Unit 3: The Cellular Basis of Life
- Last updated
- May 14, 2022
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- This page provides a schematic of an idealized liver cell, highlighting structures such as the plasma membrane, nucleus, and organelles like lysosomes and mitochondria. It also mentions components like intermediate filaments, actin filaments, and ribosomes, along with links for further discussion on each structure.
- 3.2: Cell Membranes
- This page explains the plasma membrane as a universal cell feature, acting as a boundary between the interior and environment. It consists of a phospholipid bilayer with hydrophobic tails inward and includes integral and peripheral membrane proteins. Integral proteins traverse the bilayer, while peripheral proteins are loosely attached. Some proteins are fixed in place by the cytoskeleton or extracellular matrix, and others are restricted by tight junctions.
- 3.3: The Nucleus
- This page details the nucleus of eukaryotic cells, emphasizing its dual-membraned nuclear envelope and chromosome organization. It discusses the roles of chromatin, histones, and their modifications in gene expression, alongside the distinctions between euchromatin and heterochromatin. The nucleolus is identified as crucial for ribosomal RNA synthesis, vital for ribosome assembly.
- 3.4: Ribosomes
- This page discusses ribosomes, crucial cellular structures responsible for protein synthesis by translating mRNA into polypeptides. Measuring about 20 nm, they exist as free entities or in clusters (polysomes) in eukaryotes, often attached to the endoplasmic reticulum. Mitochondria possess unique ribosomes for specific protein synthesis. Although there are structural differences among ribosomes in bacteria, eukaryotes, and mitochondria, their core functions remain consistent across these types.
- 3.5: Endoplasmic Reticulum
- This page discusses the endoplasmic reticulum (ER), a membrane-bound structure essential for producing cell membranes, lipids, and proteins. It features two types: rough ER (RER), which is involved in protein synthesis due to its ribosome presence, and smooth ER (SER), which specializes in lipid production and protein transport to the Golgi apparatus. SER is particularly significant in specific cells, such as those in the adrenal glands and liver.
- 3.6: Golgi Apparatus
- This page details the essential functions of the Golgi apparatus in processing proteins from the endoplasmic reticulum, including glycosylation and the synthesis of small peptides. It describes the mechanisms of protein migration, such as transition vesicles and Golgi cisternae movement, as well as the recycling of enzymes to the ER.
- 3.7: Centrosomes and Centrioles
- This page discusses centrioles, essential for cell division and centrosome formation, which plays a role in spindle fiber development and cytokinesis signaling. Abnormal centrosome numbers in cancer cells are linked to uncontrolled mitosis. The page also mentions anticancer drugs like vincristine and Taxol that target microtubules to disrupt cancer cell division.
- 3.8: Lysosomes and Peroxisomes
- This page discusses the role of organelles in cells, specifically focusing on lysosomes and peroxisomes. Lysosomes contain digestive enzymes and maintain acidity to prevent self-digestion, while also being involved in waste disposal; deficiencies can lead to lysosomal storage diseases. Peroxisomes are responsible for fatty acid breakdown and detoxifying hydrogen peroxide, with disorders such as X-ALD and Zellweger syndrome resulting from malfunctions.
- 3.9: Protein Kinesis
- This page discusses the role of proteins in eukaryotic cells, emphasizing their synthesis by ribosomes via mRNA translation. Proteins may remain in the cytosol or enter the rough ER, guided by signal sequences. After synthesis, they are directed to specific cellular locations using chaperones. Proper targeting and folding are crucial for cell functionality, with notable discoveries in this field receiving prestigious recognition.
- 3.10: The Proteasome
- This page discusses the importance of protein degradation alongside synthesis, highlighting the roles of lysosomes and proteasomes. Lysosomes degrade extracellular proteins from endocytosis, while proteasomes target intracellular proteins using a ubiquitin tagging system. This degradation process recycles amino acids and aids in antigen processing for immune response, involving complexes that present antigens to T cells.
- 3.11: The Cytoskeleton
- This page outlines the cytoskeleton, composed of actin filaments, intermediate filaments, and microtubules, each fulfilling distinct roles. Actin filaments enhance mechanical strength and assist in muscle contraction and cytokinesis. Intermediate filaments, such as keratins, provide structural stability. Microtubules, the largest fibers, aid in intracellular transport and chromosome movements during division, and form structures like cilia and flagella.
- 3.12: Cilia
- This page discusses cilia and flagella as essential structures for movement in eukaryotic cells, highlighting their microtubule composition and the role of dynein in mobility. Cilia are abundant, while flagella are fewer, aiding in functions such as sperm movement and clearing mucus in respiratory systems. Notably, almost all mammalian cells have a primary cilium that is important for sensory functions, with defects linked to diseases like polycystic kidney disease.
- 3.13: Animal Tissues
- This page describes the process of a fertilized egg developing into a newborn through about 41 rounds of mitosis, leading to over 100 diverse cell types categorized into four main tissue types: epithelial, muscle, connective, and nervous. Epithelial tissues protect surfaces, muscle tissues enable movement, connective tissues offer support, and nervous tissues facilitate communication, each with distinct functions and structures.
- 3.14: Adipose Tissue
- This page discusses the two types of adipose tissue in mammals: white adipose tissue (WAT) and brown adipose tissue (BAT). WAT is primarily linked to fat storage and obesity, while BAT is crucial for thermogenesis. WAT cells store energy and secrete hormones like leptin, expanding in size with obesity. Exercise might convert WAT to "beige" cells, which have BAT-like features. However, obese individuals tend to have fewer beige cells due to the insulating effects of excess WAT.
- 3.15: Junctions between Cells
- This page discusses the various types of cell junctions in animal tissues, including tight junctions, adherens junctions, gap junctions, and desmosomes, each serving unique functions like preventing substance passage, providing mechanical support, and enabling cell communication. Additionally, it mentions plasmodesmata in plants that facilitate communication and nutrient transfer between adjacent cells.
- 3.16: Plant Cells
- This page discusses plant cells, highlighting their eukaryotic nature and key structures such as a cell wall, large central vacuoles, and chloroplasts for photosynthesis. Unlike animal cells, plant cells lack centrioles and intermediate filaments. The cell wall provides structural strength, while vacuoles store various substances and maintain turgor pressure. Plasmolysis, a condition in hypertonic environments, leads to wilting as turgor pressure decreases.
- 3.17: Chloroplasts
- This page explains the structure and function of chloroplasts in plant cells, which typically contain around 50 chloroplasts with three membrane types: outer, inner, and thylakoid. Thylakoids, grouped as grana, contain essential proteins for photosynthesis. The surrounding stroma holds enzymes for the dark reactions and chloroplast genomes, while some proteins are created in the cytoplasm and transported into the chloroplasts.
- 3.18: Chlorophylls and Carotenoids
- This page explains that chlorophyll exists as types a and b, found in plants and green algae, which absorb red and violet light while reflecting green. Both types are linked to proteins in thylakoid membranes. It also mentions carotenoids, which absorb blue light and appear in various colors; they become visible in autumn and include beta-carotene, a vitamin A precursor for animals.
- 3.19: Plant Tissues
- This page explains the differentiated tissues of mature vascular plants, including meristematic, protective, parenchyma, sclerenchyma, collenchyma, xylem, and phloem. Meristematic tissue is responsible for growth, while protective tissue covers surfaces. Parenchyma stores and aids in photosynthesis, sclerenchyma offers mechanical support, and collenchyma reinforces growth areas. Xylem transports water and minerals, and phloem carries sugars and amino acids, aided by companion cells.
- 3.20: Apoptosis
- This page discusses apoptosis, the process of programmed cell death, which occurs via injury or suicide. It's crucial for development and removing harmful cells. Mechanisms influenced by internal and external signals lead to apoptosis. Cancer cells can evade this process through various means, contributing to tumor growth. Genetic defects can result in conditions like ALPS, and certain body areas utilize apoptosis to avoid immune responses.
- 3.21: Collagens
- This page discusses collagens, vital glycoproteins in animal connective tissues, highlighting 29 types in humans, primarily Types I-IV. It explains the molecular structure of collagen, featuring polypeptides in a triple helix.
- 3.22: Chromatophores
- This page discusses chromatophores, irregularly shaped pigment cells found in animals like crustaceans, cephalopods, lizards, amphibians, and some fish. Their main function is camouflage, with variations by species. Cephalopods can swiftly change color due to muscle activity, while crustaceans and amphibians have fixed shapes that change color through granule movement, regulated by hormones.
- 3.23: Diffusion, Active Transport and Membrane Channels
- This page explains cell transport mechanisms for molecules and ions through selectively permeable plasma membranes, detailing passive (diffusion) and active (energy-requiring) transport methods. It highlights facilitated diffusion via gated channels and active transport mechanisms like Na+/K+ ATPase and Ca2+ ATPases. The text also covers indirect active transport, osmosis under various conditions, and the impact of ion-channel mutations on health.
- 3.24: Endocytosis
- This page explains the cellular process of endocytosis, which includes phagocytosis and pinocytosis for absorption of materials. It highlights receptor-mediated endocytosis for acquiring specific molecules like cholesterol. It also discusses familial hypercholesterolemia's link to atherosclerosis due to mutations in the apoB gene and the transportation of hydrophobic molecules in the bloodstream.
- 3.25: Exocytosis
- This page explains exocytosis, the cellular process for restoring plasma membranes post-endocytosis by using vesicles to release contents and display proteins. Specialized cells, such as pancreatic exocrine cells, secrete proteins through this mechanism. It also describes the "kiss-and-run" method for neurotransmitter release and the role of endosomes in releasing exosomes, which are vital for immune function.
Thumbnail: A diagram of a typical prokaryotic cell. (Public Domain; LadyofHats).
