Cells: the Living Units

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PowerPoint® Lecture Slides Introduction to Cells prepared by Leslie Hendon University of Alabama, Birmingham • Several important scientists made discoveries about cells • Robert Hooke C H A P T E R 2 • Matthias Schleiden and Theodor Schwann Part 1 • Rudolf Virchow Cells: The • Cells—the smallest living units in our bodies • Organelles—“little organs”—carry on Living Units essential functions of cells

Introduction to Cells Structure of a Generalized Cell Chromatin Nuclear envelope

Nucleolus Nucleus

• Cells have three main components Plasma Smooth endoplasmic membrane • Plasma membrane—the outer boundary reticulum Cytosol

• Cytoplasm—contains most organelles Mitochondrion • Nucleus—controls cellular activities Lysosome Centrioles Rough Centrosome endoplasmic matrix reticulum Ribosomes

Golgi apparatus

Cytoskeletal Secretion being elements released from cell Microtubule by exocytosis Intermediate filaments Peroxisome

The Plasma Membrane The Plasma Membrane

Polar head of Plasma membrane (or plasmalemma) defines the phospholipid • molecule extent of the cell: separates intracellular fluid from Extracellular fluid (watery environment) extracellular fluid (watery inside from watery outside with a Glycolipid Cholesterol ‘fatty’ barrier) Nonpolar tail Glycoprotein of phospholipid molecule

Carbohydrate • Structure of the cell membrane of glycocalyx Bimolecular Fluid mosaic model (lipid bilayer) lipid layer • containing proteins • Types of membrane proteins Outward- facing layer of • Integral proteins—firmly imbedded in, or attached to lipid phospholipids bilayer Inward-facing layer of Short chains of carbohydrates attach to integral proteins phospholipids • Integral Cytoplasm proteins • Form the glycocalyx (watery environment) Filament of • Peripheral proteins—attach to membrane surface (in or out) cytoskeleton • Support plasma membrane from the cytoplasmic side Peripheral proteins

1 The Plasma Membrane Membrane Transport: moving substances across the cell membrane... These processes include: • Functions – relate to location at the interface of cell’s exterior and interior • Simple diffusion—tendency of molecules to move down their concentration gradient through the lipid bilayer • Osmosis—diffusion of water molecules across a • Provides barrier against substances outside cell membrane through small channels • Some plasma membrane molecules act as receptors • Facilitated diffusion—movement of molecules down • Determines which substances enter or leave the cell their concentration gradient through an integral protein • Membrane is referred to as being “selectively channel permeable” • Active transport—integral proteins move molecules across the plasma membrane against their concentration gradient • Endo- and exocytosis

Simple diffusion; osmosis; facilitated diffusion; and active transport: Endocytosis

Extracellular fluid Water Lipid- molecules Solutes Mechanism by which particles (chunks soluble • Endocytosis: solutes of stuff) enter cells • Phagocytosis—“cell eating”

Lipid • Pinocytosis—“cell drinking” bilayer • Receptor-mediated endocytosis • Plasma proteins bind to certain molecules

Cytoplasm • Invaginates and forms a coated pit

(a) Simple diffusion of (b) Osmosis, diffusion (c) Transport via integral proteins Pinches off to become a coated vesicle fat-soluble molecules of water through the A protein (left) that spans the • directly through the lipid bilayer. membrane may provide a phospholipid bilayer. channel across the membrane • NOTE: This is the method by which insulin and that is selective for a particular solute. Some transport proteins cholesterol and some viruses enter cells! (right) use ATP as an energy source to actively pump substances across the membrane.

Three Types of Endocytosis Exocytosis

(a) Phagocytosis (b) Pinocytosis The cell engulfs a large The cell “gulps” drops particle by forming pro- of extracellular fluid jecting pseudopods (”false containing solutes feet”) around it and en- into tiny vesicles. No • Exocytosis—a mechanism that moves closing it within a membrane receptors are used, sac called a phagosome. The so the process is phagosome then combines nonspecific. Most substances out of the cell with a lysosome, and its vesicles are protein- contents are digested. coated. Vesicle may or may not be • Substance is enclosed in a vesicle protein-coated but has receptors capable of binding Vesicle The vesicle migrates to the plasma membrane to microorganisms or solid • particles. • Proteins from the vesicles (v-SNAREs) bind with Phagosome

(c) Receptor-mediated membrane proteins (t-SNAREs) endocytosis Extracellular substances bind to specific receptor proteins in • The lipid layers from both membranes bind, and the regions of protein-coated pits, enabling the cell to ingest vesicle releases its contents to the outside of the cell and concentrate specific substances in protein-coated vesicles. The ingested substance may simply be Vesicle released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma Receptor recycled membrane in vesicles. to plasma membrane

2 Exocytosis The Cytoplasm

Extracellular Plasma membrane (a) The process fluid SNARE (t-SNARE) of exocytosis • Cytoplasm—lies internal to plasma membrane

2 There, proteins • Consists of cytosol, organelles, and inclusions, in Fused at the vesicle 1 The membrane- v- and Secretory surface (v-SNAREs) watery environment bound vesicle t-SNAREs bind with t-SNAREs vesicle Vesicle SNARE migrates to the (plasma membrane (v-SNARE) plasma membrane. proteins). • Cytosol Molecule to be secreted Jelly-like watery fluid in which other cellular elements Cytoplasm • are suspended Fusion pore formed • Consists of water with dissolved ions (salts) and

3 The vesicle and 4 Vesicle contents enzymes plasma membrane are released to the fuse and a pore cell exterior. opens up.

Cytoplasmic Organelles The Endoplasmic Reticulum and Ribosomes • Ribosomes—constructed of proteins and ribosomal RNA; not surrounded by a membrane • Site of protein synthesis • Assembly of proteins is a process called translation • Are the “assembly line” of the manufacturing plant • Endoplasmic reticulum—“network within the Smooth ER

cytoplasm”– tow types of ER: Nuclear envelope • Rough ER—ribosomes stud the external surfaces, so protein synthesis occurs here Rough ER

• Smooth ER—consists of tubules in a branching Ribosomes Cisternae network; No ribosomes are attached, therefore no (a) Diagrammatic view of smooth and rough ER (b) Electron micrograph of smooth and protein synthesis—but lipid and carb synthesis rough ER (85,000!)

Cytoplasmic Organelles Golgi Appartus

• Golgi apparatus—a stack of three to 10 disk-shaped envelopes New vesicles forming • Sorts products of rough ER and sends them to proper Cis face— “receiving” side of Transport vesicle destination from rough ER Golgi apparatus Cisternae • Products of rough ER move through the Golgi from the convex (cis) to the concave (trans) side New vesicles Is the “packaging and shipping” division of the forming • Transport manufacturing plant vesicle from trans face Trans face— “shipping” side of Secretory vesicle Golgi apparatus Transport vesicle Golgi apparatus from the Golgi apparatus (a) Many vesicles in the process of pinching off (b) Electron micrograph of the Golgi from the membranous Golgi apparatus apparatus (90,000!)

3 The sequence of events from protein More Cytoplasmic Organelles... synthesis on the rough ER to the final distribution of these proteins (1 of 2). • Lysosomes—membrane-walled sacs containing digestive enzymes Rough ER ER membrane Phagosome Plasma membrane • Digest unwanted substances Proteins in cisterna 1 Protein-containing vesicles pinch off rough ER and migrate to fuse with membranes Pathway C: of Golgi apparatus. Lysosome containing acid • Peroxisomes—membrane-walled sacs of oxidase hydrolase 2 Proteins are enzymes enzymes modified within the Vesicle becomes Golgi compartments. lysosome • Enzymes neutralize free radicals and break down

3 Proteins are then poisons packaged within different vesicle types, Secretory Golgi • Break down long chains of fatty acids depending on their vesicle Pathway B: ultimate destination. apparatus Vesicle membrane to be incorporated • Are numerous in the liver and kidneys into plasma Pathway A: membrane Vesicle contents Secretion by exocytosis Are the toxic waste removal system destined for Extracellular fluid • exocytosis

Mitochondria Mitochondria

• Mitochondria—generate most of the cell’s

energy; most complex organelle Lysosomes • More abundant in energy-requiring cells, like muscle cells and sperm • “Power plant” of the cell

Light areas are regions where materials are being digested.

More Cytoplasmic Organelles Cytoskeleton: Microfilaments (a) Microfilaments

Strands made of spherical protein • Cytoskeleton—“cell skeleton”—an elaborate subunits called actins network of rods: for support and communication Actin subunit • Contains three types of rods: 7 nm • Microtubules—cylindrical structures made of proteins • Microfilaments—filaments of contractile protein actin • Intermediate filaments—protein fibers

Microfilaments form the blue network surrounding the pink nucleus in this photo.

4 Cytoskeleton: Intermediate filaments Cytoskeleton: Microtubules (b) Intermediate filaments (c) Microtubules

Tough, insoluble protein fibers Hollow tubes of spherical protein constructed like woven ropes subunits called tubulins Tubulin subunits Fibrous subunits

10 nm 25 nm

Intermediate filaments form the purple Microtubules appear as gold networks batlike network in this photo. surrounding the cells’ pink nuclei in this photo. Copyright © 2011 Pearson Education, Inc. Figure 2.11b Copyright © 2011 Pearson Education, Inc. Figure 2.11c

More Cytoplasmic Organelles... Cytoplasmic Inclusions

• Centrosomes and centrioles • Temporary structures • Not present in all cell types • May consist of pigments, crystals of protein, and food • Centrosome—a spherical structure in the cytoplasm stores; examples ... • Composed of centrosome matrix and centrioles • Lipid droplets—found in liver cell and fat cells • Centrioles—paired cylindrical bodies • Glycosomes—store sugar in the form of glycogen • Consists of 27 short microtubules • Act in forming cilia • Necessary for karyokinesis (nuclear division)

The Nucleus The Nucleus

Surface of nuclear envelope. • The nucleus—“little nut” or “kernel”: control center of cell • DNA directs the cell’s activities Fracture Nucleus is approximate 5!m in diameter line of outer • membrane

• Nuclear envelope—two parallel membranes separated Nuclear pores by fluid-filled space Nuclear envelope Nucleus Chromatin • Nuclear pores penetrate the nuclear envelope (condensed) Nucleolus • Pores allow large molecules to pass in and out of the nucleus • Nucleolus—“little nucleus”—in the center of the nucleus

• Contains parts of several chromosomes Nuclear pore complexes. Each Nuclear lamina. The netlike pore is ringed by protein particles lamina composed of intermediate filaments formed by lamins lines • Site of ribosome subunit assembly the inner surface of the nuclear Cisternae of envelope. rough ER (b) (a)

5 Chromatin and Chromosomes Chromatin and Chromosomes Hydrogen bond • DNA (NOT a protein but another type of macromolecule Nucleotides called a nucleic acid) is a double helix is composed of The double-helix four subunits: structure of the • Thymine (T), adenine (A), cytosine (C), and guanine (G) DNA molecule: • DNA is packed with proteins Deoxyribose • DNA plus the proteins form chromatin Sugar-phosphate sugar backbone Phosphate • Each cluster of DNA and histone proteins is a nucleosome

Adenine (A) Thymine (T) Cytosine (C) Guanine (G)

Chromatin and Chromosomes

1 DNA double helix (2-nm • Extended chromatin diameter)

Is the active region of DNA where DNA’s genetic code Histones • 2 Chromatin (“beads on a string”) structure is copied onto mRNA (transcription) with nucleosomes • Condensed chromatin Linker DNA Tightly coiled nucleosomes Nucleosome (10-nm diameter; • eight histone proteins wrapped by two winds of the DNA double (a) • Inactive form of chromatin helix) • Chromosomes—highest level of organization of chromatin 3 Tight helical fiber (30-nm diameter) • Contains a long molecule of DNA 4 Looped domain structure (300-nm diameter) 5 Chromatid • 46 chromosomes (arranged in 23 pairs) are in a (700-nm diameter) typical human cell

Metaphase chromosome (at midpoint (b) of cell division)

The Cell Life Cycle The Cell Life Cycle • The cell life cycle is the series of changes a cell goes through G checkpoint Interphase 1 • (restriction point) Interphase • G1 phase—growth 1 or Gap 1 phase S • The first part of interphase Growth and DNA synthesis G2 • Cell metabolically active—growth—make proteins Growth and final G • Variable in length from hours to YEARS (egg cell) 1 preparations for Growth M division

Cytokinesis Mitosis

• Centrioles begin to replicate near the end of G1 Telophase Metaphase Anaphase Prophase • S (synthetic) phase—DNA replicates itself

• Ensures that daughter cells receive identical copies of the

genetic material (chromatin extended) Mitotic phase (M)

• G2 phase—growth 2 or Gap 2 G checkpoint • Centrioles finish copying themselves 2

• Enzymes needed for cell division are synthesized in G2

• During S (synthetic) and G2 phases, cell carries on normal activities

6 The Cell Life Cycle The Stages of Mitosis

• Cell division • Prophase—the first and longest stage of mitosis • M (mitotic) phase—cells divide during this stage • Early prophase—chromatin threads condense into • Follows interphase (G1, S, and G2) chromosomes • Cell division involves: • Chromosomes are made up of two threads called chromatids (sister chromatids) • Mitosis—division of the nucleus during cell division • Chromatids are held together by the centromere • Chromosomes are distributed to the two daughter • Centriole pairs separate from one another nuclei • The mitotic spindle forms Cytokinesis—division of the cytoplasm • • Late prophase—centrioles continue moving away • Occurs after the nucleus divides from each other • Nuclear membrane fragments

Early Prophase and Late Prophase The Stages of Mitosis

Interphase Early Prophase Late Prophase • Metaphase—the second stage of mitosis • Chromosomes cluster at the middle of the cell • Centromeres are aligned along the equator • Anaphase—the third and shortest stage of mitosis • Centromeres of chromosomes split Centrosomes (each Plasma Early mitotic Spindle pole Polar microtubule has 2 centrioles) membrane spindle Fragments of nuclear Aster envelope

Nucleolus Chromatin Chromosome Centromere Nuclear consisting of two Kinetochore Kinetochore envelope sister chromatids microtubule

Metaphase and Anaphase The Stages of Mitosis

Metaphase Anaphase Telophase and Cytokinesis • Telophase begins as chromosomal movement stops • Chromosomes at opposite poles of the cell uncoil • Resume threadlike extended-chromatin form • A new nuclear membrane forms Cytokinesis completes the division of the cell into Contractile • Nucleolus forming Nuclear ring at Spindle cleavage two daughter cells envelope furrow forming

Metaphase Daughter plate chromosomes

7 Telophase and Cytokinesis Cellular Diversity

Metaphase Anaphase Telophase and Cytokinesis • Specialized functions of cells relates to: • Shape of cell • Arrangement of organelles • Cells that connect body parts or cover organs

Contractile or transport gases Nucleolus forming Nuclear ring at Spindle cleavage envelope furrow • Fibroblast—makes and secretes protein component forming of fibers • Erythrocyte—concave shape provides surface area for uptake of the respiratory gases • Epithelial cell—hexagonal shape allows maximum Metaphase Daughter number of epithelial cells to pack together plate chromosomes

Cells that Connect Body Parts or Cover Cellular Diversity Organs

Erythrocytes • Cells that move organs and body parts • Skeletal and smooth muscle cells • Elongated and filled with actin and myosin • Contract forcefully (”shorten with force”) Fibroblasts

Epithelial cells

(a) Cells that connect body parts, form linings, or transport gases

Cells that Connect Organs and Body Parts Cellular Diversity

• Cells that store nutrients • Fat cell—shape is produced by large fat droplet in its Skeletal cytoplasm muscle Smooth • Cells that fight disease muscle cells cell • Macrophage—moves through tissue to reach infection sites

(b) Cells that move organs and body parts

8 Cells that Store Nutrients and Cells that Cellular Diversity Fight Disease • Cells that gather and process information Fat cell • Neuron—has long processes for receiving and transmitting messages

Macrophage (c) Cell that stores nutrients

Nerve cell

(e) Cell that gathers information and controls body (d) Cell that fights disease functions

Cellular Diversity Developmental Aspects of Cells

• Aging—a complex process caused by a variety of factors • Cells of reproduction • Sperm (male) – possesses long tail for swimming to • Free radical theory the egg for fertilization • Damage from byproducts of cellular metabolism • Radicals build up and damage essential molecules of cells

• Mitochondrial theory • A decrease in production of energy by mitochondria weakens and ages our cells

Sperm • Genetic theory proposes that aging is programmed by genes • Telomeres—“end caps” on chromosomes (f) Cell of reproduction • Telomerase—prevents telomeres from degrading