Describe the structures and functions of the major organelles in plant and animal cells, distinguish prokaryotic from eukaryotic cells, and relate cell structure to function (MA STE HS-LS1).

What this topic is asking

The Massachusetts STE framework (HS-LS1-2) wants you to model how the structures inside a cell work together as a system to carry out the functions of life. On the High School Biology MCAS this means more than memorizing an organelle list. You are usually shown a labeled diagram or a description of a cell and asked to identify a structure, give its function, or explain what the cell does based on which organelles it has in abundance. The big idea is structure determines function, both for the whole cell and for each organelle inside it.

The major organelles and their jobs

• Nucleus. The control center. It stores the cell's DNA and directs activities by controlling which genes are expressed. The nuclear membrane has pores that let molecules such as RNA pass in and out.
• Ribosomes. The sites of protein synthesis. They read messenger RNA and assemble amino acids into polypeptides. Some float free in the cytoplasm; others sit on the rough endoplasmic reticulum.
• Endoplasmic reticulum (ER). A network of membranes. Rough ER (with ribosomes) makes and processes proteins; smooth ER (no ribosomes) makes lipids and helps detoxify substances.
• Golgi apparatus. Modifies, sorts, and packages proteins and lipids into vesicles for delivery inside or outside the cell. Think of it as the cell's shipping department.
• Mitochondria. The sites of cellular respiration. They release energy from glucose and store it as ATP. Cells with high energy demands contain many of them.
• Chloroplasts (plants and algae only). The sites of photosynthesis. They capture light energy and use it to build sugars, and they contain the green pigment chlorophyll.
• Lysosomes. Contain digestive enzymes that break down worn-out organelles, food particles, and invading bacteria.
• Vacuoles. Storage sacs. A plant cell usually has one large central vacuole that stores water and helps keep the cell firm (turgid).
• Cell membrane. The selectively permeable boundary that controls what enters and leaves; covered in the cell membrane and transport.

Plant cells versus animal cells

Both plant and animal cells are eukaryotic and share the nucleus, mitochondria, ribosomes, ER, and Golgi. The differences are the ones the MCAS asks about most:

• Cell wall. Plant cells have a rigid cellulose cell wall outside the membrane; animal cells do not. The wall gives shape and support and stops the cell bursting when it takes in water.
• Chloroplasts. Plant cells have them and can photosynthesize; animal cells do not.
• Large central vacuole. Plant cells have a big central vacuole; animal cells have only small vacuoles, if any.

Prokaryotes versus eukaryotes

Both kinds of cell still have DNA, a cell membrane, ribosomes, and cytoplasm, and both carry out the basic processes of life. The advantage of being eukaryotic is compartmentalization: separate membrane-bound compartments let a eukaryotic cell run many different controlled reactions at the same time without them interfering. Prokaryotes are also generally much smaller.

Reading a cell from its organelles

Because the test is stimulus-driven, you are often asked to infer a cell's job from its contents. A cell packed with mitochondria has a high energy demand (muscle, sperm). A cell full of rough ER and ribosomes makes and exports a lot of protein (a gland cell). A cell with many chloroplasts photosynthesizes (a leaf cell). This is the practice of analyzing data: read the structure, then reason to the function.

Try this

Q1. Name 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. State the function of the nucleus and of the ribosome. [2]

• Cue. Nucleus: stores DNA and controls the cell's activities. Ribosome: site of protein synthesis.