Cell Structure and Function
Our understanding of cell structure and function is based on modern Cell Theory, which states:
1. all living things are made of one or more cells
2. cells are the basic units of life and all the chemical reactions of life
occur in cells
3. all cells arise from preexisting cells
Cells, because they are the basic unit of life, need to do all the things which satisfy the criteria which describes life. They need to take in materials, extract useful energy, and make their own molecules, they need to grow in an organized manner, they need to respond to stimuli, and they need to reproduce themselves.
To do these things, all cells must have some minimum number of different parts, for example they must have some kind of confining membrane to maintain its integrity. This membrane is called the plasma membrane; it is remarkably similar in all cells.
How big are cells? Well, most are far too small to be seen with the naked eye; there are exceptions such as bird eggs (which are single cells) and some long cells like muscle and nerve cells. Most are between 10 and 100 um (a micrometer is a millionth of a meter). Bacteria, which are single cells, are 0.20 um. Most animal cells are about 20 um. Why are they so small?
The answer lies in the relationship between surface area and volume; the surface area/volume hypothesis says the ratio between surface area and volume will impose size limits on cells. Cells need to exchange materials with their environment and, of course, this exchange occurs at the plasma membrane. As cells get larger, they need to exchange more materials. As the volume of a cell increases, so does the surface area, however volume increases faster than surface area. If you double the volume of a sphere or a cube, the surface area does not double. Thus as growth increases, a point is reached where the surface area can no longer service the needs of the volume of the cell. This point is about 100 um for a spherical cell. Some cells can be larger, such as bird eggs, by putting all the living parts in one corner of the cell (most of the cell is nonliving yolk and albumin) or they can be really thin, like nerve and muscle. Thin cells have more surface area relative to volume than spherical cells.
Prokaryotic versus Eukaryotic Cells
Prokaryotic (pro means first) cells are those found in bacteria and blue-green algae. These cells lack a nucleus, place their DNA in regions called nucleoids (which have no membrane around them), possess plasma membranes and often cell walls, and may have structures designed for movement. They are much simpler than Eukaryotic cells.
Eukaryotic (Eu = true) cells possess a nucleus, but the key difference is that they can coexist and cooperate as multicellular organisms. Some cells in organisms then can become specialized to perform specific tasks. Within Eukaryotic cells, different cell tasks are organized into compartments called organelles.
The Cell Boundary
At the surface of all cells lies the plasma membrane, the structure that keeps unneeded and harmful materials out and keeps useful materials in, plus it can regulate the flow of materials into and out of the cell.
One of the dynamic tasks of any cell is to move materials in and out of the cell across the plasma membrane. Inside is the intracellular fluid, the inner pool of water, nutrients, and other molecules, while outside is the extracellular fluid. Cells need to exchange materials between these two fluids to survive. Recall, that the membrane is selectively permeable. How is this accomplished?
Materials move across the membrane in one of two ways both of which are governed by gradients - differences in concentration of materials between one area and another (e.g. on either side of a cell membrane). Materials tend to move down gradients, from areas of higher concentration to areas of lower concentration. This type of movement is called passive transport.
1. Passive Transport - requires no extra energy, materials move down gradients.
a. Simple diffusion - the tendency of materials to move from areas of high to low concentration; eventually the system reaches a state of dynamic equilibrium, where molecules are still in motion, but no concentration gradient exists. This is how gases such as CO2 and O2 move across cell membranes
b. Carrier-facilitated diffusion - the movement of molecules that are large or electrically charged across membranes facilitated by carrier molecules. These carriers are proteins collectively called permeases embedded in the plasma membrane. Glucose and other chemicals may enter cells this way. It is not well understood, but it requires no energy and occurs with the gradient.
c. Osmosis - movement of water through a selectively permeable membrane. Because water is the most important substance to move across membranes, we have a special word for it, although the process is really no different than simple diffusion. Water simply moves from a region where it is present in high concentration or pressure to one where it is present in low concentration or pressure. Here concentration refers to the relative number of water molecules on either side of the membrane. Distilled water will have a higher concentration than the same volume of water with a solute dissolved in it(e.g. salt water).
Balance of Intracellular and Extracellular solutes and solvents:
Solute refers to that substance which is dissolved (suspended) in another medium (e.g. water).
Solvent refers to the medium into which the solute is dissolved.
Many cells are isotonic to their medium - there is the same concentration inside the cell as outside. Note that water will move across the membrane in isotonic solutions, but there is no net gain or loss of molecules on either side. (see Figure drawn in lecture).
If you put one of these cells in distilled water, the medium (the distilled water) is now hypotonic - the fluid outside has a higher concentration of water molecules ( or a lower concentration of solute if you prefer) than inside the cell so water will flow into the cell. The cell will swell and eventually burst in this situation. Many plants normally exist in hypotonic media; the water that moves into plant cell (often stored within the vacuole) causes the organelles to be pushed against the rigid cell walls (which do not burst) and exerting a pressure called turgor pressure. More on the positive value this has to plants later in the course.
If cells are in an environment with a higher concentration of salts (or other solutes) outside (or less water inside) water tends to move out of the cells. Such an environment is called hypertonic.
See Figures drawn in lecture for illustration of above.
2. Active Transport - requires energy; typically movement of materials against the concentration gradient. Cells need to move substances (including water) against their concentration gradient to survive; this costs energy, usually in the form of ATP.
a). Active transport may involve molecules or larger particles. Molecules are usually transported across membranes by carrier proteins in the lipid bilayer; they act like pumps. One of the most important is the sodium/potassium exchange pump. Here, for every sodium ion transported out, a potassium ion is transported in. Depending upon the cell, it takes 30-70% of a cells energy budget to work this pump. Other materials are transported by proteins that themselves move from one side of the membrane to the other; the difference between this and carrier-facilitated diffusion is energy is required.
b). Larger particles are actively transported by an out pocketing or in pocketing of the plasma membrane. In pocketing is called endocytosis (cell-eating), where the plasma membrane folds around a particle. Many single-celled organisms use this method to ingest food. Exocytosis is out pocketing of the plasma membrane; cells use this as a method to eliminate wastes.
Internal Structures of Cells -- suspended in the cytoplasm are numerous smaller compartments called organelles.
a. Cytoplasm - semifluid, organized ground material in cells; it acts as a pool of raw materials. Most (70%) is water, and the rest is proteins (mostly), carbohydrates, and nucleotides, as well as their constituents (amino acids, nucleotides, etc.).
b. Cytoskeleton - a three-dimensional network of small protein fibers. It functions to suspend organelles in the cytoplasm and allows the regulated movement of cell parts.
1. Ribosome - small structures occurring mostly in the cytoplasm. They are the site of protein synthesis. They consist of molecular complexes of ribosomal RNA and proteins. They are technically not organelles because they are not enclosed in a membrane, but are larger than molecules. They also occur in prokaryotes.
2. Nucleus - the cell's control center. It contains all the genetic information in the form of DNA. Inside the nucleus is a small dark-staining region called the nucleolus (typically 2 per cell) - it manufactures ribosomal RNA. Note: the singular for nucleolus is nucleoli.
The two chief functions of the nucleus are to a) carry hereditary information, and b) exert influence on ongoing cell activity, especially controlling the type and number of substances needed for various parts of the cell to maintain homeostasis.
Organelles of Synthesis, Storage, and Export - these consist primarily of a series of interconnected membranes similar in structure to plasma membranes; these membranes form sacs, tubes and channels for transport and storage of materials.
3. Endoplasmic reticulum - a network of flattened hollow tubules and channels; there are two
a. Smooth ER - manufactures lipids, contains enzymes that detoxify certain poisons, transports carbohydrates, lipids, and other non-proteins.
b. Rough ER - these tubules are studded with ribosomes (giving it a rough appearance) and it is where proteins are synthesized that are destined to be secreted out of the cell. (In contrast to the ribosomes suspended in the cytoplasm which produce proteins which will be used within the cell).
4. Golgi apparatus - a collection of flat sacs that transport the proteins from the rough ER to the outside of the cell. Enzymes in the Golgi apparatus modify the newly-made proteins by adding chemical groups. This material is transported to the outside in secretory vesicles which leave the cell by pinching off to the outside of the cell. For example, Lysosomes contain powerful digestive enzymes. They a) help recycle worn out cell parts by breaking them down into molecular components, b) act like miniature stomachs, by releasing enzymes to break food down to simpler molecules, and c) act as "suicide bags" by breaking and digesting entire cells from the inside-out (a process called autophagy); autophagy is useful to destroying and recycling aging or damaged cells.
5. Mitochondria - This double membrane bound organelle is often called the cell's powerhouse. This is the site where the cell's chemical energy in the form of ATP is produced by the biochemical process known as cell respiration. Carbon compounds, such as glucose, are broken down in association with the Mitochondria into CO2 and H2O by a process which releases energy that is then stored within the phosphate bonds of ATP molecules. These molecules then diffuse throughout the cell to where energy is required. This reaction requires oxygen and is called aerobic respiration - more on this later.
6. Vacuoles - Vacuoles are bound by a single membrane. Their functions include: a) taking up space and pushing other organelles closer to the plasma membrane where materials can be exchanged and also give the cell its shape, b) store waste products to be released later or used to prevent other organisms from eating them.
7. Cilia and Flagella - organelles of movement.
Flagella are whiplike microtubules jutting out of certain cells; many single-celled organisms use them for propulsion. In plants, flagella are only found in species where there are motile gametes (swimming) such as in the mosses, liverworts, ferns, cycads, and Ginko biloba.
Cilia are shorter and more numerous; they may cover the surface of some cells (e.g. Paramecium)and move in waves allowing many cells to move through a medium.
Both cilia and flagella are composed of microtubules in an arrangement of 9 tubules surrounding 2 central ones (9 + 2 arrangement).