Concept Summary - Cell Structure.pdf

Concept Summary: Cell Structure INTRODUCTION The cell is the smallest collection of matter to exhibit all of the properties of life. All organisms are composed of one or more cells. Our understanding of nature often goes hand in hand with the invention and refinement o...

Concept Summary: Cell Structure INTRODUCTION The cell is the smallest collection of matter to exhibit all of the properties of life. All organisms are composed of one or more cells. Our understanding of nature often goes hand in hand with the invention and refinement of instruments that extend our senses. The first scientist to visualize cells was Robert Hooke. In 1665, Hooke used a crude microscope to examine a piece of bark from an oak tree. He compared the structures he saw to “little rooms”—cellulae in Latin—and the term “cell” stuck. Leeuwenhoek used more refined lenses to describe living cells from blood, sperm, and ponds. INTRODUCTION TO THE CELL Microscopes reveal the world of the cell Since the days of Hooke and Leeuwenhoek, improved microscopes and techniques have vastly expanded our view of the cell. Magnification is the increase in an object’s image size compared with its actual size. Resolution is a measure of the clarity of an image. In other words, it is the ability of an instrument to show two nearby objects as separate. The light microscope (which uses magnifying lenses and visible light) is used to view cells, but it has limits of resolution. Light microscopes are best used when you want to view living specimens. For higher magnification (and resolution), you need to use electron microscopes (which use a beam of electrons). Electron microscopes have higher magnification and resolution, but the techniques used to prepare the specimen kill it. Electron microscopes are best used when you want to view extremely small specimens. There are 2 main types of electron microscopes: 1. Scanning electron microscope (SEM)—a beam of electrons scans the surface of the specimen; used to study the details of the cell’s surface 2. Transmission electron microscope (TEM)—electrons are beamed through the specimen; used to study the details of internal cell structure Cell Theory: 1. all organisms are composed of cells 2. the cell is life's basic unit of structure and function (smallest living things) 3. all cells come from pre-existing cells © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. The small size of cells relates to the need to exchange materials across the plasma membrane The main factor that limits the size of cells and makes them stay microscopic is surface area-to- volume ratio. Most substances enter and leave a cell by diffusion through the plasma (cell) membrane. With a small cell with a small volume, diffusion to the center of the cell is virtually instantaneous. When cells become very large, the time would stretch to minutes or hours. The plasma (cell) membrane is a phospholipid bilayer (remember the hydrophilic “heads” and hydrophobic “tails”) with embedded proteins. The function of the plasma membrane is to regulate what substances are allowed to enter or leave cells. The membrane uses the embedded proteins for this function. Prokaryotic cells are structurally simpler than eukaryotic cells Cellular organization varies in the cells of different organisms, but all cells resemble each other in certain fundamental ways. Prokaryotic cells are small and simple. Eukaryotic cells have membrane bound nucleus and membrane bound organelles that perform specific functions. Prokaryotic cells→ lack a membrane bound nucleus and membrane bound organelles. Include Domain Archaea (Kingdom Archaea) and Domain Bacteria (Kingdom Bacteria). Eukaryotic cells→ have a membrane bound nucleus and membrane bound organelles. Includes Domain Eukarya (Kingdoms Protista, Fungi, Plantae and Animalia). All cells have: 1. Plasma membrane 2. Genetic material (DNA) 3. Cytosol – thick, jelly-like substance in which cellular components are suspended 4. Ribosomes – site of protein synthesis in the cell All organisms have a plasma (cell) membrane, but not all cells have a cell wall. Organisms with a cell wall include all prokaryotes (Bacteria and Archaea), plants and fungi. There are 2 figures in the PPT that compare plant to animal cells. Many of the structures in these cells are the same but there are 4 structures found in plant cells but not animal cells: 1. central vacuole—large compartment that stores water and other substances to keep the pressure up in the cell 2. cell wall—provides strength and support; prevents cells from bursting when taking in water 3. chloroplasts—site of photosynthesis 4. plasmodesmata—cytoplasmic channels that connect adjacent cells There are 2 structures found in animal cells but not plant cells: centrioles and lysosomes (more on these later). © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. Eukaryotic cells are partitioned into functional compartments Membrane-enclosed organelles compartmentalize a cell’s activities. The organelles and other structures of eukaryotic cells can be organized into four basic functional groups: 1. The nucleus and ribosomes carry out the genetic control of the cell. 2. Organelles involved in the manufacture, distribution, and breakdown of molecules include the endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, and peroxisomes. 3. Mitochondria in all cells and chloroplasts in plant cells function in energy processing. 4. Structural support, movement, and communication between cells are the functions of the cytoskeleton, plasma membrane, and plant cell wall. THE NUCLEUS AND RIBOSOMES The nucleus contains the cell’s genetic instructions The nucleus houses the cell’s DNA in the form of chromosomes. It is surrounded by a double membrane called the nuclear envelope, which is a double membrane that controls the movement of substances into and out of the nucleus. Most of the time the chromosomes are in the form of chromatin (a complex of DNA and protein that is present when the cell is not dividing). Only when the cell starts to divide does the DNA begin to coil up, becoming visible as the "typical X-shaped chromosome". There is often a darkened area in the nucleus called the nucleolus, which is the site of synthesis of ribosome subunits. Ribosomes make proteins for use in the cell and for export Ribosomes are the sites of protein synthesis. They are made at the nucleolus in 2 parts (subunits). Each subunit is composed of ribosomal RNA (a nucleic acid) and proteins. There are two populations of ribosomes in the cell: a. free ribosomes - suspended in the cytoplasm, make proteins that remain in the cytoplasm b. bound ribosomes - attached to the rough ER and nucleus, make membrane proteins and proteins that will be exported from the cell. THE ENDOMEMBRANE SYSTEM Many organelles are connected in the endomembrane system Many of the membranes within a eukaryotic cell are part of the endomembrane system. Many of these organelles interact in the synthesis, distribution, storage, and export of molecules. © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. Components of the endomembrane system include: 1. the nuclear envelope 5. lysosomes 2. the plasma membrane 6. vesicles 3. the rough and smooth ER 7. vacuoles 4. Golgi apparatus The endoplasmic reticulum is a biosynthetic workshop The ER is a membranous network of tubes and sacs that functions to divide the cell into functional components. The ER also produces a variety of phospholipids, steroids, proteins, etc. Smooth ER synthesizes lipids, detoxifies foreign substances (in the liver) and stores calcium for muscle contraction. Rough ER produces membranes, and processes and packages proteins that are made by ribosomes on its surface. Processing goes like this. 1. The protein produced by a ribosome enters the rough ER 2. Sugar chains are added to the protein to produce a glycoprotein 3. The molecule is placed into a transport vesicle (a sac made of membrane material that is used to transfer segments between membranes) 4. The vesicle buds off and travels to the Golgi The Golgi apparatus modifies, sorts and ships cell products The Golgi apparatus consists of stacks of sacs in which products of the ER are processed and then sent to other organelles or to the cell surface. Golgi modifies the sugar portions of glycoproteins (adds some, removes some). Golgi makes sure that all molecules are properly marked with sugar chains. Lysosomes are digestive components within a cell Lysosomes are membrane sacs full of digestive enzymes that pinch off of Golgi and digest all sorts of biological molecules: 1. They digest old or damaged cell parts (organelles), recycling their component molecules. 2. Protists ingest food into a vacuole, which fuses with a lysosome and nutrients are released into the cytosol. 3. Substances are brought into the cell in a vesicle, which fuses with a lysosome for digestion. For example, white blood cells often ingest bacteria and destroy them using lysosomes. © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. Vacuoles function in the general maintenance of the cell Vacuoles are large vesicles have a variety of functions: 1. Some protists have contractile vacuoles that collect and expel excess water so the organism doesn’t burst. 2. Plant cells contain a large central vacuole that stores molecules and wastes and keeps the pressure up in the plant cell. 3. Other organisms have other types of small vacuoles A review of the structures involved in manufacturing and breakdown The organelles of the endomembrane system are interconnected structurally and functionally. Peroxisomes are metabolic compartments that do not originate from the endomembrane system. They contain enzymes for the breakdown of certain molecules, like fatty acids and amino acids. Hydrogen peroxide is a harmful byproduct of this type of metabolism, but the peroxisome also has the enzyme (catalase) necessary to break it down to harmless water and oxygen. ENERGY-CONVERTING ORGANELLES Mitochondria harvest chemical energy from food Mitochondria are organelles that carry out cellular respiration in nearly all eukaryotic cells. Cellular respiration is a process that converts the chemical energy of food molecules to chemical energy (ATP). Mitochondria are surrounded by a double membrane and have two internal compartments: The intermembrane space is the narrow region between the inner and outer membranes. The inner membrane is folded up and is called the cristae. The mitochondrial matrix is on the inside of the inner membrane and contains the enzymes that catalyze some of the reactions of cellular respiration. Chloroplasts convert solar energy to chemical energy Photosynthesis is the conversion of light energy from the sun to the chemical energy of sugar molecules. Chloroplasts are the photosynthesizing organelles of plants and algae. Chloroplasts have a double membrane like mitochondria. © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. On the inside of the inner membrane is a thick fluid called the stroma. There are stacks of flattened disks called thylakoids, which are the site where chlorophyll traps sunlight for photosynthesis. The disks themselves are the thylakoids and the entire stacks are called grana. Evolution connection: mitochondria and chloroplasts evolved by endosymbiosis The endosymbiont theory states that mitochondria and chloroplasts were formerly small prokaryotes that began living within larger cells. This theory explains the likely origin of the eukaryotic cell. In the eukaryotic cell, there are many membrane structures that probably arose as foldings of cell membrane that pinched off and became specialized in some type of function. Both mitochondria and chloroplasts are surrounded by a double membrane, their own unique DNA and prokaryotic- like ribosomes. This is unusual—how can it be explained? Possibly, mitochondria & chloroplasts may be descendants of once-independent prokaryotic cells that were engulfed by larger prokaryotes. For some reason they were not digested but lived inside the larger cells, providing them with certain metabolic advantages. This idea is called the Theory of Endosymbiosis. THE CYTOSKELETON AND CELL SURFACES The cell’s internal skeleton helps organize its structure and activities The cytoskeleton is a network of protein fibers extending throughout a cell. Functions of the cytoskeleton include maintenance of cell shape, anchorage and movement of organelles, amoeboid movement (crawling), and muscle contraction. It is a dynamic structure, always being formed & disassembled. The cytoskeleton includes 3 types of fibers which are classified by diameter size: 1. Microtubules—the largest of the fibers; function to shape and support the cell, form cilia and flagella, form centrioles, form motor tracks for movement of materials 2. Intermediate filaments—medium in size; function to reinforce the cell’s shape and anchor organelles 3. Microfilaments (actin)—the smallest of the fibers; involved in movement, muscle contraction, cells that can crawl (amoeboid movement) © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. Cilia and flagella move when microtubules bend Eukaryotic cilia and flagella are locomotor appendages made of microtubules in a “9 + 2” arrangement. This arrangement consists of a circle of 9 microtubule pairs surrounding 2 central microtubules. Although differences exist, flagella and cilia have a common structure and mechanism of movement. Cilia are short hair-like organelles of movement that project from the cell. They usually occur in great numbers & beat together like the oars on a large ship. Flagella are similar to cilia, but are longer and propel the cell by an undulating, whip-like motion. Usually only one or two flagella per cell. The extracellular matrix of animal cells functions in support and regulation Animal cells synthesize and secrete an elaborate extracellular matrix (ECM), which binds tissue cells together, supports the plasma membrane, and communicates with the cytoskeleton. The ECM may attach to the cell through other glycoproteins that then bind to membrane proteins called integrins. Integrins are proteins that span the membrane, attaching to the cytoskeleton on the inside and to the ECM on the outside. This provides support to the cell allows communication between the outside and inside of the cell. Three types of cell junctions are found in animal tissues Adjacent cells adhere, interact, and communicate through specialized junctions between them. 1. Tight junctions bind cells to form leakproof sheets. For example, they are found in the digestive system and function to prevent fluid from leaking into surrounding tissues. 2. Anchoring junctions fasten cells together into strong sheets of tissue. For example, they are found in the heart and provide mechanical support during contraction. 3. Gap junctions allow ions and small molecules to flow from cell to cell. For example, they are found in the heart and allow ions to flow during muscle contraction. Cell walls enclose and support plant cells A plant cell (but not an animal cell) has a rigid cell wall that protects and provides skeletal support that helps keep the plant upright and is primarily composed of cellulose. Plant cells have cell junctions called plasmodesmata that allow plant tissues to share water, nourishment, and chemical messages. © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted. See handout that summarizes the cellular organelles and their functions. © This document was created by Amanda Brammer. It is to be used by students for their own personal use only. All copyright laws apply and violators will be prosecuted.