Anatomy of a Cell Flashcards
What are the features of Eukaryotic cells?
- outer membrane
- inner cystol (fluid, gel like)
- cytoskeleton (thin & intermediate filaments and microtubules)
- organelles (membrane bound)
- inclusions
Plasmalemma/Membrane
- 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
Plasmalemma fluidity
- membrane proteins diffuse laterally in membrane
- but, many are anchored: not distributed equally
- selectively permeable (to H2O, O2… not charged ions)
Organelles
- small, intracellular organs
- essential to life
- e.g. mitochondria, nucleus, RER, SER, Golgi apparatus, lysosomes
Inclusions
- don’t necessarily have membrane
- are dispensable, may only be present as transients
Cytoskeleton
- 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
Microfilaments
- actin molecules assemble into filaments
- can later dissociate
- very dynamic
Intermediate Filaments
- bind intracellular elements together
- bind to plasmalemma
- network throughout cytoplasm
Microtubules
- hollow tubule
- composed of tubulin subunits: α, β
- can be assembled and disassembled
- originate in centrosome
- include microtubule-associated proteins
Microtubules (motorway network)
- 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
Centrosome
- microtubules originate
- at core, contains centrioles
- centrioles are mainly composed of specialized microtubule segments
Nucleus
-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
Ribosomes
- 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
Endoplasmic Reticulum
- network of interconnecting network bound compartments
- Rough Endoplasmic Reticulum (studded with ribosomes, vital in protein synthesis)
- Smooth Endoplasmic Reticulum
- more ER, more metabolically active
Polysomes
- where proteins that are to remain in the cytosol (unpackaged) are synthesised
- polysomes float free within cytosol
Protein Synthesis
- 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
Rough Endoplasmic Reticulum (RER)
- associated with protein synthesis
- associated with initiation of glycoprotein formation
(glycoproteins have carbs attached to their polypeptide chains)
Smooth Endoplasmic Reticulum (SER)
- continues processing of proteins from RER
- vital role at site of lipid synthesis
- most cells contain little SER
Golgi Apparatus
- transport vesicles arrive at Golgi from SER/RER
- Golgi cisterns modify and package macromolecules (which were synthesised in ER)
Mitochondria
- 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
Intercellular Junctions
- 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
Intercellular Junctions: Occluding
- prevent diffusion
- appear as focal region of close apposition between adjacent membranes
- also known as: tight junctions / zonula occludens
Intercellular Junctions: Anchoring
- 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
Intercellular Junctions: Junctional Complex
-close association of several types of junction found in epithelial tissues
Intercellular Junctions: Communicating
- 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
Transport Into/Out of Cell
-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
Transport: Endocytosis
- 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
Transport: Phagocytosis
- 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
