Cell Structure and Function

Cell biology on this subtest is a matching exercise. There is no reasoning to do and no process to follow, which makes it one of the cheaper topics to lock down.

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Notice what the plant cell on the right has that the animal cell does not, and notice what it still has: mitochondria appear in both.

The organelles

The three that carry the questions

Mitochondria are described as the powerhouse of the cell, and what they actually do is cellular respiration: release energy from glucose and store it as ATP. Cells that use a lot of energy, such as muscle cells, have more of them.

Ribosomes build proteins by assembling amino acids in the order the messenger RNA specifies. They are the only organelle in this table that prokaryotes also have.

The nucleus stores the genetic material and controls the cell. Red blood cells in humans are the notable exception - they lose their nucleus as they mature, which is why they cannot divide or repair themselves.

Plant against animal

Plant cells have three structures animal cells do not:

Animal cells have centrioles, which organize cell division, and plant cells generally do not.

Both have a cell membrane, cytoplasm, a nucleus, mitochondria, ribosomes, endoplasmic reticulum and a Golgi apparatus. Plants have mitochondria too - this is the most commonly missed fact in the topic. Photosynthesis makes the glucose; respiration in the mitochondria releases the energy from it. A plant needs both.

Prokaryote against eukaryote

Every living cell is one of two kinds, and the difference is the nucleus.

A prokaryote is a cell with no nucleus. Its DNA sits loose in the cytoplasm, in a region called the nucleoid, and it has none of the membrane-wrapped organelles in the table above. Bacteria are prokaryotes, and they are the only prokaryotes the test asks about.

A eukaryote is a cell with a nucleus. Its DNA is enclosed in a membrane-bound nucleus, and it has membrane-bound organelles such as mitochondria. Every cell in a plant, an animal, a fungus or a protist is a eukaryote, which includes every cell in your body.

The names carry the definitions. Prokaryote means "before nucleus"; eukaryote means "true nucleus." If the Greek does not stick, the sounds will: pro rhymes with no (no nucleus), and eu sounds like you, and you are made of eukaryotic cells.

Ribosomes are the trap. Prokaryotes have them - every cell does, because every cell makes proteins. What prokaryotes lack is the nucleus and the membrane-bound organelles.

Movement across the membrane

A small but reliable question family.

Diffusion is movement from high concentration to low, and it needs no energy.

Osmosis is the diffusion of water across a membrane, and it also needs no energy. Water moves toward the side with more dissolved material.

Facilitated diffusion is still diffusion - high to low, no energy - but through a carrier protein in the membrane, for molecules too large or charged to slip through on their own.

Active transport moves substances against the concentration gradient, from low to high, and it requires energy from ATP. Endocytosis (taking material in by wrapping membrane around it) and exocytosis (pushing it out the same way) are bulk versions, and they cost energy too.

The distinction the test asks: passive processes go down the gradient and cost nothing; active transport goes up the gradient and costs energy. Anything with "facilitated" in its name is still passive.

Hypotonic, hypertonic, isotonic

These three describe the solution outside the cell, compared with the inside, and they tell you which way water will move.

Water always moves toward the side with more dissolved material. Work it out from that rather than memorizing the table: in a hypertonic solution the outside is saltier, so water leaves. Hyper means more, as in hyperactive; the outside has more, so it pulls the water out.

Put a plant cell in pure water and water enters by osmosis until the wall stops it, which is what keeps a plant rigid. A cell with no wall in the same situation can burst.

What you can skip

The bank's 54 cell questions ask what each part does. They do not ask:

• How the membrane is built. Phospholipid bilayers and the fluid mosaic model are a textbook staple and appear in none of them. Know that the membrane controls what enters and leaves, and stop there.
• The names of the fibers. "Cytoskeleton: holds the cell's shape" is the whole of what is asked. Microtubules and microfilaments by name are not.
• Cilia, flagella, nuclear pores, vesicles. None of them come up.
• What the organelles look like. A picture you can label is less use than a list of jobs, because the question gives you the job and asks for the name.

If your study guide spends a page on any of these, skim it and move on.

Where people lose points

Thinking plant cells have no mitochondria. They do.

Confusing the cell wall with the cell membrane.

Assigning protein synthesis to the nucleus. The nucleus holds the instructions; ribosomes do the building.

Mixing up the Golgi apparatus and the endoplasmic reticulum. The ER transports, the Golgi packages and ships.

Saying osmosis requires energy. It does not; active transport does.

Forgetting that prokaryotes have ribosomes. They lack a nucleus and membrane-bound organelles, not ribosomes.

Getting hypotonic and hypertonic backward. Water moves toward more dissolved material. Hypertonic outside means water leaves and the cell shrinks.

Work one in under a minute

A muscle cell is compared with a skin cell and found to contain far more of one organelle. Which is it, and why?

Muscle cells contract constantly and need a great deal of energy, and energy is released in the mitochondria.

That reasoning - match the organelle to what the cell does for a living - answers most questions of this shape. A cell that secretes a lot of protein has abundant rough endoplasmic reticulum and Golgi; a cell that breaks down a lot of material has many lysosomes.

Where this leads

The mitochondria lead straight into cellular respiration, the nucleus into cell division and genetics, and the chloroplasts into photosynthesis.