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Biology Unit 2: Cell structure and function

Cells are the basic unit of life. Organelles divide the work, and the membrane controls what goes in and out.

Covers: Prokaryotes vs eukaryotes, organelles, membranes and transport, surface area to volume, compartmentalization

Study guide

The cell is the basic unit of life. Prokaryotic cells (bacteria and archaea) are small and lack a nucleus and membrane-bound organelles; their DNA sits in a nucleoid region, and they have ribosomes, a cell membrane and usually a cell wall. Eukaryotic cells (plants, animals, fungi, protists) are larger and compartmentalized by internal membranes.

Each organelle has a job. The nucleus holds DNA. Ribosomes make proteins; the rough ER (studded with ribosomes) makes proteins for export or membranes, and the smooth ER makes lipids and detoxifies. The Golgi apparatus modifies, sorts and ships proteins. Lysosomes digest materials, vacuoles store water and substances, mitochondria make ATP through cellular respiration, and chloroplasts (in plants) carry out photosynthesis. Mitochondria and chloroplasts have their own DNA and double membranes, evidence for endosymbiosis. Compartmentalization lets incompatible reactions happen in separate spaces and increases membrane surface area.

The plasma membrane is a phospholipid bilayer with embedded proteins, described by the fluid mosaic model. It's selectively permeable: small nonpolar molecules cross easily, while ions and large polar molecules need proteins. Passive transport moves substances down their concentration gradient without energy: simple diffusion, facilitated diffusion through channels or carriers, and osmosis (water diffusion). In a hypotonic solution water enters the cell, and in a hypertonic one it leaves. Active transport uses ATP to move substances against their gradient, like the sodium-potassium pump, and large materials move by endocytosis and exocytosis.

Cells stay small because as a cell grows, its volume increases faster than its surface area. A high surface-area-to-volume ratio speeds the exchange of materials.

The interior of the phospholipid bilayer is hydrophobic, so the parts of membrane proteins that pass through it are made mostly of nonpolar amino acids. The parts that stick out into the water on either side are polar or charged. Proteins headed for secretion, the plasma membrane or lysosomes begin with a short signal sequence, often a stretch of hydrophobic amino acids. The signal directs the ribosome to the rough ER, and the protein then moves in vesicles from the ER to the Golgi and on to its destination.

Free ribosomes in the cytosol make proteins that stay in the cytosol. Ribosomes bound to the rough ER make proteins for membranes, for secretion or for organelles like lysosomes. The two kinds of ribosome are identical; where they sit depends on the protein being made. In a hypotonic solution, a plant cell takes in water until its wall pushes back, which makes it firm (turgid). In a hypertonic solution, the membrane pulls away from the wall (plasmolysis). Facilitated diffusion has a maximum rate, because once every transporter is in use, adding more solute can't speed it up. A cell with fewer transporters therefore takes in substances more slowly.

Key ideas

• Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.
• Ribosomes make proteins; the ER and Golgi modify and ship them; mitochondria make ATP; chloroplasts do photosynthesis.
• The phospholipid bilayer is selectively permeable: small nonpolar molecules pass easily.
• Passive transport (diffusion, osmosis, facilitated diffusion) needs no energy; active transport uses ATP to move against a gradient.
• Small cells have a higher surface area to volume ratio, which speeds exchange.

Formulas and rules

Common mistakes

• Thinking osmosis moves solute. It moves water.
• Saying plant cells lack mitochondria. They have both mitochondria and chloroplasts.

Words to know

Mitochondrion

The organelle that makes ATP by cellular respiration.

Ribosome

The site of protein synthesis.

Osmosis

Diffusion of water across a selectively permeable membrane.

Active transport

Moving substances against a gradient using energy (ATP) and a carrier protein.

Prokaryote vs. eukaryote

Eukaryotes have a nucleus and membrane-bound organelles; prokaryotes don't.

Nucleus

Holds the cell's DNA.

Rough ER

ER with ribosomes; makes proteins for export or membranes.

Smooth ER

Makes lipids and detoxifies.

Golgi apparatus

Modifies, sorts and ships proteins.

Lysosome

Digests materials with enzymes.

Chloroplast

The site of photosynthesis in plant cells.

Endosymbiotic theory

Mitochondria and chloroplasts came from engulfed prokaryotes; they have their own DNA.

Fluid mosaic model

A phospholipid bilayer with moving embedded proteins.

Facilitated diffusion

Passive transport through channel or carrier proteins.

Hypertonic solution

Higher solute outside; water leaves the cell.

Endocytosis / exocytosis

Taking in or releasing large materials in vesicles.

Surface-area-to-volume ratio

Falls as a cell grows, so small cells exchange materials faster.

Compartmentalization

Internal membranes divide the cell so different reactions happen in separate spaces.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.