Plant Biology and Environmental Biology Review

Plant Biology

Reproduction (Life Cycles)

  • All plants alternate generations, meaning offspring have a different chromosome count than parents.
  • Fertilization creates the diploid generation (sporophyte).
  • Meiosis creates the haploid generation (gametophyte).
  • Offspring have a different form from their parents, even though they are the same species.

Bryophytes

  • Diploid generation alternates with haploid generation.
  • The sporophyte (2n) is parasitic on the gametophyte (1n).
  • The gametophyte is the "dominant" generation (larger, more conspicuous).

Ferns

  • Diploid generation alternates with haploid generation.
  • Both generations are photosynthetic and free-living (not parasitic).
  • The sporophyte is the dominant generation.

Flowering Plants (Angiosperms)

  • Diploid generation alternates with haploid generation.
  • The gametophyte consists of only a few hundred cells in the flower.
    • It is totally dependent on the sporophyte for nutrition.
    • It makes gametes, which fuse and form the 2n seed (the seed is the embryo that grows into the adult sporophyte).
  • The sporophyte is dominant.

Summary of Alternation of Generations

Plant TypeSporophyte (2n)Gametophyte (1n)
MossesParasiticPhotosynthetic dominant
FernsPhotosynthetic dominantPhotosynthetic
Flowering PlantsPhotosynthetic dominantParasitic inside flower

Pollination

  • Flower structure matches the pollinator.

Wind-Pollinated Plants

  • Produce small, drab flowers.
  • Make lots of pollen.
  • Examples: pine trees, grass.

Butterfly-Pollinated Plants

  • Flowers tend to be red or blue.
  • Have long, curved nectar tubes.
  • Little scent.

Moth-Pollinated Plants

  • Pale flowers.
  • Produce a sweet scent.
  • Long, curved nectar tube.
  • Open at night/evening; many close during the daytime.

Bee-Pollinated Plants

  • Bees are attracted to yellow & blue colored flowers.
  • Infrared patterns.
  • Sweet scent.
  • Nectar is easily accessible (short nectar tubes).

Fly-Pollinated Plants

  • Drab flowers.
  • Stinky smell.

Beetle-Pollinated Plants

  • Pale color.
  • Fruity or spicy scent.
  • Some provide edible petals instead of nectar.

Hummingbird-Pollinated Plants

  • Large, red flowers.
  • Sturdy, straight nectar tube.
  • Little scent.

Bat-Pollinated Plants

  • Strong stem.
  • Pale flower.
  • Strong sweet or fruity scent.

Summary of Pollination

PollinatorFlowersNectar TubeScentOther
WindSmall drabNoneNoneLots of pollen
ButterfliesRed or blueLong curvedLittleOpen 24 hours/day
MothsPaleLong curvedStrong sweetOpen at night
BeesYellow or blueShortSweetInfrared patterns
FliesSmall drabShortStinky---
BeetlesPaleModerateFruity or spicy---
HummingbirdsLarge redLong (straight)LittleBill shape matches flower
BatsLarge paleModerateStrong sweet/fruityStrong stems or hanging

Hormones

Auxin

  • Geotropism (Gravitropism):
    • Bending of stem upwards – negative geotropism.
    • Bending down of root downwards – positive geotropism.
  • Phototropism:
    • Growth toward light in stems – positive.
    • Auxin is more concentrated on the dark side.
    • Stimulates cell elongation.
  • Apical Dominance:
    • Stem tip produces auxin.
    • Auxin travels down the stem.
    • Inhibits side branches.
    • Example: avocado vs. bushes.

Gibberellins

  • Promote mitosis and starch metabolism.
  • Too much causes the plant to grow to death (no starch reserves).

Ethylene

  • Ripening (produced by damaged tissue).
  • Produced by damaged tissue.

Plant Anatomy

Stem

  • Apical meristems (buds) make new cells (mitosis).
  • Pith (“ground cells”) for support.
  • Vascular bundles:
    • Are packets of tubes for fluid transport.
    • Vascular cambium:
      • In some plants.
      • Makes more xylem & phloem.
    • Xylem:
      • Moves water and minerals up the stem.
      • Tube cells are hollow cell walls.
      • Water is a “chain” because of hydrogen bonds:
        • Pulled by evaporation in the leaves.
        • Chain would theoretically break at 450 feet.
        • Tallest tree = coastal redwood named Hyperion: 379.1 feet.
    • Phloem:
      • Cells are alive when functional (unlike xylem).
      • Moves molecules up AND down.
      • Transports sap: sugars, hormones, etc.
      • Mechanism unknown.
      • Sap (sugars) is mobilized from roots in spring.
  • Woody stems:
    • No vascular bundles.
    • Cylindrical layers of tissue.
    • External bark made by cork cambium.
    • Rings are produced by different sizes of spring & summer cells.

Roots

  • Epidermis.
  • Pith.
  • Pericycle prevents water loss.
  • Xylem, phloem, perhaps vascular cambium in the middle.

Leaves

  • Cuticle:
    • Waxy.
    • Reduces water loss.
  • Upper epidermis: top "skin" layer.
  • Mesophyll:
    • Palisade: does most of the plant’s photosynthesis.
    • Spongy:
      • Provides ventilation.
      • Air needed for its carbon dioxide.
      • Most efficient way to:
        • Moisten surface for dissolution of gasses.
        • Allow evaporation from inside leaf (evapotranspiration).
  • Lower epidermis:
    • Lower “skin.”
    • With stomates (pores, holes):
      • Prevents excessive water loss (a necessary evil).
      • Guard cells open & close stomate:
        • Lots of water: guard cells swell & open hole.
        • Little water: cells collapse over hole.
  • Vessels:
    • Xylem on top of a leaf vein.
    • Phloem on bottom.
    • No vascular cambium in leaf.

Photosynthesis

  • Remember, plants respire too!
  • CO2 + ewline sunlight + H2O \rightarrow sugars + O_2
  • Uses light
    • Visible light: 380 to 750
    • Action spectrum = what is used for photosynthesis – predominantly blues and reds
  • Light dependent reactions
    • Converts sunlight to bonds in ATP & NADPH
    • H2O \rightarrow O2
    • ADP & P -> ATP
    • NADP -> NADPH
    • light
  • Energy is now “fixed”- also called fixation
  • light-independent reactions
    • Calvin cycle or dark reactions
    • CO_2 \rightarrow sugar
    • ATP -> ADP & P
    • NADP H -> NADP
  • Don't need light, just the products of the light reactions
  • Convert energy in ATP and NADPH to a more stable form
  • Photosynthesis summary
    • H2O \rightarrow O2
    • CO2 \rightarrow C6H{12}O6
    • light energy

Diversity

Viruses

  • Some biologists consider them to be alive.
  • Not classified into classes, etc.; few detectable traits.
  • Structure:
    • Protein coat (capsule or capsid): round or hexagonal.
    • Nucleic acid (DNA or RNA): linear, 1 piece.
  • Life cycle:
    • No metabolism or growth of its own.
    • Parent virus injects its nucleic acid into host (=behavior).
      • "Normal" viruses (DNA):
        • Virus’ DNA is attached to hosts DNA.
        • Host energy & enzymes make DNA copies & protein coats.
      • Retroviruses (RNA):
        • Inject their RNA.
        • Host cell makes DNA based on this RNA (“reverse transcription”).
        • New DNA incorporated into host DNA.
        • Rest as in DNA viruses.
        • Example: HIV, Coronavirus

Domain Bacteria (= D. Eubacteria)

  • Are prokaryotes:
    • A simple type of cell.
    • No internal membranes.
  • Structure:
    • Cell wall.
    • Cell membrane (plasma membrane).
    • DNA (= chromosome; naked & circular).
    • Ribosomes (small, simple).
    • Flagellum in some: “9+0” in structure.
  • Reproduction:
    • Most cells are made by mitosis (asexual cell division).
    • Sex: in some bacteria (“conjugation”)—meaning swap DNA pieces.
  • Diversity:
    • Classification is changing radically.
    • Not sorted into kingdoms yet.
    • “True bacteria”:
      • About 26 different types aka divisions.
      • Most of these are heterotrophic: non-photosynthetic (absorb molecules for energy).
      • Important in decay, nutrient cycles, infections.
    • Division Cyanobacteria:
      • “Cyanophytes” or “blue-green-algae.”
      • Photosynthetic.
      • Some can make ammonium from N_2 in the air:
        • N needed for plants: in fertilizer (nitrate).
        • N \equiv N only can be broken by lightning, industry, cyanobacteria.
        • N_2 "fixed" in heterocyst into a dissolved form

Domain Archaea (= Domain Archebacteria)

  • Are prokaryotes.
  • But are very different from Bacteria:
    • Different photosynthetic pathway and pigment.
    • Different phospholipid in membrane.
    • rRNA of several types, one like eukaryotic rRNA.
    • DNA is circular, but with histones.
  • Found mainly in extreme environments.
  • Diversity:
    • Four divisions are thermophiles: found in hot waters (like geysers), important for rapid high-temperature DNA copying.
    • Another division includes:
      • halophiles: found in very salty waters (ex: Great Salt Lake).
      • methanogens: found in oxygen-free muds (swamps).

Summary of Domains

CharacterD. Bacteria (prokaryotic)D. Archaea (prokaryotic)D. Eukarya (eukaryotic)
Cell size1-10 μm1-10 μm5-100 μm
Cell wallyesyessome
Internal membranesneverneveralways
Chromosomessingle, circularsingle, circularmultiple, linear
Histonesneversomealways
Chlorophylldissolveddissolvedon membranes
Chloroplastsneverneversome
Mitochondrianeverneveralways
Flagella9+0variable9+2
Ribosomessmall simplevariablelarge complex

Domain Eukarya (Eukaryota)

Kingdom Protista

  • Are miscellaneous eukaryotes, unicells & colonies.
  • "Algae"
    • Life cycle
      • Most cells produced asexually (mitosis)
      • Sex has been observed in most species: genetic recombination (swapping of chromosomes)
      • Alternation of generations
        • Two-or three-phase
        • Alternating generations often look different-but not always
    • Diversity (27,000 species)
      • Six divisions based on pigments and energy storage
      • Division Chlorophyta (green algae) ex: Spirogyra, Chara
      • Division Bacillariophyta (diatoms) ex: Aulacoseira, Navicula
    • Importance
      • Oxygen production: about 60% from oceans (particularly the algae from it)
      • Food: autotrophic (self-feeding = photosynthetic)
        • Base of aquatic food chains
        • Human consumption: kelp, ice cream, Jello
      • Medicines: iodine
      • Other: agar, gels
  • "Protozoa”
    • Heterotrophic (non-photosynthetic), mostly unicells
    • Reproduction: mitosis and sex (in most)
    • Diversity (28,000 species)
      • Eleven phyla based on propulsion and spores
      • Phylum Ciliophora (cilates): Paramecium
      • Phylum Rhizopoda (amoebas): Amoeba
    • Importance
      • Most are harmless or beneficial (ex: in termites)
      • Diseases: malaria, yellow fever, cholera
  • "Slime molds" (580 species)
    • Two divisions
    • Usually unicellular, heterotrophic
    • Some are multinucleate
    • Some species aggregate when the environment goes bad
      • Ex: dry or cold
      • Then produce stalks and spores
      • Spores survive until conditions improve

Kingdom Fungi

  • Once classified with K. Plantae, but
    • Heterotrophic
    • Cell walls with chitin, not cellulose
  • Most with sex
  • Some are symbiotic: “together living” of fungus and alga
    • Fungus gets food (sugars from photosynthesis)
    • Alga gets protection, water & nutrients
    • These are lichens
  • Diversity (82,000 species)
    • Five divisions
    • True molds, yeasts & morels, mushrooms, athlete's foot & Penicillium, aquatic molds
  • Importance
    • Decay
    • Help most trees absorb water and nutrients

Kingdom Plantae

  • General
    • Virtually all are photosynthetic
    • All alternate generations
      • One haploid (gametophyte), one diploid (sporophyte)
      • Always look different
  • "Nonvascular plants"
    • No true vessels
    • Gametophyte is dominant
    • One division: D. Bryophyta (16,000 species)
    • Mosses, liverworts
  • "Lower vascular plants"
    • Low-growing
    • Division Lycophyta (1200 species): club mosses
    • Division Monilophyta (13,000 species): ferns, horsetails
  • "Gymnosperms"
    • Nonflowering vascular plants (tracheophytes)
    • Diversity (900 species in 4 divisions)
      • D. Ginkgophyta
        • Only Ginkgo biloba
        • Entire population was only 7 trees
      • D. Coniferophyta (550 species): conifers
        • Wind pollinated (makes lots of pollen)
        • Life cycle similar to angiosperms, but no flowers
        • Includes firs, pines, cedars, redwoods, cypress
  • "Angiosperms"
    • The flowering plants
    • Division Anthophyta only (235,000 species)
      • C. Monocotyledones (65,000 species): “monocots”
        • Leaves with parallel veins
        • Petals in threes
        • Includes grass, palm, lily, orchid, etc.
      • C. Dicotyledones (170,000 species): “dicots”
        • Leaves with branching veins
        • Petals in fours or fives
        • Includes oaks, pecans, herbs, poison ivy, etc.

Kingdom Animalia

  • General
    • All are multicellular (except one species)
    • Heterotrophic
    • Do not alternate generations
  • Phylum Porifera (sponges. 5000 spp.)
    • Colonial
    • No muscles or nerves
    • Six cell types
  • Phylum Cnidaria (9000 species)
    • Colony of interdependent cells
    • Specialized cells: muscles, nerves, stinging
    • Locomotion
    • Includes jellyfish, corals, anemones
  • Phylum Nematoda (10,000 species)
    • Roundworms
    • Complete digestive system
    • Protective cuticle
    • Mostly predators, some parasites
  • Phylum Platyhelminthes (13,000 species)
    • Flatworms
    • Flukes, tapeworms
  • Phylum Annelida (12,000 species)
    • Segmented worms
    • Polychaetes, leeches, oligochaetes (earthworm)
  • Other “worms” (500,000 species): 13 phyla, mostly deep sea
  • Phylum Mollusca (47,000 species)
    • Soft body, often with shell
    • Mostly marine
    • Snails, clams, cephalpods (octopus), etc.
  • Phylum Rotifera (1800 species): “wheel animalicules”
  • Phylum Arthropoda (850,000 species)
    • Chitin exoskeleton, jointed legs
    • C. Arachnida: spiders, mites, scorpions
    • C. Crustacea: shrimp, crabs, pillbugs
    • C. Merostomata (4 species) horseshoe crabs: Limulus polyphemus
    • C. Insecta (750,000 species) insects: beetles, ants, wasps, dragonflies, butterflies…
  • Phylum Echinodermata (6000 species)
    • Radial symmetry (five directions)
    • Initial development like chordates
    • Urchins, starfish, sand dollars
  • Phylum Chordata: chordates
    • Rigid dorsal notochord or backbone
    • “Lower chordates” (1300 species): barrel-like
    • sP. Vertebrata (41,700 species)
      • “Fish” (20,000 species in 3 classes): lampreys, sharks & rays, bony fish
      • Class Amphibia (2500 species)
        • Skin and eggs must be moist
        • Frogs, salamanders
      • Class Reptilia (6000 species)
        • Scaly skin and shelled eggs resist drying
        • Lizards, snakes, turtles, crocodiles
      • Class Aves (8600 species)
        • Feathers, warm-blooded
        • Birds
      • Class Mammalia (4500 species)
        • Subclass Prototheria (3 species)
          • Poor temperature regulation
          • Egg layers
          • Platypus, spiny anteaters
        • Subclass Metatheria (260 species)
          • Moderate temperature regulation
          • Marsupials
          • Opossum, kangaroos, wallabys, etc
        • Subclass Eutheria (4240 species)
          • Good temperature regulation
          • “Higher mammals”
          • Sixteen orders: bats, moles, lemurs, rabbits, carnivores, anteaters, rodents, deer & cows, horses, elephants, manatees, cetaceans, primates.

Environmental Biology

Atmospheric Issues

Urban Air Pollution

  • From combustion.
  • Mainly HC’s, CO_2
  • General symptoms: headache, nausea, lung disease, cancer.
  • Cleaning = prevention
    • Particulates: filters.
    • Chemicals: scrubbers, precipitators, incinerators.

Indoor Air

  • Radon (carcinogen)
    • Decays to radioactive materials.
    • Common in granitic areas.
    • Here, about the national average.
  • Fireplaces
    • CO, CO_2, particles of ash.
    • Heavy metals: from wrapping paper, comics.
    • Paint fumes
  • Bathroom cleaners
  • Much worse due to increased airtightness

Stratospheric Ozone

  • Ozone absorbs UV radiation.
  • Destroyed by CFCs from air conditioners, foam, etc.
  • Only two-thirds of all CFCs have reached the upper atmosphere.
  • Predictions (as of 2010)
    • 900,000 more nonfatal cancers per year (especially skin cancer).
    • 30,000 more fatal cancers per year.
    • $2 billion damage to crops, plastics, etc.
  • Antarctic ozone hole is closing - complete healing around 2065

Acidic Precipitation

  • Components
    • SO_x (sulfuric acid): from low-grade coals for power.
    • NO_x (nitric acid): vehicles.
  • Effects
    • Weakens plants.
    • In waters, kills fish, etc.
    • In cities:
      • Damages car finish.
      • Dissolves limestone.

Climate Change (Global Warming)

  • “Do you believe in climate change?” – better to ask, “Do you think that climate change is real?”
  • Greenhouse gasses: mostly CO2, CH4
  • Natural greenhouse effect: sunlight warms Earth, the atmosphere retains some heat.
  • Humans: cars/vehicles in general (50%), industry (45%), fires (5%).
  • CO_2 trends
    • 1750: 210 ppm (parts per million).
    • 2022 average: 417 ppm (highest in > 3 million years).
    • Rising by 3 ppm/yr.
    • Auto & oil industries now acknowledge their role
  • Effects
    • Temperature
      • 2024: hottest year on record
      • 2015-2024: hottest 9 years on record
      • Since 1750, + 1.7 degrees C
    • Sea level rise
      • IPCC expects 2 ft before 2100: goal < 2 degrees C (3.6 degrees F)
      • Will be complicated by ice sheet melting
        • Alaskan glaciers: 12-inch rise
        • Greenland glaciers: 20 feet rise of sea level
        • Antarctica: 180 ft rise
        • May not be gradual: if ice sheets slide into ocean
      • Threatens cities: New York, Los Angeles, Miami, San Francisco, maybe oceanfront in Memphis after this
      • Croplands: that feed about half the world’s population
    • More & stronger hurricanes that will be longer and slower-moving.
    • Shifting rainfall belt: desert in Missouri?
  • Solutions
    • Drive less, maybe more electric cars, but what source of electricity?
    • Phase out coal, oil, natural gas in favor of solar, wind, nuclear
    • Add a “greenhouse label” to products (like nutrition label information)
    • Reduce deforestation and improve reforestation

Tropical Deforestation

  • Rates
    • At least 22,000 sq miles per year
    • At least 2% of original rainforest area
    • All of Missouri in 3 years
    • Well over half gone now – probably 60%
  • Reasons
    • Huge debt of nation: sell permits to harvest wood
    • Overpopulation: need cropland, but
      • Crops fail after 3-4 years
      • Pasture fails after 6 years
    • Fuel gathering
  • Results
    • Massive erosion
    • Enhanced greenhouse effect
    • Loss of diversity
      • 50-70% of all species in 7% of Earth's land area
      • Potential food
        • 90% of modern ones were originally tropical
        • E.g., corn, potatoes, soybeans, tomatoes, peppers
      • Potential medicines
        • 40% of modern ones (digitalis, quinine)
        • HIV, cancer cures?
      • Global ecosystem stability
      • Cultural diversity
  • Solutions
    • Slow population growth
    • Forgive debt
    • Trade debt for nature: environmental clubs, world bank, tropical rain forest country
    • Enhance ecotourism
    • Use mixed cropping (leave some trees intact)

Water Issues

Wastewater Pollution (Sewage)

  • Is the biggest problem.
  • Wastewater treatment plants: need upgrading.
  • Feedlot sewage: poorly regulated.

Other Pollutants

  • Industrial: organics, metals, heat.
  • Land runoff: fertilizer, pesticides.
  • Shipping: spills, bilgewater.

Water Use

  • Use > resupply in Midwest wells

Solid Waste

Domestic

  • 4.9 lbs/person/day (for 2019).
  • Mainly paper (40%), garden (18%), food (12%), 8% each glass, wood, metals, plastics

Litter

  • Why: it’s easy and free.
  • Control
    • More bins.
    • Enforcement.
    • Despite laws: reduces by half, saves energy

Negatives of Solid Waste

  • Pests, disease, fires, blows away, cost.

Waste Reduction (3-R’s)

  • Reduce: use less.
  • Re-use: no reproduction of product (ex: deposit bottles).
  • Re-cycling
    • Re-production of the same product.
    • Saves energy and cost of mining, refining.
    • Cost is sorting & collecting.
    • Paper: 1 ton = 17 trees
    • Process: sort, liquify (melting or pulping), mold

Re-formation

  • One product is made into a different one.
  • E.g., plastics
    • Re-formed into benches, insulation, etc.
    • Can't re-form for food: cannot be sterilized.
    • Sorted by code (7 types)

Disposal

  • Composting: pile organics, let bacteria decompose.
  • Incineration
    • Generates heat, air pollution.
    • Expensive
  • Sanitary landfills
    • Residue from all of the above, plus more.
    • Covered each day.
    • Problems: settling, rainwater, ugly, stink

Hazardous Materials

Types

  • Radioactive: nuclear waste, hospitals, monitors.
  • Heavy metals: lead, mercury in paints, industrial waste.
  • Toxins (toxic waste)
    • Industrial bleaches: stored in fat, released when dieting
    • Pesticides
      • Aim: complete eradication
      • Problems
        • Resistance: some pests survive, breed (natural selection)
        • Biological amplification (magnification)
        • Dangerous
        • Long-lived
      • Alternatives: aim is to minimize damage
        • Release sterile mates
        • Release predators
        • Pheromones as attractant to traps
        • Crop resistance

General Impacts

  • Lesions, cancer, bones, kidney, nerves, liver.

Management of Hazardous Materials

  • Cradle-to-grave tracking.
  • Minimize use.
  • Legal disposal methods
    • High temperature
    • Bury in super-safe landfill (24%)
    • Deep well injection (14%)
  • Theoretical disposal
    • Shoot to sun
    • Nursing in icecap
  • 62% is disposed otherwise
  • Superfund -- EPA
    • 40,000 sites need work; 1,300 are urgent
    • 428 in progress, 1031 completed
    • Cost $400 to 500 billion

Energy Issues

Wood

  • Primary fuel for 72% of the world population.
  • Becoming scarce.
  • Renewable with care & lower population size

Fossil Fuels

  • Types
    • Coal: may be high in sulfur.
    • Oil: its cost drives economy.
    • Natural gas: clean, cheap, but often from fracking.
  • Problems: produce CO_2, Sox, particulates

Nuclear

  • Conventional fission reactors
    • U-234 split to release heat
    • Heat produces steam
    • Steam turns turbine
  • Problems
    • Expensive
      • U.S. requires the latest available technology
      • 3-12 times more cost overruns
      • Cost without federal support is 13x oil
    • Useful life is 40 years
    • Shutdown is not defined
    • No approved storage for waste: fuel rods are radioactive for > 100,000 years
  • US has around 113 reactors in various parts of the country
    • 67 were to be shut down by 2000
    • Little new construction since 1977
  • Fusion: hypothetical and expensive, but abundant & safe

Alternative Energies

  • Hydropower
    • Flowing water turns the turbine
    • Few good places left to dam
    • Tidal and river power are possibilities
  • Solar
    • Types
      • Passive: south-facing windows
      • Active: use fluid & pumps to move heat
      • Thermal-electric: sun heats oil that boils water
      • Photovoltaic: expensive
    • Problems
      • Sunshine availability
      • Collector price and maintenance
  • Wind
    • Wind turns the turbine
    • Clean, cheap
    • Scarcity of sites
  • Geothermal
    • Let earth heat water
    • Turn turbines
    • But very few sites
  • Biomass
    • Grow crops for fuel (rice, cane, grasses)
    • Burn or extract methanol
    • Should balance its own CO_2 production
  • Conservation
    • Amounts to $10 billion per year
    • Inexpensive
    • But habits are hard to change once they are established

Future of Energy (at 2010 use, and proven reserves)

  • Coal: 80% gone by 2100 AD
  • Petroleum: by 2050 AD (or 2100 at the latest)
  • Natural gas: by 2080 AD or perhaps 2110
  • Nuclear: by 2200 AD
  • Renewables (alternates)
    • Infinite timespan
    • But limited to about 35% of total needs
  • Remember: as we use up one, we’ll rely more on others, and that will accelerate its depletion…

Population

General

  • Populations naturally expand exponentially

Human

  • Second billion in 1930
  • Fourth billion in 1975
  • Sixth billion in 1996
  • Currently, 8.09 billion on the planet (April 2025)
  • Doubling time
    • Time required to double the population size
    • Now is 42 years (1.7% growth rate)