Unit 2.2 Cell Structure and Function

Cells are the building blocks of life and are responsible for all of the functions that keep living organisms alive. In this article, we will explore the similarities and differences between eukaryotic and prokaryotic cells, as well as the similarities and differences between plant and animal cells. We will also discuss the roles and characteristics of various organelles that are found within cells.

Eukaryotic vs Prokaryotic Cells

Eukaryotic cells are cells that have a true nucleus, as well as other membrane-bound organelles. These cells are typically larger and more complex than prokaryotic cells. Examples of eukaryotic cells include animal cells and plant cells. Think of eukaryotic cells like a fancy 5-star hotel with all the amenities while prokaryotic cells are like a small camping tent.

Prokaryotic cells, on the other hand, do not have a true nucleus or membrane-bound organelles. These cells are typically smaller and simpler in structure than eukaryotic cells. Examples of prokaryotic cells include bacteria and archaea.

Plant vs Animal Cells

Both plant and animal cells are eukaryotic cells, but they have some important differences. Plant cells have a rigid cell wall that surrounds the cell membrane. This cell wall provides structure and support for the cell. Plant cells also have chloroplasts, which are organelles that allow the cell to carry out photosynthesis. The cell wall of plant cells allows them to maintain their shape regardless of how much water may or may not be present in the cell. Think of plant cells as a strong fortress, able to withstand harsh conditions.

Animal cells, on the other hand, do not have a cell wall or chloroplasts. Animal cells have centrioles, which are organelles that are involved in cell division. Animal cells can also have cilia and flagella, which are used for movement. Animal cells are like a sports car, fast and agile.

Organelles and their Functions

Mitochondria: Known as the “powerhouses” of the cell, the mitochondria are responsible for producing energy for the cell through a process called cellular respiration. They are like the little power plants within the cell.

Nucleus: The nucleus is the central control center of the cell, containing the genetic material of the cell. Within the nucleus is the nucleolus which is responsible for the production of ribosomes. The nucleus is like the brain of the cell, telling it what to do.

Golgi apparatus: The golgi apparatus is responsible for processing and packaging proteins and lipids for transport to other parts of the cell. Think of it like the post office of the cell, sorting and delivering packages.

Endoplasmic reticulum: The endoplasmic reticulum (ER) is responsible for the synthesis and transport of proteins and lipids. It’s like the cell’s personal delivery service.

Ribosomes: Ribosomes are responsible for the synthesis of proteins. They are like the cell’s factory workers, producing the necessary materials for the cell.

Vacuoles: Vacuoles are sac-like structures that store nutrients and waste products. Plant cells have a large central vacuole that stores water and other materials. It’s like the cell’s pantry, storing the cell’s food supply.

Cell membrane: The cell membrane is a thin barrier that surrounds the cell, separating the cell’s contents from its environment. It’s like the cell’s bouncer, only letting in the appropriate substances. It is also called the “plasma membrane”.

Cell Transport

Cells need to move materials in and out of the cell in order to survive. There are several different types of cell transport, including active transport and passive transport.

Concentration Gradients

A concentration gradient is the difference in concentration of a substance between two different areas. In cell transport, concentration gradients drive the movement of molecules across the cell membrane. When the concentration of a substance is higher on one side of the membrane than the other, the molecules will naturally move from the area of higher concentration to the area of lower concentration.

Active Transport

Active transport is the process of moving materials across the cell membrane against a concentration gradient. This requires energy, usually in the form of ATP. An example of active transport is the sodium-potassium pump, which pumps sodium ions out of the cell and potassium ions into the cell.

Passive Transport

Passive transport is the process of moving materials across the cell membrane along a concentration gradient. This does not require energy, as the materials move from an area of higher concentration to an area of lower concentration. An example of passive transport is diffusion, which is the movement of molecules from an area of higher concentration to an area of lower concentration.

Diffusion

Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. This occurs through passive transport, as the molecules move down their concentration gradient without the need for energy.

Facilitated Diffusion

Facilitated diffusion is similar to diffusion, but it involves the use of carrier proteins in the cell membrane to move molecules across the membrane. The carrier proteins bind to the molecules, change shape, and transport them across the membrane. This also occurs along the concentration gradient.

Osmosis

Osmosis is the movement of water molecules across a selectively permeable membrane, from an area of high water concentration to an area of low water concentration. This also occurs along the concentration gradient.

Endosymbiotic Theory

Endosymbiotic theory is a scientific explanation for the origin of eukaryotic cells. It proposes that eukaryotic cells evolved from a symbiotic relationship between different types of prokaryotic cells. The theory was first proposed by Lynn Margulis in the 1960s and has since been supported by a significant amount of evidence from various fields of biology.

Evidence from Cell Structure and Function

One of the main pieces of evidence for endosymbiotic theory is the resemblance of organelles within eukaryotic cells to prokaryotic cells. For example, mitochondria, which are responsible for producing energy within eukaryotic cells, have a double membrane structure similar to that of prokaryotic cells and contain their own genetic material separate from the chromosomes of the larger cell. Additionally, mitochondria reproduce using binary fission, a method of reproduction that is similar to that of bacteria. Similarly, chloroplasts, which are found in plant cells and are responsible for photosynthesis, also have a double membrane structure and contain their own DNA. Like mitochondria, chloroplasts also reproduce using binary fission. This resemblance suggests that these organelles were once free-living prokaryotic cells that were engulfed by another cell, leading to a symbiotic relationship.

Evidence from Genetic Analysis

Another piece of evidence for endosymbiotic theory comes from genetic analysis. Studies have shown that the DNA within mitochondria and chloroplasts is more similar to that of prokaryotic cells than eukaryotic cells. Like bacteria the chromosomes found in chloroplasts and mitochondria are circular. This distinguishes them from the larger host eukaryotic cells because eukaryotes have linear chromosomes. Additionally, the genetic material within these organelles is inherited separately from the rest of the cell, further supporting the idea that they were once separate entities.

Evidence from molecular biology

Molecular biology also supports the endosymbiotic theory. The ribosomes of mitochondria and chloroplasts are similar in structure to those of prokaryotic cells, while being different from those of eukaryotic cells. Additionally, the ribosomes of mitochondria and chloroplasts are also smaller in size compared to eukaryotic ribosomes. This structural similarity suggests that mitochondria and chloroplasts have a prokaryotic origin.

In conclusion, endosymbiotic theory is supported by a significant amount of evidence from various fields of biology, including cell structure and function, genetic analysis, molecular biology and evolutionary history. The theory proposes that eukaryotic cells evolved from a symbiotic relationship between different types of prokaryotic cells, with organelles such as mitochondria and chloroplasts being examples of such symbiotic relationships. It is a widely accepted theory in the scientific community and continues to be the subject of ongoing research.