Cell Structure, Membranes, Transport, and Organelles — Comprehensive Study Notes
Membrane Permeability and Transport
Recap question: which substances pass through a phospholipid bilayer easily? Use process of elimination to identify what does not pass well.
Polar and charged molecules do not pass through easily; nonpolar or very small nonpolar molecules pass more readily.
Examples discussed:
Sodium ion: charged; does not pass through easily.
Glucose: large and polar; does not pass through easily.
Water: small and highly polar; may cross, but not very efficiently by diffusion alone.
Carbon dioxide: small, with two oxygens around a carbon; described as small and not strongly polar, so it crosses quite easily.
If another phospholipid attempted to cross the bilayer: the phosphate head (polar/charged) would hinder crossing; the lipid tails might insert, but the head would resist crossing the interior of the membrane. Thus, phosphates are ions and cannot easily cross, but the interior lipid portion could interact with the membrane.
Key takeaway: little gases (like CO₂) cross the membrane easily; small nonpolar molecules cross more easily than large polar/charged molecules.
Why transport proteins exist despite molecules like CO₂ and H₂O crossing:
Water and CO₂ can diffuse, but transport proteins often move substances against their concentration gradient or aid diffusion of substances that do not cross well.
Channels: facilitate diffusion of ions and water.
Carriers: facilitate diffusion of larger molecules like amino acids or sugars.
Pumps (ATP-driven): move substances against their concentration gradient (active transport).
Indirect active transport: pumps create ion gradients (e.g., Na⁺ out, K⁺ in) which drive secondary transport of other molecules (e.g., Na⁺-sugar symport).
Review of diffusion concepts:
Diffusion: movement from higher to lower concentration across a membrane where a gradient exists.
Osmosis: diffusion of water to the area with higher solute concentration; water moves to resolve solute gradients.
Endomembrane system: a network involved in production, processing, and export of cellular substances; contains rough ER, smooth ER, Golgi apparatus, lysosomes, and more.
Structural overview of cells (context for transport and compartments):
Plasma membrane: border of the cell; selectively permeable; contains phospholipids and proteins.
Nucleic acids (DNA/RNA): storage and transmission of genetic information.
Proteins: perform cellular functions; enzymes, transporters, structural components.
Carbohydrates: energy storage and cell signaling/structure;
Cytoplasm (cytosol): the cellular “juice” where nutrients and proteins move around.
Cell types: prokaryotes vs eukaryotes
Prokaryotes: generally smaller, lack a nucleus, simpler organization.
Eukaryotes: larger, have a nucleus, more organelles; more compartments allow specialization.
Relationship: eukaryotes are most closely related to Archaea; bacteria are kind of the odd one out in terms of relation to other domains.
Prokaryotes: Basic cell structure and features
Nucleic acids organization:
Chromosome: a single, giant circular DNA molecule; coiled in a region called the nucleoid.
Plasmids: small, circular, supercoiled DNA that bacteria exchange; often carry antibiotic resistance genes.
Cytoplasmic components:
Cytoplasm (cytosol): the cellular “juice” with salts, nutrients, and enzymes.
Plasma membrane: similar to eukaryotes but can be structurally different in bacteria.
Cell wall: external armor made of proteins/polysaccharides; in bacteria this is commonly peptidoglycan; aids structure and resistance.
Cytoskeleton: protein-based scaffolding that helps maintain shape and organize the cell; important for division.
Additional notes (bonus features seen in some bacteria):
Internal membranes: not universal, but some bacteria have internal membrane systems.
Organelles-like compartments: some bacteria have specialized compartments (e.g., magnetosomes).
External structures: flagella for movement; fimbriae for sticking to surfaces.
Important takeaway for exams:
Expect questions contrasting prokaryotes vs. eukaryotes and identifying prokaryote-specific structures (e.g., nucleoid with a single circular chromosome, presence of a cell wall, lack of a nucleus).
Be prepared for a multiple-choice item on a prokaryote-only structure.
Eukaryotes: Basic cell structure and features
Size and complexity:
Eukaryotic cells are much larger; contain many membrane-bound organelles, forming an endomembrane system that creates internal “rooms” for specialized chemistry.
Analogy: a eukaryotic cell is like a large corporation with departments; greater organization can increase efficiency but comes with higher maintenance costs.
General organelles and features (membrane-bound vs non-membrane-bound):
True (membrane-bound) organelles: nucleus, mitochondria, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, peroxisomes, chloroplasts (plants).
Non-membrane-bound organelles: ribosomes, cytoskeleton components.
Plant-specific structures: chloroplasts, large central vacuole, cell wall.
Animal-specific structures: lysosomes (more prominent in animal cells), sometimes different membrane properties.
Nucleus (brain of the cell):
Contains chromosomes; humans have 46 chromosomes.
Structure: double membrane (outer and inner membranes) with nuclear pores; nuclear lamina provides structural support.
Nucleolus: site of ribosome biogenesis (ribosome assembly).
Function summary: stores DNA, transcribes RNA, and makes ribosomes.
Fun facts: double-membrane organization; lamina around the nucleus; nucleolus is the ribosome factory.
Ribosomes:
Non-membrane-bound; made of RNA and proteins; two subunits (small and large).
Function: protein synthesis.
Locations in the cell: free in cytosol or attached to rough ER.
Endoplasmic reticulum (ER):
Rough ER: studded with ribosomes; site of protein synthesis; continuous with the nuclear envelope; lumen inside is where polypeptides are synthesized and begin folding.
Smooth ER: lacks ribosomes; lipid synthesis (including phospholipids, cholesterol) and lipid metabolism; also acts as a calcium ion reservoir.
Golgi apparatus:
Structure: flattened membrane-bound sacs called cisternae with a directional organization: cis face (near the nucleus) and trans face (away from the nucleus).
Functions: further processing and folding of proteins; post-translational modifications; packaging into vesicles for transport
Transport direction: proteins move from rough ER (cis side) → Golgi (processing) → trans face → exocytic vesicles or lysosome.
Lysosomes vs. Vacuoles:
Lysosomes (animal cells): membrane-bound vesicles containing digestive enzymes; acidic interior; degrade macromolecules and recycle components.
Vacuoles (plant cells): large central storage organelle; stores water, ions, pigments, toxins; helps maintain osmotic balance and can contribute to plant cell rigidity.
Pathways: lysosomes are part of the endomembrane system; plant cells use vacuoles for similar roles and can contain hydrolytic enzymes in some contexts.
Peroxisomes:
Single-membrane organelles; contain enzymes for oxidation-reduction (redox) reactions and detoxification; act as a silo for potentially dangerous reactions.
Mitochondria:
Double-membrane organelle with inner membrane folds called cristae and an internal matrix.
Function: major site of ATP production; converts stored energy into usable cellular energy.
Unique features: own DNA and ribosomes; evidence for endosymbiotic origin (ancestor of bacteria).
Chloroplasts (plants and some algae):
Double-membrane organelles with stacked thylakoids (grana) and stroma in the surrounding area.
Function: photosynthesis—converts CO₂, water, and light into sugars (glucose).
Unique features: own DNA and ribosomes; endosymbiotic origin; essential for energy capture in plants.
Cytoskeleton:
Non-membrane network of protein fibers that give shape, anchor organelles, and mediate movement.
Three main types:
Actin filaments: resist tension, important for cell shape and cytokinesis; essential in muscle contraction and cell movement.
Intermediate filaments: provide mechanical strength and resist tension; anchor organelles.
Microtubules: largest fibers; provide structural support, act as “pillars,” move organelles, help separate chromosomes during cell division; important in plant and animal cell division.
Cell wall (plants, fungi, some algae):
Rigid layer outside the plasma membrane; composed of cellulose (in most plants) or other polymers (e.g., chitin in fungi, lignin in wood).
Functions: structural support, protection, maintains osmotic balance to prevent bursting; contributes to rigidity and “crunch” in plant tissues.
Endomembrane system and trafficking (key concept):
The nucleus and ER are part of the internal membrane system; proteins synthesized in the rough ER are transported to the Golgi for processing, then packaged into vesicles that travel to the lysosome or the plasma membrane for secretion.
This system creates an organized pathway for production, processing, and digestion within the cell.
Plant vs Animal cells: key differences and implications
Plant cells:
Chloroplasts for photosynthesis; rigid cell walls; large central vacuole for storage and osmotic balance.
Typically have a tonoplast (vacuolar membrane) surrounding the central vacuole.
Animal cells:
Lack chloroplasts and a cell wall; rely on lysosomes for digestion and vesicular transport pathways.
Practical takeaway: differences in organelle complement reflect different life strategies (photosynthesis and structural support in plants vs rapid movement and digestion in animals).
The inside/outside question: connecting structure to function
The cell boundary and internal compartments optimize chemistry:
Compartments concentrate reactants, separate incompatible reactions, and increase efficiency.
The cost of compartments includes higher energy and resource demands and potential points of failure.
Consequences for physiology and disease:
Organelles specialize and adapt to tissue function (e.g., liver cells with many peroxisomes for detoxification; pancreatic cells with abundant rough ER and Golgi for protein export).
Changes in organelle number or function can reflect pathology or specialization (e.g., cardiac muscle cells with many mitochondria for high energy demand).
Redox biology preview and enzymes
Enzymes and energy flow:
Enzymes are central to cellular reactions; enzymes drive and regulate chemical transformations.
Free energy concept: energy available to drive chemical reactions; exists as kinetic and potential energy (contextual to reactions).
Key vocabulary introduced:
Free energy: energy available to do work in a chemical reaction; connected to kinetic and potential energy forms in a system.
Direct and indirect energy coupling: reactions can be coupled so that energetically unfavorable processes are driven by favorable ones (e.g., ATP hydrolysis) and energy is transferred via enzymes and transporters.
Redox reactions: fundamental energy-transfer reactions that involve electron transfer; important for metabolism and energy production (to be studied in more detail in the next lecture).
Electronegativity and bonding (conceptual recap):
Atoms prefer to be near electronegative elements and to complete a valence shell; sharing electrons in bonds stores energy.
Polar vs nonpolar bonds:
Nonpolar bonds can store significant energy due to electron distribution; polar bonds involve unequal sharing and are more stabilized by interactions with polar environments.
The speaker highlighted that, in the simplified model shown, nonpolar bonds were portrayed as storing more energy because electrons are pulled further from nuclei in some contexts.
Notation and future topics:
The lecture will return to enzymes, their regulation, and how environmental factors affect enzyme activity (temperature, pH, inhibitors, etc.).
A focus on redox chemistry and energetic coupling will be revisited in more depth next session.
Quick study cues and exam-oriented notes
Major exam themes to anticipate:
Distinguishing features of prokaryotes vs eukaryotes; identifying prokaryote-specific structures.
Functions and locations of major organelles; how the endomembrane system operates in protein trafficking.
Differences between plant and animal cells (chloroplasts, mitochondria, vacuoles, cell wall).
Role of cytoskeleton in maintaining shape, organizing organelles, and driving movement/division.
Pathway of a secreted protein: rough ER → Golgi → vesicles → exocytosis or lysosome.
Why transport proteins are needed: diffusion limits, concentration gradients, active transport, and secondary transport.
Memorable analogies used:
Nucleus as the “brain” and the nucleolus as the ribosome factory.
Rough ER as the protein synthesis factory with ribosomes on its surface.
Golgi as the post-office for protein processing and sorting.
Endomembrane system as the cell’s production and export center.
Cytoskeleton as the cell’s scaffolding, transport rails, and division machinery.
Important data points and named numbers mentioned in the lecture
Chromosome count (human): 46
Cytoplasmic pH range mentioned: approximately pH \, \approx \, 7 (with a noted range around 6.5\text{-}7.5)
Reference ions and small molecules discussed in transport examples: Na⁺ (charged ion), glucose (large polar), H₂O (small polar), CO₂ (small nonpolar-ish)
Structural and functional counts (conceptual, not numerical): multiple organelles with distinct membranes (two membranes for nucleus, mitochondria, chloroplasts), single-membrane organelles (ER, Golgi, lysosomes, peroxisomes, vacuoles), non-membrane-bound organelles (ribosomes, cytoskeleton)
Note on study strategy mentioned in the lecture
Use flashcards/Quizlet to memorize functions and membranes of organelles.
Create a table or table-like notes for each organelle: name, membrane status, function, key facts.
Practice with potential exam questions: identify differences between cell types, predict which cell type would have more of a given organelle, trace the secretory pathway of a protein, and recall double-membrane organelles.
Review the endomembrane system and cytoskeleton topics as they are highlighted as exam-heavy, with likely multiple-choice questions.