Cilia (singular is cilium) are structurally similar to eukaryotic flagella but are smaller and more hair-like. Cilia are found in some eukaryotic organisms. Some cilia are used for locomotion, as in the single-celled paramecium. In other organisms, the cilia act as
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a filter. Sometimes, cilia are used not to move the cell itself, but to move objects through a cell (akin to a conveyer belt).
Vacuoles are specialized organelles that are responsible for storing starch, water, and pigments. They also act as a repository for metabolic wastes. Some plant cells contain a large, central vacuole, which occupies almost the entire cell. Central vacuoles are re- sponsible for providing support, which is based on the amount of water or pressure against the cell wall. If too much water is lost in the central vacuole, a plant will lose its support and appear to droop.
Centrioles are found in all animal cells and some plant cells. These structures, which occur in pairs, are responsible for the cytoskeleton. The cytoskeleton is composed of microtubules, microfilaments, and intermediate filaments. It is with these long struc- tures that the cytoskeleton provides support, maintains the cell shape, and anchors the organelles. The cytoskeleton is also used for moving structures or products.
Within eukaryotes is an endomembrane system. In this system, the endoplasmic retic- ulum, which consists of a membrane that forms folds and pockets, connects the nu- clear envelope, the Golgi apparatus (or Golgi complex), and cell membranes. This system is often called the factory of the cell because each of the individual organelles contributes to the production and delivery of proteins, lipids, and other molecules.
The nucleus contains the blueprints for proteins. These plans are then passed to the rough endoplasmic reticulum (RER). This structure is composed of several folds of a membrane and is covered with ribosomes (these bumps are why it is called rough en- doplasmic reticulum). Once the ribosomes receive the plans, the protein is built. Some proteins will move to the Golgi complex. Other proteins will move to the smooth en- doplasmic reticulum (it is called smooth because it lacks ribosomes). These proteins instruct the organelle to build other molecules, such as lipids and carbohydrates. Like some proteins from the RER, some of these molecules will move to the Golgi com- plex.
The Golgi apparatus is the central post office area of the cell. It receives the products of the rough endoplasmic reticulum and smooth endoplasmic reticulum, packages them, and ships them to their intended destination.
Another structure found only in photosynthetic cells is the chloroplast. This special- ized structure belongs to a class of membrane-lined sacs called plastids (like the vac-
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uole). The chloroplast contains pigments and is responsible for creating food through photosynthesis.
Eukaryotic cells contain an organelle called the mitochondria, which is the site of en- ergy production. This structure is often referred to as the powerhouse of the cell. Cel- lular energy is stored in the form of adenosine triphosphate (ATP).
The ability of a cell to absorb water and nutrients is an important aspect of its sur- vival. Diffusion is the movement of solutes (dissolved molecules) in a solution or ma- trix from an area of high concentration to an area of lower concentration. Molecules move down the concentration gradient: from an area of high concentration to an area of low concentration. The greater the concentration differential, the faster the rate of diffusion. The size, shape, and composition of the solute also affect the ability of a substance to diffuse. These factors become increasingly important when considering the diffusion of substances across the cell membrane. Diffusion, being a passive process, is quite efficient across small distances. However, as distances become longer, the efficiency of diffusion decreases.
Osmosis is the movement of water across a selectively permeable membrane from an area of lower concentration (of solute) to an area of higher concentration (of solute). Remember that everything in the universe is constantly moving toward a state of equilibrium. Living cells contain a small amount of salt. For example, human cells contain 0.85% NaCl. If the solution outside the cell has this same concentration, the solution is said to be isotonic. Because there is no net difference in solutes between the inside and outside of the cell, there is no net movement of water. Higher concen- trations of solutes outside of the cell are termed hypertonic, while lower concentra- tions are termed hypotonic.
An important concept that affects how well a cell can absorb and pass material through the membrane is the surface-to-volume ratio. This formula for calculating this ratio is:
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