several important scientists made plasma membrane...

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Copyright © 2011 Pearson Education, Inc. Copyright © 2011 Pearson Education, Inc.

Chapter 2: Introduction to Cells

•  Several important scientists made discoveries about cells •  Robert Hooke •  Matthias Schleiden and Theodor Schwann •  Rudolf Virchow

•  Cells—the smallest living units in our bodies •  Organelles—“little organs”—carry on

essential functions of cells


Introduction to Cells

•  Cells have three main components •  Plasma membrane—the outer boundary •  Cytoplasm—contains most organelles •  Nucleus—controls cellular activities

•  NOTE: Important chemicals involved in cell anatomy include: •  Water and ions (simple charged particles) •  Macromolecules, such as…

•  Proteins (chains of smaller units called amino acids) •  Lipids (include fat [is 3 fatty acids + glycerol]; steroids; cholesterol;

waxes) •  Carbohydrates (sugars: mono-saccharides, poly-saccharides,

glycogen) •  ATP: adenosine triphosphate •  Nucleic acids: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid)


Structure of a Generalized Cell

Figure 2.1

Secretion being released from cell by exocytosis

Peroxisome

Ribosomes

Rough endoplasmic reticulum

Nucleus

Nuclear envelope Chromatin

Golgi apparatus

Nucleolus

Smooth endoplasmic reticulum Cytosol

Lysosome

Mitochondrion

Centrioles

Centrosome matrix

Microtubule

Cytoskeletal elements

Intermediate filaments

Plasma membrane


The Plasma Membrane

•  Plasma membrane (or plasmalemma) defines the extent of the cell: separates intracellular fluid from extracellular fluid (watery inside from watery outside with a ‘fatty’ barrier)

•  Structure of the cell membrane •  Fluid mosaic model (lipid bilayer) •  Types of membrane proteins

•  Integral proteins—firmly imbedded in, or attached to lipid bilayer

•  Short chains of carbohydrates attach to integral proteins •  Form the glycocalyx

•  Peripheral proteins—attach to membrane surface (in or out) •  Support plasma membrane from the cytoplasmic side


The Plasma Membrane

Figure 2.2

Integral proteins

Extracellular fluid (watery environment)

Cytoplasm (watery environment)

Polar head of phospholipid molecule

Glycolipid Cholesterol

Peripheral proteins

Bimolecular lipid layer containing proteins

Inward-facing layer of phospholipids

Outward- facing layer of phospholipids

Carbohydrate of glycocalyx

Glycoprotein Nonpolar tail of phospholipid molecule

Filament of cytoskeleton


The Plasma Membrane

•  Functions – relate to location at the interface of cell’s exterior and interior

•  Provides barrier against substances outside cell •  Some plasma membrane molecules act as receptors •  Determines which substances enter or leave the cell

•  Membrane is referred to as being “selectively permeable”

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Membrane Transport: moving substances across the cell membrane… These processes include:

•  Simple diffusion—tendency of molecules to move down their concentration gradient through the lipid bilayer

•  Osmosis—diffusion of water molecules across a membrane through small channels

•  Facilitated diffusion—movement of molecules down their concentration gradient through an integral protein channel

•  Active transport—integral proteins move molecules across the plasma membrane against their concentration gradient

•  Endo- and exocytosis


Figure 2.3 Membrane transport mechanisms.

Extracellular fluid

Lipid- soluble solutes

Water molecules

Lipid bilayer

Water soluble solutes Solute

ATP

Cytoplasm

Simple diffusion of fat-soluble molecules directly through the phospholipid bilayer down their concentration gradient

Osmosis, diffusion of water through the lipid bilayer and down its concentration gradient.

Facilitated diffusion An integral protein that spans the plasma membrane enables the passage of a particular solute across the membrane and down its conc gradient.

Active transport Some transport proteins use ATP as an energy source to actively pump substances across the plasma membrane against their concentration gradient.


Endocytosis

•  Endocytosis: Mechanism by which particles (chunks of stuff) enter cells • Phagocytosis—“cell eating” • Pinocytosis—“cell drinking”

•  Receptor-mediated endocytosis • Plasma proteins bind to certain molecules •  Invaginates and forms a coated pit

• Pinches off to become a coated vesicle •  NOTE: This is the method by which insulin and

cholesterol and some viruses enter cells!


Three Types of Endocytosis

Figure 2.4

Phagosome

Vesicle

Vesicle

Receptor recycled to plasma membrane

(a) Phagocytosis The cell engulfs a large particle by forming pro- jecting pseudopods (”false feet”) around it and en- closing it within a membrane sac called a phagosome. The phagosome then combines with a lysosome, and its contents are digested. Vesicle may or may not be protein-coated but has receptors capable of binding to microorganisms or solid particles.

(b) Pinocytosis The cell “gulps” drops of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein- coated.

(c) Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins in regions of protein-coated pits, enabling the cell to ingest and concentrate specific substances in protein-coated vesicles. The ingested substance may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.


Exocytosis

•  Exocytosis—a mechanism that moves substances out of the cell •  Substance is enclosed in a vesicle •  The vesicle migrates to the plasma membrane •  Proteins from the vesicles (v-SNAREs) bind with

membrane proteins (t-SNAREs) •  The lipid layers from both membranes bind, and the

vesicle releases its contents to the outside of the cell


Exocytosis

Figure 2.5

The membrane- bound vesicle migrates to the plasma membrane.

There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins).

The vesicle and plasma membrane fuse and a pore opens up.

Vesicle contents are released to the cell exterior.

(a) The process of exocytosis Extracellular

fluid Plasma membrane SNARE (t-SNARE)

Secretory vesicle Vesicle SNARE

(v-SNARE) Molecule to be secreted

Cytoplasm

Fused v- and t-SNAREs

Fusion pore formed

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3

2

4

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The Cytoplasm

•  Cytoplasm—lies internal to plasma membrane •  Consists of cytosol, organelles, and inclusions, in

watery environment

•  Cytosol •  Jelly-like watery fluid in which other cellular elements

are suspended •  Consists of water with dissolved ions (salts) and

enzymes


Cytoplasmic Organelles •  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 cytoplasm”– tow types of ER: •  Rough ER—ribosomes stud the external surfaces, so

protein synthesis occurs here •  Smooth ER—consists of tubules in a branching

network; No ribosomes are attached, therefore no protein synthesis—but lipid and carb synthesis


The Endoplasmic Reticulum and Ribosomes

Figure 2.6

Nuclear envelope

Ribosomes

Rough ER

(a) Diagrammatic view of smooth and rough ER

Smooth ER

(b) Electron micrograph of smooth and rough ER (85,000×)

Cisternae


Cytoplasmic Organelles

•  Golgi apparatus—a stack of three to 10 disk-shaped envelopes •  Sorts products of rough ER and sends them to proper

destination •  Products of rough ER move through the Golgi from

the convex (cis) to the concave (trans) side •  Is the “packaging and shipping” division of the

manufacturing plant

Copyright © 2011 Pearson Education, Inc. Copyright © 2011 Pearson Education, Inc. Figure 2.7

Golgi Appartus

Cis face— “receiving” side of Golgi apparatus

Secretory vesicle

(a) Many vesicles in the process of pinching off from the membranous Golgi apparatus

(b) Electron micrograph of the Golgi apparatus (90,000×)

Transport vesicle from the Golgi apparatus

Transport vesicle from trans face

Trans face— “shipping” side of Golgi apparatus

New vesicles forming

New vesicles forming

Cisternae Transport vesicle from rough ER

Golgi apparatus

Copyright © 2011 Pearson Education, Inc. Copyright © 2011 Pearson Education, Inc. Figure 2.8

Plasma membrane

Secretion by exocytosis

Vesicle becomes lysosome

Golgi apparatus

Rough ER ER membrane Phagosome Proteins in cisterna

Pathway B: Vesicle membrane to be incorporated into plasma membrane Pathway A:

Vesicle contents destined for exocytosis Extracellular fluid

Secretory vesicle

Pathway C: Lysosome containing acid hydrolase enzymes

Protein-containing vesicles pinch off rough ER and migrate to fuse with membranes of Golgi apparatus.

Proteins are modified within the Golgi compartments.

Proteins are then packaged within different vesicle types, depending on their ultimate destination.

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2

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The sequence of events from protein synthesis on the rough ER to the final distribution of these proteins (1 of 2).

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More Cytoplasmic Organelles...

•  Lysosomes—membrane-walled sacs containing digestive enzymes •  Digest unwanted substances

•  Peroxisomes—membrane-walled sacs of oxidase enzymes •  Enzymes neutralize free radicals and break down

poisons •  Break down long chains of fatty acids •  Are numerous in the liver and kidneys •  Are the toxic waste removal system


Lysosomes

Light areas are regions where materials are being digested.

Figure 2.9

Lysosomes


Mitochondria

•  Mitochondria—generate most of the cell’s energy

•  most complex organelle •  Contain some maternally inherited DNA •  Believed to have arisen from bacteria

•  More abundant in energy-requiring cells, like muscle cells and sperm

•  “Power plant” of the cell: release energy stored in chemical bonds and transfer energy to produce ATP


Figure 2.10 Mitochondria.

Outer mitochondrial

membrane Ribosome

Inner mitochondrial membrane Cristae

Matrix

Mitochondrial DNA

Enzymes


More Cytoplasmic Organelles

•  Cytoskeleton—“cell skeleton”—an elaborate network of rods: for support and communication

•  Contains three types of rods: •  Microtubules—cylindrical structures made of

proteins •  Microfilaments—filaments of contractile protein

actin •  Intermediate filaments—protein fibers


Cytoskeleton: Microfilaments

Figure 2.11a

(a) Microfilaments

Strands made of spherical protein subunits called actins

Actin subunit

7 nm

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

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Cytoskeleton: Intermediate filaments

Figure 2.11b

(b) Intermediate filaments

Tough, insoluble protein fibers constructed like woven ropes

10 nm

Fibrous subunits

Intermediate filaments form the purple batlike network in this photo.


Cytoskeleton: Microtubules

Figure 2.11c

(c) Microtubules

Hollow tubes of spherical protein subunits called tubulins

25 nm

Tubulin subunits

Microtubules appear as gold networks surrounding the cells’ pink nuclei in this photo.


More Cytoplasmic Organelles...

•  Centrosomes and centrioles

•  Centrosome—a spherical structure in the cytoplasm •  Composed of centrosome matrix and centrioles

•  Centrioles—paired cylindrical bodies •  Consists of 27 short microtubules •  Act in forming cilia •  Necessary for karyokinesis (nuclear division)


Cytoplasmic Inclusions

•  Temporary structures •  Not present in all cell types

•  May consist of pigments, crystals of protein, and food stores; examples ... •  Lipid droplets—found in liver cell and fat cells •  Glycosomes—store sugar in the form of glycogen


The Nucleus •  The nucleus—“little nut” or “kernel”: control center of

cell •  DNA directs the cell’s activities– provides instructions for

protein synthesis •  Nucleus is approximate 5µm in diameter

•  Nuclear envelope—two parallel membranes separated by fluid-filled space

•  Nuclear pores --- penetrate the nuclear envelope •  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 •  Site of ribosome subunit assembly


The Nucleus

Figure 2.13

Chromatin (condensed)

Nuclear envelope Nucleus Nuclear pores

Fracture line of outer membrane

Nuclear pore complexes. Each pore is ringed by protein particles

Surface of nuclear envelope.

Nuclear lamina. The netlike lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclear envelope.

Nucleolus

Cisternae of rough ER (a)

(b)

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Chromatin and Chromosomes

•  DNA (NOT a protein but another type of macromolecule called a nucleic acid) is a double helix is composed of four subunits: •  Thymine (T), adenine (A), cytosine (C), and guanine (G)

•  DNA is packed with protein molecules •  DNA plus the proteins form chromatin (“colored

stuff”) •  Each cluster of DNA and histone proteins is a

nucleosome



The double-helix structure of the

DNA molecule:

Deoxyribose sugar Phosphate

Sugar-phosphate backbone

Hydrogen bond

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

Nucleotides

Figure 2.14



•  Extended chromatin •  Is the active region of DNA where DNA’s genetic

code is copied onto mRNA (transcription)

•  Condensed chromatin •  Tightly coiled nucleosomes •  Inactive form of chromatin

•  Chromosomes—highest level of organization of chromatin •  Contains a long molecule of DNA

•  46 chromosomes (arranged in 23 pairs) are in a typical human cell


Metaphase chromosome (at midpoint of cell division)

Nucleosome (10-nm diameter; eight histone proteins wrapped by two winds of the DNA double helix)

Linker DNA

Histones

(a)

(b)

DNA double helix (2-nm diameter)

Chromatin (“beads on a string”) structure with nucleosomes

Tight helical fiber (30-nm diameter)

Chromatid (700-nm diameter)

Looped domain structure (300-nm diameter)

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2

3 4

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Figure 2.15


The Cell Life Cycle •  The cell life cycle is the series of changes a cell

goes through •  Interphase

•  G1 phase—growth 1 or Gap 1 phase •  The first part of interphase •  Cell metabolically active—growth—make proteins •  Variable in length from hours to YEARS (egg cell) •  Centrioles begin to replicate near the end of G1

•  S (synthetic) phase—DNA replicates itself •  Ensures that daughter cells receive identical copies of the

genetic material (chromatin extended) •  G2 phase—growth 2 or Gap 2 •  Centrioles finish copying themselves •  Enzymes needed for cell division are synthesized in G2

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


G1 Growth

S Growth and DNA

synthesis G2 Growth and final preparations for

division M

G2 checkpoint

G1 checkpoint (restriction point)

Mitotic phase (M)

Cytokinesis

Telophase

Anaphase

Metaphase

Prophase

Interphase

Mitosis

The Cell Life Cycle

Figure 2.16

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The Cell Life Cycle

•  Cell division •  M (mitotic) phase—cells divide during this stage

•  Follows interphase (G1, S, and G2)

•  Cell division involves: •  Mitosis—division of the nucleus during cell division

•  Chromosomes are distributed to the two daughter nuclei

•  Cytokinesis—division of the cytoplasm •  Occurs after the nucleus divides


The Stages of Mitosis •  Prophase—the first and longest stage of

mitosis •  Early prophase—chromatin threads condense into

chromosomes •  Chromosomes are made up of two threads called

chromatids (sister chromatids) •  Chromatids are held together by the centromere •  Centriole pairs separate from one another •  The mitotic spindle forms

•  Late prophase—centrioles continue moving away from each other •  Nuclear membrane fragments


Early Prophase and Late Prophase

Figure 2.17 (1 of 2)

Centrosomes (each has 2 centrioles)

Early mitotic spindle Spindle pole

Kinetochore Kinetochore microtubule

Polar microtubule

Nucleolus Centromere

Plasma membrane Fragments

of nuclear envelope Aster

Nuclear envelope

Chromosome consisting of two sister chromatids

Chromatin

Interphase Early Prophase Late Prophase


The Stages of Mitosis

•  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


Metaphase and Anaphase

Figure 2.17 (2 of 2)

Contractile ring at cleavage furrow

Nuclear envelope forming

Nucleolus forming Spindle

Metaphase plate Daughter

chromosomes

Metaphase Anaphase Telophase and Cytokinesis


The Stages of Mitosis

•  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 two daughter cells

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Contractile ring at cleavage furrow

Nuclear envelope forming

Nucleolus forming Spindle

Metaphase plate Daughter

chromosomes

Metaphase Anaphase Telophase and Cytokinesis

Telophase and Cytokinesis

Figure 2.17 Copyright © 2011 Pearson Education, Inc. Copyright © 2011 Pearson Education, Inc.

Cellular Diversity

•  Specialized functions of cells relates to: •  Shape of cell •  Arrangement of organelles

Some types of cells… •  1. Cells that connect body parts or cover

organs or transport gases •  Fibroblast—makes and secretes protein component

of fibers •  Erythrocyte—concave shape provides surface area

for uptake of the respiratory gases •  Epithelial cell—hexagonal shape allows maximum

number of epithelial cells to pack together


Cells that Connect Body Parts or Cover Organs

Figure 2.18a

Fibroblasts

Erythrocytes

Epithelial cells

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


Cellular Diversity

•  2. Cells that move organs and body parts •  Skeletal and smooth muscle cells

•  Elongated and filled with actin and myosin •  Contract forcefully (”shorten with force”)


Cells that Connect Organs and Body Parts

Figure 2.18b

(b) Cells that move organs and body parts

Skeletal muscle cell

Smooth muscle cells


Cellular Diversity

•  3. Cells that store nutrients •  Fat cell—shape is produced by large fat droplet in its

cytoplasm

•  4. Cells that fight disease •  Macrophage—moves through tissue to reach

infection sites

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Cells that Store Nutrients and Cells that Fight Disease

Figure 2.18c, d

(d) Cell that fights disease

Macrophage (c) Cell that stores nutrients

Fat cell


Cellular Diversity

•  5. Cells that gather and process information •  Neuron—has long processes for receiving and

transmitting messages

Figure 2.18e

Nerve cell

(e) Cell that gathers information and controls body functions


Cellular Diversity

•  6. Cells of reproduction •  Sperm (male) – possesses long tail for swimming to

the egg for fertilization

Figure 2.18f

Sperm

(f) Cell of reproduction


Developmental Aspects of Cells

•  Aging—a complex process caused by a variety of factors

•  Free radical theory •  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

•  Genetic theory proposes that aging is programmed by genes •  Telomeres—“end caps” on chromosomes •  Telomerase—prevents telomeres from degrading

several important scientists made plasma membrane...

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