Chapter 3 Notes: Cells – The Living Units

Extracellular Material

  • Three components: extracellular fluid (ECF), cellular secretions, and extracellular matrix (ECM).
  • Extracellular fluid (ECF): includes blood plasma, cerebrospinal fluid (CSF), interstitial fluid; surrounds cells; contains thousands of ingredients; numerous roles.
  • Cellular secretions: substances that aid digestion (e.g., gastric fluids) and lubricate (e.g., saliva, mucus).
  • Extracellular matrix (ECM): "cell glue" that helps bind cells together; most abundant extracellular material.

Cells

  • Cells are the smallest units of life; all organisms are built from one or more cells; humans have trillions of cells.
  • Cells are the structural and functional building blocks of organisms.
  • New cells arise only from existing cells.

Cell Diversity and Function

  • Different cell types have specialized functions:
    • Erythrocytes (red blood cells) and other examples: fibroblasts, epithelial cells, skeletal muscle cells, smooth muscle cells, fat cells, macrophages, nerve cells, sperm.
  • Classic roles (as illustrated in common cell-type groupings):
    • a) Cells that connect body parts, form linings, or transport gases
    • b) Cells that move organs and body parts
    • c) Cells that store nutrients
    • d) Cells that fight disease
    • e) Cells that gather information and control body functions
    • f) Cells of reproduction

Cells – Three main regions

  • Cell membrane (plasma membrane): barrier with a semi-permeable membrane.
  • Cytoplasm: intracellular fluid containing organelles.
  • Nucleus: organelle that controls cellular activities.

Structure of the Generalized Cell

  • Three main regions: cell membrane, cytoplasm, nucleus.
  • The image colors (in the diagram) identify each region; functionally, these regions correspond to the following:
    • Cell membrane: barrier and gatekeeper.
    • Cytoplasm: site of metabolism and organelle activity.
    • Nucleus: houses genetic material and coordinates cellular functions.

The Cell Membrane (Plasma Membrane)

  • Also called the cell membrane; the boundary of the cell.
  • Key components (to be detailed in subsequent sections): phospholipids, cholesterol, membrane proteins, carbohydrates.

Components of the Cell Membrane

  • Lipid bilayer: phospholipids form a bilayer that creates a hydrophobic core.
  • Cholesterol: interspersed within the bilayer, modulates fluidity and stability.
  • Membrane proteins: integral and peripheral proteins with diverse functions.
  • Carbohydrates: form part of glycoproteins and glycolipids on the extracellular surface; contribute to the glycocalyx.

Membrane Lipids: Phospholipids & Cholesterol

  • Phospholipid bilayer structure:
    • Polar hydrophilic head (includes phosphate group) facing water on both sides (extracellular and intracellular).
    • Nonpolar hydrophobic tails form the interior of the bilayer.
  • The bilayer arrangement places polar heads toward the aqueous environments and tails away from water.
  • Cholesterol is intercalated within the bilayer and helps regulate membrane fluidity and stability.

Phospholipid bilayer (illustration cues)

  • Polar heads face the water inside and outside the cell; nonpolar tails hide from water.
  • The bilayer provides some permeability characteristics to the membrane.

Membrane Proteins

  • Membrane proteins perform many tasks: transport, receptors, enzymes, CAMs, attachment to ECM/cytoskeleton, and cell-to-cell joining.
  • Types highlighted in Figure 3.3 (summary):
    • (a) Transport proteins: may provide hydrophilic channels or use ATP to pump substances across the membrane.
    • (b) Receptors for signal transduction: binding of chemical messengers (e.g., hormones) triggers cellular responses.
    • (c) Enzymes: membrane-associated enzymes catalyze chemical reactions at the membrane surface or as part of a pathway.
    • (d) Cell-to-cell recognition: glycoproteins (glycoproteins + sugars) function as identification tags (glycocalyx).
    • (e) Attachment to cytoskeleton and ECM: stabilize cell shape and position; influence movement and protein localization.
    • (f) Cell-to-cell joining: junctions between adjacent cells; some CAMs (cell adhesion molecules) mediate temporary binding and interactions.

Glycocalyx and Clinical Note

  • Glycocalyx: network of carbohydrates attached to membrane proteins and lipids forming a protective and signaling layer.
  • Clinical—Homeostatic Imbalance 3.1 (FYI, not on test): glyocalyx on some cancer cells can change rapidly, hindering immune recognition and allowing mutated cells to escape immune destruction.

Cell Junctions

  • Purpose: join plasma membranes of adjacent cells.
  • Three types and associated functions:
    • Tight junctions: impermeable seals; prevent passage of molecules between cells; provide barrier and waterproofing (example: digestive tract enzymes restriction to bloodstream).
    • Desmosomes: strong, anchoring junctions that distribute mechanical stress; common in tissues with high mechanical load (e.g., heart, skin).
    • Gap junctions: channels that connect cells allowing ions to pass directly from cell to cell; enable electrical coupling (e.g., cardiac tissue coordinating contractions).

Ways Things Move Across the Cell Membrane

  • Membrane transport types:
    • Passive transport: no added energy required; substances move down their concentration gradient.
    • Active transport: requires energy; substances can move against their concentration gradient.
    • Vesicular transport (endocytosis/exocytosis): energy-dependent, large cargo.

Active vs Passive Transport (Table Highlights)

  • Passive transport:
    • No ATP required; substances move high to low concentration (down their gradient).
    • Examples: diffusion of lipids, O2, CO2; osmotic movement of water through aquaporins.
  • Active transport:
    • Requires energy (e.g., ATP); moves substances from low to high concentration (against gradient).
    • Involves pumps and transport proteins; energy expenditure is required for transport.

Concentration Gradient

  • Definition: difference in concentration of molecules between regions.
  • Direction: typically from regions of higher concentration to regions of lower concentration.
  • Concept extended to electrochemical gradients when electrical and chemical gradients combine.

Electrochemical Gradient (Concept)

  • Gradients can be chemical, electrical, or both (electrochemical).
  • Movement across membranes can be influenced by both concentration differences and membrane potential.

Passive Membrane Transport: Three Types

  • Simple diffusion
  • Facilitated diffusion
  • Osmosis
  • Do not require ATP (energy) directly; kinetic energy drives the process; membrane permeability and channel/carrier availability modulate transport.

Diffusion (Passive Transport)

  • Definition: Movement of molecules/ions from area of higher concentration to lower concentration along the concentration gradient.
  • Driving force: intrinsic kinetic energy of molecules.

Diffusion Determinants

  • Factors influencing rate:
    • Concentration gradient: larger difference speeds diffusion.
    • Molecule size: smaller particles diffuse faster.
    • Temperature: higher temperature increases molecular speed and diffusion rate.
  • Barrier to diffusion: the cell membrane is semi-permeable and selective.
  • Lipid solubility and size determine crossing likelihood: more lipid-soluble and smaller molecules diffuse readily; larger or less lipid-soluble molecules may require carrier molecules or channels (e.g., ions).

Simple Diffusion

  • Substances diffuse directly through the lipid bilayer.
  • Examples: lipophilic molecules and gases (e.g., O2, CO2).
  • Movement along the concentration gradient: from higher to lower concentration.

Facilitated Diffusion

  • Used for molecules unable to pass directly through lipid bilayer.
  • Mechanism: use channel proteins or carrier proteins to move substances across the membrane.
  • Characteristics:
    • Carriers are limited by the number of available carrier proteins.
    • Channels are selective: leakage channels (always open) and gated channels (open/close in response to signals).
  • Main example: glucose entry into cells occurs primarily via facilitated diffusion.

Osmosis

  • Definition: diffusion of water across a selectively permeable membrane.
  • Pathways: directly through the lipid bilayer or via aquaporin channels.
  • Occurs when water can move on both sides, but the solute cannot.

Possible Outcomes of Osmosis (FYI)

  • Outcomes include normal steady-state conditions or issues that may arise depending on solute/water balance.

Passive Transport Recap (Conceptual Summary)

  • Simple diffusion: no transporter; non-specific; no energy; examples include lipids, O2, CO2.
  • Facilitated diffusion: requires carrier/channel; still no direct energy input; specificity based on protein shape.
  • Osmosis: diffusion of water through membrane or aquaporins; energy not required.

Active Transport: Primary vs Secondary & Vesicular

  • Active transport requires cellular energy to move substances across membranes.
  • Reasons for energy use include: substances cannot dissolve in lipid bilayer, are too large for channels, or need to move against their concentration gradient.
  • Two main forms:
    • Primary Active Transport: directly uses ATP hydrolysis to drive transport via pumps.
    • Secondary Active Transport (aka Cotransport): uses energy stored in ion gradients created by primary active pumps to move other substances.

Primary Active Transport

  • Mechanism: Hydrolysis of ATP energizes a pump by transferring a phosphate group, enabling transfer of substances against their gradient.
  • Classic example: Sodium-Potassium Pump (Na+/K+ ATPase).
  • Key points: Pumps are specific; ATP is the energy source; ions move against their electrochemical gradients.

Focus: Na+/K+ Pump (Primary Active Transport Example)

  • The Na+/K+ pump requires transport proteins (pumps) and energy (ATP).
  • It moves three Na+ ions out of the cell and two K+ ions into the cell.
  • Ion charges: Na+ and K+ carry positive charges.
  • Resulting inside of the cell becomes more negative relative to the outside (membrane potential).
  • It creates a gradient for Na+: higher outside the cell, lower inside the cell (Na+ gradient).
  • The pump itself maintains the Na+ gradient, which can then drive secondary transport processes.
  • Typical completion prompts (as seen in the slides):
    • 1) Na+/K+ pump involves transport proteins and energy input.
    • 2) What is pumped out: three Na+ ions.
    • 3) What is pumped in: two K+ ions.
    • 4) Charge on the ions being pumped: positive.
    • 5) Does the pump release more positives inside or outside: outside (net loss of positive charge from the cytosol).
    • 6) Is the inside of the cell membrane more negative or positive as a result: more negative.
    • 7) Does the pump leave more Na+ in the cell or outside: outside.
    • 8) Did this create a gradient for Na+: yes, gradient exists (Na+ tends to move back into the cell when allowed).

Secondary Active Transport (aka Cotransport)

  • Driven by the concentration gradient created by the primary active pumps (e.g., Na+/K+ pump).
  • Example: Na+ moving back into the cell drives the movement of other substances (e.g., glucose) against their own gradients.
  • Transport protein typically moves more than one substance at a time.

Clinical Note: SGLT2 Inhibitors

  • Diabetes medication: Sodium-Glucose Transport Protein 2 (SGLT2) inhibitors act on SGLT-2 proteins in renal proximal tubules.
  • Mechanism: reduce reabsorption of filtered glucose, lower renal glucose threshold, promote urinary glucose excretion.
  • Relevance: used to improve glycemic control and reduce cardiovascular and renal complications in type 2 diabetes (not a test focus in this context).

Chapter 3, Part 1: Quick Review Prompts (Study-Check)

  • 1) Does passive membrane transport require ATP (cellular energy)? No.
  • 2) Name three types of passive transport. Simple diffusion, facilitated diffusion, osmosis.
  • 3) Which type of cell junction allows neighboring cells to pass ions to each other? Gap junctions.
  • 4) When moving along a concentration gradient, how do molecules move? From high concentration to low concentration.
  • 5) Na+/K+ pumps move how many ions and in which direction? 3 Na+ out, 2 K+ in; this creates a negative interior relative to exterior.
  • 6) When moving against a concentration gradient, what type of transport is needed? Active transport; fuel is cellular energy (ATP).
  • 7) What makes up the basic structure of the cell membrane? A phospholipid bilayer with embedded proteins and cholesterol; associated carbohydrates (glycocalyx).
  • 8) What helps stiffen the structure of the cell membrane? Cholesterol.
  • 9) What is the smallest unit of life? The cell.
  • 10) Which cell junction exists in the heart and allows ions to pass from one cell to the next? Gap junctions.

Chapter 3, Part 1: More You Got This? (Additional Prompts)

  • 11) Name the three regions of a cell. The cell membrane, cytoplasm, and nucleus.
  • 12) Which region of the cell contains the organelles? The cytoplasm.
  • 13) Which type of cell junction keeps cells attached to each other and can distribute mechanical stress? Desmosomes.
  • 14) What is a concentration gradient? A difference in concentration between two regions.
  • 15) How is the cell membrane selectively permeable? What passes easily, what does not, and what transport do they use? Lipid-soluble substances pass readily; large or charged molecules require channels or carriers; many such molecules use facilitated diffusion or active transport to cross.
  • 16) What moves during osmosis? Water moves across a selectively permeable membrane.
  • 17) Why is facilitated diffusion needed? For substances that cannot diffuse directly through the lipid bilayer; the channels or carriers provide passage.
  • 18) The Sodium-Potassium Pump moves Na+ and K+. Do the ions gain or lose electrons? They do not undergo redox changes in this pump; they are ions transported across the membrane.
  • 19) The/ / pump moves Na+ (into or out of) the cell. This creates a concentration gradient of Na+ being (higher or lower) in the cell. Secondary active transport moves Na+ back into the cell and brings along another substance, often .
  • 20) What role do carbohydrates play in the cell membrane? Carbohydrates contribute to the glycocalyx, which is involved in cell recognition, protection, and interactions with the external environment.

Notes:

  • All explicit numerical details included from the transcript are reflected above (e.g., Na+/K+ pump stoichiometry: 3 Na+ out, 2 K+ in).
  • The notes mirror the sequence and terminology found in the transcript, with added clarifications where the slides implied general concepts (e.g., roles of glycocalyx, cell junction types, and transport mechanisms).
  • LaTeX usage: The Na+/K+ pump details are presented with explicit LaTeX formatting where appropriate (e.g., 3 Na+ out, 2 Na+ in3\ Na^+\text{ out},\ 2\ Na^+\text{ in} was the intended emphasis; spelled as text in the note to preserve readability). If you want every mathematical detail consistently formatted in LaTeX for your draft, let me know and I can convert the rest into LaTeX math where applicable.