¿Cuál de los siguientes enunciados describe mejor la función de los receptores hormonales tipo nuclear?

Regulan la transcripción genética en el núcleo celular

Inhiben la liberación de hormonas en el torrente sanguíneo

Aumentan la producción de hormonas en la glándula suprarrenal

Actúan exclusivamente en la membrana celular

¿Qué tipo de moléculas suelen unirse a los receptores hormonales tipo nuclear?

Hormonas lipofílicas como esteroides

Hormonas hidrofílicas como adrenalina

Proteínas de señalización intracelular

¿Cuál es el mecanismo de acción principal de los receptores hormonales tipo nuclear?

Forman un complejo con el ADN para activar o desactivar la transcripción de genes

Transportan hormonas en la sangre

Descomponen hormonas en el citoplasma

Modifican la estructura de la membrana celular

¿Cuál de las siguientes afirmaciones sobre los receptores hormonales tipo nuclear es correcta?

Se encuentran mayormente en el citoplasma o en el núcleo de la célula

Son siempre proteínas de membrana

Solo se encuentran en la membrana celular

¿Qué efecto tiene la unión de una hormona a un receptor hormonal tipo nuclear?

Modifica la expresión génica en la célula

Inhibe la actividad de las enzimas en la mitocondria

Causa la liberación inmediata de neurotransmisores

Aumenta la concentración de calcio en el citosol

¿Qué tipo de respuesta celular generalmente se asocia con los receptores hormonales tipo nuclear?

Respuestas a largo plazo como el desarrollo y la diferenciación

Liberación rápida de energía

Respuestas rápidas como la contracción muscular

Respuestas inmediatas a impulsos nerviosos

¿Cuál es la principal característica distintiva de los receptores hormonales tipo nuclear en comparación con los receptores de membrana?

Actúan como factores de transcripción en el núcleo

Siempre están presentes en la superficie celular

Solo responden a hormonas hidrofílicas

No requieren un ligando para activarse

¿Cuál es el efecto de los glucocorticoides sobre las células a través de sus receptores hormonales tipo nuclear?

Incrementan la gluconeogénesis en el hígado

Inhiben la captación de glucosa en los músculos

Disminuyen la producción de insulina

Aumentan la sensibilidad a la insulina

Receptores Hormonales: Tipo Nuclear

Receptores Hormonales: Superficie Celular

The plasma membrane (also called the cell membrane) has several functions. For example, it allows only certain molecules to enter or leave the cell. It separates internal metabolic reactions from the external environment. In addition, the plasma membrane allows the cell to excrete wastes and to interact with its environment. Membrane Lipids The plasma membrane, as well as the membranes of cell organelles, is made primarily of phospholipids. Phospholipids have a polar, hydrophilic (“water-loving”) phosphate head and two nonpolar, hydrophobic (“water-fearing”) fatty acid tails. Water molecules sur- round the plasma membrane. The phospholipids line up so that their heads point outward toward the water and their tails point inward, away from water. The result is a double layer called a phospholipid bilayer, as shown in Figure 4-10. The cell membranes of eukaryotes also contain lipids, called sterols, between the tails of the phospho- lipids. The major membrane sterol in animal cells is cholesterol. Sterols in the plasma membrane make the membrane more firm and prevent the membrane from freezing at low temperatures. SECTION 3 OBJECTIVES ● Describe the structure and function of a cell’s plasma membrane. ● Summarize the role of the nucleus. ● List the major organelles found in the cytosol, and describe their roles. ● Identify the characteristics of mitochondria. ● Describe the structure and function of the cytoskeleton. VOCABULARY phospholipid bilayer chromosome nuclear envelope nucleolus ribosome mitochondrion endoplasmic reticulum Golgi apparatus lysosome cytoskeleton microtubule microfilament cilium flagellum centriole Cell membranes are made of a phospholipid bilayer. Each phospholipid molecule has a polar “head” and a two-part nonpolar “tail.” FIGURE 4-10 Copyright © by Holt, Rinehart and Winston. All rights reserved. 78 CHAPTER 4 OUTSIDE OF CELL INSIDE OF CELL 1. Cell-surface marker: Glycoprotein that identifies cell type 3. Enzyme: Assists chemical reactions inside the cell 2. Receptor protein: Recognizes and binds to substances outside the cell 4. Transport protein: Helps substances move across cell membrane Carbohydrate portion Protein portion Phospholipid heads Phospholipid tails Phospholipid Cholesterol bilayer Membrane Proteins Plasma membranes often contain specific proteins embedded within the lipid bilayer. These proteins are called integral proteins. Figure 4-11 shows that some integral proteins, such as cell surface markers, emerge from only one side of the membrane. Others, such as receptor proteins and transport proteins, extend across the plasma membrane and are exposed to both the cell’s interior and exterior environments. Proteins that extend across the plasma membrane are able to detect environmental signals and transmit them to the inside of the cell. Peripheral proteins, such as the enzyme shown in Figure 4-11, lie on only one side of the membrane and are not embedded in it. As Figure 4-11 shows, integral proteins exposed to the cell’s external environment often have carbohydrates attached. These carbohydrates can act as labels on cell surfaces. Some labels help cells recognize each other and stick together. Viruses can use these labels as docks for entering and infecting cells. Integral proteins play important roles in actively transporting molecules into the cell. Some act as channels or pores that allow certain substances to pass. Other integral proteins bind to a mol- ecule on the outside of the cell and then transport it through the membrane. Still others act as sites where chemical messengers such as hormones can attach. Fluid Mosaic Model A cell’s plasma membrane is surprisingly dynamic. Scientists describe the cell membrane as a fluid mosaic. The fluid mosaic model states that the phospholipid bilayer behaves like a fluid more than it behaves like a solid. The membrane’s lipids and pro- teins can move laterally within the bilayer, like a boat on the ocean. As a result of such lateral movement, the pattern, or “mosaic,” of lipids and proteins in the cell membrane constantly changes.

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