02_cellstructfunct - Cell Structure and Function-1 CELL...

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Cell Structure and Function -1- CELL STRUCTURE and FUNCTION CONCEPTS CHECK LIST INTRODUCTION The cell theory states that the smallest unit of life is the cell, that every cell can be considered an elementary organism, and that cells can only be produced from other cells. The most important feature of a cell that limits its size is the surface area to volume ratio. Most eukaryotic cells are ~10 - 100 μm in length/diameter. Most prokaryotic cells are ~1 - 10 μm in length/diameter. Almost all cells are below the limit of resolution of the human eye (~100 - 200 μm). All cells are bounded by a plasma membrane. Prokaryotic cells lack a nucleus and other membranous organelles. Plant and animal cells have many internal structures in common, but plant cells lack lysosomes, centrioles, and gap junctions, whereas animal cells lack chloroplasts, central vacuoles, cell walls, and plasmodesmata. THE PLASMA MEMBRANE The plasma membrane is a selectively permeable barrier that separates a cell’s cytoplasm from the external environment. The basic structure of a biological membrane is a fluid phospholipid bilayer with associated proteins. Because of the selectively permeable nature of membranes, gradients of concentration, electrical charge, or both can be formed and used to do work such as active transport. THE NUCLEUS The nucleus is a membranous compartment that contains most of a eukaryotic cell’s genes. The nucleus contains most of a eukaryotic cell’s DNA, which is complexed with protein to form chromatin. The chromatin in a nucleus is present in discrete chromosomes. The nucleus is enclosed by the nuclear envelope, a double membrane system. The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum. Nuclear pore complexes are present in the nuclear envelope and allow for molecular traffic between the nucleoplasm and the cytosol. The nucleus is a site where chromosome replication occurs prior to mitosis or meiosis. Most a eukaryotic cell’s transcription occurs in the nucleus (some also occurs in mitochondria and chloroplasts). A nucleolus is a region within a nucleus where many rRNA genes are being transcribed to produce the rRNA molecules required by ribosomes. THE ENDOMEMBRANE SYSTEM The endomembrane system is composed of membranes and organelles that are either in direct physical contact or continuity with each other, or are indirectly related to each other through vesicles. The components of the endomembrane system includes the nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vacuoles, vesicles, and the plasma membrane.
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Cell Structure and Function -2- THE ENDOPLASMIC RETICULUM The endoplasmic reticulum (ER) is an extensive membranous network of tubules and sacs whose interior is continuous with the periplasmic space and whose membrane is continuous with the outer membrane of the nuclear envelope.
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02_cellstructfunct - Cell Structure and Function-1 CELL...

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