Describe the major organelles of plant and animal cells and explain how each structure supports a cellular function, distinguishing prokaryotic from eukaryotic cells (NYSSLS LS1, structure and function; systems and system models).

What this topic is asking

NYSSLS LS1 treats the cell as a system of parts. The Life Science: Biology Regents wants you to know the major organelles, to tell a prokaryotic cell from a eukaryotic one, and above all to explain how each structure supports a function. Cluster questions usually show a labelled diagram or describe a cell with an unusual feature and ask you to reason about what it does.

The major organelles

• Nucleus. Holds the cell's DNA and directs the cell by controlling which proteins are made. Bounded by a nuclear membrane with pores.
• Ribosomes. Tiny structures (free in the cytoplasm or on the rough endoplasmic reticulum) that build proteins by joining amino acids.
• Endoplasmic reticulum (ER). A network of membranes; the rough ER (studded with ribosomes) makes and folds proteins, the smooth ER makes lipids.
• Golgi apparatus. Modifies, sorts and packages proteins and lipids into vesicles for delivery.
• Mitochondria. The site of cellular respiration, releasing energy from glucose as ATP. Cells with high energy demand have many.
• Chloroplasts. Found in plant cells; the site of photosynthesis, capturing light energy to make glucose.
• Lysosomes. Contain digestive enzymes that break down waste and worn-out parts.
• Cell (plasma) membrane. The selectively permeable boundary controlling transport, covered in the cell membrane and transport.
• Cytoplasm. The fluid that holds the organelles and where many reactions occur.

Plant cells versus animal cells

Plant and animal cells share most organelles, but plant cells have three extra features:

• Cell wall, a rigid layer of cellulose outside the membrane that gives shape and support.
• Chloroplasts, for photosynthesis.
• A large central vacuole, which stores water and helps keep the cell firm (turgid).

Animal cells lack these but often have small vacuoles and structures such as centrioles. Knowing which structures are plant-only is a common cluster question.

Prokaryotic versus eukaryotic cells

The advantage of being eukaryotic is compartmentalization: membrane-bound organelles separate incompatible reactions, so a cell can digest material in lysosomes while making proteins on the ER and releasing energy in mitochondria, all at once and under different conditions. This is a systems argument the exam rewards.

Structure determines function

Every organelle is a worked example of structure fitting function. Mitochondria have folded inner membranes that pack in a large surface area for the reactions of respiration. The rough ER is covered in ribosomes because its job is to make proteins. Root cells that absorb water have no chloroplasts because they sit in the dark. When a cluster shows a cell with a striking feature, the answer is almost always: that structure suits the cell's particular job.

Try this

Q1. Identify two structures found in a plant cell but not in a typical animal cell. [2]

• Cue. Any two of: cell wall, chloroplasts, large central vacuole.

Q2. Explain why a cell that secretes large amounts of protein has a great deal of rough endoplasmic reticulum. [2]

• Cue. The rough ER, studded with ribosomes, is where proteins are made and folded; a cell that exports much protein needs a large amount of this machinery.