3.2 Life Sciences

Section 3.2: Life Sciences

Life Science at the elementary level covers the characteristics of living things, how traits are inherited, energy flow in ecosystems, human body systems, and how organisms adapt to survive.

Cell Structure and Function

Cells are the basic structural and functional units of all living things. Organisms can be unicellular (like bacteria) or multicellular (like humans and oak trees). Within cells, specialized structures called organelles perform vital life functions.

Organelle Functions

Both plant and animal cells share several key organelles:

• Nucleus: The control center of the cell. It houses the cell's genetic material (DNA) and coordinates growth, metabolism, and reproduction.
• Cell Membrane: A semi-permeable outer boundary that regulates which substances enter and exit the cell, maintaining internal balance.
• Cytoplasm: The jelly-like fluid that fills the cell and holds the organelles in place.
• Mitochondria: Often called the "powerhouse" of the cell, they conduct cellular respiration, converting glucose and oxygen into usable energy (ATP).
• Ribosomes: Small structures responsible for making proteins by translating genetic code.

Plant vs. Animal Cells

Plant cells have three key structures that animal cells lack, reflecting their sedentary, autotrophic lifestyle:

1. Cell Wall: A rigid outer layer made of cellulose that provides mechanical support, protection, and structural shape.
2. Chloroplasts: Organelles containing the green pigment chlorophyll, where photosynthesis occurs. They capture solar energy and convert it into chemical energy (glucose).
3. Large Central Vacuole: A large fluid-filled organelle that stores water and nutrients. When filled, it exerts turgor pressure against the cell wall, keeping the plant upright. Animal cells have only small, temporary vacuoles.

Table 3.2.1: Plant vs. Animal Cells

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Heredity and Genetics

Heredity is the passing of traits from parents to offspring. The instructions for these traits are encoded in DNA (deoxyribonucleic acid).

• Chromosomes: Structures of coiled DNA found in the nucleus. Humans have 46 chromosomes (23 pairs).
• Genes: Specific segments of DNA on a chromosome that code for a particular trait.
• Alleles: Different versions of a gene. For example, a gene for flower color might have a purple allele (P) and a white allele (p).
• Dominant and Recessive: Dominant alleles mask recessive alleles in heterozygous individuals.
• Genotype vs. Phenotype: Genotype is the genetic makeup of an organism (e.g., PP, Pp, pp). Phenotype is the physical expression of the trait (e.g., purple flowers, white flowers).
  • Homozygous Dominant: PP (two dominant alleles)
  • Heterozygous: Pp (one dominant, one recessive allele)
  • Homozygous Recessive: pp (two recessive alleles)

Punnett Squares

A Punnett square predicts the probability of offspring genotypes and phenotypes. For example, if two heterozygous purple flower plants (Pp x Pp) are crossed:

• The square yields: 1 PP, 2 Pp, and 1 pp.
• The genotypic ratio is 1:2:1.
• The phenotypic ratio is 3 purple to 1 white (75% purple, 25% white) because the dominant allele P masks the recessive p in the heterozygous Pp offspring.

Inherited vs. Acquired Traits

• Inherited Traits: Genetically determined characteristics passed down from parents (e.g., leaf shape, hitchhiker's thumb, blood type, naturally curly hair).
• Acquired Traits: Characteristics developed during life due to environmental influences or learning (e.g., a tree's scarred trunk, muscle mass from weightlifting, learned behaviors like riding a bicycle or speaking a language).

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Ecosystems and Food Webs

An ecosystem is a community of living (biotic) organisms interacting with their non-living (abiotic) environment.

Trophic Levels

Energy enters most ecosystems through the Sun. It flows through different feeding levels:

1. Producers (Autotrophs): Green plants and algae that capture solar energy to produce food through photosynthesis.
2. Primary Consumers: Herbivores that feed directly on producers (e.g., grasshoppers, rabbits).
3. Secondary Consumers: Carnivores or omnivores that eat primary consumers (e.g., frogs, mice).
4. Tertiary Consumers: Apex predators that eat secondary consumers (e.g., hawks, wolves).
5. Decomposers: Organisms like bacteria, fungi, and earthworms that break down dead organic matter, recycling raw nutrients back into the soil for plants to reuse.

Food Chains vs. Food Webs

• Food Chain: A linear path showing how energy is transferred from one organism to another (e.g., Oak Leaf -> Caterpillar -> Blue Jay -> Hawk).
• Food Web: A network of multiple interconnected food chains. Food webs show the true complexity of feeding relationships. If one species is removed (e.g., due to disease or pesticides), it can cause a trophic cascade, impacting many other organisms in the web.
• Energy Pyramid & the 10% Rule: Shows the total energy available at each level. Only about 10% of the energy is passed from one trophic level to the next. The other 90% is lost as heat or used by the organism for life processes (movement, respiration, growth). Thus, ecosystems can rarely support more than four or five trophic levels.

Table 3.2.2: Symbiotic Relationships

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Human Body Systems

Multicellular organisms have specialized organ systems that work together to maintain homeostasis:

• Cardiovascular/Circulatory: Transports oxygen, nutrients, and waste throughout the body via the heart and blood vessels.
• Respiratory: Exchanges oxygen and carbon dioxide between the blood and air using the lungs and trachea.
• Digestive: Breaks down food into absorbable nutrients and eliminates solid waste.
• Nervous: The brain, spinal cord, and nerves detect stimuli and coordinate bodily responses.
• Muscular & Skeletal: Provide structural support, protect vital organs, and enable movement.
• Excretory/Urinary: Filters waste from blood and excretes it as urine.
• Immune: Defends the body against viruses, bacteria, and other pathogens.

[md] System Interactions: Organ systems do not operate in isolation. During exercise, the nervous system detects low oxygen levels and signals the respiratory system to breathe faster and the cardiovascular system to pump blood quicker, delivering oxygen and glucose to active muscle cells.

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Adaptation and Natural Selection

• Structural Adaptations: Physical body characteristics that aid survival (e.g., thick blubber in polar bears, webbed feet in ducks, waxy cuticles on desert cacti).
• Behavioral Adaptations: Actions organisms take to survive (e.g., migration of monarch butterflies, hibernation of bears, playing dead in opossums).
• Natural Selection: Environmental pressures (e.g., predators, climate) select for individuals with variations that are best suited to their environment. These individuals are more likely to survive, reproduce, and pass those favorable traits to their offspring. Over generations, this leads to the adaptation of the entire species.