Flashcards · J247 · Topic 1

Cell level systems flashcards

44 cards on Topic 1, subtopics B1.1 to B1.4. Pick a subtopic or study them all.

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B1.1 Cell structures

Total magnification of a light microscope with a ×10 eyepiece and a ×40 objective: Total magnification = eyepiece × objective = 10 × 40 = ×400.
Why are cells stained for light microscopy? Give two stains.: Cells are mostly colourless, so a stain makes structures visible or colours different structures differently. Methylene blue for cheek cells; iodine solution for onion epidermis.
Preparing and viewing a slide: the key steps: Thin layer of cells in a drop of liquid; add stain; lower a cover slip at an angle to avoid air bubbles; start with the lowest-power objective; coarse then fine focus; then higher power.
An image is 45 mm long and the real length is 15 µm. Magnification?: magnification = image size ÷ actual size. 45 mm = 45 000 µm, so 45 000 ÷ 15 = ×3000.
An image is 24 mm long at ×400. Actual size in µm?: Actual size = image ÷ magnification = 24 ÷ 400 = 0.06 mm = 60 µm.
Functions of the nucleus, mitochondria, chloroplasts and ribosomes: Nucleus: contains the genetic material (chromosomes) and controls the cell. Mitochondria: enzymes for cellular respiration. Chloroplasts: chlorophyll for photosynthesis. Ribosomes: protein synthesis.
Cell membrane and plasmids: what each is or does: Cell membrane: a selective barrier with receptor molecules that detect chemical signals. Plasmids: small rings of extra DNA in bacteria, carrying genes such as antibiotic resistance.
Eukaryotic vs prokaryotic cells: Eukaryotic: nucleus, several linear chromosomes, mitochondria, larger. Prokaryotic: no nucleus (one circular chromosome free in the cytoplasm, plus plasmids), no mitochondria or chloroplasts, much smaller. Both have ribosomes.
Transmission electron microscope (TEM) vs light microscope: The TEM uses a beam of electrons and has much higher resolution, so it shows ribosomes and the inside of organelles. But specimens must be dead, thin and treated, images are black and white, and it is expensive. The light microscope resolves only to about 200 nm.
Magnification vs resolution: Magnification is how many times bigger the image is. Resolution is the ability to see two close points as separate, which decides how much fine detail is visible.
Write 0.0000048 m in standard form: 4.8 × 10⁻⁶ m. The number in front must be at least 1 and less than 10; tiny lengths have negative powers.

B1.2 What happens in cells (and what do cells need)?

Structure of DNA: A polymer made of nucleotides. Two strands form a double helix: sugar and phosphate form the outside of each strand, and the bases pair in the middle.
Nucleotide: The monomer of DNA: a sugar, a phosphate group and one of four bases (A, T, G or C).
Complementary base pairs, and the strand paired with GGATC: A pairs with T, and G with C. GGATC pairs with CCTAG.
A DNA sample is 35% thymine. Percentage of cytosine?: Adenine = 35%, so A + T = 70%. The remaining 30% is split equally, so 15% guanine and 15% cytosine.
Protein synthesis: transcription and translationHigher: Transcription: DNA unzips around the gene and an mRNA copy is made in the nucleus. The mRNA moves to the cytoplasm. Translation: at a ribosome the mRNA is read and tRNA brings amino acids, which join in order; the chain folds into the protein.
A DNA template strand reads CAT TGC. What is the mRNA?Higher: GUA ACG. RNA uses U instead of T, so A on the template pairs with U.
How can a change in DNA base sequence change an enzyme?Higher: Base sequence → triplets → order of amino acids. A different order folds into a different shape, so the active site may change and the substrate no longer fits. Some base changes give the same amino acid and have no effect.
Active site, and the lock and key model: The active site is the part of the enzyme with a specific shape that the substrate fits into. The substrate shape matches the active site, so each enzyme is specific.
Denatured: The enzyme's shape, including the active site, is permanently altered (by high temperature or extreme pH), so it cannot work. Cold only slows an enzyme, reversibly.
Effect of temperature, pH and substrate concentration on enzyme rate: Temperature: rate rises to an optimum, then falls to zero as the enzyme denatures. pH: fastest at the optimum, slower either side. Substrate concentration: rises then levels off when all active sites are occupied.
Catalase makes 45 cm³ of oxygen in 90 s. Rate?: Rate = amount ÷ time = 45 ÷ 90 = 0.50 cm³/s. If only a time is measured, 1 ÷ time is a measure of rate.

B1.3 Respiration

Cellular respiration: A chemical process in all living cells that transfers energy from fuels such as glucose to make ATP. It happens all the time, in plants too, and is enzyme-controlled.
ATP, and what cells use it for: A small molecule that carries energy to where the cell needs it; made from ADP and phosphate. Used for building molecules, muscle contraction, active transport, cell division and nerve impulses.
Why is respiration described as exothermic?: It transfers energy to the surroundings: some energy is captured in ATP and the rest is released as heat.
Aerobic respiration vs anaerobic respiration in animals and in plants and fungi: Aerobic: glucose + oxygen → carbon dioxide + water. Animals: glucose → lactic acid. Plants and fungi (yeast): glucose → ethanol + carbon dioxide.
Why does anaerobic respiration give much less ATP per glucose?: Glucose is only partly broken down, so energy is left in the lactic acid or ethanol. Anaerobic respiration gives 2 ATP per glucose, far less than aerobic.
Fermentation: Anaerobic respiration by yeast, producing ethanol and carbon dioxide. Brewing uses the ethanol; baking uses the carbon dioxide.
Investigating yeast respiration: method: Yeast in glucose solution (boiled, cooled glucose under oil for anaerobic); collect gas in a gas syringe; change one variable such as temperature; keep yeast mass and glucose volume and strength the same; rate = volume ÷ time; repeat.
Monomers of carbohydrates, proteins and lipids, and the enzymes that break them down: Carbohydrates: sugars (glucose); carbohydrase. Proteins: amino acids; protease. Lipids: 1 glycerol + 3 fatty acids; lipase. Lipids are not true polymers.
Condensation vs hydrolysis: Condensation joins two molecules with the loss of a water molecule. Hydrolysis splits a bond by adding water, which is what digestive enzymes do.
How many water molecules are released making a starch chain of 300 glucose units?: 299. A chain of n monomers has n − 1 joins.
Food tests for starch, reducing sugar, protein and lipid: Starch: iodine, orange-brown → blue-black. Reducing sugar: Benedict's in a hot water bath, blue → green, yellow, orange, brick red. Protein: biuret, blue → purple. Lipid: ethanol then water, cloudy white emulsion.

B1.4 Photosynthesis

Photosynthesis: word and balanced symbol equations: carbon dioxide + water → glucose + oxygen (light) 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ Light is absorbed by chlorophyll in the chloroplasts.
Why is photosynthesis endothermic?: It takes in energy (light) from the surroundings and stores it as chemical energy in glucose.
The two stages of photosynthesis: Stage one needs light: water is split, oxygen is released and hydrogen is made available. Stage two: the hydrogen is used to fix carbon dioxide into glucose.
Why are photosynthetic organisms called producers?: They make organic compounds from carbon dioxide and water using light energy. They are the main producers of food and biomass, so all consumers depend on them for food, energy and carbon.
Light exclusion experiment with a geranium leaf: Destarch for 24–48 h in the dark; cover part of a leaf with foil and leave in light; boil the leaf, then warm in ethanol in a water bath to remove chlorophyll; add iodine. Exposed part blue-black, covered part orange-brown.
Investigating rate with Cabomba (pondweed): Stem cut under water in sodium hydrogencarbonate solution; lamp at measured distances with a water heat shield; let it settle; count bubbles or measure gas volume in a fixed time. Gas volume is more reliable than bubble counts.
Algae in alginate beads in hydrogencarbonate indicator: what do the colours show?: Red: normal CO₂. Purple: less CO₂ (photosynthesis faster). Yellow: more CO₂ (respiration only, in the dark).
Effect of light intensity, CO₂ and temperature on the rate of photosynthesis: Light intensity and CO₂ concentration: rate rises, then levels off when another factor limits. Temperature: rate rises to an optimum, then falls fast as enzymes denature.
Inverse square law: a lamp moves from 60 cm to 30 cm. Effect on light intensity?: Light intensity ∝ 1/distance². (60 ÷ 30)² = 4, so the intensity is 4 times greater. Moving from 10 cm to 30 cm gives 1/9 of the intensity.
Two curves at different CO₂ levels overlap at low light but separate at high light. What does it show?Higher: Where the curves overlap, light intensity is limiting. Where the lower curve has flattened but the higher one keeps rising, CO₂ is limiting the lower curve. If the higher curve also flattens, a third factor (often temperature) limits it.
What did Priestley and Ingenhousz show?: Priestley: a plant restored air in which a candle had stopped burning. Ingenhousz: light was needed, and the green parts released the gas.