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Alevel Biology > Cell Structure & Membrane > Flashcards

Cell Structure & Membrane Flashcards

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1
Q

Describe the cell theory.

A
  1. *Cell is the fundamental unit of struc & function in living organisms
  2. *All living things are made up of cells
  3. Activity of an organism depends on the total activity of independent cells.
  4. Metabolism occurs within cells and it involves biochemical reactions.
  5. *All cells arise from pre-existing cells by division
  6. Cells contain hereditary information (DNA) which is passed from parent cell to daughter cells during cell division.
  7. In organism of similar species, the cells are made up of similar chemical composition
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2
Q

What is the role of the nucleus (general)?

A
  • Contains hereditary material (DNA)

- Controls cellular activities by regulating transcription and thus protein synthesis

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3
Q

What is the structure and role of the nuclear envelope?

A

Double membrane perforated with nuclear pores.

  • selectively permeable; regulates passage of substances into or out of nucleus
  • compartmentalization→ protect DNA from degradation/enzyme hydrolysis
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4
Q

What is the role of the nucleolus?

A
  • Site of synthesis of rRNA
  • Site of assembly of ribosomal subunits = rRNA + ribosomal proteins (from cytoplasm)→ exported out of nucleus into cytoplasm
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5
Q

What is the role of chromatin?

A

Carry genes that code for polypeptides

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6
Q

What is the role of ER (general)?

A
  • Compartmentalisation: ER lumen/cisternal space separated from cytosol
  • Membrane factory for the cell: grows its own membrane by adding proteins (synthesised by RER) & phospholipids (synthesised by SER) to it→ ER membrane transported in the form of transport vesicles to other components of Endomembrane system
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7
Q

What is the role of RER?

A

(- translation of mRNA into proteins/polypeptides by bound ribosomes)

  • Allow proteins to fold into their specific 3D conformation & glycosylate them / modification→ glycoproteins
  • Transports proteins synthesised by ribosomes its sf to cis face of GA via transport vesicles that bud off from RER
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8
Q

What is the structure of RER?

A
  • Flattened membranous sacs called cisternae
  • studded with ribosomes on outer surface
  • continuous with outer membrane of the nuclear envelope
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9
Q

What is the structure of SER?

A
  • Membranous tubular sacs called cisternae

- no bound ribosomes on outer sf

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10
Q

What is the role of SER?

A
  • synthesise lipids & carbs
  • Detoxify drugs and poison
  • SER called sarcoplasmic reticulum stores Ca2+
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11
Q

What is the role of the GA?

A
  1. Glycosylate proteins and lipids→ glycoproteins & glycolipids
  2. Modify existing glycoprotein/glycolipids made by ER: modify/cleaving existing sugar chain
  3. Synthesis of hydrolytic enzymes for/and lysosomes
  4. Synthesise polysaccharides eg pectin (in cell wall)→ transported in vesicles to cell membrane
  5. Sort & package completed materials into different vesicles→ target proteins to diff parts of the cell or for secretion out of cell (exocytosis)
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12
Q

What is the role of lysosomes?

A
  1. Fuse w endocytic vesicles: digest materials taken in by the cell; Fuse w phagocytic vesicle, destroying bacteria & foreign particles
    > Useful products absorbed and assimilated into cytoplasm, unwanted products released into external medium by exocytosis
  2. Release hydrolytic enzymes outside cell via exocytosis for extracellular digestion
  3. Fuse w autophagic vesicles containing worn-out/unwanted organelles to digest them & recycle the organic products
  4. Autolysis: contents released within cell, creating an acidic environment, cell undergoes self-digestion due to work of hydrolytic enzymes
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13
Q

Compare the structure of chloroplasts and mitochondria?

A
  • Size: larger vs smaller
  • Shape: lens shaped vs rod/spherical shaped
  • membrane: double membrane, intermembrane space
  • inner membrane: not folded, does not contain ATP synthase vs highly infolded to form numerous cristae, contain ATP synthase–> larger sf area for OP enzymes
  • inner membrane encloses fluid-filled cavity: 70S ribosomes, circular DNA, ATP synthase, enzymes
  • granules: starch vs phosphate
  • ATP synthase: thylakoid membrane vs inner membrane
  • photosynthetic pigments: present in thylakoid vs absent
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14
Q

What is the function of the chloroplast?

A

Site of photosynthesis: chl converts solar energy to chemical energy via LD rxn in thylakoid membrane & LID in stroma

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15
Q

What is the role of the mitochondria?

A

Site for aerobic respiration to release energy in the form of ATP

  • highly folded inner mitochondrial membrane to ↑ sf area for OP
  • matrix: site for link rxn & Krebs cycle
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16
Q

What is the role of ribosomes?

A

Site of protein synthesis, translation of mRNA to protein
- Freely floating in cytosol→ proteins function within cytosol
- Bound to ER→ proteins for
> secretion out of cell (eg digestive enzymes, insulin)
> packaging into certain organelles
> insertion into plasma membrane

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17
Q

What is the structure of centrioles?

A

Pair of hollow cylinders, each w 9 triplets of microtubules & positioned at right angles to each other

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18
Q

What is the role of centrioles?

A

During nuclear division:
- Position of pair of centrioles determines polarity of cell - Act as microtubule organising centre (MTOC) during spindle formation: organise synthesis of spindle fibres, which separate chromosomes/sister chromatids after centromere divides

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19
Q

What is the cytoskeleton made up of?

A

Tubulin (grow by adding tubulin subunits)

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20
Q

What is the role of microtubules?

A
  • Maintain shape of cells
  • Intracellular transport: Serve as tracks along which organelles equipped with motor proteins can move
  • Form spindle fibres, which move chromosomes to opposite poles in nuclear division
21
Q

What is meant by the fluid mosaic model?

A

Fluid: p.lipids & proteins held by weak hydrophobic interactions→ p.lipids and proteins free to move laterally in a layer, in a dynamic membrane
Mosaic: random arrangement of proteins, embedded amongst the dynamic p.lipid molecules, resemble a mosaic pattern

22
Q

How does each of these affect membrane fluidity?

  • Ratio of saturated to unsaturated HC chains
  • length of fatty acid
  • Presence of cholesterol
A
  • saturated→ pack more closely, more hydrophobic interactions between phospholipids→ ↑ membrane viscosity
  • higher proportion of p.lipids w unsaturated HC tail→ kinks due to cis C=C double bond, p.lipid can’t pack closely→ less hydrophobic interactions→ more fluid, freeze at lower temp
  • longer fatty acid = more hydrophobic interactions
  • Presence of cholesterol in membrane
23
Q

Describe the arrangement of p.lipid bilayer.

A

2 layers of phospholipid molecules in a bilayer.

Non-polar hydrophobic HC tails facing inwards of the membrane away from aq medium, forming hydrophobic interactions with the HC tails of other p.lipids→ hydrophobic core in bilayer structure.

Charged, hydrophillic phosphate heads will form H bonds w water & face outwards next to aq medium of cell interior/ exterior.

24
Q

Briefly describe the role of p.lipids in membranes

A
  • Regulates movement of substances moving in/out of cell, by acting as a barrier to ion, polar and large mlcs.
  • Boundary between intracellular and extracellular aq environment
  • allows compartmentalisation
25
Q

How is cholesterol embedded in membrane?

A

-OH grp aligns and interacts w charged phosphate head of p.lipid, rest interacts w and tucked into hydrophobic core of membrane via hydrophobic interactions

26
Q

What is the function of cholesterol?

A
  • Regulate membrane fluidity (i.e. stabilises membrane)
    Prevent it from being overly fluid at high temp (↑KE, move more): Restricts phospholipid movement through its (hydrophobic) interactions w phospholipids
    Prevent it from being overly firm at low temp: Prevents close packing of phospholipids
  • stabilise p.lipid bilayer, due to van der Waals interactions between lipid bilayer and the rigid fused ring struc
27
Q

How do membranes hold onto proteins?

A

non-polar, hydrophobic HC chains of p.lipid bilayer form hydrophobic interactions w non-polar hydrophobic R groups of aa found on exterior surface of the channel protein

charged hydrophilic phosphate head of phospholipid bilayer interact w charged/polar R groups of aa found on exterior surface of channel protein

28
Q

How are proteins embedded in membranes?

A

Synthesised & modified/glycosylated (for proteins) in ER→ embedded in ER membrane→ transported in vesicles to GA: further modification of glycoproteins; lipids glycosylated to form glycolipids→ Transported in vesicles to plasma membrane→ Vesicles fuse w membrane→ glycoproteins & glycolipids positioned in the plasma membrane

29
Q

What is the role of proteins in membranes?

A
  • Function as channels/carriers for facilitated diffusion and AT
    Channel proteins: hydrophilic channel/pore lined w aa w polar/charged, hydrophilic R grps→ direct diffusion of ions/mlcs across membrane, from high to low solute conc (function) e.g. aquaporins
    Carrier proteins:
    > specific solute binds to hydrophilic interior on one side of membrane→ conformational change→ solute hv access to opposite side of membrane (e.g. glucose transporter)
    > Some are pumps: use ATP to move solutes against a conc gradient
  • Function as enzymes
  • Function as receptor protein in signal transduction, ligand will bind to it
  • Stabilise membrane struc: non-covalently bonded to cytoskeleton/ extracellular matrix
30
Q

What is the role of glycoproteins in membranes?

A
  • Markers/recognition sites in cell-cell recognition & hence adhesion
  • Cell receptor: provide a point of attachment/binding sites for hormones in cell-signaling / pathogens and toxins→ gain entry into cell
31
Q

List the functions of membranes.

A
  • cell-cell recognition
  • signal transduction
  • regulate movement of substances across membrane
  • compartmentalisation
  • increase sf area for chemical rxns
  • localisation of proteins of related function along membrane
32
Q

Describe the role of membranes: cell-cell recognition & signal transduction

A

Cell-cell recognition: differentiate cells as ‘self’ or ‘non-self’ as the basis of the immune system→ cell adhesion: membrane protein of adjacent cells hook tgt→ tissues & organs (eg activation of B cell by T-helper cell)

Signal transduction: transfer info from environment into cell when ligands (specific molecules) bind to specific receptor→ cellular response
E.g. glucagon binds to glucagon receptor→ triggers chemical rxns in cell→ activate enzyme hydrolysing glycogen to glucose

33
Q

Describe the role of membranes: regulate movement os substances across membrane

A

Selectively permeable membrane act as a boundary between inside and outside of cell, between organelle and cytoplasm, & between compartments within an organelle→ compartmentalisation possible

Small, non-polar, hydrophobic molecules dissolve & directly diffuse through hydrophobic core of p.lipid bilayer
Hydrophobic core of p.lipid bilayer serves as a barrier and repels large mlcs or polar/charged mlcs/ions→ Need to be transported by transport proteins

34
Q

Why is compartmentalisation impt?

A
  • Form unique environments for highly specialised activities
  • Spatial separation of biochemical processes→ hence allows for their sequential operation in a cell
  • Accumulation of charged ions in high conc→ form chemical gradients across membranes
35
Q

Describe the role of membranes: Localisation of proteins of a related function along a membrane

A

functionally-related proteins grouped tgt so that sequential biochemical processes can occur
E.g. enzymes and proteins are grouped into PSII and PSI on thylakoid membrane of chloroplast; electrons from PS II are shuttled to PSI during photophosphorylation

36
Q

Define simple diffusion

A

Net movement of (small, non-polar, hydrophobic) mlcs/ions from a region of higher conc to region of low conc, down a conc gradient (directly through the membrane; wout transport proteins and ATP)

37
Q

How do these factors affect simple diffusion?

  • mlc size
  • solubility in h.core
  • steepness of conc gradient
  • temp
  • sf area
  • distance
A
  • Small molecular size→ ↑ rate
  • More soluble in non-polar, hydrophobic core of p.lipid bilayer→ ↑ rate
  • Steeper conc gradient→ ↑ rate
  • Higher temp→ ↑ KE of p.lipid, membrane highly fluid, more transient pores→ ↑ rate
  • ↑ Sf of cell membrane = ↑transient pores → ↑ rate
  • ↑ Distance = ↓ rate
38
Q

Define facilitated diffusion

A

Net movement of polar, charged mlcs & ions from a region of high conc to a region of low conc, down a conc gradient, through a transport protein (across membrane)

39
Q

What are the factors affecting rate of FD?

A

↑ [substance] = ↑ chance of collision between transport protein & solute → ↑ rate
↑ no. of transport proteins→ ↑ rate

40
Q

Why does the graph of rate against conc plateau off at high solute conc? (FD)

A

Saturation point, conc ↑ but rate doesn’t ↑: max facilitated diffusion demonstrates saturation kinetics; all transport proteins in use

41
Q

Define osmosis

A

Net movement of water from region of higher wp to region of lower wp, down a wp gradient, through a selectively permeable membrane

42
Q

Define active transport

A

Energy (ATP)-consuming transport of polar/charged mlcs/ions across a membrane, against conc gradient, through transmembrane carrier proteins called pumps

43
Q

What is endocytosis?

A

infolding or extension of cell sf membrane to form a vesicle (endosome) or vacuole→ allow cell to acquire macromolecules and particulate matter respectively

44
Q

Describe the process of phagocytosis

A

Filaments in cytoskeleton rearrange w utilisation of ATP, forming pseudopodia, which extends outwards to engulf large insoluble macromolecules.
Ends of pseudopodia fuse→ vacuole containing solids pinch off, moves into cytoplasm

45
Q

Describe the process of pinocytosis.

A

Small area of plasma membrane invaginates→ cell membrane fuses, forming vesicle containing liquids in cell

46
Q

Why is receptor-mediated endocytosis impt?

A

cell acquire large quantities of specific substances/ ligand, even though [ligand] in extracellular fluid may not be very high

47
Q

Describe how receptor-mediated endocytosis works

A
  • Specific molecule, ligand, binds to specific protein receptor on membrane→ ligand-receptor complexes
  • Membrane invaginates & fuses→ vesicles containing ligand-receptor complexes in cell
48
Q

Define exocytosis

A

secretion of macromolecules (e.g. waste materials, insulin in pancreas) to exterior of cell by fusion of vesicle membrane w plasma membrane

49
Q

What is ATP used for in bulk transport

A
  • Invagination/Extend membranes around materials to form vacuoles (ATP to rearrange cytoskeleton)
  • Move vesicles within cell

Alevel Biology (16 decks)

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