Cell Biology

Learning Objectives

• Understand Eukaryote Cells

• Identify Differences between Eukaryotes and Prokaryotes

• Identify Different Organelles and Their Functions

• Understand Cellular Transportation

• Understand Cellular Respiration

• Understand Cell Reproduction

The Fundamental Unit of Life

• All Living Organisms Are Made Up of Cells

• Cells form the basic structural and organizational unit of life, serving as the building blocks of all living things.

• Cellular theory, developed in the mid-19th century by scientists such as Schleiden and Schwann, states that all cells arise from pre-existing cells through cell division, emphasizing the continuity of life and the understanding that all living organisms are composed of cells, which provides insight into physiology and development.

Types of Living Cells

Prokaryotes

• Include bacteria and archaea, which are among the oldest forms of life, emerging around 3.5 billion years ago.

• Characterized as single-celled organisms that lack membrane-bound organelles; thus, they rely on a simpler cellular structure that allows them to reproduce rapidly through binary fission.

• Genetic material is found in a single, circular strand of DNA located in a region called the nucleoid.

• Offer diversity in metabolism, with some capable of anaerobic respiration, allowing them to thrive in environments devoid of oxygen, and contributing to ecological balance and nutrient cycling.

Eukaryotes

• Eukaryotic organisms belong to the domain Eucarya, which includes a wide array of life forms such as animals, plants, fungi, and protists.

• Characterized by the presence of membrane-bound nuclei and a variety of organelles, which allow for compartmentalization of cellular processes, enhancing efficiency and specialization of functions within cells.

• Eukaryotic cells can be single-celled (such as yeast and protozoa) or multicellular (like humans, trees, and mushrooms), exhibiting a great diversity of forms and functions.

Prokaryotes vs Eukaryotes

Eukaryotic Cells: Organelles

• Nucleus: Control center of the cell, containing DNA, the genetic blueprint for all cellular functions. It regulates gene expression and mediates the replication of DNA during the cell cycle.

• Nucleolus: Region within the nucleus responsible for synthesizing ribosomes, which are essential for protein synthesis.

• Ribosomes: Macromolecular complexes composed of RNA and proteins, vital for the process of translating mRNA into proteins; they exist either freely in the cytoplasm or bound to the rough endoplasmic reticulum (RER).

• Endoplasmic Reticulum (Rough and Smooth):

  • Rough ER: Studded with ribosomes, critical for protein synthesis and processing. It plays a role in the folding and modification of proteins before they are sent to the Golgi apparatus.

  • Smooth ER: Lacks ribosomes, involved in lipid synthesis and detoxification processes, helping to metabolize carbohydrates and store calcium ions.

• Golgi Apparatus: Functions as the cell's post office, modifying, sorting, and packaging proteins synthesized by the Rough ER for transport to various destinations, including the cell membrane or lysosomes.

• Lysosomes: Membrane-bound vesicles that contain hydrolytic enzymes for digesting cellular waste and debris, crucial for maintaining cellular homeostasis and participating in programs of cell death (apoptosis).

• Mitochondria: Known as the powerhouse of the cell, responsible for aerobic respiration. This process converts energy from nutrients into ATP through processes like the Krebs cycle and oxidative phosphorylation, alongside generating reactive oxygen species as by-products.

• Cytoskeleton: A network of protein filaments (microfilaments, intermediate filaments, and microtubules) providing structural support, shape, and facilitating cell movement; it also plays a vital role in intracellular transport and cellular division.

Cellular Transportation

Fluid Mosaic Model

• Describes the structure of the cell membrane as a dynamic, double-layer of phospholipids with embedded proteins, allowing for flexibility and fluidity necessary for diverse cell functions. The model emphasizes the heterogeneous nature of the membrane, which facilitates various functions, including signaling and transport.

• The membrane's selective permeability regulates the entry and exit of substances, essential for maintaining cellular homeostasis and responding to changes in the external environment.

Passive Transport

• Definition: Movement of ions or molecules across a cell membrane without energy input from the cell.

• Diffusion: Describes how molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached, such as oxygen and carbon dioxide diffusing across the membrane.

• Osmosis: The specific diffusion of water molecules through a semi-permeable membrane towards a concentrated solution, maintaining osmotic balance in cells and preventing cellular damage from osmotic pressure fluctuations.

Active Transport

• Definition: Movement of substances against their concentration gradient, requiring energy, usually in the form of ATP.

• Carrier Proteins: Specialized proteins that facilitate the transport of large molecules across the membrane through conformational changes.

• Co-transport Proteins: Utilize energy to create gradients, enabling the concurrent transport of specific molecules (e.g., glucose and sodium ions) across the membrane.

Bulk Transport

• Endocytosis: A mechanism for cellular uptake of large particles or fluids, involving the invagination of the membrane to form vesicles, crucial for nutrient uptake and immune responses.

• Exocytosis: The process by which substances are released from a cell through vesicles fusing with the cell membrane, which is essential for secretion (e.g., hormones) and waste removal.

Cellular Respiration

• Process: A series of metabolic reactions that convert biochemical energy from nutrients into ATP, with by-products of carbon dioxide (CO₂) and water (H₂O). This process is vital for providing energy for cellular activities.

• Steps:

  1. Glycolysis: Occurs in the cytoplasm, where glucose is broken down into pyruvate, yielding a small amount of ATP and NADH; it is anaerobic.

  2. Pyruvate Oxidation: Links glycolysis to the Krebs cycle occurring in mitochondria; pyruvate is decarboxylated and converted into Acetyl-CoA.

  3. Krebs Cycle (Citric Acid Cycle): A series of reactions in the mitochondrial matrix that generates NADH and FADH₂ while releasing CO₂ as a waste product; it plays a key role in energy production.

  4. Electron Transport Chain: Occurs along the inner mitochondrial membrane, where electrons from NADH and FADH₂ are used to drive ATP synthesis while consuming oxygen, ultimately forming water as a by-product; it is the major source of ATP in aerobic organisms.

Cell Division

Mitosis Steps

1. Interphase: The preparatory phase where DNA replication occurs, resulting in two identical sets of chromosomes ready for division.

2. Prophase: Chromosomes condense, and the nuclear envelope begins to break down, making the chromosomes visible, while the mitotic spindle begins to form.

3. Metaphase: Chromosomes align at the cell's equatorial plate, ensuring proper separation during division; spindle fibers attach to the kinetochores.

4. Anaphase: Sister chromatids separate and are pulled to opposite poles of the cell by spindle fibers, ensuring each daughter cell will receive an identical set of chromosomes.

5. Telophase: Chromatids reach the poles, nuclear envelopes re-form around each set, and the cytoplasm divides in a process called cytokinesis, resulting in two genetically identical daughter cells.

Meiosis

• Process: A specialized form of cell division that occurs in germ cells to produce gametes (sperm and eggs) for sexual reproduction, involving two rounds of division (Meiosis I and II). This process results in four non-identical daughter cells, contributing to genetic diversity among offspring.

• Key stages include:

  • Genetic recombination, which occurs during prophase I through crossing over, where homologous chromosomes exchange genetic material; this enhances genetic diversity.

  • Independent assortment during metaphase I, where different combinations of maternal and paternal chromosomes segregate, further increasing variability among gametes.