Proteins made from ribosomes attached to the rough endoplasmic reticulum enter the lumen of the ER and move to the smooth endoplasmic reticulum. A small vacuole (vesicle) pinches off the smooth ER and carries the protein to the Golgi apparatus, where it is further processed. ------------------------------------------------------------- Mitochondria are bounded by a double membrane. The inner membrane is folded to form little shelves, called cristae, which project into the matrix, an inner space filled with a gel-like fluid. -------------------------------------------------------------- A vacuole is a large membrane-enclosed sac that usually functions as a storage area. Plant vacuoles contain not only water, sugars, and salts but also pigments and toxic substances. The pigments are responsible for many of the red, blue, or purple colors of flowers and some leaves. ------------------------------------------------------------ The green pigment chlorophyll, found within the grana, makes chloroplasts and leaves green. Chlorophyll absorbs solar energy, and chloroplasts convert this energy into ATP molecules. ----------------------------------------------------------- Chloroplasts carry on the process of photosynthesis, in which light energy is used to produce food molecules, such as glucose. Chloroplasts take in carbon dioxide, water, and solar energy in order to produce glucose and oxygen. ------------------------------------------------------------ The energy-related organelles, chloroplasts and mitochondria, convert one form of energy into another. While chloroplasts are unique to plant cells, mitochondria are found in both plant and animal cells. ------------------------------------------------------------- Chloroplasts carry on photosynthesis, during which light energy (photo) is used to produce food molecules, like glucose (synthesis). Chloroplasts take in carbon dioxide, water, and solar energy in order to produce glucose and give off oxygen. ------------------------------------------------------------ Mitochondria are often called the powerhouses of the cell: just as a powerhouse burns fuel to produce electricity, the mitochondria convert the chemical energy of glucose products into the chemical energy of ATP molecules. -------------------------------------------------------------- Chromatin, a threadlike material, contains DNA and is found within the nucleus. At the time of cell division, chromatin condenses into rodlike structures called chromosomes. ---------------------------------------------------------- The middle lamellae, a region between cell walls, contains a sticky substance, usually pectin. Lignin is a substance found in secondary cell walls that makes them even stronger than primary cell walls. ------------------------------------------------------------ Autodigestion is important during development. For example, when a tadpole becomes a frog, the enzymes within lysosomes digest the cells of the tail, and the fingers of a human embryo are at first webbed, but they are freed from one another by lysosomal action. ------------------------------------------------------------- Lysosomes, vesicles formed by the Golgi apparatus, contain hydrolytic enzymes that can digest macromolecules. Macromolecules are sometimes brought into a cell in vesicles formed at the cell membrane. A lysosome can fuse with such a vesicle and digest its contents into simpler molecules, which then enter the cytoplasm. ------------------------------------------------------------- Special vacuoles (membrane-enclosed sacs) called peroxisomes are often attached to smooth ER, and these contain enzymes capable of detoxifying drugs. ------------------------------------------------------- A chloroplast is bounded by a double membrane. Inside the structure, there is even more membrane organized into flattened sacs called thylakoids. The thylakoids are piled up like stacks of coins, and each stack is called a granum. There are membranous connections between the grana called lamellae. The fluid-filled space about the grana is called the stroma. ------------------------------------------------------------- Inside the chloroplast, there is membrane organized into flattened sacs called thylakoids. The thylakoids are piled up like stacks of coins, and each stack is called a granum. The fluid-filled space about the grana is called the stroma. ------------------------------------------------------------- The nucleus is a large organelle that has a nuclear envelope, chromatin and nucleoli. The nuclear envelope is a double membrane that keeps the contents of the nucleus separate from the cell's cytoplasm. Pores in the nuclear envelope allow large molecules to pass into and out of the nucleoplasm, the fluid interior of the nucleus. ------------------------------------------------------------- Ribosomes are the site of protein synthesis in the cytoplasm. They can be attached to the endoplasmic reticulum or lie free within the cytoplasm. When several ribosomes are making the same protein, they are arranged in a functional group called a polysome. ------------------------------------------------------------ Both plant and animal cells have cell membranes, nuclear membranes, mitochondria, and vacuoles. Chloroplasts are found in plant cells but not in animal cells. ---------------------------------------------------------- All plants have a cell wall, located outside the cell membrane. The primary cell wall contains cellulose whereas the secondary cell walls contain lignin. The middle lamella, a region between cell walls, contains a sticky substance, usually pectin. ---------------------------------------------------------- In the process of aerobic cellular respiration, mitochondria