A cell is the smallest unit of life that can reproduce on its own. Unicellular organisms, such as amoebae, are made up of only one cell, while multicellular organisms are comprised of many cells. Cells consist of many different parts that work together to maintain the life of the cell.
The outer surface of human cells is made up of a plasma membrane, which gives the cell its shape. This membrane is primarily composed of a phospholipid bilayer, which itself is made up of two layers of lipids facing in opposing directions. This functions to separate the inner cellular environment from the extracellular space, the space between cells.
Molecules travel through the cell membrane using a number of different methods. During diffusion, molecules pass through the membrane from areas of high to low concentration. (When that molecule is water, the process is called osmosis.) Facilitated diffusion occurs with the assistance of proteins embedded in the membrane. Diffusion is known as passive transport because it does not require energy.
During active transport, proteins in the membrane use energy (in the form of ATP) to move molecules across the membrane. Usually these molecules are large or are being moved against their concentration gradient (from areas of low to high concentration).
Within the cell, specialized parts known as organelles serve individual functions to support the cell. The inside of the cell (excluding the nucleus) is the cytoplasm, which includes both organelles and cytosol, a fluid that aids in molecular transport and reactions.
The function of individual organelles can be compared to the functions of components in a city. The “power plant” for the cell is its mitochondria, which produce energy for the cell in the form of adenosine triphosphate (ATP). This process is known as cellular respiration, as it requires oxygen that is taken in from the lungs and supplied in blood. Byproducts of cellular respiration are water and carbon dioxide, the latter of which is transported into blood and then to the lungs, where it is exhaled.
The “city hall” of the cell is the cell nucleus, which is where the cell’s “instructions” governing its functions originate. The nucleus contains the cell’s DNA and is surrounded by a nuclear membrane. Only eukaryotic cells have nuclei; prokaryotic cells do not have a nucleus.
The transporting “railway” function is largely served by endoplasmic reticulum. Proteins and lipids travel along endoplasmic reticulum as they are constructed and transported within the cell. There are two types of endoplasmic reticulum, smooth and rough, which are distinguished by the fact that the latter is embedded with ribosomes. Ribosomes are sites of protein production; here, molecules produced from the nucleus-encoding proteins guide the assembly of proteins from amino acids.
The Golgi apparatus is another organelle involved in protein synthesis and transport. After a new protein is synthesized at the ribosome and travels along the endoplasmic reticulum, the Golgi apparatus packages it into a vesicle (essentially a plasma membrane “bubble”), which can then be transported within the cell or secreted outside of the cell, as needed.
Plant cells include a number of structures not found in animal cells. These include the cell wall, which provides the cell with a hard outer structure, and chloroplasts, where photosynthesis occurs. During photosynthesis, plants store energy from sunlight as sugars, which serve as the main source of energy for cell functions.
Summary of Cell Organelles
package molecules in vesicles
provide structural support
From the very earliest moments of life throughout adulthood, cell division is a critical function of cell biology. The rate of division differs between cell types; hair and skin cells divide relatively rapidly (which is why chemotherapy drugs, which target rapidly-dividing cells in an effort to destroy cancerous cells, often cause hair loss), whereas liver cells rarely divide, except in response to injury. Regardless of cell type (with the exception of reproductive cells), the process of cell division follows consistent stages, which make up the cell cycle.
The cell cycle is made up of five stages. Cells at rest, which are not dividing, are considered to be at the G0 (growth phase 0) stage of the cell cycle. Once cell division is triggered (for example, by extracellular signals in response to nearby damage, requiring new cells to replace the damaged cells), cells enter stage G1. In this stage, the organelles of the soon-to-be-dividing cell are duplicated, in order to support both daughter cells upon division. Similarly, in the next stage, S (DNA synthesis) phase, the genetic material of the cell (DNA) is duplicated, to ensure that each cell has the full complement of genetic instructions. Additional growth and protein production occurs in the subsequent stage, G2 phase.
G1, S, and G2 are collectively known as interphase, in which the cell is growing and preparing to divide; the subsequent stages in which the cell is actively dividing are stages of mitosis. The first mitotic stage is prophase, in which the newly replicated DNA condenses into chromosomes. These chromosomes are in pairs (humans have twenty-three pairs of chromosomes), with each pair joined together at the centromere.
Next, in prometaphase, the nuclear membrane breaks down. Kinetochores form on chromosomes, which are proteins that attach to kinetochore microtubules (cellular filaments) anchored at opposite ends of the cell. In metaphase, the chromosomes align along the center of the cell, perpendicular to the poles anchoring the microtubules. The alignment is such that one of each chromosome duplicates is attached to each pole by these microtubules.
In anaphase, the microtubules pull the duplicates apart from each other toward each of the poles. In the final stage of mitosis, telophase, nuclei reform in each pole of the cell, and cellular filaments contract. The process of cytokinesis divides the cell into two daughter cells, both with a full complement of genetic material and organelles.
Following mitosis, both daughter cells return to the G1 phase, either to begin the process of division once again or to rest without dividing (G0).
The process of producing gametes (ovum for women and spermatozoa for men) is similar to mitosis, except that it produces cells with only half the normal number of chromosomes. Thus, when two sex cells fuse, the resulting zygote has the proper amount of chromosomes (and genetic information from both parents). This process is known as meiosis.
In the prophase of meiosis, chromosome pairs align next to each other. At this stage, transfer of genetic material can occur between members of each pair, in a process known as homologous recombination, which can increase the genetic diversity of offspring.
In meiotic metaphase, these chromosomes align as pairs in the center of the cell, and the chromosomes are separated during anaphase. As a result, each gamete cell ends up with one copy of each chromosome pair, and thus one half of the genetic complement necessary for the zygote.