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Cells and Tissues

Cells and Tissues

Concepts of the Cell Theory
A cell is the basic structural and functional unit of living organisms. 
The activity of an organism depends on the collective activities of its cells.
According to the principle of complementarity, the biochemical activities of cells are dictated by the relative number of their specific subcellular structures.
Continuity of life has a cellular basis.

Chemical Components of Cells
Most cells are composed of the following four elements
Carbon
Hydrogen
Oxygen
Nitrogen

Cells and Tissues
Carry out all chemical activities needed to sustain life
Cells are the building blocks of all living things.
Tissues are groups of cells that are similar in structure and function.

Anatomy of the Cell
Cells are not all the same.
All cells share general structures.
All cells have three main regions
Nucleus
Cytoplasm
Plasma membrane

The Nucleus
Control center of the cell
Contains genetic material (DNA)
Three regions
Nuclear envelope (membrane)
Nucleolus
Chromatin

Nuclear envelope (membrane) 
Barrier of the nucleus
Consists of a double membrane
Contains nuclear pores that allow for exchange of material with the rest of the cell

Nucleoli
Nucleus contains one or more nucleoli
Sites of ribosome assembly
Ribosomes migrate into the cytoplasm through nuclear pores

Chromatin 
Composed of DNA and protein
Present when the cell is not dividing
Scattered throughout the nucleus
Condenses to form chromosomes when the cell divides

Plasma Membrane
Barrier for cell contents
Double phospholipid layer
Hydrophilic heads
Hydrophobic tails
Also contains proteins, cholesterol, and glycoproteins

Plasma Membrane Junctions

Tight junctions 
Impermeable junctions 
Bind cells together into leakproof sheets

Desmosomes 
Anchoring junctions that prevent cells from being pulled apart

Gap junctions 
Allow communication between cells

Cytoplasm
The material outside the nucleus and inside the plasma membrane 
Site of most cellular activities
Contains three major elements
Cytosol
Fluid that suspends other elements
Organelles
Metabolic machinery of the cell 
“Little organs” that perform functions for the cell
Inclusions
Chemical substances such as stored nutrients or cell products

Cytoplasmic Organelles


Mitochondria

“Powerhouses” of the cell
Change shape continuously
Carry out reactions where oxygen is used to break down food
Provides ATP for cellular energy

Ribosomes

Made of protein and RNA
Sites of protein synthesis
Found at two locations
Free in the cytoplasm
As part of the rough endoplasmic reticulum

Endoplasmic reticulum (ER)

Fluid-filled tubules for carrying substances
Two types of ER
Rough endoplasmic reticulum
Studded with ribosomes
Synthesizes proteins
Smooth endoplasmic reticulum
Functions in lipid metabolism and  detoxification of drugs and pesticides

Golgi apparatus
Modifies and packages proteins
Produces different types of packages
Secretory vesicles

Cell membrane components


Lysosomes

Contain enzymes produced by ribosomes
Packaged by the Golgi apparatus
Digest worn-out or nonusable materials within the cell

Peroxisomes

Membranous sacs of oxidase enzymes
Detoxify harmful substances such as alcohol and formaldehyde
Break down free radicals (highly reactive chemicals)
Replicate by pinching in half

Cytoskeleton

Network of protein structures that extend throughout the cytoplasm
Provides the cell with an internal framework
Three different types of elements
Microfilaments (largest)
Intermediate filaments
Microtubules (smallest)
Centrioles
Rod-shaped bodies made of microtubules
Direct the formation of mitotic spindle during cell division

Cellular Projections

Not found in all cells
Cilia move materials across the cell surface
Located in the respiratory system to move mucus 
Flagella propel the cell 
The only flagellated cell in the human body is sperm
Microvilli are tiny, fingerlike extensions of the plasma membrane
Increase surface area for absorption

Solutions and Transport
Solution—homogeneous mixture of two or more components
Solventdissolving medium; typically water in the body
Solutes—components in smaller quantities within a solution
Intracellular fluid—nucleoplasm and cytosol
Interstitial fluid—fluid on the exterior of the cell

Selective Permeability
The plasma membrane allows some materials to pass while excluding others.
This permeability influences movement both into and out of the cell.

Cell Physiology: Membrane Transport
Membrane transport—movement of substances into and out of the cell
Cell membranes are selectively permeable (some substances can pass through but others cannot)
Two basic methods of transport

Passive processes
No energy is required
Active processes
Cell must provide metabolic energy (ATP)

Diffusion
Particles tend to distribute themselves evenly within a solution
Movement is from high concentration to low concentration, or down a concentration gradient

Types of diffusion

Simple diffusion
An unassisted process
Solutes are lipid-soluble materials or small enough to pass through membrane pores

Osmosis—simple diffusion of water
Highly polar water molecules easily cross the plasma membrane through aquaporins

Facilitated diffusion
Substances require a protein carrier for passive transport
Transports lipid-insoluble and large substances

Filtration
Water and solutes are forced through a membrane by fluid, or hydrostatic pressure
A pressure gradient must exist
Solute-containing fluid is pushed from a high-pressure area to a lower pressure area

Active Processes

Substances are transported that are unable to pass by diffusion 
Substances may be too large
Substances may not be able to dissolve in the fat core of the membrane
Substances may have to move against a concentration gradient
ATP is used for transport

Two common forms of active transport
Active transport (solute pumping)
Vesicular transport
Exocytosis 
Endocytosis
Phagocytosis
Pinocytosis

Active transport (solute pumping)
Amino acids, some sugars, and ions are transported by protein carriers called solute pumps
ATP energizes protein carriers
In most cases, substances are moved against concentration gradients

Vesicular transport
Exocytosis
Moves materials out of the cell
Material is carried in a membranous vesicle
Vesicle migrates to plasma membrane
Vesicle combines with plasma membrane
Material is emptied to the outside

Endocytosis
Extracellular substances are engulfed by being enclosed in a membranous vescicle
Types of endocytosis
Phagocytosis—“cell eating”
Pinocytosis—“cell drinking”

Cell Life Cycle
Cells have two major periods

Interphase
Cell grows
Cell carries on metabolic processes
Cell division 
Cell replicates itself
Function is to produce more cells for growth and repair processes

DNA Replication
Genetic material is duplicated and readies a cell for division into two cells
Occurs toward the end of interphase
DNA uncoils and each side serves as a template

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
First part of cell division
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)
Transfers appropriate amino acids to the ribosome for building the protein
Ribosomal RNA (rRNA)
Helps form the ribosomes where proteins are built
Messenger RNA (mRNA)
Carries the instructions for building a protein from the nucleus to the ribosome

Transcription and Translation
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

Body Tissues
Tissues
Groups of cells with similar structure and function
Four primary types
Epithelial tissue (epithelium)
Connective tissue
Muscle tissue
Nervous tissue

Epithelial Tissues
Locations
Body coverings
Body linings
Glandular tissue
Functions
Protection
Absorption
Filtration
Secretion

Epithelium Characteristics
Cells fit closely together and often form sheets
The apical surface is the free surface of the tissue
The lower surface of the epithelium rests on a basement membrane
Avascular (no blood supply)
Regenerate easily if well nourished

Classification of Epithelia
Number of cell layers
Simple—one layer
Stratified—more than one layer
Shape of cells

Squamous
flattened
Cuboidal
cube-shaped
Columnar
column-like

Simple Epithelia
Simple squamous
Single layer of flat cells
Location - usually forms membranes
Lines body cavities
Lines lungs and capillaries
Functions in diffusion, filtration, or secretion in membranes

Simple cuboidal
Single layer of cube-like cells
Locations 
Common in glands and their ducts
Forms walls of kidney tubules
Covers the ovaries
Functions in secretion and absorption; ciliated types propel mucus or reproductive cells

Simple columnar
Single layer of tall cells
Often includes mucus-producing goblet cells
Location - lines digestive tract
Functions in secretion and absorption; ciliated types propel mucus or reproductive cells

Pseudostratified columnar
Single layer, but some cells are shorter than others
Often looks like a double layer of cells but all cells rest on the basement membrane
Location - respiratory tract, where it is ciliated
Functions in absorption or secretion

Stratified Epithelia

Stratified squamous
Cells at the apical surface are flattened
Functions as a protective covering where friction is common
Locations - lining of the:
Skin
Mouth
Esophagus

Stratified cuboidal—two layers of cuboidal cells; functions in protection

Stratified columnar—surface cells are columnar, cells underneath vary in size and shape; functions in protection
Stratified cuboidal and columnar
Rare in human body
Found mainly in ducts of large glands
Transitional epithelium
Composed of modified stratified squamous epithelium
Shape of cells depends upon the amount of stretching
Functions in stretching and the ability to return to normal shape
Location - lines organs of the urinary system

Glandular Epithelium
Gland
One or more cells responsible for secreting a particular product
Secretions contain protein molecules in an aqueous (water-based) fluid

Two major gland types
Endocrine gland
Ductless since secretions diffuse into blood vessels
All secretions are hormones
Exocrine gland
Secretions empty through ducts to the epithelial surface
Include sweat and oil glands

Connective Tissue
Found everywhere in the body
Includes the most abundant and widely distributed tissues
Functions
Binds body tissues together
Supports the body
Provides protection

Connective Tissue Characteristics
Variations in blood supply
Some tissue types are well vascularized
Some have a poor blood supply or are avascular
Extracellular matrix
Non-living material that surrounds living cells

Extracellular Matrix
Two main elements
Ground substance—mostly water along with adhesion proteins and polysaccharide molecules
Fibers
Produced by the cells
Three types
Collagen (white) fibers
Elastic (yellow) fibers
Reticular fibers
Connective Tissue Types
Bone (osseous tissue)
Composed of
Bone cells in lacunae (cavities)
Hard matrix of calcium salts
Large numbers of collagen fibers
Functions to protect and support the body

Hyaline cartilage

Most common type of cartilage
Composed of
Abundant collagen fibers
Rubbery matrix
Locations
Larynx
Entire fetal skeleton prior to birth
Functions as a more flexible skeletal element than bone

Elastic cartilage
Provides elasticity
Location
Supports the external ear
Fibrocartilage
Highly compressible
Location
Forms cushion-like discs between vertebrae

Dense connective tissue (dense fibrous tissue)
Main matrix element is collagen fiber
Fibroblasts are cells that make fibers
Locations
Tendons—attach skeletal muscle to bone
Ligaments—attach bone to bone at joints
Dermis—lower layers of the skin

Loose connective tissue types
Areolar tissue
Most widely distributed connective tissue
Soft, pliable tissue like “cobwebs”
Functions as a packing tissue
Contains all fiber types
Can soak up excess fluid (causes edema)


Adipose tissue
Matrix is an areolar tissue in which fat globules predominate
Many cells contain large lipid deposits
Functions
Insulates the body
Protects some organs
Serves as a site of fuel storage

Reticular connective tissue
Delicate network of interwoven fibers
Locations
Forms stroma (internal supporting network) of lymphoid organs
Lymph nodes
Spleen
Bone marrow

Blood (vascular tissue)
Blood cells surrounded by fluid matrix called blood plasma
Fibers are visible during clotting
Functions as the transport vehicle for materials

Muscle Tissue

Function is to produce movement
Three types
Skeletal muscle
Cardiac muscle
Smooth muscle

Muscle Tissue Types

Skeletal muscle
Under voluntary control
Contracts to pull on bones or skin
Produces gross body movements or facial expressions
Characteristics of skeletal muscle cells
Striated
Multinucleate (more than one nucleus)
Long, cylindrical cells

Cardiac muscle
Under involuntary control
Found only in the heart
Function is to pump blood
Characteristics of cardiac muscle cells
Striated
One nucleus per cell
Cells are attached to other cardiac muscle cells at intercalated disks

Smooth muscle
Under involuntary muscle
Found in walls of hollow organs such as stomach, uterus, and blood vessels
Characteristics of smooth muscle cells
No visible striations
One nucleus per cell
Spindle-shaped cells

Nervous Tissue
Composed of neurons and nerve support cells
Function is to send impulses to other areas of the body
Irritability
Conductivity
Support cells called neuroglia insulate, protect, and support neurons

Tissue Repair (Wound Healing)
Regeneration
Replacement of destroyed tissue by the same kind of cells
Fibrosis
Repair by dense (fibrous) connective tissue (scar tissue)
Whether regeneration or fibrosis occurs depends on:
Type of tissue damaged
Severity of the injury

Events in Tissue Repair
Inflammation
Capillaries become very permeable
Clotting proteins migrate into the area from the blood stream
A clot walls off the injured area
Granulation tissue forms
Growth of new capillaries
Rebuild collagen fibers
Regeneration of surface epithelium
Scab detaches

Regeneration of Tissues
Tissues that regenerate easily
Epithelial tissue (skin and mucous membranes)
Fibrous connective tissues and bone
Tissues that regenerate poorly
Skeletal muscle
Tissues that are replaced largely with scar tissue
Cardiac muscle
Nervous tissue within the brain and spinal cord

Developmental Aspects of Tissue
Epithelial tissue arises from all three primary germ layers
Muscle and connective tissue arise from the mesoderm
Nervous tissue arises from the ectoderm
With old age, there is a decrease in mass and viability in most tissues