Chapter 3: Exchanging Materials with the Environment

Chapter 3         Exchanging materials with the Environment

3.1 Exchanged Materials

3.2 Membrane as Barrier

3.3 Diffusion and Osmosis

3.4 Passive and Active Transport

3.5 Gas Exchange in Water

3.6 Adaptiation to Life on Land

3.7 Waste Removal

3.8 Human Urinary System

Chapter Summary

Review Questions

Chapter 3: Exchanging materials with the Environment

3.1 Exchanged Materials

• The cytoplasm, or interior, of cells is surrounded by a wall made of carbohydrates and proteins and a membrane made largely of phospholipids. (Fig. 3.1)
• Material needed for life must pass into this compartment to be useful.
• 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.

3.2 Membrane as Barrier

• Membranes are composed of two thin, fluid, layers of phospholipids and proteins.
• Not all molecules are equally soluble in a membrane. (Fig. 3.2)
  1. The nonpolar phospholipid tails of the lipid bilayer tend to repel charged particles such as ions but allow fat-soluble molecules to pass.
  2. Usually, the polarity, size and electric charge of molecules determine whether they can pass through a 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. (Fig. 3.3)
• Proteins and other vary 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. (Fig. 3.3)
• The structure of membranes is complex and allows them to perform many functions in the cell.
• Some proteins, called glycoproteins, are embedded in membranes and have sugars attached to them. (Fig. 3.3)
• Sugars also can be attached to the heads of membrane lipids (glycolipids). (Fig. 3.3)
• Glycoproteins and glycolipids act as antennae that receive chemical messages from other cells.

3.3 Diffusion and Osmosis

• Diffusion refers to the movement of molecules from an area of higher concentration to an area of lower concentration. (Fog. 3.4)
• Diffusion is a random process, and the entropy of the system increases as it occurs. (Fig. 3.6)
• A concentration gradient exists when there is a difference in concentration of molecules across a distance. (Fig. 3.5)
• Diffusion is a basic process underlying the movement of molecules into an out of cells.
• 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. (Fig. 3.7)
• If the concentration of water is higher inside the cell than outside, water is driven out and the cell shrinks. (Fig. 3.7)
• Outward pressure of a cell against its cell wall is called turgor. (Fig. 3.7)
• The rate of diffusion, including osmosis, depends on the size of the concentration gradient and the surface area relative to the enclosed volume.

3.4 Passive and Active Transport

• Organisms must establish and maintain concentrations of material 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.
  1. Passive transport involves diffusion without any input of energy. (Fig. 3.8a)
  2. Active transport moves substances against their concentration gradients and thus requires energy. (Fig. 3.8b)
• 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.
• 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 substances 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.
• To move vary large molecules such as proteins into or out of a cell, the cell membrane folds around the substances to be transported, making a pocket to carry it in or out of the cell. (Fig. 3.9)

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.
• 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. (Fig. 3.11 and 3.12)

3.6 Adaptation to Life on land

• Obtaining oxygen on land poses several challenges:
  1. Organisms living on land are constantly battling the tendency to dry out.
  2. 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.
• Some land-dwelling organisms  have no special gas-exchange organs. (Fig. 3.13)
• Insects use a system of small, branched air ducts to carry oxygen throughout the body. (Fig. 3.14)
• Lungs are the organs of gas exchange in many land animals, including humans. (Fig. 3.15)
• 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. (Fig. 3.16)
• 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. (Fig. 3.15)
• 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. (Fig. 3.17)
• Each stomate is surrounded by a specialized pair of guard cells which bend apart when swollen with water, opening the stomate. (Fig. 3.17)
• 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. (Fig. 3.17)

3.7 Waste Removal

• Organisms living in fresh water constantly must rid themselves of excess water. (Fig. 3.19)
• 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.
• In relatively simply organisms such as sponges and Hydra, each cell simply excretes its wastes directly through the external surface. (Fig. 3.20)
• 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.

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. (Fig. 3.21)
• 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.
• 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. (Fig. 3.21)
• The ureter drains into a holding tank, the urinary bladder. (Fig. 3.21)
• The urinary bladder is periodically drained when the urine passes through a tube called the urethra during urination. (Fig. 3.21)
• A nephron is a long, coiled tube with one cuplike end that fits over a mass of capillaries.  The other end of the nephrons opens into a duct that collects urine. (Fig. 3.21)
• The cup of the nephron is called the glomerular capsule, or Bowman’s capsule. (Fig. 3.21)
• The ball of capillaries within the cup is called a glomerulus. (Fig. 3.21)
• Collecting tubules from all the nephrons eventually empty into the ureter.
• Nephrons have three functions:
  1. Filtration
  2. Reabsorption
  3. Secretion
• Filtration occurs in the glomerulus, where the fluid portion of the blood is forced into the glomerular capsule. (Fig. 3.22)
• The filtrate includes the blood plasma, nitrogenous wastes from cells, urea, salts, ions, glucose and amino acids. (Fig. 3.22)
• Reabsorption and secretion take place in the tubule of the nephron. (Fig. 3.22)
• Cells of the tubule walls reabsorb substances needed by the body from the filtrate and return them to the blood. (Fig. 3.22)
• 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. (Fig. 3.22)
• 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. (Fig. 3.23)
• 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. (Fig. 3.24)
• 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 organisms.
• 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 process 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.
• 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.
• 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.

Review Questions –

• What function do glycoproteins perform in a cell membrane?
• What challenges do land-dwelling organisms face in relation to gas-exchange?
• How does a concentration gradient represent potential energy?
• How is the function of a stomate self-regulating?
  1. 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?