Cell Ultrastructure and Function

Cell Ultrastructure and Function

• Cells are divided into two main areas: the Nucleus and the Cytoplasm.
• Collectively, these form the protoplasm.
• Cytoplasm is mainly composed of an aqueous background material (cytosol) which is approximately 90% water.
• It contains organelles and inclusions.
• The cytoskeleton, formed by microfilaments, gives the cell shape and support.
• Actin, a contractile protein, is part of the cytoskeleton.
• The cytosol is involved in endocytosis, exocytosis, and cytoplasmic streaming (motility).
• The endoplasmic reticulum (ER) is a complex membrane system of flattened vesicles (cisternae) found around other organelles like mitochondria, or in a tubular form.
• There are two types of ER: rough ER and smooth ER.
• Rough ER has ribosomes on its outer surface, involved in protein synthesis and transport. It connects with the nuclear envelope.
• Smooth ER lacks ribosomes and is involved in lipid synthesis and detoxification, especially in the liver. It also stores calcium in skeletal muscle.
• Ribosomes are small organelles (20nm in diameter) responsible for protein synthesis.
• They are found attached to rough ER, or free in the cytoplasm, or within mitochondria and chloroplasts.
• Ribosomes are composed of two subunits containing RNA and protein.
• Ribosomes can associate to form polyribosomes (polysomes) during protein synthesis.
• The Golgi apparatus is a complex of flattened membrane-bound vesicles (cisternae) or saccules.
• It processes, sorts, and modifies proteins and lipids received from the ER.
• It is important for secretion to other locations.
• It forms lysosomes.
• Lysosomes are membrane-bound vesicles containing digestive enzymes (lysitic enzymes).
• They are involved in digesting unwanted material, old organelles and even entire cells.
• Peroxisomes are vesicles containing catalase to break down hydrogen peroxide.
• They are linked to phagocytosis.
• Mitochondria are small elongated organelles (2-5µm long) found in all eukaryotic cells.
• They have a double membrane system or envelope.
• The inner membrane folds into cristae, increasing the surface area for chemical processes.
• Mitochondria are involved in aerobic cellular respiration, converting energy-rich substrates into ATP.
• They contain limited DNA and ribosomes.
• Found in high abundance in high-energy cells (liver, muscles).
• Chloroplasts are organelles responsible for photosynthesis.
• They are enclosed by two membranes and contain thylakoids (which contain chlorophyll) stacked in grana.
• The stroma is the fluid-filled space surrounding the thylakoids.
• Chloroplasts contain their own DNA and ribosomes.
• Vacuoles are membrane-bound sacs found in plant cells.
• They store water, nutrients, waste products, and other substances.
• The central vacuole maintains turgor pressure in plant cells.
• The cellulose cell wall surrounds plant cells, providing rigidity and strength.
• Plasmodesmata are connections between plant cells, allowing cytoplasm exchange.
• Microtubules form part of the cytoskeleton and are involved in maintaining cell shape, cell division, vesicle transport, and cell motility.
• Cilia and flagella are hair-like structures involved in movement. Cilia are for moving fluids, flagella for propelling cells.
• They contain microtubules.
• Microvilli are small projections on the surface of certain cells (intestines) which increase surface area for absorption of molecules.
• Cell junctions (tight, gap, desmosomes) are important for holding together tissues in various parts of the body.