Cells: The Structural and Functional Units of Life

• A cell is the smallest unit of life able to perform all life processes.
• Cells are the building blocks for all organisms and the basis for their structure and function.

Structural Unit of Life

• All living organisms are made of one or more cells.
• Cells give structure to a body by forming tissues, organs, and organ systems in multicellular organisms.
• Even single-celled organisms, like bacteria, rely on a single cell to live.

Functional Unit of Life

• Cells conduct biological functions like metabolism, energy generation, and reproduction.
• All processes vital to life, including growth, adaptation, and repair, happen within cells.

Robert Hooke's Discoveries

• Hooke examined a thin slice of cork under a microscope.
• Cork is composed of tiny, box-like structures which Hooke named cells due to their resemblance to small rooms in a monastery.
• The structures Hooke observed were the empty cell walls of dead plant cells, which are made of cellulose.
• Hooke did not see the living contents of cells because they were absent in the cork.
• Hooke documented his findings in "Micrographia" (1665).
• Micrographia documented Hooke's illustrations and observations and marked a milestone in biology and microscopy.
• Hooke's work laid the foundation for the cell theory even though he only saw dead cells and could not identify their full significance.

Cell Theory

• All living things are composed of cells.
• A cell is the basic unit of life.
• All cells come from pre-existing cells.

Discovery of Cells

• Loewy and Siekevitz defined a cell in 1963 as the smallest unit of life
• The unit of life is enclosed by a semi-permeable membrane and able to reproduce in the right environment.
• The invention of the light microscope made the discovery of cells possible.
• Robert Hooke (1665) discovered cork is made of tiny cavities, which he called "cells" and was the first to use the term cell.
• Anton van Leeuwenhoek (1632-1723) improved microscopes and observed tiny living organisms, including bacteria, protozoa, red blood cells, and sperm.
• Anton van Leeuwenhoek noticed the nucleus in some cells.
• In 1809, Mirbel stated that all plant tissues are made of cells.
• J.B. Lamarck explained the importance of cells in living organisms.
• Robert Brown (1831) observed the nucleus in plant cells.
• Dutrochet (1837) boiled mimosa cells in nitric acid and concluded tissues are made of cells held together by adhesion.
• In 1839, Theodor Schwann concluded that all living organisms (plants and animals) are made of cells.

Cell Characteristics

• A cell is enclosed by a plasma membrane that separates its interior from the external environment and regulates the exchange of materials.
• DNA (Deoxyribonucleic Acid) inside cells is the blueprint for cellular activities which gets passed on during reproduction.
• Cells have enzymes and organelles which facilitate chemical reactions for energy production and synthesis of molecules.

Cell Diversity

• Cell shapes vary based on their function
• Red blood cells are biconcave for oxygen transport, while nerve cells are elongated for signal transmission
• Cells range in size depending on their function
• Bacteria are 1–10 µm, while human egg cells are visible to the naked eye at approximately 100 µm

Contributing Factors to Cell Diversity

• Each cell type has a specific role in an organism
• Muscle cells contract to facilitate movement, epithelial cells create protective barriers
• The number and type of organelles are based on cellular activity
• Muscle cells have mitochondria for energy, while liver cells have smooth ER for detoxification

Basic Properties of Cells

• Structural properties
• Genetic properties
• Functional properties
• Homeostasis
• Energy utilization
• Evolutionary properties
• Self-sufficiency and collaboration
• Dynamic properties

Structural Properties

• Cells are the smallest living entities that can exist independently.
• The plasma membrane separates interior from external environment, maintaining homeostasis and mediating interactions.
• Prokaryotic cells have a simple structure without membrane-bound organelles, while eukaryotic cells are complex with a nucleus and organelles, like mitochondria, Golgi apparatus, and endoplasmic reticulum.

Genetic Properties

• Cells contain DNA which is hereditary material that guides cellular functions and is passed on during cell division.
• Genes within DNA are transcribed into RNA and translated into proteins which carry out cellular activities.

Functional Properties

• Cells perform metabolic activities to convert energy from nutrients into usable forms, such as ATP through cellular respiration, or photosynthesis in plant cells.
• Anabolic and catabolic pathways regulate synthesis and breakdown of biomolecules.
• Prokaryotes divide by binary fission and eukaryotes divide by mitosis (growth) and meiosis (reproduction).
• Cells grow by synthesizing new components and dividing to form tissues and organs in multicellular organisms.

Homeostasis

• Cells maintain stable internal conditions such as pH, temperature, and ion concentration to support life processes.
• The plasma membrane controls the movement of substances into and out of the cell to preserve homeostasis through selective permeability.

Energy Utilization

• Cells acquire energy through photosynthesis in plant cells
• Cells acquire energy through cellular respiration in most eukaryotic and some prokaryotic cells.
• Energy is used for metabolic processes, reproduction, movement, and maintaining structure.

Evolutionary Properties

• All cells have a common origin, shown by universal features like genetic code, ribosomes, and ATP synthesis.
• Cells evolve to adapt through genetic mutations and natural selection.

Self-Sufficiency and Collaboration

• Single-celled organisms perform all life functions independently.
• Multicellular organisms work in coordinated systems, forming tissues, organs, and organ systems.

Dynamic Properties

• Cells sense and respond to environmental changes, such as temperature, light, or chemical signals.
• Cells interact with each other through chemical signals, hormones, or neurotransmitters to coordinate functions.
• Some cells (e.g., sperm, amoeba) move using flagella, cilia, or pseudopodia, and internally, cytoskeleton dynamics enable the movement of organelles.