Cell Ultrastructure Flashcards
Nucleus function
Largest membrane bound organelle
Storage and transmission of genetic information
Information coded in the DNA synthesises the protein determining the structure and function of the cell
Nucleus structure
Double membrane (nuclear envelope) with gaps called nuclear pores
RNA moves out via pores
DNA and proteins form chromatin- a mass of genetic material
At cell division chromatin becomes chromosomes and condenses
Nucleolus
Nucleolus
No membrane
Site of DNA transcription
Forms ribosomal RNA (rRNA)
Mitochondria function
Site of oxidative -phosphorylation
Mitochondria structure
Outer membrane- lipid synthesis and fatty acid metabolism
Inner membrane- respiratory chain (electron transport)
Matrix- Krebs cycle
Intramembranous space- nucleotide synthesis (ADP-ATP)
Vesicles structure
Very small spherical membrane bound organelles which transport and store material and exchange cell membrane between compartments
Vesicles types
Cell-surface derived pinocytotic and phagocytotic vesicles, Golgi- derived, ER-derived, lysosomes, peroxisomes
• Lysosomes - derived from Golgi, site at which proteins are degraded. H+-ATPase on endosome membrane creates low internal pH5, contain acid hydrolases that degrade proteins, formed by fusion of hydrolase vesicles and endosomes. Contain digestive enzymes
• Peroxisomes - small membrane-bound organelles containing enzymes which oxidise long-chain fatty acids. Involved in process by which fatty acids are broken down to generate ATP. Toxic hydrogen peroxide broken down by peroxisomes
Endosome- membrane bound vesicular tubular structures that live between Golgi and membrane
Vacuole function
Hold various solutions or material- can have been created, stored or excreted and that have been phagocytosed or engulfed
Vacuole structure
Chamber surrounded by a membrane- semi-permeable that only lets certain molecules through
Golgi apparatus structure
Parallel stacks of membrane
Located close to the nucleus
In most cells cannot be seen- very clear in plasma cells (perinuclear hoff)
3 parts: cis-Golgi, medial and trans-Golgi
Golgi apparatus function
Processes and modifies macromolecules synthesised in the endoplasmic reticulum
Cis-Golgi
Nuclear facing- receives from rER
protein phosphorylation occurs here
Medial Golgi
Modifies producers by adding sugars
Forms complex oligosaccharides by adding sugars to lipids and peptides
Trans-Golgi
Proteolysis of peptides into active forms and sorting of molecules into vesicles which bud from the surface
Rough endoplasmic reticulum structure
Rough due to ribosomes on surface
Highly folded flat membrane sheet
Rough endoplasmic reticulum function
Site of protein synthesis
Smooth endoplasmic reticulum structure
Highly folded, flattened membrane sheets
Smooth endoplasmic reticulum function
Site of lipid synthesis
Proteases and store synthesised proteins
Ribosome structure
2 subunits attached to the rER
Ribosome function
Acts as a large catalyst
Translate genetic code into chains of amino acids
Deposits protein into the rER to undergo further modification
3-5 amino acids per second in protein production
Cytoplasm function
Site of glycolysis
Cytoplasm structure
Fluid that fills the cell, includes the cytosol and filaments, proteins, ions and macromolecular structures as well as organelles. NA the nucleus
3 components of cytoplasms:
- Cytoskeleton with associated motor proteins
- Organelles and other multi proteins complexes
- Cytoplasmic inclusions and dissolved solute
Plasma membrane function
Controls passage of various molecules
Physical barrier
Selective permeability
Endo-/exocytosis
Cell signalling
Plasma membrane structure
Double layer of phospholipids (hydrophobic head and hydrophilic tail)
5 types attached to phosphate group:
1. Serine (PS- phosphatidyl-serine)
2. Choline (PC- phosphatidyl-choline)
3. Inositol (PI- phosphatidyl-inositol)
4. Ethanolamine (PE- phosphatidyl-ethanolamine)
5. Sphingomyelin (SM)
Cholesterol
Membrane proteins 9receptors/channels)
Carbohydrate groups which attach to proteins to form glycoproteins or glycolipids
Sphingolipids
Microtubules
25 nm
Eg tubulin (alpha and beta which arrange into groups of 13 to form hollow tubes)
Arise from centromere
Found in all cells (except erythrocytes as have no nucleus so no cell division)
Intermediate filaments
10nm
6 protein types
Anchored transmembrane proteins which can spread through tissues
6 types of intermediate filaments
- Cytokeratins: epithelial cells.
- Desmin: myocytes.
- Glial fibrillary acidic protein: astrocytic glial cells.
- Neurofilament protein: neurons.
- Nuclear laminin: nuclei of all cells.
- Vimentin: mesodermal cells.
Microfilaments
5nm
Actin forms a mesh (cell cortex) on inner surface of cell membrane
Globular G-actin polymerises into filamentous F-actin.
Centrosome structure
Made from 2 centrioles which are microtubules rings
Centrosome function
Organise microtubules and provide structure for cell
Pull chromatids apart during cell division
Lysosomes
Contain digestive enzymes
Waste disposal system and site of breakdown for most molecules
Derived from Golgi
H+ ATPase on membrane creates low pH (pH5) to enable acid hydrolysis to function
Endosomes
Membrane bound vesicular and tubular structure that live between Golgi and membrane
Peroxisomes
Small membrane bound organelles containing enzymes which oxidase long chain fatty acids
Involved in process by which fatty acids are broken down to generate ATP
Hydrogen peroxide is toxic to cells by destroyed by peroxisomes
Tight junctions
Seals neighbouring cells together i in an epithelial sheet to prevent leakage
prevent diffusion between cells- can establish a gradient for absorption or secretion of molecules from or into the gut and act as a barrier as only water and some small molecules can pass through
Adherens
Joins an actin bundle in one cell to a similar bundle in another cell
Desmosomes
Joins the intermediate filaments in one cell to a neighbour
plaques that form physical joints between cells and connect the cytoskeletons of adjacent cells - spread forces across several cells eg in epithelia exposed to abrasive forces, skin.
Gap junctions
Allows passage of small water soluble ions and molecules so cells are electrically coupled
Hemi-desmosomes
Anchor intermediate filaments in a cell to the basal lamina
Where are cytokeratins found
Epithelial cells
Where is desmin found
Myocytes
Where is vimentin found
Mesodermal cells
Where is nuclear laminin found
Nucleus of all cells
Where is neurofilament protein found
Neurones
Where is glial fibrillary acidic protein found
Astrocytic glial cells
Anti-phospholipid syndrome
• systemic auto-immune disease
• Antibodies bind to beta 2-glycoprotein-1 on cell membranes
• Initiates inappropriate blood clotting (thrombosis)
• Significant cause of pregnancy morbidity and early miscarriage
Fluid-mosaic model
mosaic as a mix of membrane proteins free to move in a sea of lipid
Why are membranes important
• cell signalling - source of lipid processors
• cell polarisation
• compartmentalisation - ionic gradients, diffusion, membrane potential
• tightly regulated - disease disrupts this
• Endo/exocytosis
• Physical barrier
• Selective permeability to control passage of molecules
• Anchors cell to extracellular matrix and adjacent cells
Glycerophospholipids
amphipathic (hydrophobic and hydrophilic)
Fatty acid tails:
• non-polar, hydrophobic,
• saturated C-C bonds,
• unsaturated cis C=C bonds lead to kink in hydrophobic tail
• significant variation in length
Glycerol backbone (3C)
Phosphate group- negative charge at physiological pH (polar, hydrophilic)
Function of lipid bilayer
• form compartments
• allow lipid-soluble substances to enter + leave cell
• prevent water-soluble substances entering + leaving cell
• make membrane flexible and self-sealing
5 types of phospholipid in membrane
- Serine (PS- phosphatidyl-serine)
- Choline (PC- phosphatidyl-choline)
- Inositol (PI- phosphatidyl-inositol)
- Ethanolamine (PE- phosphatidyl-ethanolamine)
- Sphingomyelin (SM)
Lipid bilayer
Leaflet specific orientation
Leaflet ‘flip’- thermodynamically unstable
Outer layer: Sphingomyelin and enriched for PC
Inner layer: enriched for PE. PS (signal for apoptosis) and PU majority in inner leaflet
Membrane fluidity
• movement of phospholipids and proteins
• Facilitates inter- and intra-cellular signalling (phosphoatidylinositol)
• Temperature and structure dependent phase transitions
Cholesterol in membrane
Very hydrophobic
Fit between the phospholipids and bind to the hydrophobic tails, causing them to pack more closely together. This restricts the movement of the phospholipids, meaning the membrane is less fluid and more rigid
Function of cholesterol in membrane
• reduce lateral movement of other molecules
• make membrane less fluid at high temperatures
• prevent leakage of water + dissolved ions
• help cell maintain shape e.g. Red blood cells which aren’t supported by other cells as free in blood
How can substances cross epithelial
Substances can cross epithelial by paracellular pathway (between adjacent cells)- limited by presence of tight junctions- or transcellular pathway (moves into epithelial cell diffuses through cytosol and exits across opposite membrane)
Paracellular pathways
between adjacent cells)- limited by presence of tight junctions
Trans cellular pathway
moves into epithelial cell diffuses through cytosol and exits across opposite membrane
Epithelia
• require polarisation of plasma membrane - apical vs basolateral surfaces
• permits cell specific function - secretion/absorption
• strongly adhere to neighbours - tight junctions
• Eg parietal cell (gastric pits), intestinal epithelium, nephron
Apical membrane
Lumen side of epithelial
Basolateral membrane
Blood side of epithelial
Proteins in membrane
Interspersed throughout cell-surface membrane
Embedded in 2 ways:
1. peripheral membrane proteins occur in the surface of bilayer and don’t extend completely across it. Act to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones
2. integral proteins- span whole width of bilayer and are protein channels or carriers. Channels form water-filled tubes to allow water-soluble ions to diffuse across. Carrier proteins bind to ions or molecules e.g. Glucose/Na+, then change shape in order to move these across the membrane
Peripheral membrane proteins
occur in the surface of bilayer and don’t extend completely across it. Act to give mechanical support to the membrane or, in conjunction with glycolipids, as cell receptors for molecules such as hormones
Integral proteins in membrane
span whole width of bilayer and are protein channels or carriers. Channels form water-filled tubes to allow water-soluble ions to diffuse across. Carrier proteins bind to ions or molecules e.g. Glucose/Na+, then change shape in order to move these across the membrane
Function of proteins in membrane
• provide structural support
• act as channels transporting water-soluble substances across the membrane
• allow active transport through carrier proteins
• form cell-surface receptors for identifying cells
• help cells adhere together
• act as receptors e.g. For hormones
Glycoproteins in membrane
Carbohydrate chain attached to extrinsic proteins on outersurface of membrane
Functions:
• act as recognition sites
• help cells attach to eachother, forming tissues
• allow cells to recognise one another e.g. Lymphocytes
Glycolipids in membrane
Carbohydrate section extends from phospholipid belayer to get as a cell-surface receptor for specific chemicals e.g. ABO blood system
Functions:
• act as recognition sites
• help maintain stability of membrane
• help cells attach to eachother, forming tissues
Membranes and pH
• both extremes damage protein
• Inhibits cell function critical role for acid:base homeostasis- plasma Ca2+ and cell membrane excitability/permeability
Temperature and membranes
- Below O°C = phospholipids are rigid and closely packed together as have little energy. Channel + carrier proteins deform, increasing permeability. Ice crystals may form and pierce membrane. Also, slower rate of diffusion
- Between 0-45°C = more energy so phospholipids move around more- membrane more permeable
- Above 45 °C = bilayer starts to melt + breakdown, membrane more permeable. Proteins in membrane begin to denature, damaging membrane
Lethal triad
Hypothermia
Reduced coagulation
Acidosis
Membrane permeability
• phospholipid bilayer modified by- C=C bonds, cholesterol, temperature
• Freely permeable- water (aquaporins), gases (eg CO2, N2, O2), small uncharged polar molecules (eg ammonia, urea, ethanol)
• Impermeable to- ions, charged polar molecules (eg ATP, glucose-6-phosphate), large uncharged polar molecules (eg glucose)
Uniport membrane proteins
Transport single substance
Passive
Eg glucose via Glut-1
Anti port membrane proteins
2 substances in opposite directions.
Energy from ATP hydrolysis
eg 3 Na+/ 2 K+ ATPase
Symport membrane protein
2 or more substances in the same direction.
Indirectly from ATP hydrolysis, ion gradient used
eg Na+/ glucose nutrient transport
Ion channels
• integral membrane protein
• Alpha-helix
• Hydrophobic amino acids interface with hydrophobic phospholipid tail.
• Hydrophilic pore for ion passage
• Selective: Charge and I on size
• Gated (usually)
What substances move via simple diffusion
Ketone bodies, blood gases, water (aquaporin), NH3, urea, free fatty acids
What substances move via facilitated diffusion
Glucose (GLUT-1), GLUT family
What substances move via primary active transport
Ions (Na+, K+, Ca2+, H+, HCO3-), Water-soluble vitamins- Energy direct from ATP
What substances move via secondary active transport
Ions (renal tubule; fluid balance), Symporters (Na+ + X)- Indirect energy from ion gradient
What substances move via ion channels
Voltage gated, leak channels
What substances move via pino-/phagocytosis
Vesicles
Diffusion
NET movement of particles from an area of high concentration to an area of low concentration
• Down the concentration gradient
• Random movement caused by natural kinetic energy of particles (thermal motion)
• Passive process so equilibrium is reached
• O2 and CO2 can diffuse through membrane as small and non-polar so lipid-soluble
• Flux = amount of material crossing a surface in a unit of time
Flux
amount of material crossing a surface in a unit of time
Factors affecting rate of diffusion
• steepness of concentration gradient (Co-Ci) = greater difference in concentration means a greater difference in the number of molecules passing in the 2 directions so faster rate
• temperature = particles have more kinetic energy so move faster meaning a faster rate
• surface area (A) = greater the surface area, the greater the number of particles that can diffuse across at any one moment so a faster rate, e.g. Microvilli, root - hair cells
• thickness of exchange surface (P) = the thinner the exchange surface, the faster the rate
Facilitated diffusion
• Large or charged particles diffuse through carrier or channel proteins in the membrane
• Carrier proteins: high specific e.g. For glucose only
1. When a specific molecule is present, it binds with the protein
2. this causes the carrier protein to change shape in such a way that the molecule is released to the inside of the membrane
3. The carrier then returns to its original shape
• Channel proteins: form hydrophilic pores in the membrane for charged particles, e.g. Ions, to diffuse through
• Specific to water-soluble ions. The channels are selective, opening when the ion binds to it, causing it to change shape in a way that opens one side and closes the other- ‘gated’
• Aquaporin = water channel protein
Factors affecting rate of facilitated diffusion
• concentration gradient
• number of channel or carrier proteins = when all are saturated, Vmax is reached
Which cells have a high number of carrier proteins
Neuron, kidney and muscle cells
Osmosis
the NET movement of water from a high water potential to a low water potential through a partially permeable membrane
Passive process due to kinetic energy of particles
Water potential = the concentration of soluble molecules in a solution (the tendency of water to move out of solution)
Water = 0 kPa
Water + solute = -x kPa
Osmolarity = total solute concentration of a solution
Factors affecting rate of osmosis
• water potential gradient
• thickness of exchange surface
• surface area
• Type and number of aquaporin channels
Hypertonic solution
Water moves out of cell into solution
Cell shrinks
Hypotonic solution
Water moves into cell from solution
Causes cell to swell
Isotonic
No NET movement of water
Osmolarity
Total solute concentration of a solution
Water potential
The tendency of water to move out of a solution
Active transport
the movement of molecules or ions from a region of low concentration to a region of high concentration, against the concentration gradient, using ATP and carrier proteins
1. The carrier proteins span the plasma membrane and a specific molecule or ion binds to the receptor site
2. On the inside of the cell/organelle, ATP binds to the protein, causing it to split into ADP + Pi. As a result, the protein molecule changes shape and opens to the opposite side of the membrane, releasing the molecule/ion
3. The phosphate molecule is released from the protein, which causes the protein to revert back to its original shape
Factors affecting rate of active transport
• speed of individual carrier proteins
• number of carrier proteins present
• rate of respiration- availability of ATP
Two means of coupling energy to transporters
-direct use of ATP in primary active transport (eg Na+/K+ pump)
-the use of an electrochemical gradient across a membrane to drive the process in secondary active transport (eg co transport)
Cotransport
the coupled movement of two different substances together across a cell membrane via a carrier protein e.g. Amino acids or glucose with Na+ in the mammalian ileum
1. Sodium ions are actively transported out of the epithelial cells, by the Na+/K+ pump, into the blood
2. This establishes a concentration gradient - the higher concentration of Na+ in the lumen of the ileum than inside the cell
3. Sodium ions then diffuse down their conc gradient into the epithelial cell, via a sodium-glucose co-transporter protein, carrying a glucose molecule with it
4. The concentration of glucose is now higher in the cell than the blood, so glucose diffuses by facilitated diffusion into the blood
Secondary active transport
the downhill flow of an ion is linked to the uphill movement of a second solute either in the same direction as the ion (cotransport) or in the opposite direction of the ion (countertransport).
Endocytosis
small pockets that pinch off the plasma membrane to produce intracellular, membrane bound vesicles that enclose a small volume of extracellular fluid
3 types of endocytosis
Pinocytosis
Phagocytosis
Receptor-mediated endocytosis
Pinocytosis
endocytosis vesicle encloses a small volume of extracellular fluid. Nonspecific. Usually contains ions, nutrients or small molecules
Phagocytosis
cells engulf bacteria or large particles such as cell debris from damaged tissues (pseudopodia fold around particle)
Receptor-mediated endocytosis
molecules bind to receptors on plasma membrane, causing it to undergo a conformational change which triggers endocytosis.
Exocytosis
membrane-bound vesicles in cytoplasm fuse with plasma membrane and release their contents to outside of the cell
Allows portions of membrane to be replaced and secretion of synthesised molecules from cell
Nernst equation
Determines diffusion potential
Cholera
Cholera toxin modifies GPCR —> increases cAMP —> increases ion transporters in cell membrane —> increases secretion of ions and H2O into gut —> diarrhoea and dehydration
GPCRs
Receptor- primary specificity
G protein - secondary specificity- determines 2nd messenger
Gs —> AC —> increases cAMP
Gi—| AC —> decreases cAMP
Gq —> PLC —> DAG + IP3
Which 3 types of cell are called professional antigen presenting cells
B cells
Dendritic cells
Macrophages
- What are the 3 types of cytoskeleton and what size are they each?
- Types of cytoskeleton filaments: actin 5nm, intermediate filaments eg desmin/nuclear laminin/vimentin/keratin 10nm, microtubule 25nm
Which one of the following is the role of the trans golgi in a cell?
Proteolysis of peptides and packages them into vesicles
