H2 Biology > Cell Structure > Flashcards

Cell Structure Flashcards

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1
Q

state the cell theory

A
  • all cells are made up of pre-existing cells
  • all living things are made up of one or more cells
  • a cell is the smallest unit of life
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2
Q

state the structure of the nucleus

A
  • spherical/ovoid structure, surrounded by a nuclear envelope that separes the nucleoplasm from the surrounding cytoplasm
  • double-membrane bound, outer surface of the nuclear envelope is studded with ribosomes and continuous with the RER membrane
  • nuclear pores on the nuclear envelope control the movement of substances into and out of the nucleus
  • nucleoplasm contains nucleotides, chromatin, nucleolus and enzymes (proteins)
  • nucleolus is a dense region in the nucleoplasm that is not membrane-bound; contains dna that codes for rRNA and is the location where large and small ribosomal subunits are synthesised
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3
Q

describe the functions of the nucleus

A
  • contains genetic information in the form of chromatin (DNA associated with histone proteins)
  • chromatin exists as heterochromatin (densely-stained, DNA not actively transcribed and tightly packed) and euchromatin (less densely-stained, DNA actively transcribed and less tightly-packed)
  • directs protein synthesis: transcription: synthesising mRNA according to nucleotide sequence in DNA; translation: mRNA is used as a template for protein synthesis; nucleus controls all cell activities by regulating protein and enzyme synthesis
  • nucleolus contains DNA that codes for the synthesis of rRNA, ribosomal proteins are imported from cytoplasm into nucleoplasm, large and small ribosomal subunits (consist of rRNA and ribosomal proteins) are synthesised in the nucleolus
  • large and small ribosomal subunits combine to form a functional protein in the cytoplasm during translation
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4
Q

describe the structure of the RER

A
  • interconnected, membrane-bound flattened sacs (cristenae)
  • outer membrane of RER is continuous with outer membrane of nuclear envelope and is studded with ribosomes
  • inner surface of membrane is embedded with enzymes involved in the post-translational chemical modification of proteins and folding of polypeptides into their native 3D conformation
  • protein channels act as receptors that hold ribosomes in place
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5
Q

describe the structure of SER

A
  • interconnected, membrane-bound tubules which lack ribosomes
  • outer surface of membrane is continuous with RER membrane
  • inner surface of membrane is embedded with enzymes involved in synthesis of lipids and steroids
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6
Q

describe the functions of RER

A
  • ribosomes attached to RER synthesise secretory proteins/hydrolytic enzymes/membrane proteins
  • cisternae provides suitable environmental conditions for the folding of polypeptides into their native 3D conformation and the post-translational chemical modification of proteins
  • membrane proteins are inserted into the RER membrane, which become a part of the vesicle membrane and when these vesicles fuse with their destined membranes, these membrane proteins become embedded in their destined membranes
  • polypeptides are transported to GA via transport vesicles to be secreted out of the cell
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7
Q

describe the functions of SER

A
  • synthesis of lipids and steroids, phospholipid synthesis is important to replenish/form membranes, more phospholipids are required before cell division
  • stores/releases calcium ions, impt for contraction of muscles
  • contains enzymes involved in detoxification of chemicals and drugs: enzymes add hydroxyl group to these chemicals, making them more soluble and easier to remove from the body
  • involved in carbohydrate metabolism in liver cells
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8
Q

state the general functions of ER

A

provide a large surface area for increased rate of reactions
* more ribosomes can attach to RER membrane for increased rate of protein synthesis, more enzymes for increased rate of PTCM in RER, increased rate of lipid and steroid synthesis

form an intracellular transport system to move materials from one part of the cell to another via ER lumen

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9
Q

describe the structure of GA

A
  • stack of membrane-bound flattened sacs (cisternae)
  • inner surface of membrane is embedded with enzymes involved in PTCM of lipids and proteins from ER
  • transport vesicles from ER fuse with GA cis face, adding its membrane to that of GA and releasing contents into GA cisternae
  • materials are processed as they move from cisternae to cisternae towards trans face
  • secretory vesicles containing secretory molecules at trans face pinch off from cisternae of GA
  • some vesicles move along microtubules towards cell surface membrane and release contents to exterior of cell
  • other vesicles containing hydrolytic enzymes remain in the cell and become lysosomes
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10
Q

describe the functions of GA

A

involved in packaging, sorting and modifying of substances
* proteins and lipids from ER undergo PTCM to form their final products as they move from cisternae to cisternae
* final products are packaged into membrane-bound vesicles for transport to their eventual destinations
* secretory vesicles containing secretory molecues are transported along microtubules towards cell surface membrane to release their contents into the exterior
* vesicles containing hydrolytic enzymes circulate within the cytoplasm and become lysosomes
* vesicles containing membrane proteins are transported along microtubules towards CSM, fuse with CSM and proteins are embedded in membrane
* GA is thus responsible for formation of various types of vesicles: secretory vesicles and lysosomes

major site of carbohydrate synthesis

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11
Q

describe the structure of lysosomes

A
  • membrane-bound vesicles containing hydrolytic enzymes that hydrolyse complex chemical compounds into simpler ones
  • acidic pH (optimum pH for hydrolytic enzymes)
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12
Q

describe the formation of lysosomes

A
  1. hydrolytic enzymes synthesised by ribosomes attached to RER
  2. they enter the RER and are then transported to the GA
  3. at GA, modified and packaged into membrane-bound vesicles called lysosomes
  4. lysosomes circulate in cytoplasm
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13
Q

describe the functions of lysosomes

A

intracellular digestion
* lysosomes fuse with endocytic vesicles containing material taken into the cell via endocytosis
* hydrolytic enzymes released into this newly formed secondary vesicle where intracellular digestion takes place
* digested products absorbed into cytoplasm and assimilated while undigested material is excreted out of cell via exocytosis

cell autophagy
* worn-out/defunct organelles are enclosed in a membrane
* lysosomes fuse with these vesicles and release hydrolytic enzymes
* organelles broken down by hydrolytic enzymes, soluble products absorbed into cytoplasm where they may be used to form new organelles

cell autolysis
* lysosomal membrane ruptures, releasing hydrolytic enzymes into cytoplasm, which digest cell contents, killing it in the process
* occurs in injured/dying tissue during intended cell death (apoptosis) or unintended cell death (necrosis)

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14
Q

state the structures involved in the endomembrane system

A

nuclear envelope, RER, SER, GA, endocytic vesicles, transport vesicles, secretory vesicles, lysosomes

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15
Q

describe the structure of the chloroplast

A
  • disk-shaped, biconcave, double-membrane bound
  • stroma contains grana, each granum consists of a stack of flattened sacs (thylakoids) connected by tubes to adjacent grana
  • stroma contains thylakoids, enzymes involved in Calvin cycle, 70S ribosomes, starch granules and circular DNA
  • photosynthetic pigments (carotenoids and chlorophyll), electron carriers and ATP synthase are embedded in thylakoid membranes
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16
Q

describe the function of chloroplast

A

main site of photosynthesis
* thylakoid membranes increase surface area for more photosynthetic pigments, electron carriers and ATP synthase to be embedded, increased rate of light absorption = increased rate of light-dependent reactions
* stroma contains enzymes that catalyse light-independent reactions in photosynthesis (Calvin cycle)
* 70S ribosomes synthesise chloroplast proteins encoded by chloroplast DNA (members of electron transport chain and ATP synthase)
* circular DNA codes for synthesis of some chloroplast proteins (members of the electron transport chain and ATP synthase)
* starch granules store excess glucose in the form of starch, produced during photosynthesis; in absence of sunlight, starch is converted back into glucose by enzymes and transported to cytoplasm

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17
Q

describe the structure of mitochondria

A
  • rod-shaped/cylindrical, double-membrane bound
  • outer membrane forms a smooth wall and inner membrane is highly folded to form cristae, which project into a gel-like mitochondrial matrix
  • electron carriers and ATP synthase is embedded in cristae, cristae increases surface area for increased rate of respiration
  • mitochondrial matrix contains 70S ribosomes, circular DNA and enzymes involved in the Krebs Cycle
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18
Q

describe the functions of mitochondria

A

main site of aerobic respiration
* ATP synthase and electron carriers embedded in cristae involved in oxidative phosphorylation (ATP synthesis)
* enzymes in mitochondrial matrix are involved in Krebs cycle of respiration (final breakdown of organic molecules for release of energy)
* circular DNA contains genes which codes for some mitochondrial proteins (members of the electron transport chain and ATP synthase)
* 70S ribosomes synthesises some mitochondrial proteins encoded by mitochondrial DNA
* some heat energy is released during respiration

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19
Q

describe the advantages of having membrane-bound organelles in cells

A

physically separate chemical reactions, many of which are incompatible, by allowing the localisation of specific molecules into specific compartments
* each compartments performs a different function at the same time
* allows specialisation of cells as different compartments perform different function
* nucleus contains genetic material and nuclear envelope prevents reactions in cytoplasm from altering genetic material, regulates which proteins have access to genetic information and when (gene regulation)
* photosynthesis occurs in chloroplast while respiration occurs in mitochondrion
* lysosomes are acidic and contain hydrolytic enzymes which do not interfere with other processes in cytoplasm

separate different reactions by time
* molecules are produced in one organelle and subsequently used in other reaction sequences
* enhances efficiency of processes by determining sequence of reactions taking place
* excess sugars stored as starch in starch granules to be used later
* process of transcription and translation are separated
* insulin can be stored in cytoplasm and released from cell when needed instead of being synthesised from scratch

allow high concentrations of enzymes and molecules to accumulate in specific compartments through AT
* enhances efficiency of reactions bc high concentrations of substrates and enzymes increase chances of substrates and enzymes colliding in the right orientation for reaction to occur

each membrane provides a surface for attachment of proteins for specific function
* with larger SA, more proteins can attach to membrane
* large SA of RER for attachment of ribosomes for protein synthesis
* large SA of cristae in mitochondria for attachment of ATP synthase for ATP synthesis
* proteins can be attached to membrane in a specific order and orientation, this determines the sequence of reactions taking place

each compartment provides an optimum environment for specific reactions to occur
* enhances efficiency of reactions as certain biomolecules require specific conditions in order to maintain a specific 3D conformation
* acidic environment of lysosomes optimal for hydrolytic enzymes to function

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20
Q

describe the structure of ribosomes

A
  • consist of a large and small ribosomal subunit, which combine to form a functional protein during translation
  • made up of rRNA and ribosomal proteins
  • ribosomal subunits synthesised at nucleoplasm, they exit nucleus via nuclear pores and carry out translation at RER/cytoplasm
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21
Q

describe the functions of ribosomes

A
  • ribosomes attached to RER membrane synthesise secretory proteins/membrane proteins/hydrolytic enzymes
  • polypeptides enter the RER lumen to be modified to form functional proteins
  • proteins synthesised by free ribosomes are used within the cytoplasm/transported to other organelles (nucleus, mitochondria, chloroplasts)
  • polyribosome: when many ribosomes attach to and translate the same mRNA molecule, improves efficiency of protein synthesis
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22
Q

describe the structure of centrioles

A
  • in animal cells, centrioles are found in centrosomes while plant cells have centrosomes but do not have centrioles
  • found in a region near the nucleus (centrosome)
  • found as a pair, lie perpendicular to each other
  • each centriole consists of a hollow tube, wall of each tube is made up of nine triplets of microtubules
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23
Q

describe the functions of centrioles

A

microtubule-organising centre, organises formation of meiotic and mitotic spindle during cell division
* spindle fibres assemble chromosomes at the metaphase plate during metaphase and separate the chromosomes during anaphase, facilitating the distribution of genetic material to daughter cells

give rise to basal bodies of cilia/flagella

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24
Q

state the structures always present in bacteria

A

peptidoglycan cell wall, cell surface membrane, bacterial chromosome, cytoplasm 70S ribosomes, food reserves

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25
describe the structure and function of the cell wall in bacteria
* strong and rigid due to peptidoglycan (parallal polysaccharide molecules cross-linked at regular intervals by short amino acid chains, regular cross-linking confers high tensile strength) * maintains shape and structure of bacterial cell * helps anchor appendages, which originate in the cytoplasmic membrane and protrude through the cell wall to the exterior * fully permeable to water/ions/small molecules/large enzymes/fragments of DNA
26
describe the structure and function of the cell surface membrane in bacteria
* selectively permeable, controls movement of substances into and out of the cells * in some cells, infoldings of cell surface membrane forms specialised membranes that perform many of their metabolic functions (respiratory membranes)
27
describe the function of cytoplasm in bacteria
* cell growth, metabolism and replication are carried out here * contains 70S ribosomes, bacterial chromosome, plasmids
28
describe the structure and function of a bacterial chromosome
* single, circular, double-stranded (DNA) molecule * not bound by a nuclear envelope but is found in the nucleoid region (less densely stained than rest of cytoplasm) * bacterial DNA associates with non-histone proteins and exists in a condensed supercoiled structure * genes in bacterial genome are found in a continuous stretch of DNA, with little non-coding DNA between coding genes, bacterial genes contains mostly coding genes (extrons), with introns being much rarer than in eukaryotes; * due to a lack of nuclear envelope, DNA is transcribed to form mRNA, mRNA is translated to form polypeptides before transcription is completed, transcription and translation occur concurrently
29
describe the structure of plasmids
* small, circular, double-stranded DNA molecules * contains genes that provide some benefit to bacteria but are non-essential for survival * separate from bacterial chromosome * C plasmids contains genes conferring antibiotic resistance, codes for enzymes that inactivate antibiotics * F plasmid contains genes coding for sex pillus, allow lateral transfer of F plasmid between bacteria (by conjugation)
30
describe the structure and function of ribosomes in bacteria
* 70S ribosomes scattered throughout cytoplasm, not bound to any organelles * consist of a large and small ribosomal subunit, made up of rRNA and ribosomal proteins * many ribosomes attach to and translate the same mRNA molecule: polyribosomes
31
describe the structure of the viral genome
* single-stranded/double-stranded, DNA/RNA (never both * segmented into many molecules/a single molecule * circular/linear
32
state the function of the viral genome
codes for the synthesis of viral components (capsid) and viral enzymes for its replication
33
describe the structure of capsid
* protein shell surrounding viral genome, made up of capsomeres * combined structure of viral genome and capsid is nucleocapsid * naked viruses consist of only nucleocapsid without a viral envelope * attachment proteins projecting from capsid enable binding of virus to susceptible host cell * highly symmetrical
34
state the functions of capsid
protect the virus from environmental damage and facilitates introduction of viral genome into susceptible host cells
35
describe the structure of viral envelope
* consists of phospholipids and glycoproteins (exist as glycoprotein spikes) * phospholipids are derived from cell surface membrane of host cells by budding and glycoproteins are encoded by viral genome * viral glycoproteins synthesised by host cell's ribosomes and enzymes * viral glycoproteins follow path of RER to GA to CSM
36
state the functions of viral envelope
* serve as an additional protective coat for virus * glycoproteins serve to attach virus to specific receptors on susceptible host cells * specific strains of virus will have glycoproteins that attach only to specific types of host cells (viral specificity)
37
describe the structure of a tail in bacterial cells
* found only in bacteriophages * consist of a contractile sheath, tail fibres and base plate attached to the capsid head
38
state the functions of the tail in bacterial cells
* attach bacteriophage to bacterium * contractile sheath contracts to inject phage DNA into host bacteria during infection * base plate and tail fibres enable the binding of phage to bacteria (tail fibres bind to specific receptors on specific strains of bacteria) * specific strains of bacteriophages only bind to specific types of bacteria (viral specificity)
39
state the functions of the tail in bacterial cells
* attach bacteriophage to bacterium * contractile sheath contracts to inject phage DNA into host bacteria during infection * base plate and tail fibres enable the binding of phage to bacteria (tail fibres bind to specific receptors on specific strains of bacteria) * specific strains of bacteriophages only bind to specific types of bacteria (viral specificity)
40
where are viral enzymes found?
inside the capsid are one or more viral-specific enzymes that are not found in the host cell
41
state the functions of viral enzymes
* carry out reactions that are not carried out in the host cells * involved in the infection and replication process
42
state the living characteristics of viruses
* able to reproduce at a very rapid rate * mutate and evolve over time * respond to external stimuli * contain all the necessary genetic information (DNA/RNA) for synthesis of new viruses
43
state the non-living characteristics of viruses
* acellular (do not contain cytoplasm/cellular organelles) * unable to reproduce independent of host cell * unable to obtain nutrition for growth/respire * unable to perform metabolism on their own, must use host cell's enzymes, ribosomes, nucleotides and ATP to carry out protein synthesis and DNA replication; new viral components synthesised and assembled within infected host cell * able to exist in a metabolically inert extracellular state for extended periods of time
44
explain why viruses are considered obligate parasites
* they only survive and reproduce using host cell's enzymes and other molecules * they lack rRNA and are unable to carry out transcription for protein synthesis * they lack ribosomes and are unable to carry out translation of mRNA on their own (must use host cell's ribosomes) * unable to generate/store energy in the form of ATP and derive energy for all metabolic functions from host cell * use host cell's amino acids, lipids and nucleotides
45
describe the downsides of viruses
* upon gaining entry into susceptible host cells, they control normal cell control and divert cell activity to viral-directed activity * disruption of normal cellular activities has harmful effects on host cells
46
describe the structure of phospholipids and cholesterol
* phospholipid consists of a negatively-charged phosphate group (hydrophilic) and two hydrocarbon fatty acid chains (hydrophobic) * phospholipids have both hydrophilic and hydrophobic properties so they are amphipathic in nature * phospholipids are form the phospholipid bilayer in membranes and are orientated such that the hydrophilic phosphate groups are found on either sides of the membrane facing the aqueous medium, they form hydrogen bonds with water * hydrophobic hydrocarbon chains are sandwiched between the two layers of phosphate heads and form the hydrophobic core, found within the membrane away from the aqueous medium * cholesterol molecules are found interspersed among phospholipid molecules
47
state the functions of phospholipids and cholesterol regarding regulation of membranen fluidity
* phospholipids are held together by weak hydrophobic interactions, this enables phospholipids to move within the membrane and flip-flop between the membranes, allowing membrane fluidity * membrane fluidity allows for the membrane to fold/fuse for endocytosis/exocytosis and for the insertion and function of proteins (protein's function involve change in 3D conformation) * as temperature decreases, membranes remains fluid until phospholipids settle into a closely-packed arrangement. when phospholipids settle into a closely-packed arrangement, membrane has solidified and this affects the permeability of membrane and proteins are unable to function properly
48
describe the two factors affecting fluidity of membrane
type of phospholipids * phospholipids with saturated fatty acid chains are able to pack together closely, at high temperatures this prevents the membrane from becoming too fluid (membrane has high MP) * phospholipids with unsaturated fatty acid chains have kinks in their tails where the double bond is located and they are unable to pack together as closely compared to saturated phospholipids. this enables the membrane to still be fluid at lower temperatures (membrane has low MP) * cholesterol helps stabilise membrane fluidity * cholesterol molecules restrict the movement of phospholipids, preventing the membrane from becoming too fluid at higher temperatures * cholesterol molecules hinder the close-packing of phospholipids, preventing the solidification of the membrane at lower temperatures * cholesterol decreases the permeability of membrane to polar molecules and ions
49
describe the functions of phospholipids and cholesterol in restricting movement of ions and polar molecules due to hydrophobic core of membrane
* hydrophobic hydrocarbon chains of phospholipids form the hydrophobic core, which impedes the passage of ions and polar molecules * non-polar molecules are able to dissolve and diffuse across the membrane easily without the aid of transport proteins * small polar molecules (water) are able to dissolve and diffuse across the membrane but at a very low rate * transport of polar molecules and ions across the membrane thus requires the assistance of specific transport proteins * selectively permeable membrane as specific transport proteins transport specific solutes across the membrane * polar molecules and ions can be transported across the membrane provided the transport protein specific to them are present on the CSM
50
describe the structure of proteins found in CSM
* proteins that are loosely bound on either side of the membrane are extrinsic proteins * proteins that are partly embedded in or fully penetrate through the membrane are intrinsic proteins * intrinsic proteins that penetrate all the way through the membrane are known as transmembrane proteins * globular proteins * held in position in lipid bilayer by weak hydrophobic interactions between non-polar R groups of protein and hydrophobic hydrocarbon chains of phospholipids * hydrophilic and charged areas of the protein are exposed to the aqueous environment on either side of the membrane * many membrane proteins are glycosylated to form glycoproteins
51
state the functions of glycoproteins and glycolipids
* carbohydrate portion of glycolipids and glycoproteins form the glycocalyx * cell-cell recognition: enables cell to distinguish one cell from another * cell-cell adhesion: enables cells to bind to one another to form tissues * act as receptor sites: act as receptor sites for specific chemical signals to bind to
52
state the 6 functions of membrane proteins and glycoproteins
* cell-cell adhesion: enables cells to bind to one another to form tissues * cell-cell recognition: act as identification tags, enabling cells to distinguish one cell from another * act as receptor sites: act as receptors with specific binding sites for specific chemical signals to bind to (protein has binding site with a conformation complementary to that of signal molecule); binding of external signal to receptor initiates reactions which relay signal into cell * transport: act as transport proteins that transport specific solutes across the CSM * attachment to ECM and exoskeleton: maintain cell's shape and stabilises location of certain membrane proteins * enzymatic activity: protein embedded in membrane may be an enzyme with its active site exposed to substances in adjacent solution, several enzymes in a membrane may be arranged in order to carry out sequential steps of a metabolic pathway
53
explain why the CSM is called the fluid mosaic model
* fluid refers to both the phospholipids and proteins being free to move within the membrane * phospholipids are held together by weak hydrophobi cinteractions and not strong covalent bonds * phospholipids move within the layer and flip-flop between layers * proteins free to move laterally about the membrane * mosaic refers to the scattered manner in which the membrane proteins and other membrane components are embedded in the membrane
54
describe the functions of CSM
* separates cytoplasm from external environment, enables internal chemical environment to be different from external environment * membranes act as selective barriers to regulate passage of substances in and out of cell and organelles * selectively permeable nature bc phospholipids act as hydrophobic barrier while specific membrane proteins enable specific transport of specific polar molecules/ions across the membrane; as transport is specific, membrane is selectively permeable * increase SA to increase rate of exchange of substances * cell-cell recognition: surface carbohydrates attached to CSM enable cells to recognise each other, presence of foreign proteins on membrane trigger production of antibodies for removal of foreign cells * cell-cell adhesion: CSM glycoproteins form cell-to-cell attachments which contribute to stable tissue structure * provide receptor sites: CSM proteins that form receptors will have binding sites that only specific chemical signals can bind to
55
define diffusion
net movement of particles from a region of higher concentration to a region of lower concentration
56
state the factors that affect rate of diffusion of particles across a membrane
diffusion gradient, molecular size, polarity of particles, solubility in lipids and SA of membrane
57
describe facilitated diffusion
* diffusion of substances along a concentration gradient with the assistance of transport proteins in the membrane * polar/charged substances unable to diffuse easily across the hydrophobic barrier of membrane * transport proteins speed up the rate of diffusion of these molecules across the membrane * transport proteins are specific for the solute they transport * rate at which substances diffuse into or out of the cell by facilitated diffusion depends on number of available transport proteins and diffusion gradient
58
describe the structure and function of protein carriers
* have specific binding sites which specific solutes bind to * undergo a series of conformatonal changes to transport bound solute across the membrane * can be inhibited by molecules that resemble normal solute it transport * can reach saturation: transporting solutes as fast as they can, transport reaches maximum rate
59
describe the structure and function of protein channels
* form aqueous pores that extend across the lipid bilayer and allow specific solutes to pass through them * part of protein that lines pore is hydrophilic * channels can be gated/ungated * gated channels only open when appropriate stimulus is present * ungated channels open all the time
60
define osmosis
net movement of solvent molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane
61
how do water molecules move across the cell membrane?
* water is polar and in nature and is able to diffuse across the membrane, but at a very low rate * most water molecules move across the CSM via hydrophilic pores found within intrinsic proteins (aquaporins)
62
what happens to an animal cell in a solution with higher water potential (hypotonic)?
water moves into cell, causing cell to lyse
63
what happens to a plant cell in a solution with higher water potential (hypotonic)?
* water enters cell, causing cell to be turgid * cell does not lyse due to presence of cell wall
64
why does the plant cell not burst?
rigid cell walls of plant cells exert a pressure that opposes further entry of water when cell contents start pressing against cell wall
65
what happens to an animal cell in a solution with the same water potential (isotonic)?
no net movement of water
66
what happens to a plant cell in a solution with the same water potential (isotonic)?
* no net movement of water * cell is not turgid, neither is it plasmolsed * at the point of incipient plasmolysis
67
what happens to an animal cell in a solution with lower water potential (hypertonic)?
water leaves cell, causing it to crenate
68
what happens to a plant cell in a solution with lower water potential (hypertonic)?
* water leaves cell and cell becomes flaccid * as cell contents shrivel, CSM is pulled away from cell wall (plasmolysis)
69
define active transport
* transport of substances in or out of the cell regardless of presence of concentration gradient * requires expenditure of energy in the form of ATP and protein carriers * protein carriers are coupled with a source of energy (ATP) to transport substances in and out of the cell againt the concentration gradient * anything that inhibits formation of ATP/prevents ATP from being used stops AT
70
explain the importance of active transport
* enables cell to maintain internal concentrations of small molecules that differ from concentrations in environment * allows cells to take up nutrients when concentration outside cell is low * enables cell to get rid of unwanted substances when concentration is higher outside cell * allows cell to have a higher concentration of potassium and lower concentartion of sodium ions than external environment, this uneven distribution of charged particles on opposite sides of the membrane create a membrane potential across the membrane, enabling the transmission of nerve impulses
71
define endocytosis
intake of mostly large material into a cell by invagination of CSM and its internalisation in a membrane-bound vesicle
72
define exocytosis
cell secretes large molecules by fusion of vesicle carrying these vesicles with CSM
73
state the similarities of endocytosis and exocytosis
* both require expenditure of ATP for formation and transport of vesicles * substances are contained within vesicles so they do not mix with other components of cytosol * vesicles only fuse with specific membranes, controlled transfer of molecules into and out of cell * both depend on membrane fusion, a consequence of membrane fluidity
74
describe pinocytosis
* taking fluid and solutes dissolved in that fluid into the cell via endocytosis * any solute can be taken in * small channels are formed at CS by invagination of CSM and vesicles are pinched off * pinocytic vesicle fuses with lysosome, foreign material digested by hydrolytic enzymes and digested material are absorbed into cytoplasm
75
describe phagocytosis
* ingesting large insoluble particles by endocytosis * phagocytes defend body by ingesting invading microorganisms, help to remove worn-out cells (RBCs) * partices bind to surface of phagocyte * binding induces phagocytic cell to extend pseudopodia that engulfs the particle and fuses at their tips to form a phagocytic vesicle * phagocytic vesicle fuses with lysosomes and ingested material is hydrolysed by hydrolytic enzymes * soluble products of digestion absorbed into surrounding cytoplasm * undigested material either remains in lysosome/removed from cell by vesicle moving to CS and fusing with CSM (exocytosis)
76
describe receptor-mediated endocytosis
* selectively taking up specific molecules by endocytosis * receptor proteins are embedded on CSM, cytoplasmic side of this region of CSM is lined by coat proteins * specific extracellular signal molecule binds to its specific receptor * binding of ligand to its specific receptor triggers invagination of CSM, forming a coated pit which eventually forms an endocytic vesicle * vesicle fuses with lysosome, ligand is digested and digested products are absorbed into cytoplasm while receptors are recycled back to CSM
77
describe exocytosis
* cell secretes certain macromolecules by fusion of vesicles carrying these molecules with CSM * adds membrane and membrane proteins to CSM, compensates for loss of membrane during endocytosis * secretory vesicles containing proteins pinch off from GA, transported along microtubules towards CSM, fuse with CSM and discharge contents to exterior of cll * transport of vesicles along microtubules is an energy-requiring process * after fusion, vesicle membrane becames part of CSM
78
suggest how cell membranes will be damaged by low temperature
* proteins embedded in the cell membrane will be inactivated by low temp and they cannot function * cell membranes begin to solidify as phospholipids are more tightly packed together * phospholipids are unable to move
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