Plants and puppies look completely different, but cells make up both of these organisms. Cells are found in both prokaryotes and eukaryotes, but the structures and different functions of prokaryotic and eukaryotic cells are markedly different.
Understanding cell biology will help you understanding the foundation of living things.
What Is a Cell?
Cells are the basic building blocks that make up all living organisms. However, you cannot see most individual cells without a microscope. In the 1660s, scientist Robert Hooke discovered cells by using a microscope to examine part of a cork.
If you look at the general organization of living things on earth, you will see that cells are the foundation. Cells can form tissues, which can create organs and organ systems. Different molecules and structures make up the actual cell.
Proteins consist of smaller units called amino acids. The structures of proteins can vary based on their complexity, and you can classify them as primary, secondary, tertiary or quaternary. This structure or shape determines the function of the protein.
Carbohydrates may be simple carbohydrates that provide energy for the cell, or complex carbohydrates that cells can store to use later. Plant and animal cells have different types of carbohydrates.
Lipids are a third type of organic molecule inside cells. Fatty acids make up lipids, and they can be either saturated or unsaturated. These lipids include steroids such as cholesterol and other sterols.
Nucleic acids are the fourth type of organic molecule inside cells. The two main types of nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). They contain the cell's genetic information. Cells can organize DNA into chromosomes.
Scientists believe cells developed 3.8 billion years ago after large organic molecules formed and surrounded themselves with a protective membrane. Some think that RNA was the first to form. Eukaryotic cells may have appeared after prokaryotic cells joined together to form a bigger organism. Eukaryotic cells have membrane-enclosed DNA, but prokaryotic cells do not have this and are also missing other organelles.
Gene Regulation and Expression
Genes code for proteins inside the cells. These proteins can then affect a cell's function and determine what it does.
During DNA transcription, the cell decodes the information in the DNA and copies it to make messenger RNA (mRNA). The main stages of this process are initiation, strand elongation, termination and editing. Transcriptional regulation allows the cell to control the formation of genetic material like RNA and gene expression.
During translation, the cell decodes mRNA to make amino acid chains, which can become proteins. The process includes initiation, elongation and termination. Translational regulation allows the cell to control the synthesis of proteins. Post-translational processing lets the cell modify proteins by adding functional groups to the proteins.
The cell controls gene expression during transcription and translation. The organization of chromatin also helps because regulatory proteins can bind to it and affect gene expression. DNA modifications, such as acetylation and methylation, usually happen after translation. They also help control gene expression, which is important for the development of the cell and its behavior.
Structure of Eukaryotic Cells
Like prokaryotic cells, eukaryotic cells have a plasma membrane, cytoplasm and ribosomes. However, eukaryotic cells also have a membrane-bound nucleus, membrane-bound organelles and rod-shaped chromosomes. You'll also find the endoplasmic reticulum and golgi apparatus in eukaryotic cells.
Structure of Prokaryotic Cells
Prokaryotic cells have a cell membrane, cell wall, cytoplasm and ribosomes. However, prokaryotes have a nucleoid instead of a membrane-bound nucleus. Gram-negative and gram-positive bacteria are examples of prokaryotes, and you can tell them apart because of differences in their cell walls.
Most prokaryotes have a capsule for protection. Some have a pilus or pili, which are hair-like structures on the surface, or a flagellum, which is a whiplike structure.
Cellular metabolism involves a series of chemical reactions that converts energy into fuel. The two main processes that cells use are cellular respiration and photosynthesis.
The two main types of respiration are aerobic (requires oxygen) and anaerobic (does not require oxygen). Lactic acid fermentation is a type of anaerobic respiration that breaks down glucose.
Cellular respiration is a series of processes that break down sugar. It includes four main parts: glycolysis, pyruvate oxidation, citric acid cycle or Kreb’s cycle, and oxidative phosphorylation. The electron transport chain is the last step of the cycle and where the cell makes most of the energy.
Photosynthesis is the process plants use to make energy. Chlorophyll allows a plant to absorb sunlight, which the plant needs to make energy. The two main types of processes in photosynthesis are the light-dependent reactions and the light-independent reactions.
Enzymes are molecules such as proteins that help speed up chemical reactions in the cell. Different factors can affect enzyme function, such as temperature. This is why homeostasis, or the ability of the cell to maintain constant conditions, is important. One of the roles an enzyme plays in metabolism includes breaking down larger molecules.
Cell Growth & Cell Division
Cells can grow and divide inside organisms. The cell cycle includes three main parts: interphase, mitosis and cytokinesis. Mitosis is a process that allows a cell to make two identical daughter cells. The stages of mitosis are:
During cytokinesis, the cytoplasm divides, and the two identical daughter cells form. Interphase is when the cell is either resting or growing, and it can be broken down into smaller phases:
Senescence or aging happens to all cells. Eventually, cells stop dividing. Problems with the cell cycle can cause diseases such as cancer.
Controlling gene expression affects a cell's behavior. Cell-to-cell communication allows information to spread inside an organism. It involves cell signaling with molecules like receptors or ligands. Both gap junctions and plasmodesmata help cells communicate.
There are important differences between cell development and differentiation. Cell growth means the cell is increasing in size and dividing, but differentiation means the cell becomes specialized. Differentiation is important for mature cells and tissues because this is what allows an organism to have different types of cells that perform various functions.
Cell mobility or motility can involve crawling, swimming, gliding and other movements. Often, cilia and flagella help the cell move. Motility allows cells to move into positions to form tissues and organs.
Epithelial cells line the surfaces of the human body. The connective tissue, particularly the extracellular matrix, supports epithelial cells.
The eight types of epithelial cells are:
- Simple cuboidal
- Simple columnar
- Stratified squamous
- Stratified cuboidal
- Stratified columnar
- Pseudostratified columnar
Other Specialized Cell Types
Changes in gene expression can create different cell types. Differentiation is responsible for the specialized cell types seen in advanced organisms.
Circulatory system cells include:
- Red blood cells
- White blood cells
Nervous system cells include neurons that help with nerve communication. A neuron's structure includes a soma, dendrites, axon and synapse. Neurons can transmit signals.
Nervous system cells also include glia. Glial cells surround neurons and support them. The different types of glia include:
- Ependymal cells
- Schwann cells
- Satellite cells
Muscle cells are another example of cell differentiation. The various types include:
- Skeletal muscle cells
- Cardiac muscle cells
- Smooth muscle cells
- U.S. National Library of Medicine: Genetics Home Reference: What Is a Cell?
- Scitable by Nature Education: What Is a Cell?
- YourGenome.org: What Is a Cell?
- Encyclopaedia Britannica: Cell
- British Society for Cell Biology: What Is a Cell?
- Science Museum: What Is a Cell Made Of?
- BBC Bitesize: KS3: What Are Cells?
- Khan Academy: Intro to Cells