Homeostasis, Osmoregulation, Growth, and Reproduction in Organisms

Posted on Jan 19, 2025 in Biology

Homeostasis

Homeostasis is the maintenance of relatively stable internal conditions despite changes in the external environment. In humans, the hypothalamus checks the current temperature to match the normal 37°C.

Enzyme Activity

For every 10°C rise in temperature, enzyme activity doubles.

Heat Exchange Equation

Hs = Hm + Hcd + Hcv + Hr – He

  • s (stored heat in the body)
  • m (metabolism)
  • cd (conduction)
  • cv (convection)
  • r (radiation)
  • e (evaporation)

Variation in Body Temperature

  • Poikilotherms: Body temperature varies with environmental temperature.
  • Homeotherms: Body temperature is maintained at a constant level.

Regulation of Body Temperature

Sources of energy for regulating body temperature:

  • Ectotherms
  • Endotherms

Ectotherms

Most invertebrates, fish, amphibians, and reptiles move sluggishly when temperatures drop too low. Their food needs are low, so they don’t have to look for food or expose themselves to danger.

Endotherms

Birds, mammals, sea turtles, some fish, and some invertebrates remain fully active over a wide temperature range. Cold weather doesn’t affect normal activity, but it does increase their energy and food needs.

Metabolic Response

  • Endotherm: Rises at low environmental temperatures, generating extra body heat.
  • Ectotherm: Falls at low environmental temperatures, conserving energy.

Osmosis

Water molecules move across a selectively permeable membrane from a region of high water concentration (low solute concentration) to a region of low water concentration (high solute concentration).

Osmolarity

Total solute concentration of a solution:

  • Mammals, birds, and reptiles: approximately 300 mOsm/l
  • Freshwater: 1-10 mOsm/l
  • Seawater: 1000 mOsm/l
  • Hypotonic solution: Depending on the amount of water, the cell may look enlarged or bloated.
  • Isotonic solution: The cell will remain stable.
  • Hypertonic solution: The cell will shrink because it loses water.

Extracellular Fluid

Connects all cells to the external environment; fluid outside cells. For metabolic stability, animals must keep intracellular fluid (ICF) and extracellular fluid (ECF) isosmotic.

  • Osmoconformers: Osmolarity of ICF = ECF = external environment; most marine invertebrates.
  • Osmoregulators: Use control mechanisms to keep osmolarity of ICF = ECF, but may not be equal to the external environment; most freshwater and terrestrial invertebrates, most vertebrates.

Osmoregulation and Excretion

Four steps:

  1. Filtration
  2. Reabsorption
  3. Secretion
  4. Release

Animals excrete metabolic products:

  • Nitrogenous wastes from the breakdown of proteins and nucleic acids
  • Breakdown products of proteins and toxins
  • H+ ions
  • Water
  • Ammonia: Soluble but highly toxic; must be excreted in dilute solutions (requires lots of water).
  • Urea: Soluble, relatively nontoxic; requires more energy to produce but less water to excrete.
  • Uric acid: Insoluble, nontoxic; requires even more energy to produce but conserves the most water.

Vertebrates (Kidneys)

  • Filter blood (remove metabolic waste)
  • Regulate electrolytes (salts, acids, and bases)
  • Produce urine
  • Composed of microscopic tubules

Growth and Development

  • Growth: Increase in body size (increase in cell number and/or size).
  • Development: Change in form over time.

Growth in Plants vs. Animals

  • Plants have well-defined regions of cell division (meristems), while in animals, cell division occurs throughout the entire body.

Apical Meristems

Found at stem and root tips in all plants; responsible for lengthening at tips (primary growth).

Lateral Meristems

Found in cylinders in stems and roots (in woody plants); responsible for the thickening of stems and roots (secondary growth). There are two types: vascular cambium and cork cambium.

Intercalary Meristem

Found at nodes (in grasses and related plants); responsible for lengthening (like apical meristems).

Annual Rings

Can tell us how old the tree is and what the weather was like during each year of the tree’s life.

  • Spring growth: Large cells with thin walls.
  • Summer growth: Small cells with thick walls.

Determinate and Indeterminate Growth

  • Determinate: Finite (most mammals, birds).
  • Indeterminate: Doesn’t stop (most fish, amphibians, reptiles).

Five Plant Hormones

Small amounts produced mostly in actively growing parts; transported from origin to specific plant parts.

  • Promote growth, delay senescence (auxins, gibberellins, cytokinins).
  • Inhibit growth, promote senescence (abscisic acid, ethylene).

Cell Division and Growth

Rates vary between body regions. In most body regions, cell division replaces dying cells with new cells (even after total body size is not increasing).

  • Holometabolous (Complete Metamorphosis): Growth, differentiation, and reproduction occur in separate stages; most common.
  • Hemimetabolous (Incomplete Metamorphosis): Growth and differentiation occur in the nymph stage.
  • Direct development: Juveniles look like adults (just smaller and sexually immature).

Regeneration

The natural process of replacing and restoring damaged or missing cells, tissues, organs, and even entire body parts (e.g., echinoderms).

Reproduction

Asexual Reproduction

  • One parent
  • All offspring are genetically identical to the original parent
  • Beneficial in stable (predictable) environments

Sexual Reproduction

  • Offspring produced by the fusion of gametes (sex cells)
  • Results in new genetic combinations
  • Beneficial in changing (unpredictable) environments

Nucleus

Membrane-bound organelle that contains genetic material (DNA).

Chromosome

Structure found inside the nucleus made of proteins and DNA.

DNA

Molecule inside cells that contains genetic information.

Prokaryotes

Binary fission (DNA replicates, cell elongates – DNA separates, cross wall forms, two daughter cells separate).

Protists

  • Binary fission
  • Budding
  • Multiple fission

Fungi

  • Vegetative spores
  • Mycelial fragmentation
  • Budding (yeasts)

Plant Fragmentation

A portion of the plant breaks off (nonvascular plants) and continues to grow independently.

Propagative Roots

Adventitious buds form along roots – suckers (aerial stems, many trees and shrubs).

Rhizomes

Belowground, horizontal stems (grasses).

Runners

Aboveground, horizontal stems (strawberry, spider plant).

Reproductive Leaves

Plantlets develop along leaf margins.

Apomixis

Seeds produced without the fusion of gametes (blackberry); another 2n cell in the ovule – embryo.

Animals – Budding

Offspring develop as outgrowths of the parent (sponges, cnidarians); detach or remain attached to form colonies.

Fragmentation and Regeneration

A portion of the animal breaks off (planarians, annelids, echinoderms) and grows back the rest of the body.

Parthenogenesis

Offspring develop from unfertilized eggs; some insects, fish, amphibians, reptiles, etc.; sometimes seen when females are isolated from males.

Meiosis

Produces gametes or spores with half the number of chromosomes; genetically different combinations of chromosomes.

Homologous Chromosomes

Matching pair of chromosomes (one from each parent); contain the same genes for the same traits; may contain different versions of the same gene (alleles). Meiosis I (homologous chromosomes pair up in tetrads, then separate). Meiosis II (sister chromatids separate like mitosis but with half the chromosomes).

Importance of Meiosis

  • Chromosome number stays constant in new generations.
  • Generates genetic diversity.