Lecture 3: Organelles PDF

LECTURE 3 ORGANELLES Key concepts Biologists use microscopes and tools of biochemistry to study cells. Eukaryotic cells have internal membranes that compartmentalize their functions. The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by ribosomes. The e...

LECTURE 3 ORGANELLES Key concepts Biologists use microscopes and tools of biochemistry to study cells. Eukaryotic cells have internal membranes that compartmentalize their functions. The eukaryotic cell’s genetic instructions are housed in the nucleus and carried out by ribosomes. The endomembrane system regulates protein traffic and performs metabolic functions in the cell. Mitochondria and chloroplasts change energy from one form to another. The cytoskeleton is a network of fibres that organizes structures and activities in the cell. Extracellular components and connections between cells help coordinate cellular activities. 3.1. Prokaryotic and Eukaryotic cells Cells and microscopes All organisms are made of cells. The cell is the simplest collection of matter that can be alive. Microscopes are used to visualize cell. Cell fractionation In cell fractionation cells are broken open, and separated into component parts. Centrifuges are used to separate organelles from one another by gradually increasing the centrifugation speed. Functions of organelles can be studied once they are isolated from other components. Biochemistry and cytology help correlate cell function with structure. Prokaryotic cells There are two types of cells: prokaryotic and eukaryotic. Two of the three domains of life (Bacteria and Archaea) are comprised entirely of prokaryotic cells. Protists, fungi, animals, and plants all are eukaryotic cells. Prokaryotic cells are characterized by having no nucleus and no membrane-bound organelle. DNA in an unbound cytoplasmic region called the nucleoid. Eukaryotic cells Eukaryotic cells are characterized by having DNA in a nucleus bounded by a membranous nuclear envelope and membrane-bound organelles. The cytoplasm is between the plasma membrane and nucleus. Animal and plant cells Plant and animal cells have most of the same organelles. Unique plant cell structures: chloroplasts, large central vacuoles, cell walls, plasmodesmata. Examples of Eukaryotic cells 3.2. Cellular structures Nucleus The nucleus contains most of the cell’s genes, and it is usually the most conspicuous organelle. The nuclear envelope encloses the nucleus, separating it from the cytoplasm. It is a double membrane: each membrane consists of a lipid bilayer. The nuclear pores regulate entry and exit of molecules from nucleus. The shape of the nucleus is maintained by the nuclear lamina composed of protein. Inside the nucleus In the nucleus, DNA is organized into discrete units called chromosomes. Each chromosome is composed of a single DNA molecule associated with proteins. The DNA and proteins of chromosomes are together called chromatin. Chromatin condenses to form discrete chromosomes as a cell prepares to divide The nucleolus is located within the nucleus and is the site of ribosomal RNA (rRNA) synthesis. The endomembrane system The endomembrane system regulates protein traffic and performs metabolic functions in the cell. Components of endomembrane system: nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, plasma membrane. These components are either continuous, or are connected indirectly via vesicles. The endoplasmic reticulum The endoplasmic reticulum (ER) accounts for more than half of total membrane in most eukaryotic cells. The ER membrane is continuous with the nuclear envelope. Two distinct structural regions of ER: Smooth ER, which lacks ribosomes. Rough ER, whose surface is studded with ribosomes. Rough and smooth ER The rough ER: Has bound ribosomes, which synthesize glycoproteins (proteins modified with covalently linked carbohydrates). Site for synthesis of all proteins of the endomembrane system, and for proteins to be secreted from the cell. Produces transport vesicles, which distribute lipids and proteins to other components of the endomembrane system. The ER is a “membrane factory” for the cell. The smooth ER: Synthesizes lipids. Metabolizes carbohydrates. Detoxifies drugs and poisons. Stores calcium ions. Golgi apparatus The Golgi apparatus consists of flattened membranous sacs called cisternae. Golgi apparatus functions: Modifies products of the ER. Manufactures certain macromolecules. Sorts and packages materials into transport vesicles (shipping and receiving center of the cell). Lysosome A lysosome is a membrane-bound compartment of hydrolytic enzymes that can digest macromolecules. Lysosomal enzymes can hydrolyze proteins, fats, polysaccharides, and nucleic acids. Lysosomal enzymes work best in acidic environment inside lysosome. Some types of cell can engulf another cell by phagocytosis; this forms a food vacuole. A lysosome fuses with the food vacuole (forming a phagosome) and digests the molecules. Autophagy Lysosomes also use enzymes to recycle the cell’s own organelles and macromolecules, a process called autophagy. Helps with removing damaged or malfunctioning components and prevents build up of toxic materials. Vacuoles Vacuoles are components of the endomembrane system (derived from ER and Golgi) that perform different functions depending on cell type. Food vacuoles are formed by phagocytosis. Contractile vacuoles, found in many freshwater protists, pump excess water out of cells. Central vacuoles, found in many mature plant cells, hold organic compounds and water. Plasma membrane The plasma membrane is a selective barrier. Allows passage of sufficient amounts of oxygen, nutrients, and waste to support the cell volume. General structure of a biological membrane is a phospholipid bilayer. Surface area to volume ratio As a cell increases in size, its volume grows faster than its surface area: surface area increases by a factor of n2, volume increases by a factor of n3. Small cells have a greater surface area relative to volume. If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio), the cell will eventually die. Hence growing cells tend to divide and remain small to maintain a high SA:Vol ratio suitable for survival. 3.3. Organelles for energy precessing Mitochondria and Chloroplasts Mitochondria are the site of cellular respiration, a metabolic process that uses oxygen to generate ATP. Chloroplasts, found in plants and algae, are the site of photosynthesis, a metabolic process that uses the energy from sunlight to fix carbon (from CO2) and uses it to generate energy-rich organic molecules such as glucose. Endosymbiotic theory Mitochondria and chloroplasts are not part of the endomembrane system, and are derived from prokaryotes (endosymbiotic theory). An early ancestor of eukaryotic cells engulfed a nonphotosynthetic prokaryotic cell, which formed an endosymbiont relationship with its host. The host cell and endosymbiont merged into a single organism, a eukaryotic cell with a mitochondrion. At least one of these cells may have taken up a photosynthetic prokaryote, becoming the ancestor of cells that contain chloroplasts. Mitochondria and chloroplasts resemble bacteria in numerous ways: Contain free ribosomes and circular DNA molecules. Grow and reproduce independently in cells using prakaryotic-like mechanisms. Double membranes. Mitochondria They have a smooth outer membrane and an inner membrane folded into cristae. The inner membrane creates two soluble compartments: intermembrane space and mitochondrial matrix. Mitochondria are in nearly all eukaryotic cells, even photosynthetic cells. Some metabolic steps of cellular respiration are catalyzed in the mitochondrial matrix. Cristae present a large surface area for enzymes that synthesize ATP. Chloroplasts The chloroplast is one type of a group of plant organelles, called plastids. Chloroplasts contain the green pigment chlorophyll, as well as enzymes and other molecules that function in photosynthesis. Chloroplast are surrounded by two membranes. The internal soluble compartment (within the inner membrane) is called the Stroma. Internal Thylakoid membranes contain chlorophyll and form membranous sacs. Thylakoids are stacked to form grana. Internal soluble compartment (within the Thylakoid membrane) is called the thylakoid lumen. Peroxisomes Peroxisomes are specialized metabolic compartments bounded by a single membrane. Peroxisomes are oxidative organelles. Peroxisomes produce hydrogen peroxide (for example as byproduct of fat metabolism) and convert it to water. The enzyme catalse is responsible for this. There isn’t yet a consensus about how peroxisomes are related to other organelles. 3.4. Cytoskeletal structures The cytoskeleton The cytoskeleton is a network of fibres extending throughout cytoplasm. Two functions: Support: it helps to support the cell and maintain its shape. It also organizes the cell’s structures and activities, anchoring many organelles. Movement: cytoskeleton interacts with motor proteins to produce motility. Inside the cell, vesicles can travel along cytoskeleton “tracks”. The cytoskeleton is composed of three types of structures: microtubules, microfilaments, and intermediate filaments.  Microtubules Video: https://www.youtube.com/watch?v=y-uuk4Pr2i8 Centrosomes In many cells (including animals), microtubules grow out from a centrosome near the nucleus. Centrosome is “microtubule-organizing centre”. In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring. Location and function of centrosome is important in chromosome separation during cell division. Cilia and flagella Microtubules control the beating of cilia and flagella, locomotor appendages of some cells. Cilia and flagella differ in their beating patterns. Microtubular anatomy of cell appendages Cilia and flagella share a common structure: a core of microtubules encased by the plasma membrane. The basal body that provides an anchor (it has 9 triplets of microtubules). A motor protein called dynein drives the bending motion. Dynein arms alternately grab, move, and release outer microtubules, causing doublets to curve, bending cilium or flagellum. Microfilaments Form a 3-D network called cortex just inside the plasma membrane to help support the cell’s shape.  Muscle contraction and cell motility 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. Pseudopodia (cellular extensions) extend and contract through the localized, reversible assembly and contraction brought about by actin and myosin, leading to cell movement. Video: https://www.youtube.com/watch?v=GrHsiHazpsw Cytoplasmic streaming Cytoplasmic streaming is a circular flow of cytoplasm within cells. Speeds up distribution of materials. In plant cells, it is driven by actin-myosin interactions. Intermediate filaments 3.5. Extracellular components Cell walls Most cells synthesize and secrete materials external to the plasma membrane. The cell wall is an extracellular structure that distinguishes plant cells from animal cells. Prokaryotes, fungi, and some protists also have cell walls Cell wall protects, maintains shape, and prevents excessive uptake of water in plant cells. Plant cell walls are made of cellulose embedded in other polysaccharides and protein. Plant cell wall has multiple layers: Primary cell wall. Middle lamella (layer separating the primary walls of two adjacent cells). Secondary cell wall (in some cells). Plasmodesmata are channels between plant cells. Through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell. Extracellular matrix Animal cells lack cell walls but have an elaborate extracellular matrix (ECM). Functions of the ECM: support, adhesion, movement, regulation. Made of glycoproteins such as collagen, proteoglycans, and fibronectin. Bind to receptor proteins in plasma membrane called integrins. Cell junctions Neighbouring cells in tissues, organs, or organ systems often adhere, interact, and communicate through direct physical contact. Intercellular junctions facilitate this contact. At tight junctions, membranes of neighbouring cells are pressed together, and bound together by proteins, preventing leakage of extracellular fluid. Desmosomes (anchoring junctions) function like rivets to hold cells together into strong sheets. Gap junctions (communicating junctions) provide cytoplasmic channels between adjacent cells. Thank you for staying awake Summary video to help with your studies: https://www.youtube.com/watch?v=URUJD5NEXC8