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CELL STRUCTURE
pg. 1 There are trillions of cells in the body, over 200 different types, microscopically small or, as with some nerve cells, up to 3’ long. Some, skin cells being an example, die and are replaced continuously. Nerve cells, however, live as long as you do and never reproduce themselves. All, though, have certain similar characteristics. The Cell Theory states that
- the cell is the basic structural and functional unit of life
- an organism’s vitality depends on the health of the cells within it
- a cell’s activity depends on the health of the units within it - Principal of Complementarity
- cells must continue for life to continue There are three parts of a generalized, “average” human cell. Not all cells always have all of them.
- plasma membrane – provides a boundary between the fluid outside the cell (extracellular fluid, ECF) and fluid inside the cell (intracellular fluid, ICF)
- cytoplasm – the ICF, sort of the consistency of melted Jell-o
- nucleus – contains the DNA and controls the activity of the cell; red blood cells don’t have one, ejecting theirs to make more room to carry oxygen FLUID MOSAIC MODEL – means that the structures that make up the membrane are always present but continually shifting positions PLASMA MEMBRANE – this is the membrane surrounding the cell, separating it from outer fluids (intracellular [ICF] vs. extracellular [ECF]) Phospholipid bilayer – phospholipid molecules like 2 tadpoles tail to tail, heads facing toward the ICF or ECF, tails facing each other; membrane also contains glycolipids and cholesterol in smaller amounts head – hydrophilic (attracts the water in the ICF and ECF) tail – hydrophobic Spaced along the membrane are integral proteins ; their functions include:
- act as a door way through which substances enter and exit; some molecules can dissolve right through the membrane itself, those that can’t need a protein channel
- signal transmission from a hormone to inside the cell
- join cells together – Cell Adhesion Molecule
- cell recognition – which are yours, which are invaders, enabling the immune system to know what to attack, what to leave alone
- attachment to other structures – the cell’s inner skeleton needs an anchor point Peripheral proteins – found on the inner or outer membrane surface attached loosely to the integral proteins or membrane lipids functions: 1. enzymes
- alter cell shape during division or muscle contractions
- link cell to cell glycocalyx – a “sugar coating” on the cell, allowing cells to identify one another (sperm identifies egg, WBC recognizes bacteria and viruses) microvilli – short extensions of the cell membrane that increase the cell’s exposed surface area for increased absorptive capabilities (digestive tract, kidney tubules) pg. 2
CILIA and FLAGELLA are attachments to the cell membrane. Cilia beat rhythmically to pass objects along. The fallopian tubes use ciliary motion to carry an egg to the uterus.
CYTOPLASM – between the membrane and the nucleus is a “Jello-like goo”, about the consistency of liquid soap; cellular activity occurs in it and cellular organelles are housed in it 3 elements of cytoplasm
1. cytosol – water + proteins + salts + sugars + other solutes 2. organelles – working structures, most are enclosed in a membrane similar to the plasma membrane 3. inclusions – may or may not always be there; these have a chemical rather than a functional purpose and are not necessary for cell survival (glycogen - lipids - keratin (waterproofing) - melanin [pigments])
ORGANELLES – not all of these are found in all cells; the red blood cells and fibers of the eye lens have no nucleus and few organelles
1. Mitochondria – energy power plant to provide ATP their density (many or few) reflects the cell’s energy needs has an outer and an inner membrane (inner called cristae ) – it protrudes into the matrix, where enzymes break down nutrients for energy aerobic respiration is carried on in the mitochondria contain their own DNA and RNA for replication; they divide and multiply when more are needed since they use oxygen to create ATP, red blood cells have no mitochondria 2. Ribosomes – the site of protein synthesis free ribosomes – suspended in the cytosol, making proteins for use in the cell membrane-bound – produce proteins for export from the cell or for use in the cell membrane ribosomes can switch back and forth from free to bound as needed by the cell 3. Endoplasmic Reticulum – a membrane that is continuous with the nuclear membrane; the membrane encloses cisternae There are 2 types A. Rough – studded with ribosomes that produce export proteins; abundant in secretory cells like the liver (proteins) or plasma cells (make antibodies); it also forms the proteins and phospholipids that make membranes B. Smooth – does no protein synthesis, instead its enzymes are catalysts for 1. lipid metabolism, cholesterol synthesis in liver 2. synthesis of steroid-based hormones (testosterone, estrogen) 3. absorb, synthesize, and transport fat 4. detoxify drugs, pesticides, carcinogens, alcohol 5. glycolysis in the liver 6. in skeletal and cardiac muscle the SER is called sarcoplasmic reticulum – there it stores calcium needed for muscle contractions pg.
Cells “swim” in interstitial fluid (ISF), filtered blood w/amino acids, sugars, fatty acids, vitamins, hormones, salts, neurotransmitters and such dissolved in it. Each cell must extract from the ISF what it needs. These materials must pass through the selectively permeable cell membrane. There are several ways to do so. PASSIVE TRANSPORT – no energy is used ACTIVE TRANSPORT – ATP is used PASSIVE TRANSPORT
1. Diffusion – the kinetic energy of molecules causes them bump into one another and to flow from an area of high concentration to one of low concentration. Think of it as billiard balls bouncing off each other until they’re evenly spaced from one another. Particles diffuse from high concentration areas to low concentration areas until both concentration areas are even. Molecules can diffuse into the cell if they are
- lipid soluble – these pass right through the membrane
- small enough to pass through the pore of one of the protein channels
- picked up by a protein taxi The rate of diffusion depends on
- size of particle – smaller particles diffuse more rapidly
- temperature – the higher the temperature, the more rapid the diffusion
- concentration gradient – the greater the difference between the high and low concentrations the more rapid the diffusion 4.-membrane surface area – if there is more membrane to diffuse through, diffusion will be more rapid
- membrane permeability and receptor availability – some particles dissolve through the membrane more easily than others; those that can’t need a protein channel so the more channels there are the more rapid the diffusion will be
SIMPLE DIFFUSION nonpolar molecules, if lipid soluble, (such as O, C, CO 2 , alcohol, the lipid soluble vitamins A, D, E, K) go through the membrane itself. water soluble molecules pass through pores or channel proteins on the membrane
OSMOSIS – a form of simple diffusion in which a LIQUID (water) diffuses from an area of high concentration to an area of low concentration across a membrane; the water concentration is determined by the concentration of the solutes dissolved in the waters; diffusion occurs until equilibrium is reached, but only WHEN THE MEMBRANE IS PERMEABLE TO ALL THE MOLECULES IN THE SOLUTION.
When the membrane is impermeable to some solutes, but permeable to water, the water still diffuses until the solute concentration is equal on both sides of the membrane, but one side will have more water. In animal cells osmosis will continue until solute equilibrium is reached, regardless of any possible damaging effects - cells can either gain or lose too much water, either bursting like a balloon or drying up like a raisin (remember that “concentration” of water does not mean “amount” of water) EXAMPLE: put 10 gal of water on either side of a membrane, put 5 cups of sugar in one side, 2 cups in the other; water and sugar will both move back and forth across the membrane, IF THE MEMBRANE IS PERMEABLE TO BOTH, until you have 10 gal. on each side, with 3.5 cups of sugar per side. pg. 5 TONICITY – ability to change tension in a cell by altering the water volume; solutions are either ISOTONIC , HYPERTONIC , or HYPOTONIC – the key factor governing the tonicity of a solution is its concentration of solutes that can not penetrate the membrane
Isotonic – concentration is equal to that of body fluids; diffusion is always in balance, the cell retains its shape, neither gaining nor losing water Hypertonic – ECF (extracellular fluid) has a higher concentration of solutes than the ICF (intracellular fluid); solutes can’t cross the membrane into the cell, so water crosses out; the cell shrinks (CRENATE) as water leaves Hypotonic – ECF has a lower solute concentration than the ICF, solutes don’t cross the membrane to get out, so the cell sucks in water and bursts (LYSE)
FACILITATED DIFFUSION – requires no energy expenditure Suppose a particle can’t dissolve in the lipid membrane or is too big to pass through a protein channel? Here’s how to get through: combine with a protein on one side of the membrane, let it spit you out on the other side, like a shortstop relaying a throw from the outfield. The rate of facilitated diffusion depends on the number of available receptor proteins; receptor sites are highly selective and specific as to what they bind to and carry into the cell.
FILTRATION – passive transport that relies on hydrostatic pressure, as in the high pressure in capillaries forcing fluids out into the tissues, where pressure is lower; the factor is a pressure gradient, rather than a concentration gradient ALL OF THE ABOVE ARE PASSIVE PROCESSES REQUIRING NO EXPENDITURE OF ENERGY
ACTIVE TRANSPORT This works the same as facilitated diffusion, using carrier proteins, but moves molecules against their concentration gradient, from low to high. Primary Active transport – uses ATP(Na+-K+^ pump) K+^ is 10 - 20x more concentrated in the ICF (intracellular fluid) than in the ECF (extracellular fluid). Na+^ is 10 - 20x more concentrated in the ECF than in the ICF. Na+^ diffuses in and K+^ diffuses out continuously, according to their concentration gradient. To establish the proper concentration of each on the proper side of the membrane (so the cell can be electrically charged) we need to offset that diffusion. This is especially important for nerve and muscle cells. A protein carrier uses ATP to pump 3 Na+^ out, simultaneously pumping 2 K+^ in. Both are pumped against their concentration gradients, from low concentration to high concentration. [refer to establishing Resting Membrane Potential to understand the significance of this.]
Secondary active transport – Na+^ pumped out creates a gradient (more outside, less inside) – it wants to leak back in as it diffuses back in (facilitated diffusion) it carries other substances with it; this is a passive process that relies on conditions that have been set up by an active process. pg. 6 VESICULAR TRANSPORT – transports large molecules across the membrane EXOCYTOSIS – material goes out of cell ENDOCYTOSIS – material comes in to cell
cellular respiration – food, primarily glucose, is broken down for energy (ATP); primarily a catabolic process enzymes in a cell split off the Pi to release the energy that binds it 3 steps to get the energy
- digestion
- in the cell these nutrients are used to create larger molecules (lipids, proteins, glycogen) {anabolic} OR they’re broken down to create pyruvic acid or acetylCoA {catabolic}
- in mitochondria nutrients are broken down to create water, CO 2 , and ATP {catabolic}
HOW TO GET ATP ATP (adenosine triphosphate) is an adenine molecule with three inorganic phosphates {Pi} attached by means of high energy bonds; the phosphates are all negatively charged so a lot of energy is required to hold them together; these phosphates are now like a coiled cobra ready to strike when the bonds are broken
oxidation – reduction reactions occur – an electron is lost from a particular molecule (oxidized), which is then transformed to another molecule (reduced); these are called REDOX reactions 2 main enzymes 1. dehydrogenase – removes hydrogen
- oxidase – transfers oxygen coenzymes are also needed, usually derived from Vitamin B
- nicotinamide adenine dinucleotide (NAD+)
- flavin adenine dinucleotide (FAD) 2 WAYS TO GET ATP SUBSTRATE LEVEL PHOSPHORYLATION OXIDATIVE PHOSPHORYLATION SUBSTRATE LEVEL PHOSPHORYLATION – a phosphate is transferred from an already phosphorylated substance and added to ADP to create ATP; this can occur in the cytoplasm or the mitochondria oxidative phosphorylation occurs in the mitochondria METABOLISM OF A CARBOHYDRATE
- carbohydrates are broken down to glucose C 6 H 12 O 6
- glucose enters the cell by facilitated diffusion
- a phosphate group (PO 43 - ) is removed from ATP to transform glucose into glucose - 6 – phosphate; this is different from “real” glucose so more glucose can still diffuse into the cell; G-6-P is the starting point for all catabolic or anabolic pathways pg. 8 GLYCOLYSIS doesn’t use oxygen but can occur if oxygen is present (anaerobic); occurs in the cytosol of the cell; creates 2 pyruvic acid molecules and 2 ATP molecules PHASE I
glucose enters the cell and becomes G-6-P, which is then transformed to fructose-6-phosphate F-6-P picks up another phosphate, becoming fructose - 1,6 - diphosphate P - C-C-C-C-C-C - P PHASE II F-6-P is split down the middle, creating two 3-carbon molecules glyceraldehyde - 3 - phosophate P - C-C-C dihydroxyacetate phosphate C-C-C - P PHASE III these each lose a hydrogen (are oxidized) that hydrogen goes to NAD + to make it NADH + H+ inorganic phosphates, available in the cytosol, are added to these 3-carbon fragments by high energy bonds THESE ARE LATER SPLIT OFF TO CREATE ATP (4 OF THEM) At the end of glycolysis you end up with 2 pyruvic acid molecules and 4 ATP, but you used 2 ATP in the process, so you generate a “profit” of 2 ATP if oxygen is present pyruvic acid goes into the mitochondria to enter the Krebs cycle if oxygen is not present, pyruvic acid is changed into lactic acid lactic acid can accumulate in tissues, OR it may go back to the liver where it can be reconverted to G-6-P to be stored as glycogen, OR the liver can remove a phosphate to create “real” glucose and return it to the blood stream OR it can turn it back into pyruvic acid if needed and if oxygen is present send it to the Krebs cycle lactic acid hurts if accumulated in skeletal muscle it damages cardiac muscle more quickly it damages the brain much more quickly; the brain uses only glucose, so oxygen is necessary for it not to turn into lactic acid
KREBS CYCLE takes pyruvic acid from glycolyis, or fatty acids from fat breakdown, and transfers it to the mitochondria; Krebs doesn’t need oxygen, but it is linked with another system that does, so this is AEROBIC RESPIRATION
- In the mitochondria pyruvic acid becomes acetylCoA, by this process a. a carbon is split from pyruvic acid b. H+ atom are removed to make it acetic acid c. coenzyme A is added to make it acetylCoA pg. 9
- Acetic acid (a 2-carbon acid), joins with oxaloacetic acid (a 4-carbon acid) to form citric acid (a 6-carbon acid) citric acid then goes through several chemical steps to yield 1 ATP and then steps continue to form the original oxaloacetic acid to combine with acetic acid and make acetylCoA to keep the cycle spinning
activity and growing phase) and the mitotic phase (cell division). INTERPHASE – this is the period during which the cell is performing its function, and it has 3 subphases: G 1 (gap 1), S (synthesis), G 2 G 1 – the cell is active, producing proteins or whatever else its function may be; this phase can last from minutes to years, depending on the type of cell and its rate of reproduction; if a cell no longer replicates itself it is in G 0 phase (red blood cells, neurons) during most of G 1 no cell division activity or preparation for cell division occurs; at the end of G 1 the centriole ( the organelle that organizes the mitotic spindle) begins to replicate itself S – DNA replicates itself G 2 – very brief period; enzymes and proteins needed for cell division are made and put in place; centriole replication is complete DNA REPLICATION DNA and RNA are the cell’s nucleic acids, made up primarily of carbon [C], hydrogen [H], oxygen [O], nitrogen [N], and phosphorous [P]
- the structural unit is called a nucleotide – a pentose sugar (deoxyribose or ribose), with a phosphate group attached, serving as the side “legs” of a ladder, with certain bases attached as the “rungs”. The base plus the phosphate plus the sugar together is called the nucleoside. Put 2 nucleosides (“half ladders”) together with a hydrogen bond between the bases and you have the double-stranded molecule DNA.
the 5 bases are adenine (A) A, G, C, T are used in DNA guanine (G) A, G, C, U are used in RNA cytosine (C) thymine (T) In DNA, A always binds with T, G always binds with C uracil (U) In forming RNA, G always binds with C, but A always binds with U these are called complimentary bases, because they always bind together sample DNA molecule: one strand A C G T T A C G A
opposite strand T G C A A T G C T
DNA – found in the nucleus; provides instructions for building the body’s proteins Prior to cell division it is responsible for replicating itself, so the cell can reproduce. RNA – found in the cytoplasm; does as told by DNA to produce the proteins How does RNA differ from DNA?
- different sugars (ribose vs. deoxyribose)
- different bases (uracil in RNA instead of thymine)
- RNA is outside the nucleus
- RNA is a single stranded molecule pg. 11 DNA must be copied before the cell divides
- the DNA double helix unwinds from the nucleosome, the chemical to which the strands adhere.
- DNA unwinds its helixes into individual nucleotides
- each nucleotide serves as a pattern to reproduce its “mirror image” nucleotide from materials in the nuceloplasm
THESE ORIGINAL STRANDS SEPARATE
A T A T
C G C G
G C G C
T A T A
T A T A
A T A T
C G C G
G C G C
A T A T
THESE DUPLICATE STRANDS ARE FORMED
we end up with a new DNA, exactly like the old DNA, each one with a new and a borrowed nucleotide CELL DIVISION Not all cells reproduce (skin, intestine, pancreas – constant reproduction; cardiac, nerve, red blood cells – not at all) and those that do, reproduce at different rates for regeneration or growth. Cells begin to divide when their inner volume exceeds a mathematical relationship with the circumference of its membrane [ size-to-volume ratio]. If there is too much cytoplasm the membrane isn’t large enough to supply food or remove waste. 2 phases of reproduction include mitosis – when the nucleus divides, and cytokinesis – when the cell itself becomes two cells MITOSIS – parcels out new DNA, lasts about an hour has 4 phases – prophase (the longest of these 4 phases), metaphase, anaphase, telophase CYTOKINESIS – occurs about 2/3 of the way through mitosis, at the end of telophase During the end stages of interphase the centrioles begin to replicate; after that mitosis begins, following these four steps prophase 1. chromosomes (condensed DNA) form
- centrioles migrate to opposite poles of the cell
- the nuclear membrane bursts, allowing the chromosomes to escape into the cytoplasm metaphase 1. the chromosomes from the nucleus cluster at the center (equator) of the cell anaphase 1. the chromosomes split and subsequent chromatids (split segments of DNA) migrate toward the centrioles at the opposite poles telophase 1. the chromosomes regroup in a newly membrane-encased nucleus cytokinesis occurs – the “old” cell cleaves down the middle, creating two identical new cells pg. 12 RNA AND PROTEIN SYNTHESIS During interphase, RNA in the cytoplasm carries out DNA’s instructions to produce proteins for use by the cell or the rest of the body. it takes 3 types of RNA to perform all the duties – Messenger (mRNA) Ribosomal (rRNA)
