Cell Theory. The Cellular Level Of Organization. Cell Biology. The Typical Cell. The Diversity of Cells in the Human Body.

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1 Cell Theory The Cellular Level Of Organization Chapter 3 Developed from Robert Hooke s research Cells are the building blocks of all plants and animals All cells come from the division of preexisting cells Cells are the smallest units that perform all vital physiological functions Each cell maintains homeostasis at the cellular level Homeostasis at higher levels reflects the combined, coordinated action of many cells The Diversity of Cells in the Human Body Cell Biology Organisms contain two general classes of cells: Sex Cells and Somatic Cells Sex cells (AKA germ cells or reproductive cells), are sperm in males and oocytes in females Sex cells are typically haploid and the fusion of haploid gametes produces a diploid zygote, which can develop into a new individual Somatic cells are diploid and include all cells in the body with the exception of the sex cells The Typical Cell A typical cell is surrounded by extracellular fluid. This fluid is termed the interstitial fluid a cell membrane or plasma membrane separates the cell contents, or cytoplasm from the extracellular environment. Cytoplasm can be subdivided into the cytosol, a liquid, and organelles, discrete entities within the cytosol 1

2 The cell membrane The cell membrane is the outer boundary of the cell, its functions are: The physical isolation of the cell from the extracellular fluid The regulation of exchange with the environment (ions, nutrients, wastes, secretions) Sensitivity to changes in the extracellular environment and signaling molecules Providing structural support to adjacent cells and to the tissue it is part of Membrane Structure The cell membrane is a phospholipid bilayer with associated proteins, lipids and carbohydrates. The hydrophilic heads of the phospholipids are oriented outward and the hydrophobic tails oriented towards the center of the membrane This orientation prevents most polar (hydrophilic) compounds from diffusing across the membrane and aids in the separation of the cell interior and exterior Membrane Proteins Membrane proteins can be classified according to their interactions with the cell membrane Integral proteins are structural components of the membrane and are enclosed either in part or whole by the membrane Peripheral proteins attach to the inner or outer surface but are not enclosed by the membrane 2

3 Membrane Proteins Membrane proteins can also be classified by function. Anchoring proteins attach the cell membrane to other structures and stabilize its position Recognition proteins identify cells as belonging to the body and not an invader, as such they are important in the immune response Membrane Proteins Enzymes catalyze reactions in the extracellular fluids or the cytosol Receptor proteins are sensitive to specific extracellular molecules called ligands, binding of a ligand triggers some cellular response Carrier proteins bind solutes and transport them across the membrane, the transport process may or may not require energy expenditure Channel proteins are integral proteins that contain a central pore or channel that permits the movement of water and small solutes across the membrane by diffusion Membrane Carbohydrates Proteoglycans, glycoproteins, and glycolipids Extend outside cell membrane Form sticky sugar coat (glycocalyx) Functions of the glycocalyx l Lubrication and protection Anchoring and locomotion Specificity in binding (receptors) Recognition (immune response) Into and out of the cell We ve said that the cell membrane regulates the passage of materials into and out of the cell Nothing passes through an impermeable barrier Anything can pass through a completely permeable barrier Cell membranes are selectively permeable 3

4 Transport Mechanisms Passage across the membrane is either an active or passive process Passive processes require no energy expenditure by the cell Active processes require the cell to expend energy Transport mechanisms include; diffusion, carrier-mediated transport, and vesicular transport Diffusion Diffusion is a passive process that results from the random movement of ions and molecules Diffusion results in the net movement of materials from a region of higher concentration to a region of lower concentration Such movement is said to be down a concentration gradient After the gradient is eliminated, motion continues but there is no net movement in any particular direction Diffusion Diffusion Across the Cell Membrane Diffusion across the Cell Membrane An ion or molecule can cross a cell membrane only by: Passing through a membrane channel Crossing the lipid portion of the membrane Molecules that are not lipid soluble (hydrophilic molecules) must pass through a membrane channel Since membrane channels are small, only small molecules such as H 2 O, Na +, K +,Ca +, and H + can pass through, larger organic molecules, such as glucose cannot Alcohols, lipids, and steroids, all lipid soluble compounds, can readily pass across the lipid portion of the membrane 4

5 Osmosis Osmosis is the diffusion of water across a semipermeable membrane in response to differences in solute concentration It is a passive process Osmotic pressure = force of water movement into a solution Hydrostatic pressure opposes osmotic pressure The effects of osmotic solutions on cells Cells in an isotonic (same solute concentration as the cell) solution show no net gain or loss of water Cells places in a hypertonic solution (higher solute concentration than the cell) will show a net loss of water and will shrivel Cells placed in a hypotonic solution (lower solute concentration than the cell) will show a net gain of water and will swell Osmotic flow across a cell membrane Carrier Mediated Transport Carrier mediated transport involves the binding and transporting of specific ions by integral membrane proteins Cotransport (symport) involves the transport of two substances in the same direction simultaneously (either in or out) Countertransport (antiport) move one substance in and one substance out of the cell Facilitated diffusion Facilitated diffusion is a type of carrier mediated transport In facilitated diffusion substances are passively transported across the membrane The compounds to be transported bind to a receptor site on a carrier protein and induce a conformational change that moves the molecule to the inside of the cell 5

6 Facilitated Diffusion Active Transport In active transport, ATP is expended to transport substances regardless of concentration gradient Ion pumps are carrier proteins that actively transport ions across their cell membranes An exchange pump is an ion pump that moves one ion into the cell and another in the opposite direction The Sodium-Potassium Exchange Pump Na + and K + are the principal cations in body fluids [Na + ] is high outside the cell, [K + ] is high inside the cell As a result, Na + diffuse into the cell and K + diffuse out Homeostasis in the cell depends on removing excess Na + and regaining lost K + This exchange is accomplished by the sodiumpotassium exchange pump. The carrier protein is sodium-potassium ATPase The Sodium-Potassium Exchange Pump The sodium-potassium exchange pump moves 3 Na + out and 2 K + in for each ATP expended If ATP is readily available, the rate of transport depends on [Na + ] inside the cell As the [Na + ] rises energy demands increase and may use up to 40% of the ATP produced by the cell The Sodium Potassium Exchange Pump The Sodium Potassium Exchange Pump 6

7 Secondary Active Transport Secondary active transport moves substances down their concentration gradient but also cotransports other molecules regardless of concentration gradient In effect, the concentration gradient of one substance drives the transport of the second ATP expenditure typically occurs at some later time to preserve homeostasis Vesicular transport Vesicular transport is the movement of materials into or out of cells in membranous vesicles Endocytosis is movement into the cell Receptor mediated endocytosis (coated vesicles) Pinocytosis involves fluids Phagocytosis involves engulfing larger particles by pseudopodia Exocytosis is the ejection of materials from the cell Organelles The cytoplasm of the cell contains: The fluid (cytosol) containing dissolved nutrients, ions, proteins, and wastes The organelles, structures suspended in the cytosol that perform specific cellular functions Organelles Nonmembranous organelles are not bound by a phospholipid membrane and include the cytoskeleton, microvilli, centrioles, cilia, ribosomes, proteasomes Membrane bound organelles are bound by a phospholipid membrane and include the endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, & mitochondria 7

8 The Anatomy of a Representative Cell The Endomembrane System A set of three major organelles together form a system within the cell called the endomembrane system. These organelles work together to perform various cellular jobs, including the task of producing, packaging, and exporting certain cellular products. The organelles of the endomembrane system include the endoplasmic reticulum, Golgi apparatus, and vesicles. Endoplasmic reticulum The endoplasmic reticulum is a network of intracellular membrane channels that is continuous with the nuclear envelope. It is involved in synthesis, storage, transportation and detoxification of materials The rough ER (RER) contains ribosomes that manufacture proteins that are then packaged into transport vesicles The smooth ER (SER) has no ribosomes attached and performs various functions including lipid, phospholipid, and steroid synthesis, and synthesizing steroid hormones in the reproductive organs The Endoplasmic Reticulum Figure 3.8 8

9 The Golgi Apparatus The Golgi apparatus is a series of flattened membranous discs called cisternae The Golgi apparatus modifies and packages secretions for release via exocytosis (secretory pathway) Vesicles enter forming face (cis) and exit maturing face (trans): There are three major types of vesicles: secretory vesicles: modify and package products for exocytosis membrane renewal vesicles: add or remove membrane components lysosomes: carry enzymes to cytosol The Secretory Pathway Figure 3.7 Lysosomes Lysosomes are filled with digestive enzymes and responsible for the recycling of worn out organelles and autolysis of cells Recycling Break down large molecules Attack bacteria Recycle damaged organelles Eject wastes by exocytosis Autolysis Auto- = self, lysis = break Self-destruction of damaged cells: lysosome membranes break down digestive enzymes released cell decomposes cellular materials recycle Peroxisomes Peroxisomes are smaller than lysosomes and carry a different suite of enzymes. Peroxisomes absorb and breakdown fatty acids and other organic compounds producing and then destroying the free radical H 2 O 2 in the process Proteasomes Proteasomes contain an assortment of proteases, enzymes that breakdown proteins They function to recycle damaged or denatured proteins as well as foreign proteins from infectious agents such as viruses and bacteria 9

10 Mitochondria Mitochondria are the powerhouses of the cell They are responsible for ATP production via aerobic respiration They consist of a double membrane, the outer membrane covers the organelle exterior, the inner membrane is folded into structures called cristae. Thus, a mitochondrion contains an outer compartment, between the cristae and the outer membrane, and an inner compartment within the cristae In aerobic metabolism (cellular respiration) mitochondria use oxygen to break down food and produce ATP glucose + oxygen + ADP carbon dioxide + water + ATP Glycolysis: Aerobic metabolism glucose to pyruvic acid (in cytosol) Tricarboxylic acid cycle (TCA cycle): pyruvic acid to CO 2 (in matrix) Electron transport chain inner mitochondrial membrane The cytoskeleton functions as the cell s skeleton. Think of it as a system of guy wires that provide strength, flexibility and structural integrity The Cytoskeleton 10

11 The Cytoskeleton The cytoskeleton consists of Microfilaments Microtubules Intermediate filaments Thick filaments (found in Muscle cells) The Cytoskeleton Microfilaments thin filaments composed of the protein actin Provide additional mechanical strength Interact with proteins for consistency Pair with thick filaments of myosin for muscle movement The Cytoskeleton The Cytoskeleton Intermediate filaments mid-sized between microfilaments and thick filaments Durable (collagen) Strengthen cell and maintain shape Stabilize organelles Stabilize cell position The Cytoskeleton Microtubules large, hollow tubes of tubulin protein Attach to centrosome Strengthen cell and anchor organelles Change cell shape Move vesicles within cell (kinesin and dynein) Form spindle apparatus Microvilli Microvilli are small finger shaped projections of the cell membrane that greatly increase cell surface area Microvilli contain a core of microfilaments and are anchored to the cytoskeleton Microvilli cover the surfaces of cells that are actively absorbing materials from the extracellular environment, such as those cells lining the digestive system 11

12 Centrioles organize and direct the movement of chromosomes during cell division via the spindle fibers They also help organize the cytoskeleton The cytoplasm surrounding the centrioles is called the centrosome Centrioles Cilia Cilia are long slender extensions of the cell membrane Internally they consist of 9 pairs of microtubules surrounding a central pair, this is referred to as a 9+2 array Cilia beat rhythmically to move fluids across the cell surface They are typically found on cells lining the respiratory tract where their movement moves mucus to clear dust and dirt from the respiratory tract Ribosomes Ribosomes are the organelles responsible for the manufacture of proteins They consist of a large ribosomal subunit and a small ribosomal subunit They contain ribosomal RNA (rrna) They can be free in the cytosol or fixed on a membrane (RER) The Nucleus The nucleus serves as the control center for the cell The nucleus contains DNA DNA Ais only packaged dinto chromosomes when the cell is ready to divide at other times the DNA exists as chromatin 12

13 Information Storage The DNA in the nucleus contains genes Genes are instructions for making proteins Genes dictate both the sequence and identity of the amino acids in proteins Genes are turned into proteins through the processes of transcription and translation Transcription What is a transcript? In transcription a copy of the DNA instructions (nucleotide sequence) on a gene is transcribed into an RNA copy (also a nucleotide sequence) The DNA gene stays on the chromatin, the RNA copy, called messenger RNA (mrna), leaves the nucleus and goes to a ribosome in the cytoplasm where translation occurs Translation What is a translation? Translation is the process by which the nucleotide sequence of mrna is translated into an amino acid sequence of a protein. Once we have the amino acid sequence the protein can become functional and voila! We re in business The Cell Cycle Cell division produces 2 genetically identical diploid daughter cells from one diploid cell Cell division is essential to the growth, development and survival of the organism. Many cells undergo a genetically programmed death, or apoptosis, after a certain period of time Thus, cell populations must be maintained by cell division 13

14 The Cell Cycle Cells cycle between a dividing phase called the mitotic or M phase and a nondividing phase called Interphase This regular alternation between M phase and Interphase is termed the cell cycle (see p98) In interphase a cell performs its normal functions Interphase is divided into 4 phases, the last 3 of which are only performed by cells that will divide Interphase In G 1 the cell conducts its normal functions and also replicates organelles and synthesizes additional cytoplasm so that the daughter cells will be normal in both size and function. In the S phase DNA replication occurs In G 2 protein synthesis and centriole replication occurs Interphase before S Interphase after S 14

15 Mitosis Once the cell is ready to reproduce it enters mitosis Mitosis specifically refers to division of the nucleus, cytokinesis, the division of the cytoplasm, occurs concurrently Collectively, mitosis and cytokinesis are cell division Mitosis Mitosis is divided into four phases Prophase (early and late) Metaphase Anaphase Telophase Although mitosis is described in stages, it is one continuous process without pauses Prophase Chromosomes condense and become visible In the cytoplasm the mitotic spindle forms, this array of microtubules called spindle fibers will pull the sister chromatids apart to separate daughter cells In animals, spindle fibers extend between two centrosomes which contain the spindle organizing structures called centrioles Prophase (continued) The nuclear envelope breaks down. a spindle fiber from each centrosome attaches to one of the two sister chromatids at a structure called a kinetochore Centrosomes begin moving to opposite poles of the cell Spindle fibers pull chromosomes toward the center of the cell 15

16 Metaphase Centrosomes at opposite poles Chromosomes aligned at middle of the cell on the metaphase plate Each chromatid attached to a spindle fiber that runs from its kinetichore to one of the two poles. Anaphase Spindle fibers shorten and pull sister chromatids apart to create independent chromosomes The sister chromatids, now daughter chromosomes, are ultimately pulled to opposite poles of the cell Telophase Nuclear envelope reforms around chromosomes, chromosomes decondense Spindle fibers disintegrate Mitosis i is complete and Cytokinesis i begins 16

17 Cytokinesis Cleavage Furrow Cytokinesis is the division of the cytoplasm and the formation of two daughter cells In animals it begins with the formation of a cleavage furrow In plants, cytokinesis begins as a cell plate forms in the middle of the dividing cell Mitotic Rates In general, the longer a cell lives the slower its mitotic rate Muscle cells and neurons either never divide or do so only under special circumstances Other cells, such as skin cells and the lining of the digestive tract are maintained by stem cells which retain the ability to divide and differentiate Regulation of the Cell Cycle Several critical points in the cell cycle are regulated These critical points are termed cell-cycle checkpoints. Cell cycle checkpoints normally insure that DNA replication and mitosis occur only when conditions are favorable and the process is working correctly. Cell-Cycle Checkpoints G 2 checkpoint M Mitosis Pass this checkpoint if: cell size is adequate chromosome replication is successfully completed G 2 Sec econd gap First gap G 1 Metaphase checkpoint Pass this checkpoint if: all chromosomes are attached to mitotic spindle G 1 checkpoint DNA synthesis S Pass this checkpoint if: cell size is adequate nutrient availability is sufficient growth factors (signals from other cells) are present 17

18 Cancer: unchecked cell division Human beings suffer some 200 types of cancer Mutations in genes that encode cell cycle proteins can lead to unregulated cell growth unregulated cell growth results in tumor or neoplasm formation Cancer: unchecked cell division In a benign tumor the cells usually remain within an envelope of connective tissue that can be surgically removed In a malignant tumor the cells do not remain confined but spread by invasion and may disperse to form new tumors by a process called metastasis Cancer: unchecked cell division Normal cells often become malignant when genes involved with cell growth, differentiation, and division mutate Such mutant genes are called oncogenes Cancer cells lose their resemblance to normal cells and can become abnormally large or small As tumors grow, organ function deteriorates, and death ensues 18