Anatomy of a Cell

Question 1

What are the features of Eukaryotic cells?

Answer 1

• outer membrane
• inner cystol (fluid, gel like)
• cytoskeleton (thin & intermediate filaments and microtubules)
• organelles (membrane bound)
• inclusions

Question 2

Plasmalemma/Membrane

Answer 2

• bimolecular layer of amphipathic phospholipid molecules
• contains integral proteins (cell inserts into membrane)
  e. g. receptors, transporters, channels, enzymes, cell attachment proteins
• also contains peripheral proteins and cholesterol
• endocytose and exocytose through membrane

Question 3

Plasmalemma fluidity

Answer 3

• membrane proteins diffuse laterally in membrane
• but, many are anchored: not distributed equally
• selectively permeable (to H2O, O2… not charged ions)

Question 4

Organelles

Answer 4

• small, intracellular organs
• essential to life
• e.g. mitochondria, nucleus, RER, SER, Golgi apparatus, lysosomes

Question 5

Inclusions

Answer 5

• don’t necessarily have membrane

- are dispensable, may only be present as transients

Question 6

Cytoskeleton

Answer 6

• filamentous cytoskeletal proteins
  1. Microfilaments (actin protein)
  2. Intermediate filament
  3. Microtubules (tubulin protein)
• attach to cell membrane/each other, by anchoring/joining proteins = scaffolding

Question 7

Microfilaments

Answer 7

• actin molecules assemble into filaments
• can later dissociate
• very dynamic

Question 8

Intermediate Filaments

Answer 8

• bind intracellular elements together
• bind to plasmalemma
• network throughout cytoplasm

Question 9

Microtubules

Answer 9

• hollow tubule
• composed of tubulin subunits: α, β
• can be assembled and disassembled
• originate in centrosome
• include microtubule-associated proteins

Question 10

Microtubules (motorway network)

Answer 10

• polar
• the proteins dynein and kinesin attach to microtubules, move along them
• dynein and kinesin associate with membrane of organelles, drag them
• kinesin: moves towards cell periphery
  dynein: (direct) towards cell centre

Question 11

Centrosome

Answer 11

• microtubules originate
• at core, contains centrioles
• centrioles are mainly composed of specialized microtubule segments

Question 12

Nucleus

Answer 12

-enclosed by nuclear envelope (inner/outer nuclear membrane, contain pores)
-inner membrane, perinuclear cistern, outer membrane
-outer membrane: ribosomes (is continuous with RER)
-contains chromosomes, RNA synthesis location
(mRNA, tRNA transcribed here. rRNA transcribed in nucleolus)
-contains euchromatin: more dispersed DNA, actively undergoing transcription
-contains heterochromatin: more condensed, not undergoing transcription

Question 13

Ribosomes

Answer 13

• formed in nucleolus
• instrumental in protein synthesis
• made up of small subunit (binds RNA) & large subunit (catalyses peptide bond formation)
• export of ribosomes:depends on nuclear core complex

Question 14

Endoplasmic Reticulum

Answer 14

• network of interconnecting network bound compartments
• Rough Endoplasmic Reticulum (studded with ribosomes, vital in protein synthesis)
• Smooth Endoplasmic Reticulum
• more ER, more metabolically active

Question 15

Polysomes

Answer 15

• where proteins that are to remain in the cytosol (unpackaged) are synthesised
• polysomes float free within cytosol

Question 16

Protein Synthesis

Answer 16

• free ribosome attaches to mRNA
• growing peptide chain is inserted into pore in ER, if ER signal peptide sequence is present
• growing peptide forms in ER, so signal sequence is removed
• Ribosome detaches, most proteins then enter SER

Question 17

Rough Endoplasmic Reticulum (RER)

Answer 17

• associated with protein synthesis
• associated with initiation of glycoprotein formation

(glycoproteins have carbs attached to their polypeptide chains)

Question 18

Smooth Endoplasmic Reticulum (SER)

Answer 18

• continues processing of proteins from RER
• vital role at site of lipid synthesis
• most cells contain little SER

Question 19

Golgi Apparatus

Answer 19

• transport vesicles arrive at Golgi from SER/RER

- Golgi cisterns modify and package macromolecules (which were synthesised in ER)

Question 20

Mitochondria

Answer 20

• oblong, cylindrical organelles (contained in all cells)
• contain own DNA
• outer & inner membrane
• inner membrane is extensively folded = cristae (increases surface area)
• generate ATP, synthesise certain lipids and proteins

Question 21

Intercellular Junctions

Answer 21

• specialized membrane structure
• link individual cells together (form functional unit)
  1. Occluding Junctions: form a diffusion barrier
  2. Anchoring Junctions: provide mechanical strength
  3. Communicating Junctions: allow movement of molecules between cells

Question 22

Intercellular Junctions: Occluding

Answer 22

• prevent diffusion
• appear as focal region of close apposition between adjacent membranes
• also known as: tight junctions / zonula occludens

Question 23

Intercellular Junctions: Anchoring

Answer 23

• adherent junctions (link submembrane actin bundles of adjacent cells)
• transmembrane cadherin molecules bind to each other, and to the actin of cytoskeleton
• also known as: zonula adherens
• Desmosomes/macula adherens: link submembrane intermediate filaments of adjacent cells

Question 24

Intercellular Junctions: Junctional Complex

Answer 24

-close association of several types of junction found in epithelial tissues

Question 25

Intercellular Junctions: Communicating

Answer 25

- allow selective diffusion of molecules between adjacent cells - also known as: gap junctions - is a circular patch studded with hundreds of pores - pores produced by "connexon" proteins - found in epithelia, some smooth and cardiac muscle: is critical for spread of excitation

Question 26

Transport Into/Out of Cell

Answer 26

-material moves across cell membrane in variety of ways: Diffusion Via transport proteins (pumps or channels) ``` Vesicular transport (incorporation into vesicles) e.g. endocytosis, phagocytosis ```

Question 27

Transport: Endocytosis

Answer 27

- material from extracellular space can be incorporated into cell by endocytosis - cell membrane invaginates, fuses: newly made endocytotic vesicle (endosome) buds into the cell - this process is often receptor mediated -exocytosis works in reverse fashion to discharge material

Question 28

Transport: Phagocytosis

Answer 28

- bacteria (or larger particulate material) from the extracellular space can be incorporated into cell by phagocytosis: - bacterium binds to cell surface receptors - triggers extensions of cell to engulf it - forms phagosome - phagosome binds with lysosome carrying digestive enzymes - produces phagolysosome