MEDI 101 lecture 3 and 4 (2nd week)

Nucleus

• Membrane-bound; contains DNA

• Surrounded by the nuclear envelope, which has nuclear pores & encloses a jelly-like fluid called nucleoplasm.

• controls gene expression

• site of DNA replication and transcription

• controls cell activities

Cytoplasm

• Jelly-like material filling the cell

• Contains cytosol, organelles, and cytoskeleton

• Site for biochemical reactions & internal transport

Nucleolus

Makes ribosomal RNA and ribosomal subunits

Plasma membrane

• thin flexible boundary around the cell

• made of phospholid bilayer + proteins

• controls entry/exit of substances and cell communication

chromatin

• loose network of DNA threads scattered throughtout the nucleus when a cell is not dividing

• complex network made of chains of DNA wrapped around proteins called histones

• when a cell is dividing, the threads condense to form chromosomes

cytoplasm

• the gelatinous liquid that fills the inside of a cell

• contains cell organelles and structures, solutes and nutrients, inclusion bodies (lipid droplets in fat cells, glycogen granules in liver cells, melanin, mucus in goblet cells)

endoplasmic reticulum

• large continuous network of membrane-bound sacs and tubules that is connected to the nuclear envelope and extends throught the cytoplasm

rough ER

• network of membranous sacs covered with ribosomes

• synthesizes proteins for secretion, cell membranes, lysosomes

• begins protein folding and modification

• sends newly synthesized proteins to the golgi apparatus

smooth ER

• lacks ribosomes

• synthesizes lipids and phospholipids

• important in steroid hormone synthesis

• participates in drug and toxin detoxification

• stores and releases calcium

• specialized SER in muscle = sarcoplasmic reticulum

golgi apparatus

• consists of stacks of flattened membrane sacs

• receives proteins and lipids from ER

• modifies proteins and lipids

• sorts molecules according to their destination

• packages them into transport or secretory vesicles

• contributes to the formation of lysosomes

lysosomes

• membrane-bound organelles containing digestive enzymes

• maintain an acidic internal environment

• Breaks down macromolecules, foreign material, damaged and old organelles

• participate in autophagy

• recycle cellular components

mitchondria

• surrounded by a double membrane

• inner membrane forms folds called cristae

• the thinner fluid-filled space contains enzymes, mitochondrial DNA and ribosomes. this allows mitochondria to produce some of their own proteins

• major cite of aerobic cellular respiration

• produces most cellular ATP

• involved in fatty-acid oxidation

• helps regulate apoptosis

• participates in calcium regulation

peroxisomes

• small membrane-bound organelles

• contain oxidative enzymes

• break down very-long-chain fatty acids

• produce hydrogen peroxide during oxidation reactions

• contains catalase, which converts into water and oxygen

• protects cells from oxidative damage

proteasomes

• tiny barrel-shaped structure that contains proteases, which degrade unneeded damaged, or faulty cytoplasmic proteins by cutting them into small peptides

• then it is recycled to the rough ER

centrosome

• the major microtubule organizing centre

• located near the nucleus

• helps form the mitotic spindle

• important for chromosome separation during cell division

• centrioles contribute to the formation of cilia and flagella

cytoskeleton

• is a dynamic network of protein fibers that maintians cell shape

• provides mechanical support

• enables cell movement and intracellular transport

• participates in cell division

microfilaments

• made of actin

• cell movement and shape changes

• muscle contraction with myosin

• forms the cleavage furrow during cytokinesis

intermediate filaments

• keratin and other proteins

• provide mechanical strength and stability

• protect cells against mechanical stress

• keratin supports epithelial cells

microtubules

• made of alpha and beta TUBULIN

• provide tracks for intracellular transport

• form cilia and flagella

• form mitotic spindle during cell division

tight junctions

• seal cells together, preventing leakage of molecules between them

• where to find: epithelial lining of the intestine and the urinary system

desmosomes

• provide strong adhesion to resist mechanical stress from being pulled apart

• where to find: epidermis and cardiac cells

gap junctions

• allow direct communication between cells via connexons (protein channels)

• permit passage of ions, small metabolites and signaling molecules

• most cells have it (neurons, epithelial cell, smooth muscle and cardiac cells) except sperm cells, skeletal muscles and RBC

plasma membrane

• consists of two lipid layers (phospholipid bi-layer)

• selectively permeable barrier that allows some substances to pass through it while excluding others

• contains various types of protein, lipids and carbohydrates attached to proteins and lipids on the external surface

polar heads

• the ______ of phospholipid molecules are hydrophilic and are attached to water on the inner and outer surfaces of the membrane

nonpolar tails

• the ______ being hydrophobic, avoid water and line up in the center of the membrane

intracellular fluid

makes up about 2/3 of total body water

extracellular fluid

makes up about 1/3 of total body water

interstitial fluid

• fluid between and around the cells

• allows exchange of nutrients, gases, and wastes between blood cells

plasma

• liquid portion of blood

• located inside blood vessels

• contains relatively high concentrations of plasma proteins

diffusion

• movement of molecules/ions down a concentration gradient due to kinetic energy with no required ATP

simple diffusion

• through a lipid bilayer

• examples: oxygen, carbon dioxide, nitrogen, fatty acids, steroids, fat-soluble vitamins

facilitated diffusion

• transport via membrane proteins (channels/carriers)

• examples: glucose, fructose, galactose, some vitamins, ions (potassium, sodium, chlorine, calcium)

osmosis

• passive movement of water across a selectively permeable membrane from to low concentration

• special for water

active transport

• movement against concetration gradient

• requires ATP + carrier proteins

primary active transport

• uses ATP hydrolysis directly

• pumps ion against gradient

• examples: sodium, potassium, calcium, hydrogen, chlorine

secondary active transport

• uses energy from sodium or hydrogen gradient

• Antiport: calcium and hydrogen out of cell

• symport: glucose (SGLT), amino acids into cell

endocytosis

• bringing substances into cell

• an active transport process where a cell brings large molecules, fluids, or whole particles inside by folding its cell membrane inward to form a tiny sac called a vesicle

exocytosis

• secretion of contents via vesicle fusion

• an active transport process where a cell expels large molecules or waste by fusing internal vesicles with the plasma membrane

transcytosis

• movement across a cell (endocytosis + exocytosis)

• macromolecules move across the inside of a cell by being swallowed on one side in a tiny bubble and pushed out on the other side

Membrane potential

• the difference in electrical charge between the inside and the outside of a cell

• Factors determining this: ion concentration gradient and ion permeability of the membrane

Resting membrane potential

• is the electrical voltage difference across the cell membrane when the cell is at rest

• The inside of the cell is negative relative to the outside

• Results from the unequal distribution of ions across the membrane

Leak channels

• randomly open and close

• Allow passive ion movement

• Location: present in nearly all cells: dendrites, cell bodies and axons

Ligand gated channels

• open in response to chemical (ligand) binding important in synaptic transmission

• Location: dendrites of sensory neurons, Dendrites and cell bodies of interneurons and motor neurons

Mechanically gated channels

• open in response to mechanical stimuli such as touch, pressure, vibration and tissue stretch

• Location: dendrites of sensory neurons, touch receptors, pressure receptors and some pain receptors

Voltage gated channels

• open in response to changes in membrane potential (change in voltage)

• Essential for a action potential generation

• Location: axons of all neuron types

Graded potential

• small, local changes in membrane potential

• Occurs when ligand-gated or mechanically gates channels open/close

• Triggered by a stimulus

• Depolarizing (less negative, graph goes upward

• Hyperpolarizing (more negative, graph goes downward

• Location: dendrites, cell body, sensory receptors

• Post-synaptic, short distance signaling only and can summate

Action potential

• Rapid sequence of events that reverses the membrane potential and restores it to the resting state

• Generated at the axon hillock when threshold is reached

• All-or-none principle with no summation

• Propagates along the axon

• Ion channels involved: voltage gated sodium and voltage gated potassium channels

  
  

Depolarization

• triggered when the threshold is exceeded, causing rapid sodium influx and the membrane become less negative

Repolarization

• potassium efflux restores membrane potential and returns toward -70 mV

After-hyperpolarization

• Membrane becomes more negative than the resting level due to continued potassium efflux

Refractory period

• state of resistance

• Complete insensitivity to another stimulus

• From beginning of action potential until near end of repolarization

• Note: a stronger-than-threshold stimulus can initiate another action potential

Nerve impulse conduction

• the movement of an electrical and chemical signal along a axon

• Resulting action potential causes an electric current that stimulates adjacent portions of the membrane

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continuous conduction

• occurs in unmyelinated neurons

• Action potential propagates along the entire axon membrane

• Slow conduction ~1m/s

Saltatory conduction

• occurs in myelinated neurons

• Electrical charge moves along the axon rather than across the membrane

• Action potential occurs only at unmyelinated regions: nodes of ranvier, where the gated channels are concentrated, jumping from one node to another

• Fast conduction: up to 100m/s

Synapses

Points of contact between nerve cells where signals are transmitted from one cell to another

Electrical synapes

• action potentials conduct directly between the plasma membranes of adjacent neuron through structures called gap junctions

• This has faster communication and synchronization

Chemical synapses

• presynaptic and postynaptic neurons are separated by the synaptic cleft (a space filled with interstitial fluid)

• Presynaptic neuron converts an electrical signal into a chemical signal

• Postsynaptic neuron receives the chemical signal and in turn generates an electrical signal

Postsynaptic potential

• local changes in membrane potential in the postsynaptic neuron

• occur in response to neurotransmitter release, causing a graded potential

• They can summate (temporal and spatial)

Excitatory PSP

• depolarization

• Moves the membrane closer to threshold classically due to sodium influx

Inhibitory PSP

• hyperpolarization

• Moves the membrane away from the threshold, typically due to potassium influx or chlorine influx

Synaptic integration

• the combining of EPSPs and IPSPs on a neuron