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Transcript
What is a cell? • All living things possess levels of organization. Organism Organ System Organ Tissue Cell Images : Copyright © The McGraw-Hill Companies, Inc. What is a Cell? • Cells are the building blocks of all organisms. – They are the smallest living parts of an organism. – They are the smallest structures that can be classified as living. • An organism can be unicellular, consisting of a single cell, or multicellular, consisting of many cells. The Cell Theory • Cells are very small, so small that they were not discovered until microscopes were invented in the mid-seventeenth century. • In 1665, Robert Hooke examined a piece of cork using a basic, homemade microscope. • Hooke observed a matrix of tiny rooms, which he called cells. • Another scientist, Antoni van Leeuwenhoek, was the first to observe and describe unicellular organisms. Drawing of cork (dead plant tissue) by Robert Hooke that appeared in Micrographia The Cell Theory • Advances in microscopy led to the Cell Theory • The theory states that: – A cell is the most basic unit of life – All living things are composed of cells – All cells arise from pre-existing cells Cell Organism Organs Tissues Images : Copyright © The McGraw-Hill Companies, Inc., Pearson Education, Inc. The Cell Theory • Advances in microscopy led to the Cell Theory • The theory states that: – A cell is the most basic unit of life – All living things are composed of cells – All cells arise from pre-existing cells Images : Copyright © The McGraw-Hill Companies, Inc., Pearson Education, Inc. Why are cells so small? • The maximum size of a cell is limited by the amount of surface area it needs to obtain nutrients and to exchange gases with its environment. • In the center of every cell is the machinery (e.g. DNA, enzymes, ribosomes) that the cell needs to function. • All of these components must communicate with one another. Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • A cell’s surface provides the only way to pass substances into and out of the cell. • As a cell’s size increases, both its surface area and volume increase, as well. • However, the volume increases faster than the surface area, making exchange of substances less efficient the larger the cell becomes. Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • A cell’s surface area is determined according to the following calculation: Surface Area = 4 x π x (radius)2 Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • A cell’s volume is determined according to the following calculation: Volume = 4/3 x π x (radius)3 Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • As a cell’s size increases, its radius increases. • Surface area is calculated by squaring the radius. • Volume is calculated by cubing the radius. Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • Both surface area and volume increase as cell size increases but volume increases faster than surface area. Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • For a small cell, the ratio between its surface area and its volume is large enough to permit efficient transfer of materials. • For a large cell, however, the surface area to volume ratio is much smaller. Images : Copyright © The McGraw-Hill Companies, Inc. Surface Area to Volume Ratio • In a small cell, surface area exceeds volume (SA:V is >1). • In a large cell, volume exceeds surface area (SA:V is <1). Images : Copyright © The McGraw-Hill Companies, Inc. • By remaining small, a cell has enough surface area to accommodate its volume, and thus all of the machinery contained within it. Images : Copyright © The McGraw-Hill Companies, Inc. Prokaryotes vs. Eukaryotes • All organisms can be divided into two groups, based on their cell type: – Prokaryotes – Eukaryotes Images : Copyright © The McGraw-Hill Companies, Inc. Prokaryotes vs. Eukaryotes • Prokaryotes are the simplest organisms and have the simplest cells. • All Prokaryotes are unicellular and belong to the domain – Archaea or – Bacteria Images : Copyright © The McGraw-Hill Companies, Inc. Prokaryotes vs. Eukaryotes • Eukaryotes include unicellular and multi-cellular organisms. • Eukaryotes belong to the domain Eukarya – Kingdom Plantae – Kingdom Fungi – Kingdom Animalia – Kingdom Protista Images : Copyright © The McGraw-Hill Companies, Inc. Prokaryotic Cells • Prokaryotic cells are structurally simpler, and smaller than eukaryotic cells. • A prokaryotic cell lacks a nucleus, the membrane-bound organelle where DNA is stored. • A prokaryotic cell is enclosed by a plasma membrane, but has no distinct interior components. – No membrane-bound organelles Images : Copyright © The McGraw-Hill Companies, Inc. Prokaryotic Cells • Prokaryotic cells contain ribosomes. • Ribosomes are the sites of protein synthesis, but are not considered to be organelles because they are not bound by a membrane. Images : Copyright © The McGraw-Hill Companies, Inc. Eukaryotic Cells • Eukaryotic cells are larger and structurally more complex than prokaryotic cells. • Every eukaryotic cell has a nucleus, a membrane-bound organelle where the cell’s DNA is stored. Images : Copyright © The McGraw-Hill Companies, Inc. Eukaryotic Cells • The interior of a eukaryotic cell is divided into functional compartments. • Membrane-bound organelles perform specialized functions within the cell and are connected via an extensive internal membrane system. • The organelles are anchored in place by a cytoskeleton. Images : Copyright © The McGraw-Hill Companies, Inc. Eukaryotic Cells • Eukaryotic cells are further divided into: – Plant cells – Animal cells Images : Copyright © The McGraw-Hill Companies, Inc. Plant Cells • Plant cells, as well as those of many protists, have a sturdy cell wall that surrounds their plasma membrane. • The cell wall is made of cellulose. • Animal cells do not possess cell walls. Plasma membrane Images : Copyright © The McGraw-Hill Companies, Inc. Plant Cells • Plant cells (and photosynthetic protists) also possess chloroplasts, the organelle responsible for photosynthesis. • Animal and fungal cells do not contain chloroplasts, and are therefore not capable of performing photosynthesis. Images : Copyright © The McGraw-Hill Companies, Inc. Plant Cells • Plant cells (and some protist cells) also possess a large central vacuole. • The central vacuole may store water, chemicals), or waste products. • The central vacuole usually occupies ~30% of the cell’s volume, but can occupy as much as 80% in specialized cells. Images : Copyright © The McGraw-Hill Companies, Inc. Plant Cells • Central vacuoles help maintain pressure against the cell wall, which keeps the plant in an upright position, and pushes the chloroplasts closer to the membrane, and so closer to the light! Images : Copyright © The McGraw-Hill Companies, Inc. Animal Cells • In contrast, animal cells do not possess cell walls, chloroplasts, plasmodesmata or central vacuoles. • Instead, animal cells contain food vacuoles, which are involved in the engulfing of food particles, while other vacuoles store and transport lipids and proteins out of the cell. Images : Copyright © The McGraw-Hill Companies, Inc. Nucleus Ribosomes No: • Food vacuole • Centrioles Central vacuole Chloroplast Cell wall Plasma membrane A Plant Cell Cell wall of adjacent cell Images : Copyright © Pearson Education, Inc. NUCLEUS: An Animal Cell Nuclear envelope Nucleus Lysosome Centriole Plasma membrane Ribosomes No: • Central Plasmavacuole membrane • Chloroplasts Mitochondrion• Cell Wall Images : Copyright © Pearson Education, Inc. The Cell: Organelles Cell Organelles • Eukaryotic cells contain organelles, membranebound components with specialized functions. Images : Copyright © The McGraw-Hill Companies, Inc. The Nucleus • One of the most important organelles in a eukaryotic cell is the nucleus. • The nucleus is the cell’s control center. • The nucleus contains most of the cell’s DNA and controls the cell’s activities by directing protein synthesis. Images : Copyright © The McGraw-Hill Companies, Inc. The Nucleus • Eukaryotic DNA is divided among several long molecules, or chromosomes, which consist of a complex of DNA and proteins. • The chromosomes are made up of chromatin, which packages the DNA into smaller volumes and contains genetic instructions to direct cell activities. Images : Copyright © The McGraw-Hill Companies, Inc. chromosome chromatin loops chromatin loop DNA double helix solenoid nucleosome Images : Copyright © The McGraw-Hill Companies, Inc. Ribosomes • Ribosomes carry out protein synthesis via the genetic instructions encoded in the DNA. • A single cell can contain a few million ribosomes! • Ribosomes consist of a large subunit and a small subunit, made up of ribosomal RNA and protein. Images : Copyright © The McGraw-Hill Companies, Inc. The Endoplasmic Reticulum • The endoplasmic reticulum (ER) is an extensive system of internal membranes surrounding the nucleus. • The ER is further differentiated into: – Rough endoplasmic reticulum – Smooth endoplasmic reticulum • The surface of the rough ER is studded with ribosomes, while the smooth ER lacks ribosomes. Images : Copyright © The McGraw-Hill Companies, Inc. The Endoplasmic Reticulum Smooth ER Nuclear envelope Ribosomes Rough ER Images : Copyright © Pearson Education, Inc. The Endoplasmic Reticulum • The ribosomes bound to the rough ER produce proteins which enter the ER membrane and are transported to other organelles or secreted by the cell. • The smooth ER is embedded with enzymes that aid in the production of lipids and carbohydrates. Images : Copyright © Pearson Education, Inc. Rough Endoplasmic Reticulum • Insulin – the hormone from the pancreas responsible for converting glucose into glycogen - is an example of a protein produced by ribosomes, and secreted by each cell via the rough endoplasmic reticulum. Images : Copyright © Pearson Education, Inc. Smooth Endoplasmic Reticulum • Cells in the ovaries and testes are rich in smooth ER for their role in the production of steroid sex hormones. • Smooth ER contains enzymes which process drugs and other potentially harmful substances. Images : Copyright © Pearson Education, Inc. The ER creates a series of interconnected channels but also isolates some spaces by creating membrane-enclosed sacs known as vesicles. Vesicle budding off of the ER Protein inside transport vesicle Ribosome Sugar chains Polypeptid e Glycoprotein Rough Images : Copyright © Pearson Education, Inc. The Golgi Complex • After biomolecules are produced in the ER, they are passed along to flattened stacks of membranes known as Golgi bodies. • The number of Golgi bodies in a cell ranges from one or a few in protists, to 20 or more in animal cells, and several hundred in certain plant cells! • Collectively, these Golgi bodies are referred to as the Golgi complex or Golgi apparatus. • Proteins and lipids manufactured on the ER membranes are transported through ER channels and packaged into transport vesicles that bud off the ER. • The vesicles fuse with the Golgi bodies, releasing their contents. Images : Copyright © The McGraw-Hill Companies, Inc. • Within the Golgi complex, many of the proteins and lipids are tagged with carbohydrates. • The molecules collect at the end of the Golgi complex, where they pinch off into vesicles and are transported to other regions of the cell or to the plasma membrane for secretion. Images : Copyright © The McGraw-Hill Companies, Inc. • The Golgi complex receives, sorts, chemically alters and packages important molecules manufactured by the ER. • One side of the Golgi stack serves as a receiving dock, while the other serves as a shipping facility, where vesicles bud off to transport the prepared materials elsewhere. Receiving side Transport vesicle from the ER Transport vesicle from the Golgi Shipping side Images : Copyright © Pearson Education, Inc. Lysosomes • Organelles known as lysosomes emerge from the Golgi complex containing a concentrated mix of enzymes manufactured in the rough ER to break down macromolecules within the cell. • Lysosomes are the recycling centers of the cell, digesting worn-out organelles and cellular components. Images : Copyright © Pearson Education, Inc. Mitochondria: ATP Production • Eukaryotic organisms extract energy from organic molecules in a process known as cellular respiration, whereby the chemical energy of food (e.g. glucose) is converted into the chemical energy of ATP. • ATP or adenosine triphosphate is the main energy source for cellular work. – the molecular currency by which energy is transferred within cells for metabolism. Mitochondria: ATP Production • Small quantities of ATP are produced within cellular cytoplasm in a process known as glycolysis. • However, much larger quantities of ATP are produced in the presence of oxygen within specialized organelles known as mitochondria. Images : Copyright © The McGraw-Hill Companies, Inc. Mitochondria: ATP Production • The mitochondria are sausage-shaped organelles, bound by two membranes. • The outer membrane of the mitochondrion (singular) is smooth, while the inner membrane is bent into numerous folds called cristae. • The cristae and the resulting inner matrix and surface area make oxidative metabolism possible. Images : Copyright © The McGraw-Hill Companies, Inc. Mitochondria: ATP Production • Mitochondria contain their own DNA. – Different from cellular DNA stored in the nucleus. – Similar to bacterial DNA! • Mitochondria reproduce in a manner similar to the asexual reproduction of prokaryotes. • All mitochondria contain two phospholipid bilayer membranes. Images : Copyright © The McGraw-Hill Companies, Inc. • The unique characteristics of the mitochondria provide evidence for endosymbiosis. • Mitochondria are believed to have once existed as free-living prokaryotes which were engulfed and retained by ancient eukaryotic cells approximately 1.5 billion years ago. • Chloroplasts, the organelles responsible for photosynthesis, are also believed to have evolved via endosymbiosis. Images : Copyright © Pearson Education, Inc. • Rather than being broken down by digestive enzymes, the engulfed prokaryotes provided their hosts with advantages from their special metabolic abilities: oxidative metabolism and photosynthesis! Images : Copyright © The McGraw-Hill Companies, Inc. The Cytoskeleton • Organelles do not float freely within the cell. • Rather, they are supported and transported by a dense network of protein fibers called the cytoskeleton. Images : Copyright © The McGraw-Hill Companies, Inc. The Cytoskeleton • The cytoskeleton: – Anchors organelles to fixed locations in the cell. – Provides a framework that supports the shape of the cell. – Organizes cellular activities by providing a scaffold that anchors ribosomes and enzymes. – Acts as tracks along which organelles can move. Protein Fibers of the Cytoskeleton • The protein fibers of the cytoskeleton are constantly being formed and disassembled. • There are three different types of protein fibers: – Microfilaments – Intermediate filaments – Microtubules Images : Copyright © The McGraw-Hill Companies, Inc. Microfilaments • Microfilaments are long, slender rods of a protein called actin. • Microfilaments are made of two actin molecules twisted together into a double chain. 7 nanometers thick Images : Copyright © Pearson Education, Inc. Intermediate Filaments • Intermediate filaments are composed of overlapping proteins that form a rope-like structure. • These structures serve to reinforce cell shape and to anchor certain organelles in place. 10 nanometers thick Images : Copyright © Pearson Education, Inc. Microtubules • Microtubules are straight, hollow tubes composed of the protein tubulin. • Microtubules shape and support the cell, and act as tracks along which organelles can travel. 25 nanometers Images : Copyright © Pearson Education, Inc. Cell Movement • Cellular movement is dependent upon the movement of microfilaments, microtubules or both. • Microfilaments play a major role in determining the shape of cells because they can form and dissolve very rapidly. • The ability of microfilaments to form and dissolve quickly enables some cells to crawl. • Other cells use flagella or cilia for movement. Flagella and Cilia • Flagella are long, singular appendages used for locomotion. • Cilia are short, numerous appendages that enable cell movement, and generate currents. • Both flagella and cilia are structurally similar, consisting of a circle of nine microtubule pairs surrounding two central microtubules. – This 9+2 arrangement is a fundamental feature of eukaryotic cells. 9+2 Arrangement of Microtubules 1 2 9 3 8 7 4 6 5 Images : Copyright © The McGraw-Hill Companies, Inc. Flagella • The undulating flagellum on a human sperm cell propels the cell in a swimming motion toward the egg. Images : Copyright © Pearson Education, Inc. Cilia • Cilia on cells lining the human windpipe create currents that sweep mucus containing foreign debris out and away from the lungs. Images : Copyright © Pearson Education, Inc. All Cells Contain… • Regardless of cell type (prokaryotic or eukaryotic, plant or animal, etc), ALL cells contain: – DNA – Ribosomes – Plasma membrane