Sub Cellular Organelles

Question 1

Ribosome structure

Answer 1

Macromolecular structure 20x30 nm in size

Two subunits bound to strand of mRNA- core of the ribosomal subunit is a highly folded rRNA chain associated with more than 30 unique proteins.

Large subunit formed of 3 chains of rRNA and 50 other basic proteins.

Question 2

Ribosome functions

Answer 2

Assemble polypeptides from amino acids using tRNA and mRNA. Provide structural support and position tRNAs in the correct reading frame.

Peripheral proteins seem to stabilise the catalytic core.

Question 3

What is a polyribosome + function?

Answer 3

Many ribosomes bind to mRNA, increase the speed of transcription.

Polysomes bind to the endoplasmic reticulum translate mRNAs coding for membrane proteins of the ER.

Question 4

Rough Endoplasmic reticulum structure

Answer 4

Network of membrane-limited channels called cistern that extends from the nucleus throughout most of the cytoplasm.

Covered in polyribosomes that synthesise proteins.

parallel stacks of flattened cisternae

Question 5

Rough ER location

Answer 5

prominent in cells specialised for protein secretion. Pancreatic acing cells, fibroblasts and plasma cells.

Question 6

Rough ER function

Answer 6

production of membrane associated proteins. Primary glycosylation and other postranslational modifications.

ER-associated degradation of proteins that cannot be assembled or folded using chaperones- unsalvageable proteins translocated back to cytosol, conjugated to ubiquitin and then degraded by proteasome.

Question 7

Smooth endoplasmic reticulum structure

Answer 7

cistern are more tubular and sac like. Found continuous with the RER, no polyribosomes present. Interconnect channels

Question 8

Smooth endoplasmic reticulum functions (3)

Answer 8

enzymes in SR perform synthesis of phospholipids and steroids

other SER enzymes, including those of the cytochrome P450 family, allow detoxification of potentially harmful exogenous materials such as alcohol

SER vesicles responsible for sequestration and controlled release of Ca2+- assumes a specialised form called sarcoplasmic reticulum

Question 9

Golgi apparatus structure

Answer 9

Multi-layered membranous organelle near the nucleus. Consists of many smooth membranous saccules, some vesicular and others flattened. All contain enzymes and proteins being processed.

Contain different levels- cis, medial and trans

Question 10

Golgi apparatus function

Answer 10

completes postranslation modifications that began in the endoplasmic reticulum. Packages and addresses the proteins to their proper destinations.

Question 11

Different directions of vesicle transport

Answer 11

Vesicles moving towards the Golgi fuse with the cis face, vesicles leaving the Golgi come from the trans face

Question 12

Lysosome structure

Answer 12

Membrane limited vesicles that contain about 40 different hydrolytic enzymes.

Question 13

Lysosome function

Answer 13

Sites of intracellular digestion.

During digestion of macromolecules, released nutrients diffuse into the lysosome. Indigestible material is retained in a small vesicular body called a residual body.

Autophagy- degrade excess organelles

Question 14

Endosome structure

Answer 14

membrane bound vesicle that originates at the trans Golgi surface- contains molecules on the way to lysosomal degradation or recycle back to golgi apparatus. Formed of multivesicular bodies, fuse with lysosomes.

Question 15

endoscope function

Answer 15

provide an environment for material to be sorted before it reaches degradative lysosomes.

Question 16

peroxisome structure

Answer 16

spherical organelles enclosed by a single membrane . Contain enzymes producing and degrading hydrogen peroxide.

Question 17

Peroxisome function

Answer 17

contain oxidases that remove hydrogens and add to oxygen forming hydrogen peroxide

Contain catalases that break down hydrogen peroxide.

Question 18

Mitochondria structure

Answer 18

two membranes- outer membrane and inner folded membrane called a cristae. Outer membrane contains many transmembrane proteins called porins that form channels that allow small molecules, such as pyruvate to move from the cytoplasm to the inter membrane space.

Matrix containing enzymes specialised for aerobic respiration

Question 19

Mitochondria function

Answer 19

aerobic respiration producing ATP

Role in apoptosis- releases cytochrome C from intermembrane space which initiates apoptosis.

Question 20

What is ubiquitin + function?

Answer 20

Small regulatory protein that tags proteins for breakdown

Question 21

Cytoskeleton definition

Answer 21

Complex arrangement of microtubules, microfilaments and intermediate filaments that are present in the cytoplasm.

Question 22

Microtubule structure

Answer 22

• 25nm in diameter
• long hollow cylinders made of alpha and beta tubulin
• rigid and straight
• contain microtubule organising centre (MTOC)

Question 23

Microtubule functions

Answer 23

Stabilise cell shape due to rigid structure

form the tracks for kinesin based transport- away from cell nucleus and dynein transport towards the nucleus

Question 24

How do microtubules grow?

Answer 24

Two ends, one plus and one minus end.

Growth is faster at the plus end. Energy for growth is derived from GTP bound to the incoming tubule subunits.

Question 25

Explain dynamic instability

Answer 25

Continuous cycles of polymerisation and depolymerisation. Microtubules decrease in length if depolymerisation exceeds growth.

Question 26

What are MAPs + function?

Answer 26

Microtubule association proteins that bind to and stabilise microtubules to enhance stability. Bind more strongly in adults, allowing neuronal stability- more loosely in embryonic development.

Question 27

What are MTOCs and what occurs?

Answer 27

Microtubule Organisation Centres. Minus end embedded in the in the MTOC- plus end sticks out and grows- example of a centriole and the microtubules are the spindle fibres.

Question 28

Microfilament structure + location

Answer 28

Composed of flexible G actin that assembles in the presence of potassium and magnesium ions into a double stranded helix of F actin.
Concentrated beneath plasma membrane

plus end- fast growth and shrinkage
minus end- slow growth and shrinkage

Question 29

Microfilament functions

Answer 29

allow cell movement, allowing neutrophils to hunt bacteria- tread milling due to growing actin

protruding actin filament causes crawling cell, to engulf another cell

involved in muscle movement

myosin motors use F actin to transport cargo, usually towards + end

Question 30

Intermediate filament structure + function

Answer 30

10-12 nm in diameter, stable.

subunits often coiled, rodlike dimers that form antiparallel tetramers which self assemble into cable like bundles

increase mechanical stability in cell structure

Question 31

Examples of intermediate filaments, their location and function

Answer 31

Keratin- form large bundles called tonofibrils which attach to different cell junctions. Also present in stratum corneum preventing dehydration of the skin. keratin networks held by dilsulphide bridges

neurofilament- main intermediate filament in neurons, maintain axon shape. Form crosslinks to provide axon with tensile strength. Level of neurofilament expression controls the diameter of the axon, thus conductance velocity.

lamin- present in cell nucleus, form structural network called the nuclear lamina- provide anchoring point for chromosomes.

Question 32

metabolic functions of peroxisomes

Answer 32

breakdown very long chain fatty acids through beta oxidation, before shuttling to mitochondria

Question 33

2 main processes in peroxisome

Answer 33

enzymes that oxidise compounds, forming hydrogen peroxide

hydrogen peroxide is then converted by a catalase into water and oxygen, to prevent the formation of free radicals