7 Inside the Cell: Organelles, Transport, and Cytoskeleton

Flashcards reviewing prokaryotic and eukaryotic cell structure, organelle functions, endomembrane protein transport, and cytoskeletal filaments from Concepts 7.1-7.6.

How many cells compose the human body, and what are the typical size ranges for eukaryotic versus bacterial cells?

The human body is made of $50\text{--}90\text{ trillion}$ cells. Eukaryotic cells range from $0.005\text{--}0.1\,\text{mm}$ ($5\text{--}100\,\mu\text{m}$), while bacterial cells are less than $0.003\,\text{mm}$.

How do total surface area and surface-to-volume ratio change when a cell is divided into smaller units while total volume remains constant?

Dividing a larger object into smaller units increases total surface area (e.g., from $150$ to $750$) and increases the surface-to-volume ratio (e.g., from $1.2$ to $6$).

What geometric formulas are used to calculate the surface area and volume of a spherical cell?

Surface area is calculated as $4\pi r^2$ and volume is calculated as $\frac{4}{3}\pi r^3$.

What are the primary structural differences between prokaryotic and eukaryotic cells?

Prokaryotes lack a defined nucleus (having an unbound nucleoid region) and are smaller, whereas eukaryotes possess a true nucleus surrounded by a nuclear membrane and contain complex membrane-bound organelles.

What are the main structural components and functions of the nucleus?

The nucleus holds most cellular DNA, is surrounded by a nuclear membrane with nuclear pores controlling entry/exit, maintains shape via the nuclear lamina, and contains a nucleolus for rRNA synthesis and ribosome subunit assembly.

What is the difference between free and bound ribosomes, and why are ribosomes not considered true organelles?

Free ribosomes synthesize proteins that function in the cytosol, while bound ribosomes synthesize proteins destined for membrane insertion or secretion. Ribosomes are not true organelles because they are not surrounded by membranes.

Which cellular organelles compose the endomembrane system?

The endomembrane system consists of the Endoplasmic Reticulum (Smooth and Rough), Golgi Apparatus, Lysosomes, and Vacuoles.

What are the distinct functions of the Smooth Endoplasmic Reticulum versus the Rough Endoplasmic Reticulum?

Smooth ER (lacking ribosomes) carries out lipid processing, calcium ion storage, and detoxification of drugs and poisons. Rough ER (studded with ribosomes) processes proteins destined for secretion, membrane insertion, or transport to other organelles.

What is the primary function of the Golgi apparatus in protein transport?

The Golgi apparatus modifies, stores, and ships proteins by adding specific molecular tags (molecular "zip codes") to direct them to their proper destinations.

How do lysosomes perform digestive functions, and at what pH do their enzymes operate?

Lysosomes break down cellular waste via phagocytosis (engulfing solid particles) and autophagy (recycling internal cellular materials). Proton pumps maintain an acidic internal environment with a $\text{pH } \approx 5.0$ optimal for lysosomal enzymes.

What functions does the central vacuole perform in plant cells?

The central vacuole stores inorganic ions, enables plant cells to increase in size without synthesizing extra cytoplasm, and helps the plant maintain structural rigidity.

What metabolic functions are carried out by peroxisomes, and how is toxic hydrogen peroxide detoxified?

Peroxisomes perform redox reactions to break down fatty acids and detoxify alcohol and other toxins. These reactions produce toxic $H_2O_2$, which is broken down into safe components by the enzyme catalase.

What structural feature of the mitochondrial inner membrane optimizes its function in cellular respiration?

The inner membrane features extensive folding (cristae) which increases the surface area available for cellular respiration and ATP production.

What are the key structural components and functions of chloroplasts?

Chloroplasts possess a double membrane, contain chlorophyll, their own DNA, and ribosomes, and serve as the site of photosynthesis.

What signal sequences and transport proteins direct nuclear import and export as shown in the nuclear transport model?

Nuclear import requires a cargo protein with a Nuclear Localization Sequence (NLS) binding to importin. Nuclear export requires a cargo protein with a Nuclear Export Sequence (NES) binding to exportin along with Ran-GTP.

How are nascent proteins directed into the Endoplasmic Reticulum during translation?

Proteins destined for the ER feature a signal peptide (ER signal sequence). A Signal-Recognition Particle (SRP) binds the signal peptide and escorts the ribosome to an SRP receptor on the ER membrane, allowing the growing polypeptide to enter the ER lumen.

What are the structural properties, diameter, protein subunit, and functions of microfilaments?

Microfilaments consist of two intertwined strands of actin ($7\,\text{nm}$ diameter). Their functions include maintaining cell shape under tension, muscle contraction (via myosin), cytoplasmic streaming, cell motility via pseudopodia, and forming the cleavage furrow during cell division.

What are the structural properties, diameter, protein subunits, and functions of intermediate filaments?

Intermediate filaments consist of fibrous proteins supercoiled into thicker cables ($8\text{--}12\,\text{nm}$ diameter) composed of proteins such as keratins. Their functions include maintaining cell shape (tension-bearing), anchoring the nucleus and other organelles, and forming the nuclear lamina.

What are the structural properties, diameter, protein subunits, and functions of microtubules?

Microtubules are hollow tubes consisting of $13$ columns of tubulin dimers ($\alpha\text{-tubulin}$ and $\beta\text{-tubulin}$) with a diameter of $25\,\text{nm}$ ($15\,\text{nm}$ lumen). Their functions include maintaining cell shape (compression-resisting girders), driving cell motility in cilia or flagella, moving chromosomes during cell division, and guiding organelle movements.

How do motor proteins interact with microtubules to transport cellular cargo?

An ATP-powered motor protein binds to a specific receptor on a vesicle or organelle and "walks" along the microtubule track of the cytoskeleton.