Cell Structure and Function: Study Guide (Biology 2.1)

A cell is a set of connected workspaces

A eukaryotic cell is not a bag of evenly mixed chemicals. It is divided into compartments, each surrounded by a membrane and each holding its own set of enzymes and conditions. Keeping reactions apart lets a cell build proteins in one place, digest old material in another and release energy in a third, all at the same time and without interference.

In this topic, ask two questions about every structure: what is it made of or how is it shaped, and how does that shape help the whole cell? Exam questions on this topic almost always want that link between structure and function, not a list of names.

Ribosomes: protein builders in every cell

A ribosome is made of ribosomal RNA (rRNA) and protein, arranged as a large and a small subunit. It has no membrane, so it is not a membrane-bound organelle.

• Job: a ribosome reads a messenger RNA (mRNA) sequence and joins amino acids in the order it specifies, making a polypeptide (Topic 1.7).
• Where: some ribosomes float free in the cytosol and make proteins that stay there. Others sit on the surface of the rough ER and make proteins that will be exported, built into membranes or sent to lysosomes.
• Everywhere: ribosomes are found in all known forms of life: bacteria, archaea and eukaryotes. Such a complex structure, with the same basic job in every cell, is best explained by common ancestry: all living things inherited it from a shared ancestor.

The endomembrane system: one route, many stations

The endomembrane system is a group of membranes and membrane-bound compartments that work together. It includes:

• the nuclear envelope (the double membrane around the nucleus, joined to the ER)
• the endoplasmic reticulum (ER), rough and smooth
• the Golgi complex
• lysosomes and vacuoles
• transport vesicles, small membrane sacs that carry material between the others
• the plasma membrane

Together these parts modify, package and transport proteins, lipids and polysaccharides around the cell and to its surface. Vesicles bud off one membrane and fuse with the next, so material moves without ever crossing a membrane.

Endoplasmic reticulum: rough and smooth

The ER is a network of membrane tubes and flattened sacs that runs through the cytoplasm. Its membranes give the cell mechanical support, helping it keep its shape, and its inner space acts as a channel for transport within the cell.

• Rough ER looks rough because ribosomes are bound to its membrane. As each new protein is made, it is threaded into the ER’s inner space. This keeps proteins for export or for membranes in their own compartment, away from the cytosol. Rough ER is therefore part of compartmentalization and of protein synthesis.
• Smooth ER has no ribosomes. It makes lipids, including phospholipids for new membranes, and detoxifies harmful substances, for example by chemically changing drugs so they can be removed.

Golgi complex: finishing and dispatch

The Golgi complex is a stack of flattened membrane sacs. Vesicles from the ER arrive at one face of the stack, and products leave from the other face. While they pass through, the Golgi:

• makes sure new products are correctly folded and chemically modified. One common change is glycosylation, the addition or trimming of sugar chains on proteins. These changes can decide what a protein does and where it goes.
• packages proteins into vesicles for trafficking: some go to the plasma membrane to be released, some become part of the membrane and some go to lysosomes.

A useful way to think of it: the rough ER is the factory floor; the Golgi is the finishing, labelling and dispatch room.

Lysosomes: digestion and controlled self-destruction

A lysosome is a membrane-enclosed sac full of hydrolytic enzymes. These enzymes break polymers down by hydrolysis (Topic 1.3): proteins to amino acids, nucleic acids to nucleotides and so on. Lysosomes digest food taken in by the cell, bacteria engulfed by white blood cells, and the cell’s own worn-out organelles, so the monomers can be reused. The lysosome membrane keeps these enzymes away from the rest of the cell.

Lysosomes also take part in apoptosis, programmed cell death. This is an orderly, controlled way of removing cells the body no longer needs, such as the tissue between the developing fingers of a human embryo.

Vacuoles: storage and support

Vacuoles are membrane-bound sacs with many roles.

• A plant cell usually has one large central vacuole, which often fills a third or more of the cell’s volume and in some cells much more. It stores water and dissolved nutrients. Water inside it pushes outward on the cell wall. This outward push is turgor pressure, and it keeps non-woody parts of the plant firm. When a plant is short of water, its vacuoles lose water, turgor pressure falls and the plant wilts.
• Animal cells have vacuoles that are smaller and more numerous. They store materials such as water, ions or food particles.

Mitochondria and chloroplasts: two membranes, two energy roles

Both of these organelles have a double membrane: an outer membrane and an inner membrane, with a narrow space between them. Two membranes create separate compartments, so different steps of a process can happen in different places.

Mitochondria

Mitochondria are the site of most steps of aerobic cellular respiration, which releases energy from food molecules and stores it in ATP (Unit 3).

• The outer membrane is smooth.
• The inner membrane is highly folded. The folds are called cristae. Many of the proteins that make ATP sit in the inner membrane, so more membrane means more of them. Folding packs a large membrane area into a small organelle, so ATP is made more efficiently.
• The two membranes separate two compartments: the intermembrane space and the matrix. Different reactions of respiration happen in each.

Cells with high energy demands, such as muscle cells, tend to have many mitochondria with densely packed cristae.

Chloroplasts

Chloroplasts are found in plants and photosynthetic algae. Like mitochondria, they have a double membrane. They are the site of photosynthesis, which captures light energy and stores it in sugars. Inside, a third system of membranes (stacks of thylakoids) holds the light-absorbing pigments; you will meet these in Unit 3.

Structure and function at a glance

Structure	Key structural feature	Main function	Where found
Ribosome	rRNA and protein, no membrane	builds proteins from mRNA	all cells
Rough ER	membrane with bound ribosomes	makes proteins for export, membranes and lysosomes; keeps them in a compartment	eukaryotes
Smooth ER	membrane without ribosomes	lipid synthesis, detoxification	eukaryotes
Golgi complex	stack of flattened sacs	folds, modifies (e.g. glycosylation), sorts and packages	eukaryotes
Lysosome	sac of hydrolytic enzymes	digestion, recycling, role in apoptosis	animal cells mainly
Vacuole	membrane-bound sac	plants: turgor and storage; animals: storage	plants (large), animals (small)
Mitochondrion	double membrane, folded inner membrane	aerobic respiration, ATP synthesis	almost all eukaryotes
Chloroplast	double membrane, internal thylakoids	photosynthesis	plants and photosynthetic algae

Worked example 1: following a protein through a gland cell

Question. Researchers give a dish of gland cells a short pulse (5 minutes) of a radioactive amino acid, then replace it with normal amino acids (the chase). Only proteins made during the pulse are radioactive. At each time, they measure where the label is. The table gives the percentage of the label found in four places. (Fictional data.)

Time after start / min	5	20	45	90	150
Rough ER / %	86	41	12	5	3
Golgi complex / %	10	47	38	11	4
Secretory vesicles / %	4	10	44	46	18
Outside the cell / %	0	2	6	38	75

(a) State the order in which the labelled protein passes through the compartments, using the data. (b) Calculate the mean rate at which label appeared outside the cell between 45 and 150 minutes. (c) Explain why the protein is first found in the rough ER and not in the cytosol. (d) Predict the result at 150 minutes if a drug stopped secretory vesicles fusing with the plasma membrane.

(a) Find when each compartment peaks. Rough ER: 86% at 5 min. Golgi: 47% at 20 min. Vesicles: 46% at 90 min (44% at 45 min). Outside: still rising at 150 min. Order: rough ER → Golgi complex → secretory vesicles → outside the cell, matching Figure 1. Each column adds up to 100%, which is a quick check that no reading is missing.

(b) Change outside = 75 − 6 = 69 percentage points. Time = 150 − 45 = 105 min. Rate = 69 ÷ 105 = 0.66 percentage points per minute. (The unit is “percentage points per minute” because the data are percentages of the label, not amounts.)

(c) The protein is made by ribosomes bound to the rough ER. As the chain grows, it is threaded into the ER’s inner space. This keeps a protein meant for export in the endomembrane system, so it can be modified and packaged on its way out.

(d) The protein would still be made and processed, so label would leave the rough ER and Golgi. But it would build up in secretory vesicles, and very little would appear outside the cell. This prediction follows the structure of the route: each step depends on the one before.

Check. The rough ER loses 86 − 12 = 74 percentage points between 5 and 45 minutes, while the Golgi and vesicles gain. Label moves along; it does not disappear, which fits a single route.

Worked example 2: making a claim from organelle data

Question. Three fictional mammalian cell types, P, Q and R, were measured under an electron microscope. The table shows the percentage of each cell’s volume taken up by three organelles. One cell type secretes large amounts of digestive enzymes (proteins). One breaks down drugs and makes lipids. One contracts all the time. Make a claim about which is which and support it with evidence and reasoning.

Cell type	Rough ER / %	Smooth ER / %	Mitochondria / %
P	22	2	8
Q	3	15	20
R	1	1	35

Step 1: find what stands out. Compare across rows, not just down columns.

• P: rough ER 22%, the highest of the three. Ratio of rough to smooth ER = 22 ÷ 2 = 11.
• Q: smooth ER 15%, the highest. Ratio of rough to smooth ER = 3 ÷ 15 = 0.2.
• R: mitochondria 35%, about 4.4 times the share in P (35 ÷ 8) and 1.75 times that in Q (35 ÷ 20).

Step 2: claim. P secretes digestive enzymes, Q breaks down drugs and makes lipids, and R contracts all the time.

Step 3: evidence and reasoning.

1. P. Digestive enzymes are proteins made for export. Ribosomes on rough ER make such proteins and thread them into the endomembrane system for packaging and secretion. P has by far the most rough ER, so it has the most capacity for this job.
2. Q. Smooth ER makes lipids and detoxifies drugs. Q has the most smooth ER, 7.5 times as much as P (15 ÷ 2 = 7.5).
3. R. Contraction uses ATP continuously. Mitochondria make most of a cell’s ATP by aerobic respiration, so a cell that contracts all the time needs many of them. R has the largest share of mitochondria.

Step 4: limits. The claim is supported, not proved. These are single measurements of each cell type, and volume is only one measure: two cells with the same mitochondrial volume might differ in how folded their inner membranes are. A stronger test would measure the function directly, for example the amount of enzyme each cell type secretes per hour.

Common misconceptions

• “Ribosomes are membrane-bound organelles found only in eukaryotes.” Ribosomes have no membrane and are found in every cell, including bacteria.
• “Rough ER and smooth ER are separate organelles with unrelated jobs.” They are parts of one continuous network. Rough ER is rough only because ribosomes are attached.
• “The Golgi makes proteins.” Ribosomes make proteins. The Golgi modifies, sorts and packages products it receives from the ER.
• “Proteins move from the ER to the Golgi by crossing membranes.” They travel inside vesicles that bud off one membrane and fuse with the next.
• “The inner membrane of a mitochondrion is folded so the organelle can hold more fluid.” The folds increase the area of membrane, which holds the proteins that make ATP.
• “Plant cells have chloroplasts instead of mitochondria.” Plant cells have both. Chloroplasts make sugars; mitochondria release energy from them.
• “Apoptosis is accidental cell death.” It is programmed and controlled, and lysosomes help carry it out.
• “A high volume of an organelle proves a cell’s function.” It supports a claim; direct measurement of the function is stronger evidence.

Where this leads

The plasma membrane at the end of the secretory route is the subject of Topic 2.3, and Topic 2.10 asks how internal compartments may have arisen. Next, Topic 2.2, Cell Size, explains why cells stay small and why folded membranes, like cristae, matter so much for exchange. Test yourself now with the practice questions, then use the revision notes and the checklist to consolidate.