FALL SEMESTER 2024

Instructor: Anthony Uzwiak

uzwiak@dls.rutgers.edu

Cellular Biology

Cellular biology digs deeper into the system and function of the cell.

Topics included:

1. Describe the structure and function of the cell membrane

2. Describe types of non-active and active transport through the plasma membrane

3. Describe the contents of the cytoplasm and its organelles

4. Describe the structure and function of the endomembrane system

5. Describe gene expression

6. Describe the components and functions of the cytoskeleton

Functions of the Cell

Cells are the basic units of all living things. In humans, cells can vary in size, structure, and function, but all share several common characteristics.

Cell Theory

1. Cells as Units: Cells are the fundamental functional and structural units of living organisms.

   1. All life forms are made up of cells.

2. Activity Dependence: The overall activity of an organism relies on both the individual and collective activities of its cells.

3. Biochemical Activities: The biochemical activities of cells are determined by their subcellular structures, reflecting the principle of complementarity.

   1. The structure of a cell influences its function.

4. Continuity of Life: The continuity of life is fundamentally based on cellular processes.

   1. All living things come from pre-existing cells.

Generalized Cell Structure

Major Parts:

• Nucleus: Contains genetic material— directs cell activity

  • Located near the center of the cell

  • Consists of nucleoplasm surrounded by a nuclear envelope.

    • Nuclear envelope is composed of two membranes separated by a space

    • Nuclear envelope has nuclear pores, openings where molecules move between the nucleus and cytoplasm

• Cytoplasm: Houses organelles and is the site of most cellular activities.

  • Contains organelles

• Plasma Membrane: The outer boundary of the cell that regulates the entry and exit of substances.

Organelles:

• Ribosomes: Located on the (rough) endoplasmic reticulum, serves as site of protein synthesis.

  • Free ribosomes float freely into the cytoplasm and synthesize proteins used in the cell

• Endoplasmic reticulum: Network of membranous tubules and sacs (cisternae) within the cytoplasm

  • Continuous with nuclear envelope

  • Two regions— smooth ER and rough ER

    • Smooth ER: Lacks ribosomes

      • Calcium storage

      • Synthesis of lipids, phospholipids, and steroids

      • Carbohydrate metabolism

      • Detoxifies drugs

    • Rough ER: protein synthesis (has ribosomes)

      • Ribosomes attached to ER synthesize secretory proteins

      • Growing polypeptide is threaded through ER membrane into cisternal space

      • Protein folds into native conformation

      • Protein departs in a transport vesicle pinched off from the ER

• Golgi apparatus: Modifies, packages, and distributes proteins from the rough ER (packaging system)

• Organelle of stacked, flattened membranous sacs

  • Has polarity

    • Cis face— receives transport vesicles from rough ER

    • Trans face— pinches off vesicles

  • Rough ER products are modified as they mouth through golgi apparatus

• Lysosome: Contains digestive enzymes and breaks things down

  • Digest all major classes of macromolecules

  • Digest bacteria

  • Digest organelles that are no longer functioning (autophagy)

• Peroxisome: Breaks down hydrogen peroxide using the enzyme catalase

• Proteasomes: Large protein complexes containing enzymes that break down and recycle other proteins in the cell

• Mitochondria: Converts oxygen into ATP (energy)

  • The powerhouse of the cell

Endomembrane System

The endomembrane system is the network of membranes within a cell that synthesize, modify, and transport the degradation of biomolecules. The system facilitates the transport of materials within the cell via vesicles.

The several key structures of the system are:

• Nuclear Envelope

• Endoplasmic Reticulum

• Golgi Apparatus

• Lysosomes

• Vacuoles

Mitochondria

The mitochondria has a double membrane (similarly to the plasma membrane.)

• The outer membrane is permeable to small solutes.

• The inner membrane contains embedded proteins involved in cellular work.

  • The material within the inner membrane is called the matrix.

• The cristae are folds of the inner membrane.

Cytoplasmic Organelles

The cytoplasm is the cellular material outside of the nucleus but inside the plasma membrane. It is made up of cytosol, organelles, and inclusions.

• Cytosol is the fluid portion of the cytoplasm. It is a viscous solution.

• Inclusions are non-functioning chemicals substances that are unique to cell types.

Cytoskeleton

Within the cell also contains the cytoskeleton, which supports the cell and holds the nucleus + other organelles into place. It is a network of fibers throughout the cytoplasm that form a framework for support, movement, and regulation.

Functions of the cytoskeleton are:

1. Mechanical support to maintain shape

2. Allows cell to change shape when environment changes

3. Associated with motility

4. Regulatory role in transmitting signals from cell’s surface to its interior

The components of the cytoskeleton are responsible for changes in cell shape and movement of cell organelles. It is constructed from three types of fibers.

• Microtubules (thickest)

  • Hollow tubes composed primarily of protein units called tubulin

  • Radiate from cell’s center

  • Determine cell shape

  • Provide tracks for organelle movement

  • Involved in separation of chromosomes during cell division

• Actin Filaments/Microfilaments (thinnest)

  • Made up of contractile protein (actin)

  • Attach to cytoplasmic side of plasma membrane

  • Participate in muscle contraction

  • Localized cell contraction

• Intermediate Filaments

  • Most stable and permanent cytoskeletal element

  • Act as guy wires to resist pulling forces on the cell

  • Fix organelle position

Plasma Membrane

The plasma membrane serves five different important functions to the life/regulation of a cell.

1. Serves a boundary separating the cytoplasmic (intracellular) substances from outside substances (extracellular)

2. Enclose and supports the cell

3. Attaches cells to extracellular environment, or to other cells

4. Cells communicate and recognize each other through plasma membranes

5. Regulates what comes in and out of the cell

The plasma membrane is composed of a bilayer of phospholipids. The (polar, charged) hydrophobic tails are directed towards each other while the (nonpolar, uncharged) hydrophilic heads are directed toward the cytoplasm or extracellular fluid.

This structure of the plasma membrane is described as the fluid mosaic membrane. This is because the plasma membrane is neither rigid nor static in structure. It is highly flexible and can change shape and composition.

This fluidity allows:

1. Important means of distributing molecules within plasma membrane

2. If slight damage occurs, membrane can be repaired as phospholipids reassemble around damaged sites

3. Fluid nature enables membranes to fuse with one another

There are little carbohydrates in the plasma membrane. However, the carbs can combine with lipids to form glycolipids— and combine with proteins to form glycoproteins.

Glycocalyx is a combination of glycolipids, glycoproteins, and carbohydrates on the plasma membrane. It is involved in cell adhesion, protection, and signaling, allowing cells to recognize and interact with each other.

Membrane Proteins

Basic structure and some functions of the plasma membrane are determined by lipids, however many other functions are determined by proteins— depending on location or attachment, proteins can be integral or peripheral.

Integral membrane proteins penetrate deeply into the lipid bilayer—extending from one surface to the other.

• Includes channels and carriers

Peripheral membrane proteins are attached to the membrane surface.

• Involved in enzymatic activity and structural support

Functions of membrane proteins include:

1. Transport

2. Enzymatic activity

3. Receptor sites

4. Intercellular junctions

5. Cell-cell recognition

6. Cytoskeletal and extracellular matrix attachment

Membrane Junctions and Transport

In order to communicate between cells, cell membranes have membrane junctions. The three types of junctions are tight junctions, desmosomes, and gap junctions.

• Tight junctions create an impermeable barrier between adjacent cells. Substances are prevented from passing through. They are barriers.

• Desmosomes are anchoring junctions that provide mechanical stability to tissues. They provide structural support.

• Gap junctions allow the passage of small molecules and ions. They allow passage.

The cell membrane is selectively permeable. This means that the membrane can allow certain substances to pass while blocking others.

There are multiple ways in which substances move through the membrane. These three transport methods are called passive, active, and bulk transport.

Passive Transport: Does not require energy

Passive Transport (Filtration): When water and solutes are forced through membrane or capillary by pressure

• Diffusion— movement down a concentration gradient; movement of solutes from an a area of high concentration to lower concentration

  • A concentration gradient changes over a distance in a particular direction

  • Results from intrinsic kinetic energy

  • Affected by temperature and molecular size

  • Movement is random

  • Continues until dynamic equilibrium is reached

• Osmosis— diffusion of a water (solvent) through a selectively permeable membrane; water movement

  • Hypertonic solution: solution with greater solute concentration than inside a cell

  • Hypotonic solution: solution with a lower solute concentration than inside a cell

  • Isotonic solution: equal parts solute in and out of the cell

    • Osmolarity— total concentration of all solutes in a solution

    • Osmotic pressure—amount of pressure required to prevent net movement of water into a solution

• Facilitated diffusion— carrier proteins move substances across membrane

  • Occurs when molecules are too large to diffuse through membrane pores

  • Selective— not just size and lipid solubility (specific)

  • Limited by number of carrier molecules present (saturated)

Active Transport: Transport process that requires energy provided by ATP

• Cell uses energy to move substances across membrane

• Transport molecules harvest energy from ATP to pump molecules against concentration gradients

  • Symport— same direction

  • Aniport— opposite direction

• Sodium-potassium (Na+ K+ pump) exchange one substance for another using active transport

Bulk Transport (aka Vesicular Transport): Movement of large quantities of substances into or out of the cell using vesicles (membrane-bound sacs).

• Exocytosis: Substance is released from vesicle

  • Fuses with membrane and releases contents to outside

• Endocytosis: Large substances enclosed by the membrane and taken into cell

  • Phagocytosis— “cell eating” of large particles

    • Solid particles are ingested and phagocytic vesicles are formed

    • Important in eliminating harmful substances from the body

  • Pinocytosis— “cell drinking” of fluids

    • Similar to phagocytosis, except that vesicles are smaller and material inside the vesicle is liquid

Endocytosis can exhibit specificity. The plasma membranes contain specific receptor molecules that recognize certain substances and allow them to be transported in or out of the cell via phagocytosis or pinocytosis. Receptor-mediated endocytosis is a term used to describe the specific uptake of molecules via receptors.