10.1 Cell Structure and Transport

10.1 Cell Structure and Transport

Quick Answer: Every living cell is bounded by a selectively permeable plasma membrane. Eukaryotic cells package DNA in a nucleus and generate ATP mainly in mitochondria; plant cells also run photosynthesis in chloroplasts and are reinforced by a cellulose cell wall. Materials cross membranes by passive transport (no cell energy: diffusion, osmosis, facilitated diffusion) or active transport (ATP-driven pumps and vesicle traffic). On Praxis 5442, expect plant-vs-animal organelle contrasts and “energy required?” transport classification items.

Life Science is about 30% of Praxis Middle School Science (5442). Cell structure and membrane transport are foundational: they explain how organisms obtain energy, maintain internal conditions, and specialize into tissues. Middle-grades students often confuse “wall” with “membrane,” or assume all transport needs energy. Your job as a teacher—and as a test-taker—is to keep those distinctions crisp.

Shared cell parts and the major eukaryotic organelles

All cells have a plasma membrane, cytoplasm, ribosomes, and genetic material (DNA). In eukaryotes (plants, animals, fungi, many protists), DNA is enclosed in a double-membrane nucleus. The nucleus houses chromosomes, controls gene expression, and is bounded by a nuclear envelope with pores that regulate RNA and protein traffic.

Three organelles dominate middle-school items:

Teaching trap: Students often say mitochondria are “only in animals.” Correct that: plants have mitochondria too—they burn sugars at night and in non-photosynthetic tissues. Chloroplasts are the plant-specific (or algae-specific) addition for capturing light.

Other high-yield structures: ribosomes (protein synthesis; free or on rough ER), endoplasmic reticulum and Golgi apparatus (processing and shipping proteins/lipids), vacuoles (storage; large central vacuole in plant cells), and lysosomes (digestive enzymes, especially emphasized in animal cells).

Membranes vs walls

The plasma membrane is a phospholipid bilayer with embedded proteins. It is selectively permeable: small nonpolar molecules (O₂, CO₂) cross relatively freely; ions and large polar molecules need channels, carriers, or vesicles. The membrane is flexible and present in both plant and animal cells.

A cell wall is a rigid outer layer outside the plasma membrane. Plant walls are primarily cellulose; fungal walls use chitin; bacterial walls use peptidoglycan. Animal cells lack cell walls. Walls provide support and protection and help plant cells resist osmotic swelling (turgor), but they are not the primary selective barrier—the membrane still controls most molecular traffic.

Passive transport (no cellular ATP spent on the movement itself)

Passive transport moves substances down their concentration gradient (high → low). Three forms appear constantly on middle-school assessments:

1. Simple diffusion — molecules spread through the bilayer or open space (e.g., O₂ into a cell).
2. Facilitated diffusion — membrane proteins (channels/carriers) help polar molecules or ions move down their gradient (e.g., glucose via a transporter). Still passive: no ATP required for the downhill move.
3. Osmosis — diffusion of water across a selectively permeable membrane toward the side with higher solute concentration (lower free water).

Osmosis scenarios are exam favorites. Place a cell in:

• Isotonic solution — water in = water out; volume stable.
• Hypotonic solution — outside has lower solute; water enters. Animal cells may lyse; plant cells become turgid (wall prevents bursting).
• Hypertonic solution — outside has higher solute; water leaves. Animal cells crenate; plant cells undergo plasmolysis (membrane pulls from wall).

Active transport (requires energy)

Active transport moves substances against their concentration gradient (low → high) or packages bulk cargo, using ATP (or another energy-coupling mechanism). Classic examples:

• Protein pumps (e.g., sodium–potassium pump in animal cells) that maintain ion gradients for nerve and muscle function.
• Endocytosis / exocytosis — vesicle engulfing or exporting large particles or volumes.

Classroom and Praxis framing

A strong teaching-scenario stem might show a model of a plant cell and ask which structure would be absent in a cheek cell, or present a dialysis-tubing demo and ask whether glucose movement is diffusion or active transport. Anchor answers in gradient direction and ATP requirement, not vocabulary memorization alone. When diagrams appear, match labels to function first, then to plant/animal presence.

Keep chloroplast work here at the organelle identity level; detailed light/dark reactions belong with energy transformations in the next life-science chapter. For 5442, mastery means: name the big three organelles by job, contrast wall vs membrane, and classify transport as passive or active with a reason.