Cell Ultrastructure and Function Overview

Cell Ultrastructure and Function

• Cells are divided into two broad areas: the Nucleus and Cytoplasm. Collectively, these form the protoplasm.
• Cytoplasm is an aqueous background, containing organelles and inclusions.
• The cytosol is the aqueous component of the cytoplasm and is about 90% water.
• The cytosol is transparent and structureless, but contains microfilaments, forming the cytoskeleton.
• The cytosol contains contractile protein actin, and is involved in endocytosis/exocytosis and motility (cytoplasmic streaming).
• The cytosol contains a wide range of biochemicals like salts, amino acids, glucose etc, in true solution.
• Larger molecules like proteins are in colloidal solution in the cytosol. Sol (non-viscous) and Gel (viscous) are forms of this solution.
• The Cytoplasm may also act as a site for metabolic pathways, such as glycolysis.
• Membranes are described in previous notes.
• The Endoplasmic Reticulum (ER) is a complex system of flattened membrane bound vesicles (cisternae).
• The ER is arranged in stacks, concentric patterns around other organelles (like mitochondria), or in a tubular form.
• There are two distinct types of ER.
• Rough ER has ribosomes attached to the external cisternal membrane, and is involved in protein synthesis and transport. It connects to the nuclear envelope.
• Small vesicles containing newly synthesised proteins pinch off from the Rough ER, and may either fuse with the Golgi Apparatus, or the cell membrane.
• Smooth ER has no ribosomes associated with the cisternal membrane. It is involved in lipid transport (including steroids), detoxification (liver), and calcium storage (skeletal muscle cells).
• Ribosomes are 20nm in diameter, function as protein synthesis sites. They are found in association with the Rough ER, or alone in the cytoplasm. These are also found inside mitochondria and chloroplasts. Ribosomes are composed of two subunits, both containing RNA and Protein.
• Ribosomes may associate to form chains (polyribosomes)
• Golgi Apparatus modifies proteins, lipids, and carbohydrates.
• The Golgi Apparatus is a stack of flattened plate-like cisternae (saccules).
• New cisternae form on the outer edge of the concave stack, from vesicle fusion.
• The Golgi Apparatus discharges contents into vesicles which then fuse with plasma membrane.
• Lysosomes are membrane-bound vesicles (0.2-0.5 m diameter).
• Lysosomes contain digestive enzymes (lysitic enzymes).
• Lysosomes are used to degrade old/surplus organelles, or breakdown whole cells and tissues, and are linked to phagocytosis (1° & 2° Lysosomes).
• Mitochondria are small elongated organelles (2-5 m length) found in eukaryotic cells. They're involved in aerobic cellular respiration.
• Mitochondria are bounded by a double membrane system (envelope), with a space between them.
• The inner mitochondrial membrane is folded inward to form cristae, which project into a matrix. The inner membrane's large surface area allows for contact with enzymes in the matrix. The cristae have stalked ATPase particles covering their surface.
• Aerobic cellular respiration produces ATP from ADP and inorganic phosphate ions, through the oxidation of energy-rich substrates (pyruvic acid, derived from sugars like glucose through glycolysis in the cytoplasm).
• Mitochondria contain limited DNA and limited capacity for protein synthesis. They also contain ribosomes, abundant in cells needing lots of energy (liver & muscles).
• Chloroplasts are found in plant cells and are involved in photosynthesis.
• Chloroplasts are surrounded by two membranes. The inner membrane folds into grana, stacks of thylakoids. The stroma is the fluid surrounding the grana.
• Thylakoids contain chlorophyll and electron carriers, where the light reactions of photosynthesis occur (reducing NADP/ATP).
• Chloroplasts contain limited DNA for protein synthesis and ribosomes.
• Vacuoles are large, single fluid-filled spaces in plant cells. They store salts, sugars, and pigments. The cell wall also interacts with the vacuole and creates turgor pressure.
• Plant cells have a cell wall, a rigid structure that provides shape and support. It's made of cellulose, and plant cells have plasmodesmata joining them, regions of cytoplasm to connect cells.
• Cilia and flagella are used for locomotion in protozoa, feeding (sponges), and reproduction (sperm). Cilia are for movement of ova in mammals.
• Microtubules are a component of the cytoskeleton in cells.
• Cell junctions, such as tight junctions, gap junctions, desmosome junctions, are used to connect and hold neighboring cells together. Microvilli are finger-like projections present in cells, for increasing surface area for absorption (e.g., small intestine).