Tour of the Cell

• The cell is the fundamental unit of function in living systems in biology & the simplest collection of matter that can live
• Cells are studied using microscopes and biochemistry tools such as subcellular fractionation
• The light microscope cannot resolve finer than 0.2 micrometres
• The electron microscope can show the ultrastructure of cells
• Staining with iodine or methylene blue is very important
• Cell fractionation involves taking apart & separating cell components in order to study cell structure & function. Cells are blended (homogenization) & centrifuged causing cell components to settle to the bottom of the tube
• The nucleus contains most genes in eukaryotic cells. It is surrounded by the nuclear envelope, which is a double membrane perforated by pores. DNA is organised into chromosomes made up of chromatin. Chromatin fibres condense to form chromosomes in preparation for division. A typical diploid human cell has 46 chromosomes in its nucleus.
• Ribosomes are complexes of rRNA & protein which carry out protein synthesis. Free ribosomes are suspended in the cytosol. Bound ribosomes are attached to the outside of the endoplasmic reticulum or nuclear envelope. They are structurally identical but have different roles. Free ribosome's proteins function in the cytosol & bound ribosome's proteins are inserted into membranes for packing within organelles or secretion from the cell.
• The endomembrane system consists of the nuclear envelope, the endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles, and the plasma membrane.
• The endoplasmic reticulum is an extensive network of membranes that accounts for more than half the total membrane in eukaryotic cells. It consists of membranous tubules and sacs called cisternae. The ER membrane separates the ER lumen from the cytosol and is continuous with the nuclear membrane. The smooth ER has no ribosomes, the rough ER has ribosomes on the outer surface and produces proteins for secretion. Most secretory proteins are glycoproteins. They depart wrapped in transport vesicles. The rough ER is also the membrane factory 🏭
• The Golgi Apparatus is the centre for manufacturing, warehousing, sorting, and shipping. Transport vesicles arrive here to be modified, stored, and sent to other parts of the cells. Extensive in cells specialised for secretion
• Lysosomes are membranous sacs containing hydrolytic enymes used to digest macromolecules, made by the rough ER and processed by the Golgi apparatus. They have an acidic pH at which enzymes work best and fuse with vacuoles during phagocytosis.
• Food vacuoles are formed by phagocytosis. Contractile vacuoles are used to pump excess water out of the cell by protists. Mature plant cells have a large central vacuole containing cell sap which stores proteins and ions.
• Mitochondria are the sites of cellular respiration. They have 2 membranes and contain DNA coding for synthesis of own ribosomes. They are semiautonomous, growing and reproducing within the cell. The number of mitochondria often correlates with the cell's level of metabolic activity. They are between 1 and 10 micrometres long. The outer membrane is smooth while the inner membrane has cristae thus creating a larger surface area. The inner matrix contains enzymes that catalyse some steps of cellular respiration
• Chloroplasts are specialised members of the plastids family. They contain chlorophyll which functions in photosynthesis. They are lens-shaped and between 2 and 5 micrometres. Their contents are partitioned from the cytosol by a 2 membrane envelope. Inside are thylakoids, a membrane system of flattened interconnected sacs called grana. The fluid outside the thylakoids is the stroma, containg DNA, ribosomes, and enzymes. They move along the cytoskeleton.
• The peroxisome is a specialised metabolic compartment bounded by a single membrane. It contains enzymes that produce hydrogen peroxide and breakdown fatty acids for cellular respiration. In the liver, they detoxify poisons. Special kinds called glyoxysomes are found in fat-storing tissues of plant seeds
• The cytoskeleton is a network of fibres which gives mechanical support to the cells & maintains its shape. It is important for animal cells and is stabilized by opposing forces. It provides anchorage points for organelles and cytosolic enzyme molecules. It can be quickly dismantled and moved. Cell motility generally requires the interaction of the cytoskeleton with motor proteins. There are three types of molecular structures: Microtubules, microfilaments, and intermediate filaments
• Extracellular components include: cell wall, microfibrils of cellulose, and the extracellular matrix
• Cell communication is facilitated by channels perforated in the cell wall called plasmodesmata which the cytosol can pass through
• Intercellular junctions include: tight junctions, desmosomes, gap junctions
• Autophagy is a cellular process that involves the degradation and recycling of cellular components, such as organelles and macromolecules. Lysosomes play a crucial role in autophagy.