Discover BiologySIXTH EDITION

CHAPTER 4Cell Membranes, Transport,

and Communication

© 2015 W. W. Norton & Company, Inc.

Anu Singh-Cundy • Gary Shin

CHAPTER 4Cell Membranes, Transport,

and Communication

MYSTERIOUS MEMORY LOSS

4.1 The Plasma Membrane as Gate and Gatekeeper

In diffusion, substances move passively down a concentration gradient

Some small molecules can diffuse through the phospholipid bilayer

4.2 Osmosis

4.3 Facilitated Membrane Transport

Channel proteins move substances passively

Carrier proteins bind to molecules to help them cross the membrane

Passive carrier proteins mediate facilitated diffusion

Active carrier proteins move materials against a concentration gradient

4.4 Exocytosis and Endocytosis

Exocytosis moves bulk materials out of the cell

Endocytosis brings bulk materials into the cell

4.5 Cellular Connections

4.6 Cell Signaling

BIOLOGY MATTERS: OSMOSIS IN THE KITCHEN AND GARDEN

APPLYING WHAT WE LEARNED: CHOLESTEROL IN THE BRAIN

Mysterious Memory Loss• Membranes are at the

forefront of transport and communication in the cell.

• Cholesterol is a normal component of our cell membranes.

• Certain drugs can affect the ability of membranes to perform their functions.

Why was the astronaut losing his memory? Can low cholesterol be a problem? Does cholesterol do

anything good for us, or is it all bad?

• The plasma membrane helps maintain the unique life-giving internal chemistry of a cell.

• Dysfunction and death follow if a cell fails to maintain its special internal environment.

The Plasma Membrane Manages a Cell’s Relationship with Its Surroundings

Plant cells placedin a syrupy solutionlose water and thecytoplasm shrinks.Water management

is a matter of life and death for a cell.

Cytoplasm Cytoplasm

The Plasma Membrane Is BothGate and Gatekeeper

• The selective permeability of the plasma membrane ensures that the cell interior is chemically different than the extracellular environment.

• Cells must expend a large amount of energy to maintain the chemical balance between the interior and exterior of the cell.

• The proteins and the phospholipids make up a selectively permeable barrier, which allows only certain substances to enter or leave the cell.

• Transport proteins span the width of the plasma membrane and provide pathways by which specific materials can enter or leave.

The Plasma Membrane Is Composed of a Phospholipid Bilayer with Embedded Proteins

• Passive transport is the spontaneous movement of a substance and can take place without an input of energy.

• Active transport is the movement of a substance in response to an input of energy.

Transport of Substances Is Either Passive or Active

In Diffusion, Substances Move Passively Down a Concentration Gradient

• Diffusion is the passive transport of a substance from a region of higher concentration to a region of lower concentration.

• Equilibrium is reached when concentration gradients disappear and therefore diffusion ceases (substances continue to move, but net movement ceases).

The Rate of Diffusion Is Affected by Various Factors

• The greater the difference in concentration between two points, the higher the rate of diffusion.

• Heat increases the rate of diffusion.

• Small substances diffuse faster than larger substances.

High concentration

Low concentration

Uniformconcentration

throughout solution

Maintaining a Proper Water Balance Is Vital for Every Cell

• Osmosis refers to the diffusion of water molecules (from an area of lower solute concentration to an area of higher solute concentration) across a selectively permeable membrane.

• Osmosis refers to the movement of water molecules, not the movement of solutes; however, it is the differences in solute concentration that drives the osmotic movement of water molecules.

Low solute concentration

High solute concentration

H2O

• A hypotonic solution has a lower solute concentration than the cytosol of a cell, which causes water to flow into the cell and make it swell.

• A hypertonic solution has a higher solute concentration than the cytosol of a cell, which causes water to flow out of the cell and make it shrink.

• An isotonic solution has an equal concentration of solute inside and outside the cell.

Water Can Enter or

Leave a Cell Through Osmosis

Substances Move Across Membranes in a Variety of Ways

• Without energy, or with an absolute need for energy (passively, or actively). • Without the help of transport proteins or with the help of transport proteins (simple

diffusion, or facilitated diffusion).

• Very small molecules, such as oxygen and carbon dioxide, can cross cell membranes without assistance through simple diffusion.

• Most hydrophobic molecules can pass through the cell membrane’s hydrophobic interior.

Some Small Molecules Can Diffuse through the Phospholipid Bilayer

Transport ProteinsEnable Facilitated Diffusion

• Facilitated diffusion is the passive transmembrane movement of a substance with the assistance of membrane transport proteins:– Channel proteins– Carrier proteins

Channel Proteins Transport Substances Passively

• Channel proteins carry out facilitated diffusion:

- They move substances down a concentration gradient, without an energy input, selecting cargo (mainly ions) on the basis of size, shape, and chemical characteristics.

Carrier Proteins Transport Substances Passively or Actively

• Carrier proteins transport specific molecules across the plasma membrane based on the shape of the molecule, and are of two types:– Passive carrier proteins– Active carrier proteins

Glucose transporters (red) move glucoseinto or out of a cell via passive transport.

Passive Carrier Proteins Mediate Facilitated Diffusion

• Passive carrier proteins assist in the diffusion of molecules and ions from an area of higher concentration to an area of lower concentration.

• An important passive carrier of glucose is a protein called GLUT, which helps maintain blood glucose levels through facilitated diffusion.

Active Carrier Proteins Move Materials Against a Concentration Gradient

• Active carrier proteins use ATP to pump ions or molecules across the membrane against a concentration gradient.

• An important active carrier protein called the sodium-potassium pump plays a vital role in maintaining the ratio of sodium-potassium concentration by exporting sodium from the cell and importing potassium ions against a concentration gradient.

Exocytosis and Endocytosis Are Cellular Processes for Exporting or Importing Materials

• Exocytosis: substances to be exported from a cell are packaged into transport vesicles that fuse with the plasma membrane and expel the contents into their surrounding.

• Endocytosis brings substances into the cell by wrapping them in a section of the plasma membrane that eventually breaks free inside the cell.

Endocytosis

Endocytosisof yeast cellby immune

cell

The Different Types of Endocytosis Are Specialized for Importing Specific Substances

• Pinocytosis is a nonspecific type of endocytosis that brings in all the material in an immediate area.

• Receptor-mediated endocytosis uses specialized receptor proteins to determine which substances are incorporated into the vesicle created by the plasma membrane.

• Phagocytosis is used to ingest large particles, such as a bacteria or viruses; white blood cells use phagocytosis to engulf and destroy foreign substances.

Our Cells Use Receptor-Mediated Endocytosis to Take Up LDL Particles Selectively

• Cholesterol is hydrophobic, and must be packaged with proteins (apoliproteins) to form low-density lipoprotein particles (LDL particles) for transport in blood.

• LDL particles docks with LDL receptors in the plasma membrane.

• The docking triggers endocytosis of the entire complex.

• Once inside the cell, the LDL particle is degraded and the released lipids used by the cell.

• The LDL receptor remains intact and is shipped back to the plasma membrane to receive yet more LDL particles.

• LDL receptors are defective or lacking in patients with familial hypercholesterolemia.

Cell Junctions Are Plasma Membrane Structures That Link Cells to Their Surroundings

Main cell junctions in animal cells:• Tight junctions • Anchoring junctions • Gap junctions

• Tight junctions are formed by belts of proteins that create leak-proof sheets of cells, which can be found in the skin and lining of the body cavities.

Tight junction proteins (red) are located all along the surface of these pig kidney cells. Cells like these line the millions of

tubules in our kidneys that form and concentrate urine.

• Gap junctions are direct cytoplasmic connections between two cells formed by protein-lined tunnels that span the intercellular space between adjacent cells.

• Gap junctions facilitate the rapid passage of ions and small molecules, including electrical signals, between adjacent cells.

Cell Junctions Anchor Cells and Serve as Rapid Communication Routes

• Anchoring junctions form patches of proteins that extend through the plasma membrane and link cells that typically undergo heavy structural stress.

• Plasmodesmata are tunnels through the cell wall that connect the plasma membranes and cytoplasm of adjacent cells.

• They enable rapid communication between plant cells, in a manner similar to gap junctions.

Many Plant Cells Are Linked by Plasmodesmata

• A target cell receives a signal through receptor protein located on the plasma membrane (cell surface receptors) or in the cytoplasm (intracellular receptors).

• Signals received at the cell surface must be relayed internally through a series of events known as a signal transduction pathway.

Cells Communicate by Releasing and SensingSignaling Molecules

BIOLOGY MATTERS:

OSMOSIS IN THE KITCHEN AND GARDEN

• One way to preserve food is to createa hypertonic medium, which causes osmotic water loss from fungal and bacterial cells (plasmolysis).

• Adding sugar (as in jams) or salt (as in cured meats) are some ways of creating ahypertonic environment.

• Fertilizer contains concentrated minerals, and and if not diluted properly, it can kill a plant by causing osmotic water loss from the roots.

• Plants suffering from “fertilizer burn” look wilted.

APPLYING WHAT WE LEARNED: CHOLESTEROL IN THE BRAIN

• Cholesterol is found in the cell membranes of all animals with a backbone (vertebrates); it plugs the space between phospholipids to make membranes less leaky.

• Most of the body’s cholesterol is made and exported by the liver.

• The membranes of brain cells—the plasma

membrane as well as organellar membranes—have the most cholesterol.

• Glial cells in the brain cholesterol helps neurons make connections among themselves.

• Certain types of statin drugs may interfere with the ability of brain tissue to make cholesterol, at least in some people.

A quarter of all the cholesterol in the body is in the brain.

List of Key Terms: Chapter 4active carrier protein (p. 94)active transport (p. 87)anchoring junction (p. 97)carrier protein (p. 94)cell junction (p. 97)channel protein (p. 92)concentration gradient (p. 88)diffusion (p. 87)endocytosis (p. 96)exocytosis (p. 95)facilitated diffusion (p. 92)facilitated transport (p. 92)gap junction (p. 98)hormone (p. 99)hypertonic solution (p. 92)hypotonic solution (p. 91)isotonic solution (p. 92)osmoregulation (p. 92)osmosis (p. 90)passive carrier protein (p. 94)passive transport (p. 87)phagocytosis (p. 97)

pinocytosis (p. 96)plasmodesma (p. 98)receptor (p. 96)receptor-mediated endocytosis (p. 96)selective permeability (p. 86)signal transduction pathway (p. 99)signaling molecule (p. 99)simple diffusion (p. 89)target cell (p. 99)tight junction (p. 97)transport protein (p. 86)

The main difference between active and

passive transport is that

A. passive transport moves substances up a

concentration gradient.

B. active transport moves substances down a

concentration gradient.

C. active transport requires energy.

D. passive transport requires energy.

Class Quiz

Class Quiz

If a human red blood cell is placed in pure water, it will

A. swell and burst.B. shrink in size.C. lose water because of osmosis.D. gain water through active transport.

Class Quiz_____ involves the engulfing of specific molecules by the plasma membrane.

A. Endocytosis

B. Pinocytosis

C. Exocytosis

D. Receptor‑mediated endocytosis

Relevant Art from Other Chapters

All art files from the book are available in JPEG and PPT formats online and on the

Instructor Resource Disc

The Sodium/Potassium Pump

The sodium-potassium pump creates and maintains the large, but opposite, concentration gradients of sodium and potassium ions across the plasma membrane

in animal cells.

The Sodium/Potassium Pump

4.1 Concept Check

1. What is meant by the selective permeability of biological membranes?

ANSWER: Biological membranes allow some substances to enter or leave the cell, but block other substances.

4.1 Concept Check

2. How is passive transport different from active transport by biological membranes?

ANSWER: Passive transport occurs spontaneously and moves substances down their concentration gradient without the input of energy. Active transport, which moves substances against their concentration gradient, requires the cell to expend energy.

4.2 Concept Check

1. Compare simple diffusion and osmosis.

ANSWER: Osmosis is a special case of simple diffusion; it is the passive movement of water across a selectively permeable membrane.

4.2 Concept Check

2. Is your blood hypertonic, hypotonic, or isotonic for all the trillions of cells in your body?

ANSWER: Isotonic; severe illness would result if ourinternal fluids did not match the osmotic state of our cells.

4.3 Concept Check

1. Explain why ions, such as Na+, cannot move across a phospholipid bilayer unassisted.

ANSWER: The electrical charge on ions makes them hydrophilic, so they cannot cross the lipid bilayer, which is hydrophobic.

4.3 Concept Check

2. Compare channel proteins and carrier proteins.

ANSWER: Both types of transport proteins facilitate the movement of substances across the membrane, but channel proteins allow passive movement of materials and do not need energy input; carrier proteins transport a great variety of substances either passively or actively.

4.4 Concept Check

1. Which process is more selective in terms of the cargotransported: pinocytosis or receptor-mediated endocytosis?

ANSWER: Receptor-mediated endocytosis

4.4 Concept Check

2. What is the fate of LDL particles that bind to cell surface receptors?

ANSWER: They are internalized via receptor-mediatedendocytosis and taken apart in the lysosome.

4.5 Concept Check

1. What is the main function of tight junctions?

ANSWER: To create a leak-proof sheet of cells

4.5 Concept Check

2. How are the gap junctions of animal cells similar to the plasmodesmata found in plant cells?

ANSWER: Both are cytoplasmic connections that act as tunnels to allow ions and small molecules to move between neighboring cells.

4.6 Concept Check

1. Would you expect the receptor for a small lipid signal, such as the hormone testosterone, to be located within the cytoplasm or in the plasma membrane? Would it require signal transduction? Explain.

ANSWER: Within the cytoplasm; because lipid signalsare hydrophobic and can cross the plasma membrane, they do not dock with cell surface receptors; as a result, signal transduction is not required.

4.6 Concept Check

2. Compare hormone signaling with signaling by aneurotransmitter.

ANSWER: Hormones are long lasting and widely broadcast throughout the body. Neurotransmitters are released very close to their target cells and are quickly removed.