Cell structure and organelles: flashcards, quiz and summary

Flashcards

1. 1What are the three parts of the cell theory?

   All living things are made of one or more cells, the cell is the basic unit of structure and function in living things, and all cells come from existing cells.

2. 2How do prokaryotic and eukaryotic cells differ in their internal structure?

   Prokaryotes have no nucleus and no membrane-bound organelles, and their DNA sits in the nucleoid. Eukaryotes have a nucleus with linear chromosomes and many membrane-bound organelles.

3. 3Why do plant cells need a large central vacuole and a cell wall?

   The central vacuole pushes against the wall through turgor pressure, which keeps the plant firm. The rigid cell wall supports the cell outside the membrane.

4. 4How does the rough ER work with the Golgi apparatus to produce proteins for export?

   Ribosomes on the rough ER make and fold proteins, then ship them in vesicles to the Golgi apparatus. The Golgi modifies, sorts and packages them into vesicles for delivery.

5. 5Why are mitochondria and chloroplasts sometimes called the energy organelles of the cell?

   Mitochondria carry out cellular respiration to make most of the cell's ATP. Chloroplasts carry out photosynthesis in plants and algae, capturing light energy to make sugar.

6. 6What structure in the cell membrane explains why small nonpolar molecules pass through easily but ions do not?

   The hydrophobic tails of the phospholipid bilayer form a barrier to charged and large polar molecules, so ions need transport proteins, while small nonpolar molecules such as O2 and CO2 pass through directly.

7. 7What does the fluid mosaic model describe about the cell membrane?

   The membrane is fluid because phospholipids and many proteins drift sideways. It is a mosaic because proteins are scattered through the bilayer like tiles.

8. 8What evidence supports the endosymbiotic theory for mitochondria and chloroplasts?

   They have their own circular DNA, bacteria-like ribosomes and a double membrane. They divide by a process like binary fission, and they are roughly bacteria-sized.

9. 9Why does a cell's surface area to volume ratio drop as the cell grows?

   Volume increases faster than surface area as a cell grows. Since exchange happens across the surface but needs grow with volume, larger cells have less surface per unit of volume.

10. 10Why do cells divide or add microvilli instead of simply growing larger?

    Growing larger lowers the surface area to volume ratio, so exchange becomes too slow for the cell's needs. Dividing keeps cells small, and folds like microvilli increase surface area without increasing volume.

Practice quiz

Pick an answer to check it.

1. 1Which organelle has a double membrane with its inner layer folded into cristae?

   1. Mitochondrion
   2. Chloroplast
   3. Lysosome
   4. Golgi apparatus
   Show the answer

   A. Mitochondrion Mitochondria have a double membrane, and the inner membrane folds into cristae where cellular respiration occurs. Chloroplasts also have a double membrane, but their inner structures are thylakoids, not cristae.

2. 2Where is the DNA of a bacterial cell located?

   1. Packed into the nucleolus
   2. Free in the nucleoid region without a membrane
   3. Inside a membrane-bound nucleus
   4. Inside the mitochondria only
   Show the answer

   B. Free in the nucleoid region without a membrane Prokaryotes have no nucleus, so their DNA sits in the nucleoid, a region not enclosed by a membrane. The nucleolus is found only in eukaryotic nuclei and makes ribosome parts.

3. 3Which job belongs to the smooth ER rather than the rough ER?

   1. Making proteins for export
   2. Detoxifying drugs and storing calcium
   3. Assembling ribosome subunits
   4. Folding newly made proteins
   Show the answer

   B. Detoxifying drugs and storing calcium Smooth ER has no ribosomes and handles lipid making, detoxification of drugs and poisons, and calcium storage. The rough ER makes and folds proteins, and the nucleolus assembles ribosome subunits.

4. 4A cube-shaped cell grows so that its side length doubles from 1 to 2. What happens to its surface area to volume ratio?

   1. It doubles to 12:1
   2. It stays at 6:1 because the shape is still a cube
   3. It falls from 6:1 to 3:1
   4. It rises from 6:1 to 3:1
   Show the answer

   C. It falls from 6:1 to 3:1 A cube with side 1 has surface area 6 and volume 1, a ratio of 6:1. A cube with side 2 has surface area 24 and volume 8, a ratio of 3:1, so the ratio drops as the cell grows.

5. 5Which membrane property is explained by phospholipids and proteins drifting sideways?

   1. Selective permeability to ions
   2. Rigidity from cholesterol
   3. Fluidity
   4. Cell recognition by carbohydrate chains
   Show the answer

   C. Fluidity The fluid part of the fluid mosaic model means that phospholipids and many proteins can move sideways within the bilayer. Carbohydrate chains help cells recognize each other, and selective permeability describes which substances can cross.

6. 6Which observation best supports the idea that mitochondria were once free-living bacteria?

   1. They have linear chromosomes like a nucleus
   2. They are found only in animal cells
   3. They have no membrane at all
   4. They have their own circular DNA and bacteria-like ribosomes
   Show the answer

   D. They have their own circular DNA and bacteria-like ribosomes Circular DNA and bacteria-like ribosomes match prokaryotes, which supports the endosymbiotic theory. Mitochondria have a double membrane, and they are found in both plant and animal cells.

7. 7Which material forms the cell walls of fungi?

   1. Actin
   2. Peptidoglycan
   3. Cellulose
   4. Chitin
   Show the answer

   D. Chitin Fungal cell walls are made of chitin. Plant cell walls are made of cellulose, bacterial walls contain peptidoglycan, and actin is a protein in the cytoskeleton, not a wall material.

Test traps

What teachers like to test on this, and the mistakes students make.

• Saying that plant cells lack mitochondria: they have mitochondria too, and chloroplasts make sugar while mitochondria break it down for ATP.
• Saying prokaryotes have no DNA or no ribosomes: they have both, but they lack a nucleus and membrane-bound organelles.
• Confusing the rough ER and smooth ER: rough ER has ribosomes and makes and folds proteins, while smooth ER has no ribosomes and makes lipids and detoxifies drugs.
• Mixing up the cell wall and the cell membrane: a plant cell has both, and the wall sits outside the membrane.
• Reversing the surface area to volume ratio: it drops as a cell grows, so smaller cells exchange materials more efficiently.
• Describing the membrane as rigid instead of fluid, or forgetting that ions need transport proteins to cross the bilayer.

Summary

• All living things are made of one or more cells, the cell is the basic unit of structure and function, and all cells come from existing cells.
• Prokaryotes such as bacteria and archaea have no nucleus and no membrane-bound organelles, with DNA in a nucleoid and often plasmids, while eukaryotes have a nucleus with linear chromosomes and many membrane-bound organelles.
• Plant cells have a cell wall, chloroplasts and a large central vacuole, while animal cells have no wall, usually have centrioles and lysosomes, and change shape more easily.
• The rough ER makes and folds proteins that are exported or used in membranes, the smooth ER makes lipids and detoxifies drugs, and the Golgi apparatus modifies, sorts and packages them into vesicles.
• Mitochondria carry out cellular respiration to make most of the cell's ATP, and chloroplasts carry out photosynthesis in plants and algae.
• The cell membrane is a phospholipid bilayer that is selectively permeable, and the fluid mosaic model describes proteins scattered through a membrane whose lipids and proteins can drift sideways.
• The endosymbiotic theory proposes that mitochondria and chloroplasts began as free-living prokaryotes engulfed by a larger cell, supported by their own circular DNA, bacteria-like ribosomes and double membranes.
• As a cell grows, its volume increases faster than its surface area, so the surface area to volume ratio drops, which is why cells stay small, divide or add folds like microvilli.

Key terms

Prokaryote

A cell with no nucleus and no membrane-bound organelles, such as a bacterium or archaean, usually with one circular chromosome in the nucleoid.

Eukaryote

A cell with a nucleus holding linear chromosomes and many membrane-bound organelles, found in animals, plants, fungi and protists.

Nucleoid

The region of a prokaryotic cell where its DNA is located, not enclosed by a membrane.

Ribosome

A non-membrane structure that makes proteins. Free ribosomes make proteins for the cytoplasm, and ribosomes on the rough ER make proteins for export or membranes.

Rough ER

Endoplasmic reticulum studded with ribosomes that makes and folds proteins, then ships them in vesicles.

Golgi apparatus

Stacked flat sacs that modify, sort and package proteins and lipids into vesicles.

Cristae

The folds of the inner membrane of a mitochondrion, where many reactions of cellular respiration take place.

Phospholipid bilayer

A double layer of phospholipids in the cell membrane, with hydrophilic heads facing the water on both sides and hydrophobic tails facing each other inside.

Selectively permeable

Describes a membrane that lets some substances pass freely, such as O2 and CO2, while ions and large polar molecules need transport proteins.

Surface area to volume ratio

The surface area of a cell divided by its volume. It falls as a cell gets larger, which limits how efficiently the cell can exchange materials.

How to study it

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