Eukaryotic cell structure Flashcards
What is the function of the eukaryotic organelles?
They provide specialised environments.
Facilitate processes with different chemical requirements
How do the organelles work?
They cooperate (protein trafficking) or act autonomously to accomplish a given function
How many compartments in an organelle?
An organelle has one or more compartments
What are the main eukaryotic organelles?
Cytoplasm/cytosol
Mitochondria
Lysosomes
Describe cytoplasm as an organelle
- cytoplasm = area contained by the plasma membrane+ everything excluding the nucleus
- contains many membrane bound compartments (these membrane facilitate distinct environments within the organelle)
Cytosol definition and composition and function
- cytosol=largest compartment of the cell
- composition: aqueous gel, densely packed molecules
- function –> site of many chemical reactions: EARLY STEPS IN NUTREINTS BREAK DOWN, ATP GENERATION (ANAEROIC GLYCOLYSIS), MANUFACTURING OF PROTEINS, SIGNAL TRANSDUCTION
MITOCHONDRIA
- where is it present
- structure
- Present in all eukaryotic cells
- function: cellular respiration (pyruvate oxidation) ; Mitochondria play an important role in cell death mechanisms
- STRUCTURE: cylinders reassembling bacteria, exhibit high degree of plasticity and movement. Mitochondria remain fixed in some cells associated at sites requiring most energy
How many mitochondria present in a cell?
- Number of mitochondria can vary within a cell and between cell type
- Liver ~1000-2000/cell; 20% cell volume
- Numbers can increase in muscle cells
What are the components of mitochondria?
- outer membrane
- intermembrane space
- inner membrane
- matrix
Function and structure of outer membrane in mitochondria
- separation of external and internal environments
- role in cell death (disruption of outer membrane allows proteins from intermembrane space to leak into cytosol )
- structure: similar to plasma membrane
Function and structure of intermembrane space in mitochondria
- contains cytochrome C: essential component of the electron transport chain as it carries an electron
- Function: Important in apoptosis, Cytochrome C release from mitochondria into cytosol induces a series of biochemical reactions that result in cell death
Inner membrane in mitochondria: structure and function
- STRUCTURE: Highly folded to form cristae –> Increase surface area of inner membrane (5x vs. outer membrane)–> enhance ability to produce ATP
- Mitochondria of cells with high ATP demand contain more cristae c.f. typical mitochondria (EG MUSCLE CELLS VS SKIN CELLS)
Structure and function of matrix in mitochondria
- Contains most of mitochondrial proteins/enzymes (Pyruvate and fatty acid oxidation, citric acid cycle)
- contains mitochondrial ribosomes, tRNA and mtDNA and several copies of the mitochondrial genome
- 37 total genes that encode: tRNA and rRNA and Inner membrane proteins
Lysosomes
- what are they?
- structure
- where are they produced from?
- Principal sites of intracellular digestion
- Produced from Golgi
- Contain many hydrolytic enzyme (lipases, Carbohydrases, Nucleases, Proteases )
- pH dependent activity
Function of lysosomes
- digest macromolecules from: phagocytosis (external pathogens); endocytosis (receptor recycling); autophagy (excess/old organelles)
- specialised environment for enzyme function
Phagocytosis definition
- Phagocytosis is a specific form of endocytosis by which cells internalise solid matter, including microbial pathogens.
- Phagocytes of the immune system include macrophages, neutrophils and dendritic cells
Endocytosis definition
Endocytosis is uptake of macromolecules and particles from the surrounding medium. Internalized material is surrounded by an area of plasma membrane, that form a vesicle
Autophagy definition
Autophagy is an intracellular degradation system that delivers (dysfunctional) cytoplasmic components to the lysosome
Endsomes definition
-endosomes= membrane bound organelles that sort and deliver internalised material from cell surface
Process of formation of endocytotic vesicles
- plasma membrane buds inwards –> plasma membrane pinches off to form intracellular endocytotic vesicle
- can fuse with lysosomes and be recycled at the surface
Exocytosis definition
=counterpart of endocytosis
-a form of active transport; molecules (such as proteins) are transported out of the cell or receptors are inserted into the cell membrane
Function of endosomes
- internalisation of nutrients
- regulation of cell surface protein expression
- uptake and digestion of extracellular debris
- can be exploited by pathogens
Which organisms depend on endocytic uptake?
- most viruses depend on endocytic uptake, vesicular transport through the cytoplasm
Why do cells require endocytosis?
-most molecules required for cell function are large, polar that can’t pass through the hydrophobic portion of the plasma cell membrane by passive means
What are the 3 types of endocytosis?
- pinocytosis
- receptor-mediated endocytosis
- phagocytosis
Pinocytosis
- small particles are taken in by a cell by splitting off small vesicles from the cell surface
- nonspecific transport of substances
- the cell takes in surrounding fluids
- active transport (requires energy)
Receptor-mediated endocytosis: Clathrin/caveolin-mediated endocytosis
- receptor binds ligand
- captured by clathrin coated pit
- mature pit buds off forming a coated vesicle
example: LDL uptake via LDL receptor
Phagocytosis
- a cell engulfs a solid particle to form an internal compartment known as a phagosome
- phagosome= vesicle around particle
- pseudopodium = cell protrusion
- eliminate debris/pathogens (phagocytes of the immune system) via lysosome
What are the professional phagocytes?
- white blood cells: neutrophils, monocytes, macrophages, mast cells, dendritic cells
- have receptors to detect marked pathogens
endoplasmic reticulum definition
=single compartment extending from the outer layer of nuclear envelope into the cytoplasm
- most extensive membrane system within eukaryotic cells (50% of the total membrane)
- consists of 2 functional compartments: RER and SER
- RER is covered in protein manufacturing ribosomes; cells that secrete large amounts of proteins have large amounts of RER: eg B-lymphocyte that secretes antibodies vs fibroblast cell
- SER lacks ribosomes and is involved in lipid manufacture
Sarcoplasmic reticulum
- the ER in striated muscle cells in the heart and skeletal muscle
- it has a specialised function
- it forms an extensive network surrounding muscle sarcomeres and supplies them with calcium ions
- tubular network
- smooth muscle cells are spindle shaped, with the widest part 2-5microns and the length ranging up to 500 microns in visceral muscle and 150 microns in vascular tissues
Ribosomes
=site of protein synthesis (translation)
- link amino acids together in the order specified by the messenger RNA molecules
- two major components: the small ribosomal unit, which reads RNA and the large subunit which joins the aminoacids to form a polypeptide chain
- amino acids are selected, collected and carried to the ribosome by transfer RNA molecules
- tRNA enters one part of the ribosome and bind to the messenger RNA chain
Function of the endoplasmic reticulum
- PROTEIN MANUFACTURE
- PROTEIN TARGETTING
- STEROID PRODUCTION
- DRUG DETOXIFICATION
- CA2+ STORAGE AND RELEASE
PROTEIN MANUFACTURE
- protein production = generation of a polypeptide chain (antibodies secreted by leukocytes and insulin by pancreatic beta cells)
- post translational modification: glycosylation (carbohydrates attached to proteins) for folding and stability
PROTEIN TARGETTING
- proteins begin synthesis in cytosol but their destination is varied: secretion, cytosolic, specific organelles
- proteins that are destined for certain organelles or for secretion enter the ER first
- targeting signals enable the cellular transport machinery to correctly position a protein inside or outside the cell; continuous stretch of amino acids residues=signal peptides; polypeptide chain or infolded protein. signal sequence: short stretch of aa, located at the N-terminus, direct proteins to their destination. no short stretch –> cytosolic protein
Steroid production, drug detoxification, Calcium storage
- SER
- storage of proteins and sites of action of many enzymes
- membrane (lipid) synthesis (most cells)
- cytochrome p450 enzymes present in most tissues and particular liver
- calsequestrin (calcium binding protein of the sarcoplasmic reticulum)
Structure of GOLGI APPARATUS
- flattened membrane enclosed in disc-shaped sacs (cisternae)
- located near the nucleus
- cis (entry) and trans (exit) face
- proteins travel from the ER to Golgi and within Golgi via transport vesicles
- incoming transport vesicle (cis)
- outgoing transport vesicle(trans)
Where is the Golgi apparatus larger and more numerous
Golgi apparatus is large and more numerous in cells that synthesize and secrete large amount of substances: antibody-secreting plasma B cells
Function of the GOLGI APPARATUS
- SECRETORY PATHWAY: exocytic pathway-proteins are destined for secretion into the extracellular space or PM; a mannose-6-phosphate label is added to proteins destined for lysosomes (eg. digestive enzymes)
- lysosome formation (through the endocytic pathway): early endosome–> late endosome–> lysosome
- PROTEIN MODIFICATION AND SORTING: it packages proteins synthesised in ER into membrane bound vesicles, before the vesicles are sent to their destination –> vesicles bud off from the trans-Golgi (lysosome); addition of carbohydrates or phosphorylation of oligosaccharides
How are proteins shipped to their intended destinations?
Proteins are sorted and shipped to their intended destinations by their placement into one of at least 3 different types of vesicles:
1) Exocytotic vesicle: contains proteins destined for extracellular release; after packaging, these vesicles bud off and immediately move towards the plasma membrane where they fuse and release contents into the extracellular space in a process known as constitutive secretion
2) Secretory vesicles: vesicles contain proteins destined for extracellular release; after packaging, these vesicles bud off and are stored in the cell until a signal is given for their release –> when the appropriate signal is received they move towards the plasma membrane and fuse to release their contents = process known as regulated secretion
3) Vesicles contain proteins and ribosomes destined for the lysosome, a degradative organelle containing acid hydrolases. These proteins include both digestive enzymes and membrane proteins. The vesicle fuses with the late endsome and then the contents are transferred to the lysosome via the unknown mechanism
