CELLULAR FORM AND FUNCTION

Modified cell theory:

• Cells are basic units of life

• Cells come from existing cells

• Physiology of the organism is controlled at the cellular level

• Structure and function is variable

• Controlled at the level of genetic function

Membrane structure

• Functions of plasma membranes

1. Contains and separates → keeps cell and its components in tact and is the boundary between cells

2. Recognition → contains receptors

3. excitability/irritability → the ability to respond to a stimulus from the environment

4. Transport → provides a surface through which substances can enter and exit the cell via diffusion, facilitated diffusion, active transport and endocytosis

*membranes are selectively permeable*

Fluid mosaic model of membrane structure

The PM is invisible to a light microscope but, appears as a double line under a e-microscope

Membranes: composed of lipids and proteins

• The inner pm faces the cytosol + cytoplasm + intracellular fluid

• The outer pm faces another cell + extracellular fluid/ matrix

• Membranes structure is based on the physical and chemical behavior of phospholipids in H2O

• Amphipathic : hydrophilic head and hydrophilic tail organized into bilayer membranes

• Phospholipids form micelles in water and vesicles within cells

Cholesterol : abundant in animal membranes and provides fluidity and plugs holes

• Often associates with sphingolipids and forms less fluid lipid rafts

• Important for other membrane functions

• Cholesterol is absent from plants

The lipid bilayer is fluid

• Lipids slide laterally

• Bilayer lipids are solvent for dissolved proteins

• Many membrane lipids are glycolipids

• PM protein classification based on structure

Integral and intrinsic proteins

• Extended into bilayer

• Transmembrane proteins go across the PM

Peripheral and extrinsic proteins

• At the edge of the bilayer

• Glycoproteins and carbohydrates are common in membranes

• Glycocalyx the fuzzy-sticky cell coating of the carbohydrate chains that extend from the surface of many cells

Functions of PM proteins

• Transport - transmembrane proteins  from pores / channels that transport substances in and out of cells (ion channels)

• Receptors - recognize and bind a specific substance / ligand

• Ligand = molecule that forms a complex

• hormones , neurotransmitters

• Involves a signal transduction pathway

• Can be blocked or stimulated by drugs

• Attachment to the cytoskeleton or extracellular matrix

• Enzyme activity small intestine, ATP, cell signaling

• Cell adhesion / joining via cell adhesion molecules - important for development and repair

• Cell recognition - critical for the immune system development and repair

Intercellular junctions - hold the cells of tissues together

• Tight junctions : membranes of adjacent cells become fused together to form a protein reinforced structure which resembles a belt that closes off the intercellular space. (common in intestines)

• Gap junction : connection between cells which forms a channel that allows small molecules to pass from cell to cell

• Ions, glucose, common in cardiac muscle

• Desmosome : protein reinforced junction ; plaque present in one PM area, like a “spot weld”

• Hemidesmosomes - link the cell to the ECM

• Adherins junctions - glycoproteins join cells together via bely like protein layers that may attach to the microfilaments

Cytosol: the cell fluid, contains molecules in solution/ dissolved, colloids / milky but wont settle and particles in suspension / will settle

Cytoplasm: the cytosol plus all the organelles