Cells and Cell Theory
What advantages does small size give to a cell?
Many cellular processes occur by diffusion, which is efficient over short distances, but less efficient over long distances. Since all materials going in and out of a cell must pass through the plasma membrane, the greater the surface area of this membrane, the faster a given quantity of molecules can pass through. Smaller cells have a much greater surface-to-volume ratio than larger cells and therefore can "feed" all areas of the cell in less time.
What is "surface-to-volume ratio," and how does it affect cell size?
The surface-to-volume ratio is a mathematical relationship between the volume of an object and the amount of surface area it has. This ratio often plays an important role in biological structures. Think of a cell as a sphere:
The surface area of a sphere can be calculated by
4ð r2
where r is the radius of the sphere.
Volume of a sphere can be calculated by
4/3 ð r3.
An increase in r will increase the surface area by a power of two, but increase the volume by a power of three. This means that the volume will increase much faster than the surface area. This puts an upper limit on the size of a cell, because if the cell volume gets too big, there won't be enough membrane to transport the amount of food in and wastes out to support that large cell size.
What is the difference between prokaryotic and eukaryotic cells?
Prokaryotic cells are more simple: they are usually much smaller and don't have a nucleus or any other membrane-bound organelles. Bacteria are prokaryotes. Eukaryotic cells are much more complex, are usually larger, and have a nucleus and several other membrane-bound organelles that allow them to compartmentalize their functions. All multicellular plants and animals are eukaryotes. A helpful trick to remember is that "you" are a "eu"karyote.
Are there any single-celled eukaryotes?
Yes--yeast, for example. Yeast are single-celled organisms, but they do contain a membrane-bound nucleus, mitochondria, and other organelles.
What are the advantages and disadvantages of prokaryotic compared to eukaryotic cells?
Although prokaryotes may seem more primitive than eukaryotes, they are among the most successful species on our plant and comprise a very large percentage of the total mass of all living things on earth. Simple, small, and single-celled organisms can reproduce quickly and evolve quickly. Prokaryotes can generate millions of progeny in a short period of time. In addition, some have evolved to thrive in extreme conditions in which no eukaryote can live. Some can also form a protective capsule enabling them to survive periods of adverse conditions.
However, they have no membrane-bound organelles, so they do not have some of the cellular functions that eukaryotes do. Prokaryotes are also unable to join together to form a multicellular organism with specialized tissues. The multicellular human body has a brain, eyes, and an immune system that allow it to take in and process information about its environment and protect itself from pathogens; prokaryotes don't have these functions.
How did eukaryotic organelles evolve?
The endosymbiotic theory suggests that eukaryotic cells may have arisen from prokaryotic cells living in what is called mutualistic symbiosis. Mutualistic symbiosis is a relationship in which two or more organisms live together and each benefit from the partnership. According to the endosymbiotic theory, the first eukaryote began to evolve when one ancient prokaryote lived inside another ancient prokaryote, both benefiting from the arrangement. The inner cell might be able to perform aerobic respiration and provide its host with an efficient way to harness the chemical energy in food. The host could, in turn, provide the inner cell with protection and a supply of resources. It is easy to imagine how such an inner cell might have evolved into the mitochondria inside eukaryotic cells. Other eukaryotic organelles could have evolved in similar ways.
Cells and Cell Theory
cell
The basic unit of life. A living cell is composed of cytoplasm (which contains organelles, such as ribosomes) and is surrounded by a semi-permeable membrane. During part or all of its life, a cell also contains genetic material (DNA). Organisms may be composed of one cell or many. A cell may be eukaryotic, in which case its genetic material is enclosed by an internal membrane to form a structure called a nucleus. A cell may be prokaryotic, in which case its genetic material is not enclosed by a membrane.
cell theory
The theory that all living organisms are made of cells and all cells arise from pre-existing cells.
cell wall
A rigid structure that surrounds the plasma membrane of some cells (plant, fungal, and bacterial cells). Plant cell walls contain fibers of the polysaccharide cellulose. Bacterial cell walls contain a protein and carbohydrate compound called peptidoglycan. Fungal cell walls contain the polysaccharide chitin. In both eukaryotes and prokaryotes, the cell wall provides the cell with support and protection.
cellular work
Activities that occur in a cell which are necessary to the cell's survival. Cellular work requires energy and includes activities such as growth, reproduction, movement, and protein synthesis.
chloroplast
Organelle that is the site of photosynthesis. Chloroplasts contain chlorophyll.
endosymbiotic theory
The theory that the first eukaryotic cells originated from a symbiotic relationship between different types of ancient prokaryotes.
eukaryotic cell
A cell that contains a membrane-bound nucleus and membrane-bound organelles. Eukaryotic cells tend to be larger and more complex than prokaryotic cells.
micron
The unit of measurement often used to describe cellular structures. A micron is equal to one one-millionth of a meter (1 micron = 0.000001 meters).
mitochondrion
(pl: mitochondria) Organelle that is the site of aerobic cellular respiration. Mitochondria are the "powerhouses" of the cell.
nucleus
The organelle that acts as the cell's control center. It contains most of the cell's genetic information (DNA).
organelles
Structures within the cell that carry out particular cellular tasks.
plasma membrane
The membrane surrounding a cell that separates the cytoplasm from the cell's external environment. It acts as a selective barrier, allowing only certain ions and molecules to enter or leave the cell.
prokaryotic cell
A cell that lacks a membrane-bound nucleus and other membrane-bound organelles. Prokaryotic cells tend to be smaller and less complex than eukaryotic cells. Bacteria are examples of prokaryotes.
ribosome
Organelle that is the site of protein synthesis.