Cell Modifications

General Biology 1 — Module 5: Cell Modifications

I. Cell Modifications

Cell modification, also known as cell differentiation or specialization in the module, is a process that occurs after cell division in which newly formed cells become structurally modified to perform specific functions effectively and efficiently.

Three types of cell modifications

1. Apical modifications

Location: Upper or free surface of epithelial cells

Responsible for absorption, secretion, and movement.

Examples: Cilia, flagella, microvilli, stereocilia.

2. Basal modifications

Location: Bottom or underneath epithelial cells

Responsible for support, attachment, anchoring, and fluid or ion transport.

Examples: Basal lamina, basal infoldings, hemidesmosomes.

3. Lateral modifications

Location: Sides of adjacent epithelial cells

Responsible for cell-to-cell attachment, sealing, and communication.

Examples: Tight junctions, desmosomes, gap junctions, adherens junctions.

II. Apical Modifications

1. Cilia

• Structure: Short, hair-like, membrane-bound projections that cover much of the apical surface.

• Movement: Beat in a coordinated, wave-like motion.

• Functions:

  • Help microscopic organisms move (locomotion).

  • Create currents that assist feeding.

  • Trap and remove dirt from the respiratory tract.

  • Move the ovum toward the uterus through the fallopian tubes.

• Examples: Respiratory epithelium and fallopian tubes.

2. Flagella

• Structure: Long, hair-like structures; longer than cilia.

• Main function: Locomotion or movement of cells.

• Cells possessing flagella are called flagellates.

• Composition:

  • Prokaryotic flagella — flagellin.

  • Eukaryotic flagella — microtubules.

• Example: Sperm cells (spermatozoa).

3. Microvilli

• Also known as the brush border or striated border.

• Structure: Small, finger-like projections that are shorter and narrower than cilia.

• Composition: Parallel bundles of actin filaments held together by the proteins villin and fimbrin.

• Main function: Increase surface area for more efficient absorption and secretion.

• Locations:

  • Simple columnar epithelium lining the small intestine.

  • Simple cuboidal epithelium lining the proximal tubules of the kidneys.

4. Stereocilia

• Structure: Very long microvilli.

• Functions: Absorption.

• Locations: Male reproductive structures, including the epididymis and ductus deferens.

Comparison of apical modifications

Memory tip: Cilia move substances, flagella move cells, microvilli increase surface area, and stereocilia assist absorption.

III. Basal Modifications

1. Basal lamina

• Location: Underneath epithelial cells.

• Functions:

  1. Supports the overlying epithelium.

  2. Acts as a filter, allowing water and small molecules to pass through.

  3. Helps anchor epithelial cells to the underlying connective tissue.

  4. Limits contact between epithelial cells and other cell types in the tissue.

2. Basal infoldings

• Location: Basal surface of epithelial cells.

• Functions:

  1. Actively transport fluids and ions.

  2. Increase the surface area available for transport.

• Important characteristic: Numerous mitochondria are found in the folds, providing energy for active transport.

3. Hemidesmosomes

• Location: Basal surface, connecting epithelial cells to the basement lamina.

• Structure: Protein structures or filaments.

• Function: Provide a strong connection between epithelial cells, the basement lamina, and the underlying connective tissue.

Comparison of basal modifications

IV. Lateral Modifications

Lateral modifications occur along the sides of epithelial cells. They help cells attach to one another, prevent unwanted fluid movement, and communicate.

1. Tight junctions

• Also known as zonula occludens.

• Formed by the proteins claudin and occludin.

• Create a tight, zipper-like seal between adjacent cells.

• Main function: Block the flow of fluids between epithelial cells.

• Examples:

  • Intestines: Help keep digestive juices out of the body.

  • Kidneys: Help keep urine within kidney tubules.

2. Desmosomes

• Also known as macula adherens.

• Form connections between adjacent cells.

• Connect to the intermediate filaments of neighboring cells' cytoskeletons.

• Main function: Provide strong cell-to-cell attachment.

• Common location: Skin cells.

3. Gap junctions

• Formed by connexin proteins that create pores between cells.

• Directly connect the cytoplasm of two cells through channels.

• Main function: Allow molecules and ions to pass between cells for communication.

• Importance: Especially important in cardiac and smooth muscles because ions must flow between cells for coordinated contraction.

4. Adherens junctions

• Also known as zonula adherens.

• Located beneath tight junctions.

• Main function: Provide a strong bond between the sides of adjacent epithelial cell membranes.

Comparison of lateral modifications

Memory tip: Tight = seal, desmosome = link, gap = communicate, adherens = bond.

V. Specialized Cell Modifications

The module also discusses specialized cells in plants and animals. Their structures are adapted to their particular functions.

A. Specialized cells in plants

1. Root hairs

• Projections from root-hair cells.

• Absorb water and minerals from the soil.

• Numerous root hairs increase the absorptive surface area of the roots.

• Contain many mitochondria to support the high energy requirement of active transport.

2. Xylem vessels

• Long tubes formed by individual cells joined end to end.

• Have thick cell walls impregnated with lignin, making them strong and impermeable.

• Transport water and minerals from the roots to other parts of the plant.

• Provide structural support.

• Help keep leaves positioned horizontally toward the sun, enabling sufficient light absorption for photosynthesis.

3. Guard cells

• Specialized cells found in the epidermis.

• Surround the stomata.

• The inner cell wall is thicker than the outer wall.

When turgid: Guard cells bend outward, opening the stomata.

When flaccid: Guard cells bend inward, closing the stomata.

B. Specialized cells in animals

4. Erythrocytes (red blood cells)

• Specialized blood cells that lack a nucleus, according to the module.

• Contain hemoglobin, a respiratory pigment containing iron.

• Hemoglobin binds oxygen loosely, allowing oxygen to be carried in the blood.

• An insufficient number of red blood cells may lead to anemia.

5. Neurons (nerve cells)

• Specialized cells of the nervous system.

• Have dendrites, axons, and a cell body.

Dendrites: Receive electrical signals from sensory receptors or other neurons and transmit them toward the cell body.

Axons: Transmit signals to other neurons or other cells.

- Main role: Send messages throughout the body through electrical impulses.

6. Muscle cells

• Contain the proteins actin and myosin.

• These proteins allow muscles to contract.

• Main role: Produce movement through contraction.

7. Sperm cells

• Specialized reproductive cells necessary for human reproduction.

• Predominantly made up of a nucleus.

• Highly motile.

• Contain mitochondria that provide energy for movement.

• Use a flagellum to move.

VI. Important Terms to Memorize