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Questions and Answers
What is the estimated number of cells in the average adult human body?
What is the term for the process by which cells become specialized in function?
What is the primary function of the plasma membrane?
What is the term for the proteins attached to the phospholipid bilayer?
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What is the term for the movement of molecules through the plasma membrane?
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What is the term for the proteins that span the phospholipid bilayer?
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What is the typical thickness of membranes?
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What is the primary component of the phospholipid bilayer?
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What is the primary function of cholesterol in the plasma membrane?
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What is the term for the carbohydrates attached to phospholipids and proteins?
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Which type of protein is incorporated directly within the lipid bilayer?
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What is the primary mechanism of transport for small, nonpolar molecules across the plasma membrane?
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What is the approximate proportion of proteins in the plasma membrane by weight?
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What is the role of transmembrane proteins in the plasma membrane?
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What is the orientation of the hydrophobic fatty acid chains in the phospholipid bilayer?
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Which type of molecule can diffuse through the membrane readily?
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What is the purpose of channel-linked receptors?
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What type of receptors bind to hydrophilic signaling molecules?
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What occurs when a ligand binds to a G-protein–coupled receptor?
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What type of protein is usually associated with hydrophilic signaling molecules?
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What is the function of enzymatic receptors?
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What are cells that bear receptors for a specific ligand referred to as?
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Match the following components with their functions in the plasma membrane:
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Match the following characteristics with the phospholipid bilayer:
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Study Notes
The Plasma Membrane – Structure
- Membranes range from 7.5 to 10 nm in thickness and are only visible in the electron microscope.
- Phospholipids are amphipathic, consisting of two nonpolar fatty acid chains linked to a charged polar head with a phosphate group.
- Phospholipids are most stable when organized into a double layer (bilayer) with hydrophobic fatty acid chains in the middle and hydrophilic polar head groups contacting water.
- Cholesterol molecules insert among phospholipid fatty acids, restricting their movements and modulating membrane fluidity.
Lipids & Cholesterol in the Membrane
- Proteins make up ~50% by weight in the plasma membrane.
- Integral proteins are incorporated directly within the lipid bilayer, while peripheral proteins are bound to one of the two membrane surfaces, particularly on the cytoplasmic side.
The Plasma Membrane – Transmembrane Proteins & Membrane Transport
- The plasma membrane is the site where materials are exchanged between the cell and its environment.
- Small molecules cross the membrane through diffusion, which transports nonpolar molecules directly through the lipid bilayer.
- Lipophilic (fat-soluble) molecules diffuse through membranes readily, while water diffuses very slowly.
Cell Differentiation
- The average adult human body consists of nearly 40 trillion cells, which exist as hundreds of histologically distinct cell types, all derived from the zygote.
- The first zygotic cellular divisions produce cells called blastomeres, which give rise to all tissue types of the fetus.
- Most cells of the fetus undergo a specialization process called differentiation, where they express specific genes that mediate cytoplasmic activities, becoming efficiently organized in tissues with specialized functions.
The Plasma Membrane – Roles/Functions
- The plasma membrane functions as a selective barrier regulating the passage of materials into and out of the cell.
- One important role of the cell membrane is to keep constant the ion content of cytoplasm, which differs from that of the extracellular fluid.
The Plasma Membrane – Signal Reception & Transduction
- Each cell type in the body contains a distinctive set of cell surface and cytoplasmic receptor proteins that enable it to respond to a complementary set of signaling molecules in a specific, programmed way.
- Receptors for hydrophilic signaling molecules, including polypeptide hormones and neurotransmitters, are usually transmembrane proteins in the plasmalemma of target cells.
- Three important functional classes of receptors are:
- Channel-linked receptors that open associated channels upon ligand binding to promote transfer of molecules or ions across the membrane.
- Enzymatic receptors that induce catalytic activity in associated peripheral proteins upon ligand binding.
- G-protein–coupled receptors that stimulate associated G-proteins upon ligand binding, which then bind guanine nucleotide GTP and are released to activate other cytoplasmic proteins.
Cell Structure and Differentiation
- The average adult human body consists of nearly 40 trillion cells.
- These cells exist as hundreds of histologically distinct cell types, all derived from the zygote.
- The process of cell differentiation leads to cells specializing in specific activities and changing their shape accordingly.
The Plasma Membrane
- The plasma membrane (cell membrane or plasmalemma) consists of phospholipids, cholesterol, and proteins.
- The membrane has oligosaccharide chains covalently linked to many of the phospholipids and proteins.
- The plasma membrane defines the outer limit of the cell and regulates the passage of materials into and out of the cell.
Roles and Functions of the Plasma Membrane
- The plasma membrane functions as a selective barrier, facilitating the transport of specific molecules.
- It keeps the ion content of the cytoplasm constant, which differs from that of the extracellular fluid.
- Membrane proteins perform recognition and signaling functions, playing a key role in cell interactions.
Structure of the Plasma Membrane
- The membrane ranges from 7.5 to 10 nm in thickness and is only visible in the electron microscope.
- Membrane phospholipids are amphipathic, consisting of nonpolar fatty acid chains linked to a charged polar head.
- Phospholipids are most stable when organized into a double layer (bilayer) with the hydrophobic fatty acid chains located in a middle region away from water.
Lipids and Cholesterol in the Membrane
- Molecules of cholesterol, a sterol lipid, insert at varying densities among the closely-packed phospholipid fatty acids, restricting their movements and modulating the fluidity of all membrane components.
Proteins in the Membrane
- Proteins are major constituents of membranes (~50% by weight in the plasma membrane).
- Integral proteins are incorporated directly within the lipid bilayer, whereas peripheral proteins are bound to one of the two membrane surfaces, particularly on the cytoplasmic side.
Transmembrane Proteins and Membrane Transport
- The plasma membrane is the site where materials are exchanged between the cell and its environment.
- Small molecules cross the membrane by diffusion, channels, and carrier proteins.
- Diffusion transports small, nonpolar molecules directly through the lipid bilayer.
- Channels are multipass proteins forming transmembrane pores through which ions or small molecules pass selectively.
- Carrier proteins bind small molecules and translocate them across the membrane via conformational changes.
- Membrane pumps are enzymes engaged in active transport, utilizing energy from the hydrolysis of ATP to move ions and other solutes across membranes.
Signal Reception and Transduction
- Cells in a multicellular organism communicate with one another to regulate tissue and organ development, control growth and division, and coordinate their functions.
- Cells form communicating gap junctions that couple the cells and allow exchange of ions and small molecules.
- Cells use different types of receptors to detect and respond to various extracellular molecules and physical stimuli.
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Description
Learn about the structure and composition of the plasma membrane, including phospholipid bilayers and cholesterol molecules.