5.1 Cell membranes are composed of lipids, proteins, and carbohydrates.
Phospholipids have both hydrophilic and hydrophobic regions. As a result, they spontaneously form structures such as micelles and bilayers when placed in an aqueous environment. page 90
Membranes are fluid, meaning that membrane components are able to move laterally in the plane of the membrane. page 92
Membrane fluidity is influenced by length of fatty acid chains, presence of carbon–
Many membranes also contain proteins that span the membrane (transmembrane proteins) or are temporarily associated with one or other layer of the lipid bilayer (peripheral proteins). page 93
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5.2 The plasma membrane is a selective barrier that controls the movement of molecules between the inside and the outside of the cell.
Selective permeability results from the combination of lipids and proteins that makes up cell membranes. page 96
Passive transport is the movement of molecules by diffusion, the random movement of molecules. There is a net movement of molecules from regions of higher concentration to regions of lower concentration. page 96
Passive transport can occur by the diffusion of molecules directly through the plasma membrane (simple diffusion) or be aided by protein transporters (facilitated diffusion). page 96
Active transport moves molecules from regions of lower concentration to regions of higher concentration and requires energy. page 97
Primary active transport uses energy stored in ATP; secondary active transport uses the energy stored in an electrochemical gradient. page 98
Animal cells often maintain size and shape by protein pumps that actively move ions in and out of the cell. page 99
Plants, fungi, and bacteria have a cell wall outside the plasma membrane that maintains cell size and shape. page 100
5.3 Cells can be classified as prokaryotes or eukaryotes, which differ in the degree of internal compartmentalization.
Prokaryotic cells lack a nucleus and other internal membrane-
Prokaryotic cells include bacteria and archaeons and are much smaller than eukaryotes. page 101
Eukaryotic cells have a nucleus and other internal compartments called organelles. page 101
Eukaryotes include animals, plants, fungi, and protists. page 102
5.4 The endomembrane system is an interconnected system of membranes that includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles, and plasma membrane.
The nucleus, which is enclosed by a double membrane called the nuclear envelope, houses the genome. page 105
The endoplasmic reticulum is continuous with the outer nuclear envelope and manufactures proteins and lipids for use by the cell or for export out of the cell. page 105
The Golgi apparatus communicates with the endoplasmic reticulum by transport vesicles. It receives proteins and lipids from the endoplasmic reticulum and directs them to their final destinations. page 105
Lysosomes break down macromolecules like proteins to simpler compounds that can be used by the cell. page 107
Protein sorting directs proteins to their final destinations in or out of the cell. page 108
Proteins synthesized on free ribosomes are sorted after translation and proteins synthesized on ribosomes associated with the rough endoplasmic reticulum are sorted during translation. page 108
Proteins synthesized on free ribososomes are often sorted by means of a signal sequence and are destined for the cytosol, mitochondria, chloroplasts, or nucleus. page 108
Proteins synthesized on ribosomes on the rough endoplasmic reticulum have a signal sequence that is recognized by a signal-
5.5 Mitochondria and chloroplasts are organelles involved in harnessing energy, and likely evolved from free-
Mitochondria harness energy from chemical compounds for use by both animal and plant cells. page 111
Chloroplasts harness the energy of sunlight to build sugars. page 111
Describe how lipids with hydrophilic and hydrophobic regions behave in an aqueous environment.
In an aqueous environment, the polar hydrophilic head group readily interacts with the polar water molecules. In contrast, the nonpolar hydrophobic tail does not readily interact with water and instead interacts with other nonpolar tail groups or hydrophobic molecules. For example, a micelle forms when the polar head group of a lipid interacts with water and the hydrophobic tails of the lipids interact with each other, excluding the water. Lipids can also form bilayers and liposomes (Fig. 5.3).
Describe two types of association between proteins and membranes.
Proteins can associate with membranes in the following ways: (1) Integral membrane proteins are permanently associated with the membrane and cannot be removed without destroying the membrane itself. Most integral membrane proteins span the cell membrane, and therefore they have both hydrophilic and hydrophobic regions. (2) Peripheral membrane proteins are temporarily associated with the membrane and can easily be experimentally separated. These proteins can be associated with either the internal or external side of the membrane. They are mostly hydrophilic and interact with the polar heads of the lipid bilayer, or the hydrophilic regions of integral membrane proteins.
Describe an experiment that demonstrates that proteins move in membranes.
An experiment designed to show that proteins move in membranes can use the FRAP technique. FRAP stands for fluorescence recovery after photobleaching. First, the proteins embedded in the cell membrane are labeled with fluorescent dye molecules. A laser is then used to bleach part of the cell so that it no longer fluoresces. Eventually, the fluorescently labeled proteins from other parts of the cell move into the bleached area and cause it to fluoresce once again. If the proteins did not move in membranes, that area would stay bleached for the life of the cell.
Name three parameters that need to be stably maintained inside a cell.
Three parameters that need to be stably maintained inside a cell are pH, salt concentration, and volume.
Explain the role of lipids and proteins in maintaining the selective permeability of membranes.
Lipids help maintain the selective permeability of the membrane by preventing charged molecules and ions from diffusing freely into the cell. They also allow molecules like gases and small polar molecules to diffuse freely through the membrane. Molecules like proteins and polysaccharides are too large to cross the plasma membrane without help. Proteins in the membrane help transport these larger molecules by acting as channels and carriers that move molecules into and out of the cell. Each kind of channel or carrier is specific for one or a few molecules.
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Distinguish between passive and active transport mechanisms across cell membranes.
Passive transport into and out of cells works through diffusion (the random movement of molecules). When there is a concentration difference (concentration gradient) of a particular molecule across the cell membrane, the molecule moves from the area of higher concentration to the area of lower concentration. When a molecule cannot move across the plasma membrane on its own, it is sometimes able to passively diffuse through channels or carriers in the lipid bilayer in a process called facilitated diffusion (Fig. 5.10). In contrast, active transport is used by the cell to move a molecule into or out of the cell against its concentration gradient. Molecules move through transport proteins embedded in the cell membrane. This type of transport requires energy, either directly (primary active transport, Fig. 5.12) or indirectly (secondary active transport, Fig. 5.13).
Describe three different ways in which cells maintain size and shape.
Cells maintain size and shape in the following ways: (1) Cells can use active transport to maintain the intracellular solute concentration so that it equals the extracellular solute concentration (e.g., red blood cell). (2) The cell wall of plants, fungi, and bacteria helps maintain the cell’s size and shape by providing a rigid structure surrounding the cell membrane. (3) Some single-
Compare the organization, degree of compartmentalization, and size of prokaryotic and eukaryotic cells.
Prokaryotic cells lack a nucleus and extensive internal compartmentalization. They contain plasmids that carry additional genes that can be transferred to other bacteria. Prokaryotic cells are also small (usually 1–2 micrometers in diameter or smaller). Because of their small size, they have a large surface-
Name the major organelles in eukaryotic cells and describe their functions.
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 are involved in the breakdown of fatty acids and the synthesis of certain types of phospholipid. Mitochondria harness the chemical energy of organic molecules for the cell. Chloroplasts convert the energy of sunlight into chemical energy of organic molecules. Vacuoles are specialized for water uptake. See Fig. 5.17.
Explain how a protein ends up free in the cytosol, embedded in the plasma membrane, or secreted from the cell.
Proteins produced on free ribosomes in the cytosol are directed to their final destination through particular amino acid sequences called signal sequences. These proteins are sorted after they have been translated. Proteins destined for the nucleus, mitochondria, and chloroplasts have specific signal sequences (e.g., nuclear localization signals will direct the protein to the nucleus). Proteins with no signal sequence remain in the cytosol. Proteins produced by ribosomes on the rough endoplasmic reticulum end up in the lumen of the endomembrane system or embedded in its membrane. They may also be secreted out of the cell. These proteins are sorted as they are translated. They are initially translated by a ribosome in the cytosol, but a signal sequence in the growing protein directs the ribosome to a channel on the rough ER. As the protein is translated, it is threaded through the channel. These proteins are destined for the ER lumen, Golgi apparatus, lysosomes, or for secretion outside the cell. If the protein contains an additional signal sequence called a signal anchor sequence, it remains in the ER membrane as it is synthesized, rather than passing entirely into the ER lumen.