The Cell

Cell organization: overview

• The topic is the cell and its basic structure.

• The cell consists of three major parts:

  • Plasma membrane (cell membrane)

  • Nucleus (located inside the cell)

  • Cytoplasm (the region between the nucleus and the plasma membrane)

• The cytoplasm has two components:

  • Organelles: structures with specific functions (except for the nucleus, which is itself an organelle)

  • Cytosol: the intracellular fluid, a thicker fluid between the nucleus and the plasma membrane, primarily water with dissolved solutes

• The nucleus houses DNA, and DNA codes for the proteins the cell will use

• The cytoplasm between the nucleus and the plasma membrane contains organelles and cytosol; the organelles perform cell-specific functions

The plasma membrane: structure and function

• The plasma membrane is a phospholipid bilayer: two layers of phospholipids forming the boundary of the cell

• Phospholipid structure:

  • Polar head (hydrophilic) that loves water and faces the aqueous environments

  • Fatty acid tails (hydrophobic) that avoid water and face toward each other

• This arrangement creates a semipermeable barrier separating extracellular fluid from intracellular fluid

• Fluids around the cell:

  • Extracellular fluid (outside the cell) also called interstitial fluid (interstitium) in many contexts

  • Intracellular fluid (inside the cell)

• Ion distributions across the membrane:

  • Inside the cell: high potassium concentration (e.g., $K^+$)

  • Outside the cell: high sodium concentration (e.g., $Na^+$)

  • The level of sodium inside the cell is relatively low, and potassium outside the cell is relatively low

• Significance of the ion distribution:

  • Potassium tends to move out if unimpeded, but the membrane is fatty (lipid) and restricts passage of ions like $K^+$, contributing to the membrane potential

  • Maintaining this balance is essential for nervous system function and muscle contraction

• The membrane’s role as a semipermeable barrier is crucial for life

• Lipids in the membrane beyond phospholipids:

  • Cholesterol (a steroid fat) is also present and contributes to membrane rigidity

  • Cholesterol has a four-ring structure and is nonpolar; it provides some rigidity without making the membrane completely rigid

• The membrane is often described by the fluid mosaic model:

  • A sea of phospholipids with floating (moving) proteins embedded in or associated with the membrane

  • Cholesterol provides structural support within this mosaic

• Membrane proteins:

  • Integral (transmembrane) proteins: span the membrane from one side to the other; removal would damage the membrane

  • Peripheral proteins: attached to one side of the membrane; can be removed without destroying the membrane

• Functions of membrane proteins:

  • Channels and transporters (transport channels) to move substances across the membrane

  • Receptors for signaling molecules (e.g., hormones)

  • Markers that identify cells (cell recognition by the immune system)

  • Enzymes that catalyze reactions on the membrane surface

  • Structural support and roles in cell adhesion and junctions

• Polar vs nonpolar transport:

  • Nonpolar (lipid-soluble) substances can typically diffuse through the lipid bilayer

  • Polar molecules generally cannot diffuse through the lipid bilayer and require carrier proteins or channels

• Glucose transport as a key example:

  • Glucose is polar and cannot cross the nonpolar lipid bilayer unaided

  • It enters many body cells through integral proteins that act as glucose carriers (channels) with a three-dimensional shape specific for glucose

  • In many body cells, glucose transport requires the hormone insulin to facilitate entry via these carrier proteins

  • This connection to insulin is directly relevant to diabetes: in type 1 diabetes (insulin deficiency), glucose cannot efficiently enter body cells, impairing ATP production via this pathway

• Integral transport proteins and hormone dependence:

  • The presence of insulin can regulate glucose entry into cells via these transport carriers

• Peripheral proteins and their roles:

  • Located on only one side of the membrane (inside or outside) and can be removed without destroying the membrane

• Glycolipids and glycoproteins:

  • Some fats in the membrane have sugars attached (glycolipids) and some membrane proteins have sugars attached (glycoproteins)

  • These sugar-bearing molecules serve multiple roles, including markers for cell recognition and adhesion

  • They are introduced in the chemistry portion of the course, but their presence in the membrane is important for identifying cells and aiding cell adhesion

• Summary of membrane components and functions:

  • Primary lipids: phospholipids (most common)

  • Other lipids: cholesterol (adds rigidity)

  • Proteins: integral (transmembrane) and peripheral (on one side)

  • Carbohydrate-bearing lipids and proteins: glycolipids and glycoproteins (markers and adhesion)

  • The membrane acts as a semi-permeable barrier and a site for chemical reactions, receptors, and markers, and it participates in cell junctions

• Additional emphasis:

  • The membrane determines what gets in and out

  • It is crucial for maintaining ion balance (e.g., $K^+$ inside, $Na^+$ outside) that underpins nerve impulses and muscle activity

  • It can serve as a site for cellular reactions and for recognition and signaling via membrane proteins

Cytoplasm: composition and components

• Location and scope:

  • The cytoplasm is the region of the cell between the nucleus and the plasma membrane

• Two main components:

  • Organelles: discrete structures with specific cellular functions

  • Cytosol: the intracellular fluid

• Cytosol characteristics:

  • A thicker, viscous fluid composed mainly of water with dissolved solutes

  • It suspends organelles and provides the medium for metabolic reactions

• Organelles function as discrete units with specialized roles essential to cell survival and operation

Nucleus: storage and gene expression

• The nucleus is an internal organelle housing the cell's DNA

• DNA codes for proteins the cell uses; gene expression leads to synthesis of these proteins

• The nucleus is separated from the cytoplasm by the nuclear envelope (not detailed in transcript but implied by context)

Cell junctions: connections between cells

• Cell junctions are protein-based structures that physically connect two cells and support function

• Three major types: 1) Gap junctions:

  • Involve a protein that spans both cell membranes, forming a channel between adjacent cells

  • Allow direct chemical communication and passage of small molecules between cells

  • Important in tissues like cardiac muscle, where cells communicate and function as a unit
    2) Tight junctions:

  • Proteins connect adjacent cells so that water and other fluids cannot pass between them

  • Important in organs that require a contained lumen (e.g., epithelial layers) to prevent leakage of fluids
    3) Desmosomes:

  • Protein connections that tightly bind neighboring cells together, resisting tearing and stretching

  • Critical in tissues that experience mechanical stress, such as skin, expanding uterus during pregnancy, and the bladder as it stretches

Glycolipids and glycoproteins: sugars attached to lipids and proteins

• Glycolipids: lipids with attached sugars

• Glycoproteins: proteins with attached sugars

• Functions of glycolipids and glycoproteins:

  • Markers for cell recognition and identity (cell labeling for immune recognition and tissue compatibility)

  • Create sticky or adhesive regions around the cell surface to aid in cell adhesion

• These sugar-bearing molecules are part of the membrane's complex structure and contribute to cell signaling, recognition, and cohesion

Key takeaways and implications

• The plasma membrane is more than a simple barrier; it is a dynamic, semi-permeable, functional interface for communication, transport, signaling, and adhesion

• Polar molecules require transport proteins (channels or carriers) to cross the membrane; nonpolar molecules can generally diffuse through the lipid bilayer

• Membrane proteins provide a wide range of functions: transport, receptors, markers, enzymes, and structural roles, including involvement in cell junctions

• The ion gradient (high $K^+$ inside, high $Na^+$ outside) is fundamental for electrical excitability and signal transmission in nerve and muscle tissues

• The interaction between glucose transport and insulin illustrates how membrane biology directly connects to metabolism and disease (e.g., diabetes)

• Cell junctions enable tissue integrity and coordinated function, with gap junctions enabling intercellular communication, tight junctions preventing leakage, and desmosomes providing mechanical resilience

• Glycolipids and glycoproteins add specificity and adhesion properties to the cell surface, influencing immune recognition and cellular organization

Connections to foundational principles and real-world relevance

• Foundational concepts: selective permeability, membrane composition, protein function, and cell-to-cell communication

• Real-world relevance: understanding insulin and glucose transport informs diabetes pathology; gap junctions relate to cardiac physiology; tight junctions are essential in organ integrity; desmosomes are critical for tissue stretch and resilience