mechanically supporting the nuclear envelope

Question 12

What are ribosomes?

Answer 12

• • Ribosomes are involved in the cell’s protein synthesis (joining amino acids together according to the instructions in the mRNA sequence)
• • Ribosomes are made of a mix of proteins and ribosomal RNA (which is produced in the nucleolus)
• • These subunits then exit the nucleus through the nuclear pores to the cytoplasm, where a large and a small subunit can assemble into a ribosome

Question 13

How does the nucleus direct protein synthesis?

Answer 13

• • directs protein synthesis by synthesizing messenger RNA (mRNA) according to instructions provided by the DNA
• • The mRNA is then transported to the cytoplasm via the nuclear pores
• • Once an mRNA molecule reaches the cytoplasm, ribosomes translate the mRNA’s genetic message into the primary structure of a specific polypeptide

Question 14

What’s the difference between free and bound ribosomes?

Answer 14

• • Free ribosomes are suspended in the cytoplasm and typically involved in making proteins that function within the cytoplasm (ex. enzymes that catalyze sugar breakdown)
• • Bound ribosomes are attached to the endoplasmic reticulum (ER) associated with the nuclear envelope and generally make proteins that are destined for insertion into membranes, for packaging within certain organelles such as lysosomes, or for export from the cell (secretion) (ex. cells of the pancreas that secrete digestive enzymes frequently have a high proportion of bound ribosomes

Question 15

What does the endomembrane system include?

Answer 15

• • nuclear envelope,
• • the endoplasmic reticulum,
• • the Golgi apparatus,
• • lysosomes,
• • various kinds of vesicles and vacuoles
• • plasma membrane

Question 16

What is the endoplasmic reticulum?

Answer 16

• • consists of a network of membranous tubules and sacs called cisternae which separates the internal compartment of the ER (lumen or cisternal space) from the cytosol
• • Smooth ER: outer surface lacks ribosomes
• • Rough ER: studded with ribosomes on the outer surface of the membrane and thus appears rough

Question 17

What is the function of the smooth ER?

Answer 17

• • synthesis of lipids (sex hormones of vertebrates and the various steroid hormones secreted by the adrenal glands)
• • metabolism of carbohydrates,
• • detoxification of drugs and poisons (by adding a hydroxyl group to make it soluble -alcohol induces the proliferation of the smooth ER which increases rate of detoxification and thus tolerance to these drugs. Also, because detoxification enzymes have a broad action, the proliferation of smooth ER in response to one drug can increase the need for higher dosages of other drugs as well)
• • storage of calcium ions (When a muscle cell is stimulated by a nerve impulse, calcium ions rush back across the ER membrane from the lumen into the cytosol and trigger contractions)

Question 18

What is the function of the rough ER?

Answer 18

• • assemble protein which then depart from the ER in membrane bound vesicles
• • As a polypeptide chain grows from a bound ribosome, the chain is threaded into the ER lumen through a pore as it folds into its functional shape in the lumen
• • Most secretory proteins are glycoproteins, proteins with carbohydrates covalently bonded to them because of the enzymes in the ER membrane which depart from the ER in transport vesicles
• • grows in place by adding membrane proteins and phospholipids (made from precursors in cytoplasm) to its own membrane
• • The ER membrane expands and portions of it are transferred in the form of transport vesicles to other components of the endomembrane system.

Question 19

What is the function of the Golgi apparatus?

Answer 19

• • Products travel in transport vesicles from the ER to the Golgi apparatus.
• • One side of the Golgi apparatus (cis face) functions as a receiving dock for the product (vesicles fuse with the membrane) and the other (trans face) as a shipping dock (gives rise to vesicles that pinch off)
• • Products are modified (according to particular tags or sequences of amino acids) as they go from one side of the Golgi apparatus to the other and travel in vesicles to other sites
• • also manufactures some macromolecules

Question 20

What are the functions of lysosomes?

Answer 20

• • A lysosome is a membranous sac containing digestive enzymes to hydrolyze macromolecules
• • Hydrolytic enzymes and lysosomal membrane are made by rough ER and then transferred to the Golgi apparatus for further processing
• • the three-dimensional shapes of these proteins in the lysosomal membrane protect vulnerable bonds from enzymatic attack
• • Lysosomes also use their hydrolytic enzymes to recycle the cell’s own organic material, a process called autophagy; a damaged organelle or small amount of cytosol becomes surrounded by a double membrane (of unknown origin), and a lysosome fuses with the outer membrane of this vesicle and the lysosomal enzymes then degrade it (cell can continually renew itself because of this)
• • slow rate of lysosomal action is associated with a range of neurological conditions like alzheimers disease

Question 21

What is the function of vacuoles?

Answer 21

• • large vesicles derived from the endoplasmic reticulum and Golgi apparatus
• • food vacuoles are formed by phagocytosis of microorganisms or particles to be used as food by the cell
• • protists have contractile vacuoles that pump excess water out of the cell, thereby maintaining a suitable concentration of ions and molecules inside the cell
• • In plants, vacuoles may have digestive functions, contain pigments, or contain poisons that protect the plant.
• • The central vacuole plays a major role in the growth of plant cells, which enlarge as the vacuole absorbs water; the solution inside the central vacuole, called cell sap, is the plant cell’s main repository of inorganic ions

Question 22

What is the function of mitochondria?

Answer 22

• • the sites of cellular respiration, the metabolic process that uses oxygen to drive the generation of ATP
• • The inner membrane divides the mitochondrion into two internal compartments
    1) The inter-membrane space is the narrow region between the inner and outer membranes.
    2) The mitochondrial matrix contains the mitochondrial DNA, ribosomes, and many enzymes that catalyze some of the reactions of cellular respiration.
• • the number correlates with the cell’s level of metabolic activity
• • The outer membrane is smooth, but the inner membrane is convoluted, with infoldings called cristae (increase SA of the inner membrane)
• • Defects in one or more of the proteins that participate in cellular respiration, decrease the amount of ATP the cell cane make (mitochondrial myopathy which causes weakness, fatigue, muscle deterioration)

Question 23

What is the function of chloroplasts?

Answer 23

• • contain the green pigment chlorophyll, along with enzymes and other molecules that function in the photosynthetic production of sugar
• • Chloroplasts are partitioned into compartments:
    1) Between the outer and inner membrane is a thin inter-membrane space.
    2) Inside the inner membrane is stroma (equivalent to mitochondrial matrix); Contains the chloroplast DNA, ribosomes, enzymes; a network of interconnected sacs called thylakoids; Stacks of thylakoids are called a granum.
• • The membranes of the chloroplast divide the chloroplast space into three compartments: the intermembrane space, the stroma, and the thylakoid space
• • The chloroplast is a specialized member of a family of closely related plant organelles called plastids (like amyoplast and chloroplast)

Question 24

What is the endosymbiont theory?

Answer 24

• • states that an early ancestor of eukaryotic cells engulfed an oxygen-using nonphotosynthetic prokaryotic cell (mitochondrion)
• • Eventually, the engulfed cell formed a relationship with the host cell in which it was enclosed, becoming an endosymbiont and over the course of evolution they merged into a single organism (a eukaryotic cell with a mitochondrion)
• • At least one of these cells may have then taken up a photosynthetic prokaryote, becoming the ancestor of eukaryotic cells that contain chloroplasts

Question 25

What is the evidence for endosymbiosis?

Answer 25

• • rather than being bounded by a single membrane like organelles, mitochondria and typical chloroplasts have two membranes
• • like prokaryotes, mitochondria and chloroplasts contain ribosomes
• • Very similar morphology of bacteria, mitochondria and chloroplasts
• • Self-replicating (can only come from pre-existing ones)
• • Genetic Information encoded in DNA
• • Transcribe and translate to produce proteins from genetic material
• • Can generate ATP through electron transport processes

Question 26

What are peroxisomes?

Answer 26

• • a specialized metabolic compartment bounded by a single membrane and contain enzymes that remove hydrogen atoms from various substrates and transfer them to oxygen to produce hydrogen peroxide (but have enzymes to covert this to oxygen since H2O2 is toxic)
• • Some peroxisomes use oxygen to break fatty acids down to smaller molecules that are transported to mitochondria for fuel
• • Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to oxygen
• • Specialized peroxisomes called glyoxysomes are found in the fat-storing tissues of plant seeds that contain enzymes that initiate the conversion of fatty acids to sugar

Question 27

What are cytoskeletons?

Answer 27

• • a network of fibres extending throughout the cytoplasm
• • The eukaryotic cytoskeleton, which plays a major role in mechanical support/shape, and is composed of three types of structures:
    1) microtubules
    2) microfilaments
    3) intermediate filaments
• • Some types of cell motility (movement) involve the cytoskeleton (generally requires interaction of the cytoskeleton with motor proteins)
• • motor proteins that attach to receptors on vesicles can “walk” the vesicles along microtubules or sometimes microfilaments (ex. how vesicles containing neurotransmitter molecules migrate to the tips of axons)
• • The cytoskeleton also manipulates the plasma membrane, bending it inward to form food vacuoles or other phagocytic vesicles

Question 28

What are microtubules?

Answer 28

• • constructed from a protein called tubulin; each tubulin protein is a dimer (a molecule made up of two subunits); a tubulin dimer consists of two slightly different polypeptides, α-tubulin and B -tubulin
• • Microtubules grow in length by adding tubulin dimers which can also be disassembled to build microtubules elsewhere
• • Because of the orientation of tubulin dimers, the two ends of a microtubule are slightly different (one end can accumulate or release tubulin dimers at a much higher rate than the other so it is called the “plus end”)
• • In animal cells, microtubules grow out from a centrosome which have a pair of centrioles, each composed of nine sets of triplet microtubules arranged in a ring (helps organize microtubules)
    1) involved in the separation of chromosomes during cell division
    2) shape and support the cell
    3) serve as tracks for vesicle movement (ones that have motor proteins attached)

Question 29

What are cilia and flagella?

Answer 29

• • microtubule-containing extensions that project from some cells (extending away from the cell and pushing the membrane with them)
• • (the ciliated lining of the trachea sweeps mucus containing trapped debris out of the lungs; the cilia lining the oviducts help move an egg toward the uterus)
• • A flagellum has an undulating motion like the tail of a fish. In contrast, cilia work more like oars in a crew boat
• • Both cilia and flagella have nine doublets of microtubules arranged in a ring; in the centre of the ring are two single microtubules (“9” + “2” pattern)
• • The microtubule assembly of a cilium or flagellum is anchored in the cell by a basal body, which is structurally very similar to a centriole, with microtubule triplets in a “9+0” pattern (the basal body of the fertilizing sperm’s flagellum enters the egg and becomes a centriole)

Question 30

What are nonmotile cilia?

Answer 30

• • non motor primary cilium act as signal receiving antenna by transmitting molecular signals from the cell environment to its interior, which triggers signalling pathways that may lead to changes in the cell’s activities (crucial to brain function and embryonic development)
• • Nonmotile primary cilia have a “9+0” pattern, lacking the central pair of microtubules

Question 31

How do motile cilia and flagella move?

Answer 31

– dyneins: a large motor protein that is attached to each outer microtubule doublet. A typical dynein protein has two “feet”that “walk” along the microtubule of the adjacent doublet, using ATP for energy. The outer doublets and two central microtubules are held together by flexible cross-linking proteins and the walking movement of the dynein protein is coordinated so that it happens on one side of the circle at a time

Question 32

What are microfilaments?

Answer 32

• • a twisted double chain of actin subunits
• • seem to be present in all eukaryotic cells.
• • the structural role of microfilaments in the cytoskeleton is to bear tension (pulling forces)
• • cortical microfilaments (3 dimensional network) helps support the cells shape (gives the outer cytoplasmic layer a semisolid consistency)
• • cell motility; actin filaments that interact with myosin proteins (a motor protein) cause contraction of muscle cells

Question 33

What are intermediate filaments?

Answer 33

• • larger than the diameter of microfilaments but smaller than that of microtubules
• • Unlike microtubules and microfilaments, which are found in all eukaryotic cells, intermediate filaments are only found in the cells of some animals, including vertebrates
• • bearing tension (like microfilaments)
• • unlike microfilaments and microtubules which are consistent in diameter and composition, intermediate filaments are constructed from a particular molecular subunit belonging to a family of proteins whose members include the keratins
• • are more permanent fixtures of cells than are microfilaments and microtubules
• • chemical treatments remove microfilaments/microtubules but leave intermediate filaments with cells retaining its original shape suggests that especially sturdy and that they play an important role in reinforcing the shape of a cell and fixing the position of certain organelles (nucleus sits within a cage of intermediate filaments)

Question 34

What is the extracellular matrix?

Answer 34

• • the meshwork surrounding animal cells which helps hold cells together in tissues and protects and supports the plasma membrane
• • consists of glycoproteins, polysaccharides, and proteoglycans synthesized and secreted by the cells
• • the most abundant glycoprotein in the ECM is collagen, which forms strong fibres outside the cells
• • The collagen fibres are embedded in a network woven out of proteoglycans (a proteoglycan molecule consists of a small core protein with many carbohydrate chains covalently attached, so that it may be up to 95% carbohydrates). Large proteoglycan complexes can form when hundreds of proteoglycan molecules become noncovalently attached to a single long polysaccharides molecule
• • Some cells are attached to the ECM by glycoproteins such as fibronectin. Fibronectin bind to cell surface receptors called integrins that are built into the plasma membrane
• • Integrins are in a position to transmit signals between the ECM and the cytoskeleton and thus to integrate changes occurring outside and inside the cell

Question 35

What are plasmodesmata?

Answer 35

• • channels (pores) that connect cells; cytosol passing through the plasmodesmata joins the internal chemical environments of adjacent cells
• • Water and small solutes can pass freely from cell to cell, and certain proteins/RNA molecules
• • The macromolecules transported to neighbouring cells appear to reach the plasmodesmata by moving along fibres of the cytoskeleton

Question 36

What are the three main types of cell junction in animal cells?

Answer 36

• • tight junctions: the plasma membranes of neighbouring cells are very tightly pressed against each other and bound by specific proteins and prevents leakage of extracellular fluid (found in epithelial cells)
• • Desmosome (anchoring junctions): fasten cells together like tight sheets. Intermediate filaments anchor desmosomes in the cytoplasm (muscle tears involves involve rupture of desmosomes)
• • Gap junctions (communicating junctions): provide cytoplasmic junctions from one cell to an adjacent cell (similar to plasmodesmata)

Question 37

How do cell membranes form?

Answer 37

• • Phospholipids, the key ingredient of biological membranes, spontaneously self-assemble into simple membranes
• • Membranes are not static sheets of molecules locked rigidly in place; most of the lipids and some of the proteins can shift about laterally or may flip-flop across the membrane, switching from one phospholipid layer to the other

Question 38

What is the fluidity of cell membranes?

Answer 38

• • Many phospholipids are made from unsaturated fatty acids that have kinks in their tails (fluid membrane) and saturated phospholipids pack tightly together and form viscous membrane
• • The temperature at which a membrane solidifies depends on the types of lipids it is made of; the membrane remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails because they can’t pack tightly
• • In animal cell membranes, cholesterol helps maintain membrane integrity (at high temperatures, cholesterol makes the membrane less fluid by restraining phospholipid movement but cholesterol also hinders the close packing of phospholipids by lowering the temperature required for the membrane to solidify

Question 39

How does membrane fluidity affect its function?

Answer 39

• • the fluidity of a membrane affects both its permeability and the ability of membrane proteins to move to where their function is needed
• • When a membrane solidifies, its permeability changes, and enzymatic proteins in the membrane may become inactive if their activity requires movement within the membrane. However, membranes that are too fluid cannot support protein function either
• • Ex. fishes that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails, enabling their membranes to remain fluid

Question 40

What are integral and peripheral proteins?

Answer 40

• • Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic interior (the hydrophobic regions of the protein are imbedded while the hydrophilic regions are exposed to the aqueous solution on either side of the membrane)
• • Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to exposed parts of integral proteins

Question 41

How are membrane proteins held in place?

Answer 41

– On the cytoplasmic side of the plasma membrane, some membrane proteins are held in place by attachment to the cytoskeleton. And on the extracellular side, certain membrane proteins are attached to fibres of the extracellular matrix

Question 42

How is the membrane a fluid mosaic?

Answer 42

• • A single cell may have cell-surface membrane proteins that carry out several different functions
• • Thus, the membrane is not only a structural mosaic, with many proteins embedded in the membrane, butalso a functional mosaic

Question 43

How are proteins in a cell membrane important in the medical field?**

Answer 43

• • a protein called CD4 on the surface of immune cells helps the human immunodeficiency virus (HIV) infect these cells, leading to acquired immune deficiency syndrome (AIDS)
• • Comparing the genes of resistant people with the genes of infected individuals, researchers learned that resistant people have an unusual form of a gene that codes for an immune cell-surface protein called CCR5
• • although CD4 is the main HIV receptor, HIV must also bind to CCR5 as a “co-receptor” to infect most cells. An absence of CCR5 on the cells of resistant individuals, due to the gene alteration, prevents the virus from entering the cells
• • Discovery of the CCR5 co-receptor provided a safer target for development of drugs that mask this protein and block HIV entry

Question 44

How do cells recognize each other?**

Answer 44

• • Cells recognize other cells by binding to molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane
• • some of these carbohydrates (which are short chains of fewer than 15 monosaccharides), are covalently bonded to lipids, forming molecules called glycolipids while most are covalently bonded to proteins, which are thereby glycoproteins
• • The carbohydrates on the extracellular side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual.
• • For example, the four human blood types designated A, B, AB, and O reflect variation in the carbohydrate part of glycoproteins on the surface of red blood cells

Question 45

How is the membrane selectively permeable?

Answer 45

• • Selective permeability of a membrane depends upon lipid bilayer barrier, and the specific transport proteins embedded within
• • Small molecules and ions move across the membrane in both directions
• • non-polar molecules are hydrophobic therefore they can dissolve in the lipid bilayer of the membrane and cross it easily, without the aid of membrane proteins while polar (hydrophilic) molecules such as glucose and other sugars pass only slowly through a lipid bilayer

Question 46

What are transport proteins?

Answer 46

• • Specific ions and a variety of polar molecules can’t move through cell membranes on their own; however, these hydrophilic substances can avoid contact with the lipid bilayer by passing through transport proteins
• • some transport proteins, called channel proteins, function by having a hydrophilic channel that certain molecules or atomic ions use as a tunnel through the membrane (ex. aquaporins for water molecules)
• • Other transport proteins, called carrier proteins, hold onto their passengers and change shape in a way that shuttles them across the membrane (such a change in shape may be triggered by the binding and release of the transported molecule)
• • transport proteins are specific for the substance it translocates (ex. a specific carrier protein in the plasma membrane of red blood cells transports glucose across the membrane but is so selective it rejects fructose, a structural isomer of glucose)

Question 47

What is diffusion?

Answer 47

• • Molecules have a type of energy called thermal energy, due to their constant motion
• • One result of this motion is diffusion, the movement of particles of any substance so that they spread out evenly into the available space
• • In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated (substance will diffuse down its concentration gradient, the region along which the density of a chemical substance increases or decrease)
• • One important example is the uptake of oxygen by a cell performing cellular respiration

Question 48

What is passive transport?

Answer 48

• • The diffusion of a substance across a biological membrane because the cell does not have to expend energy to make it happen, the concentration gradient itself represents potential energy and drives diffusion
• • Molecules don’t have to be small to passively diffuse (ex. steroids because they are hydrophobic/nonpolar lipids)

Question 49

What is osmosis?

Answer 49

– The diffusion of free water across a selectively permeable membrane, whether artificial or cellular

Question 50

What is tonicity and the three types?

Answer 50

• • the ability of a surrounding solution to cause a cell to gain or lose water
• • depends in part on its concentration of solutes that cannot cross the membrane (nonpenetrating solutes) relative to that inside the cell
• • isotonic: If a cell without a cell wall is immersed in an environment that is isotonic to the cell, there will be no net movement of water across the plasma membrane (diffusion in and out will occur at the same rate)
• • hypertonic: a solution that has more non-penetrating solutes will cause the cell to lose water, shrivel and possibly die
• • hypotonic: a solution that has less non-penetrating solutes will cause water to enter the cell faster than it leaves, and the cell will swell and lyse (burst)

Question 51

What is osmoregulation?

Answer 51

– In hypertonic or hypotonic environments, however, organisms that lack rigid cell walls must have other adaptations for osmoregulation, the control of solute concentrations and water balance

Question 52

What happens to tonicity of cells with cell walls?

Answer 52

• • When such a cell is immersed in a hypotonic solution the cell wall helps maintain the cell’s water balance
• • the relatively inelastic cell wall will expand only so much before it exerts a back pressure on the cell, called turgor pressure, that opposes further water uptake
• • At this point, the cell is turgid (very firm), which is the healthy state for most plant cells
• • If a plant’s cells and their surroundings are isotonic, there is no net tendency for water to enter, and the cells become flaccid (limp); the plant wilts
• • in a hypertonic environment, a plant cell, like an animal cell, will lose water to its surroundings and shrink. As the plant cell shrivels, its plasma membrane pulls away from the cell wall at multiple places. This phenomenon, called plasmolysis

Question 53

What is facilitated diffusion?

Answer 53

– polar molecules and ions impeded by the lipid bilayer of the membrane that diffuse passively with the help of transport proteins that span the membrane

Question 54

What are ion channels?**

Answer 54

• • Channel proteins that transport ions are called ion channels.
• • Many ion channels function as gated channels, which open or close in response to a stimulus
• • For some gated channels, the stimulus is electrical (In a nerve cell an ion channel opens in response to an electrical stimulus, allowing a stream of potassium ions to leave the cell)
• • Other gated channels open or close when a specific substance other than the one to be transported binds to the channel (ex. neurotransmitters)

Question 55

What is active transport?

Answer 55

• • To pump a solute across a membrane against its gradient requires work; the cell must expend energy
• • The transport proteins that move solutes against their concentration gradients are all carrier proteins rather than channel proteins. This makes sense because when channel proteins are open, they merely allow solutes to diffuse down their concentration gradients rather than picking them up and transporting them against their gradients

Question 56

How does ATP hydrolysis provide the energy for active transport?

Answer 56

e. g. the sodium-potassium exchange pump
- - The imbedded protein will not only bind to the Na, but also an ATP molecule which it will hydrolyze and then attach the energized phosphate onto itself (the protein) so that the actual shape changes which leads to the movement of the Na onto the other side of the membrane. The phosphate remains attached and then the K will bind and then the phosphate is gonna detach which will cause the protein to change back into its original shape and the K will be released into the cell

Question 57

What is membrane potential?

Answer 57

• • All cells have voltages across their plasma membranes (electrical potential energy)
• • The cytoplasmic side of the membrane is negative in charge relative to the extracellular side because of an unequal distribution of anions and cations on the two sides (this voltage across a membrane is membrane potential)

Question 58

What’s the electrochemical gradient?**

Answer 58

• • Because the inside of the cell is negative compared with the outside, the membrane potential favours the passive transport of cations into the cell and anions out of the cell
• • Thus, two forces drive the diffusion of ions across a membrane: a chemical force (the ion’s concentration gradient) and an electrical force (the effect of the membrane potential on the ion’s movement)
• • An ion diffuses not simply down its concentration gradient but, more exactly, down its electrochemical gradient (ex. the concentration of Na+ inside a resting nerve cell is much lower than outside it. When the cell is stimulated, gated channels open that facilitate Na+ diffusion. Sodium ions then “fall” down their electrochemical gradient, driven by the concentration gradient of Na+ and by the attraction of these cations to the negative side (inside) of the membrane)
• • In this example, both electrical and chemical contributions to the electrochemical gradient act in the same direction across the membrane, but this is not always so
• • In cases where electrical forces due to the membrane potential oppose the simple diffusion of an ion down its concentration gradient, active transport may be necessary

Question 59

What’s an electrogenic pump?**

Answer 59

• • A transport protein that generates voltage across a membrane is called an electrogenic pump (sodium-potassium pump pumps three sodium ions out of the cell for every two potassium ions it pumps into the cell)
• • The main electrogenic pump of plants, fungi, and bacteria is a proton pump, which actively transports protons (hydrogen ions, H+) out of the cell

Question 60

What is cotransport?**

Answer 60

• • In a mechanism called cotransport, a transport protein (a cotransporter) can couple the “downhill” diffusion of the solute to the “uphill” transport of a second substance against its own concentration gradient
• • This is analogous to water that has been pumped uphill and performs work as it flows back down

Question 61

What is exocytosis?

Answer 61

• • the cell secretes certain molecules by the fusion of vesicles with the plasma membrane
• • A transport vesicle that has budded from the Golgi apparatus moves along microtubules of the cytoskeleton to the plasma membrane. When the vesicle membrane and plasma membrane come into contact, specific proteins rearrange the lipid molecules of the two bilayers so that the two membranes fuse
• • The contents of the vesicle spill out of the cell, and the vesicle membrane becomes part of the plasma membrane
    ex. neurons use exocytosis to release neurotransmitters, pancreatic cells use it to release insulin

Question 62

What is endocytosis?

Answer 62

• • the cell takes in molecules and particulate matter by forming new vesicles from the plasma membrane
• • A small area of the plasma membrane sinks inward to form a pocket. Then, as the pocket deepens, it pinches in, forming a vesicle containing material that had been outside the cell (essentially the reverse of exocytosis)
• • the three types of endocytosis: phagocytosis (“cellular eating”), pinocytosis (“cellular drinking”), and receptor-mediated endocytosis (Human cells use receptor-mediated endocytosis to take in cholesterol. LDLs bind to LDL receptors on plasma membranes and then enter the cells by endocytosis)