PDF Cells simplest structural units Cell differentiation - StudentVIP

Cells – simplest structural units

Cell differentiation  4 types of specialised cells;

▪ muscle cells – produce force and movement ▪ nerve cells – electrical signalling

▪ epithelial cells – secrete and absorb ions and molecules

▪ connective tissue – connect, anchor, support

- Specialised cells associate together to form tissues - organs  4 kinds of tissues in various proportions

- organ systems  collection of organs that perform an overall function

System physiology  relationship between different systems (systems do not work in isolation)  eg. renal system and cardiovascular system  renal system controls fluid/salt balance and osmotic balance, cardiovascular system circulates nutrients and oxygen and generates pressure (eg. blood pressure is affected by osmotic balance)

Homeostatic Control Systems;

▪ homeostasis results from the operation of compensatory mechanisms

▪ negative feedback = response pushes variable in the opposite direction to the original change (inhibitory)

▪ positive feedback = response increases the disturbance even further (stimulatory)  less common  hard to control  can play a role in diseases

****control systems minimise change but cannot maintain complete constancy****

Components of Homeostatic Control Systems;

- receptor, afferent pathway, integrating centre, efferent pathway, effector - pathways can be neural or hormonal

Positive feedback  propagation of an action potential along a nerve axon;

Physical Properties of Cells;

Cell membranes – barrier to the movement of substances into and out of the cell Composed of;

- phospholipids - glycolipids - cholesterol

- proteins  channels, pumps, enzymes Phospholipids;

- hydrophilic polar head - hydrophobic non-polar tail

- form lipid bilayer with hydrophobic tails in the middle and polar hydrophilic heads facing to the outside

- mycelles can form (1 layer, with polar heads on outside and tails on inside) - lipid bilayer is highly flexible/fluid

fluidity depends on;

- cholesterol – rigid molecule that decreases fluidity - unsaturated carbon chains – increase fluidity

- distribution of different types of lipids is not uniform in the cell

Movement of substances across cell membranes;

- membranes act as barriers to the movement of many substances  important for cellular function

- semi-permeable membrane (permeable substances move through)

- physical properties of substance (composition, size, charge, etc.,) affects ability to pass through membrane

Passive;

Lipids  permeable to hydrophobic tails of membrane and can pass through Water  move through pores or channels of cell membrane

Active;

- energy is often required when molecules are going to a place of higher concentration of that kind of molecule

diffusion;

▪ most important physical phenomenon responsible for the movement of substances at a cellular level

fick’s law of diffusion  defines quantitatively the ease with which substance can permeate a membrane

Membrane Flux;

if the substance (molecule) is electrically charged (eg. an ion) then the driving force (in fick’s law) is also the electrical gradient (electrochemical gradient). The electrical gradient is assumed to be constant within the membrane and is defined by;

Passive diffusion through lipid bilayers

- dependant on degree of solubility in the lipid - related to the potential to form H-bonds - permeability constant PS = DS BS/x

- rate of passage is linearly related to concentration facilitated diffusion (passive);

- occurs through pores

- allows movement of ions, water, small molecules

- molecules move down concentration or electrical gradient - has maximum rate (eg. reaches saturation) as only a certain amount can move through a pore at a time

- channels/pores may be specific to certain molecules Voltage gated channels;

Types – Na+, K+ and Ca2+ channels

Ligand-gated channels;

Ca2+ channels

receptors  chemoreceptors (taste, smell, light), hormone receptors (eg. adrenalin receptors), neuropeptides

Stretch activated channels;

Types – pressure sensitive touch receptors (eg. in fingertips especially), hair cells (hearing)

Active Transport Characteristics;

- saturation kinetics  reaches a maximum rate of transport - requires energy in the form of ATP, either directly or indirectly - transport of molecules against a concentration gradient - generally highly selective

Primary active transport;

- Na+/K+ pump maintains distribution of high intracellular K+ and low intracellular Na+

relative to extracellular concentrations (pump requires ATP)

- electrogenic pump  pumps out more + charge than it lets in (eg. pumps out 3Na+ for every 2K+ it lets in)

- net transfer of + charge to outside of cell Secondary active transport;

- doesn’t use ATP directly  energy source is an electrochemical gradient (established (using energy) by the Na+/K+ pump)

- movement of an ion down its electrochemical gradient is coupled to the transport of another molecule

- transporters have 2 binding sites

- movement of the ion is always downhill

- movement of the actively transported solute is always uphill - co-transport = solute moves in same direction as ion

- counter-transport = solute moves in opposite direction to ion

Special transport;

****does not require the molecules to pass through the cell membrane****

endocytosis = plasma membrane pinches off to import an extracellular substance into the cell  includes fluid endocytosis, absorptive endocytosis (bind to membrane proteins and are taken into cell when the membrane invaginates)  these two are often referred to as pinocytosis (cell drinking), phagocytosis (cell engulfs large particles, such as bacteria) exocytosis = membrane bound vesicles in cytoplasm fuse with plasma membrane to release their contents to the outside of the cell

functions of exocytosis;

▪ way to replace portions of plasma membrane lost in endocytosis

▪ provides a route for impermeable cell products to be secreted

Special transport;

Epithelial Transport;

- epithelial cells line hollow organs or tubes (eg. the gut) - regulate absorption and secretion across these surfaces

luminal membrane (aka apical/mucosal membrane) – faces the hollow/fluid filled chamber/cavity (eg. faces into the intestine)

basolateral membrane – faces extracellular/interstitial fluid (outside), usually adjacent to network of blood vessels

Molecules can cross epithelial layer in two ways;

paracellular pathway – through the extracellular spaces between the cells

transcellular pathway – through the cell, across the luminal and basolateral membranes