5.1
什么是膜？
生活的一个关键特征是将外部环境与内部空间分开的能力。为此，细胞已经进化出半透膜，其调节生物分子的通过。此外，细胞膜定义了细胞的形状和与外部环境的相互作用。真核细胞膜还用于将内部空间区分成细胞器，包括细胞核，内质网和高尔基体的内膜结构。
膜主要由亲水头和两个疏水尾组成的磷脂组成。这些磷脂自组装成双层，…
Membranes are dynamic layers composed primarily of phospholipids, proteins, and carbohydrates that enclose a cell, forming selectively permeable boundaries and interior compartments. One component, phospholipids, are polar molecules that spontaneously arrange, often as a continuous bilayer about seven nanometers thick. The hydrophilic, or water loving, polar heads face the outside and inside of the cell.
The hydrophobic, or water fearing, non-polar tails line up in the middle to avoid exposure to water. While phospholipids provide the basic structure, it's how the other components, the membrane proteins and carbohydrates, associate with them that mostly contribute to the functional properties. Depending on the cell's needs, some proteins function as receptors to transduce signals in the cell's environment.
For example, a cell surface receptor binds a signal in the extracellular space and generates intracellular signals. Others connect as transporters to provide a path for different types of molecules to cross. Additional proteins may serve as structural links to connect the cytoskeleton to the extracellular matrix or adjacent cells.
Finally, carbohydrates will bind to either the phospholipids forming glycolipids or to proteins on the membranes, creating glycoproteins. Both serve important functions, such as binding hormones or neurotransmitters as well as allowing cells to recognize each other.
Q1: What are the main components that make up a cell membrane?
Cell membranes are composed of three primary components: phospholipids, proteins, and carbohydrates. Phospholipids form the basic structural framework, arranging into a bilayer about seven nanometers thick. Membrane integral and peripheral proteins provide functional properties like signaling and transport, while carbohydrates bind to lipids and proteins to enable cellular recognition and communication.
Q2: How are phospholipids arranged in a cell membrane?
Phospholipids spontaneously self-assemble into a bilayer structure with hydrophilic, water-loving polar heads facing outward toward the aqueous environment inside and outside the cell. Hydrophobic, water-fearing non-polar tails orient toward the membrane's center, avoiding water exposure. This arrangement creates a selectively permeable boundary approximately seven nanometers thick.
Q3: What role do membrane proteins play in cell function?
Membrane proteins serve multiple critical functions depending on cellular needs. Some function as receptors that bind external signals and generate intracellular responses. Others act as transporters, providing pathways for molecules to cross the membrane. Additional proteins serve as structural links connecting the cytoskeleton to the extracellular matrix or adjacent cells.
Q4: How do carbohydrates contribute to membrane function?
Carbohydrates bind to phospholipids forming glycolipids or to proteins forming glycoproteins on the membrane's exterior surface. These carbohydrate modifications enable cells to recognize each other and bind hormones or neurotransmitters. The unique patterns of glycoproteins and glycolipids allow immune cells to distinguish self from non-self.
Q5: What is the relationship between membrane structure and selective permeability?
The phospholipid bilayer's hydrophobic core prevents polar molecules from freely crossing the membrane, creating selective permeability. This arrangement allows polar molecules to interact with the hydrophilic heads but blocks passage through the hydrophobic interior. Membrane proteins provide alternative pathways for molecules that cannot cross the lipid bilayer directly.
Q6: How do cell membranes enable cellular compartmentalization?
In eukaryotic cells, membranes separate the external environment from internal space and create distinct compartments called organelles. The endomembrane system includes the nucleus, endoplasmic reticulum, and Golgi apparatus, each bounded by membranes. This compartmentalization allows cells to organize and regulate different biochemical processes in specialized spaces.
Q7: How do transmembrane receptor proteins communicate signals across the membrane?
Transmembrane receptor proteins span the entire cell membrane, with binding sites on the external surface. When a signaling molecule binds to the extracellular domain, the receptor undergoes a conformational change that generates an intracellular signal. This mechanism allows cells to respond to external chemical messages without the signaling molecule entering the cell.