An introduction to cells and tissues Cells are living building blocks of all multicellular organisms

Aoife Gowran, TCD 2011

Chapter 2-Cellular Physiology ‘Human Physiology’, Sherwood

Principles of the Cell Theory •  All new cells and new life arise only from preexisting cells •  Cells of all organisms are fundamentally similar in structure and function •  An Organism’s structure and function ultimately depend on individual and collective structural characteristics and functional capabilities of its cells

THE CELL IS THE BASIC UNIT OF LIFE Basic functions of cell •  chemical production of energy (ATP) •  obtains O2/nutrients from surrounding environment •  eliminates waste materials from cell •  manufacture of new proteins •  functional activities of each cell depend on specific structural properties of the cell

Basic Cell Functions •  Control exchange of materials between cell & its surrounding environment •  Sensing and responding to changes in surrounding environment •  Reproduction –  Exception, Nerve cells and muscle cells lose their ability to reproduce during their early development

Specialist functions of cell DIFFERENTIATION

•  Muscle cells contract •  Bone cells deposit calcified matrix •  Cells in retina of eye detect light

Mammalian cells Sperm

Ovum

Neurones

5µm

100µm

400µm

Ultrastructure of mammalian cell ‘Typical’ Cell

10µm

Nucleus •  largest single organized cell component •  Enclosed by a double-layered nuclear envelope •  Stores genetic material (genes) in the form of DNA • Acts as a central control-point of cell function •  Dictates which proteins should be made by cell

DNA - deoxyribonucleic acid

C= cytosine G= guanine A= adenine T= thymine T is a U (uracil) in mRNA

DNA packaging

1

2 Chromosomes •  46 in human (23 pairs) •  Genes located within chromosomes •  Each gene codes for a protein •  20,000-25, 000 genes

The genetic code Gene (DNA)

Nucleic acid bases A -adenosine T -tyrosine (uracil in mRNA) G -guanine C -cytosine

Amino acids (20)

M

P

K

M -methione (ATG/AUG)

Mature protein

• All cells contain same genes • Different cell types “switch on” different genes • Express (make) different proteins • Allows different cell types to have specific functions - muscle cells contract - bone cell deposit extracellular matrix - cells lining GI tract secrete digestive enzymes

Making new proteins Flow of genetic material

DNA transcription

mRNA

(messenger ribonucleic acid)

translation

New Protein FUNCTION

Cytoplasm Includes all material inside the cell except the nucleus 3 Components 1.  Cytosol - intracellular fluid; semi gelatinous, contains nutrients and proteins, ions and waste products. 2. Inclusions - particles of insoluble material; direct contact with cytosol e.g.,protein fibers, ribosomes and proteasomes. 3. Organelles - membrane bound compartments; play specific roles in the function of the cell e.g., production of energy

Several ORGANELLES are contained within the cytoplasm

Membranous: Mitochondria Endoplasmic reticulum Golgi complex Lysosomes Peroxisomes Non-membranous: ribosomes, vaults, cytoskeleton

Mitochondria Small spherical; double membrane Outer & inner membrane Folded into leaflets - cristae Central lumen – mt Matrix Inter membrane space

•  Energy organelle –  Major site of ATP (adenosine triphosphate) production –  Contains enzymes for citric acid cycle and electron transport chain

ATP is used by the cell for cellular processes: - Movement/Mechanical work - Transport of molecules against a concentration gradient -  Biochemical reactions -  Synthesis of new chemical compounds

Cellular Energy Production Cytosol

Glycolysis anaerobic aerobic

mt Matrix

mt Inner Membrane

Citric Acid Cycle

Electron Transport Chain

Endoplasmic reticulum

Endoplasmic Reticulum (ER) •  Elaborate fluid-filled membranous system distributed throughout the cytosol •  Primary function –  Protein and lipid manufacture

•  Two types –  Smooth ER (lipid synthesis) •  Mesh of tiny interconnected tubules

–  Rough ER (protein synthesis) •  Projects outward from smooth ER as stacks of relatively flattened sacs •  Surface has attached ribosomes

Golgi complex

Golgi Complex •  Closely associated with ER •  Consists of a stack of flattened, slightly curved, membrane-enclosed sacs called cisternae •  Number of Golgi complexes per cell varies with the cell type •  Functions “Protein post production” –  Processes raw materials (RER) into finished products –  Sorts and directs finished products to their final destinations –  Packages secretory vesicles to release by exocytosis

Lysosomes & Peroxisomes

Lysosomes Cytoplasmic vesicle, high pH Contain digestive enzymes

Function Digestive system of the cell They breakdown old worn out organelles, bacteria, abnormal proteins Products of digestion can be reused

Cytoskeleton Microtubules Micro filaments Intermediate filaments

Functions Cell shape Internal organization Intracellular transport Assembly of cells into tissues Movement

CYTOSKELETON Outside cell

membrane

Inside cell cytoskeleton

Gives cell mechanical support and structure Supports organelles within cell Allows cell to move

Cytoskeleton Element

Function

Microtubules

• Transport secretory vesicles • Movement of specialized cell projections • Form mitotic spindle during cell division • Contain Tubulin protein

Microfilaments • Contractile systems • Mechanical stiffeners • Contain 2 chains of Actin protein

Intermediate filaments

• Help resist mechanical stress • Contain Keratin

Plasma Membrane

Plasma Membrane Structure •  Fluid lipid bilayer embedded with proteins –  Most abundant lipids are phospholipids •  Polar end of phospholipid is hydrophilic •  Nonpolar end of phospholipid is hydrophobic

•  Also has small amount of carbohydrates –  On outer surface only •  Cholesterol –  Tucked between phospholipid molecules –  Contributes to fluidity and stability of cell membrane

Plasma Membrane •  Extremely thin layer of lipids and protein that forms outer boundary of every cell •  Controls movement of molecules between the cell and its environment •  Participates in joining cells to form tissues and organs •  Plays important role in the ability of a cell to respond to changes in the cell’s environment

How do cells respond to stimulus???

Chemical

Physical

- Insulin -drug

-stretch -fluid flow

Sensor membrane protein

Biochemical pathways

GENES

Response (movement)

New protein - Long-term adaptation - Differentiation (become specialized) - Programmed cell death

EXTRACELLULAR MATRIX (ECM) • Biological “glue” • Secreted by cells • Meshwork of fibrous proteins in a watery gel-like substance composed of carbohydrate

1.

COLLAGEN

tensile strength

2.

ELASTIN

allows stretch and recoil

3.

FIBRONECTIN

cell adhesion holds cell in position

Bone - hard extracellular matrix

Skin -flexible

Lung - elastic

TISSUES OF THE BODY A group of cells functioning together is a TISSUE Exception Fusion of gametes  fertilized egg

ONE CELL ALONE IS NOT ENOUGH

Blastocyst

Tissue types

function dependent on structure structure dependent on protein composition

1. NEURAL - neurones 3. MUSCLE - myocytes Blood Ciliated Simple Stratified

Basement membrane

Bone

squamous cuboidal columnar

2. EPITHELIUM - epithelial

Cartilage

4. Connective tissues

Nervous tissue –  Consists of cells specialized for initiating and transmitting electrical impulses –  Found in brain, spinal cord, and nerves

Epithelial tissue –  Consists of cells specialized for exchanging materials between the cell and its environment –  Organized into two general types of structures •  Epithelial sheets •  Secretory glands

Muscle tissue –  Specialized for contracting which generate tension and produce movement –  Three types of muscle tissue •  Skeletal muscle –  Moves the skeleton

•  Cardiac muscle –  Pumps blood out of the heart

•  Smooth muscle –  Encloses and controls movement of contents through hollow tubes and organs

Connective tissue –  Connects, supports, and anchors various body parts –  Distinguished by having relatively few cells dispersed within an abundance of extracellular material –  Examples •  Tendons •  Bone •  Blood •  Cartilage

Cell-To-Cell Adhesions •  Adhesions bind groups of cells into tissues and package them into organs •  Once arranged, cells are held together by three different means –  Extracellular matrix •  Serves as biological “glue” •  Major types of protein fibers interwoven in matrix –  Collagen, elastin, fibronectin

–  Cell adhesion molecules in cells’ plasma membranes (CAMs) temporary adhesion –  Specialised cell junctions more permanent adhesion

Specialized Cell Junctions •  Three types of specialized cell junctions – Desmosomes – Tight junctions (impermeable junctions) – Gap junctions (communicating junctions)

Desmosomes •  Act like “spot rivets” that anchor two closely adjacent nontouching cells •  Most abundant in tissues that are subject to considerable stretching

Tight junctions •  Firmly bond adjacent cells together •  Seal off the passageway between the two cells •  Found primarily in sheets of epithelial tissue •  Prevent undesirable leaks within epithelial sheets

Gap junctions •  Small connecting tunnels formed by connexons •  Especially abundant in cardiac and smooth muscle •  In nonmuscle tissues permit unrestricted passage of small nutrient molecules between cells •  Also serve as method for direct transfer of small signaling molecules from one cell to the next

Blisters Stress shears the proteins connecting the different layers in the skin separate

Necrosis

Apoptosis Cell Renewal

Cell death

Tissue homeostasis (remodelling)

Blastocyst (totipotent)

Stem cells

Neural stem cells (multipotent)