Cellular structure/processes Flashcards
What the cell structure basics
basic structural, biological, functional units that comprises an organism
Smallest self-replicating life-form
The main levels of organization in the body, from the simplest to the most complex are:
Cells > tissues > organ > organ system > organism
What are the basic constituents of cells
- Plasma membrane
- Cytoplasm: everything in cell membrane except nucleus)
- Fluid suspension
- Composition: cytosol (liquid found inside of cells), organelles
What is the Cytosol?
Intracellular fluid
- Composition: dissolved/suspended organic, inorganic chemicals; macromolecules; pigments; organelles are in the cytosol
- the cytosol is the fluid surrounding it.
- Site of most cellular activity.
Where is the site of most cellular activity?
The Cytosol (in cytoplasm)
Site of most cellular activity
What is the composition of ribosomes?
rRNA, ribosomal proteins
Where can you find ribosomes
Can exist freely in the cytoplasm/bound to endoplasmic reticulum (forms rough endoplasmic reticulum)
What is the purpose of ribosomes
Turns mRNA into protein via translation
What subunits are ribosomes organised into?
- Organized into two subunits (40’s, 60’s)
- Small subunit: binding sites for mRNA, tRNA
- Larger subunit: has ribosome to catalyse peptide bond formation (for bonds between amino acids)
What is the endoplasmic reticulum?
Also, what is its appearance?
membrane-enclosed organelle
Appearance: a stack of membranous. Flattened disks (cisterns)
Rough endoplasmic reticulum (RER) structure
Contain bound ribosomes on the surface
Rough endoplasmic reticulum cisterna continuous with nuclear envelope.
What is the function of Rough endoplasmic reticulum
Site of packaging, folding of proteins Designated for secretion, lysosomal degradation,
plasma membrane insertion, proteins packed into vesicles, sent to Golgi apparatus for further modification
Smooth endoplasmic reticulum structure
No ribosomes
Smooth endoplasmic reticulum function
Site of making lipids, steroid synthesis (Glans), ions storage (muscles), glycogen metabolism, Detoxification (liver).
Golgi apparatus purpose/function
Golgi apparatus
Post-translational modification site (e.g. phosphorylation, glycosylation, sulfonation) of proteins, lipids hormones
→ sorted, packed into secretory vesicles → secreted out of cell/lysosomal fusion/plasma membrane insertion
Golgi apparatus Structure
Membrane-enclosed organelle
Appearance: a collection of fused, flattened sacs (cisterns) with associated vesicles, vacuoles
Two sides
Cis-side: receives proteins from Rough endoplasmic reticulum (entry)
Trans side: opposite side, releases vesicles towards the plasma membrane (Exit)
what are the Golgi apparatus side(s) functions
Two sides
Cis-side: receives proteins from Rough endoplasmic reticulum (entry)
Trans side: opposite side, releases vesicles towards the plasma membrane (Exit)
Mitochondria Structure
Double membrane-enclosed organelle;
Outer smooth membrane: Inner membrane:
Inner membrane space: space between the inner, outer membrane
Mitochondria Purpose
synthesizes ATP for cell via aerobic respiration
In cytoplasm glucose undergoes glycolysis, glucose is cleaved into pyruvate.
Pyruvate enters mitochondria > citric acid cycle (Krebs cycle), electron transport chain (which require oxygen)
In glucose absence, mitochondria can use fatty acids as fuel via beta-oxidation (only medium-sized fatty acids used; longer ones chopped by peroxisome)
Mitochondria number: correlates with cell activity/energy/requirements.
Nucleus Structure
Large, membrane-enclosed organelle present in all cells except mature erythrocytes (RBC)
Most cells contain one nucleus; some cells have more (e.g. muscle cells, osteoclasts, hepatocytes)
Usually spherical, may take on other shapes
Lobulated (e.g. polymorphonuclear leukocytes)
Elongates (e.g. columnar epithelium)
Nucleus purpose
Contains genetic material (DNA, tightly packed into chromatin); coordinates cellular activities
Cell membrane structure
Semipermeable membrane made from phospholipid bilayer; surrounds cell cytoplasm
Cell membrane: phospholipid bilayer structure
Two-layered polar phospholipid molecules comprising two parts
Negatively charged phosphate “head” (hydrophilic; orientated outwards)
Fatty acid “tail” (hydrophobic orientated (inwards)-
why is the phospholipid layer Semipermeable?
Allows passage of certain molecules through the membrane (02, C02 etc)
Denies passage of others (large molecules such as proteins, glucose)
Certain molecule transportation (Ions, H2O) allowed through embedded membrane proteins (ion channels, pumps)
What is meant by “Selective permeability of the cell membrane”
Cell membrane controls which molecules enter and leave:
Passive transport:
Active transport: energy is required
In relation to energy, What is passive transport?
Passive transport: no energy required to pass cell membrane
In relation to energy, What is Active transport:
Active transport: energy is required to cross cell membrane = adenosine triphosphate (ATP)
Explain passive transport
Simple diffusion
Random molecular motion
Small nonpolar molecules move from high concentration -> low concentration
What affects diffuse flux (Ficks law)
Three factors
Concentration gradient
- Larger differences in solute concentration on each side of the membrane -> high driving force -> high net diffusion
- Equal concentrations -> no net diffusion (e.g. , movement between alveoli and blood)
Membrane surface area
- Increase surface area available for diffusion -> increase diffusion rate; vice versa (e.g. microvilli in small intestines amplify the surface area -> increase nutrient, water distribution)
The distance separating each side of the membrane (e.g. thickness)
- Increase distance molecules must travel -> decrease of net diffusion; vice versa (e.g. pulmonary oedema -> increase distance between compartments -> decrease net diffusion)
How does concentration affect diffuse flux? (Ficks Law)
Concentration gradient
Larger differences in solute concentration on each side of the membrane -> high driving force -> high net diffusion
Equal concentrations → no net diffusion (e.g. , movement between alveoli and blood)
How does membrane surface area affect diffuse flux (Ficks law)
Membrane surface area
Increase surface area available for diffusion -> increase diffusion rate; vice versa (e.g. microvilli in small intestines amplify the surface area -> increase nutrient, water distribution)
How does The distance separating each side of the membrane affect diffuse flux (Ficks law)
The distance separating each side of the membrane (e.g. thickness)
Increase distance molecules must travel -> decrease of net diffusion; vice versa (e.g. pulmonary oedema -> increase distance between compartments -> decrease net diffusion)
What is Facilitated diffusion?
Uses transport proteins (e.g. channels, carrier proteins)
Allows larger/polar molecules to move across the membrane
What are Channels on the cell membrane?
Non-specific, open to allow water, small polar molecules through (e.g. voltage-gated calcium channel)
What are Carrier proteins on cell membranes
Very specific, only allow certain molecules to bind (e.g. glucose transporter (GLUT4)
Explain primary active transport
Primary
Uses ATP
Enzymes called ATPases use ATP as fuel; (e.g. Na+/K+-ATPase, ATPase, H⁺/K⁺ ATPase)
May create concentration/electrochemical gradients
Explain secondary active transport
Secondary
Uses existing electrochemical gradients
One solute, normally Na+ moves with concentration gradient through transporter -> supplies energy transporter needs to -> another solute against concentration gradient in same/opposite direction as Na+ (e.g. sodium-glucose SGLT1 transporter)
Explain Bulk transport
AKA vesicular transport
Endocytosis
Cell membrane invaginates, pulling something in from outside e.g. pathogen phagocytosis)
Exocytosis
Vesicle inside cell pushes something (e.g. hormone secretion)
What is the Extracellular Matrix
The environment surrounding the cells
Varies between tissues (epithelial, connective, and nervous)
What are the major molecules in the extracellular matrix?
Adhesive proteins
Structural proteins
Proteoglycans
What are Adhesive proteins
Adhere cells together (communication with extracellular fluid)
e.g. integrating cadherins
What is the purpose of Structural proteins?
- Give tissues tensile, compressive strength
- Collagen
- Resists tension, can stretch
- Starts as procollagen → cleaves into tropocollagen → arranged into fibrils
- Four major types of structural proteins: type 1(bone, skin, tendon). Type 2 (cartilage), type 3 (reticulin, blood vessels), Type 4 (basement membrane)
- Elastin
- Elastic, returns tissue to original shape
- Keratin
- Tough, found in hair and nails
What are Proteoglycans?
Fill space between cells, hydrate, cushion cells
Consists of a protein core with sugar chains
What is the purpose of Cell-Cell junctions
Protein structures that physically connect cells
Improve cellular communication, tissue structure; allow transport of some substances between cells, create an impermeable barrier for others
Only found between immobile cells; abundant in epithelial tissue (e.g. in the skin)
What are the three junction types?
Tight junctions
Adherens junctions
Gap junctions
What are Tight junctions?
e.g. in the gastrointestinal tract/brain
Seal adjacent-cell plasma membranes, especially near the apical surface; prevent the passage of water. Small proteins, bacteria Formed by claudins, occluding embedded in cellular plasma membranes
In “leaky” epithelia, tight junctions may allow certain molecules to pass (e.g. K+. Na+, -CL in kidneys proximal tubules - due to ion pores)
What are Adherens junctions?
e.g. in skin
Anchor cells together, provide strength; consist of three major components
Actin filaments: provide cellular shape
Protein plaques: anchor membrane bind to actin filaments
Cadherins: attach to protein plaques, connect to cadherins on other cells.
What are Gap junctions?
E.g. in heart
Connect adjacent cells, allow rapid communication; formed by connexins -> create tubular structure (allows charged particles to pass)
In cardiac myocytes: gap junctions create coordinated heart contractions
In infected cells: gap junctions send cytokines to neighbouring cells, triggering apoptosis, preventing infectious spread (“bystander effect”)
Endocytosis and exocytosis
ransports material in/out of cell
Requires adenosine triphosphate (ATP) for energy
What does endocytosis do?
Three types are:
Cell engulf extracellular material
Three types are: phagocytosis, pinocytosis & receptor-mediated endocytosis
What does phagocytosis do?
Aka cell eating
Used by white blood cells (e.g. macrophages, neutrophils)
Process:
- The cell extends arm-like projects (AKA pseudopods) around the target
- Cell membrane slowly engulfs target, invaginates to form a vesicle
- Vesicle separates from cell membrane to form a phagosome
- Phagosome fuses with lysosome, target is digested
- Debris released by exocytosis
What is pinocytosis
Aka cell drinking
Edges of invagination come together to form a vesicle
Motor proteins use ATP to carry vesicles into the cytosol
What is receptor-mediated endocytosis?
Used by cells to take in specific molecules (e.g. iron, cholesterol)
Process
- Clathrin-covered pits/coated pits with receptors bind certain molecules
- Edges of put come together, clathrin proteins link up
- Vesicle pinches off; clathrin detaches, return to the cell membrane
- Vesicle merges with endosome to separate receptors into the second vesicle
What is Exocytosis?
Cells expel material into extracellular space (e.g. neurotransmitters, hormone)
Last phagocytosis step
Process
- Golgi apparatus creates vesicle from various proteins, lipids, hormones
- Motor proteins use ATP to carry vesicle along cytoskeleton
- Vesicle pressed against cell membrane until rupture -> spills contents into extracellular space.
What is Osmosis?
Passive water-flow across selectively permeable (semipermeable) cellular membrane; primarily determined by solute concentration differences (osmotic pressure)
Factors affecting water movement across the membrane?
- Molecules (e.g. water molecules, ions) tend to move around kinetic energy) + movement is disordered, random (entropy) → larger solutes tend to block openings in a semipermeable membrane
- If solute ions positively charged, they attract slightly negatively charged oxygen atoms in water molecule; if solute ions are negatively charged they attract slightly positively charged hydrogen atoms in water molecule
→ water molecules partially attached to the ion
→ movement through membrane impeded
- Water molecules tend to move from hypotonic side (more water/less solutes) to hypertonic side (less water/more solute)
What relation does the selectively permeable membrane have on Osmosis?
Allows small molecules (e.g. water) across, but not larger molecules/ions
In an isotonic solution..
Side A= Side B
If solute concentration is the same on each side of membrane -> net water movement across membrane is zero (equilibrium)
In Hypertonic/hypotonic solution….
Side a> side B or Side B >Side A
If the solute concentration is greater on one side (hypertonic) -> net water migration across the membrane is from the hypotonic side toward the hypertonic side
Cellular effects on hypertonic, hypotonic solution
Red blood cells in hypertonic solution → net movement of water molecules out of cell → cell shrinks (cremation)
Red blood cell in hypotonic solution→ net movement of water molecules into cell → cell swells may burst (lyses)
Resting membrane potential: Electric potential across cell membrane
- Given weighted (based on membrane permeability) sum of equilibrium potentials for all ions.
- High concentrations of Na+, -Cl, Ca+ outside the cell; high concentrations of k+, -A (various anions) inside cell -> concentration gradients are established Sodium-potassium pump uses ATP to move two K+ ions into cell, three Na+ ions out.
- Potassium concentration = 150mMol/L inside cell, 5mMol/L outside cell.
Resting membrane potential: Concentration gradients
Concentration gradients establish electrostatic gradients
- Concentration gradient pushes potassium out through potassium leaky channels, inward rectifier channels
- Anions remain in cells -> negative charge builds up -> potassium is pulled back into cell.
Resting membrane potential: Equilibrium (Nernst) potential:
Equilibrium (Nernst) potential: electrostatic gradient equal to concentration gradient (-92mV for potassium)
Value is flipped for negative ions
Resting membrane potential is sum of equilibrium potentials of major ions multiplied by their membrane permeabilities.
Cell signalling pathway stages
Cell signalling pathway stages
- Reception: ligand binds to receptor
- Transduction: receptor changes activating intracellular molecules
- Response: signal triggers a response in target cell
Ion channel receptors
Ion channels that open specific ligands bind
Allow ions (e.g. chloride, calcium, sodium, potassium) to flow through
The resulting shift in electrical charge distribution triggers a response
cytoskeleton & intracellular motility
Non-membrane bound organelles comprising complex protein filament network
Provide structural stability, shape, organisation, intracytoplasmic motility, cell motility
Cytoskeleton protein filament network Types
Microfilaments
Microtubules
Intermediate filaments
Nuclear Envelope
- Encloses separates the nucleus from the cytoplasm
- Composed of the selectively permeable membrane phospholipid bilayer
Consists of:
- Nuclear pores
- Outer membrane
- Inner membrane
Nuclear pores:
- Form where membranes fuse together at various intervals
- Each pore is lined with a nuclear pore complex (nucleoporin) to facilitate communication between the nucleus, cytoplasm
- Allow bidirectional macromolecule movement
Outer membrane
Anchoring proteins that hold nucleus in place with cytoplasm
Continuous with rough endoplasmic reticulum
Inner membrane
Covered by the nuclear lamina
Thin filamentous protein network, creates web within the nucleus; provide support for chromatin
Nucleosome
Eight histones packed together in four stacks of two; DNA wraps around them twice
Strung on strand of DNA-like “beads on string”
Two chromatin types
Euchromatin: loosely packed DNA, actively being transcribed into RNA
Heterochromatin: densely packed DNA, inactive (not being transcribed)
Nucleolus
- Dense non-membrane-bound structure; some cells have more than one nucleolus
- Contains rDNA -> transcribes into rRNA
- Assembles ribosomal subunits
Nucleoplasm
- Protoplasmic material
- Composed of complex water, molecule, ion mixture
- Contains nucleolus, chromatin
Chromatin
- Helical fibre
- Composed of 46 DNA molecules wrapped around proteins (histones)
- Histones help regulate DNA, gene expression
- Chromosomes become visible as chromatin fibres become tightly coiled during cellular division
