Cells, Tissues, Organs, and Organ Systems

• Cell is the basic membrane-bound unit containing life's fundamental molecules and the structural and functional unit of life.
• Living things are either unicellular (one cell) or multicellular (two or more cells).
• Single-celled organisms perform all functions within themselves; multicellular organisms have cells that cooperate for specialized tasks.
• Unicellular organisms include bacteria, yeasts, and archaea.
• Multicellular organisms include mammals, arthropods, poriferans, and plants.
• Mycoplasmas are the smallest known cells.
• The largest cell is the unfertilized ostrich egg, measuring 15-18 cm long and wide.
• The longest cell is the nerve cell, and the largest cell in the human body is the female ovum.
• Cells metabolize nutrients, synthesize molecules like ATP, and replicate to replace worn-out cells.
• Cells are closed compartments where different chemical reactions occur simultaneously under precise control.
• In multicellular organisms, cells differentiate to perform distinct functions, necessitating constant communication.
• Cells exchange nutrients and wastes and cooperate with other cells.
• Similar cells form tissues to perform specific functions.
• Tissues cooperate to form organs.
• Organs assemble into organ systems to sustain life.

Types of Cells

• Two cell types exist based on internal structure: prokaryotic and eukaryotic.

Prokaryotic Cells

• Found in some living organisms with distinct characteristics, including:
• Lacking a true nucleus; they have a nucleoid region where genetic material is suspended in the cytoplasm.
• Possessing single, circular DNA.
• Being exclusively unicellular.
• Reproducing asexually via binary fission.
• Having a small cell size, ranging from 0.1 to 0.5 µm in diameter.
• Containing either DNA or RNA as genetic material.
• Known to live and survive in extreme environmental conditions.
• Lacking membrane-bound organelles.
• Simple cell structure and activities.
• Possessing a cell wall mostly of peptidoglycan.
• Having a capsule that protects against external agents.
• Having pilus for adherence to the external environment.
• Flagella enables locomotion.
• Possessing a plasmid, a circular DNA distinct from chromosomal DNA that carries genes for antibiotic resistance.
• It has inclusion as a storehouse of nutrients and gas vacuoles.
• Examples include bacteria and archaea.

Eukaryotic Cells

• Eukaryotic cells have a true nucleus separated from the cytoplasm housing the genetic material.
• Presence of a double linear DNA structure.
• Found in multicellular and some unicellular organisms like yeast and euglena.
• Capable of sexual and asexual reproduction.
• Larger cell size than prokaryotes, ranging from 10–100 µm in diameter.
• Possessing mostly DNA as genetic material.
• Known to live and survive in moderate environmental conditions.
• Presence of membrane-bound organelles like mitochondria and Golgi apparatus.
• Complex cell structure and activities.
• A cell wall, if present, is mostly made of cellulose, chitin, glucans, and glycoproteins.
• Examples include fungi, protists, animals, and plants.

Cellular Structures and Functions

• Eukaryotic cells are more complex, with a well-organized structure for complex activities.
• Main parts include the cell membrane, nucleus, and cytoplasm.

Cell Membrane

• Also called the plasma membrane, it encloses the cell and acts as a selective barrier.
• Composed of:
  • Phospholipid bilayer: This membrane has two phospholipid layers with hydrophilic heads and hydrophobic tails. Hydrophilic heads face the cytoplasm and external environment, while hydrophobic tails face inward.
  • Proteins: They are embedded in the phospholipid bilayer for specific functions.
    • Channel proteins act like pores for specific ions or molecules.
    • Carrier proteins bind and transport specific molecules across the membrane.
    • Receptor proteins bind to signal molecules on the cell surface to trigger responses within the cell.
  • Carbohydrates: Attached to proteins and lipids on the outer surface, playing a role in cell recognition and communication.

Functions of Cell Membrane

• Selective permeability allows essential nutrients to enter and wastes to exit.
• Protects the cell from damage and leakage.
• Maintains cell shape and provides support.
• Proteins act as receptors for cell communication and response to external signals.
• Carbohydrates act as identification tags for cell recognition, important for the immune system.

Cell Nucleus

• A membrane-bound organelle in the cell's center acts as its control unit.
• Contains only one nucleus.
• Components:
  • Nuclear Envelope: Separates DNA from the cytoplasm.
  • Nucleolus: A region of RNA and proteins aiding ribosome synthesis.
  • Nuclear pore: Channels for molecule transport between the nucleus and cytoplasm.
  • Chromatin/Chromosome: Genetic material, the DNA molecule bounded by histone.
  • Nucleoplasm: Gelatinous substance (karyoplasm) within the nuclear envelope with water, salts, enzymes, and organic molecules.

Functions of Nucleus

• Contains DNA, the hereditary material.
• Sends signals for cell growth, maturation, reproduction, or death.
• Protects DNA from the external environment.
• Serves as the site for RNA formation needed for protein synthesis.

Cytoplasm

• A thick, clear, jelly like substance that fills the inside of a cell.
• Composed of cytosol, filaments, water, salts, and organic molecules.
• Houses all cell components in a suspended manner.
• Encloses most intracellular organelles.
• Facilitates cell protein production and most of the cell's chemical reactions, including glycolysis.
• Associates with all cell contents except the nucleus in eukaryotic cells; includes genetic material in prokaryotic cells.

Function of Cytoplasm

• Maintains cell turgidity, helping cells hold their shape.
• Aids chemical reactions required for the cell's life processes.

Mitochondria

• Known as the powerhouse of the cell and the site for ATP production.
• Double membrane-bound, rod-shaped organelle 0.5 to 1.0 micrometer in diameter.
• Has its own DNA containing genes needed for ATP production.
• Contains an outer membrane, an inner membrane, and the matrix.
• The outer and inner membranes are made of proteins and phospholipid layers separated by the intermembrane space.
• The outer membrane contains porins.
• The inner membrane has many folds called cristae to increase surface area, aiding efficient ATP production. Only permeable to oxygen and ATP molecules.
• Viscous fluid containing enzymes, proteins, ribosomes, inorganic ions, mitochondrial DNA, nucleotide cofactors, and organic molecules.
• Enzymes present in matrix are crucial in ATP synthesis.

Functions of Mitochondria

• Primary function: energy production via oxidative phosphorylation.
• Promotes new cell growth and multiplication by releasing necessary energy.
• Aids detoxification of ammonia in liver cells.
• Plays a vital role in apoptosis or programmed cell death.
• Maintains adequate calcium ion concentration within cell compartments.

Endoplasmic Reticulum

• A membrane-bound organelle involved in protein synthesis, protein folding, lipid and steroid synthesis, carbohydrate metabolism, calcium storage, and the transportation of substances.
• Found in every type of eukaryotic cell except sperm cells and red blood cells.
• The largest organelle in the cell.
• Can be rough (with ribosomes) or smooth (without ribosomes). Rough ER has ribosomes attached. Lies adjacent to the nucleus, and its membrane continues with the nuclear envelope. Plays a role in protein synthesis, folding, and sorting.
• Smooth ER is involved in the synthesis of lipids, including cholesterol and phospholipids, new membrane production, steroid hormones, transport of products of Rough ER. Regulates calcium ion concentration in some cells and contributes to detoxification.

Golgi Apparatus

• Also known as Golgi bodies or Golgi complex.
• A membrane-bound organelle found near the endoplasmic reticulum and nucleus that receives, modifies, sorts, and packages proteins and lipids produced by the ER, and transports them to their final destinations.
• Comprises a 5 to 8-cup-shaped series of compartments known as cisternae.
• Animal cells usually 10-20 Golgi stacks per cell connected by tubular connections.

Function of Golgi Apparatus

• Receives and modifies proteins from the rough endoplasmic reticulum and can modify it through glycosylation, phosphorylation, sulfation, and cleavage.
• Sorts and packages modified proteins into membrane-bound vesicles for transport to lysosomes and plasma membranes.
• Takes part in lipid transport and lysosome formation.
• Acts as the site for the synthesis of glycolipids, sphingomyelin.

Lysosome

• Spherical-shaped membrane-bound organelle, functions as the cell's digestive system to degrade external material and obsolete cell components and are known as cell suicide bags.
• Contains hydrolytic enzymes capable of breaking down biological macromolecules for engulfing and digesting foreign bodies.
• The organelle's lumen has a Ph of 4.5 and 5.0 (acidic).
• Lumen contains cellular debris and over 50 hydrolytic enzymes produced in the rough endoplasmic reticulum.
• Enzymes produces mix w/ mannose 6-phosphate to fix them into vesicles.
• Nuclear genes control lysosomal enzyme production; mutations can cause over 30 genetic diseases, such as Gaucher's disease.
• Besides breaking down biomolecules, lysosomes are involved in energy metabolism, counting discharged materials, cell signaling, and plasma membrane restoration.

Peroxisome

• A membrane-bound organelle found in all eukaryotic cells and a main site for oxygen utilization apart from mitochondria and also contains over 60 oxidative enzymes,
• Also contains catalase and urate oxidase.
• Produces hydrogen peroxide as a byproduct, which is toxic and is converted into water and oxygen by catalase.
• Catalase utilizes H2O2 generated by other enzymes in the organelle to oxidize a variety of other substrates including phenols, formic acid, formaldehyde, and alcohol, by the "peroxidative" reaction producing water as a harmless molecule.
• Provides a safe location for the oxidative metabolism of certain molecules, and the reactions are important in liver and kidney cells for detoxifying toxic molecules.
• About 25% of the ethanol that is consumed is oxidized to acetaldehyde.
• A major function of the oxidative reactions performed is the breakdown of fatty acid and amino acid molecules.
• ẞ oxidation shortens alkyl chains of fatty acids, converting them to acetyl CoA.
• Acetyl CoA is then exported from the peroxisomes to the cytosol for reuse in biosynthetic reactions.
• In mammalian cells, B oxidation occurs in both mitochondria and peroxisomes but in yeast and plant cells, it occurs only in peroxisomes.
• An essential biosynthetic function of animal peroxisomes is to catalyze the first reactions in the formation of plasmalogens.
• Deficiency of plasmalogens causes profound abnormalities in the myelination of nerve cells
• Peroxisomes are diverse organelles that contain different enzymes.
• They can adapt to changing conditions.
• Yeast cells grown on sugar have small peroxisomes, wheras yeast cells on methanol develop large peroxisomes.
• Yeast cells on fatty acids, develop large peroxisomes that break down fatty acids to acetyl CoA by ẞ oxidation.

Functions of Peroxisomes

• Specialized to carry out oxidative reactions using molecular oxygen.
• Generate hydrogen peroxide, then destroy the excess with catalase.
• In animals, important in synthesizing plasmalogens for nerve cell myelination.
• Contribute to the biosynthesis of membrane lipids known as plasmalogens.
• In plant cells, they recycle carbon during photorespiration and convert fatty acids to carbohydrates during seed germination.
• Firefly peroxisomes possess luciferase and promote bioluminescence and find potential food.
• Catalase is present to detoxify the hydrogen peroxide produced by the peroxisome's beta-oxidation of lipids in peroxisomes. Lysosomes function by endocytosis, phagocytosis, and autophagy. Lysosomal proteins come from the rough ER with the aid of nuclear genes. Peroxisomes derive their proteins from the cytosol.

Vacuole

• A membrane-bound cell organelle found in both animal and plant cells and performs functions such as storage, ingestion, digestion, excretion, and expulsion of excess water.
• The surrounding membrane is known as tonoplast.
• In animal cells, they are small and help sequester waste products by getting rid of harmful toxins or clears the extracellular space of those harmful toxins
• In plant cells, vacuoles help maintain water balance.

Function of Vacuole

• Stores salts, minerals, pigments, and proteins, filled with cell sap.
• Filled with protons to maintain an acidic environment,
• Large number of lipids are also stored within the vacuoles.
• Filled with water, exerting turgor pressure that provides shape and helps withstand extreme conditions.
• Involved in endocytosis and exocytosis of unwanted particles

Plastid

• A double-membraned cell organelle that play a role in the manufacturing and storing of food; there are three types.
  • Chromoplasts: Color plastids found in flowers and fruits, responsible for distinctive colors.
  • Leucoplasts: Colorless plastids mainly used for storing starch, lipids, and proteins within plant cells.
  • Chloroplasts: Green-colored plastids containing chlorophyll that captures sunlight and converts it into energy, releasing oxygen.
• Also contain pigments (Carotenoids) that trap solar energy and pass it to chlorophyll.
• Range between 4-6 µm in diameter and 1-3 µm in thickness and only found in green plants and algae cells
• Are the food producers of plants mostly found in parenchyma cells of the mesophyll located in the leaves of the plants
• Circular DNA molecules like mitochondria, which makes them semi autonomous
• Structures consist of a a double membrane, and internal membrane known as as the thylakoid membrane.
• Inner membranes have stroma and grana
• Grana is made up of about 20-30 thylakoids and different protein complexes including photosystem I, photosystem II, and ATP synthase.

Cell Wall

• A permeable non-living component of a cell, covering the outmost layer of a cell and separating the interior contents of the cell from the exterior environment.
• Cell wall provides shape, support, and protection to the cell and its organelles.
• Exists in plant, fungi and prokaryotic organisms.
• Fungi possess cell walls, are made up of chitin, exoskeletons of arthropods.
• Bacteria and archaea contain cell walls made of polymers: peptidoglycans, lipoproteins and lipopolysaccharides.
• Prevents lysis with eukaryotes
• Plant cell are arranged in three layers and made up of minerals a nd carbs like like pectin, cellulose, hemicellulose.

Primary Cell Wall

• Situated closest to the inside of the cell and is the first-formed cell wall
• Made up of cellulose, so the cell can stretch and be permeable

Functions of Cell Wall

• Provides definite shape, strength, and rigidity to plant,bacteria and fungi of all sizes.
• Provides rigid building blocks to stems, stems, and leaves.
• Protects the organisms against mechanical stress and physical shocks.
• Controls cell expansion due to the intake of water.
• Helps in preventing water loss from the cell.
• Provides a porous medium for circulation of water, minerals, and nutrients.
• Storage site for regulatory molecules and microbes.
• Acts as a barrier between the components and the environment.

Cytoskeleton

• A non-membranous network of fibers made of filamentous proteins, that helps maintain cell shape and internal organization of the cell's parts
• Provides mechanical support that enables cells perform functions like division and movement.
• Three major classes of elements include microtubules, actin filaments, and intermediate filaments.
• Microtubules are the largest filament type (25 nm in diameter), made of tubulin protein forming hollow.
• Minus ends of microtubules are anchored in microtubule organizing centers.

Ribosomes

• A non-membranous organelle that produces of both RNA and protein

• Is the site of synthesis in the cell, the proteins synthesized in the cytoplasm are the same as the synthesized in bound ribosomes

• Is the location where where mRNA binds and is decoded, and where amino acids get added.

• Cells from different tissues contain organelles in different proportions.

• Animal cells do not have cell walls and chloroplast.

• Liver cells (hepatocytes) and muscles (myocytes) contain endoplasmic reticulum, peroxisomes, and mitochondria.

• Mature differentiated cells are also important

Explore the relationship between cells, tissues, organs, and organ systems. Understand cell characteristics, unicellular vs. multicellular organisms, and cell size. Learn about cellular differentiation, prokaryotes, antibiotic resistance, and cell membrane barriers.