YAW BBL_WebCT Introduction and skeletal_Lecture1 PDF

Dr Yasser Abdel-Wahab (Module Coordinator) WEEK 1 Introduction to Basic Anatomy and Physiology including anatomy of the Skeletal & Integumentary systems Aims: To give an overview of the basic concepts of anatomy and physiology including anatomy of the skeletal system Lecture Outlines:...

Dr Yasser Abdel-Wahab (Module Coordinator) WEEK 1 Introduction to Basic Anatomy and Physiology including anatomy of the Skeletal & Integumentary systems Aims: To give an overview of the basic concepts of anatomy and physiology including anatomy of the skeletal system Lecture Outlines: 1. Overview of human anatomy 2. Anatomy of the skeletal system, 3. Anatomy of the integumentary system. 4. Introduction to anatomy and physiology 5. Major systems of the body and their function 6. Physiology as an intergrative science 7. Homeostasis’ and regulation of homeostasis 8. Cell communication 9. Reference of chemistry terms 10. Basic components of a human cell 11. Plasma membrane and transport 12. Osmosis The Intended Learning outcomes are: Introduction to anatomy of the various body systems Understand the basic structure and function of bone, including the four cell types which constitute bone tissue. Understand the classification of bone and the divisions of the skeletal system. Outline the bones of skull and vertebrae Describe the basic structure and function of the integument (skin). Outline the layers of the epidermis and underlying dermis. Give an overview of the accessory structures of the integument. Define physiology and anatomy and appreciate the various specialities of each discipline. Identify the major systems of the human body and their general function. Explain the concept of homeostasis and the components of feedback systems. Describe how negative and positive feedbacks contribute to homeostatic regulation. Give and overview of communication and integration within the body, with particular reference to cell signalling. List the principal components of a human cell. Describe the functions of the plasma membrane and associated proteins. Describe the various transport mechanisms that cells use to facilitate the absorption or removal of substances. 1  Dr Yasser Abdel-Wahab (Module Coordinator) Overview of anatomy of the human body The human body is made up of a number of anatomical and physiological systems which are listed below. The skeletal system Major organs: bones, cartlidge, associated ligaments and bone marrow Functions: Provide support and protection for other tissues, stores calcium and other minerals, forms blood cells. The nervous system Major organs: brain, spinal cord, peripheral nerves and sensory organs Functions: Directs immediate responses to stimuli, coordinates or moderates activities of other organs systems, provides and interprets sensory information about external conditions. Muscle Major organs: skeletal muscle, associated tendons and aponeuroses (tendinous sheets) Functions: Provides movement, support and protection for other tissues, generates heat that maintains body temperature. The cardiovascular system Major organs: heart, blood and blood vessels Functions: Distributes blood cells, water, nutrients, waste products, and gases (oxygen and carbon dioxide; distributes heat and assists in controlling body temperature. The respiratory system Major organs: nasal cavities, sinuses, larynx, trachea, bronchi, lungs and alveoli Functions: Delivers air to alveoli of lungs for gas exchange, provides oxygen to and removes carbon dioxide from blood stream, produces sounds for communication. The endocrine system Major organs: pituitary gland, thyroid, pancreas, adrenal glands, gonads (ovaries and testes) and endocrine tissue in other systems 2  Dr Yasser Abdel-Wahab (Module Coordinator) Functions: Directs long-term changes in the activities of other organ systems, adjusts metabolic activity and energy use by the body, controls many functional and structural changes during development. The gastrointestinal system Major organs: Teeth, tongue, pharynx, esophagus, stomach, small intestine, large intestine, liver, gallbladder and pancreas Functions: Processes and digests food, absorbs and conserves water, absorbs nutrients, stores energy reserves. The renal system Major organs: kidneys, ureters, urinary bladder and urethra Functions: excretes waste products from blood, controls water balance by regulating urine volume, stores urine prior to voluntary elimination, regulates blood ion concentrations and pH. The reproductive system Major male organs: testes, epididymis, ductus deferens, seminal vesicles, prostate gland, penis and scrotum Major female organs: ovaries, uterine tubes, uterus, vagina, labia, clitoris, mammary glands Functions male: Produce sperm and male sex hormones (testosterone) Functions female: Produce female sex cells (oocytes) and hormones, supports developing embryo from conception to delivery, provides milk to feed newborn infants. The integumentary system Major organs: skin, hair, sweat glands and nails Functions: Protects against environmental hazards, helps regulate body temperature, and provides sensory information. The lymphatic system Major organs: spleen, thymus, lymphatic vessels, lymph nodes, tonsils. Functions: Defends against infection and disease, returns tissue fluid to the bloodstream. Anatomy of the Skeletal System Functions of bone: Bone has a number of important functions as listed below 1. Support – groups of bones provide framework to which tissues an organs can attach 2. Protection – for example, the skull protects the brain, while ribs protect the heart and lungs. 3. Movement – skeletal muscle attached to the bones move them by contracting/relaxing 4. Mineral homeostasis – bones store calcium and phosphate which can be released into blood to maintain normal levels 5. Site of blood cell production – red marrow within the bones contain progenitor cells for formation of blood cells (red and white cells and platelets). 3  Dr Yasser Abdel-Wahab (Module Coordinator) 6. Storage of energy (yellow marrow) – lipid can be stored in yellow marrow within bones as an energy reserve. Structure of the skeletal system: The main structural elements of the skeletal system are 1. Bone 2. Cartlidge - 3. Bone marrow – red and yellow bone marrow within the bone cavity 4. Periosteum – membrane covering bone surface, which links to connective tissue joining bones at joints Histology of bone: bone is composed of 4 main cell types as listed below  Osteoprogenitor cells – divide to form cells which become osteoblasts and are important in fracture repair  Osteoblasts – these cells produce new bone matrix (osteogenesis)  Osteocytes – these are mature bone cells and make up most of the bone cells.  Osteoclasts – these cells remove and recycle bone matrix. They are very large and can have more than 50 nuclei. This diagram shows the steps involved in formation of bone by osteoblasts Classification of bone - is on the basis of shape:  Long bones – long and slender, located in arms, hands, legs and feet, the largest being the femur bone of the thigh.  Short bones – examples include carpal bones of wrist and tarsal bones of ankle.  Flat bones – examples include sternum, ribs, scapulae and the bones on top of the skull  Irregular bones – complex in shape (short, flat, notched or ridged surfaces), such as vertebrae of spine. 4  Dr Yasser Abdel-Wahab (Module Coordinator) Divisions of the Skeletal System: The skeletal system can be divided into the axial skeleton (bones which form longitudinal axis of the body, 80 bones in total) and appendicular skeleton (bones of limbs and supporting elements like pelvis, 126 bones in total). The diagrams below show the bones of the axial skeleton (left) and appendicular skeleton (right). The skull Infant skull -superior view The infant skull bones (see diagram right, REDRAW) are separated by fontanels (soft spots) which allow the brain to grow. By the age of about 1.5 – 2 yrs these fontanels become smaller and eventually the skull bones fuse at the sutures (coronal, sagittal, squamous and lamboid sutures). The main bones of the adult skull are also shown in the diagram below (REDRAW). The 8 cranial bones include: The frontal bone – forms the anterior of the skull and upper orbits of the eye socket. 5  Dr Yasser Abdel-Wahab (Module Coordinator) The 2 parietal bones – form much of superior and lateral surface of the skull The occipital bone – forms much of the posterior and inferior region of the skull The 2 temporal bones – form lateral region covering the inner ear and linked to zygomatic arch The sphenoid bone – forms part of the floor of skull, unites the cranial and facial bones, and strengthens the sides of the skull The ethmoid bone – forms anterior mid floor of skull and roof of nasal cavity The 14 facial bones include: The maxillary bone – also known as upper jaw bone The zygomatic bones – form part of rim and lateral orbit of eye socket and form part of zygomatic arch The nasal bones – support superior portion of nose and connects to cartlidge forming external nares (nose) The lacrimal bones – form part of medial wall of eye socket The mandible – also known as lower jaw bone Adult skull -superior view Adult skull -lateral view The vertebral column (spine) The vertebral column is made up of 26 bones, 24 vertebrae, the sacrum and the coccyx (tail bone), as shown in this diagram. The vertebral column has 4 curves which define the regions of the spine, 1. the cervical curve 2. the thoracic curve 3. the lumbar curve 4. the sacral curve 6  Dr Yasser Abdel-Wahab (Module Coordinator) Vertebrae have 3 basic parts: 1. vertebral body – part which transfers weight along axis of vertebral column. Each vertebral body is separated from its adjacent verterbrae by intervertebral discs (pads of fibrous cartlidge) 2. vertebral arch – bone arching from the vertebral body around the vertebral foramen which holds the spinal cord. The walls of the arch are called pedicles and the roof of the arch is called the lamina. Spinous processes project posteriorly from the point of the vertebral arch and can be felt under the skin of the back. 3. articular processes – These arise at junction between the pedicles and the lamina and form both superior and inferior articular processes on each side of the vertebra. The inferior articular process of one vertebra will articulate with the superior articular process of the verterbra below. The last vertebra of one region usually resembles the first vertebra of the next region. The cervical vertebrae There are 7 cervical vertebrae (C1 – C7). They extend from the occipital bone of the skull to the thorax. They support the weight of the head and are relatively small compared to the other vertebrae. A typical cervical vertebra is shown in the diagram below The first cervical vertebra C1 is called the atlas (as it holds up the head) and vertebra C2 is called the axis. The body of C1 and C2 fuse during development to form the Dens of axis which allows rotation of the atlas and skull. C1 has no body or spinous process The thoracic vertebrae There are 12 thoracic vertebrae (T1 – T12) as shown in the diagram (REDRAW). They - are much larger than the cervical vertebrae - have heart shaped bodies and smaller foramens than C1-C7. The ribs articulate with T1-T12 at the costal facets on the side of the vertebral body. T1 - T8 articulate with 2 pairs of ribs and have and both inferior and superior costal facets on each side. T1 - T10 also have transverse costal facets for articulation with the ribs T9 – T11 have only one costal facet on each side as they only articulate with 1 pair of ribs. 7  Dr Yasser Abdel-Wahab (Module Coordinator) The spinal process of C7 resembles that of T1. The gaps separating the pedicels of adjacent vertebrae can be seen and is called the intervertebral foramen The lumbar vertebrae There are 5 lumbar vertebrae (L1 – L5) as shown in the diagram below. They are the largest vertebrae as they must support the weight of the body. The vertebral body is thick and oval shaped. There are no costal or transverse costal facets. The vertebral foramen is triangular shaped. They have large downward projecting spinous processes to with large surface area for attachment of lower back muscles. The sacrum is made up of 5 fused sacral vertebrae which are completely fused by ages of 25-30 yrs. The coccyx is made up of 3 – 5 coccygeal vertebrae which start to fuse by about 26yrs of age. The appendicular skeleton The appendicular sketelon consists of the pelvic and pectoral girdles and the bones of the limbs and extremities. The girdles attach the bones of the limbs to the axial skeleton. Anatomy of the integumentary system The integumentary system is a major organ or organ system which consists of: The cutaneous membrane (skin) which has two components: epidermis (superficial epithelium) which is the outer layer of the skin dermis (connective tissue layer under the epidermis). Accessory structures including which originate in the dermis and protrude through the epidermis: Hair Nails Multicellular exocrine glands 8  Dr Yasser Abdel-Wahab (Module Coordinator) A layer known as the hypodermis or subcutaneous layer lies under the dermis and separates the integument from the underlying tissues and organs (muscles, etc). The dermis has rich blood supply and innervation, with sensory receptors which relay signals to the CNS based on touch, pressure, temperature and pain. Main functions of the integumentary system There are 6 main functions of the integumentary system: Protection – protects underlying tissues and organs from external environment, including keeping microorganisms out. Excretion – excretes salt, water and organic waste in form of sweat Maintenance – maintains body temperature Detection – detects touch, pressure, temperature, pain and sends sensory information to CNS to elicit response. Synthesis – synthesizes vitamin D by conversion of 7-dehydrocholesterol by UV light Storage – stores nutrients. The epidermis The epidermis is the outer layer of skin and the main epithelial cell type in epidermis is the keratinocyte. Keratinocytes form several layers: 4 layers in thin skin (approx 0.08 mm thick) 5 layers in thick skin (palms of hand and soles of feet, and up to 6 times the thickness of thin skin). The 5 layers of keratinocytes which make up the epidermis (skin) are: Stratum germinativum – inner layer attached to basement membrane separating dermis and epidermis; has epidermal ridges which extend into dermis to increase contact area between epidermis and dermis; area between adjacent ridges filled with dermal papillae (raised areas of dermis). Epidermal ridge patterns cause contours in skin surface (e.g. fingerprints). Large Basal cells (stem cells in the germinativum) make up most of this layer and divide to replace the upper layers of epidermis. Melanocytes (pigment cells) are scattered throughout germinativum and produce brown skin pigment. Stratum spinosum – lies above the stratum germinativum. Daughter cells from division of basal cell enter this layer and may further divide to increase thickness of the epidermis. Has 8-10 layers of cells. Adjacent keratinocytes are attached strongly by desmosomes. Contains Langerhans cells which stimulate immune defense against microorganisms and superficial skin cancers. Stratum granulosum – lies above the stratum germinativum. Has 3-5 layers of keratinocytes and most cells no longer undergoing cell division. Cells in this layer produce keratin (fibrous protein which makes up hair and nails) and keratohyalin. While keratin fibers are being produced the cells in this layer become thin and flat, have thickening and reduced permeability of cell membrane and eventually die and dehydrate. This results in the formation a tight fibrous layer. 9  Dr Yasser Abdel-Wahab (Module Coordinator) Stratum lucidum – present in thick skin. Cell are filled with keratin, are flattened and densely packed and forms a glassy (clear layer) layer over the granulosum. Stratum corneum – This forms the outer layer of skin and can consist of 15-30 layers of dead keratin filled cell. The cells in these layers remain tightly connected by desmosomes. The stratum corneum provides a protective layer over the dividing layers of the germinativum, spinosum and granulosum, and cells remain in this layer for ~2 weeks before being shed in sheets of cells. It takes between 15 and 30 days for cells to move from inner germinativum layer to corneum layer. Water can penetrate the stratum corneum from the interstitial layer and be lost by evaporation from the surface of the skin (~500 ml/day). The outer surface is maintained by secretions from the sebaceous glands and sweat glands. The Dermis The dermis lies under the epidermis and is made up of 2 layers which contain blood vessels, lymphatic vessels and nerves: The superficial papillary layer – loose connective tissue layer containing capillaries and sensory neurons. This layer forms the dermal papillae which extend between epidermal ridges. Deep reticular layer – contains a meshwork of dense irregular connective tissue and bundle of collagen fibers which extend into the papillary layer and also into the underlying subcutaneous layer. Sweat glands and hair follicles originate in epidermis and extend into dermis. The subcutaneous layer This layer of loose connective tissue is abundant in adipocytes (fat cells) and stabilizes the position of the skin in relation to underlying tissues and organs. The upper region of the subcutaneous layer contains arteries and veins which supply the dermis and epidermis. The abundant adipocytes help to reduce heat loss, act as energy reserves, and provide protection (like shock absorbers). The accessory structures of the integument These include, hair and hair follicles, nails, sebaceous glands and sweat glands Hair and hair follicles Hair: originates in hair follicles and the human body has ~5 million hairs over most of the body surface. There are two type of hair 10  Dr Yasser Abdel-Wahab (Module Coordinator) Vellus hair – fine hairs located over most of body surface Terminal hair – thick and pigmented hair such as that on head and eyebrows. Hair colour is derived from the pigment produced by melanocytes at the hair papilla. Hair grows in cycles (2-5 yr for scalp hair at rate of ~0.33 mm/day). Hair follicle becomes inactive at end of cycle and the hair comes away from the follicle. The follicle then produces new hair at the start of the new cycle. Hair follicles: extend deep into dermis and even into subcutaneous layer. Follicles consist of: Hair papilla – connective tissue containing capillaries and nerves located at base of follicle. Hair bulb – epithelial cells surrounding the hair papilla Hair matrix – epithelial cell layer which divide and undergo keratinisation to form hair. The dividing cells are pushed towards skin surface. Inner cells of matrix from medulla (core) of hair and have soft flexible keratin. Cells at edge of matrix form stiff outer cortex (contains hard keratin). Dead cells at surface of hair form hard outer cuticle of hair. Follicle walls: There are several layers in the follicle wall. Internal root sheath – produced by cells at edge of hair matrix and surrounds hair root and deep portion of hair shaft. External root sheath – extend from hair matrix to skin surface and is similar to outer layer of epidermis. Cells at join between sheath and matrix resemble stratum germinativum cells. Glassy membrane – thick basement membrane surrounded by outermost dense connective tissue sheath of the hair follicle. Nails Nails cover the dorsal tips of finger and toes and protect tips from distortion. Nail root – epithelial fold which extends close to periosteum of fingertip bone and is site of nail production. Nail body – main part of nail made up of dead tightly compressed keratin rich cells which lie on nail bed and is are bounded by nail groves and nail folds at the sides and base of the nail. Cuticle – portion of stratum corneum of nail root which extends over the nail. Hyponychium – thick portion of stratum corneum over which free edge of nail sits. Exocrine glands There are two different exocrine glands in the skin. These are: Sebaceous (oil) gland: These are holocrine glands (release their products by rupture of cells closest to duct) which produce oily lipid secretion into hair follicles and skin surface. 11  Dr Yasser Abdel-Wahab (Module Coordinator) Contraction of arrector pili muscle of hair squeezes gland to force secretion from duct into the hair follicle. The secretion is known as sebum and contains triglycerides, cholesterol, proteins and electrolytes. Sebum lubricates the hair and skin and inhibits bacterial growth Sweat glands: There are 2 types of sweat gland Apocrine sweat glands – coiled tubular glands which communicate with hair follicles found in armpits, groin and around nipples. They produce a sticky, cloudy sometime odorous secretion. Myoepithelial cells surround the secretory cells of the gland and contract to cause glandular secretion. Merocrine sweat glands – more numerous than apocrine glands with approx 2-5 million in adult skin, being most concentrated on the palms of hands and soles of feet. These are also coiled tubular glands but secrete directly onto skin surface. Sweat produced by these glands has a pH of 4 – 6.8 and contains water (99%), sodium chloride and other electrolytes, organic nutrients, and waste products. Mecrocrine sweat functions to: Cool skin surface and reduce body temperature by evaporation of the sweat (perspiration). Excrete excess water and electrolytes (some drugs are also excreted) Protect from environmental hazards by diluting harmful chemicals and inhibiting microbial growth. Introduction to physiological concepts Definitions Anatomy Comes from Greek word anatome (to cut up). Study of structure and relationships between structures. Physiology Greek origin, literally means “knowledge of nature”. Aristotle (384-322 B.C.) used the word in its literal sense. Hippocrates (c.a. 460-377 B.C.), the father of medicine, interpreted it as “the healing power of nature”. Subdivisions of anatomy and physiology Physiology Systemic physiology Cell physiology Functions of specific Functions of living cells organ systems Special physiology Pathophysiology Functions of specific Effects of disease on organs organ/system function 12  Dr Yasser Abdel-Wahab (Module Coordinator) Life processes Metabolism Sum of all the chemical processes that occur in the body. Responsiveness The ability to detect and respond to changes in the external or internal environment. Movement Includes motion of the whole organism, individual organs, single cells, or even organelles within cells. Growth Increase in size resulting from increase in number or size of cells, or both. Differentiation Change of cell from unspecialized to specialized state. Reproduction Formation of either new cells for growth, repair, or replacement, or the production of a new individual. Major systems of the body The major body systems and their function are listed in the table below. 13  Dr Yasser Abdel-Wahab (Module Coordinator) This diagram shows the overall body composition of each body system  The muscular system account for the about 44% of total body composition by weight.  The skeletal system is next at about 20%.  Then the integumentary system (skin) at 16%.  The reproductive system and endocrine systems each account for only 0.15% of body composition by weight. Physiology: the integrative science In physiology, all or most of the organs systems communicate and function by intergration with the other body organ systems. This diagram gives a general overview of the intergration between the body systems, e.g. the digestive system absorbs water and nutrients, which enter the blood stream requiring the heart to pump the blood around the body. Excess water is secreted by the kidneys and urinary system, but also through sweat (integumentary system). Many functions of the organ systems and their process are regulated by the nervous system and endocrine system. As shown in this lower diagram, intracellular fluid of most cells in the body communicates with the extracellular fluid surrounding them. The extracellular fluid communicates with the protective cells of the body amd with the exchange cells to take in materials from the outside of the body and take in materials essential for cells. Exchange cells also allow communication between cells external environment such as changes in termperature. 14  Dr Yasser Abdel-Wahab (Module Coordinator) Homeostasis and its regulation Homeostasis is: Process by which a nearly stable environment is maintained in the body, so that cellular metabolic functions can proceed at maximum efficiency. Maintained by effectors (generally muscles or glands), which are regulated by sensory information from the internal environment. Relationship with pathophysiology Opposite in meaning. Health reflects homeostasis, pathophysiology (abnormal function) marks deviation from homeostasis. Maintenance of homeostasis is by either: - Negative feedback Can be considered as a homeostatic loop. Where the internal environment deviates from a set point, control system responds with a counterchange, or reversal (hence “negative”), which restores homeostasis. - Positive feedback Cannot be considered as a homeostatic loop. Where the stimulus (stress) produces a response which reinforces, exaggerates or enhances its effects. Accelerates processes which must be completed rapidly (e.g. childbirth). Feedback systems (Loops) Feedback systems Involve cycle of events in which information about the status of a condition is monitored and reported (fed back) to a control centre. Feedback systems have three basic components: – A control centre. – A receptor. – An effector. Control centre Determines the set-point at which a factor should be maintained, and determines an appropriate response. Receptor Monitors deviations from the set-point, and then sends information (input) to the control centre. Effector Receives information (output) from the control centre and generates a response (effect). 15  Dr Yasser Abdel-Wahab (Module Coordinator) Analogies for homeostasis A balancing act Circus performer perched on board that is resting on a ball To stay on the ball, he can’t hold still; his weight is continuously shifting from one leg to the other. As long as he moves his arms and weight within a limited, physical range, he can maintain his balance on the ball. However if his left arm goes up too high or his weight shifts too far to the right, he loses his balance and falls to the ground. Keeping ourselves from falling off the ball is the basis for homeostatic regulation. Popular misconception In spite of the literal meaning of homeostasis, humans are never physiologically in a state of “unchanging sameness” unless we are dead. Homeostasis is a completely dynamic process, one that is constantly fluctuating between normal limits. Thermostatic control of room temperature Requirements: An original stimulus (a change in the environment) Pathway is one way (receptor can only receive information about the environment, it cannot respond. The effector can only respond if the control centre sends a message to do so. The crucial link between effector and receptor is feeback. Note: This is the same pathway modelled in the nervous system and in a reflex arc. Afferent (toward the centre) Efferent (away from the centre) 16  Dr Yasser Abdel-Wahab (Module Coordinator) Negative feedback loop (e.g. temperature regulation)  Increase in body temperature disturbs normal homeostasis.  Temperature sensors in skin send signal to control centre in hypothalamus  Hypothalamus sends response to skin via efferent neurons to increase sweat secretion and dilation of blood vessels close to skin surface.  This causes increased heat loss by evaporation of sweat and radiation of heat through skin from dilated blood vessels.  This returns body temperature to normal maintaining homeostasis. Positive feedback loop (e.g. labour and childbirth)  Fully grown foetus distorts/stretches walls of the uterus.  This stimulates stretch receptors in uterus wall to send signal to control centre (endocrine centre) in brain.  Brain send effector signal to stimulate contraction of uterine wall muscles.  The contraction further distorts the uterus which sends more signals via stretch receptors to brain to enhance effector response.  This then helps to contract the uterus until childbirth is complete. 17  Dr Yasser Abdel-Wahab (Module Coordinator) Communication and integration Homeostasis Maintained by communication within and between systems. Relies on chemical and electrical signals. Maintained by local or long distance pathways. Basic methods of cell-to-cell communication 1. Direct cytoplasmic transfer of electrical and chemical signals through gap- junctions connecting adjacent cells (connexin proteins pores joining cytolplasm of adjacent cells form these junctions). 2. Local chemical communication through paracrines (act on neighbouring cells), autocrines (act on same cell as produced signal) or neuromodulators (released from nerve endings). 3. Long-distance communication using both chemical (hormonal, travels in blood) and electrical signals (travel via nerves). Reference of Chemistry Terms Polar – a molecule that carries a small positive charge at one end and a corresponding negative charge at the other is called polar. The water molecule is a typical polar molecule. Note these charges are much weaker than those carried by ions. Ions and polar molecules dissolve readily in polar solvents such as water. Such molecules are called hydrophilic (water-loving). Non-polar – a molecule that is electrically neutral. Oils and fats are typical non-polar substances. Ionic compounds such as salts, and strongly polar molecules such as water, do not readily dissolve in or mix with non-polar solvents. Substances that do not mix with water are called hydrophobic (water-hating). Some substances, such as steroid hormones and some alcohols, can dissolve in both polar and non polar solvents, and can act as solvents for both hydrophobic and hydrophilic substances. Some large molecules can have distinct hydrophilic and hydrophobic parts; these molecules are called amphiphilic (loving both). The phospholipids that constitute the plasma membrane, as well as many transmembrane proteins, exploit this property. 18  Dr Yasser Abdel-Wahab (Module Coordinator) Basic concepts in cell physiology Principal components of a human cell 1. Plasma (cell) membrane – Outer membrane separating the internal components from the extracellular environment. 2. Cytosol – Intracellular fluid containing dissolved proteins, enzymes, nutrients, ions and other molecules. 3. Organelles – Highly organized and specialized structures suspended in the cytosol, such as cell nucleus (contains DNA), endoplasmic reticulum (translation of RNA into protein occurs in ER), mitochondria (generate energy for the cells), golgi apparatus (packages proteins) etc. 4. Inclusions – Temporary structures in the cytoplasm which contain secretions and storage products of the cell (such as Zymogen granules in exocrine cells of pancreas which store digestive enzymes). The diagram below shows the anatomy of a typical cell (REDRAW) Cells contain a central nucleus which has a central nucleolus surrounded by nucleoplasm. The nucleus has an outer porous nuclear envelope to allow communication between nucleus and cytoplasm. The cytoplasm around the nucleus contains the endoplasmic reticulum (ER) which can contain ribosome (which translates mRNA into protein) and is responsible for synthesis of proteins and other molecules such as lipids (like cholesterol) and carbohydrates. ER which contains ribosomes is known as rough endoplasmic reticulum (RER), while that without ribosomes is known as smooth ER (SER). Golgi apparatus store, modify and package the products of the endoplasmic reticulum. Lysosomes and peroxisomes are vesicle in the cytoplasm which contain enzymes which remove damaged organelles, pathogens and neutralize toxic compounds. The cytoplasm also contain cytoskeleton (give support to cell and transport materials within cell), centrosomes (organise microtubules of cytoskeleton and aid cell division), and ribosomes (can be free or fixed to endoplasmic reticulum). The cells can also have membrane extensions called mircovilli and cilia which increase the surface area for absorption and aid in movement of materials over cell surface. 19  Dr Yasser Abdel-Wahab (Module Coordinator) Plasma membrane Gatekeeper regulating the passage of substances into and out of the cell. Described by the fluid mosaic model, where the membrane is a mosaic of proteins floating like icebergs in a sea of lipids. General functions include: – Physical isolation. – Regulation of exchange with environment. – Communication between cell and environment. – Structural support. This diagram shows the anatomy of a typical cell membrane Functions of membrane proteins Structural proteins Maintain shape by linking membrane to cytoskeleton. Form part of cell-to-cell connections holding tissues together. Enzymes Catalyze chemical reactions occuring on external surface or immediate internal surface. Receptors Receptor proteins on outer surface of cell form important part of body’s chemical signalling system. Each receptor is specific of certain molecules (ligands) or families of molecules. Ligand binding to receptor usually triggers a cascade of events. Transporters Transport proteins fall into two categories: channels and carriers. Channel proteins have water-filled passages which link the intracellular and extracellular compartments. Carrier proteins never form a direct connection between the intracellular and extracellular fluid, carry solutes across the membrane by changing conformation, or shape. 20  Dr Yasser Abdel-Wahab (Module Coordinator) Membrane permeability Permeability Determines which substances can enter or leave cell cytoplasm. Cell membranes are selectively permeable, that is, they permit the free passage of some materials and restrict passage of others. Passage across the membrane is either passive or active. – Passive processes Move ions or molecules without any energy expenditure by the cell. – Active processes Require energy expenditure as ATP. Main transport processes 1. Diffusion – Passive process resulting from random motion and collisions of ions and molecules. 2. Filtration – Passive process occuring when hydrostatic pressure forces fluid and solutes across a membrane barrier. 3. Carrier-mediated transport – Either passive or active process requiring the presence of specialized integral membrane proteins. 4. Vesicular transport – Active process involving the movement of materials within small membranous sacs, or vesicles. This diagram shows different types of diffusion across the plasma membrane  Lipids can diffuse through the lipid membrane of cells.  Ions and small soluble molecules diffuse through membrane channels. Other compound which can not diffuse freely across membrane need to be transported by carrier molecules in the cell membrane. 21  Dr Yasser Abdel-Wahab (Module Coordinator) Osmosis: a special case of diffusion As net diffusion of water across a membrane is so important, it has been given a special name, osmosis. As such, osmosis is the response to a concentration gradient as shown below. Stage 1 Equal volumes in both compartments. Solution in compartment 1 is more concentrated. Semi-permeable membrane does not allow glucose to cross. Stage 2 Osmosis stops when concentrations of glucose in both compartments are equal. Stage 3 A force must be applied to stop osmosis. The amount of force required to stop osmosis is called the osmotic pressure. Reading Lists:  Martini FH & Nath JL (2008) Fundamentals of Anatomy and Physiology (Eighth Edition), San Francisco, Pearson Benjamin Cummings.  Martini FH (2008) Fundamentals of Anatomy and Physiology Applications Manual (Seventh Edition), San Francisco, Pearson Benjamin Cummings Benjamin Cummings. 22