Cell Structure and Function

Eukaryotes

Eukaryotic cells contain a nucleus that directs functions and activities of the cell's organelles.
Eukaryotes are cells categorized as having a nucleus and a variety of other membrane-enclosed organelles. Eukaryotes include plants, animals, fungi, and protists. Eukaryotic cells have greater mass than prokaryotic cells and have far more internal complexity. Within each eukaryotic cell, internal membranes create multiple compartments. These compartments, called organelles, carry out specific functions through independent chemical reactions.

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

Eukaryotic cells differ widely in size and shape. Nerve cells (neurons) carry messages from the brain to other parts of the body. Blood cells transport oxygen and nutrients to the cells. Cardiac muscle cells are found in the heart. Skin surface cells provide a layer of protection.

Membrane-Bound Organelles

Eukaryotic cells contain a variety of organelles, each with specific functions within the cell. The complexity of eukaryotic cells can be compared to the various body systems of mammals.

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

Multicellular organisms have a variety of cells that have to maintain close connections with other cells to form tissues in order to fulfill their function in the organism. The type of connections between cells depends on the function that the resulting tissue will have within the organism. A cell junction is a point at which cells are bound together, usually connecting cells to other cells. The types of cell junctions include tight junctions, anchoring junctions, and gap junctions.
  • 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.
Cells are bound to each other at cell junctions. The three types of cell junctions are tight junctions, anchoring junctions, and gap junctions.
There are two other common structures in cells. Eukaryotic cells have a cytoskeleton that determines their shape. The cytoskeleton is a network of filaments that gives the cell its shape and forms the support network for cell functions, such as cell division. The cytoskeleton is formed by a network of different types of protein filaments and extends throughout the cell. The filaments of the cytoskeleton form the support network for the transport of materials within the cell, aid in cell movement, and form the spindle involved in separating chromosomes during cell division. The centrosome is a structure in the cytoplasm of animal cells that coordinates the formation of microtubules, which allow cell division to proceed during reproduction. It is composed of a ring of protein tubes called microtubules, which are also part of the cytoskeleton. The centrosome directs the formation of the spindle involved in cell division. Together, the centrosome and the spindle ensure that each new cell has an equal number of chromosomes by pulling the chromatids apart during reproduction.

Plant Cells

Plant cells are eukaryotic cells with a cell wall and organelles (called chloroplasts) involved in photosynthesis.
While both animals and plants are composed of eukaryotic cells, their cells have different structures and functions. Plant cells are generally rectangular and tend to be larger than animal cells. Plant cells, like animal cells, join together to form tissues. Plants have far fewer types of tissues than higher order animals, but the basic cell structure of plants (such as algae and a giant sequoia) are the same.

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.

Plant Cell

Specialized structures of plants cells include a stiff cell wall for protection and chloroplasts that facilitate photosynthesis. Being eukaryotic, plant cells also contain a nucleus.

Animal Cells

Animal cells are eukaryotic cells that generally lack a cell wall.
Animal cells are eukaryotic cells enclosed within a plasma membrane and contain a variety of membrane-bound organelles suspended in the cytoplasm. Like plant cells, animal cells have a nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, and ribosomes. They may also have centrioles, cilia or flagella, and lysosomes, which are not usually found in plant cells.

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.

Animal Cell

Animal cells are eukaryotic. Specialized structures of animal cells include lysosomes, the "digestive system" of the cell. Animal cells may also have cilia or flagella, which assist in movement of the cell within the fluids around it.

Comparison of Plant and Animal Cells

Plant and animal cells share some structures. The differences in structures help each type of cell carry out its specific functions.