Objectives
- Perceive the inseparability of structure and function in living organisms.
- Know the names and functions of cytoplasmic components.
- Know the subunits of each cytoplasmic component and their roles in its function.
- List the cell’s functions and explain the role of each cytoplasmic component in each function.
- Recognize a cell’s cytoplasmic components in a micrograph and hence predict the cell’s function(s).
- Predict which structures are present in a cell from its function.
- Predict the functional deficit(s) that accompany specific structural aberrations.
- Predict the cytoplasmic component(s) likely to be involved in a functional deficit.
- Explain and give examples of cell differentiation.
MAX-Yield™ Study Questions
1. Compare prokaryotic and eukaryotic cells in terms of the presence of a nucleus, histones, and cytoplasmic organelles, as well as overall size (I.B).
3. List the major biochemical constituents of cell membranes and sketch their organization as described in the fluid mosaic model of membrane structure (II.A and B; Fig. 2–1).
4. Explain why a membrane’s phospholipid bilayer has a trilaminar appearance in transmission electron micrographs (TEMs) (II.A.1).
5. Compare the locations of peripheral and integral membrane proteins in relation to the lipid bilayer and the methods required to isolate them from cell membranes (II.A.2.a and b; Fig. 2–1).
6. Name three membrane receptor types that transduce signals across the plasma membrane. Compare them in terms of the number of passes they make through the plasma membrane, the effects of ligand binding on their conformation, and the role of enzymes in signal transduction (II.C.2.a–c; Figs. 2–2, 2–3).
7. Explain how signal transduction by steroid hormone receptors differs from that by membrane receptors in terms of ligand type, receptor location, and binding between the receptor and DNA (II.C.2.a–d).
8. Compare phagocytosis and pinocytosis in terms of the way that vacuoles or vesicles are formed, the types of materials endocytosed, and the relative size of the vacuoles and vesicles (II.C.3.a and b).
9. List the steps in receptor-mediated endocytosis, beginning with ligand-receptor binding and ending with receptor return to the cell surface (II.C.3.c; Fig. 2–4).
10. Compare organelles and cytoplasmic inclusions in terms of the presence of limiting membranes, enzyme content, active or passive role in cell function, and their relative constancy in the cytoplasm (III).
Outer mitochondrial membrane
Inner mitochondrial membrane
Cristae
Inner membrane (F1) subunits
Intermembrane space (III.A.1.c)
Intercristal space
Intracristal space
Matrix
Matrix granules
ATP synthase (III.A.1.b)
Citric acid cycle enzymes
Electron transport system (III.A.1.b)
13. Name the substances and structures in the mitochondrial matrix that resemble those in prokaryotic cells and duplicate eukaryotic cell components found elsewhere in eukaryotic cells (III.A.1.d).
14. Compare the mitochondrial cristae of most cells with those of steroid-secreting cells and cells with a high metabolic rate (III.A.1.b).
17. List the major biochemical and structural components of ribosomes, and name their sites of synthesis and assembly (III.B.1).
20. What is the difference between the functions of free polyribosomes and those of polyribosomes attached to the rough endoplasmic reticulum (RER) (III.B.2)?
21. Compare the RER with smooth endoplasmic reticulum (SER) in terms of the presence of ribosomes (III.C.1.a and 2.a), shape of cisternae (III.C.1.a and 2.a), functions (III.C.1.b and 2.b), and cell types in which each is typically abundant; include examples (III.C.1.c and 2.c).
22. List the steps in RER-associated protein synthesis and posttranslational modification, beginning with ribosome attachment to mRNAs for proteins destined for secretion and ending with the transfer vesicle budding for transport to the Golgi complex (II.D; III.B.2, C.1.a and b).
Cisternae
Cis face
Trans face
Transfer vesicles
Condensing vacuoles
Secretory granules
Site of selective osmium deposition
25. Compare primary and secondary lysosomes in terms of size, appearance, and contents (III.F.1 and 2).
26. Compare primary lysosomes (III.F) and peroxisomes (III.G) in terms of size, content, appearance, and function.
28. Trace the steps in the ingestion and digestion of extracellular materials, beginning with endocytosis (II.C.3) and ending with residual body formation (III.E and F.1–3).
29. Describe how an inherited deficiency or lack of a particular lysosomal enzyme affects the intracellular concentrations of substrates of that enzyme (III.F).
Diameters (III.I.1.a and 2.a)
Major protein components (III.I.1.a and 2.a)
Functions (III.I.1.b and 2.b)
Subunit polymerization and depolymerization (III.I.1.a,b and 2.a,b)
Contractile capacity (III.I.1.b and 2.b)
Location in the cell (III.I.1.c and 2.c)
Associated motor proteins (III.I.1.b and 2.b)
31. Compare dynein and kinesin in terms of their typical direction of movement along microtubules and their association with the RER and Golgi complex (III.I.1.b).
32. Explain the main function of intermediate filaments and give their diameter in nanometers (III.I.3.a and b).
33. List five intermediate filament protein types and the cell types in which each is found (III.I.3.a).
34. Compare centrioles, basal bodies, cilia, and flagella in terms of microtubule number and organization (III.J–N).
36. Compare cilia and flagella in terms of their structure, length, motion, and typical number per cell (III.J–L).
37. Explain the concept of membrane trafficking and the role of SNAREs and coat proteins in the process (II.D).
Synopsis
Cells are the structural and functional units of life (and of disease processes) in all tissues, organs, and organ systems. Each cell’s capabilities and limitations are implicit in its structure.
There are two basic cell types. Prokaryotes are small, single-celled organisms (e.g., bacteria) that lack a nuclear envelope, histones, and membranous organelles. Eukaryotic cells exist primarily as components of multicellular organisms. This chapter covers the basic structural and functional features of eukaryotic cells. Specific human cell types are described in later chapters.
Eukaryotic cells have three major components:
Cell membranes (II) separate a cell from its environment and form distinct functional compartments (nucleus, organelles) in the cell. The outer cell membrane is called the plasma membrane, or plasmalemma.
The cytoplasm (III) surrounds the nucleus and is enclosed by the plasma membrane. It contains the structures and substances that decode the instructions of DNA and carry on the cell’s activities.
The membrane-limited nucleus (Chapter 3) contains the DNA, which carries the genetic code for protein synthesis and thus for all cell activities. It also has components that determine which parts of the genetic code are used and that deliver coded information to the cytoplasm.
Three activities basic to living organisms are nourishment, growth, and reproduction. Functions directed toward these activities are described in this chapter and in Chapter 3. More specialized cell functions receive detailed treatment in subsequent chapters.
Lipids in cell membranes include phospholipids (Fig. 2–1, E), sphingolipids, and cholesterol (Fig. 2–1, A). Phospholipids (e.g., lecithin) are the most abundant form. Each phospholipid molecule has a polar (hydrophilic), phosphate-containing head group (Fig. 2–1, G) and a nonpolar (hydrophobic) pair of fatty acid tails (Fig. 2–1, F). Membrane phospholipids are arranged in a bilayer, with their tails directed toward one another at the center of the membrane. In electron micrographs (EMs) of osmium-stained tissue, a single membrane, or unit membrane, has two dark outer lines with a lighter layer between them. This trilaminar appearance reflects the deposition of reduced osmium on the hydrophilic head groups.
Proteins may contribute more than 50% of membrane weight. Most membrane proteins are globular and belong to one of two groups:
Integral membrane proteins (Fig. 2–1, C and D) are tightly lodged in the lipid bilayer; detergents are required to extract them. They are folded, with hydrophilic amino acids in contact with the membrane phospholipids’ phosphate groups and hydrophobic amino acids in contact with the fatty acid tails. Some protrude from only one membrane surface (Fig. 2–1, D). Others, called transmembrane proteins (Fig. 2–1, C), penetrate the entire membrane and protrude from both sides. Some transmembrane proteins, such as protein-3-tetramer, are hydrophilic channels for the passage of water and water-soluble materials through hydrophobic regions. Some transmembrane proteins pass multiple times through the bilayer to form channels and receptors. Cryofracture preparations often split plasma membranes through the hydrophobic region, between the ends of the phospholipids’ fatty acid tails (Fig. 2–1). Most integral proteins exposed in this way remain in the side closest to the cytoplasm, termed the P (protoplasmic) face. The membrane half nearest to the environment, the E (ectoplasmic) face, usually appears smoother.
Peripheral membrane proteins (Fig. 2–1, H) are ionically associated with the inner or outer membrane surface and are released in high-salt solutions; some are globular, some filamentous. In erythrocytes, examples on the cytoplasmic surface include adapter proteins like spectrin, which helps maintain membrane integrity, and ankyrin, which links spectrin to protein-3-tetramer.
Carbohydrates occur on plasma membranes mainly as oligosaccharide moieties of glycoproteins (Fig. 2–1, B) and glycolipids. Membrane oligosaccharides have a characteristic branching structure and project from the cell’s outer surface, forming a surface coat called the glycocalyx that participates in cell adhesion and recognition.
The fluid mosaic model describes biologic membranes as “protein icebergs in a lipid sea”. Integral proteins exhibit lateral mobility and may rearrange through their association with peripheral proteins, cytoskeletal filaments within the cell (III.I), membrane components of adjacent cells, and extracellular matrix components. Integral proteins may diffuse to and accumulate in one membrane region. Membrane asymmetry refers to differences in chemical composition between the bilayer’s inner and outer halves. Oligosaccharides occur only on the plasma membrane’s outer surface. Phospholipid asymmetries also occur. The outer half has more phosphatidyl choline and sphingomyelin and the inner half has more phosphatidyl serine and phosphatidyl ethanolamine.
Selective permeability. Cell membranes separate the internal and external environments of a cell or organelle, preventing the intrusion of harmful substances, the dispersion of macromolecules, and the dilution of enzymes and substrates. This selective permeability is essential for maintaining the functional steady state, or homeostasis, required for cell survival. Homeostatic mechanisms attributable to cell membranes maintain optimal intracellular concentrations of ions, water, enzymes, and substrates. Three mechanisms allow selected molecules to cross membranes.
Passive diffusion. Some substances (e.g., water and lipids) can cross the membrane in either direction following a concentration gradient, without the cell expending energy.
Facilitated diffusion. Some molecules (e.g., glucose) are helped across the membrane by a membrane component. This facilitated diffusion is often unidirectional, but it follows a concentration gradient and requires no energy.
