HKDSE Biology flashcards
HKDSE Biology · Build recall with a focused review session.
33 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Magnification formula
- Magnification = image size ÷ actual size. Rearranged, actual size = image size ÷ magnification. Convert all measurements to the same unit before dividing, e.g. 1 mm = 1000 µm.
- Resolution vs magnification
- Magnification enlarges the image; resolution is the smallest distance at which two points are still seen as separate. High magnification without high resolution produces a large but blurred image.
- Prokaryotic cell features
- No membrane-bound nucleus (DNA lies free in the nucleoid), no mitochondria, chloroplasts or endoplasmic reticulum, 70S ribosomes, and usually a cell wall of peptidoglycan plus a capsule and flagella in some species.
- Eukaryotic cell features
- DNA enclosed in a nuclear envelope, 80S ribosomes, and membrane-bound organelles including mitochondria, endoplasmic reticulum, Golgi apparatus and, in plants, chloroplasts and a large central vacuole.
- Benedict's test
- Detects reducing sugars such as glucose and maltose. Add Benedict's solution and heat in a water bath; a positive result changes colour from blue through green, yellow and orange to brick-red precipitate.
- Iodine test for starch
- Add a few drops of iodine in potassium iodide solution to the sample. A positive result is a colour change from yellowish-brown to blue-black, indicating starch is present.
- Biuret test for protein
- Add Biuret reagent (or sodium hydroxide then copper(II) sulphate) to the sample. A positive result is a colour change from blue to violet or purple, showing peptide bonds are present.
- Emulsion test for lipid
- Shake the sample with ethanol, then pour into water. A positive result is a cloudy white emulsion, indicating lipid is present. The grease-spot test gives a translucent spot on filter paper.
- Enzyme denaturation
- Above the optimum temperature, hydrogen bonds and other interactions holding the enzyme's tertiary structure break, so the active site changes shape and the substrate no longer fits; the change is usually irreversible.
- Osmosis definition
- The net movement of water molecules across a partially permeable membrane from a region of higher water potential (less concentrated solution) to a region of lower water potential (more concentrated solution), without energy input.
- Active transport
- Movement of ions or molecules across a membrane against their concentration gradient, using carrier proteins and energy from ATP, for example mineral uptake by root hair cells.
- Water potential
- The tendency of water to move from one place to another. Pure water at atmospheric pressure has a water potential of zero; adding solute lowers it, so solutions have negative water potential values.
- Plasmolysis
- When a plant cell is placed in a solution of lower water potential, water leaves by osmosis, the vacuole shrinks and the protoplast pulls away from the cell wall; the cell becomes flaccid then plasmolysed.
- Turgor pressure
- In a plant cell placed in a solution of higher water potential, water enters the vacuole, pushing the protoplast against the rigid cell wall; the resulting turgidity supports non-woody plant parts.
- Mitosis stages
- Prophase (chromosomes condense, nuclear envelope breaks down), metaphase (chromosomes line up on the equator), anaphase (sister chromatids separate to opposite poles), telophase (nuclei re-form), followed by cytokinesis.
- Meiosis and genetic variation
- Two divisions producing four haploid cells. Crossing over in prophase I and independent assortment of homologous chromosomes in metaphase I generate new allele combinations in gametes.
- Aerobic respiration equation
- C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. It occurs in glycolysis (cytoplasm), the Krebs cycle (mitochondrial matrix) and oxidative phosphorylation on the inner mitochondrial membrane.
- Anaerobic respiration in humans
- Glucose is converted to lactic acid in muscle cells when oxygen is insufficient, releasing a small amount of energy. Lactic acid is later broken down in the liver using oxygen (oxygen debt).
- Photosynthesis equation
- 6CO2 + 6H2O → C6H12O6 + 6O2, driven by light energy absorbed by chlorophyll. Light-dependent reactions occur on the thylakoid membranes; the Calvin cycle occurs in the stroma.
- Limiting factor in photosynthesis
- The factor in shortest supply that restricts the rate when all others are adequate. Common limiting factors are light intensity, carbon dioxide concentration and temperature; the graph plateaus when another factor becomes limiting.
- Gas exchange in leaves
- Stomata, opened and closed by guard cells, allow CO2 in and O2 and water vapour out. Spongy mesophyll cells have large air spaces to increase the surface area for diffusion.
- Alveoli adaptations
- Large total surface area, thin one-cell-thick walls, moist surface, and a dense capillary network maintaining a steep concentration gradient, giving efficient gaseous exchange by diffusion.
- Villi adaptations
- Finger-like projections of the small intestine that increase surface area; each has a thin epithelium, a rich blood supply and a lacteal to absorb glucose and amino acids into blood and lipids into lymph.
- Transpiration pull
- Loss of water vapour from leaves through stomata lowers water potential in mesophyll cells, drawing water up the xylem as a continuous column; cohesion and adhesion help maintain the column under tension.
- Translocation in phloem
- Transport of sugars and amino acids from sources (e.g. leaves) to sinks (e.g. roots, fruits) through sieve tubes with companion cells, driven by pressure differences between source and sink.
- Blood components
- Plasma carries dissolved substances; red blood cells (no nucleus in mammals, contain haemoglobin) transport oxygen; white blood cells defend against pathogens; platelets are cell fragments that help clotting.
- Double circulation advantage
- Blood passes through the heart twice per circuit, so oxygenated blood from the lungs is pumped at high pressure to the body while deoxygenated blood returns to the lungs at lower pressure.
- Nervous vs hormonal control
- Nervous responses are electrical and rapid, acting on specific target cells via neurones and neurotransmitters. Hormonal responses are chemical, slower and longer-lasting, travelling in the blood to target organs.
- Homeostasis definition
- Maintenance of a relatively constant internal environment, such as body temperature, blood glucose and water potential, through negative feedback involving receptors, a control centre and effectors.
- Negative feedback
- A change in a factor is detected by receptors and triggers a response that reverses the change, restoring the set point; for example, insulin lowers raised blood glucose after a meal.
- Mendel's law of segregation
- The two alleles of a gene separate during meiosis so each gamete receives only one allele. Fusion of gametes at fertilisation restores the diploid number and produces predictable phenotypic ratios.
- Monohybrid cross ratio
- Crossing two heterozygous individuals (Aa × Aa) for a gene showing complete dominance gives a genotypic ratio of 1 AA : 2 Aa : 1 aa and a phenotypic ratio of 3 dominant : 1 recessive.
- Natural selection
- Individuals with inherited variations better suited to the environment survive and reproduce more successfully, so favourable alleles increase in frequency in later generations, causing evolution of the population.