Questiond: Cell Biology: Cell Theory, Membrane Structure, Transport, and Endomembrane System

Comprehensive question-and-answer flashcards covering cell theory, prokaryotic vs. eukaryotic features, membrane structure and fluidity, passive and active transport, and the eukaryotic endomembrane system.

What three observations form the cell theory?

1) All organisms are made up of cells, 2) The cell is the fundamental unit of life, and 3) Cells come from preexisting cells.

How does the biconcave shape of a human red blood cell support its function?

Its biconcave shape allows the cell to alter its shape to pass through narrow blood vessels and provides a high surface area relative to its volume for efficient oxygen uptake and release.

What are the key structural and spatial differences between prokaryotic and eukaryotic cells?

Prokaryotic cells lack a nucleus, house a circular chromosome in a nucleoid, and are small ($1\text{--}2\,\mu\text{m}$) with high surface-area-to-volume ratios. Eukaryotic cells contain a double-membraned nucleus, multiple linear chromosomes, internal organelles, and spatially separate transcription (in the nucleus) from translation (in the cytoplasm).

What is the chemical structure of a phospholipid, and why is it described as amphipathic?

A phospholipid consists of a glycerol backbone attached to a polar, hydrophilic phosphate head group and two nonpolar, hydrophobic fatty acid tails. Having both hydrophilic and hydrophobic domains within a single molecule makes it amphipathic.

How do phospholipids behave when placed in water at neutral $\text{pH } 7$ depending on their molecular shape?

Lipids with bulky heads and single hydrophobic tails are wedge-shaped and pack into spherical micelles. Lipids with less bulky heads and two tails are rectangular and form bilayers or closed spherical liposomes.

How do fatty acid tail length and saturation affect membrane fluidity?

Longer fatty acid tails decrease fluidity due to greater surface area for van der Waals interactions. Saturated fatty acid tails (no double bonds) pack tightly and reduce fluidity, whereas unsaturated fatty acids (with double-bond kinks) reduce packing tightness and increase fluidity.

What dual effect does cholesterol have on animal cell membrane fluidity across different temperatures?

Cholesterol constitutes about $30\%$ by mass of animal cell membrane lipids. At high temperatures, its rigid ring structure decreases fluidity by restricting phospholipid mobility; at low temperatures, it increases fluidity by preventing tight packing of phospholipids.

How do integral membrane proteins differ from peripheral membrane proteins?

Integral membrane proteins are permanently associated with cell membranes and usually span the lipid bilayer as transmembrane proteins. Peripheral membrane proteins temporarily associate with lipid head groups or integral proteins through weak noncovalent interactions.

What experimental technique proved protein mobility in membranes, and what model was proposed by S. Jonathan Singer and Garth Nicolson in 1972?

Fluorescence recovery after photobleaching (FRAP) demonstrated lateral protein movement in the membrane. Singer and Nicolson proposed the fluid mosaic model, describing the lipid bilayer as a dynamic structure containing a mosaic of lipids, proteins, and carbohydrates.

What is the difference between simple diffusion and facilitated diffusion?

In simple diffusion, hydrophobic or small nonpolar molecules move directly through the lipid bilayer down their concentration gradient. In facilitated diffusion, molecules move down their concentration gradient through transport proteins (channels or carriers).

How do channel proteins and carrier proteins function during passive transport?

Channel proteins provide an opening (which can be gated by chemical or electrical signals) for molecules to pass based on charge and shape. Carrier proteins bind to specific molecules, undergo a conformational change, and transfer the molecule across the membrane.

What is osmosis, and how do aquaporins facilitate water transport?

Osmosis is the net movement of a solvent (such as water) across a selectively permeable membrane toward an area of higher solute concentration. Aquaporins are membrane channel proteins that allow water to move rapidly across the membrane by facilitated diffusion.

How does the sodium--potassium pump function as a primary active transport mechanism?

The sodium--potassium pump directly uses chemical energy from ATP breakdown to actively pump $Na^+$ ions out of the cell and $K^+$ ions into the cell against their respective concentration gradients.

How does secondary active transport utilize electrochemical gradients?

Primary active transport pumps (such as proton pumps using ATP) establish an electrochemical gradient ($H^+$ chemical and electrical difference). Transporter proteins then use the movement of $H^+$ down its electrochemical gradient to drive another molecule against its concentration gradient.

What happens to a red blood cell when placed in hypertonic versus hypotonic solutions?

In a hypertonic solution (higher solute concentration outside), water leaves the cell by osmosis and the cell shrinks. In a hypotonic solution (lower solute concentration outside), water enters the cell by osmosis and the cell lyses (bursts).

What are the primary structural components of cell walls in plants, fungi, and bacteria?

Plant cell walls are composed of cellulose (a polymer of glucose), fungal cell walls are made of chitin, and bacterial cell walls are composed primarily of peptidoglycan (a polymer of amino acids and sugars).

What is turgor pressure and how does it benefit plant cells in hypotonic conditions?

Turgor pressure is the force exerted by water pressing against the rigid cell wall as water enters by osmosis. The wall pushes back, stopping net osmosis and providing structural support that keeps the plant upright.

Which organelles belong to the eukaryotic endomembrane system?

The endomembrane system comprises the nuclear envelope, endoplasmic reticulum (ER), Golgi apparatus, lysosomes, cell membrane, and the vesicles that transport material between them.

What structural feature of the nuclear envelope regulates molecular traffic in and out of the nucleus?

Nuclear pores are large protein complexes located where the inner and outer lipid bilayers of the nuclear envelope meet, regulating the transport of mRNA out of the nucleus and proteins (such as transcription factors) into the nucleus.

What are the structural and functional differences between the rough ER and smooth ER?

Rough ER is studded with ribosomes and synthesizes transmembrane proteins, organelle proteins, and secreted proteins. Smooth ER lacks ribosomes and is the site of fatty acid, phospholipid, and steroid hormone biosynthesis, as well as drug detoxification.

What are the primary functions of the Golgi apparatus, and what occurs during glycosylation?

The Golgi apparatus modifies proteins and lipids, sorts them to their final destinations, and synthesizes carbohydrates. Glycosylation is the covalent linkage of sugars to lipids or proteins in the Golgi apparatus to create glycolipids and glycoproteins.

How do lysosomes maintain an acidic internal environment for macromolecule degradation?

Lysosomal membranes contain proton pumps that actively pump $H^+$ inside to keep the internal pH at an acidic level of about 5, which is optimal for hydrolytic degradation enzymes while protecting the cytosol (pH ~7) from degradation.