Bio 120 Midterm Study Guide PDF

**Biology 120 Midterm Study Guide** Biology: "bios" meaning life, and "logy" meaning the knowledge of There are unicellular and multicellular organisms The Cell Theory: 1. All organisms are composed of one or more cells 2. The cell is the basic structural and functional unit of all organi...

**Biology 120 Midterm Study Guide** Biology: "bios" meaning life, and "logy" meaning the knowledge of There are unicellular and multicellular organisms The Cell Theory: 1. All organisms are composed of one or more cells 2. The cell is the basic structural and functional unit of all organisms 3. Cells arise only from the division of preexisting cells Topic 1: Prokaryotic and Eukaryotic Cells \*\*Prokaryotic Cells: \*\* \- \*\*Structure\*\*: Simple and smaller, typically ranging from 0.1 to 5.0 micrometers in diameter. Shaped as spherical, rodlike, and spiral. Move using a long flagellum \- \*\*Organelles\*\*: Lack membrane-bound organelles, which means they do not have a nucleus or other organelles like mitochondria or chloroplasts. \- \*\*Genetic Material\*\*: DNA is in a region called the nucleoid, and usually consists of a single circular chromosome. They might also have small, circular DNA molecules called plasmids. Information from DNA is copied into messenger RNA (mRNA) molecules and carried to ribosomes in the cytoplasm, which assemble amino acids into proteins \- \*\*Cell Wall\*\*: Most prokaryotes have a rigid cell wall outside the plasma membrane, often made of peptidoglycan (in bacteria). The plasma membrane is typically surrounded by a rigid external cell wall coated with polysaccharides (glycocalyx- "sugar coat"). When the glycocalyx is loosely associated with cells, it is a slime layer; when it is firmly attached, it is a capsule. The plasma membrane contains molecular systems that metabolize food molecules into ATP \- \*\*Reproduction\*\*: Primarily reproduce asexually through binary fission. \- \*\*Examples\*\*: Bacteria and Archaea. \*\*Eukaryotic Cells: \*\* \- \*\*Structure\*\*: More complex and larger, typically ranging from 10 to 100 micrometers in diameter. \- \*\*Organelles\*\*: Contain membrane-bound organelles, including a nucleus which houses the cell's DNA. Other organelles include mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and in plant cells, chloroplasts. Eukaryotic ribosomes are either free in the cytosol or attached to membranes. The mitochondria are the organelles in which cellular respiration occurs \- \*\*Genetic Material\*\*: DNA is organized into multiple linear chromosomes within the nucleus. \- \*\*Cell Wall\*\*: Present in plant cells (made of cellulose) and fungi (made of chitin). Animal cells lack a cell wall. \- \*\*Reproduction\*\*: Reproduce both sexually and asexually, involving complex processes such as mitosis and meiosis. The Eukaryotic nucleus is more complex than the nucleoid of the prokaryotic cell \- \*\*Examples\*\*: Plants, animals, fungi, and protists. -the nucleus contains most of the cell's DNA and is usually the most conspicuous organelle \- the nuclear envelope encloses the nucleus, separating it from the cytoplasm The Nucleus - Nuclear Envelope: (nuclear membrane) is made up of two lipid bilayer membranes that in eukaryotic cells surround the nucleus - Nuclear pore: (complex) is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm - Nucleolus: is the distinct structure present in the nucleus of eukaryotic cells. Primarily, it participates in assembling the ribosomes - Chromatin: is a complex of DNA and proteins that form chromosomes within the nucleus of eukaryotic cells. - Nuclear lamina- is a dense fibrillar network of intermediate filaments and membrane associated proteins located in the inner nuclear membrane Ribosomes- protein factories in the cell - All living cells contain ribosomes - Ribosomes - Ribosomes are particles made of ribosomal RNAs (rRNAs) and proteins - Ribosomes carry out protein synthesis in two locations: - In the cytosol (free ribosomes) - On the outside of the endoplasmic reticulum (ER) or the nuclear envelope (bound ribosomes) - Each ribosome is composed of two subunits, a large subunit and a small subunit - Ribosomes serves as the site of protein synthesis via decoding the message, and the formation of peptide bonds Endomembrane System - Components of the endomembrane system: - Nuclear envelope - Endoplasmic reticulum - Golgi apparatus - Lysosomes - Vacuoles - Plasma membrane - These components are either continuous or connected via transfer by vesicles Endoplasmic Reticulum (ER) (inside the endomembrane system)- Biosynthetic Factory - The endoplasmic reticulum (ER) accounts for more than half of the total membrane in many eukaryotic cells - The ER membrane is continuous with the nuclear envelope - There are two distinct regions (types) of ER: - Smooth ER, which lacks ribosomes - Rough ER, with ribosomes studding its surface - ER with distinct domains, consisting of tubules, sheets, the nuclear envelope, exit sites, organelle contact sites *etc.* Rough ER - Functions of the Rough ER: - Has bound ribosomes - Is involved in modification and folding of proteins, which are distributed by transport vesicles - Manufactures membranes by producing membrane proteins and phospholipid molecules - Is a membrane factory for the cell - Function of the Smooth ER: - Synthesizes lipids - Regulates carbohydrate metabolism in some cells - Stores calcium - Detoxifies toxins and drugs The Golgi Apparatus- Shipping and Receiving center - the Golgi apparatus consists of flattened membranous sacs called cisternae - functions of the Golgi apparatus: - Modifies products of the ER - Manufactures certain macromolecules - Sorts and packages materials into transport vesicles Lysosomes- Digestive Compartments - A lysosome is a membranous sac of hydrolytic enzymes - Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids - Lysosomes also use enzymes to recycle organelles and macromolecules, a process called autophagy Phagocytosis- lysosomes associated process- Phagocytosis - Phagocytosis is the cellular process by which a cell uses its PM to engulf a large particle, giving rise to an internal compartment (lysosome) called the phagosome Autophagy - Autophagy is the conserved degradation of the cell that removes unnecessary or dysfunctional components through a lysosome-dependent regulated mechanism - It allows the orderly degradation and recycling of cellular components Vesicles- A basic tool used by cell for organizing cellular substances - Vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a membrane layer - Vesicles form naturally during the processes of secretion (exocytosis), uptake endocytosis), and the transport of materials within the plasma membrane Vacuoles- Diverse Maintenance Compartments - Vacuoles are membrane-bound sacs with varies functions - A plant cell or fungal cell may have one or several vacuoles - Food vacuoles are formed by phagocytosis - Contractile vacuoles, found in many freshwater protists, pump excess water of out cells - Central vacuoles, found in many mature plant cells, hold organic compounds and water The endomembrane system is a complex, interconnected and dynamic player in the cell's compartmental organization Mitochondria and Chloroplasts- Energy Conversion from One Form to Another - Mitochondria are the sites of cellular respiration - Chloroplasts, found only in plants and algae, are the sites of photosynthesis - Mitochondria and chloroplasts are not part of the endomembrane system - Peroxisomes are oxidative organelles Mitochondria- Chemical Energy Conversion - Mitochondria are in nearly all eukaryotic cells - They have a smooth outer membrane, and an inner membrane folded into cristae - The inner membrane creates two compartments: intermembrane space and mitochondrial matrix - Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix - Cristae present a large surface area for enzymes that synthesize ATP Chloroplasts- Capture of Light Energy - The chloroplast is a member of a family of organelles called plastids - Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis - Chloroplasts are found in leaves and other green organs of plants and in algae - Chloroplast structure includes: - Outer and inner membranes - Thylakoids - Thylakoid membranous sacs - Stroma, the internal fluid Peroxisome- Oxidation and Lipid Metabolism - Peroxisomes are specializing metabolic compartments bounded by a single membrane - Peroxisomes carry out oxidative reactions - Peroxisomes produce hydrogen peroxide and convert it to water Cytoskeleton- a network of fibers that organizes structures and activities in the cell - The cytoskeleton is a network of fibres extending throughout the cytoplasm - It organizes the cell's structures and activities, anchoring many organelles - It is composed of three types of molecular structures: - Microtubules- the thickets of the three components of the cytoskeleton - Microfilaments- also called actin filaments, are the thinnest components - Intermediate filaments- fibers with diameters in the middle range Roles of the Cytoskeleton: Support, Motility, and Regulation - The cytoskeleton helps to support the cell and maintain its shape - It interacts with motor proteins to produce motility - Recent evidence suggest that the cytoskeleton may help regulate biochemical activities - Inside the cell, vesicles can travel along "monorails" provided by the cytoskeleton Microtubule Structure and Function - Maintenance of cell shape - Guiding movement of organelles - Making up the internal structure of cilia and flagella - Cell motility - Chromosome movements in cell division - Organelle movements Microtubules- Centrosomes and Centrioles - In many cells, microtubules grow out from a centrosome near the nucleus - The centrosome is a "microtubule-organizing center" - In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring Microtubules- Cilia and Flagella - Cilia and flagella are motile cellular appendages found in most microorganisms and animals, but not in higher plants - They aid in cell movement and help move substances around - Cilia is a membrane-bound organelle found on most types of eukaryotic cell; it has the shape of a slender threadlike projection that extends from the surface of the cell body. - Flagella found on sperm cells of eukaryotes and many protozoans have a similar structure to motile cilia that enables swimming through liquids; they are longer than cilia and have a different undulating motion - Cilia and flagella differ in their beating patterns - Cilia and flagella share a common ultrastructure: - A core of microtubules sheathed by the plasma membrane - A basal body that anchors the cilium or flagellum - A moto protein called dynein, which drives the bending movements of a cilium or flagellum Difference Between Cilia and Flagella ------------------------------------------------------------------------------------------ --------------------------------------------------------------------------------------- Cilia Flagella The number of cilia is comparatively more (typically ranges in the thousands) The number is comparatively less (usually ranges from 1-8) Cilia are usually shorter in length Flagella are comparatively longer in length The beating pattern of cilia is very complicated- it can move is a wide range of motions The beating pattern of flagella involves circular, wave like or propellor-like motion Found in eukaryotic cells Found in prokaryotic and eukaryotic cells Cilia are of two types: non-motile cilia and motile cilia Flagella are pf three types: bacterial, archaeal, and eukaryotic flagella Microfilament Structure and Function - Maintenance of cell shape - Changes in cell shape muscle contraction - Cytoplasmic streaming - Cell motility (pseudopodia) - Cell division (cleavage furrow formation) - Actin works with another protein called myosin to regulate muscle movements, cell division, cell tension, cytoplasmic streaming etc. - They form 3D network just inside the plasms membrane to help support the cell's shape and keep organelles in place within the cells Microfilaments (Actin Filaments) - Microfilaments are made of two intertwined strands of actin (G-actin monomer) - Microfilaments are solid rods about 7nm in diameter, built as a twister double chain of actin subunits - Microfilaments that function in cellular motility contain the protein myosin in addition to actin - In muscle cells, thousands of actin filaments are arranged parallel to one another - Thicker filaments composed of myosin interdigitate with the thinner actin fibers - Muscle contraction thus results from an interaction between the actin and myosin filaments that generates their movement relative to one another Microfilaments- Cytoplasmic Streaming - Cytoplasmic streaming is a circular flow of cytoplasm within the cells - This streaming speeds distribution of materials within the cell - In plant cells, actin-myosin interaction and sol-gel transformation drive cytoplasmic streaming - Intermediate Filament Structure and Function - Maintenance of cell shape - Anchorage of nucleus and certain other organelles - Formation of nuclear lamina \*\*Key Differences: \*\* 1\. \*\*Nucleus\*\*: \- Prokaryotic: No true nucleus; DNA is in the nucleoid region. \- Eukaryotic: DNA is enclosed in a membrane-bound nucleus. 2\. \*\*Organelles\*\*: \- Prokaryotic: Lack membrane-bound organelles. \- Eukaryotic: Have various membrane-bound organelles to compartmentalize functions. 3\. \*\*Size\*\*: \- Prokaryotic: Generally smaller. \- Eukaryotic: Generally larger. 4\. \*\*DNA Structure\*\*: \- Prokaryotic: Usually a single circular chromosome. \- Eukaryotic: Multiple linear chromosomes. 5\. \*\*Cell Division\*\*: \- Prokaryotic: Binary fission. \- Eukaryotic: Mitosis and meiosis (more complex processes). 6\. \*\*Examples\*\*: \- Prokaryotic: Includes bacteria and archaea. \- Eukaryotic: Includes plants, animals, fungi, and protists. Understanding these differences is crucial for recognizing how different life forms function at the cellular level and is a fundamental aspect of cell biology. Discovery of Cells Antonie van Leeuwenhoek, often called the \"Father of Microbiology,\" made groundbreaking contributions to the study of cells and microorganisms through his improvements to the microscope. In the 1670s, he was the first to observe and describe microorganisms, which he called \"animalcules,\" including bacteria and protozoa, found in samples like pond water and dental plaque. He also observed red blood cells, sperm, and muscle fibers. Leeuwenhoek\'s detailed observations confirmed the existence of microscopic life, laying the groundwork for later developments in cell theory. By examining diverse samples, his work challenged notions like spontaneous generation and advanced the understanding of life processes at the microscopic level. Robert Hooke, an English scientist, is best known for his discovery of cells. In 1665, using a microscope, he observed the structure of cork and described the tiny, box-like compartments he saw as \"cells,\" a term he coined. Although these were actually the cell walls of dead plant tissue, Hooke\'s work was pivotal in making the concept of cells a fundamental part of biology. His observations and detailed illustrations in his book \"Micrographia\" laid the foundation for cell theory and expanded the scientific community's understanding of life\'s microscopic structures. Basic Features of Cell Structure and Function Cell structure and function are fundamental concepts in biology. Here are the basic features: Cell Structure 1\. \*\*Cell Membrane\*\*: A protective barrier that regulates what enters and exits the cell, composed of a lipid bilayer with embedded proteins. 2\. \*\*Cytoplasm\*\*: The gel-like substance inside the cell, where organelles are suspended, and biochemical processes occur. 3\. \*\*Nucleus\*\*: The control center of the cell containing genetic material (DNA). It regulates cell activities and stores instructions for protein synthesis. 4\. \*\*Organelles\*\*: Specialized structures within the cell that perform specific functions: \- \*\*Mitochondria\*\*: Energy production through cellular respiration. \- \*\*Ribosomes\*\*: Protein synthesis. \- \*\*Endoplasmic Reticulum (ER)\*\*: \- \*\*Rough ER\*\*: Studded with ribosomes, synthesizes proteins. \- \*\*Smooth ER\*\*: Lacks ribosomes, synthesizes lipids and detoxifies substances. \- \*\*Golgi Apparatus\*\*: Modifies, sorts, and packages proteins and lipids for transport. \- \*\*Lysosomes\*\*: Contain enzymes for digestion and waste removal. \- \*\*Peroxisomes\*\*: Break down fatty acids and detoxify harmful substances. 5\. \*\*Cytoskeleton\*\*: A network of fibers that maintains cell shape, provides support, and facilitates movement. 6\. \*\*Cell Wall (in plant cells)\*\*: A rigid outer layer that provides structural support and protection. \#\#\# Cell Function 1\. \*\*Metabolism\*\*: All chemical reactions occurring within the cell, including energy production and utilization. 2\. \*\*Protein Synthesis\*\*: The process of creating proteins based on genetic instructions, crucial for cell structure and function. 3\. \*\*Transport\*\*: Movement of substances in and out of the cell, including passive (diffusion, osmosis) and active (transport proteins) mechanisms. 4\. \*\*Cell Communication\*\*: Interaction with other cells and the environment through signaling molecules and receptors. 5\. \*\*Reproduction\*\*: Cells can reproduce through processes like mitosis (for growth and repair) and meiosis (for gamete formation). Understanding these features helps explain how cells operate, maintain homeostasis, and contribute to the overall function of organisms. Magnification and Resolution \#\#\# Magnification and Resolution in Microscopes \*\*Magnification\*\* refers to the process of enlarging the appearance of an object. It is quantified as the ratio of the size of the image produced by the microscope to the actual size of the object. For example, a magnification of 100x means the image appears 100 times larger than the real object. \*\*Resolution\*\* (or resolving power) is the ability of a microscope to distinguish between two points that are close together. It defines how clearly an image can be seen and is determined by the quality of the optics and the wavelength of light used. Higher resolution allows for the visualization of finer details within a specimen. \#\#\# Key Points \- \*\*Magnification\*\*: Increases size; does not necessarily improve detail. \- \*\*Resolution\*\*: Determines clarity and detail; higher resolution = clearer image. \- \*\*Trade-off\*\*: Often, increasing magnification can lead to a decrease in resolution if not paired with adequate optical quality. \#\#\# Types of Microscopes 1. \*\*Light Microscopes\*\*: Use visible light; can magnify up to about 1000x. Resolution is limited (about 200 nm). a. Reflected light b. Transmitted light c. Fluorescence Types of Light Microscopes: - Bright field microscopy - Differential interference contrast microscopy (DIC) - Fluorescence microscopy - Confocal microscopy 2. \*\*Electron Microscopes\*\*: Use electron beams; can achieve much higher magnifications (up to 1,000,000x) and better resolution (down to 1 nm), allowing for detailed visualization of cellular structures. d. Transmission e. Scanning - Transmission electron microscopy (TEM) - Scanning electron microscopy (SEM) In summary, effective microscopy relies on both magnification and resolution to provide clear and detailed images of specimens. Microscopy Fluorescence Microscopy \#\#\# Fluorescence Microscopy Summary Fluorescence microscopy is a powerful imaging technique used to visualize biological samples by detecting fluorescently labeled structures. In this method, a sample is treated with fluorescent dyes or proteins that emit light when excited by specific wavelengths, usually from a light source like a laser or mercury lamp. \*\*Key Features: \*\* \- \*\*Fluorescent Labels\*\*: Cells or tissues are tagged with fluorescent molecules, allowing specific components to be highlighted. \- \*\*Excitation and Emission\*\*: The microscope uses filters to isolate the excitation light and capture the emitted fluorescence, providing high contrast images. \- \*\*Applications\*\*: Commonly used in cell biology, neurobiology, and molecular biology to study cellular processes, protein interactions, and disease mechanisms. Fluorescence microscopy enables researchers to observe dynamic biological processes in real time and at a high spatial resolution, making it an essential tool in modern biomedical research. Confocal Laser Scanning Microscopy \#\#\# Confocal Laser Scanning Microscopy Summary Confocal laser scanning microscopy (CLSM) is an advanced imaging technique that provides high-resolution, three-dimensional images of biological specimens. It uses a focused laser beam to illuminate a specific point within a sample, capturing the emitted fluorescence from that point while blocking out-of-focus light. This results in images with greater clarity and contrast compared to traditional fluorescence microscopy. \*\*Key Features: \*\* \- \*\*Optical Sectioning\*\*: CLSM can create thin optical sections of a sample, allowing for detailed imaging of complex structures without physical slicing. \- \*\*3D Reconstruction\*\*: By compiling multiple optical sections, researchers can generate detailed 3D representations of the sample. \- \*\*Enhanced Resolution\*\*: The technique improves spatial resolution and depth penetration, making it suitable for studying intricate cellular arrangements. CLSM is widely used in fields like cell biology, neuroscience, and developmental biology, enabling detailed analysis of cellular processes and interactions within tissues. Advantages: - Live cell imaging - Real time imaging - Multicolor imaging - High-resolution and less background in thick tissues - 3-D imaging Electron Microscopy \#\#\# Electron Microscopy Summary Electron microscopy (EM) is a powerful imaging technique that uses beams of electrons instead of light to achieve extremely high magnifications and resolutions, allowing for the detailed visualization of cellular structures and materials at the nanometer scale. \*\*Key Features: \*\* \- \*\*High Magnification\*\*: EM can magnify specimens up to 1,000,000x, revealing fine details not visible with light microscopy. \- \*\*Types\*\*: \- \*\*Transmission Electron Microscopy (TEM)\*\*: Electrons pass through thin specimens, providing detailed internal views. \- \*\*Scanning Electron Microscopy (SEM)\*\*: Electrons scan the surface of samples, producing 3D images with surface detail. \- \*\*Resolution\*\*: EM offers resolution down to about 1 nanometer, making it ideal for studying the ultrastructure of cells, viruses, and materials. Electron microscopy is widely used in biological research, materials science, and nanotechnology, providing insights into the fine structure and composition of specimens. Cell Structure and Function Elements Common to All Living Cells - Plasma Membrane (cell membrane) - Cytoplasm - Chromosome - Ribosomes - Cytoskeleton The basic structure and function of the plasma membrane - All cells are surrounded by the plasma membrane, a bilayer made of phospholipid molecules with embedded protein molecules - Semipermeable plasma membrane controls the flow of substances into and out of a cell - The lipid bilayer is a hydrophobic barrier to water-soluble substances - Selected substances can penetrate cell membranes through transport protein channels - Selective transport of ions and water-soluble molecules maintains the specialized internal environments required for cellular life Internal organization - A central region of all cells contains DNA molecules, which store hereditary information (genes) - The cytoplasm (between the plasma membrane and the central region) describes all material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus - Cytosol is the aqueous portion of the cytoplasm in an intact cell, containing water, dissolved ions and various organic molecules Plant and Animal Cells - Plant and animal cells have most of the same organelles - In animal cells, but not plants cells: - In plant cells, but not animal cells: -Chloroplasts Extracellular Components and Connection between Cells - Most cells synthesize and secrete materials that are external to the plasma membrane - These extracellular structures include: - Cell walls of plants - The extracellular matrix (ECM) of animal cells - Intercellular junctions Cell Walls of Plants - The cell wall is an extracellular structure that distinguished plant cells from animal cells - The cell wall protects the plant cell, maintains its shape, and prevents excessive uptake of water - Plant cell walls are made of cellulose fibers embedded in other polysaccharides and proteins - Plant cell walls have multiple layers: - Primary Cell wall: relatively thin and flexible - Middle lamella: thin layer between primary walls of adjacent cells - Secondary cell wall (in some cells): added between the plasma membrane and the primary cell wall - Plasmodesmata are channels between adjacent plant cells The Extracellular Matrix (ECM) of Animal Cells - Animal cells lack cell walls but are covered by an elaborate extracellular matrix (ECM) - The ECM is made up of glycoproteins and other macromolecules - Functions of the ECM: - Support - Adhesion - Movement - Regulation Intercellular Junctions - Neighboring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact referred to as intercellular junctions - Intercellular junctions facilitate this contact - There are some differences in the ways that plant and animal cells do this - Plasmodesmata are junction between plant cells, whereas animal cell contacts include tight junction, gap junctions, and desmosomes Plants: Plasmodesmata - Plasmodesmata (PD) are membrane-lines pored that connect adjacent cells to mediate symplastic communication in plants - Plasmodesmata facilitate transport of small molecules such as ions, hormones, and photosynthesis from cell to cell - Intercellular trafficking/transport via plasmodesmata is highly regulated - Plasmodesmata establish an interconnected cytoplasm called the symplast, through which metabolites, hormones, proteins, and RNAs are exchanged between Animals: Tight Junctions, Desmosomes, and Gap Junction - Intercellular junctions are elaborate structures formed by integral membrane proteins connected to the actin cytoskeleton by linker or adaptor proteins - At tight junctions, membranes of neighboring cells are pressed together, preventing leakage of extracellular fluid - Desmosomes (anchoring junctions) fasten cells together into strong sheets - Gap Junctions (communication junctions) provide cytoplasmic channels Cell Membranes and Transport Cell Membrane and Functions - The plasma membrane is the boundary that separates the living cell from its nonliving surroundings - The PM exhibits selective permeability, allowing some substances to cross it more easily than others - Fluid Mosaic Model- Cellular membranes are fluid mosaics of lipids and proteins - Phospholipids are the most abundant lipid in the plasma membrane, forming a phospholipid bilayer - Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions - The fluid mosaic model sates that a membrane is a fluid structure with a "mosaic" of various proteins embedded in it The Fluidity of Membranes - Phospholipids in the plasma membrane can move within the bilayer - Most of the lipids, and some proteins, drift laterally - Rarely does a molecule flip-flop transversely across the membrane - Flippase moves an outer phospholipid leaflet to the inner phospholipid leaflet. Floppase does the opposite - As temperatures cool, membranes switch from a fluid state to a solid state - The temperature at which a membrane solidifies depends on the types of lipids - Temperature fluctuates of growth environment adjust the fatty acid composition of the membrane - Membranes rich in unsaturated fatty acids are more fluid that those rich in saturates fatty acids - The enzyme desaturase catalyzes the formation of double bonds in the tails of membrane phospholipids - Membranes must be fluid to work properly; they are usually about as fluid as salad oil - The steroid cholesterol, acting as essential building blocks of the plasma membranes, plays pivotal roles in maintaining the structural integrity and regulating the fluidity of cell membranes - The cholesterol has different effects on membrane fluidity at different temperatures - At warm temperatures, cholesterol retrains movement of phospholipids - At cool temperatures, it maintains fluidity by preventing tight packing Membrane Proteins and Their Functions - A membrane us a collage of different proteins embedded in the fluid matrix of the lipid bilayers - Proteins determine most of the membrane's specific functions - Peripheral proteins are not embedded - Integral proteins penetrate the hydrophobic core and often span the membrane - Integral proteins that span the membrane are called transmembrane proteins - Six major functions of membrane proteins: - Transport - Enzymatic activity - Signal transduction - Cell-cell recognition - Intercellular joining - Attachment to the cytoskeleton and the extracellular matrix (ECM) Membrane Transport - Movement across membranes: Passive Transport- diffusion of a substance across a membrane with no energy investment - Diffusion is the tendency for molecules to spread out evenly into the available space - Although each molecule moves randomly, diffusion of a population of molecules may exhibit a new movement in one direction and finally reaches a dynamic equilibrium. - Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another - No work must be done to move substances down the concentration gradient - The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen Osmosis- Effect on Water Balance - Osmosis is the diffusion of water across a selectively permeable membrane - The direction of osmosis is determined only by a difference in total solute concentration - Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Water Balance of Cells Without Walls - Tonicity is the ability of a solution to cause a cell to gain or lose water - Isotonic Solution: Solute concentration is the same as that inside the cell; no net water movement across the PM - Hypertonic Solution: solute concentration is the same as that inside the cell; cell loses water - Hypotonic Solution: solute concentration is less than that inside the cell; cell gains water - Animals and other organisms without rigid c ell walls have osmotic problems in either a hypertonic or hypotonic environment. To maintain their internal environment, such organisms must have adaptations for osmoregulation, the control of water balance - In plant cells, due to osmosis, water will diffuse into the vacuole, placing pressure on the cell wall Facilitates Diffusion: Passive transport Aided by Proteins - In facilitates diffusion, transport proteins speed movement of molecules across the plasma membrane - Chanel proteins provide corridors that allow a specific molecule or ion to cross the membrane - Carrier proteins undergo a subtle change in shape that translates the solute- binding site across the membrane Active Transport- Using Energy to Move Solutes against their Gradients - Active transport moves substances against their concentration gradients - Active transport requires energy, usually in the form of ATP - Active transport is performed by specific proteins embedded in the membranes - The sodium-potassium pump is one of the most important active transport systems in animal cells Three classes of transporters for active transport: Uniporters, Symporters, and Antiporters - Uniporter: Move one molecule across a cell membrane independent of the other molecules - Symporter: move two molecules in the same direction through a protein channel - Antiporter: (exchanger or counter-transporter) move two molecules in the opposite direction through a protein channel Cotransport- Couples Transport by a Membrane Protein - Cotransport occurs when active transport of a solute indirectly drives transport of another solute - Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell Two Types of Active Transport - Primary (direct) Active Transport- in which the energy is derived directly from the breakdown of ATP - Secondary (indirect) Active Transport- (co-transport) that uses an electrochemical gradient created by the primary active transport Bulk Transport across the PM by: Exocytosis and Endocytosis - Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins - Large molecules, such as polysaccharides and proteins, cross the membrane via vesicles Exocytosis - An active process of moving materials from within a cell to the exterior of the cell - In exocytosis, transport vesicles migrate to the membrane fuse with it, and release their contents - Many secretory cells use exocytosis to export their products Endocytosis - An active cellular process in which substances are brought into the cell - In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane - Endocytosis is a reversal of exocytosis, involving different proteins - Three types of endocytosis: - Phagocytosis ("cellular eating"): Cell engulfs particle in a vacuole - Pinocytosis ("cellular drinking): Extracellular fluids or small particles are ingested by the cell - Receptor-mediated endocytosis: Binding of ligands to receptors triggers vesicle formation The Cell Cycle: Mitosis and Meiosis The Cell Cycle - The cell cycle, or cell-division cycle, is the series of events that take place in a cell that causes it to divide into two daughter cells - The division cycle of most cells consists of four coordinated processes: cell growth, DNA replication, distribution of the duplicated chromosomes to daughter cells, and cell division - In bacteria, cell growth and DNA replication take place throughout most of the cell cycle, and duplicated chromosomes are distributed to daughter cells in association with the plasma membrane; In eukaryotes, however, the cell cycle is more complex and consists of four discrete phases The Cell Cycle in Prokaryotic Organisms - The chromosome process occupies most of the cell cycle in rapidly diving prokaryotic cells - Replicated chromosomes are distributed actively to the daughter cells in binary fission - The bacterial cell cycle is divided into three stages: - B period: the period extends from the end of cell division to the beginning of DNA replication - C period: the period required for chromosome replication - D period: the time between the completion of chromosome replication and the completion of cell cycle - Mitosis (the process in which a eukaryotic cell nucleus split in two) has evolved from and early for of binary fission The Bacterial Cell Cycle- binary fission - Prokaryotic cells undergo a cycle of binary fission involving coordinated cytoplasmic growth, DNA replication, and cell division, referred to as the B, C and D periods, respectively, producing two daughter cells from an original parent cell - Replication of the bacterial chromosome consumes most of the time in the cell cycle - It begins at a single site called the origin of replication (on) through reaction catalyzed by enzymes located in the middle of the cell - Once the ori is duplicated, the two origins actively migrate to the two ends of the cell - Divisions of the cytoplasm then occurs through a partition of cell wall material that grows inward until the cell is separated into two parts - Believed that ancestral division process was binary fission and mitosis evolved from that process - Variations in mitotic apparatus is modern-day organisms show possible intermediates in evolutionary pathway The Eukaryotic Cell Cycle - The eukaryotic cell cycle consists of four distinct phases: G1 phase, S phase (synthesis), G2 phase (collectively known as interphase) and M phase (mitosis and cytokinesis) - Chromosomes are the genetic units divided by mitosis - Interphase extends from the end of one mitosis to the beginning of the next mitosis - After interphase, mitosis proceeds in five stages - Cytokinesis completes cell division by dividing the cytoplasm between daughter cells Mitotic Cell Division - Chromosomes: Nuclear units of genetic information that are divided and distributed by mitotic cell division - Chroma= colour (describing their strong staining by particular dyes) - Some=body - Chromatids: replication of DNA of each individual chromosomes creates two identical molecules called sister chromatids - Chromatin: A complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells - Ploidy: the number of chromosomes sets in a cell or species - Diploid (2n) (i.e. humans contain 23 pairs of chromosomes, for a total of 46 chromosomes) - Haploid (1n) Interphase from End of One Cycle to beginning of Another - The formation of a new daughter cell marks the beginning of the cell cycle- the first and longest phase is interphase and comprises three phases of the cell cycle - Three Phases of Interphase 1\. G1 Phase (gap 1 phase, or Growth 1 phase): The cell carries out its function, and in some cases grows 2\. S phase (Synthesis phase): DNA replication and chromosome duplication occur 3\. G2 phase (Gap 2 phase, or Growth 2 phase): Brief gap in the cell cycle when cell growth continues, and the cell prepares for mitosis and cytokinesis 4\. G0 phase (quiescence): G0 is a resting phase where the cell has left the cycle and has stopped dividing Mitosis- Mitotic Cell Cycle - Following interphase, mitosis process in five stages: - Prophase - Prometaphase - Metaphase - Anaphase - Telophase - Chromosome Segregation- the equal distribution of daughter chromosomes into each of the two daughter cells that results from cell division - Spindle- the spindle apparatus is the cytoskeletal structure that forms during cell division to separate sister chromatids between daughter cells. It is composed of hundreds of proteins and microtubules comprise the most abundant components of the machinery - Centrosome- the main microtubule organizing center of a cell, which organizes the microtubules cytoskeleton during interphase and positions many of the cytoplasmic organelles - Centromere- the region of a chromosome to which the microtubules to the spindle attach, via the kinetochore, during the cell division - Kinetochore- are large protein assemblies that connect chromosomes to microtubules of the mitotic and meiotic spindles to distribute the replicated genome from a mother cell to its daughters - Cleavage Furrow- in cytokinesis, a groove that girdles the cell and gradually depends on until it cuts the cytoplasm into two parts - Cell Plate- new cell wall forms during cytokinesis in terrestrial plants. This process entails the delivery of Golgi-derived and endosomal vesicles carrying cell wall and cell membrane components to the plane of cell division and the subsequent fusion of these vesicles within this plate. Mitotic Cell Division- Mitosis - Prophase - Chromosomes condense into compact, rodlike structures, that become visible under the light microscopes - Each chromosome is doubled as a result of replication - The centrosome has divided into two parts, which are generating the spindle as they separate - Spindle forms in the cytoplasm - Prometaphase - Nuclear envelope breaks down - Spindle enters former nuclear area - Microtubules from opposite spindle poles attach to two kinetochores of each chromosome - Metaphase - Spindle is fully formed - Chromosomes align at spindle midpoint - Each sister chromatid pair is held in position by opposing forces: the kinetochore microtubules pulling to the poles and the cohesins binding the sister chromatids together - Anaphase - Separase cleaved the cohesion ring holding sister chromatids together - Spindle separates sister chromatids and moved them to opposite spindle poles - Chromosome segregation is complete - Telophase - Chromosomes decondense - Return to extended state typical of interphase - New nuclear envelope forms around chromosomes - The cytoplasm is beginning to divide by furrowing - Cytokinesis- is the physical process of cell division, which divides the cytoplasm of parental cell into two daughter cells Meiosis - Meiosis: The process sexually reproducing organisms use to create gametes with half the required genetic material to create a zygote - Bivalent and Tetrad: a bivalent is one pair of chromosomes (homologous chromosomes) in a tetrad. A tetrad is the association of a pair of homologous chromosomes (4 sister chromatids) physically held together by at least one DNA crossover - Crossing-over: Genetic recombination that occurs during meiosis, leading to novel forms - Chiasmata: A point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands - Recombination- the mixing of genetic material from different strands of DNA - Synapsis- An event that occurs during meiosis in which homologous chromosomes pair with their counterparts and remain bound due to the exchange of genetic information - Synaptonemal Complex- A protein and RNA structure that aids in forming the connections during synapsis of homologous chromosomes Cell Division- Meiosis - Sexual reproduction requires a fertilization event in which two haploid gametes unite to create a diploid cell called a zygote - Meiosis is the process by which haploid cells are produced from a cell that was original diploid - Meiosis reduces the number of chromosomes sets from diploid to haploid - Like mitosis, meiosis is preceded by the replication of chromosomes - Meiosis takes place in two sets of cell divisions, called meiosis I and meiosis II - The two cell divisions result in four daughter cells, rather than the two daughter cells in mitosis - Each daughter cell has only half as many chromosomes as the parent cell The stages of Meiosis - In the first cell division (meiosis I) homologous chromosomes separate - Meiosis I result in two haploid daughter cells with replicated chromosomes - In the second cell division (meiosis II), sister chromatids separate - Meiosis II results in four haploid daughter cells with unreplicated chromosomes Interphase - Like mitosis, meiosis begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle - Meiosis I is preceded by interphase, in which chromosomes are replicated to form sister chromatids - The sister chromatids are genetically identical and joined at the centromere - the single centrosome replicated, forming two centrosomes The Stages of Meiosis I - Division in Meiosis I occur in four phases: - Prophase I - Metaphase I - Anaphase I - Telophase I Prophase I - Prophase I typically occupy more than 90% of the time required for meiosis - Chromosomes begin to condense - In synapsis, homologous chromosomes loosely pair up, aligned gene by gene - In crossing over, non-sister chromatids exchange DNA segments - Each pair of chromosomes forms a tetrad, a group of four chromatids - Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred. Metaphase I - At metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole - Microtubules from one pole are attached to the kinetochore of one chromosomes of each tetrad - Microtubules from the other pule are attached to the kinetochore of the other chromosome Anaphase I - In anaphase I, pairs of Homologous chromosomes separate - One chromosome moved toward each pole, guided by the spindle apparatus - Sister chromatids remain attached at the centromere and move as one unit toward the pole Telophase I and Cytokinesis - In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids - Cytokinesis usually occurs simultaneously, forming two haploid daughter cells - In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms - No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated The Stages of Meiosis II - No S phase between meiosis I and II - Sorting events of meiosis II are similar to those of mitosis - Sister chromatids are separated during anaphase II, unlike anaphase I Prophase II - Meiosis II is very similar to mitosis - In prophase II, a spindle apparatus forms - In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate Metaphase II - At metaphase II, the sister chromatids are arranged at the metaphase plate - Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical - The kinetochores of sister chromatids attach to microtubules extending from opposite poles Anaphase II - At anaphase II, the sister chromatids separate - The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles Telophase II and Cytokinesis - In telophase II, the chromosomes arrive at opposite poles - Nuclei form, and the chromosomes begin decondensing - Cytokinesis separates the cytoplasm - At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes - Each daughter cell is genetically distinct form the parent cell Three Events Unique to Meiosis (all three in meiosis I): - Synapsis and crossing over in prophase I: homologous chromosomes physically connect and exchange genetic information - At the metaphase I, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes (sister chromatids) - At the anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are carried to opposite poled of the cell A comparison of Mitosis and Meiosis - Mitosis conserved the number of chromosome sets, producing cells that are genetically identical to the parent cell - Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell - The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis A Comparison of Mitosis, Meiosis I, and Meiosis II ---------------------------------------------------- --------------------------------------------------------------------------------------------- --------------------------------------------------------------------- --------------------------------------------------------------------------------------------- Event Mitosis Meiosis I Meiosis II Synapsis during prophase: No Yes, bivalents are formed No Crossing over during prophase: Rarely commonly rarely Attachment to poled at prometaphase: A pair of sister chromatids is attached to both poles. A pair of sister chromatids is attached to just one pole A pair of sister chromatids is attached to both poles. Alignment along the metaphase plate: Sister chromatids bivalents Sister chromatids Type of separation at anaphase: Sister chromatids separate. A single chromatid, now called a chromosome, moves to each pole Bivalents separate. A pair of sister chromatids moves to each pole. Sister chromatids separate. A single chromatid, now called a chromosome, moves to each pole Meiosis I - Prophase I- replicated chromosomes condense and bivalents form as the nuclear membrane breaks down - Prometaphase I- spindle apparatus complete, and the chromatids are attached to kinetochore microtubules - Metaphase I- segregation of homologues occurs - Connections between bivalents break, but not the connections that hold sister chromatids together - Each joined pair of chromatids migrates to one pole, and homologous pair of chromatids moves to the opposite pole - Telophase I- sister chromatids have reached their respective poles and decondense and nuclear membranes reform - Cytokinesis - Original diploid cell had its chromosomes in homologous pairs, while the two cells produced at the end of meiosis I are haploid- they do not have pairs of homologous chromosomes Meiosis II - No S phase between meiosis I and meiosis II - Sorting event of meiosis II are similar to those of mitosis - Sister chromatids are separated during anaphase II, unlike anaphase I