Cell Structure
Background
Cells represent the most basic biological units of all organisms, whether it be simple, single-celled organisms like bacteria, or large, mu…
Life, from the largest organisms to the most minuscule, is comprised of cells. Cells are the building blocks for life and provide the materials needed for life to exist.
While cells from all walks of life contain structures that are similar, such as DNA and protein, there are two different cell types based on how such structures are organized internally. Prokaryotic cells, those of bacteria and archaea, are simple in their organization. For example, while their genome is densely packed, it is not surrounded by a membrane. Eukaryotic cells, those making up animals, plants, and fungi are more complex. They compartmentalize their internal structures into organelles, membrane-bound structures that carry out specific functions. In contrast to prokaryotes, eukaryotic DNA is enclosed in a nuclear membrane. Other organelles include the endoplasmic reticulum, the mitochondrion, and the lysosome.
The first cells on earth appear in the fossil record from around 3.7 billion years ago and were thought to have been prokaryotic. Eukaryotic cells first appeared around 2.4 billion years ago when an amoeba like cell engulfed a bacterial cell and formed a stable coexistence. Over time, this cell within cell evolved to form structures such as the chloroplast and the mitochondrion.
Plants and animals also have diverse metabolic and structural needs, leading to further differences in their cells. Because plants need to make their own energy, plant cells contain chloroplasts that aid in photosynthesis. The plant cell also includes a large vacuole to store proteins, metabolites, and water. Lastly, plant cells also have a rigid cell wall, necessary for growth and water retention.
Since cells are so small, they can only be seen using microscopy. One of the simplest ways to view cells is through the bright field microscope. This microscope contains a light source, a condenser lens to focus the light, an ocular lens to view the specimen, and an objective lens for magnification. A microscope is not the only thing needed to visualize the invisible. The structures within the cells are transparent, so the cells must be stained with dyes. For example, safranin is used to stain cell nuclei red.
In this laboratory exercise, you will use a compound microscope to visualize cheek and plant cells.
Life, from the largest organisms to the most minuscule, is comprised of cells. Cells are the building blocks for life and provide the materials needed for life to exist.
While cells from all walks of life contain structures that are similar, such as DNA and protein, there are two different cell types based on how such structures are organized internally. Prokaryotic cells, those of bacteria and archaea, are simple in their organization. For example, while their genome is densely packed, it is not surrounded by a membrane. Eukaryotic cells, those making up animals, plants, and fungi are more complex. They compartmentalize their internal structures into organelles, membrane-bound structures that carry out specific functions. In contrast to prokaryotes, eukaryotic DNA is enclosed in a nuclear membrane. Other organelles include the endoplasmic reticulum, the mitochondrion, and the lysosome.
The first cells on earth appear in the fossil record from around 3.7 billion years ago and were thought to have been prokaryotic. Eukaryotic cells first appeared around 2.4 billion years ago when an amoeba like cell engulfed a bacterial cell and formed a stable coexistence. Over time, this cell within cell evolved to form structures such as the chloroplast and the mitochondrion.
Plants and animals also have diverse metabolic and structural needs, leading to further differences in their cells. Because plants need to make their own energy, plant cells contain chloroplasts that aid in photosynthesis. The plant cell also includes a large vacuole to store proteins, metabolites, and water. Lastly, plant cells also have a rigid cell wall, necessary for growth and water retention.
Since cells are so small, they can only be seen using microscopy. One of the simplest ways to view cells is through the bright field microscope. This microscope contains a light source, a condenser lens to focus the light, an ocular lens to view the specimen, and an objective lens for magnification. A microscope is not the only thing needed to visualize the invisible. The structures within the cells are transparent, so the cells must be stained with dyes. For example, safranin is used to stain cell nuclei red.
In this laboratory exercise, you will use a compound microscope to visualize cheek and plant cells.
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Q1: What are the main differences between prokaryotic and eukaryotic cells?
Prokaryotic cells, found in bacteria and archaea, lack a membrane-bound nucleus and have densely packed DNA in a nucleoid region. Eukaryotic cells, found in animals, plants, and fungi, contain a true membrane-bound nucleus and compartmentalize internal structures into organelles. Eukaryotic cells are typically larger, ranging from 10-100 micrometers, compared to prokaryotes at 0.1-5.0 micrometers.
Q2: How did eukaryotic cells originate according to the endosymbiotic theory?
Eukaryotic cells first appeared approximately 2.4 billion years ago when an amoeba-like cell engulfed a bacterial cell, forming a stable coexistence. The engulfed bacteria evolved into mitochondria, the energy-producing organelles. Over time, this cell-within-cell arrangement led to the development of other specialized structures like chloroplasts, enabling the diversity of eukaryotic life.
Q3: What are organelles and what role do they play in eukaryotic cells?
Organelles are membrane-bound structures within eukaryotic cells that carry out specific functions. They allow eukaryotic cells to compartmentalize and specialize their internal processes. Key organelles include the nucleus, which contains DNA; mitochondria, which produce energy; the endoplasmic reticulum, which synthesizes proteins and lipids; and lysosomes, which digest cellular materials.
Q4: What structural features distinguish plant cells from animal cells?
Plant cells contain chloroplasts for photosynthesis and possess a rigid cell wall made of cellulose for structural support and water retention. They also have larger vacuoles to store water and maintain cell pressure. Animal cells lack these structures, resulting in rounded, irregular shapes, while plant cells typically appear rectangular due to their rigid cell walls.
Q5: Why must cells be stained before viewing under a microscope?
Cell structures are naturally transparent, making them invisible under a light microscope without staining. Biological dyes like safranin and methylene blue selectively bind to cellular components such as DNA and nuclei based on their molecular composition. Staining allows researchers to distinguish between different organelles and cell types, enabling detailed observation of cell structure.
Q6: How does bright-field microscopy work and what are its key components?
Bright-field microscopy uses a halogen light source directed through a condenser lens to focus light on a specimen. The objective lens magnifies the image, and the ocular lens allows viewing. The stage holds the specimen, and coarse and fine focus knobs adjust clarity. This simple technique is ideal for viewing stained cells and is commonly used in undergraduate laboratory exercises.
Q7: What is the purpose of immersion oil in microscopy and how does it improve image clarity?
Immersion oil has the same refractive index as glass, allowing light to pass through it as effectively as glass would. When placed between the coverslip and objective lens, it eliminates the glass-air interface that normally scatters light and reduces image clarity. This technique enables better visualization of specimens at high magnification by improving light transmission and reducing optical distortion.