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Unit 2: Cell Structure and Function

Cell Organelles

Fundamental Units of Life/Study of Cells/Surface Area

All organisms are made of cells
The cell is the simplest collection of matter that can be alive
Cell structure is related to cellular function
All cells are related by their descent from earlier cells
Everything that happens to the body originates from an effect on the cells
How Cells are Studied
Usage of Microscopes
Light Microscopes - see most cells, organelles, bacteria, etc.
Electron Microscopes - viruses, ribosomes, proteins, etc.
Scanning Microscope (SEMs) - scans surface of an object, tells what an object looks like on a surface, use for viruses, proteins
Transmission Microscope (TEMs) - look inside of an object
Cell Fractionation - uses centrifuge (spins fast) to separate contents based upon density split cell into its components, isolates organelles
Begins with homogenization (break apart cell membranes, blenderize)
Put in centrifuge and separate contents after spinning, get pellet in bottom (most dense part) Nucleus of each cell is bottom
Pour supernatant on top of nuclei, centrifuge again, find mitochondria and chloroplasts in pellet
Pour out supernatant on top again to find membrane pellet
Find Ribosomes in last pellet
Why are cells so small?
Cells take in three things thru membrane: nutrients, O2, and H2O
CO2 and waste leave the cells thru the membrane
Cells need as much surface area as possible (the amount of membrane you have) more surface area = faster rate of exchange
Smaller cells have more surface area compared to volume (which is amount of stuff in cell)
More efficient at exchange, for respiration and waste excretion
Surface area vs. Volume (ratio):
Larger the volume of a cell the more surface area it needs to exchange materials necessary for life to continue
Nucleus (brain of the cell) can only control a certain amount of cell processes, control smaller cell better than bigger cell
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Cell Membrane

The Cell membrane is selectively permeable - only certain things move through the membrane
Amphipathic Molecule: (polar and nonpolar) Made up of phospholipid has a head (phosphate group PO4, POLAR attracts to water) and two fatty acid nonpolar tails (hydrocarbons)
One straight (saturated) and one bent (unsaturated, removed hydrogens) fatty acid
One half hydrophilic and one half hydrophobic
Phospholipid bilayer self assemble into membrane polar heads face water and lipids face each other on the inside
Fluid Mosaic patchwork of proteins that are embedded in cell membrane
“Fluid” refers to phospholipids, the phospholipids float around in membrane, not rigid structure proteins are also mobile
Phospholipids can move laterally along membrane but can infrequently flip flop sides
Different membranes have different amounts of saturated vs unsaturated fatty acids, and determine viscosity. saturated fats are solid, unsaturated fats are more liquid (because of kinks, can’t stack)
In arctic, use more unsaturated fats for liquid membrane, avoid freezing
In desert, to combat too much liquidness, have more saturated fats
Membranes change over time depending on environment
Cholesterol (solid four ring carbon structure) - stops membrane from becoming too fluid, restrains movement of phospholipid
Excessive cholesterol (solid) can form plaques
Membrane Proteins
Peripheral Proteins - “outside’ proteins, does not span the membrane, found on one side of the membrane (in or out of the cell) or the other
Polar because outside membrane
Integral Proteins - (transmembrane protein) span the entire membrane (embedded)
Parts of the protein in bilayer are nonpolar, outside has polar regions (amphipathic)
Signal transduction (receptors)
To transport quickly thru bilayer, must be very small or nonpolar (e.g. fats, steroid hormones, H2O, CO2, O2)
Large and polar things need Transport proteins (integral) to get thru membrane (glucose, amino acids)
Transport proteins extreme specific, like enzymes, only transport certain types of substances, allow hydrophilic substances across the membrane → shape determined by sequence amino acids
Channel proteins have a hydrophilic channel that certain molecules or ions can use as a tunnel
Polar channel but outside of channel is nonpolar
Aquaporins - facilitate the passage of water, make water move faster but still can fit thru membrane cuz small
Carrier Proteins bind to molecules and change shape to shuttle them across the membrane
Changes shape, opens on one side to let it go thru and then reopen inside the membrane
Enzymatic activity
Cell-cell recognition
Glycoprotein MHC - major histocompatibility (work with tissues) complex (integral in cell membrane)- with carbohydrate (glucose) attached to it are like billboards on outside of cells (every person has a unique glycoprotein, that identifies self)
Recognize by binding to surface molecules often with carbs
If a foreign cell enters the body, if it doesn’t have the unique glycoprotein (unless they are twins), immune system attacks it
Membranes carbs are covalently bonded to proteins forming MHC
Intercellular joining
Attachment of the cell to the extracellular matrix (ECM): integrins

Active and Passive Transport

Passive Transport- no energy required, material moves from high concentration from low concentration == going down the concentration gradient,
Own kinetic energy drives reaction
Diffusion: movement of molecules from high cons. to low cons. (O2, CO2)
Once equilibrium is met, the net concentrations remain equal but are still moving
E.g. chemiosmosis: diffusion of protons
Osmosis: diffusion of water
Look at water vs solute, if you have the same amount of solution on either side, but more solute on one side, that means there is more water on the other side, so water will move
Defies gravity
Tonicity: ability of a surrounding solution to cause a cell to gain or lose water i.e. amount of solute, hypertonic and hypotonic refer to the solution
Hypertonic: more solute=less water: dehydrates
Cells in hypertonic solution: Water leaves the cell, and shrivels
Hypotonic: less solute=more water: hydrates
Cells in hypotonic solution: Water enters the cell and swells (could lyse cells or when the cell bursts)or when the because these a higher concentration of water on the outside,
Isotonic: solution with equal concentrations
Osmolarity of blood: natural concentration of all solutes in your blood (salt [electrolytes], and sugar, etc.)
Osmoregulation: kidneys (excretory system) perform or regulate blood volume, if you have excess water, you pee out the rest to stop you from exploding
Plant cells have cell walls and stop lysing
Hypo: Instead become turgid (swollen) cells, better for photosynthesis
Cells can’t lyse because of cell walls, just flex outward and become RIGID and hard
Iso: cells become flaccid
Hyper:Plasmolyzed, cell membrane pulls off oc cell wall and kills plant cell
Facilitated Diffusion - passive transport of large polar things by a transport protein down a concentration gradient (amino acids, sugar, etc.)
Active Transport - pump against gradient, in the opposite direct, must invest energy (ATP), usually to build a concentration gradient
Sodium-Potassium Pump - ion pump
part of nervous system, pumps sodium out of neuron and potassium in the neuron → pumps ions (atoms with charge)
Ion Pumps (aka a proton pump (proton pumps are a type of electrogenic pump) or electrogenic pump) type of active transport protein that uses ATP to pump ions in one direction,
Establish Membrane Potential: difference in charge outside and inside a membrane
Voltage: The difference in charge, generated by ion pumps, can generate electricity
Electrochemical gradient - chemical gradient, the actual amount of ions on either side, and the electrical gradient which is the difference in charge
Simultaneously create two gradients, physical concentration and voltage difference
All types of organisms use proton pumps → protons are used to get work done (respiration and photosynthesis)
Plants use proton pumps for cotransport in plant cells
Phloem tubes transport sugar, but high concentration of sugar in phloem, so sugar wants to diffuse into cells, but cells need to load the phloem with more sugar, so active cotransport pumps sugar into phloem
Proton pumps pump protons out of the phloem, and protons want to diffuse back into the phloem, then second cotransporter that transports sucrose and protons, sucrose cannot go through without protons, protons pull sucrose into phloem, uses diffusion of protons in order to load sugar up its gradient into the phloem, used in roots
Plants need to live in slightly acidic soil for more protons in a proton gradient to absorb nutrients (nitrate) from the soil

Bulk Transport

movement of extremely large things in and out of the cell, cannot fit through protein channels
Endocytosis - takes big things in, phospholipids in membrane create a vesicle around the object, membrane caves in
Phagocytosis: cell eating
How WBCs (macrophages) engulf bacteria and viruses make food vesicles
And cells are sent to lysosome for destruction (hydrolysis)
Pinocytosis: same thing as phagocytosis with liquids
Receptor-Mediated Endocytosis when substance is binded to a receptor and substance and receptor are brought in
Exocytosis - sends big things out (like proteins, using transport vesicles from Golgi UPS) Golgi made out of phospholipid bilayer, can form vesicles easily fuses with membrane and releases contents
Most organelles are membrane bound for vesicle formation and transportation

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