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Biology: Organismal Biology: Structure and Function

Biology · Work through your exam, one topic at a time.

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Organismal Biology: Structure and Function

Organismal Biology: Structure and Function examines how animals and plants maintain internal stability, exchange gases and materials, transport substances, and coordinate responses. Items typically ask you to predict a physiological consequence from anatomy or from a feedback loop.

Core rules

  • Homeostasis: negative feedback reverses a deviation, such as insulin lowering blood glucose; positive feedback amplifies a change, as in oxytocin during childbirth or the luteal surge of LH.
  • Circulation: arteries carry blood away from the heart and veins carry it back, regardless of oxygen content. Cardiac output equals heart rate times stroke volume. Mean arterial pressure is approximately cardiac output times total peripheral resistance.
  • Gas exchange: diffusion depends on partial-pressure gradients and surface area. Countercurrent exchange in fish gills maintains a gradient along the entire gill; mammalian alveoli maximize surface area and keep diffusion distance short.
  • Osmoregulation: water follows osmotic gradients. Freshwater fish gain water and lose ions; marine bony fish lose water and gain ions. Kidneys adjust water and solute reabsorption under ADH and aldosterone.
  • Plant transport: xylem moves water and minerals upward by cohesion–tension and transpiration; phloem moves sugars by pressure flow from source to sink. Guard cells open stomata when turgid.

Worked example

A person has a heart rate of 70 beats per minute and a stroke volume of 70 mL per beat. Cardiac output = 70 × 70 = 4,900 mL min⁻¹, or 4.9 L min⁻¹. If total peripheral resistance is 20 mmHg per L min⁻¹, mean arterial pressure is approximately 4.9 × 20 = 98 mmHg. The reasoning shows why a fall in stroke volume can be offset by a rise in heart rate, and why vasoconstriction raises pressure without directly changing cardiac output.

Traps

  • Calling the pulmonary artery oxygen-poor without noting that it is still an artery because it leaves the heart.
  • Confusing negative feedback with positive feedback in endocrine or immune examples.
  • Reversing xylem and phloem directions or forgetting that phloem transport is source-to-sink, not always downward.
  • Ignoring surface-area-to-volume ratios when explaining heat loss, diffusion, or metabolic rate.
  • Treating stomatal opening as unconditional; water stress and ABA promote closure.