Cell Structure and Organisation

Cell - basic unit of biological organisation

Characteristics:

• Generate any protein products of genome
• Selective gene expression - reflects specialisation
• Cell division and mitosis
• Metabolism
• Apoptosis
• Communicate with external environment

Cells bound by cell membrane - impermeable to water

Isolates cell components from external environment - regulate composition = stability

Membrane consists of phospholipid bilater - outer hydrophilic globular head and inner hydrophobic fatty acid chains

• Phospholipid - globular head of glycerol, phosphate and organic parts where fatty acid attached
  • Organic part either choline, serine, ethanolamine, inositol
• Spingomyelin - glycerol replaced by amino alcohol (spingosine)
• Cholesterl - steroid, provides rigidity
• Glycolipid - carbohydrate-linked fatty acids

Fluid Mosaic Model - lipids laterally mobile, each phospholipid can exchange places with neighbour but rarely move across bilayers

Proteins/glycoproteins either integral (through entire membrane) or peripheral (only one side). Most tethered to membrane cytoskeleton and integral membrane proteins have hydrophobic amino acids to anchor them.

Functions:

• Cell adhesion for tissue formation - different cell types have different cell adhesion systems
• Receptors for receiving external signals or endocytosis
• Transport proteins - control entry/exit of molecules/ions

Protein transporter types:

• Uniporter - transports 1 molecule along concentration gradient
• Symporter - transports 2 molecules in same direction, one against one along
• Antiporter - transports 2 molecules in different directions, one against one along

Epithelial cells - asymmetric w/ distinct apical, lateral and basal plasma membrane domains = POLARITY

Connect via lateral membranes to form epithelial sheets

Tight junctions formed of adhesion molecules (actin) - brings epithelial cells together and seals space

Specialised cell membrane w/ septum before division

No membrane bound organelles

DNA not membrane bound, in central nucleoid

Cytosol (cytoplasm) - ribosomes not organised into compartments

Mesosomes - invaginations of plasma membrane where secretion/DNA synthesis occurs

DNA forms chromatin in nucleus

Nuclear membrane continuous with endoplasmic reticulum

Histones - proteins forming associations w/ DNA = nucleosomes

Lamins - filamentous mesh on inner surface providing structural function and transcriptional regulation

Nuclear pores - act as filters for passage between nucleus and cytoplasm

• Ions, small molecules diffuse across
• Larger molecules actively transported
• mRNA coupled to proteins (mRNPs) - uncoil, move through pore, immediately associated w/ ribsomes for translation
• RNPs leave nucleus have nuclear export sequence (NES)
• Transcription factors enter nucleus have nuclear localisation signal (NLS)
• NES and NLS bind to shuttle proteins - control passage across pore
• Nuclear basket - regulates size of molecules passing in nucleus

Polarised epithelial cells that have biosynthetic pathway

Biosynthetic pathway - interconnected route - rough ER -> golgi -> secretory vesicle

Secretory substance secreted at apical domain

Smooth ER involved in synthesis of fatty acids/phospholipids

Modifies proteins

cis region - vesicles from ER fuse

Medial region - proteins mature through

Trans region - proteins packaged into secretory vesicles

Acidic organelles - contain hydrolytic enzymes for protein degradation and re-use of amino acids

Associated with phagocytosis, autophagy and endocytosis

Acidic state achieved by H+ ion ATPase pump and Cl- ion channel

Site of ATP production in aerobic respiration

Double membrane - outer contains porin channel protein = permeable

Inner membrane - forms cristae rich in ATP production complexes for electron transport chain

Central matric - mitochondrial DNA coding for specific enzymes for respiration - evidence for cyanobacterial endosymbiosis evolution

Site of photosynthesis

Thylakoid membrane system w/ grana (disc-shaped membrane sticks w/ chlorophyll)

Stroma matrix w/ starch granules

One in each plant cell

Occupies up to 80% of cell volume - store for water, ions, nutrients and waste

pH acidic due to membrane proton pumps and Cl- ion channels

Structural role - osmotic pressure leads to hydrostatic pressure and plant rigidity

Membrane surrounding vacuole called tonoplast