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Chapter Introduction

Living Systems as Compartments

3.1Exchanged Materials

3.2Membrane as Barrier

How Cells Exchange Materials

3.3Diffusion and Osmosis

3.4Passive and Active Transport

Exchange in Multicellular Organisms

3.5Gas Exchange in Water

3.6Adaptation to Life on Land

3.7Waste Removal

3.8Human Urinary System

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A Discuss the structure and function of membranes in living organisms.

Learning Outcomes

By the end of this chapter you will be able to:

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.


What molecular processes are responsible for exchange?

Exchanging Materials with the Environment How do living organisms

exchange materials with their surroundings?

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


• Their cytoplasm, or interior, of cells is surrounded by a wall made of carbohydrates and proteins and a membrane made largely of phospholipids.


3.1 Exchanged Materials

• Materials needed for life must pass into this compartment to be useful.

A Bacillus megaterium bacterium (x30,500)


• 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.


3.1 Exchanged Materials (cont.)


• Membranes are composed of two thin, fluid, layers of phospholipids and proteins.

3.2 Membrane as Barrier

• 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


• 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.

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.

3.2 Membrane as Barrier (cont.)


• 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


• Diffusion refers to the movement of molecules from an area of higher concentration to an area of lower concentration.


3.3 Diffusion and Osmosis

• Diffusion is a random process, and the entropy of the system increases as it occurs.


• A concentration gradient exists when there is a difference in concentration of molecules across a distance.


3.3 Diffusion and Osmosis (cont.)

• Diffusion is a basic process underlying the movement of molecules into and out of cells.




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).


• 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.




• 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.






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 MaterialsInitially 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.


• The rate of diffusion, including osmosis, depends on the size of the concentration gradient and the surface area relative to the enclosed volume.




• 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.

3.4 Passive and Active Transport


– Passive transport involves diffusion without any input of energy.

– Active transport moves substances against their concentration gradients and thus requires energy.


• 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.

3.4 Passive and Active Transport (cont.)


• Facilitated diffusion makes transport more specific and speeds up the rate, but it does not work against the gradient.


• 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.




• 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


• 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.




Large molecules are transported into a cell by endocystosis (a), and out of a cell by exocytosis (b). Both processes require energy.


• Cellular respiration is an important supply of energy for metabolism and other cell activities in most organisms.


3.5 Gas Exchange in Water

• 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.


• 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.


3.5 Gas Exchange in Water (cont.)



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.


• Obtaining oxygen on land poses several challenges:

• 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.


3.6 Adaptation to Life on Land

– Organisms living on land are constantly battling the tendency to dry out.

– Land organisms must dissolve gases in water on the exchange membrane.


• Some land-dwelling organisms have no special gas-exchange organs.


3.6 Adaptation to Life on Land (cont.)

Planaria (flatworms) (a), and earthworms (b), have no special gas-exchange organs. Gases are exchanged directly through their skin.


• 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.


• 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.




• 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.


• 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.




• 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.




• 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.




• 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).




• Organisms living in fresh water constantly must rid themselves of excess water.

3.7 Waste Removal


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).


• 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.

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.




• 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.




• The human urinary system is an example of how waste removal is critical to maintaining homeostasis.

3.8 Human Urinary System

• 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.


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.


• 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.


• 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.


• 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.


• 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.




• 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.




• 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.




• 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.


• 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.






Water content of the blood is controlled by ADH from the hypothalamus and pituitary gland.


• 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

• 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.

• A living system is a single or series of protected compartments.


Summary (cont.)

• 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.

• Exocytosis and endocytosis are responsible for exporting or importing large materials, respectively.


Summary (cont.)

• 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.

• In humans, the kidneys are the major organs for removing waste products from the internal environment.


Reviewing Key TermsMatch the term on the left with the correct description.

___ diffusion

___ osmosis

___ turgor

___ endocytosis

___ cuticle

___ homeostasis

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

d

a

e

b

f

c


Reviewing Ideas1. What function to glycoproteins perform in a cell

membrane?

Glycoproteins act as antennae that receive chemical messages from other cells.


Reviewing Ideas2. 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 Concepts3. 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 Concepts4. 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.


Synthesize5. 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.


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