Reading for Week 4

Chapter 5: 5.1-5.5 (Smartbook only over 5.1-5.3)

Chapter 6: 6.1-6.4

Chapter 5 Summary

5.1 Plasma Membrane Structure and Function

The fluid-mosaic model describes the interactions of the components of the plasma membrane. In the lipid bilayer, phospholipids are arranged with their hydrophilic (polar) heads adjacent to water, and their hydrophobic (nonpolar) tails buried in the interior. The lipid bilayer has the consistency of oil but acts as a barrier to the entrance and exit of most biological molecules. Membrane glycolipids and glycoproteins are involved in marking the cell as belonging to a particular individual and tissue.

A variety of proteins may be embedded within the plasma membrane. The hydrophobic portion of an integral protein lies in the lipid bilayer of the plasma membrane, and the hydrophilic portion lies at the surfaces. Some functions include the following:

Channel proteins: Form a channel through the membrane to move molecules. Examples are aquaporins that allow rapid movement of water across a membrane.
Carrier proteins: Interact with a target molecule, causing a change in shape, which allows movement of the molecule through the membrane.
Cell recognition proteins: Glycoproteins involved in cellular identification.
Receptor proteins: Detect specific molecules in the environment and invoke a cellular response.
Enzymatic proteins: Carry out metabolic reactions within the plasma membrane.
Junction proteins: Form connections between cells.

The plasma membrane of a cell is selectively permeable and isolates the cell from the external environment. In doing so, the membrane establishes a concentration gradient for each type of molecule. These molecules are then moved across the membrane by active, passive, or bulk transport.

5.2 Passive Transport Across a Membrane

Diffusion is the passive movement of molecules from a higher to a lower concentration. In a solution, the solute diffuses within the solvent until it is evenly distributed. Some molecules (lipid-soluble compounds, water, and gases) simply diffuse across the membrane. No metabolic energy is required for diffusion to occur.

The diffusion of water across a selectively permeable membrane is called osmosis. During osmosis, osmotic pressure directs water to move across the membrane into the area of higher solute (less water) content per volume. Tonicity is a measure of the relative solute concentration.

In an isotonic solution, cells neither gain nor lose water.
In a hypotonic solution, cells gain water. This accounts for turgor pressure of plants and hemolysis in red blood cells.
In a hypertonic solution, cells lose water, which can cause crenation and plasmolysis (Table 5.3).

Other molecules are transported across the membrane either by a channel protein or by carrier proteins that span the membrane. During facilitated transport, a substance moves down its concentration gradient. No energy is required.

5.3 Active Transport Across a Membrane

During active transport, a carrier protein acts as a pump that causes a substance to move against its concentration gradient. One example is the sodium–potassium pump, which carries Na⁺ to the outside of the cell and K⁺ to the inside of the cell. Energy in the form of ATP molecules is required for active transport to occur.

Larger substances can enter and exit a cell by exocytosis and endocytosis.

Exocytosis involves secretion.
Endocytosis includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Receptor-mediated endocytosis makes use of receptor proteins in the plasma membrane. Once a specific solute binds to receptors, a coated pit becomes a coated vesicle. After losing the coat, the vesicle can join with the lysosome or, after discharging the substance, the receptor-containing vesicle can fuse with the plasma membrane.

5.4 Modification of Cell Surfaces

Animal cells have an extracellular matrix (ECM), which influences their shape and behavior. The amount and character of the ECM vary by tissue type. Some animal cells have junction proteins that join them to other cells of the same tissue.

Adhesion junctions and tight junctions help hold cells together.
Gap junctions allow the passage of small molecules between cells.

Plant cells have a freely permeable cell wall, with cellulose as its main component. Plant cells are also joined by narrow, membrane-lined channels called plasmodesmata, which span the cell wall and contain strands of cytoplasm that allow materials to pass from one cell to another.

5.5 Cell-to-Cell Communication

All cells, whether they are prokaryotes or eukaryotes, utilize signals to communicate with other cells. These signals are delivered using signaling molecules called chemical messengers, which interact with specific receptors within the membrane of the target cell.

Once activated, a signal transduction pathway, often with the aid of G proteins and second messengers, relays the signal to a target protein, which is then often activated by adding or removing phosphates.

Chapter 6 Summary

6.1 Cells and the Flow of Energy

Energy is the ability to do work or bring about a change. Energy may exist in several forms.

Kinetic energy is the energy of motion, and potential energy is stored energy.
Chemical energy is a form of potential energy that is stored within the chemical bonds of molecules.

Two laws of thermodynamics are basic to understanding energy in biological systems.

The first law of thermodynamics states that energy cannot be created or destroyed but can only be transferred or transformed.
The second law of thermodynamics states that one usable form of energy cannot be completely converted into another usable form, such as heat. As a result of these laws, we know that the entropy of the universe is increasing and that only a flow of energy from the sun maintains the organization of living organisms.

6.2 Metabolic Reactions and Energy Transformations

The term metabolism encompasses all the chemical reactions occurring in a cell. Considering individual reactions, only those that result in a negative free-energy difference—that is, the products have less usable energy than the reactants—occur spontaneously. Such reactions, called exergonic reactions, release energy.

An endergonic reaction, which requires an input of energy, occurs only when coupled to an exergonic process. The energy released from the exergonic reaction must exceed the net energy cost of the endergonic reaction in order for the overall reaction to proceed. Hydrolysis of ATP, an exergonic reaction, is commonly used to drive energetically unfavorable metabolic reactions.

The energy currency of the cell is ATP (adenosine triphosphate). As cells use ATP for cellular reactions, ADP (adenosine diphosphate) is generated. Metabolic reactions in the cell can break down glucose and other macromolecules to release energy and regenerate ATP.

6.3 Metabolic Pathways and Enzymes

A metabolic pathway is a series of reactions that proceeds in an orderly, step-by-step manner. Enzymes speed reactions by lowering the energy of activation when they form a complex with their substrates.

The induced fit model explains how the substrate interacts with the active site of the enzyme. The three-dimensional shape of the enzyme is important. When denaturation occurs, an enzyme may not be able to interact as efficiently with the substrate. Most enzymes are proteins, but some, called ribozymes, may be RNA molecules.

Generally, enzyme activity increases as substrate concentration increases; once all active sites are filled, maximum reaction rate has been achieved. Environmental factors, such as temperature or pH, can affect the shape of an enzyme and therefore its function.

Many enzymes need cofactors or coenzymes to carry out their reactions. Vitamins are often used to build coenzymes, such as NAD⁺, FAD, and NADP⁺.

Enzyme inhibition regulates the activity of many metabolic pathways.

In noncompetitive inhibition, a molecule binds to an allosteric site on the enzyme and alters its activity.
In competitive inhibition, an inhibitor interferes with the binding of the substrate at the enzyme’s active site.

6.4 Oxidation–Reduction Reactions and Metabolism

The overall equation for photosynthesis is the opposite of that for cellular respiration. Both processes involve oxidation–reduction reactions. Redox reactions are a major way in which energy is transformed in cells.

OIL RIG
Oxidation Is Loss
Reduction Is Gain

During photosynthesis, carbon dioxide is reduced to glucose, and water is oxidized. Glucose formation requires energy, and this energy comes from the sun. Chloroplasts capture solar energy and convert it to the chemical energy of ATP molecules, which are used along with hydrogen atoms to reduce carbon dioxide to glucose.

During cellular respiration, glucose is oxidized to carbon dioxide, and oxygen is reduced to water. This reaction releases energy, which is used to synthesize ATP molecules in all types of cells. Energy flows through all living organisms.

Photosynthesis is a metabolic pathway in chloroplasts that converts solar energy to chemical energy stored in carbohydrates, and cellular respiration is a metabolic pathway completed in mitochondria that converts this energy into ATP. Eventually, the energy within ATP molecules becomes heat.

Cellular respiration is an aerobic process that requires oxygen and gives off carbon dioxide. Cellular respiration involves oxidation. The air we inhale contains the oxygen, and the food we digest after eating contains the carbohydrate glucose needed for cellular respiration.