Chapter Menu Chapter Introduction Living Systems as Compartments 3.1 Exchanged Materials 3.2 Membrane as Barrier How Cells Exchange Materials 3.3 Diffusion and Osmosis 3.4 Passive and Active Transport Exchange in Multicellular Organisms 3.5 Gas Exchange in Water 3.6 Adaptation to Life on Land 3.7 Waste Removal 3.8 Human Urinary System Chapter Highlights Chapter Animations Learning Outcomes By the end of this chapter you will be able to: A Discuss the structure and function of membranes in living organisms. B Describe how materials are exchanged across membranes. C Explain how various organisms are adapted to maintain water balance while processing nitrogenous wastes. D Relate the structure of the human nephron to its function. Exchanging Materials with the Environment How do living organisms exchange materials with their surroundings? What molecular processes are responsible for exchange? A scuba diver breathing under water with the aid of an apparatus Exchanging Materials with the Environment • The surface of an organism is a barrier against destructive forces. • Food, water, waste, and communication signals must be allowed to pass through the barrier if the organism is to survive. A scuba diver breathing under water with the aid of an apparatus Living Systems as Compartments 3.1 Exchanged Materials • Their cytoplasm, or interior, of cells is surrounded by a wall made of carbohydrates and proteins and a membrane made largely of phospholipids. • Materials needed for life must pass into this compartment to be useful. A Bacillus megaterium bacterium (x30,500) Living Systems as Compartments 3.1 Exchanged Materials (cont.) • Organisms and their cells need water. • Cells need the correct balance of ions, such as sodium (Na+), magnesium (Mg+2), calcium (Ca+2), hydrogen (H+), chloride (Cl–), and potassium (K+). • Carbon dioxide is needed in autotrophs to build food molecules. • Nutrients must enter cells to supply energy and building material for cell components. • Some hormones are needed to transmit messages. • Wastes, such as ammonium ion (NH4+), must exit. Living Systems as Compartments 3.2 Membrane as Barrier • Membranes are composed of two thin, fluid, layers of phospholipids and proteins. • Not all molecules are equally soluble in a membrane. – The nonpolar phospholipid tails of the lipid bilayer tend to repel charged particles such as ions but allow fat-soluble molecules to pass. – Usually, the polarity, size, and electric charge of molecules determine whether they can pass through a membrane. A selectively permeable membrane Living Systems as Compartments 3.2 Membrane as Barrier (cont.) • Charged molecules such as the ions H+ or Ca+2 can pass through only with the help of special proteins, called transport proteins, that are embedded in the membrane. • Proteins and other very large molecules cannot pass through a membrane without special processes. • By limiting entry, a membrane is selectively permeable, which means that it regulates the exchange of materials in a very specific way. Living Systems as Compartments 3.2 Membrane as Barrier (cont.) • The structure of membranes is complex and allows them to perform many functions in the cell. • Some proteins, called glycoproteins, are embedded in membranes have sugars attached to them. • Sugars also can be attached to the heads of membrane lipids (glycolipids). • Glycoproteins and glycolipids act as antennae that receive chemical messages from other cells. The fluid-mosaic model of a membrane’s structure How Cells Exchange Materials 3.3 Diffusion and Osmosis • Diffusion refers to the movement of molecules from an area of higher concentration to an area of lower concentration. • Diffusion is a random process, and the entropy of the system increases as it occurs. How Cells Exchange Materials 3.3 Diffusion and Osmosis (cont.) • A concentration gradient exists when there is a difference in concentration of molecules across a distance. • Diffusion is a basic process underlying the movement of molecules into and out of cells. How Cells Exchange Materials 3.3 Diffusion and Osmosis (cont.) Molecules move from an area of higher concentration to an area of lower concentration until the concentration is the same throughout. In (a), a crystal of potassium permanganate (KMnO4) was dropped into a glass of water. The molecules diffuse through the water (b) until they are evenly distributed throughout (c). How Cells Exchange Materials 3.3 Diffusion and Osmosis (cont.) • Concentration gradients across cell membranes provide potential energy to drive many cellular processes. • The potential energy is based on the concentration gradient of substances. • If the substance in question is charged, an electric potential also forms across the membrane. How Cells Exchange Materials 3.3 Diffusion and Osmosis (cont.) • Movement of water down its concentration gradient is a special form of diffusion called osmosis. • If the concentration of water outside the cell is higher than inside, water moves in, and the cell swells. • If the concentration of water is higher inside the cell than outside, water is driven out and the cell shrinks. • Outward pressure of a cell against its cell wall is called turgor. How Cells Exchange Materials 3.3 Diffusion and Osmosis (cont.) The motion of molecules in a glass container is random, but the net result is movement from an area of higher concentration to one of lower concentration. Initially, a barrier separates the two bulbs, with gas molecules (and potential energy) concentrated on the right side. When the barrier is removed, molecules begin to appear in the left-hand bulb. How Cells Exchange Materials Initially the cells are in a solution with the same concentration of dissolved material as is found inside the cells. This is called an isosmotic solution. The animal cell can survive only fairly small variations from this concentration. How Cells Exchange Materials 3.3 Diffusion and Osmosis (cont.) • The rate of diffusion, including osmosis, depends on the size of the concentration gradient and the surface area relative to the enclosed volume. How Cells Exchange Materials 3.4 Passive and Active Transport • Organisms must establish and maintain concentrations of materials inside their cells that may differ from concentrations resulting from diffusion. • Membranes are permeable to many substances only with the help of transport proteins, which assist movement passively or actively. – Passive transport involves diffusion without any input of energy. – Active transport moves substances against their concentration gradients and thus requires energy. How Cells Exchange Materials 3.4 Passive and Active Transport (cont.) • Simple diffusion of neutral molecules such as oxygen or carbon dioxide into or out of a cell is a form of passive transport. • Facilitated diffusion is passive transport that occurs with the help of transport proteins in the membrane. • Facilitated diffusion makes transport more specific and speeds up the rate, but it does not work against the gradient. How Cells Exchange Materials 3.4 Passive and Active Transport (cont.) • Active transport requires energy to move substances, in addition to the help of transport proteins. • Sources of energy include the hydrolysis of ATP and coupling the movement of one substance against its gradient to the movement of another down its gradient. How Cells Exchange Materials 3.4 Passive and Active Transport (cont.) • Maintaining specific gradients across cell membranes is essential to keep internal conditions in a range that permits life functions. • Many necessary substances could not enter or leave cells without active transport. Passive and active transport How Cells Exchange Materials 3.4 Passive and Active Transport (cont.) • To move very large molecules such as proteins into or out of a cell, the cell membrane folds around the substance to be transported, making a pocket to carry it in or out of the cell. – Endocytosis is a useful way for unicellular organisms or very simple multicellular organisms to get food into their internal environment. – Exocytosis helps cells remove waste materials and specific molecules into the external environment. How Cells Exchange Materials 3.4 Passive and Active Transport (cont.) Large molecules are transported into a cell by endocystosis (a), and out of a cell by exocytosis (b). Both processes require energy. Exchange in Multicellular Organisms 3.5 Gas Exchange in Water • Cellular respiration is an important supply of energy for metabolism and other cell activities in most organisms. • Oxygen is essential for cellular respiration, and carbon dioxide is given off as a waste product. • The correct balance of these two important molecules must be regulated carefully. Exchange in Multicellular Organisms 3.5 Gas Exchange in Water (cont.) • Gas exchange happens by diffusion across a membrane when the gases are dissolved in water. • As with most exchange processes, efficiency requires a large surface area relative to volume. • In fish, breathing through gills is very efficient because they have a large surface area made up of many fine, threadlike filaments. Exchange in Multicellular Organisms 3.5 Gas Exchange in Water (cont.) Fish gills are thin filaments supported by bony structures and richly supplied with blood vessels. Each filament is made of disks that contain numerous capillaries. Water flows past these disks in directions opposite (countercurrent) to the flow of blood through the capillaries. A covering over the gills, called the operculum, protects the delicate filaments. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land • Obtaining oxygen on land poses several challenges: – Organisms living on land are constantly battling the tendency to dry out. – Land organisms must dissolve gases in water on the exchange membrane. • Many species of land organisms have evolved exchange surfaces in an interior space which protects the surface from excess evaporation caused and still allows a large area for exchange. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • Some land-dwelling organisms have no special gas-exchange organs. Planaria (flatworms) (a), and earthworms (b), have no special gasexchange organs. Gases are exchanged directly through their skin. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • Insects use a system of small, branched air ducts to carry oxygen throughout the body. In insects, gas exchange occurs through branching air tubes called tracheae (singular: trachea). Air flows in and out of tracheae through openings called spiracles. The spiracles can close to retain water and keep foreign particles out. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • Lungs are the organs of gas exchange in many land animals, including humans. • Lungs minimize the effects of drying out by eliminating the one-way flow of oxygen that is so efficient in gills. • Because the concentration difference is not great, the gas-exchange efficiency of lungs is much less than that of gills. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • The air you breathe passes through your nose, where it is filtered by hairs lining the nasal cavities, moistened, and warmed. • It then travels through branched passageways to reach millions of microscopic cavities in the lungs called alveoli. Scanning electron micrograph of alveoli, x415. Capillaries in the alveolar walls provide a close relationship between blood and air. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • Oxygen and carbon dioxide diffuse across the alveolar walls and the walls of the capillaries. • The numerous alveoli of the lungs provide an enormous amount of surface area for gas exchange. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • Another water-conservation strategy of terrestrial (land-dwelling) organisms involves barriers that limit the permeability of the outside of the organism itself. • Air-breathing vertebrates and arthropods, plants, and fungi all have surface waxes and lipids that minimize water loss by evaporation. • In plants, cells along the surface of a leaf secrete a waxy substance that forms a water-repellent covering called the cuticle. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • In plants, gases normally move into and out of the leaf tissue through openings known as stomates on the leaf surface. • Each stomate is surrounded by a specialized pair of guard cells which bend apart when swollen with water, opening the stomate. • This opening allows carbon dioxide to diffuse in and water vapor and oxygen to exit. The loss of water by this pathway is called transpiration. Exchange in Multicellular Organisms 3.6 Adaptation to Life on Land (cont.) • As osmosis results in the loss of water from the guard cells, they shrink and draw toward one another, closing the stomate. Guard cells act as gates around the stomates in the leaf surface. When open (a), they allow water vapor to escape and carbon dioxide to enter the leaf. When water loss in the plant is higher than its replacement, the guard cells droop toward one another. This action closes the stomate (b). Exchange in Multicellular Organisms 3.7 Waste Removal • Organisms living in fresh water constantly must rid themselves of excess water. Contractile vacuoles in Paramecium rid the cell of excess water. The vacuoles (a) expand as water fills them through radiating canals (b, c). The vacuoles then contract and eject the water from the organism (d). Exchange in Multicellular Organisms 3.7 Waste Removal (cont.) • In addition to water, a variety of waste products must be removed from cells and organisms, including excess salts and carbon dioxide. • The exchange of materials, including the removal of wastes, is essential to maintaining homeostasis, the balanced and controlled conditions in the internal environment of an organism. Exchange in Multicellular Organisms 3.7 Waste Removal (cont.) • In relatively simple organisms such as sponges and Hydra, each cell simply excretes its wastes directly through the external surface. • In more complex animals, special organs have evolved for excretion and maintaining water balance in larger organisms. Exchange in Multicellular Organisms 3.7 Waste Removal (cont.) • Metabolism produces toxic nitrogenous waste, such as ammonia (NH3), which must be disposed of. – The high solubility of ammonia makes it a safe excretory product in freshwater and saltwater protists and animals. – Mammals, some fishes, and amphibians excrete nitrogenous wastes chiefly as urea. – Uric acid, an almost insoluble and nontoxic form of nitrogenous waste, is an adaptation of birds and many desert reptiles. Exchange in Multicellular Organisms 3.8 Human Urinary System • The human urinary system is an example of how waste removal is critical to maintaining homeostasis. • The excretory tubules of humans, the nephrons, are collected into compact organs, the kidneys. • The two kidneys are the major organs in mammals responsible for processing the waste products of metabolism. • The urinary system is composed of the kidneys, the blood vessels that serve them, and the plumbing that carries fluid formed in the kidneys out of the body. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) (a), The human urinary system. (b), A section through the human kidney. (c), An enlarged view of one nephron with its surrounding capillaries. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Blood to be filtered enters the kidneys via the renal artery and leaves via the renal vein. • The waste fluid, urine, leaves the kidneys through a tube called the ureter. • The ureter drains into a holding tank, the urinary bladder. • The urinary bladder is periodically drained when the urine passes through a tube called the urethra during urination. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • A nephron is a long, coiled tube with one cuplike end that fits over a mass of capillaries. The other end of the nephron opens into a duct that collects urine. • The cup of the nephron is called the glomerular capsule, or Bowman’s capsule. • The ball of capillaries within the cup is called a glomerulus. • Collecting tubules from all the nephrons eventually empty into the ureter. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Nephrons have three functions: filtration, reabsorption, and secretion. • Filtration occurs in the glomerulus, where the fluid portion of the blood is forced into the glomerular capsule. • The filtrate includes the blood plasma, nitrogenous wastes from cells, urea, salts, ions, glucose, and amino acids. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Reabsorption and secretion take place in the tubule of the nephron. • Cells of the tubule walls reabsorb substances needed by the body from the filtrate and return them to the blood. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Secretion occurs as cells of the tubule wall selectively remove from the surrounding capillaries substances that were left in the plasma after filtration or returned by reabsorption. • The cells then secrete these substances into the filtrate. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Reabsorption accounts for 85% of the salt, water, and other substances processed by the kidney. • The remaining 15% is regulated by hormones or nervous-system controls. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Excretion of sodium and potassium is regulated by aldosterone, a hormone secreted by the adrenal gland. All of the potassium ions in the filtrate are reabsorbed into the blood by the time the filtrate passes the nephron loop. Under the influence of aldosterone, potassium ions are secreted back into the filtrate near the collecting duct, where they are excreted with the urine. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • Feedback regulation is a process in which substances (such as aldosterone) inhibit their own formation and to maintain balance and stability • The hypothalamus in the brain detects a drop in blood pressure and stimulates the pituitary gland to release antidiuretic hormone (ADH) into the bloodstream. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) Water content of the blood is controlled by ADH from the hypothalamus and pituitary gland. Exchange in Multicellular Organisms 3.8 Human Urinary System (cont.) • The kidneys can also remove excess salt from the body, but only in small amounts. • The kidneys remove nitrogenous wastes from the blood as urea, help regulate blood pressure, regulate water-salt balance, conserve blood glucose, and excrete excess salt, within limits. Summary • A living system is a single or series of protected compartments. • The internal conditions are usually different from conditions outside the organism. • Internal conditions must be carefully balanced with regard to nutrients and wastes, a condition known as homeostasis. • The cell membrane is selectively permeable, which helps it control an organism’s exchange of substances with the environment. • The physical processes of diffusion and osmosis are responsible for movement of substances into and out of cells. • Transport proteins in the membrane can help specific substances cross the membrane barrier. Transport is either passive or, if it requires energy, active. Summary (cont.) • Exocytosis and endocytosis are responsible for exporting or importing large materials, respectively. • Gas exchange is an essential aspect of living processes. Exchange surfaces must be kept moist, and the ratio of surface area to volume affects the efficiency of exchange by diffusion. • Land organisms must balance the need for large surface area of the exchange membranes against the danger of drying out. • Wastes must be expelled from all living systems. Nitrogenous wastes are particularly toxic and may be excreted as ammonia, urea, or uric acid. • Contractile vacuoles in unicellular organisms force wastes out of the cell. Summary (cont.) • In humans, the kidneys are the major organs for removing waste products from the internal environment. • The nephron is the functional unit of the kidney. • Hormones assist the urinary system in regulating ion balance, water levels in the blood, and blood pressure. Reviewing Key Terms Match the term on the left with the correct description. ___ diffusion d ___ osmosis a ___ turgor e ___ endocytosis b ___ cuticle f ___ homeostasis c a. the movement of water through a selectively permeable membrane b. the cellular uptake of materials in which the plasma membrane surrounds and engulfs extracellular materials c. state of balance within an internal environment d. the movement of a substance down its concentration gradient e. a cell’s swelling against its cell wall caused by internal pressure f. the waxy outer layer covering the surfaces of most land-dwelling plants Reviewing Ideas 1. What function to glycoproteins perform in a cell membrane? Glycoproteins act as antennae that receive chemical messages from other cells. Reviewing Ideas 2. What challenges do land-dwelling organisms face in relation to gas-exchange? Organisms living on land are constantly battling the tendency to dry out. Land organisms must also dissolve gases in water on their exchange membrane. Using Concepts 3. How does a concentration gradient represent potential energy? The barrier formed by a membrane can act like a dam that holds back the water of a lake. In a cellular compartment, the membrane may hold back ions. A great amount of potential energy is stored in this way, just as the water behind a dam has the potential to rush out if the dam is opened. Using Concepts 4. How is the function of a stomate self-regulating? Each stomate is surrounded by a specialized pair of guard cells, which function as gates. When guard cells are swollen with water, they bend apart, opening the stomate. This opening allows carbon dioxide to diffuse in and water vapor and oxygen to exit. If a plant loses more water than it can take in through its roots, the plant wilts. As osmosis results in the loss of water from the guard cells, they shrink and draw toward one another, closing the stomate and minimizing further water loss. Synthesize 5. If a person is dehydrated, why can’t pure water simply be injected into them? Ringer’s solution is commonly injected directly into the blood stream to help fight dehydration. What allows this to be safe? Injecting pure water would cause a high water concentration in the blood. This would cause the blood cells to swell and possibly burst as water rushes down its concentration gradient and into the cells. Ringer’s solution is isometric, containing the same concentration of dissolved material as found inside the blood cells. To navigate within this Interactive Chalkboard product: Click the Forward button to go to the next slide. Click the Previous button to return to the previous slide. Click the Section Back button return to the beginning of the section you are in. Click the Menu button to return to the Chapter Menu. Click the Help button to access this screen. Click the Speaker button where it appears to listen to a glossary definition of a highlighted term. Click the Exit button to end the slide show. You also may press the Escape key [Esc] to exit the slide show. Click the Biology Online button to access the online features that accompany this textbook at BSCSblue.com. This Web site will open in a separate browser window. Chapter Animations A selectively permeable membrane The fluid-mosaic model of a membrane’s structure Passive and active transport A selectively permeable membrane The fluid-mosaic model of a membrane’s structure Passive and active transport End of Custom Shows This slide is intentionally blank.