Unit 2A Characteristics of Life

2025-2026 Honors Biology

Atoms

• The basic building blocks of matter. Atoms combine to form molecules.

• Not alive

Macromolecules

• Large, complex molecules essential for life, including proteins, nucleic acids, carbohydrates, and lipids.

• Not alive

Cell

• The smallest unit of life, capable of performing all life processes such as metabolism, growth, and reproduction. Cells can be unicellular or part of multicellular organisms.

• Alive.

• Ex. muscle cell

Tissue

• A group of similar cells that work together to perform a specific function in an organism.

• Alive.

• Ex. muscle tissue

Organ

• A part of an organism that is typically self-contained and has a specific vital function.

• Alive.

• Ex. heart

Organ System

• A group of anatomically connected organs that work together to perform one or more complex functions.

• Alive.

• Ex. Circulatory

Organism

• an individual animal, plant, or single-celled life form.

• Alive

• Human

Simplest Organisms

One-celled (unicellular)

Autotroph

• can produce their own food/nutrients directly from sunlight (photosynthesis) and can then convert the food/nutrients into ATP through cellular respiration

• Ex. flower

Heterotrophs

• cannot produce their own nutrients/food directly from sunlight. Must ingest other organisms/food which is converted into ATP through cellular respiration

• Ex. bunny

A.T.P

Adenosine Triphosphate

Cellular Respiration

• produces energy for the organism in the form of ATP molecules

• Two types:

  • Anaerobic

  • Aerobic

• Happens in BOTH autotrophs and heterotrophs

Complex organisms

Multi-celled with differentiation (cell specialization)

Ingestion

taking in food

Digestion

Chemically breaking food down

Anaerobic

• no oxygen required; used primarily by simple organisms

• not effective

Aerobic

requires oxygen

Photosynthesis

Conversion of sunlight energy and CO2 into organic molecules/nutrients used for food for organisms

Metabolism

All the chemical processes by which organisms take in and transform energy/matter from the environment

Stimulus

Change in environment that causes a response

Response

reaction to stimuli

Homeostasis

• Steady states of unchanging, stable internal conditions

• Important

• Ex. temperature, pH, water balance 

Regulation

All processes which help an organism maintain homeostasis

Evolution

long term response to environmental stimuli; Species adapt and change in response to challenging environmental conditions

Unicellular

increase in size

Multicellular

increase in number of cells

Development

Changes an organism goes through as it reaches mature adult form; involves different ion/specialization of cells, tissues, etc. 

Reproduction

• The creation of new organisms

• Not necessary to Life of an individual

• Essential for the species

Asexual reproduction

• One parent

• No genetic variation

• Energetically inexpensive

Sexual Reproduction

• Two parents

• Genetic variation

• Energetically expensive form of reproduction

DNA

• Molecule that carries the individuals genetic code

• Determines the identity of the organism and its physical traits

Gene

Specific region of DNA that contributes to the expression of a particular trait

Life

defined by a group of characteristics