Cellular Structures, Functions, and Tissue Organization

Cells: Fundamental Units of Life

Cells are the basic structural and functional units that constitute living things. Based on their distinct structure, cells are classified as prokaryotic (e.g., bacteria) and eukaryotic (found in other living organisms). Eukaryotic cells are generally larger and more complex than prokaryotic cells. The most significant difference is the presence of a nucleus in eukaryotic cells, indicating a more evolved structure.

Essential Cellular Functions

Cells perform three primary functions:

• Nutrition: The process of obtaining materials for growth and energy to perform various cellular activities.
• Interaction (Relationship): The ability of cells to perceive and respond to environmental stimuli, allowing for adaptation.
• Reproduction: The process of obtaining two daughter cells from an initial parent cell through cell division.

Major Components of a Eukaryotic Cell

Cell Membrane: The Protective Barrier

The cell membrane surrounds the cytoplasm of the cell. It is flexible and has selective permeability, controlling what enters and exits the cell. Plant cells, in addition to a cell membrane, possess a cell wall primarily composed of cellulose.

Cytoplasm: The Cellular Matrix

The cytoplasm is the gel-like fluid located between the cell membrane and the nucleus. It houses the cellular organelles.

Nucleus: The Control Center

The nucleus is enclosed by a nuclear membrane that regulates the exchange of substances with the cytoplasm. It contains the cell's DNA and the nucleolus.

DNA: Genetic Information Blueprint

DNA (Deoxyribonucleic Acid) is a molecule containing the complete genetic information of the organism. Before cell division, DNA condenses to form chromosomes.

Nucleolus: Ribosome Production Site

The nucleolus is located within the nucleus and is responsible for manufacturing ribosomes.

Key Organelles and Their Roles

Organelles found in the cytoplasm are responsible for performing various specialized functions within the cell:

• Mitochondria: Organelles where cellular respiration occurs, producing ATP (adenosine triphosphate). The process can be summarized as: Organic matter + O₂ → CO₂ + H₂O + ATP.
• Chloroplasts: Organelles where photosynthesis takes place, found only in plant cells and some protists. The process can be summarized as: CO₂ + H₂O + Light Energy + Mineral Salts → Organic Matter + O₂.
• Ribosomes: Organelles, manufactured in the nucleolus, responsible for protein synthesis.
• Vacuoles: Organelles used for storing water, nutrients, and waste products.
• Golgi Apparatus (Golgi Complex): A structure where proteins and lipids are modified, sorted, and packaged for transport.
• Cytoskeleton: A network of protein filaments and tubules in the cytoplasm that gives shape and coherence to the cell and allows for cellular movement.

Transport Across the Cell Membrane

Substances enter and leave the cell through the cell membrane. These substances are often processed by organelles. Small molecules can cross the membrane in two main ways:

Passive Transport: Diffusion & Osmosis

This is the movement of molecules from an area of higher concentration to an area of lower concentration until they are homogeneously distributed, without energy expenditure by the cell. Osmosis is a specific type of diffusion involving the movement of water across a selectively permeable membrane to equalize solute concentrations on both sides.

Active Transport: Energy-Reliant Movement

This process involves the cell membrane in transporting substances, often against their concentration gradient. Therefore, it consumes cellular energy (ATP).

Understanding Cellular Tissues

Tissues are groups of similar cells that are organized to perform a specific function. When different tissues group together, they form organs. Organs working together form organ systems (e.g., the digestive system). Blood vessels are also composed of tissues.

Epithelial Tissue: Covering & Protection

Epithelial tissue is composed of closely packed cells, often in geometric shapes. It covers and protects the internal and external surfaces of organs and body cavities. Example: skin.

Connective Tissue: Support & Connection

Connective tissue is characterized by cells scattered within an extracellular matrix. It supports, connects, or separates different tissues and organs. Examples: cartilaginous tissue, bone tissue.

Muscle Tissue: Movement & Contraction

Muscle tissue is composed of elongated cells (muscle fibers) whose primary function is movement, achieved through their ability to contract. Examples include:

• Smooth Muscle Tissue: Produces involuntary contractions; found in the walls of internal organs like the digestive tract.
• Striated (Skeletal) Muscle Tissue: Produces voluntary contractions and is responsible for body movements.
• Cardiac Muscle Tissue: Similar in appearance to striated muscle but produces involuntary contractions; found exclusively in the heart.

Nervous Tissue: Information Transmission

Nervous tissue is composed of specialized cells called neurons, often star-shaped, which have the function of transmitting electrochemical signals (information). Neurons, along with cardiac muscle cells, are notable for their limited ability to reproduce in mature organisms. Nervous tissue forms the brain, spinal cord, and nerves.