Cell Structure and Signaling

Basic Unit of Life: Cells are the smallest structural and functional units of living organisms.
Types of Cells:
- Prokaryotic: No nucleus; smaller in size (e.g., bacteria).
- Eukaryotic: Contains a nucleus; larger and more complex (e.g., plant and animal cells).
Major Organelles:
- Nucleus: Contains genetic material; controls cell activities.
- Mitochondria: Powerhouse of the cell; site of ATP production.
- Endoplasmic Reticulum:
  - Rough ER: Studded with ribosomes; involved in protein synthesis.
  - Smooth ER: Synthesizes lipids; detoxifies substances.
- Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
- Plasma Membrane: Semi-permeable barrier; regulates entry and exit of substances.

Definition: Communication process that governs basic cellular activities and coordinates cell actions.
Types of Signaling:
- Autocrine: Cells respond to substances they secrete themselves.
- Paracrine: Signals affect nearby cells.
- Endocrine: Hormones travel through the bloodstream to target distant cells.
Components of Signaling:
- Receptors: Proteins on the surface or inside cells that bind to signaling molecules (ligands).
- Second Messengers: Molecules that relay signals received from receptors to target molecules inside the cell (e.g., cAMP).
Outcomes: Usually results in changes in gene expression, metabolism, or cellular function.

Purpose: Essential for growth, repair, and reproduction.
Types:
- Mitosis: Process of somatic cell division resulting in two identical daughter cells (prophase, metaphase, anaphase, telophase).
- Meiosis: Specialized cell division producing gametes (sperm and eggs); involves two rounds of division, resulting in four non-identical cells.
Cell Cycle Phases:
- Interphase: Cells prepare for division; consists of G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).
- M Phase: Mitosis and cytokinesis occur.

Definition: The process by which a less specialized cell becomes a more specialized cell type.
Importance: Essential for the development of multicellular organisms; enables formation of various cell types (e.g., muscle cells, nerve cells).
Mechanisms:
- Gene Regulation: Differential gene expression leads to specialized functions.
- Signaling Pathways: External signals influence differentiation pathways.
Stem Cells: Undifferentiated cells with the potential to develop into various cell types; categorized into:
- Totipotent: Can form all cell types including extra-embryonic tissues.
- Pluripotent: Can form nearly all cell types but not extra-embryonic tissues.
- Multipotent: Limited to specific lineages (e.g., blood stem cells).

Definition: All biochemical reactions that occur within a cell to maintain life.
Types:
- Catabolism: Breakdown of molecules to extract energy.
- Anabolism: Synthesis of all compounds needed by cells.
Key Pathways:
- Glycolysis: Breakdown of glucose to pyruvate; generates ATP and NADH.
- Krebs Cycle (Citric Acid Cycle): Processes acetyl-CoA; produces NADH, FADH2, and ATP.
- Oxidative Phosphorylation: Uses electron transport chain; produces the majority of ATP in aerobic respiration.
Regulation: Metabolic pathways are tightly regulated by enzymes, hormones, and available substrates.

Cells are the fundamental building blocks of all living organisms.
Prokaryotic cells lack a nucleus and are generally smaller than eukaryotic cells. Examples include bacteria.
Eukaryotic cells possess a nucleus and are more complex, with a variety of internal structures called organelles. Examples include plant and animal cells.
The nucleus is the control center of the cell, housing the genetic material (DNA).
Mitochondria are responsible for energy production through cellular respiration.
The endoplasmic reticulum (ER) is a network of membranes involved in protein synthesis (rough ER) and lipid synthesis and detoxification (smooth ER).
The Golgi apparatus processes, sorts, and packages proteins and lipids for transport within or outside the cell.
The plasma membrane acts as a barrier, regulating the passage of substances into and out of the cell.

Cell signaling is the process of communication between cells, crucial for maintaining homeostasis and coordinating cellular activities.
There are several types of signaling based on the distance between the signaling cell and the target cell:
- Autocrine signaling: Cells respond to signals they produce themselves.
- Paracrine signaling: Signals affect neighboring cells.
- Endocrine signaling: Signals (hormones) travel through the bloodstream to distant target cells.
Signaling molecules (ligands) bind to receptors on the cell surface or inside the cell.
Second messengers, such as cyclic AMP (cAMP), relay signals from receptors to target molecules within the cell.
Cell signaling results in a variety of cellular responses, including changes in gene expression, metabolism, or cell function.

Cell division is essential for growth, repair, and reproduction.
Mitosis is the process of somatic cell division, resulting in two identical daughter cells. It involves four stages: prophase, metaphase, anaphase, and telophase.
Meiosis is a specialized cell division that produces gametes (sex cells) - sperm and eggs. It involves two rounds of division, resulting in four genetically diverse daughter cells.
The cell cycle consists of two main phases:
- Interphase: The cell prepares for division, involving G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).
- M Phase: Mitosis and cytokinesis occur, dividing the cell and its contents into two daughter cells.

Cell differentiation is the process by which unspecialized cells undergo changes to develop specialized functions, forming different cell types.
This process is essential for the development of multicellular organisms.
Cell differentiation is driven by:
- Gene regulation: Different genes are expressed in different cells, leading to unique functions.
- Signaling pathways: External signals influence the differentiation pathways of cells.
Stem cells are undifferentiated cells with the potential to develop into various cell types.
- Totipotent stem cells can form all cell types, including extra-embryonic tissues.
- Pluripotent stem cells can form nearly all cell types but not extra-embryonic tissues.
- Multipotent stem cells are limited to specific cell lineages, such as blood stem cells.

Cell metabolism encompasses all biochemical reactions that occur within a cell to sustain life.
Catabolism involves the breakdown of complex molecules to release energy.
Anabolism involves the synthesis of new molecules, required for cellular growth and function.
Some key metabolic pathways include:
- Glycolysis: The breakdown of glucose into pyruvate, producing ATP and NADH.
- Krebs Cycle (Citric Acid Cycle): Processes acetyl-CoA, producing NADH, FADH2, and ATP.
- Oxidative Phosphorylation: Uses the electron transport chain to produce the majority of ATP in aerobic respiration.
Metabolic pathways are tightly regulated by enzymes, hormones, and the availability of substrates.