Molecules to Organisms

What this reading is about

"Molecules to Organisms" is MCAS Reporting Category 1 — the biggest slice of the test. It covers everything from the parts of a cell to how cells get energy. This block reviews four sub-topics in one pass:

1. Cell types and organelleA small part inside a cell that does a specific job. functions.
2. The four macromolecules and enzymeA protein that speeds up a chemical reaction in the body. rules.
3. How things cross the cell-membraneThe flexible outer boundary of every cell — controls what enters and leaves. (transport).
4. How cells make and use energy (photosynthesisPlants use light, CO2, and water to make glucose and oxygen. and cellular-respirationCells break down glucose with oxygen to make ATP energy.).

Part 1: Cells

Two kinds of cells

Every living thing is made of cells. There are two main types:

• prokaryoteA cell with no nucleus — its DNA floats freely inside. — no nucleusThe control center of a eukaryotic cell — holds the DNA.. DNA floats freely. Bacteria are prokaryotes. They are small and simple.
• eukaryoteA cell with a nucleus and membrane-bound organelles inside. — has a nucleus. DNA is inside the nucleus. Animals, plants, and fungi are eukaryotes. They are bigger and more complex.

Key organelles

Eukaryotic cells have organelles — small parts inside the cell, each with a specific job:

• nucleusThe control center of a eukaryotic cell — holds the DNA. — the control center. Holds the DNA.
• Ribosome — makes proteinA large molecule made of amino acids — does many jobs in cells.. Found in ALL cells (prokaryotes too).
• mitochondriaThe organelle that makes ATP — the cell's energy source. — makes atpThe energy molecule — powers nearly everything cells do. (energy). The inner membrane has folds for more surface area. Found in both plant and animal cells.
• chloroplastThe organelle in plant cells where photosynthesis happens. — does photosynthesisPlants use light, CO2, and water to make glucose and oxygen.. Found only in plant cells.
• cell-membraneThe flexible outer boundary of every cell — controls what enters and leaves. — the flexible outer boundary of every cell. Controls what enters and leaves.

Part 2: Macromolecules

The four types

All living things are built from four types of large molecules. All of them have carbon as the backbone — carbon is what makes a molecule "organic."

• carbohydrateA sugar or starch molecule — the body's quick energy source. — sugars and starches. Building block: glucoseA simple sugar — the building block of carbohydrates. (a monosaccharide). Quick energy. Words ending in -ose = carbohydrate.
• lipidA fat or oil — stores energy and makes up cell membranes. — fats and oils. Building block: fatty acid + glycerol. Long-term energy storage and cell membranes.
• proteinA large molecule made of amino acids — does many jobs in cells. — does many jobs (enzymes, antibodies, structure). Building block: amino acid.
• Nucleic acid — DNA and RNA. Building block: nucleotide. Holds genetic information.

Enzymes

An enzymeA protein that speeds up a chemical reaction in the body. is a protein that speeds up a chemical reaction. Words ending in -ase are enzymes (lipase, sucrase, amylase). Enzymes work best at one specific temperature — too hot or too cold and they stop working. On a graph, enzyme activity looks like a bell curve.

Part 3: Transport across the membrane

The cell-membraneThe flexible outer boundary of every cell — controls what enters and leaves. is selectively permeable — it lets some things through but not everything. There are three main ways molecules cross:

• diffusionThe spreading of particles from where there are many to where there are few. — molecules move from HIGH concentration to LOW concentration. No energy needed. Small molecules (like O₂) can pass right through.
• osmosisThe diffusion of water across a membrane — high to low. — the diffusion of water across a membrane. Still high to low, still no energy.
• active-transportMoving molecules against the gradient — requires ATP energy. — molecules move from LOW to HIGH concentration (against the gradient). Requires atpThe energy molecule — powers nearly everything cells do. energy and a protein pump. The sodium-potassium pump is the classic example.

Part 4: Cellular energy

The two equations

These two processes are opposites. The products of one are the reactants of the other:

• photosynthesisPlants use light, CO2, and water to make glucose and oxygen.: 6 CO₂ + 6 H₂O + light energy → glucoseA simple sugar — the building block of carbohydrates. + 6 O₂. Happens in the chloroplastThe organelle in plant cells where photosynthesis happens..
• cellular-respirationCells break down glucose with oxygen to make ATP energy.: glucose + 6 O₂ → 6 CO₂ + 6 H₂O + atpThe energy molecule — powers nearly everything cells do.. Happens in the mitochondriaThe organelle that makes ATP — the cell's energy source..

Plants do BOTH — photosynthesis during the day (when there is light) and cellular respiration all the time. Animals only do cellular respiration.

Connecting it all

These four sub-topics connect. Cells need energy → energy comes from cellular respiration → respiration needs glucose → plants make glucose via photosynthesis → photosynthesis happens in chloroplasts (organelles) → the glucose and oxygen cross the membrane via transport. One topic feeds the next.