Plasma Membrane and Cytoplasm: Comprehensive Study Notes

Plasma Membrane: Phospholipid Bilayer

• Perspective and location: focus on a small region of the cell membrane; the membrane separates the extracellular space (outside) from the cytoplasmic side (inside) as drawn in the slide.
• Exterior vs interior faces: extracellular face/side is the outer leaflet facing the extracellular fluid; cytoplasmic side faces the cytoplasm.
• Phospholipids: the basic building blocks of the membrane
  • Each phospholipid has a hydrophilic (charged) phosphate head and hydrophobic fatty acid tails.
  • Lipids arrange into a sheet (the lipid bilayer) with heads facing the aqueous environments and tails tucked inward.
  • The bilayer forms the basic component of the plasma membrane, and is sometimes also called a lipid bilayer.
• Formation and orientation of the bilayer:
  • Hydrophilic heads face water-based environments (extracellular fluid outside; cytoplasm inside).
  • The hydrophobic tails face inward, away from water, creating a hydrophobic core that acts as a barrier to charged molecules.
  • The bilayer can form a sheet, curve, and extend as a thin tube or vesicle; it is the border of the cell.
• Thickness and scale:
  • Thickness of the lipid bilayer is about $t \approx 10\,\text{nm}$.
  • In more intuitive terms, $10\,\text{nm} = 0.01\,\mu\text{m}$, which is about $\frac{1}{1000}$ of a typical cell diameter (typical cells are about $10-100\,\mu\text{m}$ in diameter).
• Size and visibility: this thickness is extremely small and typically observable with electron microscopy.
• Membrane as a barrier:
  • The extracellular region and cytoplasm are both water-based and charged.
  • The central hydrophobic region of the phospholipid bilayer presents a barrier to charged or polar molecules, making the membrane selectively permeable.
• Concept of membrane-bound organelles:
  • Many organelles (mitochondria, rough endoplasmic reticulum, Golgi) have membranes with similar lipid bilayer structure.
  • These organelles can be visualized as small "bags within the larger bag" that is the plasma membrane.
• Membrane dynamics:
  • Lipids can drift within the bilayer; they can flip-flop (flip from one leaflet to the other) in principle, though this course will not cover flip-flop mechanics.
  • Proteins embedded in the membrane can also move laterally and interact with each other.
• Membrane proteins: diversity and roles (overview)
  • Proteins can be embedded in the membrane (integral/transmembrane) or attached to its surface (peripheral).
  • Integral proteins span the membrane; peripheral proteins attach on one side.
  • Some proteins are transmembrane (across the lipid bilayer) and are often called transmembrane proteins.
  • The exterior domain (extracellular domain) and cytoplasmic domain refer to the parts of a membrane protein exposed to the outside and inside of the cell, respectively.
  • Proteins provide a wide range of functions and are diverse in shape and size.
• Protein functions in the membrane (illustrative categories):
  • Anchors/connect cytoskeleton to the extracellular matrix or to other cellular components.
  • Receptors detect extracellular signals (e.g., insulin). Binding of a ligand outside the cell causes a conformational change in the receptor, which can propagate a signal to the inside by altering the cytoplasmic domain.
  • Enzymes may be located on the membrane (either extracellularly or cytoplasmically) to catalyze reactions near the membrane.
  • Carriers (transporters) physically move specific molecules across the membrane by binding and releasing them on the other side.
  • Channels form tunnels that allow ions or small molecules to diffuse through the membrane; these can be leak channels (always open) or gated channels (open or close in response to signals).
  • Some membrane proteins participate in recognition and binding of extracellular ligands or cells.
• Illustrative analogy to cellular signaling:
  • If you imagine a broom handle protruding through the ceiling, the bottom end (inside the cell) moves when the top end (outside) is perturbed by a ligand (e.g., insulin) binding to the extracellular domain, signaling the inside to respond.
• Specific examples of membrane protein functions:
  • Receptors like insulin receptors: insulin binds on the extracellular domain, causing conformational change and downstream signaling that activates intracellular enzymes.
  • Enzymatic domain locations: enzymatic activity can be on the extracelluar portion or the cytoplasmic portion, with different signaling outcomes.
• Carbohydrates and the glycocalyx:
  • Carbohydrates attach to membrane proteins (glycoproteins) or lipids (glycolipids).
  • Carbohydrates are predominantly on the extracellular surface and not on the cytoplasmic side.
  • The dense carbohydrate layer on the outside is called the glycocalyx, a “carbohydrate lawn” that covers the cell surface.
  • ABO blood typing is determined by carbohydrate patterns on the surface of red blood cells, which will be discussed in AMP 2.
• Glycocalyx functions and interactions:
  • The glycocalyx contributes to cell recognition, protection, and interactions with other cells and molecules; it can modulate receptor-ligand binding (e.g., insulin interactions may depend on carbohydrate patterns).
• Cytoplasm and cytosol: definitions and distinctions
• Cytoplasm overview:
  • By definition, cytoplasm is the entire region inside the cell excluding the nucleus; it includes cytosol plus organelles.
• Cytosol overview:
  • Cytosol is the fluid component of the cytoplasm: the intracellular liquid (water) with ions, buffers, and organic molecules (sugars, amino acids, etc.).
  • The terms cytoplasm and cytosol are often used interchangeably in practice, though technically cytoplasm = cytosol + organelles.
• Membrane-bound vs non-membrane-bound organelles:
  • Some organelles are membrane-bound (nucleus, mitochondria, Golgi, rough ER) and are enclosed by lipid bilayers with embedded proteins.
  • Non-membranous organelles exist as well (filamentous components of the cytoskeleton): they lack a surrounding lipid bilayer.
• The cytoskeleton: three major components
  • Microfilaments: smallest diameter; primarily actin; important for cell movement and shape changes; microvilli core is made of microfilaments.
  • Intermediate filaments: intermediate diameter; provide mechanical strength and resilience.
  • Microtubules: largest diameter; hollow tubes; involved in transport and motor-based movement; provide tracks for motor proteins.
• Surface projections and their cores
  • Microvilli: small projections on the cell surface; core composed of microfilaments; function to increase surface area for absorption.
  • Cilia: larger, numerous projections; core composed of microtubules; can move fluid or mucus across the cell surface (e.g., mucus transport in the respiratory tract).
  • Flagella: typically a single long projection (e.g., sperm tail); structurally similar to cilia; also built on microtubules.
• Distinguishing microvilli from cilia/flagella
  • Protein core: microvilli (microfilaments) vs. cilia/flagella (microtubules).
  • Mobility: microvilli do not actively move; they respond to external flows; cilia and flagella can actively beat and move fluid or propel the cell.
• Voluntary motion distinction
  • Cilia and flagella can be moved by the cell; microvilli primarily serve to increase surface area and do not generate movement themselves.
• Additional cytoskeletal components and organelles mentioned
  • Centrioles: important for cell division (orientation and organization of the spindle).
  • Ribosomes: sites of protein synthesis; some ribosomes are free in the cytoplasm, others are attached to the rough endoplasmic reticulum; ribosomes assemble amino acids into protein chains.
  • Inclusions: catch-all term for various cytoplasmic molecules such as lipid droplets, glycogen, melanin, and other stored or pigment-containing inclusions.
• Quick recap and connections
  • The plasma membrane is a dynamic, semi-permeable barrier composed of a phospholipid bilayer with embedded and peripheral proteins, carbohydrates, and a glycocalyx on the exterior.
  • The bilayer’s hydrophobic core creates a selective barrier to solutes, with proteins providing transport, signaling, and structural roles.
  • The glycocalyx plays a key role in cell recognition and interaction, including ABO antigen patterns on red blood cells.
  • The cytoplasm consists of the cytosol and organelles, with cytoskeletal networks that give structure, enable movement, and support membrane dynamics.
  • Surface projections (microvilli, cilia, and flagella) have distinct protein cores and functions: microfilaments versus microtubules.
  • Organelles and inclusions contribute to cellular organization, storage, and function; ribosomes perform protein synthesis; centrioles organize division; inclusions store or pigment molecules.

$1\text{ nm} = 10^{-9}\text{ m}, \quad 1\ \mu\text{m} = 10^{-6}\text{ m}.$

The membrane thickness is $t \approx 10\ \text{nm}$, which equals $t = 0.01\ \mu\text{m}$, and corresponds to about $\frac{t}{D<em>{\text{cell}}} \approx \frac{10\text{ nm}}{10\text{ μm}} = \frac{1}{1000}$ for a typical cell diameter $D</em>{\text{cell}} \approx 10-100\ \text{μm}$.

• In practice, many cell biologists use cytoplasm and cytosol interchangeably, though technically cytoplasm includes organelles while cytosol is the fluid component. This distinction is noted but not crucial for most coursework discussions.

• The slide also emphasizes an overarching theme: membranes are fluid and dynamic, with components capable of lateral movement and functional interactions essential for cellular signaling and transport.