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Cell Membrane Structure, Function: Glycocalyx, Transport, and Origin of Life Experiments, Lecture notes of Biology

Johns Hopkins University (JHU)Biology

An overview of cell membrane structure and function, including the roles of carbohydrates, lipids, and proteins in cell recognition and transport. It covers topics such as the glycocalyx, electrochemical gradients, bulk transport (endocytosis and exocytosis), and the miller-urey experiments related to the origin of life. The document also discusses passive and active transport mechanisms, including diffusion, osmosis, and the sodium-potassium pump. It is a valuable resource for understanding the fundamental processes of cell biology. This document also covers the endosymbiotic theory and the structure of plant and animal cells. It also covers the experiment of pasteur and the experiment of frye-edidin. It also covers the different types of solutions: hypertonic, isotonic and hypotonic.

Typology: Lecture notes

2024/2025

Available from 06/09/2025

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Chapter 5: Structure and Function of Plasma Membranes
5.1 Components and Structure
Fluid Mosaic Model
-In the 1950s, researchers were able to discover that the plasma membrane’s core
consisted of a double, rather than a single, layer.
-The fluid mosaic model describes the plasma membrane structure as a mosaic of
components – including phospholipids, cholesterol, proteins, and carbohydrates – that
gives the membrane a fluid character.
-A plasma membrane’s principle components are lipids (phospholipids and cholesterol),
proteins, and carbohydrates attached to some of the lipds and proteins. A phospholipid is
a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group.
-Carbohydrates are present only on the plasma membrane’s exterior surface and are
attached to proteins, forming glycoproteins, or attached to lipids, forming glycolipids.
Phospholipids
-The membrane’s main fabric comprises amphiphilic, phospholipid molecules. The
hydrophilic or “water-loving” areas of these molecules are in contact with the aqueous
fluid both inside and outside the cell.
-Hydrophobic, or water-hating molecules, tend to be non-polar. They intereact with other
non-polar molecules in chemical reactions, but generally do not interact with polar
molecules.
-The phospholipids’ hydrophilic regions form hydrogen bonds with water and other polar
molecules on both the cell’s exterior and interior, Thus, the membrane surfaces that face
the cell’s interior and exterior are hydrophilic. In contrast, the cell membrane’s interior is
hydrophobic and will not interact with water.
-A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid
molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the
third carbon. Head area -> phosphate, Tail -> fatty acid
-Amphiphilic: Molecule possessing a polar or charged area and a nonpolar or uncharged
area capable of interacting with both hydrophilic and hydrophobic environments.
-Phospholipids heated in an aqueous solution usually spontaneously form small spheres or
droplets (micelles or liposomes), with their hydrophilic heads forming the exterior and
their hydrophobic tails on the inside.
Proteins
-Integral Proteins: Protein integrated into the membrane structure that interacts
extensively with the membrane lipids’ hydrocarbon chains and often spans the membrane
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Download Cell Membrane Structure, Function: Glycocalyx, Transport, and Origin of Life Experiments and more Lecture notes Biology in PDF only on Docsity!

Chapter 5: Structure and Function of Plasma Membranes 5.1 Components and Structure Fluid Mosaic Model

  • In the 1950s, researchers were able to discover that the plasma membrane’s core consisted of a double, rather than a single, layer.
  • The fluid mosaic model describes the plasma membrane structure as a mosaic of components – including phospholipids, cholesterol, proteins, and carbohydrates – that gives the membrane a fluid character.
  • A plasma membrane’s principle components are lipids (phospholipids and cholesterol), proteins, and carbohydrates attached to some of the lipds and proteins. A phospholipid is a molecule consisting of glycerol, two fatty acids, and a phosphate-linked head group.
  • Carbohydrates are present only on the plasma membrane’s exterior surface and are attached to proteins, forming glycoproteins , or attached to lipids, forming glycolipids. Phospholipids
  • The membrane’s main fabric comprises amphiphilic, phospholipid molecules. The hydrophilic or “water-loving” areas of these molecules are in contact with the aqueous fluid both inside and outside the cell.
  • Hydrophobic , or water-hating molecules, tend to be non-polar. They intereact with other non-polar molecules in chemical reactions, but generally do not interact with polar molecules.
  • The phospholipids’ hydrophilic regions form hydrogen bonds with water and other polar molecules on both the cell’s exterior and interior, Thus, the membrane surfaces that face the cell’s interior and exterior are hydrophilic. In contrast, the cell membrane’s interior is hydrophobic and will not interact with water.
  • A phospholipid molecule consists of a three-carbon glycerol backbone with two fatty acid molecules attached to carbons 1 and 2, and a phosphate-containing group attached to the third carbon. Head area -> phosphate, Tail -> fatty acid
  • Amphiphilic : Molecule possessing a polar or charged area and a nonpolar or uncharged area capable of interacting with both hydrophilic and hydrophobic environments.
  • Phospholipids heated in an aqueous solution usually spontaneously form small spheres or droplets (micelles or liposomes), with their hydrophilic heads forming the exterior and their hydrophobic tails on the inside. Proteins
  • Integral Proteins : Protein integrated into the membrane structure that interacts extensively with the membrane lipids’ hydrocarbon chains and often spans the membrane

- Peripheral Proteins : Protein at the plasma membrane’s surface either on its exterior or interior side o Along with integral proteins they may serve as enzymes, as structural attachments for the cytoskeleton’s fibers, or as part of the cell’s recognition sites. Scientists sometimes refer to these as “cell-specific” proteins. The body recognizes its own proteins and attacks foreign proteins associated with invasive pathogens. Carbohydrates

  • Carbohydrates are the third major plasma membrane component. They are always on the cell’s exterior surface and are bound either to proteins (forming glycoproteins) or to lipids (forming glycolipids).
  • Along with peripheral proteins, carbohydrates form specialized sites on the cell surface that allows cells to recognize each other.
  • We collectively refer to these carbohydrates on the cell’s exterior surface—the carbohydrate components of both glycoproteins and glycolipids – as the glycocalyx (meaning “sugar coating”). The glycocalyx is highly hydrophilic and attracts large amounts of water to the cell’s surface. This aids in the cell’s interaction with its watery environment and in the cell’s ability to obtain substances dissolved in the water. Membrane Fluidity
  • Sterols (such as cholesterol in animal cells) are an additional membrane constituent that assists in maintaining fluidity. They lie alongside the phospholipids in the membrane and tend to dampen temperature effects on the membrane.
  • Thus, they function as a buffer, preventing lower temperatures from inhibiting fluidity and preventing increased temperatures from increasing fluidity too much.
  • Cholesterol also serves other functions, such as organizing clusters of transmembrane proteins into lipid rafts. 5.2 Passive Transport - Plasma membranes are selectively permeable —they allow some substances to pass through, but not others. - The most direct forms of membrane transport are passive. o Passive transport is a method of transporting material through a membrane that does not require energy. Substances move from an area of higher concentration to an area of lower concentration o A physical space in which there is a single substance concentration range has a concentration gradient. Selective Permeability
  • Plasma membranes are asymmetric. On the membrane’s interior, some proteins serve to anchor the membrane to cytoskeleton’s fibers. There are peripheral proteins on the membrane’s exterior that bind extracellular matrix elements. Carbohydrates, attached to
  • Solubility : As we discussed earlier, nonpolar or lipid-soluble materials pass through plasma membranes more easily than polar molecules, allowing a faster diffusion rate.
  • Surface area and plasma membrane thickness : Increased surface area increases the diffusion rate, whereas a thicker membrane reduces it.
  • Distance travelled : The greater the distance that a substance must travel, the slower the diffusion rate. This places an upper limitation on cell size. A large, spherical cell will die because nutrients or waste cannot reach or leave the cell’s center, respectively. Therefore, cells must either be small in size, as in the case of many prokaryotes, or be flattened, as with many single-celled eukaryotes.
  • A variation of diffusion is the process of filtration. In filtration, material moves according to its concentration gradient through a membrane. Sometimes pressure enhances the diffusion rate, causing the substances to filter more rapidly.
  • Solute : Substance dissolved in a liquid to form a solution Facilitated Transport
  • Facilitated Transport : Process by which material moves down a concentration gradient (from high to low concentration) using integral membrane proteins
  • The transported material first attaches to protein or glycoprotein receptors on the plasma membrane’s exterior surface. This allows removal of material from the extracellular fluid that the cell needs. The substances then pass to specific integral proteins that facilitate their passage.
  • Channels o Transport protein : Membrane protein that facilitates a substance’s passage across a membrane by binding it. They function as either channels for the material or carriers. In both cases, they are transmembrane proteins. o Channel protein : Membrane protein that allows a substance to pass through its hollow core across the plasma membrane. Passage through the channel in this hollow core allows polar compounds to avoid the plasma membrane’s nonpolar central layer that would otherwise slow or prevent their entry into the cell. o Aquaporin : Channel protein that allows water through the membrane at a very high rate o Channel proteins are either open at all times or they are “gated”, which controls the channel’s opening. When a particular ion attaches to the channel protein it

may control the opening, or other mechanisms or other substances may be involved.

  • Carrier Proteins o Carrier protein : Membrane protein that moves a substance across the plasma membrane by changing its own shape. o This protein binds a substance and, thus triggers a change of its own shape, moving the bound molecule from the cell’s outside to its interior. Depending on the gradient, the material may move in the opposite direction. Carrier proteins are typically specific for a single substance. o When all of the proteins are bound to their ligands, they are saturated and the rate of transport is at its maximum. Increasing the concentration gradient at this point will not result in an increased transport rate. o Channel proteins transport much more quickly than carrier proteins. Osmosis
  • Osmosis : Transport of water through a semipermeable membrane according to the water’s concentration gradient across the membrane that results from the presence of solute that cannot pass through the membrane. o The water travels to the side where it is less concentrated (high concentration to low concentration). Osmosis continues until the water’s concentration gradient goes to zero or until the water’s hydrostatic pressure balances the osmotic pressure Tonicity
  • Tonicity : Amount of solute in a solution. It describes how an extracellular solution can change a cell’s volume by affecting osmosis. A solution’s tonicity often directly correlates with the solution’s osmolarity.
  • Osmolarity : Describes the solution’s total solute concentration. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles.

Electrochemical Gradient

- Because ions move into and out of cells and because cells contain proteins that do not move across the membrane and are mostly negatively charged, there is also an electrical gradient , a difference of charge, across the plasma membrane. - Cells have higher concentrations of potassium and lower concentrations of sodium than the extracellular fluid. Thus in a living cell, the concentration gradient of sodium tends to drive it into the cell, and its electrical gradient (a positive ion) also drives it inward to the negatively charged interior. - The electrical gradient of potassium also drives it into the cell, but the concentration gradient of potassium drives potassium out of the cell. We call the combined concentration gradient and electrical charge that affects an ion its electrochemical gradient. Moving Against a Gradient

  • The energy cells use to move substances against a concentration or electrochemical gradient comes from ATP generated through the cell’s metabolism
  • Primary active transport moves ions across a membrane and creates a difference in charge across the membrane, which is directly dependent on ATP.
  • Secondary active transport does not directly require ATP; instead, it relies on the movement of material due to the electrochemical gradient established by primary active transport. Proteins: Active Transport
  • An important membrane adaptation for active transport is the presence of specific proteins or pumps to facilitate movement: there are three protein types of transporters. o A uniporter carries one specific ion or molecule. o A symporter carries two different ions or molecules, both in the same direction. o An antiporter also carries two different ions or molecules, but in different directions. o All of these transporters can also transport small, uncharged organic molecules like glucose.
  • Some examples of pumps for active transport are Na+ -K+ ATPase, which carries sodium and potassium ions, and H+ -K+ ATPase, which carries hydrogen and potassium ions. Two other pumps are Ca2+ ATPase and H+ ATPase, which carry only calcium and only hydrogen ions, respectively Primary Active Transport
  • The primary active transport that functions with the active transport of ions such as sodium and potassium allows secondary active transport to occur. The second transport method is still active because it depends on using energy as does primary transport.
  • The sodium-potassium pump moves K+ into the cell while moving Na+ out at the same time, at a ratio of three Na+ for every two K+ ions moved in. The Na+ -K+ ATPase exists in two forms, depending on its orientation to the cell’s interior or exterior and its affinity for either sodium or potassium ions.
  • The process consists of the following six steps
    1. With the enzyme oriented towards the cell’s interior, the carrier has a high affinity for sodium ions. Three ions bind to the protein.
    2. The protein carrier hydrolyzes ATP and a low-energy phosphate group attaches to it.
    3. As a result, the carrier changes shape and reorients itself towards the membrane’s exterior. The protein’s affinity for sodium decreases and the three sodium ions leave the carrier.
    4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
    5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the cell’s interior.
    6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions move into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.
  • As a result of this process, there are more sodium ions outside the cell than inside and more potassium ions inside than out. For every three sodium ions that move out, two potassium ions move in. This results in the interior being slightly more negative relative to the exterior.
  • The sodium-potassium is, therefore, an electrogenic pump , a pump that creates a charge imbalance, creating an electrical imbalance across the membrane and contributing to the membrane potential. This difference in charge is important in creating the conditions necessary for the secondary process. Secondary Active Transport (Co-transport)
  • As sodium ion concentrations build outside of an animal cell because of the primary active transport process, this creates an electrochemical gradient. If a transport protein allows these ions to move, the sodium ions will pull through the membrane.
  • This movement can transport other substances that can attach themselves to the transport protein through the membrane. Many amino acids, as well as glucose, enter a cell this way.
  • Similar processes happen in cells that use a proton pump as their primary electrogenic pump. The potential energy that accumulates in the stored hydrogen ions translates into kinetic energy as the ions move down their electrochemical gradient, and that energy is used to transport other substances against their gradient.

5.4 Bulk Transport

Exocytosis

  • Exocytosis is the opposite of the previous processes in that its purpose is to expel material from the cell into the extracellular fluid.
  • Waste material is enveloped in a membrane and fuses with the plasma membrane’s interior. This fusion opens the membranous envelope on the cell’s exterior, and the waste material expels into the extracellular space.
  • Other examples of cells releasing molecules via exocytosis include extracellular matrix protein secretion and neurotransmitter secretion into the synaptic cleft by synaptic vesicles. This Strange Microbe May Mark One of Life’s Great Leaps - Bacteria and archaea have no nuclei, lysosomes, mitochondria, or skeletons. - In the late 1900s, researchers discovered that mitochondria were free-living bacteria at some point in the past. Somehow they were drawn inside another cell, providing new fuel for their host. - Asgard archea was found to contain no structure and just DNA and proteins. o This finding suggests that the proteins that eukaryotes used to build complex cells started out doing other things, and only later were assigned new jobs. - It was speculated that a series of Agard archaea on the seafloor dragged bacteria into a web of tentacles, drawing them into even more intimate association. Ultimately, it swallowed the bacteria, which evolved into the mitochondria fueling every complex cell. Long-Awaited Cystic Fibrosis Drug Could Turn Deadly Disease Into a Manageable Condition
  • The most common mutation for cystic fibrosis is the protein misfolding and not being able to reach the right spot in the cell. Lecture
  • Taylor’s Idea was that cell wall would gradually become eukaryotic flagellum o New cell wall in some lineages
  • Vertical transfer doesn’t have to be sexual. Reproduction
  • Lateral transfer is taken from another species. No reproduction 9/ What does it mean to be living?
  • Made of cells
  • Carbon-based
  • Growth and Metabolism
  • Reproduction
  • Genetic Material
  • Adaptation and Recent research on Prometheoarchaeum has shown that they have genes that code for proteins closely related to those in eukaryotes. Evidence that eukaryotes can gain genes from their endosymbionts. Theory of Abiogenesis (Oparian and Haldane)
  • 1920’s
  • Oparin: Russian biochemist in soviet unions
  • Haldane: British physiologist. Proposed similar theory in 1929
  • Needed: o Synthesis of macromolecules, esp. proteins o Separation from their environment o Heredity and replication machinery
  • Proposed that at the beginning when you have the mostly inorganic composition of the atmosphere of the earth
  • Exception of methane
  • Have random reactions happening, forming the simplest organic compounds. Later random reactions using the simple organic compounds formed more complex ones i.e. amino acids, nucleotides, carbohydrates
  • Need a reaction for polymerization for peptides, lipids, carbocarbohydrates o Produces genetic material for replication and heredity
  • Lipids could form a membrane barrier that separates water inside from water outside o Produces an isolated system separated from the outside, beginning of a cell
  • Through random reactions could form the molecules you need Miller-Urey experiments 1950’s
  • Experiments to simulate what could have happened
  • Methane is an organic gas the rest are not
  • Atmosphere: CH4, NH3, H20, H2 GASES
  • No O
  • Did not include carbon dioxide
  • Liquid water
  • Bombarded with electrical energy: simulated what happens with lightning in reactions
  • Sampled liquid water to see what new things came in the solution.
  • Results:

▪ 1989 nobel prize ▪ Suggested that the earliest form of cells were RNA based cells o John sutherland’s group (2020): maybe a mix of DNA purines and RNA pyrmidines at first. And then DNA became a primary form of genetic material ▪ Questions the dominance of RNA as the beginning of genetic material Cow Burps

- Studying whether an altered diet can make cattle burp and fart less methane – one of the most harmful greenhouse gases and a major contributor to climate change. - Inside a cow’s stomach is an oxygen-free environment with a steady temperature. Microbes decompose and ferment materials like cellulose, starch and sugars. - Methane is a main biproduct of the enzymes that help break down the food. The gas can’t be turned into energy, so as it builds up, a cow must burp, sending little puffs of pollution into the atmosphere. - Allicin is a chemical that targets enzymes in the cow’s gut that create methane. Too much could harm the cow’s ability to process food, or give the milk and meat a garlic flavor. 9/ Plasma Membrane and Function

  • Lipids in membrane are phospholipids
  • Cholesterol prevent water molecules or anything that would dissolve in water molecules from slipping between phospholipid tails Cis formation
  • Causes the kink in unsaturated fatty acids The fluid mosaic model
  • Fluid structure
  • “mosaic” of various proteins and lipids (singer and Nicolson 1972)
  • Lateral drift: both lipids and protein move sideways very easily. Happens about 10^ times per second
  • Transverse flip-flop is rare: going from one side of the membrane to the other. Happens about once per month o This is more rare because flip-flop requires more energy since the hydrophilic head has to pass through hydrophobic tail region which is difficult due to repulsion
  • Integral membrane proteins may have one or more alpha helices that span the membrane (examples 1 and 2), or they may have beta-sheets that span the membrane (example 3) - Frye-Edidin Experiment o The scientists grew human cells in one dish and mouse cells in another. They used a technique to attach fluorescent labels to some of the proteins on the outside of cells. They labeled some of the proteins in the human cells with a fluorescent blue dye, while labeling the proteins on the mouse cells with a red dye. Then, they used a virus to trick the cells into fusing together. These hybrid cells that were half human, half mouse did not survive for very long, but they did live just long enough to show us something about membranes. At first, just after the cells had fused, all of the blue label was segregated on one half of the hybrid cell, while the red label was on the other half. However, very quickly, the labels began to intermix with each other and within 40 minutes, the blue and red labels were evenly distributed throughout the surface of the hybrid cell. o The quick mixing of the fluorescent labels means that the proteins that are on the surface of the cell are not fixed in place – they can and do diffuse rapidly around the exterior of the cell, while still being embedded in the plasma membrane (redistribution of proteins across lipid bilayer). This led to the development of the fluid-mosaic model. o Question : Do these membrane proteins move laterally? o Alternative Hypothesis : The membrane proteins will move laterally o Null Hypothesis : The membrane proteins will not move laterally over time o Prediction : If we tag the proteins with fluorescent dots, when the cells fuse, then you’ll see an even distribution of the fluorescent dots on the new cell.
  • Membrane carbohydrates o Glycolipids

o Ligand-Gated Channel ▪ nACh receptors on channels ▪ Ach binds to receptor, pore opens and ACh plus ions come in o Voltage gated channel changes shape depending on the voltage on the membrane

  • Active transport o The cell will spend energy since it is moving a solute against its gradient o The source of energy comes from some kind of energy source such as ATP o Electrogenic pumps (causes negative charges across the membrane) ▪ Proton pump ● Only moves one ion. Positive charges leave the cell. ● Not the major electrogenic pump in animal cells ▪ Sodium potassium pump ● 3 sodium ions leave the cell and only 2 potassium ions come inside ● Uses energy to pump both sodium and potassium against their gradients ● Primary active transport moves ions across a membrane, creating an electrochemical gradient (electrogenic transport) ● Sodium binding and removal and potassium changes protein shape ● Mutations in the pump have consequences for health o If the concentration of sucrose is higher inside a cell than outside, sucrose will enter the cell only by active transport because it’s going against the gradient which requires. o A plant cell typically has a membrane potential of -100 mV, negative inside. If the concentration of Ca2+ is higher outside a plant cell than inside, Ca2+ will enter the cell via facilitated diffusion through channels. Due to the fact that it’s an ion cannot cross on its own. Both electrical and chemical gradient favored the same direction. o A plant cell typically has a membrane potential of -100 mV, negative inside. If the concentration of NO3- is higher outside a plant cell than inside, will NO3- enter the cell via diffusion down its concentration gradient? ▪ It depends due to osmotic pressure. Bilayer has nonpolar parts. Since the nonpolar and polar don’t like interaction, will keep the polar particles in respective places despite concentration gradient

▪ In this case electrical gradient favors moment out of the cell and chemical favors movement in the cell ● Two gradients oppose each other so direction depends ● Depends on which gradient is stronger o Channels are for diffusion, pumps are for active transport o Two kinds of active transport ▪ Primary active transport ● Protein transporting a substance? ● Source of energy? ▪ Secondary active transport ● Involves both active and passive transport ● Transport is indirectly driven by ATP hydrolysis o Relies on pumps that use ATP to move something against its gradient to bring it back down on the gradient ● How do these transporters work? o Ex: Na+ - glucose cotransport

  1. Initial state: open to outside
  2. Two Na+ from outside the cell bind
  3. Binding of Na+ allows glucose to bind
  4. Conformational change allows protein to open to the inside of the cell
  5. Na+ Are released inside; Na+/K+ pump pumps them out again
  6. Loss of Na+ is followed by release of glucose due to conformational change
  7. Release of glucose allows the protein to return to initial state o Cotransport : coupled transport by a membrane protein o Symport : driving ion and solute move in same direction o Antiport : driving ion and solute move in opposite directions o Standard volt meter can be used to measure membrane potential. One electrode in the cell and another out of the cell. Membrane potential is the potental between solution inside the cell and outside cell (difference)
  • Null hypothesis states no statistical significance. Alternative hypothesis states that there is one.
  1. What are the dependent and independent variables in this study? How did the researchers standardize the other variables in the study? a. DV: Success rate b. IV: Attendance in the program c. Standardized variables: number of students, year chosen, age
  2. Consider the study described below. What are the problems associated with this study? Explain. If you were to redesign this study, what would be the independent variable? The dependent variable? The sample size? The control treatment? Be prepared to do this kind of analysis for any given scenario. 150 patients with Crohn’s disease are given an experimental treatment in which they all drink “cocktails” containing thousands of pinworm eggs. Within one week, 72% of the patients report a reduction in their abdominal pain. Is ingesting pinworm eggs an effective treatment for the symptoms of Crohn’s disease? a. There are no standardized variables so it can’t be concluded what relieved stomach pain. b. Sample Size: 1,000 people c. IV: Administration of the cocktail d. DV: Abdominal Pain e. Control Treatment: Age/gender; time (1 week); how many cocktails they drink a day
  3. Below are three experiments. For each experiment: a. Identify the alternative and null hypotheses being tested. b. What, if anything, is the control treatment (positive or negative) in the experiment? What other variables did the investigators attempt to standardize in the experiment? What else should they have standardized in the experiment? EXPERIMENT 1: The scientist fertilizes one lake with phosphorus and takes 12 water samples 1 month later to determine the amount of chlorophyll in the lake. The scientist also takes 12 samples from a control lake of the same size. The average chlorophyll level is much higher in the fertilized samples than in the control samples, so the scientist concludes that phosphorus has a significant effect on production of chlorophyll by photosynthetic organisms. EXPERIMENT 2: The scientist takes 12 different lakes and measures their chlorophyll levels in May. The scientist then fertilizes all the lakes with phosphorus and re-measures chlorophyll levels in July. Because the average chlorophyll concentration is much greater in July, the scientist concludes that phosphorus has a significant effect on production of chlorophyll by photosynthetic organisms.

EXPERIMENT 3: Because the scientist has a limited budget to buy fertilizer, s/he fertilizes the 6 smallest lakes in the area with phosphorus and measures the change in chlorophyll concentration over time. Because the change in these lakes is significantly greater than the change in 6 larger control lakes, the scientist concludes that phosphorus has a significant effect on production of chlorophyll by photosynthetic organisms.

  • 1: Null Hypothesis: Addition of phosphorous to a lake has no effect on chlorophyll levels o Alternative hypothesis: Addition of phosphorous to a lake increases chlorophyll levels o Control Treatment (Negative): The lack of phosphorous in the second lake o Standardized Variables: 12 samples from both lakes o They should have standardized the time period when the sample was taken from the lake.
  • 2: Null: Phosphorous will have no effect on production of chlorophyll by photosynthesis o Alternative: Phosphorous will have a significant effect on production of chlorophyll by photosynthesis o Control Treatment (Positive): All the lakes have been given phosphorous o Standardized Variable: All the chlorophyll levels were measured in May o They should have split up the lakes into control groups, instead of simply fertilizing all of them.
  • 3: Null: Phosphorous will have no effect on production of chlorophyll by photosynthesis o Alternative: Phosphorous will cause an increase in the production of chlorophyll by photosynthesis o Control Treatment (Positive): Fertilizing all the lakes o Standardized Variables: All the lake chlorophyll levels were measured over the same time period
  1. What are the fundamentals of the cell theory as put together by Schleiden, Schwann, and Virchow?
  • The fundamentals are: i Cells are the fundamental units of life ii All life is composed of cells iii All living cells arise from preexisting cells iv No spontaneous generation
  1. How do Pasteur’s experiments support Virchow’s hypothesis?
  • Microbial life grows only in flasks exposed to microorganisms. There is no “spontaneous generation” of life in the sterile flask. An environment without life remains lifeless.
  1. Review Pasteur’s experiment. What are two alternative hypotheses he was testing? Pick one of them and write the null hypothesis for that alternative hypothesis. What