The Cell

Edited Notes by Jill Hunter and Leigh Eisenman

Figures drawn by Abby Marsh - click on figures for detailed view.


Some topics in this lecture: Parts of the Cell, The Cell Cycle, Mitosis, Meiosis, Development, Cell Components

All living organisms consist of cells -- whether these organisms contain one cell or a billion, cells give life to all creatures and growing organisms. There are different functions and purposes for cells, and there are different kinds of cells. The purpose of this section is to give greater insight to the cell as a functioning organism.

Parts of the Cell

Although all cells have different individual functions, they are similar in their makeup. All cells have cell membranes which serve to keep the cell separated from its environment and hold the cell together. Plant cells also have a cell wall and contain chloroplasts. The cell wall is a rigid structure that gives plants their shape and integrity. Chloroplasts are responsible for carrying out photosynthesis to convert sunlight, carbon dioxide, and water into sugar.

A cell contains individual structures inside of it. These structures are called organelles. Cells are classified into two types -- prokaryotic and eukaryotic. Eukaryotic cells are distinguished by the fact that they have nuclei. In general, there is one nucleus in each eukaryotic cell, and it contains the genetic material for the cell in the form of chromatin.

Prokaryotic cells tend to be small in size. Although their size does vary, they are about 1-10 mm in diameter. Instead of reproducing sexually, they reproduce by fission -- so each offspring contains genetic information identical to that of its single parent. Prokaryotic cells are enclosed by a cell wall made of complex polysaccharides, which are many sugar molecules linked together in a complex way. This polysaccharide wall provides the prokaryotic cell with protection and support. The DNA in prokaryotic cells is usually a long circular molecule, and it is not associated with proteins.

Eukaryotic cells have a nucleus which contains the cellular genes in the form of chromatin. Chromatin is a complex of DNA and proteins. During mitosis the chromatin forms chromosomes.

The nucleus serves as the "brain" of the cell. It contains the genetic information of the cell. Within the nucleus is a nucleolus, whose function it is to produce ribosomes. The nucleolus does not have a membrane around it. Ribosomes are composed of RNA and protein and their purpose is to synthesize polypeptides from amino acid subunits. The nucleus, which is membrane bound, has nuclear pores on its surface. The nuclear pores regulate the flow of material into and out of a nucleus.

The mitochondria, another organelle found in eukaryotic cells, are often referred to as the "powerhouse" of the cell. Here, the food we eat is transformed into fuel or energy for the cell and our bodies. Mitochondria have their own DNA. It is thought that this came about as a result of a symbiotic relationship that existed a long time ago between a primitive prokaryotic organism (which would evolve into a mitochondrion) and a primitive eukaryotic cell host. A symbiotic relationship is one in which two organisms help each other survive. The pre-mitochondrion "invaded" the host cell and provided the host cell with efficient energy production. In return, the host cell provided the mitochondrion with food. Ultimately, this arrangement became permanent and the original prokaryotic organism lost much of its DNA. The mitochondrion is the leftover result of the original prokaryote.Mitochondria also contain ribosomes, which are very similar to prokaryotic ribosomes and different from the ribosomes found in the rest of the cell.

Chloroplasts, like mitochondria, are thought to be the result of an endosymbiotic relationship. Photosynthesis occurs within chloroplasts utilizing products encoded in the chloroplast DNA as well as products encoded in the nuclear DNA. The chloroplast also has its own ribosomes that, like mitochondrial ribosomes, resemble prokaryotic ribosomes.

The endoplasmic reticulum is a system of membranes in the cell that are involved in a variety of processes such as protein synthesis and transport, fat metabolism and steroid synthesis.

The Golgi apparatus is a part of the membrane system within the cell as well and works closely with the endoplasmic reticulum. The Golgi Apparatus modifies proteins and brings them to the cell surface where they can be secreted. Secretions include hormones, enzymes, antibodies and other molecules.

Lysosomes are membrane bound vesicles whose purpose is to help with the digestion of molecules, and they are able to do this because of their acidic interior which contains digestive enzymes. They also make sugars, amino acids, and bases which help create the foundation of the very macromolecules they digest.

The cell membrane of animal cells is very important in that it protects the organelles and also keeps unwanted particles from entering the cell body. It is made of a phospholipid bilayer which has proteins floating in it. The proteins span the membrane and touch both the inside and outside of the cell. Those that are on the outside of the cell include blood group proteins, antibodies, and HLA antigens. Their function is to interact with molecules outside the cell. This includes the ability to serve as protein receptors for hormones, bind to other cells in wound healing and in the immune response, and transport molecules into and out of the cell. The membrane has an electrical gradient across it, the force of which can be used to move material in and out of the membrane. There is 100 mV across the membrane, but the membrane is so thin that this is equivalent to about 10,000 volts per centimeter. Electrical impulses in nerve cells and muscle cells are carried along the cell through the cell membrane. Organisms like an electric eel can generate enough voltage to kill a person -- all by pumping ions into and out of cells across a cell membrane!

The Cell Cycle

The life cycle of a cell consists of growth and division. There are several stages to the cell cycle:

G1 phase

S phase

G2 phase -- cell prepares itself for mitosis by synthesizing needed components

Some cells remain in interphase their whole lives because they do not divide. Two such examples are nerve cells and adult muscle cells. The long "G1" phase is sometimes called G0 phase.

Control of the Cell Cycle

Mitosis

Mitosis

Interphase

  • cells are not dividing
  • chromosomes are decondensed (called chromatin) and their information is available to the cell for synthesizing products
  • cells spend most of their time in this intermediate non-mitotic state
  • during interphase (in S phase), all the cell's DNA is duplicated -- resulting in 4 copies of each gene instead of the normal 2 in a diploid cell

Prophase

  • chromatin begins to coil and condense to form chromosomes
  • each chromosome appears to have two strands (each containing a single molecule of DNA)
  • each strand is called a chromatid
  • each chromatid is attached to its sister chromatid at the centromere
  • at this stage, the number of chromosomes (containing a pair of chromatids) is considered to be equal to the number of centromeres
  • the two chromatids are the result of DNA replication that takes place just before mitosis starts.
  • the nuclear envelope disappears
  • the nucleolus disappears
  • in cytoplasm, the spindle apparatus forms
  • eventually the spindle guides the separation of sister chromatids into the two daughter cells

Metaphase

  • spindle grows and forms attachments to the chromosomes at the centromeres
  • chromosomes move to an equatorial plate (metaphase plate) which is formed along the midline of the cell between the poles
  • chromosomes are at their most condensed state now
  • metaphase chromosomes can be stained and will show distinctive banding patterns

Anaphase

  • centromeres divide to create two chromosomes instead of a pair of attached chromatids
  • spindle fibers shorten and the sister chromosomes are drawn to the opposite poles of the cell
  • poles of the spindle apparatus are pushed apart as the cell elongates
  • anaphase results in the exact division of chromosome, distributing one complete diploid complement of genetic information to each daughter cell

Telophase

  • nuclear envelopes reassemble and surround each set of daughter chromosomes
  • nucleoli reappear inside the newly formed nuclei
  • in animal cell, a furrow appears around the cell that eventually pinches the cell into two new cells
  • in plants, a cell plate forms between the two daughter nuclei as the cell wall divides the cell
  • chromosomes decondense in the daughter cells to become chromatin and the cells are once again in Interphase

Meiosis

In meiosis, the process is quite similar to mitosis. However, another cell division takes place in which there is no extra DNA replication step. Instead of having a pair of genes (as in a diploid cell), there is only one copy of each gene (a haploid cell). This one copy of genetic information produces gametes of either sperm or eggs. Thus, only one copy of a gene is passed on to each gamete. It is not until the sperm and egg join that there will be two halves of genetic information. This process is the basis for all of Mendel's laws.

Development

Many eukaryotic organisms do not exist as one-celled bodies, and generally, the cells multiply and develop into a more complex organism. This development comes about by way of two processes -- growth and differentiation.

During the growth process, cells reproduce and grow more cells through mitosis. This process occurs daily, and even many times a day as cells continually need to be replenished as others die.

As cells form more complex organisms, they must produce different types of cells that result in a functioning integrated organism. A group of cells of the same type form a larger structure called a tissue (e.g. -- skin, muscle). Multiple tissues can form an organ (e.g. -- eye, kidney). This variation among cell groups is known as differentiation. Cells that are capable of differentiating into many different cell types are called totipotent.

The genetic information these cells contain is used sparingly and only at the key moments necessary during the developmental process. This "gene control" regulates the growth and development of the body. Genetic information that is not properly regulated causes vital problems in cell growth and can lead to diseases such as cancer.

When cells become a part of a tissue or organ, they lose their ability to work alone and cannot survive without the support of other cells. They become dependent on other cells to work as a group to govern the whole. In doing so, they are capable of astonishing things in the body. For example, our blood serves as a messenger for all types of nutrients throughout the body, carries oxygen to (and removes CO2 from) different parts of the body. It is through our blood and its immune system that we fight off diseases. Our muscles have the ability to contract, and our nerves can carry and receive electrical signals. Our digestive system can extract nutrients from the foods we eat. Our kidneys remove waste. The main organs of our senses provide us with sight, hearing, and the abilities to smell and taste. All of these abilities create the working body.

Cell Components

Cells have large organic molecules and small molecules. The more complex molecules known as polymers are those such as carbohydrates, nucleic acids, and proteins. Carbohydrates are assembled by linking together sugars. Complex linking results in large molecules such as starch and cellulose. Carbohydrates can be attached to proteins and thereby alter the functioning of the protein. DNA and RNA are nucleic acids that are made by linking together nucleotides. DNA contains the genetic information, and RNA conveys that information to the rest of the cell and directs the synthesis of proteins.

Proteins are vital molecules that are long chains of amino acids held together through a unique bond known as the peptide bond. Because each protein has such a unique shape, this shape is easily recognized by other molecules enabling proteins to perform unique functions. Enzymes are catalytic proteins which carry out many vital processes to obtain energy from food, keep the flow of ions in and out of cells occurring, carry oxygen, and create new molecules such as DNA, RNA, lipids, carbohydrates, and other proteins. Proteins provide a structural backbone for the cell as well and provide a means of moving molecules within the cells.

Microtubules are rod-shaped fibers that exist in cells and serve as railroad tracks. Within the cell they move vesicles, mitochondria, and other materials and particles throughout the cell.

Actin fibers exist within a single cell and give the cell its shape through its constant interaction with myosin also present in cell.

Cell-Cell Interations

Electrical interactions

  • Nerve, muscle interaction
  • Sensor cells (ex. perception of light in the retina)

Chemical interactions

  • 1. Neurotransmitters (cause vesicles at end of neuron to fuse with membane when the chemical gradient changes)
  • 2. Hormones

Direct Interactions: cell-cell junctions that allow molecules to flow from one cell to another; ex. In muscle, the fusion of thousands of cells whose membranes dissolve to form synsytium

Small molecules are also important to the cell, and these have different purposes. They include water, salts, sugars, lipids, amino acids, and nucleotides. The presence of salts allows for creating and maintaining the electrical balance within the cell -- inside and outside the membrane. Sugars serve as energy sources to the cell. Excess energy is stored as fat in the form of lipids. Some small amino acids can serve as neurotransmitters which move from cell to cell and transfer signals between nerves. Nucleotides are the building blocks of DNA and RNA.

Other Aspects of Cells

It is important to understand how cells interact with one another. One such interaction is through electrical signals. Electrical signals transmitted between cells in the nervous system allow us to feel something. When we touch something, an electrical signal is sent through our nerve cells to our brain. Our brain processes the information and acknowledges the sensation of touch. Another type of cellular communication is through chemical transmitters. Chemical transmission is seen in the action of hormones -- chemicals that are released from one tissue in the body and act at other sites. For example, the hormone estrogen is released in the female from the ovaries and acts an many sites in the body during the menstrual cycle. Testosterone and estrogen are both involved in development.

The process of contact inhibition is another important concept. This is the process in which cells "realize" that it is time to stop multiplying. This is vital in tissue and organ growth, for malfunction of this process results in disease. This is the process that occurs when wounds heal -- growth stops when the cells meet and join together to heal the wound.

When contact inhibition does not occur, cells become "immortal" in a sense as they cannot recognize that they must stop growing. This is what occurs in cancer -- the cells continue to reproduce at a rapid rate and cannot be stopped unless they are all destroyed through removal of an organ or a process such as chemo-therapy.

Cell Mortality - normal cells are mortal and can continue to grow and divide from 50=100 times before dying

  • there is a natural limit to how many times cells can divide
  • biochemical errors accumulate during cell division and reproduction which eventually result in cell death
  • immortality of cancer cells shows there is a way to keep cells alive
  • cells can become immortal if transformed by infecting them with a virus
  • cell death is part of normal development (ex. In fetus, cells of webs between fingers and toes die before birth)
  • cell death important for regenerating tissues or bones after injury