Cells - Basic Units of the Body

Cells, Part 1

Cells as Basic Units

• The cell serves as the fundamental unit of structure and function within the body.
• Cells are measured in micrometers.
• Differentiated cells possess specialized characteristics.
• Cell size and shape vary, reflecting their inter-related structure and function.

Variety in Cell Structure and Function

• Nerve cells have long extensions which enable them to conduct electrical impulses across the body.
• Epithelial cells are organized in sheets to protect underlying cells.
• Muscle cells contain contractile proteins that enable contraction and movement.

Composite Cell

• A composite cell includes components found in most cells.
• Major parts of a cell include the nucleus, cytoplasm, and cell membrane.

Nucleus

• Contains genetic material and directs cell activities.

Cytoplasm

• Composed of organelles with specific functions, suspended in cytosol.

Cell Membrane

• The outer boundary of the cell.

Cell Membrane

• The cell membrane, also known as the plasma membrane, is the outer boundary of the cell.
• It maintains cell integrity and separates intracellular fluid (cytosol) from extracellular fluid.
• It is selectively permeable, regulating substance entry and exit.
• It facilitates signal transduction, allowing the cell to receive and respond to messages.
• Composed mainly of lipids and proteins, with some carbohydrates.

Cell Membrane Lipids and Proteins

Phospholipid Bilayer

• Acts as the cell membrane framework.
• Composed of water-soluble (hydrophilic) heads that form surfaces and water-insoluble (hydrophobic) tails that form the interior.
• The bilayer is permeable to lipid-soluble substances but not to water-soluble substances.

Cholesterol

• Stabilizes the membrane and helps maintain its impermeability to water-soluble substances.
• Some lipids and proteins can move, exhibiting "fluid mosaic" properties.

Membrane Proteins

• Perform various functions, including forming pores, channels, receptors, enzymes, facilitating cell contact and identification, and acting as Cell Adhesion Molecules (CAMs).

Carbohydrates

• Involved in cell recognition and interaction and act as self-markers.

Cytoplasm and Organelles

Cytoplasm

• Consists of networks of membranes and organelles suspended in cytosol.
• Cytoplasm = Cytosol + Organelles

Cytosol

• The fluid portion of the cytoplasm.

Organelles

• Tiny solid structures with specific functions within the cell.
• Contains the cytoskeleton, a supporting framework of protein rods and tubules.

Cytoplasmic Organelles

Ribosomes

• Composed of protein and RNA.
• Found free in the cytoplasm or on the Rough Endoplasmic Reticulum (RER).
• Provide structural support and enzymatic activity to link amino acids during protein synthesis.

Endoplasmic Reticulum (ER)

• A network of membrane-bound sacs, canals, and vesicles.
• Forms a tubular transport system.
• Rough ER contains ribosomes and is involved in protein synthesis.
• Smooth ER lacks ribosomes and is involved in lipid synthesis.

Cytoplasmic Organelles: Vesicles and Golgi Apparatus

Vesicles

• Membranous sacs that store or transport substances.

Golgi Apparatus

• Consists of flattened, membranous sacs.
• Refines, packages, and delivers proteins made on the RER.

Organelle Interaction Example

• Milk secretion exemplifies the interaction among rough and smooth ER, Golgi apparatus, and transport vesicles.

Cytoplasmic Organelles: Mitochondria, Lysosomes, and Peroxisomes

Mitochondria

• Membrane-bound, fluid-filled sacs.
• House chemical reactions that extract energy from nutrients through cellular respiration, producing ATP.
• Referred to as the "powerhouse of the cell."

Lysosomes

• Small membranous sacs.
• Contain enzymes that digest proteins, carbohydrates, nucleic acids, bacteria, debris, and worn-out cell parts.
• Function as the cell's "garbage disposals."

Peroxisomes

• Membranous sacs similar to lysosomes.
• Contain enzymes that digest lipids, alcohol, and hydrogen peroxide.

Other Cytoplasmic Structures: Cytoskeleton

• Microfilaments, microtubules, and intermediate filaments are thread-like structures in the cytoplasm that form the cytoskeleton.

Microfilaments

• Tiny rods of actin.
• Provide cellular movement, such as muscle contraction.

Microtubules

• Larger tubes of tubulin.
• Provide rigidity to maintain cell shape.
• Make up cilia, flagella, and centrioles.
• Help move organelles.

Intermediate Filaments

• Composed of several proteins.
• Provide cytoskeletal structure.
• Support the nuclear envelope.

Other Cytoplasmic Structures: Centrosome

Centrosome

• Also known as the "central body"; consists of 2 centrioles.
• Located in the cytoplasm near the nucleus.

Centrioles

• Cylindrical structures composed of microtubules.
• Produce spindle fibers during cell division, which distribute chromosomes to forming daughter cells.

Other Cytoplasmic Structures: Cilia and Flagella

Cilia

• Motile extensions of the cell membrane.
• Consist of microtubules in a cylindrical pattern.
• Form a "fringe" on the surface of certain epithelial cells.
• Shorter than flagella but very abundant when present.
• Beat back and forth in a coordinated manner.
• Propel mucus in the respiratory tract and propel the egg toward the uterus.

Flagella

• Another type of motile extension from the cell membrane.
• Similar in structure to cilia but much longer.
• The flagellum causes the entire cell to move.
• The tail of a sperm cell is the only flagellum in a human cell.
• Each cell has only 1 flagellum.

Cell Nucleus

• Contains genetic material and controls cell activity.

Nuclear Envelope

• A double-layered membrane surrounding the nucleus.
• Separates the nucleoplasm from the cytoplasm.
• Nuclear pores allow the passage of certain substances.

Nucleolus

• A dense body of RNA and protein.
• The site of ribosome production.

Chromatin

• Consists of the cell's chromosomes, each containing DNA wound around proteins.
• Stores information for protein synthesis.