Structure and Function of Cells 3 Structure and Function of Cells 1

Cell Doctrine All living things are composed of cells A single cell is the smallest unit that exhibits all of the characteristics of life All cells come only from preexisting cells

Cells Are Classified According to Their Internal Organization Prokaryotic Cells Plasma membrane No nucleus Cytoplasm: fluid within membrane No true organelles Eukaryotic Cells Plasma membrane Nucleus: membrane bound information center Cytoplasm: fluid within membrane Organelles: membrane bound structures with specialized functions All human cells are eukaryotic

A eukaryotic animal cell has a large nucleus and numerous small Figure 3.1 Plasma membrane Cell wall Cytoplasm Organelles Nucleus A eukaryotic animal cell has a large nucleus and numerous small organelles. The cytoplasm is enclosed by a flexible plasma membrane. Prokaryotic cells such as this bacterium have a rigid cell wall surrounding the plasma membrane. The genetic material is not surrounded by a membrane, and there are no organelles in the cell. The elongated bacterium in the center of the photo is about to divide in two, as its genetic material is concentrated at both ends of the cell. Figure 3.1 Eukaryotes versus prokaryotes. 4

Cell Structure Reflects Cell Function Though eukaryotic cells are remarkably similar, there are structural differences Examples: Muscle cells Contain numerous organelles providing energy needed for muscle contraction Nerve cells Long and thin to carry impulses over distance Small size is efficient

A portion of several muscle cells of the heart ( 1,500). Figure 3.2 A portion of several muscle cells of the heart ( 1,500). Nerve cells of the central nervous system ( 830). Figure 3.2 Human cells vary in shape. Cells lining a tubule of a kidney ( 250). 6

Cells Remain Small to Stay Efficient Small cells have a higher surface to volume ratio High surface to volume ratio promotes efficiency in Acquisition of nutrients Disposal of wastes

One large cell. Eight small cells. Cell with microvilli Figure 3.3 One large cell. Eight small cells. Cell with microvilli on one surface. Figure 3.3 Cell size and plasma membrane shape affect surface area and volume. 8

Visualizing Cells with Microscopes Cells cannot be seen without magnification Microscopes enable visualization and study of cells Light microscope Magnifies up to 1000 Transmission electron microscope Magnifies up to 100,000 Scanning electron microscope Provides 3-D view of cell surface

The light microscope (LM). The transmission electron microscope (TEM). Figure 3.4 The light microscope (LM). The transmission electron microscope (TEM). Figure 3.4 Visualizing cells with microscopes. The scanning electron microscope (SEM). 10

A Plasma Membrane Surrounds the Cell Separates a cell from its environment Selectively permeable Permits movement of some substances into and out of the cell, but blocks others Enables transfer of information between environment and cell

A Plasma Membrane Surrounds the Cell Plasma membrane is a lipid bilayer Phospholipids: polar head and nonpolar tail Cholesterol: makes membrane a bit more rigid Proteins: provide means of transport through membrane Carbohydrates: recognition patterns for cells and organisms Non-rigid Fluid mosaic

Extracellular environment Figure 3.5 Extracellular environment Carbohydrate groups Receptor protein Channel protein (always open) Gated channel Protein (closed position) Figure 3.5 The plasma membrane. Lipid bilayer Glycoprotein Phospholipid Transport protein Cytoskeleton filaments Cytoplasm Cholesterol 13

Molecules Cross the Plasma Membrane in Several Ways Passive transport—cell does not need to expend energy for this Diffusion Osmosis Active transport—cell must expend energy Bulk transport Involves membranous vesicles to move larger substances Endocytosis Exocytosis

Passive Transport: Principles of Diffusion and Osmosis Passive transport: transports a substance without having to expend energy Passive transport relies on diffusion Diffusion: movement of molecules from a region of high concentration to a region of low concentration High concentration  Low concentration “Down” the gradient

cm inches 5 2 4 3 1 2 1 10 minutes 1 hour 24 hours Figure 3.6 Figure 3.6 Diffusion. 1 10 minutes 1 hour 24 hours 16

Osmosis: Diffusion of Water Osmosis: the diffusion of water across a selectively permeable membrane Water moves from an area of low solute concentration to an area of high solute concentration Osmotic pressure: fluid pressure required to exactly oppose osmosis

Diffusion of water (osmosis) Pressure-induced water movement Figure 3.7 Osmotic pressure Glucose Water Selectively permeable membrane Figure 3.7 Generation of osmotic pressure by osmosis. Diffusion of water (osmosis) Pressure-induced water movement 18

Passive Transport Moves with the Concentration Gradient Diffusion directly through the lipid bilayer Small uncharged nonpolar molecules Example: O2, CO2, urea Diffusion through protein channels in the bilayer Some always open, others are “gated” Example: H2O, ions Facilitate transport (facilitated diffusion) Membrane transport protein changes shape and transports molecule through the bilayer Highly specific Example: glucose

Facilitated transport. Figure 3.8 Higher concentration Lower concentration Diffusion through the lipid layer. Lipid-soluble molecules such as O2 and CO2 diffuse freely through the plasma membrane. Diffusion through channels. Some polar and charged molecules diffuse through protein channels that span the membrane. Water is a typical example. Facilitated transport. Certain molecules bind to a protein, triggering a change in protein shape that transports the molecule across the membrane. Glucose typically enters cells by this method. Figure 3.8 The three forms of passive transport. 20

Active Transport Requires Energy Active transport moves substances from an area of lower concentration to an area of higher concentration Transported substance moves against the concentration gradient Requires a membrane protein (transporter) Requires ATP or other energy source Example: sodium-potassium pump

In active transport using ATP, energy derived from the Figure 3.9 ADP  PI ATP Figure 3.9 Active transport. In active transport using ATP, energy derived from the breakdown of ATP is used to change the shape of the carrier protein. Some carrier proteins use energy derived from the downhill transport of one molecule to transport another molecule uphill. In this example, the energy to transport the square molecules comes from the facilitated transport of the spearhead molecules. 22

Endocytosis and Exocytosis Move Materials in Bulk Used to move larger molecules Endocytosis: brings substances into the cell As substance enters, it is surrounded by a membrane forming a membrane-bound vesicle Exocytosis: expels substances from the cell Substance is contained within a membranous vesicle, which then fuses with the membrane, releasing the substance to the external environment

Endocytosis. In endocytosis, material is surrounded Figure 3.10 Extracellular environment Plasma membrane Cytoplasm Vesicle Endocytosis. In endocytosis, material is surrounded by the cell membrane and brought into the cell. Figure 3.10 Endocytosis and exocytosis. Exocytosis. In exocytosis, a membranous vesicle fuses with the plasma membrane, expelling its contents outside the cell. Photomicrograph showing various stages of endocytosis. 24

Information Can Be Transferred Across the Plasma Membrane Receptor proteins span membrane—required for transmission of information to and from cell Receptor sites (on receptor proteins)—interact specifically with signal molecules A change is triggered within the cell as a result of binding of signal molecule to receptor site Different cell types have different receptor proteins

Extracellular environment Figure 3.11 Extracellular environment Receptor site Figure 3.11 Receptor protein action. Substrate Product Cytoplasm 26

The Sodium–Potassium Pump Helps Maintains Cell Volume Sodium (Na+)–potassium (K+) pump expels unwanted ions (Na+), stockpiles needed ones (K+), and maintains cell volume ATP is used to expel three Na+ for every two K+ brought into the cell Increase in cell volume  more water in cytoplasm, accomplished by decreasing pumping and allowing more sodium inside cell Decrease in cell volume  less water in cytoplasm, accomplished by increasing pumping and expelling more sodium ions

Figure 3.12a Extracellular fluid Cytoplasm Sodium ions bind to binding sites accessible only from the cytoplasm. 2 Binding of three cytoplasmic Na+ to the sodium-potassium pump stimulates the breakdown of ATP. 7 Na+ Most of the potassium diffuses out of the cell, but sodium diffuses in only very slowly. Na+ Na+ Na+ Na+ 3 K+ Na+ Energy released by ATP causes the protein to change its shape, expelling the sodium ions. Na+ K+ Na+ ATP ADP + Pi Na+ K+ Cytoplasm Na+ Na+ K+ K+ 6 K+ Figure 3.12a The cell membrane contains Na+-K+ pumps, and also channels that permit the rapid outward diffusion of K+ but only a slow inward diffusion of Na+. Potassium is transported into the cell, and the sodium binding sites become exposed again. 4 The loss of sodium exposes two binding sites for potassium. K+ K+ K+ 5 Potassium binding triggers another change of shape. The cell membrane contains Na+-K+ pumps, and also channels that permit the rapid outward diffusion of K+ but only a slow inward diffusion of Na+. 28

The rate of transport by the Na+-K+ pumps determines cell volume. Figure 3.12b Key: Active transport of Na+ Sodium-potassium pump Diffusion of K+ Diffusion of Na+ Diffusion of H2O H2O Na+ Na+ Na+ H2O H2O In the steady-state, the rate of outward sodium transport equals the rate of inward diffusion. When the rate of outward sodium transport exceeds inward diffusion, water diffuses out and the cell shrinks. When the rate of outward sodium transport is less than the rate of inward diffusion, water diffuses in and the cell swells. Figure 3.12b The rate of transport by the Na+-K+ pumps determines cell volume. The rate of transport by the Na+-K+ pumps determines cell volume. 29

Isotonic Extracellular Fluid Also Maintains Cell Volume Tonicity: relative concentration of solutes in two fluids Isotonic Extracellular and intracellular solute concentrations are equal Cells maintain a normal volume in isotonic extracellular fluids Regulatory mechanisms maintain extracellular fluid that is isotonic with intracellular fluid

Isotonic Extracellular Fluid Also Maintains Cell Volume Variations in tonicity Hypertonic Extracellular solute concentration higher than intracellular solute concentration Water will diffuse out of cell Cell may shrink and die Hypotonic Extracellular solute concentration lower than intracellular solute concentration Water will diffuse into cell Cell may swell and burst

Water movement into and out of human red blood Figure 3.13 Isotonic Hypertonic Hypotonic 9 grams of salt in 1 liter of solution 18 grams of salt in 1 liter of solution Pure water Water movement into and out of human red blood cells placed in isotonic, hypertonic, and hypotonic solutions. The amount of water movement is indicated by the sizes of the arrows. Figure 3.13 The effect of extracellular fluid tonicity on cell volume. Scanning of electron micrographs of red blood cells placed in similar solutions. 32

Nucleus Mitochondrion Lysosome Cytosol Peroxisome Centrioles Figure 3.14 Cytosol Semifluid gel material inside the cell Nucleus Information center for the cell. Contains DNA Peroxisome Destroys cellular toxic waste Centrioles Microtubular structures involved in cell division Cytoskeleton Structural framework of the cell Smooth endoplasmic reticulum Primary site of macro- molecule synthesis other than proteins Rough endoplasmic reticulum Primary site of protein synthesis by ribosomes Golgi apparatus Refines, packages, and ships macromolecular products Figure 3.14 A typical animal cell. Secretory vesicle Membrane-bound shipping container Ribosomes Site of protein synthesis Mitochondrion Produces energy for the cell Plasma membrane Controls movement of materials into and out of cell Lysosome Digests damaged organelles and cellular debris 33

The Nucleus Controls the Cell Function: Contains the genetic information of the cell Controls all of the activities of the cell Structural features: Double-layered nuclear membrane Nuclear pores Chromosomes/chromatin Nucleolus

A transmission electron micrograph Figure 3.15 Nuclear pores Nucleolus Nuclear membrane Nuclear membrane Figure 3.15 The nucleus. A transmission electron micrograph ( 6,000) of the nucleus of an animal cell 35

Ribosomes Are Responsible for Protein Synthesis Site of protein synthesis Location Free: floating in cytoplasm These ribosomes synthesize proteins for immediate use in the cell Bound: attached to outer surface of endoplasmic reticulum These ribosomes synthesize proteins that will be transported to other organelles or exported from the cell

Endoplasmic Reticulum (ER) Is the Manufacturing Center Highly folded membranous network Two types of endoplasmic reticulum (ER) Rough ER Has ribosomes on surface Protein manufacturing, particularly those that will be secreted from the cell Smooth ER No ribosomes on surface Lipid synthesis, including the synthesis of some hormones Packaging of proteins and lipids for delivery to Golgi apparatus

Nucleus Rough ER Vesicle Smooth ER Figure 3.16 Figure 3.16 The endoplasmic reticulum (ER). 38

Golgi Apparatus: Refines, Packages, and Ships Refines synthesized products Packaging and shipping center Products are packaged into vesicles and shipped to other locations within the cell or to the cell membrane for export

Smooth ER Golgi apparatus Vesicle Lysosome Secretory vesicle Plasma Figure 3.17 Smooth ER Golgi apparatus Vesicle Lysosome Secretory vesicle Figure 3.17 The Golgi apparatus. Plasma membrane 40

Vesicles: Membrane-bound Storage and Shipping Containers Four types 1. Secretory vesicles 2. Endocytic vesicles 3. Peroxisomes Contain enzymes that detoxify wastes produced by the cell 4. Lysosomes Contain digestive enzymes

Harmless waste Residual body Figure 3.18 Harmless waste Alcohol Peroxisome Golgi apparatus Cell toxic waste Lysosome Residual body Figure 3.18 Lysosomes and peroxisomes. Bacterium Plasma membrane 42

Mitochondria Provide Energy “Power plant” of the cell The number of mitochondria within a cell will vary with the cell’s energy requirement Surrounded by a double membrane Inner membrane is highly folded Site of cellular respiration Utilizes O2 and produces CO2 Generates ATP

The structure and overall function of a mitochondrion. Figure 3.19 O2 ADP Nutrients from foodstuffs Pi Inner membrane Figure 3.19 Mitochondria. Outer membrane ATP CO2 The structure and overall function of a mitochondrion. A photomicrograph of a mitochondrion. 44

Fat and Glycogen: Sources of Energy Triglycerides Long-term energy storage in animals Glycogen Carbohydrate storage Short-term energy storage in animals Stored in muscle cells and liver cells

The Cytoskeleton Supports the Cell Microtubules: tiny hollow tubes of protein Microfilaments: thin solid fibers of protein Microtubules and microfilaments form framework that supports the cell Cytoskeleton also supports and anchors other cellular structures

Glycoprotein Plasma membrane Microfilaments Microtubule Golgi Figure 3.20 Glycoprotein Plasma membrane Microfilaments Figure 3.20 The cytoskeleton. Microtubule Golgi apparatus Mitochondrion 47

Cilia and Flagella Are Specialized for Movement Short, many Found on cells lining airways Flagella Long, single Enable spermatozoa to swim Cilia and flagella have similar internal structure Centrioles Short rod-like microtubular structures near nucleus Play important role in cell division

Figure 3.21 Microtubule pair Figure 3.21 Flagella. 49

Cells Use and Transform Matter and Energy Metabolism: sum of all chemical reactions in an organism Two types of metabolic pathways: Anabolism: Assembly of larger molecules from smaller ones Requires energy (ATP input) Catabolism: Larger molecules are broken down Releases energy (ATP output)

Glucose Provides the Cell with Energy Energy in glucose is used to generate ATP One glucose molecule may yield 36 ATP In absence of glucose, other carbohydrates, fats, and protein can be catabolized to generate ATP ATP is a more readily used “pocket change” form of energy. Glucose and other fuel molecules must be “cashed in” for ATP ATP can then be used to do cellular work

(carbon dioxide + water) Figure 3.23 C6 H12 O6 + (6) O2 (glucose + oxygen) (6) CO2 + (6) H2O (carbon dioxide + water) (36) ADP + (36) Pi 36 ATP Figure 3.23 Glucose provides energy for the cell. Anabolism Transport Muscle contraction 52

Glucose Provides the Cell with Energy Cellular respiration: the breakdown of glucose in the presence of oxygen to yield ATP Four stages of cellular respiration Glycolysis Preparatory step Citric acid cycle Electron transport system

Citric acid cycle Electron transport system Prepatory step Glycolysis Figure 3.24 Energy Energy Energy Energy Citric acid cycle Electron transport system Prepatory step Glycolysis Figure 3.24 Cellular respiration: An overview. ATP ATP ATP 54

Glycolysis: Glucose Is Split Into Two Pyruvate Molecules Occurs in the cytoplasm Series of 10 reactions that split glucose into two molecules of pyruvic acid (pyruvate) Requires investment of two ATP, four ATP are produced Net ATP yield: two High-energy electrons and hydrogen ions (H) are removed and picked up by a coenzyme NAD, forming NADH Later in the cell respiration process, the NADH will be used to generate additional ATP

2 Glyceraldehyde-3-phosphate (PGAL) Figure 3.25 Glycolysis 1 Glucose (6-carbon) ATP Energy investment steps ADP ATP ADP 2 Glyceraldehyde-3-phosphate (PGAL) (3-carbon) 2 NAD+ Figure 3.25 Glycolysis. 2 NADH + 2 H+ 2 ADP 2 ATP Energy-yielding steps 2 ADP 2 ATP 2 Pyruvate (3-carbon) 56

The Preparatory Step: Pyruvate Is Converted to Acetyl CoA Pyruvate (from glycolysis) enters mitochondria Pyruvate converted to acetyl group and CO2 High-energy electrons and hydrogen ions are removed and picked up by a coenzyme NAD, forming NADH Acetyl group is joined to coenzyme A to form acetyl CoA Acetyl CoA will enter citric acid cycle

Preparatory step Pyruvate (3-carbon) CO2 NAD NADH  H Acetyl group Figure 3.26 Preparatory step Pyruvate (3-carbon) CO2 NAD NADH  H Figure 3.26 The preparatory step. Acetyl group (2-carbon) Coenzyme A Acetyl CoA 58

The Citric Acid Cycle (Krebs Cycle) Harvests Energy Occurs in mitochondria Cyclic series of eight reactions which break down the acetyl CoA Molecule (oxaloacetate) that accepts acetyl CoA is regenerated at the end of one full turn of the cycle High-energy electrons are extracted to form NADH and FADH2 Produces two ATP and four CO2 per glucose molecule

Citric acid cycle Figure 3.27 Acetyl CoA CoA Acetyl group (2-carbon) NADH Oxaloacetate (4-carbon) NAD Citric acid cycle NAD CO2 NADH Figure 3.27 The citric acid cycle. Fumarate (4-carbon) -ketoglutarate (5-carbon) CO2 NAD FADH2 Succinate (4-carbon) NADH ADP FAD ATP 60

The Electron Transport System Produces ATP Located in inner mitochondrial membrane Takes electrons from NADH and FADH2 Movement of electrons from one electron carrier to the next releases energy that is harvested to generate ATP Final electron acceptor is O2, which forms water upon receiving electrons and hydrogen ions ATP is generated by ATP synthase enzyme Process also known as oxidative phosphorylation

Electron transport system ATP Synthase Figure 3.28 H Outer membrane H H H H H H H H H H H H H H Inner membrane of mitochondrion H e e e Figure 3.28 The electron transport system and oxidative phosphorylation. FADH2 FAD NADH NAD 1/2O2  2H  2e H2O ADP  Pi ATP H Electron transport system ATP Synthase 62

Summary of Energy Production from Glucose Over 20 enzyme-catalyzed reactions Approximately 36 ATP (net) produced from each molecule of glucose Oxygen (O2) consumed, carbon dioxide (CO2) produced Cellular respiration: cellular process that uses O2 and produces CO2 in the process of making ATP

Mitochondrion Electron transport chain and oxidative phosphorylation Figure 3.29a 2 NADH 2 ATP to shuttle electrons from NADH in cytosol to NADH within mitochondrion Mitochondrion 2 NADH 6 NADH 2 FADH2 Electron transport chain and oxidative phosphorylation Glycolysis 2 Acetyl CoA Citric acid cycle 2 Pyruvate Preparatory step Glucose 2 ATP  4 ATP  2 ATP  about 34 ATP to initiate glycolysis by substrate-level phosphorylation by substrate-level phosphorylation by oxidative phosphorylation Figure 3.29a Most of the ATP generated during cellular respiration is synthesized in the electron transport system. About 36 ATP Most of the ATP generated during cellular respiration is synthesized in the electron transport system. 64

Fats and Proteins Are Additional Energy Sources Glycogen (storage form of glucose) Can be rapidly catabolized to glucose which then participates in cellular respiration 1% of total energy reserves Fats: 78% of total energy reserves Triglycerides have twice the energy of carbohydrates Proteins: 21% of total energy reserves Have the same amount of energy as carbohydrates

Acetyl CoA Preparatory step Figure 3.30 Fats Glycogen Protein Glucose Amino acids Carbon backbone NH3 Urea (waste) Fatty acids Glycerol Pyruvate Citric acid cycle Acetyl CoA Electron transport system Preparatory step Figure 3.30 Metabolic pathways for fats, glycogen, and proteins as sources of cellular energy. (2) many ATP ATP 66

Anaerobic Pathways Make Energy Available without Oxygen Cellular respiration cannot continue in the absence of O2 Glycolysis will continue, pyruvate will build up Pyruvate will be converted to lactic acid Lactic acid buildup in muscles will cause a burning sensation Two ATP produced per molecule of glucose When O2 is available, lactic acid will be metabolized aerobically

Glucose Mitochondrion (Glycolysis) (2) ATP Lactic acid Pyruvate Figure 3.31 Glucose (Glycolysis) (2) ATP Lactic acid buildup Pyruvate Mitochondrial metabolism blocked without oxygen Mitochondrion Figure 3.31 Anaerobic metabolism. 68