The Cell

Chapter 3 Cells Vary in shape Measured in micrometers Differ in size Vary in shape Measured in micrometers Over 200 Types in Human Body

(a) Cells that connect body parts, form linings, or transport gases Erythrocytes Fibroblasts Epithelial cells (a) Cells that connect body parts, form linings, or transport gases Nerve cell Skeletal Muscle cell Smooth muscle cells (e) Cell that gathers information and control body functions (b) Cells that move organs and body parts Sperm Macrophage (f) Cell of reproduction Fat cell (c) Cell that stores nutrients (d) Cell that fights disease Figure 3.1

Cell Theory All living things are composed of cells They are the smallest units of life Cells come only from pre-existing cells Organismal functions depend on individual and collective cell functions Biochemical activities of cells are dictated by their specific subcellular structures

Chromatin Nuclear envelope Nucleolus Nucleus Smooth endoplasmic reticulum Plasma membrane Mitochondrion Cytosol Lysosome Centrioles Centrosome matrix Rough endoplasmic reticulum Ribosomes Golgi apparatus Secretion being released from cell by exocytosis Cytoskeletal elements • Microtubule • Intermediate filaments Peroxisome Figure 3.2

Cell Membrane Phospholipid bilayer (75%) outer limit of cell controls what moves in and out of cell selectively permeable Phospholipid bilayer (75%) water-soluble “heads” form surfaces water-insoluble “tails” form interior permeable to lipid-soluble substances Cholesterol stabilizes the membrane (20%) Lipid Rafts? Proteins receptors pores, channels, carriers enzymes CAMS self-markers

Cell Membrane

A protein (left) that spans the membrane (a) Transport A protein (left) that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Some transport proteins (right) hydrolyze ATP as an energy source to actively pump substances across the membrane. Figure 3.4a

(b) Receptors for signal transduction Signal A membrane protein exposed to the outside of the cell may have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone. The external signal may cause a change in shape in the protein that initiates a chain of chemical reactions in the cell. Receptor Figure 3.4b

(c) Attachment to the cytoskeleton and extracellular matrix (ECM) Elements of the cytoskeleton (cell’s internal supports) and the extracellular matrix (fibers and other substances outside the cell) may be anchored to membrane proteins, which help maintain cell shape and fix the location of certain membrane proteins. Others play a role in cell movement or bind adjacent cells together. Figure 3.4c

(d) Enzymatic activity Enzymes A protein built into the membrane may be an enzyme with its active site exposed to substances in the adjacent solution. In some cases, several enzymes in a membrane act as a team that catalyzes sequential steps of a metabolic pathway as indicated (left to right) here. Figure 3.4d

(e) Intercellular joining Membrane proteins of adjacent cells may be hooked together in various kinds of intercellular junctions. Some membrane proteins (CAMs) of this group provide temporary binding sites that guide cell migration and other cell-to-cell interactions. CAMs Figure 3.4e

(f) Cell-cell recognition Some glycoproteins (proteins bonded to short chains of sugars) serve as identification tags that are specifically recognized by other cells. Glycoprotein Figure 3.4f

Roles of Membrane Receptors Contact signaling—touching and recognition of cells Ex. normal development and immunity Chemical signaling—interaction between receptors and ligands (neurotransmitters, hormones and paracrines) G protein–linked receptors—ligand binding activates a G protein, affecting an ion channel or enzyme or causing the release of an internal second messenger, such as cyclic AMP

Cascade of cellular responses (metabolic and structural changes) 1 Ligand (1st messenger) binds to the receptor. 2 The activated receptor binds to a G protein and activates it. 3 Activated G protein activates (or inactivates) effector protein (e.g., an enzyme) by causing its shape to change. Extracellular fluid Effector protein (e.g., an enzyme) Ligand Receptor 4 Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell Inactive 2nd messenger G protein GDP Active 2nd messenger 5 Second messengers activate other enzymes or ion channels 6 Activated kinase enzymes Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various cell responses. Cascade of cellular responses (metabolic and structural changes) Intracellular fluid Figure 3.16

Intercellular Junctions Tight junctions Desmosomes Gap junctions

(a) Tight junctions: Impermeable junctions prevent molecules Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Interlocking junctional proteins Intercellular space (a) Tight junctions: Impermeable junctions prevent molecules from passing through the intercellular space. Figure 3.5a

(b) Desmosomes: Anchoring junctions bind adjacent cells together Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Plaque Intermediate filament (keratin) Linker glycoproteins (cadherins) (b) Desmosomes: Anchoring junctions bind adjacent cells together and help form an internal tension-reducing network of fibers. Figure 3.5b

(c) Gap junctions: Communicating junctions allow ions and small mole- Plasma membranes of adjacent cells Microvilli Intercellular space Basement membrane Intercellular space Channel between cells (connexon) (c) Gap junctions: Communicating junctions allow ions and small mole- cules to pass from one cell to the next for intercellular communication. Figure 3.5c

Cytoplasmic Organelles Endoplasmic Reticulum Rough ER Smooth ER Ribosomes Free- Cytosol and Mito. Membrane Bound- Integral or Export

(a) Diagrammatic view of smooth and rough ER Smooth ER Nuclear envelope Rough ER Ribosomes (a) Diagrammatic view of smooth and rough ER Figure 3.18a

Cytoplasmic Organelles Golgi apparatus Lysosomes Peroxisomes Mitochondria Endosymbiotic bacteria????

Golgi at Work

Cytoplasmic Organelles Centrosome

Cytoplasmic Organelles Cilia- produced from centrosome short hair-like projections propel substances on cell surface Flagellum long tail-like projection provides motility to sperm

Cellular Extensions Microvilli Fingerlike extensions of plasma membrane Increase surface area for absorption

Microvillus Actin filaments Terminal web Figure 3.28

Cytoplasmic Organelles Vesicles membranous sacs store substances Microfilaments and microtubules protein rods and tubes support cytoplasm allows for movement of organelles

Cell Nucleus control center of cell nuclear envelope nucleolus chromatin

Chromatin (condensed) Nuclear pores Nuclear envelope Nucleus Chromatin (condensed) Nucleolus Cisternae of rough ER (a) Figure 3.29a

Surface of nuclear envelope. Fracture line of outer membrane Nuclear pores Nucleus Nuclear lamina. The netlike lamina composed of inter- mediate filaments formed by lamins lines the inner surface of the nuclear envelope. Nuclear pore complexes. Each pore is ringed by protein particles. (b) Figure 3.29b