Flashcards · IB Diploma Programme Biology · Topic 2

Form and function flashcards

62 cards on Topic 2, subtopics B1.1 to B4.2. Pick a subtopic or study them all.

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B1.1 Carbohydrates and lipids

Condensation and hydrolysis: Condensation joins two molecules with the release of water. Hydrolysis breaks a bond by splitting water into −H and −OH.
Starch, glycogen and cellulose compared: Starch (plants): α-glucose; coiled, amylopectin branched; energy store. Glycogen (animals): α-glucose; coiled and highly branched; energy store, quickly mobilised. Cellulose (plant cell walls): β-glucose; straight chains; structural strength.
Why is cellulose so strong?: β-glucose units alternate in orientation, giving straight chains. Chains lie in bundles cross-linked by many hydrogen bonds, which together give high tensile strength.
Glycoproteins and the ABO blood groups: Carbohydrate chains on glycoproteins face outwards from the plasma membrane as recognition markers. The ABO antigen depends on the terminal sugar; group O cells lack A and B antigens. People have antibodies against the ABO antigens they do not carry.
Saturated, monounsaturated and polyunsaturated fatty acids: Saturated: no C=C; straight; high melting point. Monounsaturated: one C=C; bent; lower melting point. Polyunsaturated: two or more C=C; more bends; lowest melting point.
Why is fat used for long-term energy storage and glycogen for short-term?: Fat releases about twice as much energy per gram (values supplied in questions, about 37 kJ g⁻¹ against 17 kJ g⁻¹), so stores are lighter; it is insoluble and also insulates. Glycogen is broken down to glucose more quickly.

B1.2 Proteins

Structure of a generalised amino acid: A central (alpha) carbon carrying an amine group (NH₂), a carboxyl group (COOH), a hydrogen and a variable R-group. Only the R-group varies.
How many different tripeptides can be made from the 20 amino acids?: 20³ = 8000. In general 20ⁿ sequences for a chain of n amino acids.
Essential and non-essential amino acids: Essential amino acids cannot be made and must be eaten; non-essential ones can be made from other amino acids. Vegan diets need a mix of plant proteins, as some are short of essential amino acids.
Denaturation of a protein: A change in three-dimensional shape, from heat or pH change, that stops the protein working; usually irreversible. Peptide bonds are not broken, so the primary structure survives.
Bonds that hold secondary and tertiary structure: Secondary (α-helix, β-pleated sheet): hydrogen bonds in regular positions along the backbone. Tertiary: interactions between R-groups: hydrogen, ionic and disulfide bonds and hydrophobic interactions. (HL only)
Insulin, collagen and haemoglobin: structure and function: Insulin: two chains, globular, soluble hormone. Collagen: three chains in a triple helix, fibrous, strong and insoluble. Haemoglobin: four chains each with a haem group, conjugated and globular, carries oxygen. (HL only)

B2.1 Membranes and membrane transport

Why is a phospholipid bilayer a barrier?: Amphipathic phospholipids form a bilayer, heads to water and tails together. The hydrophobic core of fatty acid tails repels ions and polar molecules, so it is a barrier to them and to large molecules.
Simple diffusion, facilitated diffusion and active transport: Simple diffusion: between phospholipids, down a gradient, no protein, no ATP. Facilitated diffusion: through a channel protein, down a gradient, no ATP. Active transport: by a pump protein using ATP, can be against a gradient.
Osmosis: Net movement of water across a membrane that is impermeable to the solute, towards the higher solute concentration. Aquaporins speed it up but do not decide the direction.
Integral and peripheral proteins: Integral: embedded in one or both lipid layers. Peripheral: attached to one surface only.
How does cholesterol affect an animal cell membrane?: It sits between phospholipids and modulates fluidity: it stabilises the membrane when warm and stops it stiffening when cold. (HL only)
Sodium-dependent glucose cotransporter: Na⁺ moves into the cell down its gradient and brings glucose with it. This is indirect active transport: the gradient is made by the sodium–potassium pump using ATP. Used in the small intestine and the nephron. (HL only)

B2.2 Organelles and compartmentalization

Organelle, and structures that are not organelles: An organelle is a discrete subunit of a cell with a specific function (nuclei, vesicles, ribosomes, mitochondria, plasma membrane). The cell wall, cytoskeleton and cytoplasm are not organelles.
Advantages of compartmentalization: Enzymes and substrates are concentrated; each compartment can have its own conditions such as pH; incompatible reactions are kept apart. Example: lysosomes hold hydrolytic enzymes that would damage the cytoplasm.
Why does separating the nucleus from the cytoplasm matter in eukaryotes?: Transcription happens in the nucleus and the mRNA can be modified before it is exported for translation. In prokaryotes ribosomes meet mRNA immediately, so no modification is possible first.
Cell fractionation: Cells are broken up into a homogenate and spun in an ultracentrifuge; denser structures form a pellet at lower speeds, so organelles can be separated and studied.
How is a mitochondrion adapted for aerobic respiration?: Small intermembrane space, so protons build up quickly into a steep gradient; cristae give more surface for electron transport chains and ATP synthase; the matrix concentrates Krebs cycle enzymes and substrates. (HL only)
Route of a protein made for secretion: Made by ribosomes on rough ER; enters the ER lumen and folds; carried in a vesicle to the Golgi apparatus, which processes and sorts it; a vesicle takes it to the plasma membrane and exocytosis releases it. (HL only)

B2.3 Cell specialization

Two properties of stem cells: They can divide endlessly, and they can differentiate along different pathways.
Totipotent, pluripotent and multipotent: Totipotent: every cell type, including a whole organism (earliest embryo). Pluripotent: most body cell types, not a whole organism. Multipotent: a limited range of related cell types (adult tissue).
Stem cell niche, with two examples: A microenvironment that maintains stem cells or promotes their proliferation and differentiation. Examples: bone marrow (haematopoietic stem cells give blood cells) and the hair follicle.
Why does surface area to volume ratio limit cell size?: For a cube, SA:V = 6 ÷ side, so it falls as the cell grows: volume (the need) rises faster than surface area (exchange). A 3 µm cube has a ratio of 2 µm⁻¹; a 6 µm cube, 1 µm⁻¹.
How do cells with the same genome become different types?: Differentiation depends on which genes are expressed. In an early embryo, gradients mean cells in different positions receive different signals, so they express different genes. Cells do not gain or lose genes.
Type I and type II pneumocytes: Type I: extremely thin, so the diffusion distance for gas exchange is short. Type II: lamellar bodies release surfactant into the alveolar lumen, reducing surface tension. (HL only)

B3.1 Gas exchange

Four properties of a gas-exchange surface: Permeable, thin, moist and with a large surface area.
How are concentration gradients kept steep at gas-exchange surfaces?: Dense networks of blood vessels, continuous blood flow, and ventilation with air (lungs) or water (gills).
Inhalation: muscles, volume and pressure: Diaphragm contracts and flattens; external intercostal muscles contract, moving the ribs up and out. Thorax volume rises, pressure falls below atmospheric, and air flows in. Quiet exhalation is passive.
Tidal volume 0.60 L, inspiratory reserve 2.8 L, vital capacity 4.5 L. What is the expiratory reserve volume?: Vital capacity = tidal + inspiratory reserve + expiratory reserve, so ERV = 4.5 − 0.60 − 2.8 = 1.1 L.
Factors affecting the rate of transpiration: Rate rises with light, temperature and wind, and falls as humidity rises, because each changes stomatal opening or the water vapour gradient.
Bohr shift, and foetal haemoglobin: More CO₂ shifts the oxygen dissociation curve to the right (lower affinity), so more oxygen is released in actively respiring tissue. Foetal haemoglobin has a higher affinity (curve to the left), so it loads oxygen from maternal blood. (HL only)

B3.2 Transport

How are arteries adapted?: Thick walls with elastic fibres and smooth muscle and a narrow lumen. Elastic stretch and recoil keep pressure up and smooth the flow between beats.
Capillaries and fenestrations: Capillary walls are one cell thick and the lumen about one red cell wide: short diffusion distance, large total surface area. Fenestrations are pores through the endothelium in sites needing rapid exchange (glomerulus, villi, endocrine glands).
Causes and consequences of coronary occlusion: Damage to the artery lining; plaque builds up (atherosclerosis); the lumen narrows and a clot may form; less oxygen reaches cardiac muscle: angina if partial, heart attack if blocked. Risks: smoking, high blood pressure, high cholesterol, saturated fat, inactivity.
How is water pulled up the xylem?: Water evaporates from mesophyll cell walls and leaves through stomata; water is drawn from the xylem through leaf cell walls by capillary action, creating tension (negative pressure potential); cohesion keeps the column continuous as it is pulled up.
How are xylem vessels adapted?: No cell contents and missing or incomplete end walls for free flow; lignified walls that resist collapse under tension; pits so water can move in and out sideways.
Single circulation (bony fish) and double circulation (mammals): Fish: heart, gills, body, heart; blood passes through the heart once per circuit and pressure is low after the gills. Mammals: right side, lungs, left side, body; pressure is restored by the left ventricle. (HL only)

B3.3 Muscle and motility (HL only)

What happens to the bands of a sarcomere during contraction?: A band (whole length of thick filaments): unchanged. I band: shorter. H zone: narrows or vanishes. Sarcomere (Z to Z): shorter. The filaments slide; they do not shorten. (HL only)
Cross-bridge cycle in muscle: Ca²⁺ from the sarcoplasmic reticulum binds troponin; tropomyosin moves, exposing binding sites on actin; myosin heads bind; the power stroke pulls actin and sheds ADP and phosphate; ATP attaches and the head lets go; ATP hydrolysis resets the head. (HL only)
Role of titin: A giant elastic protein linking thick filaments to Z lines. It stores energy when stretched, helps the sarcomere recoil and prevents overstretching. (HL only)
Motor unit: One motor neuron plus the muscle fibres it controls; all its fibres contract together. Small units give fine control, large units strong force, and recruiting more units gives more force. (HL only)
Roles of cartilage, synovial fluid, ligaments and tendons in a synovial joint (hip): Cartilage reduces friction and absorbs shock; synovial fluid lubricates and cushions; ligaments join bone to bone and stabilise; tendons join muscle to bone and transmit force. (HL only)
External and internal intercostal muscles: External intercostals raise the ribs up and out (inhalation); internal intercostals pull them down and in (forced exhalation). They are antagonistic: contraction of one stretches the other. (HL only)

B4.1 Adaptation to environment

Range of tolerance: The span of values of an abiotic variable within which a species can survive. Survival range is wider than the range for good growth or breeding.
Biome: A group of ecosystems with similar communities because of similar abiotic conditions and convergent evolution.
Adaptations of marram grass to sand dunes: Rolled leaves, sunken stomata and hairs trap humid air and cut transpiration; a thick cuticle reduces evaporation; long roots and rhizomes reach water and anchor the plant; it grows upwards to survive burial by sand.
Abiotic conditions needed by coral reefs: Shallow, clear water (light for the photosynthetic algae in coral tissue); warm water, roughly 23 to 29 °C; stable salinity near that of the open ocean; slightly alkaline pH, about 8.1, for building calcium carbonate skeletons.
How is a transect used to study tolerance?: Lay a line or belt transect along an environmental gradient; at fixed intervals record species abundance in quadrats and measure the abiotic variable; describe the correlation. Correlation does not show cause.

B4.2 Ecological niches

Ecological niche: The role of a species in its ecosystem: its biotic and abiotic interactions, including how it obtains food. Habitat is only where it lives.
Obligate anaerobes, facultative anaerobes and obligate aerobes: Obligate anaerobe: harmed or killed by oxygen. Facultative anaerobe: grows with or without oxygen, usually better with it. Obligate aerobe: cannot grow without oxygen.
Holozoic and saprotrophic nutrition: Holozoic (all animals): ingest food, digest it internally, absorb and assimilate. Saprotrophic (some fungi and bacteria): secrete enzymes onto dead matter and absorb the products; decomposers.
Mixotrophic nutrition: Both autotrophic and heterotrophic, as in Euglena. Obligate mixotrophs need both modes; facultative mixotrophs can switch between them.
Herbivore adaptations and plant defences: Herbivores: chewing mouthparts, piercing mouthparts (stylet into phloem), detoxifying enzymes. Plants: physical defences (thorns, spines, tough or hairy leaves) and chemical defences (toxic secondary compounds in leaves and seeds).
Fundamental niche: The potential niche set by a species' adaptations and tolerance limits, with no competitor.
Realized niche: The niche actually occupied when competing species are present. It is usually smaller than the fundamental niche and never larger.
Competitive exclusion: the two possible outcomes when two species compete for the same limiting resource: Two species cannot occupy exactly the same niche in the same place. Either one species is eliminated from the area, or both survive but each is restricted to part of its fundamental niche.
A species tolerates 10 to 34 °C alone, but with a competitor it is found only at 16 to 28 °C. Realized niche as a percentage of the fundamental niche?: Fundamental width 34 − 10 = 24 °C; realized width 28 − 16 = 12 °C. 12 ÷ 24 × 100 = 50%.