Week 2 LS 7A Chapter 5

After the system described above reaches equilibrium, what will be the concentration of glucose on side B?

0.75 M side a 1m sucrose 0.5 glucose side b 1m glucose 0.5 sucrose

electrochemical gradient

A gradient that has both charge and chemical components is known as an electrochemical gradient

secondary active transport

Because the movement of the coupled molecule is driven by the movement of protons and not by ATP directly, this form of transport is called secondary active transport. Secondary active transport uses the potential energy of an electrochemical gradient to drive the movement of molecules; by contrast, primary active transport uses the chemical energy of ATP directly. The use of an electrochemical gradient as a temporary energy source is a common cellular strategy. For example, cells use a sodium electrochemical gradient generated by the sodium-potassium pump to transport glucose and amino acids into cells. In addition, cells use a proton electrochemical gradient to move other molecules and, as we discuss in Chapter 7, to synthesize ATP. Because small ions cannot cross the lipid bilayer, many cells use a transport protein to build up the concentration of a small ion on one side of the membrane. The resulting concentration gradient stores potential energy that can be harnessed to drive the movement of other substances across the membrane against their concentration gradient. Secondary active transport. Protons are pumped across a membrane by (a) primary active transport, resulting in (b) an electrochemical gradient, which drives (c) the movement of another molecule against its concentration gradient

STRUCTURE OF CELL MEMBRANES

Cells are defined by membranes, membranes define spaces within many cells that allow them to carry out their diverse functions. Lipids are the main component of cell membranes. They have properties that allow them to form a barrier in an aqueous (watery) environment. Proteins are often embedded in or associated with the membrane, where they perform important functions such as transporting molecules. Carbohydrates can also be found in cell membranes, usually attached to lipids (glycolipids) and proteins (glycoproteins).

Eukaryotes structure

Eukaryotes have a remarkable internal array of membranes. These membranes define compartments, called organelles, that divide the cell contents into smaller spaces specialized for different functions. Fig. 5.17a shows a macrophage, a type of animal cell, with various organelles. The endoplasmic reticulum (ER) is the organelle in which proteins and lipids are synthesized. The Golgi apparatus modifies proteins and lipids produced by the ER and acts as a sorting station as they move to their final destinations. Lysosomes contain enzymes that break down macromolecules such as proteins, nucleic acids, lipids, and complex carbohydrates. Peroxisomes also contain many different enzymes and are involved in important metabolic reactions, including the breakdown of fatty acids and the synthesis of certain types of phospholipid. Mitochondria are specialized organelles that harness energy for the cell. Many cell membranes that define these organelles are associated with a protein scaffold called the cytoskeleton that helps cells to maintain their shape and serves as a network of tracks for the movement of substances within cells (Chapter 10). shows a typical plant cell. In addition to the organelles described above, plant cells have a cell wall outside the plasma membrane, vacuoles specialized for water uptake, and chloroplasts that convert energy of sunlight into chemical energy. The entire contents of a cell other than the nucleus make up the cytoplasm. The jelly-like internal environment of the cell that surrounds the organelles inside the plasma membrane is referred to as the cytosol.

In the absence of the sodium-potassium pump, the extracellular solution becomes hypotonic relative to the inside of the cell. Poisons such as the snake venom ouabain can interfere with the action of the sodium-potassium pump. What are the consequences for the cell?

If the sodium-potassium pump is made inactive by poison, the cell will swell and even burst, as the intracellular fluid becomes hypertonic relative to the outside of the cell and water moves into the cell by osmosis.

Why is the transporter in the figure above considered to be an example of "secondary transport"?

It is driven by the proton gradient that was created by energy from ATP.

lipids

Lipids freely associate with one another because of extensive van der Waals forces between their fatty acid tails lipids are able to move in the plane of the membrane, the membrane is said to be fluid. The degree of membrane fluidity depends on which types of lipid make up the membrane. In a single layer of the lipid bilayer, the strength of the van der Waals interactions between the lipids' tails depends on the length of the fatty acid tails and the presence of double bonds between neighboring carbon atoms. The longer the fatty acid tails, the more surface is available to participate in van der Waals interactions. The tighter packing that results tends to reduce lipid mobility. Likewise, saturated fatty acid tails, which have no double bonds, are straight and tightly packed—again reducing mobility (Fig. 5.4a). The double bonds in unsaturated fatty acids introduce kinks in the fatty acid tails, reducing the tightness of packing and enhancing lipid mobility in the membrane

cell size

Many cells maintain size and composition using active transport. Consider human red blood cells placed in a variety of different solutions (Fig. 5.14). If a red blood cell is placed in a hypertonic solution (one with a higher solute concentration than that inside the cell), water leaves the cell by osmosis and the cell shrinks. By contrast, if a red blood cell is placed in a hypotonic solution (one with a lower solute concentration than that inside the cell), water moves into the cell by osmosis and the cell lyses, or bursts. Animal cells solve the problem of water movement in part by keeping the intracellular fluid isotonic (that is, at the same solute concentration) as the extracellular fluid. Cells use the active transport of ions to maintain equal concentrations inside and out, and the sodium-potassium pump plays an important role in keeping the inside of the cell isotonic with the extracellular fluid. Human red blood cells avoid shrinking or bursting by maintaining an intracellular environment isotonic with the extracellular environment, the blood Paramecium and some other single-celled organisms contain contractile vacuoles that solve this problem. Contractile vacuoles are compartments that take up excess water from inside the cell and then, by contraction, expel it into the external environment. The mechanism by which water moves into the contractile vacuoles differs depending on the organism. The contractile vacuoles of some organisms take in water through aquaporins, while the contractile vacuoles of other organisms first take in protons through proton pumps, with water following by osmosis.

osmosis

Osmosis. Osmosis is the net movement of a solvent such as water across a selectively permeable membrane from an area of lower solute concentration to an area of higher solute concentration.

Prokaryotes vs. Eukaryotes

Prokaryotes, including bacteria and archaeons, lack a nucleus; eukaryotes, including animals, plants, fungi, and protists, have a nucleus (Fig. 5.16). The presence of a nucleus in eukaryotes allows for the processes of transcription and translation to be separated in time and space. This separation in turn allows for more complex ways to regulate gene expression than are possible in prokaryotes . Prokaryotes do not have a nucleus—that is, there is no physical barrier separating the genetic material from the rest of the cell. Instead, the DNA is concentrated in a discrete region of the cell interior known as the nucleoid. Cholesterol belongs to a group of chemical compounds known as sterols, which are molecules containing a hydroxyl group attached to a four-ringed structure. In eukaryotes other than mammals, diverse sterols are synthesized and present in cell membranes. Most prokaryotes do not synthesize sterols, but some synthesize compounds called hopanoids. These five-ringed structures are thought to serve a

What are proteins and what do they do?

Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body's tissues and organs. Proteins are made up of hundreds or thousands of smaller units called amino acids, which are attached to one another in long chains. There are 20 different types of amino acids that can be combined to make a protein. The sequence of amino acids determines each protein's unique 3-dimensional structure and its specific function. protein functions Function Description Example Antibody Antibodies bind to specific foreign particles, such as viruses and bacteria, to help protect the body. Immunoglobin G (IgG) (illustration) Enzyme Enzymes carry out almost all of the thousands of chemical reactions that take place in cells. They also assist with the formation of new molecules by reading the genetic information stored in DNA. Phenylalanine hydroxylase (illustration) Messenger Messenger proteins, such as some types of hormones, transmit signals to coordinate biological processes between different cells, tissues, and organs. Growth hormone (illustration) Structural component These proteins provide structure and support for cells. On a larger scale, they also allow the body to move. Actin (illustration) Transport/storage These proteins bind and carry atoms and small molecules within cells and throughout the body. Ferritin (illustration)

A container is divided into two compartments by a membrane that is fully permeable to water and small ions. Water is added to one side of the membrane (side A), and a 5% solution of sodium chloride (NaCl) is added to the other (side B). If allowed to reach equilibrium, which of the following would you predict?

The NaCl concentration on side A and side B will each be 2.5%.

net movement of water

The net movement of a solvent such as water across a selectively permeable membrane such as the plasma membrane is known as osmosis. As in any form of diffusion, water moves from regions of higher water concentration to regions of lower water concentration (Fig. 5.11). Because water is a solvent within which nutrients such as glucose or ions such as sodium or potassium are dissolved, water concentration drops as solute concentration rises. Therefore, it is sometimes easier to think about water moving from regions of lower solute concentration toward regions of higher solute concentration. Either way, the direction of water movement is the same. During osmosis, the net movement of water toward the side of the membrane with higher solute concentration continues until it is opposed by another force. This force could be pressure due to gravity (in the case of Fig. 5.11) or the cell wall

A container is divided into two compartments by a membrane that is fully permeable to water but not to larger molecules. Water is added to one side of the membrane (side A), and an equal volume of a 5% solution of glucose is added to the other (side B). What would you predict will happen?

The water level on side B will increase and on side A will decrease.

After the system described above reaches equilibrium, what can you predict about the water levels?

The water will be higher on side A than on side B.

Which of the following correctly describes the movement of water across a selectively permeable membrane during osmosis?

The water will move from high water concentration to low water concentration. The water will move from low solute concentration to high solute concentration

A container is divided into two compartments by a membrane that is fully permeable to water and small ions. Water is added to one side of the membrane (side A), and a 5% solution of sodium chloride (NaCl) is added to the other (side B). In which direction will water molecules move? In which direction will sodium and chloride ions move? When the concentration is equal on both sides, will diffusion stop?

Water molecules move in both directions, but the net movement of water molecules is from side A to side B. Water moves from regions of higher water concentration to regions of lower water concentration. Likewise, sodium and chloride ions move in both directions, but the net movement of sodium and chloride ions is from side B to side A. Movement of water and ions results from diffusion, the random motion of substances. Even when the concentration of all molecules is the same on the two sides, diffusion still occurs, but there is no net movement of water molecules or ions.

In the image shown above, an active transport proton pump drives protons out of the cell using energy from ATP. Under some circumstances pumps like this can be run in reverse. If this pump could be reversed, what would be the result in the cytoplasm? Question 3 choices Choice

a decrease in pH and an increase in ATP

Homeostasis and plasma membrane

active maintenance of a constant environment is known as homeostasis, and it is a critical attribute of cells and of life itself. How does the plasma membrane maintain homeostasis? The answer is that it is selectively permeable (or semipermeable). This means that the plasma membrane lets some molecules in and out freely; it lets others in and out only under certain conditions; and it prevents other molecules from passing through at all.membrane's ability to act as a selective barrier is the result of the combination of lipids and embedded proteins of which it is composed. The hydrophobic interior of the lipid bilayer prevents ions as well as charged or polar molecules from diffusing freely across the plasma membrane. Furthermore, many macromolecules such as proteins and polysaccharides are too large to cross the plasma membrane on their own. By contrast, gases, lipids, and small polar molecules can freely move across the lipid bilayer. Protein transporters in the membrane allow the export and import of molecules, including certain ions and nutrients, that cannot cross the cell membrane on their own. For example, cells in your gut contain membrane transporters that specialize in the uptake of glucose, whereas nerve cells have different types of ion channel that are involved in electrical signaling.

In the example illustrated in the image above, a substance is moved _______BLANK its concentration gradient using the energy of ______________BLANK.

against; an electrochemical gradient

The defining characteristics of active transport are that this category of transport moves substances _________BLANK their concentration gradient and requires ___________BLANK.

against; energy

Which of the following would be the best analogy for an electrochemical gradient across a cellular membrane?

battery

Cholesterol (lipid)

cholesterol is amphipathic, with both hydrophilic and hydrophobic groups in the same molecule. In cholesterol, the hydrophilic region is simply a hydroxyl group (-OH) and the hydrophobic region consists of four interconnected carbon rings with an attached hydrocarbon chain (Fig. 5.5). This structure allows cholesterol to insert into the lipid bilayer so that its head group interacts with the hydrophilic head group of phospholipids, while the ring structure participates in van der Waals interactions with the fatty acid chains. Cholesterol increases or decreases membrane fluidity depending on temperature. At temperatures typically found in a cell, cholesterol decreases membrane fluidity because the interaction of the rigid ring structure of cholesterol with the phospholipid fatty acid tails reduces the mobility of the phospholipids. However, at low temperatures, cholesterol increases membrane fluidity because it prevents phospholipids from packing tightly with other phospholipids. Thus, cholesterol helps maintain a consistent state of membrane fluidity by preventing dramatic transitions from a fluid to solid state.

Simple diffusion of a molecule down its concentration gradient requires an input of energy to the system.

false

plasma membrane

fundamental, defining feature of all cells. It is the boundary that defines the space of the cell, separating its internal contents from the surrounding environment. But the plasma membrane is not simply a passive boundary or wall. Instead, it serves an active and important function. The environment outside the cell is changing all the time. In contrast, the internal environment of a cell operates within a narrow window of conditions, such as a particular pH range or salt concentration. It is the plasma membrane that actively maintains intracellular conditions compatible with life.

Molecules that are _____________BLANK and _____________BLANK are able to move across the cell membrane via simple diffusion.

hydrophobic; small

As molecules move down their concentration gradient, from a more ordered state to a less ordered state, entropy:

increasing

Phospholipid

major types of lipid found in cell membranes are phospholipids, introduced in Chapter 2. Most phospholipids are made up of a glycerol backbone attached to a phosphate group and two fatty acids (Fig. 5.2). The phosphate head group is hydrophilic ("water-loving") because it is polar, enabling it to form hydrogen bonds with water. By contrast, the two fatty acid tails are hydrophobic ("water-fearing") because they are nonpolar and do not form hydrogen bonds with water. Molecules with both hydrophilic and hydrophobic regions in a single molecule are termed amphipathic. They spontaneously arrange themselves into various structures in which the polar head groups on the outside interact with water and the nonpolar tail groups come together on the inside away from water. This arrangement results from the tendency of polar molecules like water to exclude nonpolar molecules or nonpolar groups of molecules. The shape of the structure is determined by the bulkiness of the head group relative to the hydrophobic tails. For example, lipids with bulky heads and a single hydrophobic fatty acid tail are wedge-shaped and pack into spherical structures called micelles (Fig. 5.3a). By contrast, lipids with less bulky head groups and two hydrophobic tails form a bilayer (Fig. 5.3b). A lipid bilayer is a structure formed of two layers of lipids in which the hydrophilic heads are the outside surfaces of the bilayer and the hydrophobic tails are sandwiched in between, isolated from contact with the aqueous environment. Phospholipid structures. Phospholipids can form (a) micelles, (b) bilayers, or (c) liposomes when placed in water. The bilayer structure forms spontaneously, dependent solely on the properties of the phospholipid and without the action of an enzyme, as long as the concentration of free phospholipids is high enough and the pH of the solution is similar to that of a cell. The pH is important because it ensures that the head groups are in their ionized (charged) form and thus suitably hydrophilic. Thus, if phospholipids are added to a test tube of water at neutral pH, they spontaneously form spherical bilayer structures called liposomes that surround a central space (Fig. 5.3c). As the liposomes form, they may capture macromolecules present in solution

Primary active transport

many of the molecules that cells require are not highly concentrated in the environment. Although some of these molecules can be synthesized by the cell, others must be taken up from the environment. In other words, cells have to move these substances from areas of lower concentration to areas of higher concentration. The "uphill" movement of substances against a concentration gradient, called active transport, requires energy. The transport of many kinds of molecules across membranes requires energy, either directly or indirectly. In fact, most of the energy used by a cell goes into keeping the inside of the cell different from the outside, a function carried out by proteins in the plasma membrane. Page 98 During active transport, cells move substances through transport proteins embedded in the cell membrane. Some of these proteins act as pumps, using energy directly to move a substance into or out of a cell. A good example is the sodium-potassium pump (Fig. 5.12). Within cells, sodium is kept at concentrations much lower than in the exterior environment; the opposite is true of potassium. Therefore, both sodium and potassium have to be moved against a concentration gradient. The sodium-potassium pump actively moves sodium out of the cell and potassium into the cell. This movement of ions takes energy, which comes from the chemical energy stored in ATP. Active transport that uses energy directly in this manner is called primary active transport. Note that the sodium ions and potassium ions move in opposite directions. Protein transporters that work in this way are referred to as antiporters. Other transporters move two molecules in the same direction, and are referred to as symporters or cotransporters.

cell wall

organisms have a cell wall external to the plasma membrane. The cell wall plays an important role in maintaining the shape of these cells.

Passive Transport (Diffusion)

simplest form of movement into and out of cells is passive transport. Passive transport works by diffusion, which is the random movement of molecules. Molecules are always moving in their environments.Diffusion, the movement of molecules due to random motion. Net movement of molecules results only when there are concentration differences. When a molecule moves by diffusion through a membrane protein and bypasses the lipid bilayer, the process is called facilitated diffusion. Diffusion and facilitated diffusion both result from the random motion of molecules, and net movement of the substance occurs when there are concentration differences (Fig. 5.10). In the case of facilitated diffusion, the molecule moves through a membrane transporter, whereas in the case of simple diffusion, the molecule moves directly through the lipid bilayer.Membrane transporters are of two types. The first type is a channel, which provides an opening between the inside and outside of the cell within which certain molecules can pass, depending on their shape and charge. Some membrane channels are gated, which means that they open in response to some sort of signal, which may be chemical or electrical (Chapter 9). The second type of transporter is a carrier, which binds to and then transports specific molecules. Membrane carriers exist in two conformations, one that is open to one side of the cell, and another that is open to the other side of the cell. Binding of the transported molecule induces a conformational change in the membrane protein, allowing the molecule to be transported across the lipid bilayer, Water itself also moves into and out of cells by passive transport. Although the plasma membrane is hydrophobic, water molecules are small enough to move passively through the membrane to a limited extent by simple diffusion. In addition, many cells have specific protein channels, known as aquaporins, for transporting water molecules. These channels allow water to move more readily across the plasma membrane by facilitated diffusion than is possible by simple diffusion

Diffusion is best described as the random movement of molecules influenced by:

the thermal energy of the environment and energy transferred from molecular collisions in the cell.

At equilibrium, there will be no net movement of molecules across the cell membrane.

true