Life Science 7A - Cell Organization & Membranes (Week 1b)

Do all prokaryotic cells lack DNA or ribosomes?

No. Prokaryotes have DNA and ribosomes; they simply lack a membrane-bound nucleus and typical membrane-bound organelles.

How do plant and animal cells differ in common introductory models?

Plant cells typically have a cellulose cell wall, chloroplasts in photosynthetic tissues, and a large central vacuole. Animal cells lack cell walls and chloroplasts and often have smaller vesicles/vacuoles.

Define organelle, cytoplasm, and cytosol.

Organelle: specialized cell structure, often membrane-bound. Cytoplasm: contents between the plasma membrane and nucleus, including cytosol and organelles. Cytosol: the fluid portion of the cytoplasm.

What is the nucleus's main function?

Stores most eukaryotic DNA and is a major site of gene regulation and transcription.

What do mitochondria do, and which cells tend to have many?

They perform most cellular aerobic respiration and produce much of the cell's ATP. Cells with high energy demands, such as muscle cells, often have many mitochondria.

What do chloroplasts do, and where are they found?

They carry out photosynthesis in plants and algae. They are common in photosynthetic tissues, such as leaf cells, but not in every plant cell.

What is the rough endoplasmic reticulum (rough ER) specialized for?

It has ribosomes and helps synthesize and process proteins destined for secretion, membranes, or certain organelles.

What is the smooth endoplasmic reticulum (smooth ER) specialized for?

It participates in lipid synthesis, detoxification, and calcium storage, depending on cell type.

What does the Golgi apparatus do?

Modifies, sorts, and packages proteins and lipids into vesicles for delivery to their destinations or secretion.

What do lysosomes do?

Use acidic enzymes to digest and recycle macromolecules, damaged cell parts, and material taken up by the cell.

What are vacuoles used for in plant cells?

The large central vacuole stores water and solutes, helps maintain turgor pressure, and can store or break down materials.

How can organelle abundance help you infer a cell's function?

Look for structures matching the cell's job: many mitochondria suggest high ATP demand; abundant rough ER and Golgi suggest protein production/secretion; many chloroplasts suggest photosynthesis.

A cell model shows abundant rough ER, Golgi, and secretory vesicles. What function is likely prominent?

Producing, processing, and exporting proteins, such as a cell specialized to secrete enzymes or hormones.

A cell model shows many mitochondria but little rough ER. What does this suggest?

High energy demand may be more important than large-scale protein secretion, though organelle abundance alone does not prove a cell's identity.

What is a triglyceride made of, and what is its main role?

One glycerol joined to three fatty acids. Triglycerides are major long-term energy-storage molecules and also provide insulation/cushioning.

What is a fatty acid structurally?

A hydrocarbon chain with a terminal carboxyl group. The chain is mostly nonpolar; the carboxyl group is polar and can be ionized.

How do saturated and unsaturated fatty acids differ?

Saturated fatty acids have no carbon–carbon double bonds in the hydrocarbon chain and are relatively straight. Unsaturated fatty acids have one or more double bonds, often creating kinks.

How does a phospholipid differ from a triglyceride?

A phospholipid typically has glycerol, two fatty-acid tails, and a phosphate-containing polar head. A triglyceride has three fatty-acid tails and is mainly for energy storage.

Why are phospholipids amphipathic?

Their phosphate-containing head is polar/hydrophilic, while their fatty-acid tails are nonpolar/hydrophobic.

What is cholesterol's basic structure and role in animal membranes?

It has a small polar hydroxyl group and a large rigid hydrophobic steroid-ring region plus a hydrocarbon tail. It helps regulate membrane fluidity and stability.

Describe the fluid mosaic model of the plasma membrane.

A dynamic phospholipid bilayer with proteins and other molecules embedded in or associated with it. Lipids and many proteins can move laterally; the membrane is not a rigid, uniform sheet.

How can membrane proteins associate with the lipid bilayer?

Integral proteins are embedded in the bilayer; transmembrane proteins span it. Peripheral proteins associate with the membrane surface or other proteins. Some proteins are lipid-anchored.

Why do phospholipids form a bilayer in water?

Hydrophilic heads interact with water while hydrophobic tails avoid water and pack together. This arrangement lowers the exposure of tails to water.

What happens to amphipathic lipids in a hydrophobic environment?

They may orient their hydrophobic tails toward the surrounding nonpolar environment and shield their polar heads, potentially forming reverse structures such as reverse micelles. The arrangement depends on the molecules and environment.

How does increasing temperature usually affect membrane fluidity?

It increases phospholipid movement and usually increases membrane fluidity.

How do longer fatty-acid tails affect membrane fluidity?

Longer tails generally increase van der Waals contact between tails, making the membrane less fluid at a given temperature.

How do unsaturated tails affect membrane fluidity?

Cis double bonds create kinks that prevent tight packing, generally increasing fluidity.

How does cholesterol affect membrane fluidity?

It buffers fluidity: at high temperatures it restrains phospholipid movement and reduces fluidity; at low temperatures it prevents tight packing and helps prevent solidification.

A membrane model changes from short, unsaturated tails to long, saturated tails. Predict the fluidity change.

Fluidity decreases because longer tails have more intermolecular contact and saturated tails pack more tightly.