bio - the cell

Introduction to Cells

Cells are the smallest unit of life, first observed in the 1600s. The term "cellulae" means "little rooms" in Latin, inspired by their appearance when viewed under a microscope. Understanding cell structure is essential for studying biology.

Scale of Cells

• Relative sizes of various biological cells:

  • Frog eggs visible to the naked eye.

  • Human eggs and smaller cells visible under light microscopes.

  • Organelles, like nuclei and mitochondria, are significantly smaller.

  • Mitochondrion is about 1/100th the diameter of a typical animal cell.

• Cells must balance size to allow for adequate exchange of materials through the plasma membrane.

Size and Exchange of Materials

• Cell size relates to the exchange of materials across the plasma membrane.

• Ideal cell size reflects a balance—neither too large nor too small, allowing ample organelle accommodation while facilitating material exchange.

• Watch a Khan Academy video for a comprehensive explanation of surface area-to-volume ratio important for cell function.

Importance of Membranes

• Plasma membranes are vital structures in all cells, composed of a phospholipid bilayer.

  • Phospholipid Bilayer:

    • Hydrophilic heads are water-attracting, aligning outward.

    • Hydrophobic tails repel water, aligning inward, creating a barrier.

• Membranes are selectively permeable, controlling material movement in and out of cells.

• Transport proteins facilitate molecule movement across membranes, critical for cell functions.

Types of Cells

Prokaryotic Cells:

• Simpler and smaller than eukaryotic cells.

• Lack membrane-bound organelles; genetic material is located in a nucleoid region.

• Ribosomes, present in both cell types, differ slightly, a target for many antibiotics.

• Example organism: Bacteria.

Eukaryotic Cells:

• More complex cells, containing membrane-bound organelles including a true nucleus.

• Organelles are surrounded by membranes, which facilitate compartmentalization of cellular processes.

Common Structural Features of Cells

• Both cell types have:

  • Genetic material: Prokaryotic cells possess circular chromosomes; eukaryotic cells have linear chromosomes within the nucleus.

  • Plasma membrane surrounds the cell and controls internal environment.

Functional Groupings of Eukaryotic Organelles

Eukaryotic cells contain a diverse range of organelles grouped into four main functional categories:

1. Control Center: Nucleus and Ribosomes (holds genetic information and synthesizes proteins).

2. Organelles that synthesize molecules (e.g., endoplasmic reticulum, Golgi apparatus).

3. Organelles that generate energy (e.g., mitochondria, chloroplasts).

4. Structural support organelles (e.g., cytoskeleton, cell wall in plants).

Organelles Involved in Cell Function

• Nucleus:

  • A prominent organelle, often visible with a light microscope, that holds all of the cell's DNA and directs activity through RNA.

• Nucleolus:

  • Located at the center of the nucleus, responsible for assembling ribosome subunits.

• Ribosomes:

  • Synthesize proteins under the direction of RNA and can be free-floating in the cytoplasm or attached to the endoplasmic reticulum.

• Endoplasmic Reticulum (ER):

  • A membranous network with two parts: rough (with ribosomes for protein synthesis) and smooth (for lipid production and toxin processing).

• Golgi Apparatus:

  • Processes and ships proteins produced in the ER using numerous membranes for transport.

• Lysosomes:

  • Digestive organelles that break down materials, recycling cellular components.

• Vacuoles:

  • Large vesicles with various functions including storage, especially prominent in plant cells, facilitating growth.

Energy Harvesting Organelles

• Mitochondria:

  • The powerhouse of the cell, involved in carrying out cellular respiration in nearly all eukaryotic cells. This process will be discussed in further detail in Chapter 6. Mitochondria have two membranes – an outer membrane and an inner membrane which has numerous folds to enhance its function in cellular respiration.

• Chloroplasts:

  • Found in plant cells, not in animal cells. Chloroplasts are crucial for photosynthesis, converting light energy into chemical energy. Their role will be further elaborated on in Chapter 7.

Cytoskeleton and Structural Support

• The cytoskeleton is a complex structure that provides structural support, movement, and communication within and between cells. It comprises three types of filaments:

  • Microfilaments: The smallest size.

  • Intermediate filaments: Intermediate in size.

  • Microtubules: The largest of the three.

• Functions of the cytoskeleton include maintaining cell shape, anchoring and moving organelles, and facilitating muscle contraction.

Cilia and Flagella

• Both involved in movement but differ in size and structure. Cilia are small hair-like structures on the cell surface, while flagella are longer, whip-like tails. Microtubules bend to enable movement in both of these organelles.

Differences Between Plant and Animal Cells

• Plant cells have a rigid cell wall in addition to a plasma membrane, which provides structural support and protection. This cell wall is predominantly composed of cellulose.

• In contrast, animal cells do not have cell walls or chloroplasts. They do contain lysosomes, which break down food or damaged organelles.

Conclusion

Understanding these organelles, their functions, and their distinctions between plant and animal cells is crucial for comprehending cellular processes and structures.