Cell Structure & Membrane Flashcards
Describe the cell theory.
- *Cell is the fundamental unit of struc & function in living organisms
- *All living things are made up of cells
- Activity of an organism depends on the total activity of independent cells.
- Metabolism occurs within cells and it involves biochemical reactions.
- *All cells arise from pre-existing cells by division
- Cells contain hereditary information (DNA) which is passed from parent cell to daughter cells during cell division.
- In organism of similar species, the cells are made up of similar chemical composition
What is the role of the nucleus (general)?
- Contains hereditary material (DNA)
- Controls cellular activities by regulating transcription and thus protein synthesis
What is the structure and role of the nuclear envelope?
Double membrane perforated with nuclear pores.
- selectively permeable; regulates passage of substances into or out of nucleus
- compartmentalization→ protect DNA from degradation/enzyme hydrolysis
What is the role of the nucleolus?
- Site of synthesis of rRNA
- Site of assembly of ribosomal subunits = rRNA + ribosomal proteins (from cytoplasm)→ exported out of nucleus into cytoplasm
What is the role of chromatin?
Carry genes that code for polypeptides
What is the role of ER (general)?
- 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
What is the role of RER?
(- 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
What is the structure of RER?
- Flattened membranous sacs called cisternae
- studded with ribosomes on outer surface
- continuous with outer membrane of the nuclear envelope
What is the structure of SER?
- Membranous tubular sacs called cisternae
- no bound ribosomes on outer sf
What is the role of SER?
- synthesise lipids & carbs
- Detoxify drugs and poison
- SER called sarcoplasmic reticulum stores Ca2+
What is the role of the GA?
- Glycosylate proteins and lipids→ glycoproteins & glycolipids
- Modify existing glycoprotein/glycolipids made by ER: modify/cleaving existing sugar chain
- Synthesis of hydrolytic enzymes for/and lysosomes
- Synthesise polysaccharides eg pectin (in cell wall)→ transported in vesicles to cell membrane
- Sort & package completed materials into different vesicles→ target proteins to diff parts of the cell or for secretion out of cell (exocytosis)
What is the role of lysosomes?
- 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 - Release hydrolytic enzymes outside cell via exocytosis for extracellular digestion
- Fuse w autophagic vesicles containing worn-out/unwanted organelles to digest them & recycle the organic products
- Autolysis: contents released within cell, creating an acidic environment, cell undergoes self-digestion due to work of hydrolytic enzymes
Compare the structure of chloroplasts and mitochondria?
- 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
What is the function of the chloroplast?
Site of photosynthesis: chl converts solar energy to chemical energy via LD rxn in thylakoid membrane & LID in stroma
What is the role of the mitochondria?
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
What is the role of ribosomes?
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
What is the structure of centrioles?
Pair of hollow cylinders, each w 9 triplets of microtubules & positioned at right angles to each other
What is the role of centrioles?
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
What is the cytoskeleton made up of?
Tubulin (grow by adding tubulin subunits)
What is the role of microtubules?
- 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
What is meant by the fluid mosaic model?
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
How does each of these affect membrane fluidity?
- Ratio of saturated to unsaturated HC chains
- length of fatty acid
- Presence of cholesterol
- 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
Describe the arrangement of p.lipid bilayer.
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.
Briefly describe the role of p.lipids in membranes
- 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
How is cholesterol embedded in membrane?
-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
What is the function of cholesterol?
- 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
How do membranes hold onto proteins?
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
How are proteins embedded in membranes?
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
What is the role of proteins in membranes?
- 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
What is the role of glycoproteins in membranes?
- 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
List the functions of membranes.
- 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
Describe the role of membranes: cell-cell recognition & signal transduction
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
Describe the role of membranes: regulate movement os substances across membrane
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
Why is compartmentalisation impt?
- 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
Describe the role of membranes: Localisation of proteins of a related function along a membrane
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
Define simple diffusion
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)
How do these factors affect simple diffusion?
- mlc size
- solubility in h.core
- steepness of conc gradient
- temp
- sf area
- distance
- 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
Define facilitated diffusion
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)
What are the factors affecting rate of FD?
↑ [substance] = ↑ chance of collision between transport protein & solute → ↑ rate
↑ no. of transport proteins→ ↑ rate
Why does the graph of rate against conc plateau off at high solute conc? (FD)
Saturation point, conc ↑ but rate doesn’t ↑: max facilitated diffusion demonstrates saturation kinetics; all transport proteins in use
Define osmosis
Net movement of water from region of higher wp to region of lower wp, down a wp gradient, through a selectively permeable membrane
Define active transport
Energy (ATP)-consuming transport of polar/charged mlcs/ions across a membrane, against conc gradient, through transmembrane carrier proteins called pumps
What is endocytosis?
infolding or extension of cell sf membrane to form a vesicle (endosome) or vacuole→ allow cell to acquire macromolecules and particulate matter respectively
Describe the process of phagocytosis
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
Describe the process of pinocytosis.
Small area of plasma membrane invaginates→ cell membrane fuses, forming vesicle containing liquids in cell
Why is receptor-mediated endocytosis impt?
cell acquire large quantities of specific substances/ ligand, even though [ligand] in extracellular fluid may not be very high
Describe how receptor-mediated endocytosis works
- Specific molecule, ligand, binds to specific protein receptor on membrane→ ligand-receptor complexes
- Membrane invaginates & fuses→ vesicles containing ligand-receptor complexes in cell
Define exocytosis
secretion of macromolecules (e.g. waste materials, insulin in pancreas) to exterior of cell by fusion of vesicle membrane w plasma membrane
What is ATP used for in bulk transport
- Invagination/Extend membranes around materials to form vacuoles (ATP to rearrange cytoskeleton)
- Move vesicles within cell
