Cells: The Basic Units of Life

- All cells come from preexisting cells and have certain processes, types of molecules, and structures in common.

- The first cells may have arisen from aggregates of macromolecules in bubbles.

- To maintain adequate exchanges with its environment, a cell’s surface area must be large compared with its volume.

- Cells can be visualized by various methods using microscopes.

- All cells are surrounded by a plasma membrane.

- All prokaryotic cells have a plasma membrane, a nucleoid region with DNA, and a cytoplasm that contains ribosomes, water, and dissolved proteins and small molecules.

- Some prokaryotes have additional protective structures: cell wall, outer membrane, and capsule. Some prokaryotes contain photosynthetic membranes or mesosomes, and some have flagellaor pili.

- Like prokaryotic cells, eukaryotic cells have a plasma membrane, cytoplasm, and ribosomes. However, eukaryotic cells are larger and contain many membrane-enclosed organelles.

- The membranes that envelop organelles in the eukaryotic cell are partial barriers, ensuring that the chemical composition of the interior of the organelle differs from that of the surrounding cytoplasm.

- Organelles can be isolated by cell fractionation.

- The nucleus is usually the largest organelle in a cell. It is surrounded by a double membrane, the nuclear envelope, which disassembles during cell division. Within the nucleus, the nucleolus is the source of the ribosomes found in the cytoplasm. Nuclear pores have a complex structure.

- The nucleus contains most of the cell’s DNA, which associates with protein to form chromatin. Chromatin is diffuse throughout the nucleus until just before cell division when it condenses to form chromosomes.

- The endomembrane system is made up of a series of interrelated compartments enclosed by membranes.

- The rough endoplasmic reticulum has attached ribosomes that synthesize proteins. The smooth endoplasmic reticulum lacks ribosomes and is associated with the synthesis of lipids.

- The Golgi apparatus receives materials from the rough ER by means of vesicles that fuse with its cis region. Vesicles originating from the trans region of the Golgi contain proteins targeted to different cellular locations. Some of these vesicles fuse with the plasma membrane and release their contents outside the cell.

- Lysosomes contain many digestive enzymes. Lysosomes fuse with the phagosomes produced by phagocytosis to form secondary lysosomes in which engulfed materials are digested. Undigested materials are secreted from the cell when the secondary lysosome fuses with the plasma membrane.

- Mitochondria are enclosed by an outer membrane and an inner membrane that folds inward to form cristae. Mitochondria contain the proteins needed for cellular respiration.

- The cells of photosynthetic eukaryotes contain chloroplasts. These organelles are enclosed by double membranes and contain an internal system of thylakoids organized as grana.

- Thylakoids within chloroplasts contain the chlorophyll and proteins that harvest light energy for photosynthesis.

- Both mitochondria and chloroplasts contain their own DNA and ribosomes and are capable of making some of their own proteins.

- The endosymbiosis theory of the evolutionary origin of mitochondria and chloroplasts states that these organelles originated when larger prokaryotes engulfed, but did not digest, smaller prokaryotes. Mutual benefits permitted this symbiotic relationship to be maintained, allowing the smaller cells to evolve into the eukaryotic organelles observed today.

- Peroxisomes and glyoxysomes contain special enzymes andcarry out specialized chemical reactions inside the cell.

- Vacuoles are prominent in many plant cells and consist of a membrane-enclosed compartment full of water and dissolved substances. By taking in water, vacuoles enlarge and provide the pressure needed to stretch the cell wall and provide structural support for the plant.

- The cytoskeleton within the cytoplasm of eukaryotic cellsprovides shape, strength, and movement. It consists of three interacting types of protein fibers.

- Microfilaments consist of two chains of actin units that together form a double helix. Microfilaments strengthen cellular structures and provide the movement in animal cell division, cytoplasmic streaming and pseudopod extension. Microfilaments may be found as individual fibers, bundles of fibers, or networks of fibers joined by linking proteins.

- Intermediate filaments are formed of keratins and are organized into tough, ropelike structures that hold organelles in place within the celland add strength to cell attachments in multicellular organisms.

- Microtubules are composed of dimers of the protein tubulin.

They can lengthen and shorten by adding and losing tubulin dimers. They are involved in the structure and function of cilia and flagella, both of which have a characteristic “9 + 2” pattern of microtubules.

- The movements of cilia and flagella result from the binding of the motor protein dynein to the microtubules. Dynein and another motor protein, kinesin, also bind to microtubules to move organelles through the cell.

- Centrioles, made up of triplets of microtubules, are involved in the distribution of chromosomes during cell division.

- Materials external to the plasma membrane provide protection, support, and attachment for cells in multicellular systems.

- The cell walls of plants consist principally of cellulose. They are pierced by plasmodesmata that join the cytoplasm of adjacent cells.

- In multicellular animals, the extracellular matrix consists of different kinds of proteins, including proteoglycans. In bone and cartilage, the protein collagen predominates.