In eukaryotic cells, the DNA is separated from the rest of the cell in the nucleus by the nuclear membrane. This separation is important in the regulation of protein synthesis, as the cellular structures that produce proteins are found outside the nucleus. The nuclear membrane limits the type of molecules that can reach the DNA and also limits the type of molecules that can leave the nucleus and reach the protein synthesis machinery of the cell. Eukaryotic cells have more precise regulation of the production of proteins and their RNA intermediates than do prokaryotes, in which the DNA is not sequestered from the rest of the cell.One feature of eukaryotic cells, particularly within multicellular organisms, is the variety of shapes and sizes of these cells. The shape of the cell is related to the function it performs. Nerve cells, for example, have fingerlike projections that allow them to communicate over a distance. In addition, cells aggregate to form larger structures, such as tissues and organs, that perform specific functions.
Types of Human Cells
The nucleus fulfills a similar function as an animal's brain, directing a cell's growth, division for reproduction, activities, and death. The nucleus houses most of the DNA. Within the nucleus is the nucleolus (plural, nucleoli), a site within the nucleus where ribosomal subunits are assembled before export to the cytoplasm. RNA carries the message encoded on DNA from the nucleus into the cytoplasm, the combination of cytosol, organelles, and other components in the interior of the cell.
An endomembrane system is a system of membranes suspended within eukaryotic cells to partition the cells into functional compartments, such as the Golgi apparatus and endoplasmic reticulum. The endoplasmic reticulum (ER) is a network of membranes that helps process molecules in a cell and transports cell materials. The endoplasmic reticulum may be rough, if studded with ribosomes, or smooth, without ribosomes. The Golgi apparatus is an organelle that attaches chemical markers to molecules produced in the endoplasmic reticulum in order to transport the molecules to their places inside or outside a cell. It is a group of stacked and flattened membrane sacs. Molecules move from the endoplasmic reticulum through the successive sacs of the Golgi apparatus.
A lysosome is an organelle that digests bacteria that enter a cell, eliminates toxins, and recycles unneeded cell materials. A peroxisome is a structure in eukaryotes that transforms fatty acids into sugars and aids chloroplasts in oxidizing plant sugars. Peroxisomes also contain enzymes involved in the degradation of lipids and alcohols.
Called the power plant of eukaryotic cells, a mitochondrion (plural, mitochondria) is an organelle that changes energy from food into energy a cell can use. It has a smooth outer membrane and an inner membrane that is highly folded. A eukaryotic cell has several mitochondria, which are the site of cellular respiration, an activity that removes chemical energy from food. Mitochondria also have their own DNA and can replicate independently of the cell's replication. Both plant and animal cells derive energy from cellular respiration.
Plant cells have chloroplasts that are involved in photosynthesis, the transformation of radiant energy from the sun into chemical energy in the bonds of sugars. A chloroplast is a membrane-bound organelle found in plants and some other organisms that captures energy from light and converts it into chemical energy. Chloroplasts have two membranes and groups of stacked membrane sacs inside. Chloroplasts have their own DNA and can replicate independently of the cell's replication.
Junctions and Other Structures
- Tight junctions seal two cells together to form a tissue barrier that blocks unwanted substances from moving between cells, similar to a watertight seal.
- Anchoring (or adhesion) junctions link one cell to its neighbor via their cytoskeletons. Desmosomes bind cells in animals.
- Gap junctions allow materials to move through cells. Molecules of water and oxygen, as well as electrical signals, pass through gap junctions of cells. When a muscle flexes, the signal to flex moves along neurons through gap junctions to the brain. Plasmodesmata connect cells in plants.
Plants are autotrophs; they produce their own food and form the primary level in most food chains. Food production occurs because plant cells have specialized membrane-bound organelles called chloroplasts. Chloroplasts contain chlorophyll, which provides the plant's green hue and is the plant's means of absorbing light energy from the sun. The sun's energy is combined with water and carbon dioxide in the plant's process of photosynthesis. The end products of photosynthesis are sugars and waste oxygen.
While chloroplasts perform the primary task of food production, other organelles play a role in cellular metabolism. Vacuoles are membrane-bound sacs, much like a bubble, that store nutrients and waste products. Water stored in vacuoles helps keep plant stems, limbs, and branches rigid. The crunch of a carrot comes from the vacuole's ability to maintain turgor pressure, the pressure of water against the plasma membrane, within each cell. That pressure supports the cell wall and the overall structure of the plant. When the vacuoles do not retain sufficient water, plants wilt. Organelles called peroxisomes have a variety of tasks related to conversion of waste from cellular metabolism. These organelles transform fatty acids into sugars and aid chloroplasts in oxidizing plant sugars.Plant cells have stiff cell walls that are made of cellulose, which protect both the cell itself, and the plant as a whole. Plant cells join together to form tissues, such as the vascular tissues phloem and xylem. The phloem transports sugars away from the leaves, while the xylem transports water, minerals, and other nutrients upward from the roots. Other types of cells form protective tissue layers to help plants retain water.
A centriole is made up of a ring of nine groups of fused microtubules, long strands of protein that form tubes. These structures are part of the cytoskeleton, a lattice-like structure that protects cells. In an animal cell, a centrosome consists of two centrioles arranged perpendicular to each other. Centrosomes are organelles that divide and migrate during reproduction. They provide strength to a cell's structure and pull apart chromatids (threadlike strands containing a double helix of DNA) when cells divide.
Some animal cells also contain cilia or flagella, structures that are anchored in the cell membrane. Cilia and flagella push fluids along the surfaces of cells. Flagella are thin, whiplike tails that assist cells in movement. A sperm cell, for example, has a flagellum to enable it to move to find and fertilize an egg. Giardia is a common protist parasite that has a flagellum. Cilia are tiny hairlike structures found on the surface of unicellular animals, such as paramecia, and function in locomotion. Cilia are also found on the cells lining the human respiratory tract, where they help remove debris from the lungs.
Animal cells contain lysosomes, which are organelles that hold enzymes created by the cell that break down complex molecules such as proteins. Lysosomes fuse with vacuoles that contain material derived from within the cell, such as damaged organelles, or ingested from outside the cell. Once the membranes fuse, the contents of the vacuoles are exposed to the enzymes in the lysosome and are degraded. The components can then be reused by the cell.Animal cells exhibit a variety of shapes and sizes that are related to their function. Elongated smooth muscle cells, round white blood cells, and spiky neurons all have the same basic cellular components.