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Cell Review. Diffusion & Osmosis Lab. Diffusion. Molecules are in constant, random motion Molecules move from higher concentration to lower concentration (down their concentration gradient) Meaning, they spread out Example: when the smell of a candle fills a room. Osmosis.

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## Cell Review

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**Diffusion**• Molecules are in constant, random motion • Molecules move from higher concentration to lower concentration (down their concentration gradient) • Meaning, they spread out • Example: when the smell of a candle fills a room**Osmosis**• A specific type of diffusion • Diffusion of water across a selectively permeable membrane • Water will diffuse toward the higher solute concentration**Types of Solutions Based on Solute Concentration**• Hypertonic—higher solute concentration • Isotonic—equal solute concentrations • Hypotonic—lower solute concentration**Water Potential**• Tendency of water to leave one place for another • Water moves from higher to lower water potential • Affected by pressure and amount of solute**Calculating Water Potential**• Water potential (Ψ) = pressure potential (Ψp) + solute potential (Ψs ) • Pressure potential (Ψp) is the pressure exerted by the rigid cell wall (limits further water uptake) • Solute potential (Ψs ) is solute concentration • As solute is added, solute potential becomes more negative • Causes the water potential to decrease**Exercise 1: Diffusion**• Filled bag with sugar and starch • Put bag into beaker with water & iodine • Each molecule moves from high to low concentration (unless it is too large to fit through the membrane) • When iodine comes into contact with starch, it changes color to blue-black**Exercise 2: Osmosis**• You filled 6 dialysis bags with increasing concentrations of sucrose • You placed the bags into distilled water • You compared the initial weigh of each bag with its final weight • And calculated the percent change in mass**Exercise 3: Water Potential of Potato Cores**• Potatoes were placed in varying concentrations of sucrose solutions • You calculated the percent change in mass**Mitosis**• Development of an adult from a single fertilized egg & growth and repair of tissues • Parent cell produces two genetically identical daughter cells**Cell Cycle**• Interphase • Prior to mitosis • Chromosomes in nucleus are replicated • Mitosis (karyokinesis) • Nucleus divides • Cytokinesis • Cytoplasm and organelles divide**Interphase**• Majority of the cell cycle • Cell is growing and metabolizing • At the end, new DNA is synthesized • Chromosomes are replicated • Each chromosome of the homologous pair is then composed of two sister chromatids**Mitosis (Karyokinesis)**• Nucleus divides • Several subphases • Prophase • Metaphase • Anaphase • telophase**Cytokinesis**• Cytoplasm and organelles divide • Two identical daughter cells are formed**Timing the Stages**• 1. Determine the approximate duration of the entire cycle for the cells you are studying. (Your instructor will provide this information.) • 2. Looking at the slide, count and record the number of cells in the field of view that are in each phase. • 3. Determine the total number of cells counted. • 4. Determine the percent of cells that are in each phase. • 5. To calculate the time (in minutes) for each phase, multiply the percent of cells in that phase by the number of minutes for the whole cycle.**We will practice with the slide of onion root cells below.**Looking at the cells marked with an X, count the number of cells in each phase. (In lab, you will count at least 200 cells by moving your slide so that you view several fields.) • The average time for onion root tip cells to complete the cell cycle is 24 hours = 1440 minutes. To calculate the time for each stage: • % of cells in the stage X 1440 minutes = number of minutes in the stage**Meiosis**• Similar phases to mitosis • DNA is replicated in interphase • However, meiosis is followed by 2 divisions • Resulting in 4 daughter cells with half the chromosomes of the parent cell**Independent Assortment**• Maternal and paternal chromosomes line up randomly • In humans there are 223 possible combinations • Ensures genetic variation**Crossing Over**• Occurs during prophase I of meiosis • Replicated homologous pair comes together during synapsis • Sections of the chromosomes are exchanged**Analysis of Results**• What is the % of crossovers? • From that %, you can draw a map of the genes on a chromosome • More crossovers = greater map distance • What is the map distance between the gene for spore color and the centromere? (percent of crossovers divided by 2)

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