Cells: the Living Units

Chapter 3 Cells: The Living Units

Cell Theory

•Cell - structural and functional unit of life

•Organismal functions depend on individual and collective cell functions

•Biochemical activities of cells dictated by their shapes or forms, and specific subcellular structures

•Continuity of life has cellular basis

Generalized Cell

• All cells have some common structures and functions

• Human cells have three basic parts:

–Plasma membrane—flexible outer boundary

–Cytoplasm—intracellular fluid containing organelles

–Nucleus—control center

Nucleus

•Largest organelle; genetic library with blueprints for nearly all cellular proteins

•Responds to signals; dictates kinds and amounts of proteins synthesized

•Most cells uninucleate; skeletal muscle cells, bone destruction cells, and some liver cells are multinucleate; red blood cells are anucleate

•Three regions/structures

The Nuclear Envelope

•Double-membrane barrier; encloses nucleoplasm

•Outer layer continuous with rough ER and bears ribosomes

•Inner lining (nuclear lamina) maintains shape of nucleus; scaffold to organize DNA

•Pores allow substances to pass; nuclear porecomplex line pores; regulates transport of large molecules into and out of nucleus

Nucleoli

•Dark-staining spherical bodies within nucleus

•Involved in rRNA synthesis and ribosome subunit assembly

•Associated with nucleolar organizer regions

–Contains DNA coding for rRNA

•Usually one or two per cell

Chromatin

•Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%)

•Arranged in fundamental units called nucleosomes

•Histones pack long DNA molecules; involved in gene regulation

•Condense into barlike bodies called chromosomes when cell starts to divide

Plasma Membrane

•Lipid bilayer and proteins in constantly changing fluid mosaic

•Plays dynamic role in cellular activity

•Separates intracellular fluid (ICF) from extracellular fluid (ECF)

–Interstitial fluid (IF) = ECF that surrounds cells

Membrane Lipids

•75% phospholipids (lipid bilayer)

•5% glycolipids

•20% cholesterol

Membrane Proteins

•Allow communication with environment

•½ mass of plasma membrane

•Most specialized membrane functions

•Some float freely

•Some tethered to intracellular structures

•Two types:

–Integral proteins

–Peripheral proteins

Six Functions of Membrane Proteins

1. Transport

2. Receptors for signal transduction

3. Attachment to cytoskeleton and extracellular matrix

4.Enzymatic activity

5. Intercellular joining

6. Cell-cell recognition

The Glycocalyx

•"Sugar covering" at cell surface

–Lipids and proteins with attached carbohydrates (sugar groups)

•Every cell type has different pattern of sugars

–Specific biological markers for cell to cell recognition

–Allows immune system to recognize "self" and "non self"

–Cancerous cells change it continuously

Cellular Extensions

•Microvilli

–Minute, fingerlike extensions of plasma membrane

–Increase surface area for absorption

–Core of actin filaments for stiffening

Plasma Membrane

•Cells surrounded by interstitial fluid (IF)

–Contains thousands of substances, e.g., amino acids, sugars, fatty acids, vitamins, hormones, salts, waste products

•Plasma membrane allows cell to

–Obtain from IF exactly what it needs, exactly when it is needed

–Keep out what it does not need

Cytoplasm

•Located between plasma membrane and nucleus

–Composed of

•Cytosol

•Organelles

•Inclusions

Cytoplasmic Organelles

•Membranous• Nonmembranous

–Mitochondria  Cytoskeleton

–Peroxisomes  Centrioles

–Lysosomes  Ribosomes

–Endoplasmic reticulum

–Golgi apparatus

•Membranes allow crucial compartmentalization

Mitochondria

•Double-membrane structure with inner shelflike cristae

•Provide most of cell's ATP via aerobic cellular respiration

–Requires oxygen

•Contain their own DNA, RNA, ribosomes

•Similar to bacteria; capable of cell division called fission

Ribosomes

•Granules containing protein and rRNA

•Site of protein synthesis

•Free ribosomes synthesize soluble proteins that function in cytosol or other organelles

•Membrane-bound ribosomes (forming rough ER) synthesize proteins to be incorporated into membranes, lysosomes, or exported from cell

Endoplasmic Reticulum (ER)

•Interconnected tubes and parallel membranes enclosing cisterns

•Continuous with outer nuclear membrane

•Two varieties:

Rough ER

•External surface studded with ribosomes

•Manufactures all secreted proteins

•Synthesizes membrane integral proteins and phospholipids

•Assembled proteins move to ER interior, enclosed in vesicle, go to Golgi apparatus

Smooth ER

•Network of tubules continuous with rough ER

•Its enzymes (integral proteins) function in

–Lipid metabolism; cholesterol and steroid-based hormone synthesis; making lipids of lipoproteins

–Absorption, synthesis, and transport of fats

–Detoxification of drugs, some pesticides, carcinogenic chemicals

–Converting glycogen to free glucose

–Storage and release of calcium

Golgi Apparatus

•Stacked and flattened membranous sacs

•Modifies, concentrates, and packages proteins and lipids from rough ER

Lysosomes

–Spherical membranous bags containing digestive enzymes (acid hydrolases)"Safe" sites for intracellular digestion

•Digest ingested bacteria, viruses, and toxins

•Degrade nonfunctional organelles

•Metabolic functions, e.g., break down and release glycogen

•Destroy cells in injured or nonuseful tissue (autolysis)

Break down bone to release Ca2+

Peroxisomes

•Membranous sacs containing powerful oxidases and catalases

•Detoxify harmful or toxic substances

•Catalysis and synthesis of fatty acids

•Neutralize dangerous free radicals (highly reactive chemicals with unpaired electrons)

–Oxidases convert to H2O2 (also toxic)

–Catalases convert H2O2 to water and oxygen

Endomembrane System

•Overall function

–Produce, degrade, store, and export biological molecules

–Degrade potentially harmful substances

•Includes ER, Golgi apparatus, secretory vesicles, lysosomes, nuclear and plasma membranes

Cytoskeleton

•Elaborate series of rods throughout cytosol; proteins link rods to other cell structures

–Three types:

Microfilaments

•Thinnest of cytoskeletal elements

•Dynamic strands of protein actin

•Each cell has a unique arrangement of strands

•Dense web attached to cytoplasmic side of plasma membrane is called terminal web

–Gives strength, compression resistance

•Involved in cell motility, change in shape, endocytosis and exocytosis

Intermediate Filaments

•Tough, insoluble, ropelike protein fibers

•Composed of tetramer fibrils

•Resist pulling forces on cell; attach to desmosomes

•E.g., neurofilaments in nerve cells; keratin filaments in epithelial cells

Microtubules

•Largest of cytoskeletal elements; dynamic hollow tubes; most radiate from centrosome

•Composed of protein subunits called tubulins

•Determine overall shape of cell and distribution of organelles

•Mitochondria, lysosomes, secretory vesicles attach to microtubules; moved throughout cell by motor proteins

Centrosome and Centrioles

•"Cell center" near nucleus

•Generates microtubules; organizes mitotic spindle

•Contains paired centrioles

–Barrel-shaped organelles formed by microtubules

•Centrioles form basis of cilia and flagella

Cellular Extensions

•Cilia and flagella

–Whiplike, motile extensions on surfaces of certain cells

–Contain microtubules and motor molecules

–Cilia move substances across cell surfaces

– Longer flagella propel whole cells (tail of sperm)

Cell Diversity

•Over 200 different types of human cells

•Types differ in size, shape, subcellular components, and functions

Membrane Transport

•Plasma membranes selectively permeable

–Some molecules pass through easily; some do not

•Two ways substances cross membrane

–Passive processes

–Active processes

Types of Membrane Transport

•Passive processes

–No cellular energy (ATP) required

–Substance moves down its concentration gradient

•Active processes

–Energy (ATP) required

–Occurs only in living cell membranes

Passive Processes

•Two types of passive transport

–Diffusion

•Simple diffusion

•Carrier- and channel-mediated facilitated diffusion

•Osmosis

–Filtration

•Usually across capillary walls

Tonicity

•Tonicity: Ability of solution to alter cell's water volume

–Isotonic: Solution with same non-penetratingsolute concentration as cytosol

–Hypertonic: Solution with higher non-penetrating solute concentration than cytosol

–Hypotonic: Solution with lower non-penetrating solute concentration than cytosol

Membrane Transport: Active Processes

•Two types of active processes

–Active transport (solute pumping)

–Vesicular transport

•Both require ATP to move solutes across a living plasma membrane because

–Solute too large for channels

–Solute not lipid soluble

–Solute not able to move down concentration gradient

Active Transport

•Requires carrier proteins (solute pumps)

–Bind specifically and reversibly with substance

•Moves solutes against concentration gradient

–Requires energy

Sodium-Potassium Pump

•Na+ and K+ channels allow slow leakage down concentration gradients

•Na+-K+ pump works as antiporter

–Pumps against Na+ and K+ gradients to maintain high intracellular K+ concentration and high extracellular Na+ concentration

•Maintains electrochemical gradients essential for functions of muscle and nerve tissues

•Allows all cells to maintain fluid volume

Vesicular Transport

•Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles

•Requires cellular energy (e.g., ATP)

Vesicular Transport

•Functions:

–Exocytosis—transport out of cell

–Endocytosis—transport into cell

•Phagocytosis

•Pinocytosis

–Transcytosis

–transport into, across,

and then out of cell

–Vesicular trafficking

–transport from one area

or organelle in cell to another

Cell Life Cycle

Cells have two major periods

Interphase

Cell division

Interphase

Events of Cell Division

Mitosis—division of the nucleus
Results in the formation of two daughter nuclei
Cytokinesis—division of the cytoplasm
Begins when mitosis is near completion
Results in the formation of two daughter cells

Stages of Mitosis

Prophase
Centrioles migrate to the poles to direct assembly of mitotic spindle fibers
DNA appears as double-stranded chromosomes
Nuclear envelope breaks down and disappears
Metaphase
Chromosomes are aligned in the center of the cell on the metaphase plate

Anaphase
Chromosomes are pulled apart and toward the opposite ends of the cell
Cell begins to elongate
Telophase
Chromosomes uncoil to become chromatin
Nuclear envelope reforms around chromatin
Spindles break down and disappear

Cytokinesis

Begins during late anaphase and completes during telophase
A cleavage furrow forms to pinch the cells into two parts

Protein Synthesis

Gene—DNA segment that carries a blueprint for building one protein
Proteins have many functions
Building materials for cells
Act as enzymes (biological catalysts)
RNA is essential for protein synthesis

Role of RNA

Transfer RNA (tRNA)

Ribosomal RNA (rRNA)

Messenger RNA (mRNA)

Transcription

Transfer of information from DNA’s base sequence to the complimentary base sequence of mRNA
Three-base sequences on mRNA are called codons

Translation

Base sequence of nucleic acid is translated to an amino acid sequence

Amino acids are the building blocks of proteins

Extracellular Materials

•Body fluids—interstitial fluid, blood plasma, and cerebrospinal fluid

•Cellular secretions—intestinal and gastric fluids, saliva, mucus, and serous fluids

•Extracellular matrix–most abundant extracellular material

Developmental Aspects of Cells

•All cells of body contain same DNA but cells not identical

•Chemical signals in embryo channel cells into specific developmental pathways by turning some genes on and othersoff

•Development of specific and distinctive features in cells called cell differentiation

Apoptosis and Modified Rates of Cell Division

•During development more cells than needed produced (e.g., in nervous system)

•Eliminated later by programmed cell death (apoptosis)

–Mitochondrial membranes leak chemicals that activate

caspases DNA, cytoskeleton degradation  cell death

–Dead cell shrinks and is phagocytized

Apoptosis and Modified Rates of Cell Division

•Organs well-formed and functional before birth

•Cell division in adults to replace short-lived cells and repair wounds

•Hyperplasia

•Atrophy

Theories of Cell Aging

•Wear and tear theory—Little chemical insults and free radicals have cumulative effects

•Mitochondrial theory of aging–free radicals in mitochondria diminish energy production

•Immune system disorders—autoimmune responses; progressive weakening of immune response; C-reactive protein of acute inflammation causes cell aging

Theories of Cell Aging

•Most widely accepted theory

–Genetic theory-cessation of mitosis and cell aging programmed into genes

•Telomeres (strings of nucleotides protecting ends of chromosomes) may determine number of times a cell can divide

•Telomerase lengthens telomeres

–Found in germ cells; ~ absent in adult cells