Cells, Membranes And Organelles
The aqueous internal environment of the cell is separated from the aqueous external medium by an envelope of fat molecules (lipids) known as the plasma membrane. About half the cell is filled with cytosol, a viscous, protein-rich fluid between the internal structures. These consist of organelles which are themselves enclosed by lipid membranes, and components of the cytoskeleton such as microtubules and actin filaments which provide structural stability. The reticular appearance of the cell interior is due to organelles whose membranes are folded to maximize surface area. These include the rough endo- plasmic reticulum and Golgi apparatus, which are involved in protein assembly, and the smooth endoplasmic reticulum which serves as a store for intracellular Ca2+ and is the major site of lipid production (Fig. 3a).
The nucleus (Fig. 3a) contains the chromosomes and nucleolus, a membrane-less structure responsible for production of ribosomes. Ribosomes translocate to the rough endoplasmic reticulum (giving it its appearance), where they are responsible for protein assembly. The endoplasmic reticulum and Golgi apparatus perform post-trans- lational processing of new proteins. This includes trimming amino acid chains to the right length, protein folding, addition of polysac- charide chains (glycosylation) and identification of improperly folded proteins, which are tagged for subsequent destruction by lysosomes. Proteins are delivered from the Golgi apparatus to specific intracellular destinations. For example, receptor and structural proteins are sent to the membrane and digestive enzymes to lysosomes, and molecules for extracellular action are packaged into secretory vesicles. Lysosomes contain acid hydrolase enzymes which catabolize macromolecules.
They work optimally at pH 5.0, and as cytosolic pH is ∼7.2, anyleaking into the cytosol cannot attack the cell inappropriately. Lysosomes digest unwanted and defective proteins, recycling raw materials and preventing accumulation of rubbish.
Membranes and membrane proteins Membrane lipids (mostly phospholipids) comprise a hydrophilic (water-loving) head, with two short hydrophobic (water-repelling) fatty acid tails (Fig. 3b). In an aqueous medium they self-organize into a bilayer with the heads facing outwards and the tails inwards (Fig. 3b). They diffuse freely within each layer (lateral diffusion) so the membrane is fluid. The hydrophobic interior and hydrophilic exte- rior of the membrane means that lipid-soluble (hydrophobic) sub- stances such as cholesterol incorporate into the membrane, whilst molecules with both hydrophobic and hydrophilic domains such as proteins can be tethered part in and part out of the membrane (the fluid mosaic model; Fig 3b). Many such molecules provide signalling, transport or structural functions. The latter are provided by proteins such as spectrin, which binds to the inner layer and forms an attachment framework for the cytoskeleton. Lipid-soluble molecules such as O2 and CO2, and small molecules such as water and urea readily pass through the lipid bilayer. However, larger molecules such as glucose and polar (charged) molecules such as ions cannot, and their transport is mediated by transporter and ion channel membrane proteins (Chapter 4). Membrane proteins also undergo lateral diffusion and move around the membrane. However, the cell can control exactly which proteins insert into which portion of the membrane. For example, cells lining the kidney tubules are polarized so that Na+ –K+ ATPase transporters (Chapters 4 and 33) are located only on one side of the cell. Most cells are covered by a thin gel-like layer called the glycocalyx, containing glycoproteins and carbohydrate chains extending from the membrane and secreted proteins (Fig. 3b). It protects the membrane and also plays a role in cell function and cell–cell interactions.
Membrane proteins associated with cell signalling include enzymes bound to the inner surface such as phospholipases, which produce arachidonic acid (a precursor of some second messenger molecules), and adenylyl cyclase, which generates the second messenger cyclic adenosine monophosphate (cAMP). cAMP activates protein kinase enzymes to initiate numerous changes in cell function by phosphor- ylating membrane and intracellular proteins. Transmembrane proteins (Fig. 3b) penetrate the entire thickness of the bilayer, and include receptors and ion channel proteins. The intramembrane segments are composed of hydrophobic amino acid residues and the extra- and intra-cellular portions predominantly of hydrophilic residues. Receptors include those that bind growth factors and regulate gene transcription, and the superfamily known as G-protein–coupled receptors (GPCRs). The latter possess seven membrane-spanning segments and detect neurotransmitters or hormones in the extracellular medium. On binding the appropriate molecule, they activate specific mem- brane-associated GTP-binding proteins (G-proteins), which cleave guanosine triphosphate (GTP) to guanosine diphosphate (GDP), and depending on type (e.g. Gs, Gi, Gq), activate or inhibit other membranebound signalling enzymes such as adenylyl cyclase. Transmembrane proteins such as integrins and cadherins provide structural and signalling links with other cells and the extracellular matrix (Fig. 3b). Their cytosolic ends bind to components of the cytoskeleton, including protein kinases which can initiate, for example, altered gene transcription or changes in cell shape.
Mitochondria and energy production Mitochondria use molecular oxygen to, in effect, burn sugar and small fatty acid molecules to produce adenosine triphosphate (ATP), which is used by all energy-requiring cellular reactions. Glucose is first converted to pyruvate in the cytosol by glycolysis, producing in the process a small net amount of ATP and reduced nicotinic adenine dinucleotide (NADH). Glycolysis does not require O2, so when O2 is limited, this anaerobic respiration can supply some ATP, with NADH being reoxidized to NAD+ by metabolism of the pyruvate to lactate (Fig. 3c). However, under normal conditions where there is sufficient O2, oxidative phosphorylation in the mitochondria produces ∼15 fold more ATP for each glucose molecule than does glycolysis. Pyruvate and fatty acids transported into the mitochondrial matrix act as substrates for enzymes that drive the citric acid (Krebs’) cycle, which generates NADH and the waste product CO2. The electron transport chain, a series of enzymes in the inner mitochondrial membrane, then uses molecular O2 to re-oxidize NADH to NAD+. In doing so, it generates a H+ ion gradient across the inner membrane which drives the ATP synthase (Fig. 3c). Note that mitochondria are not solely devoted to ATP production, as they are also involved in other cellular processes, including Ca2+ homeostasis and signalling.