Cell Structures and Their Functions

Hello! This is the first lesson in a course that builds from cells to whole organisms, ecosystems, and scientific investigations. We begin with cell structure because the same question will recur throughout biology: how does a structure help something do its job?

Today you’ll study the major components of plant and animal cells and practise explaining why each component is suited to its function. That is more useful in an MYP response than memorising a list of labels. Allow about 35–40 minutes, including the video and time to inspect the diagrams.

A cell is an organised working system

Plant and animal cells contain structures that perform different tasks. Some provide boundaries or support; others hold genetic information, make proteins, or release energy for cellular activities. Their arrangement matters as much as their names.

Begin with this short visual tour. Look for the structures shared by plant and animal cells before considering what the plant cell has in addition.

The diagrams below contain more detail than you need to memorise at once. Use them first to locate the main structures; we will return to a few of the smaller ones later.

Structures both plant and animal cells need

Start at the outside of the animal cell diagram. Its cell membrane is a thin, flexible boundary. It separates the cell’s interior from its surroundings and controls which substances enter and leave. This is not an airtight seal: cells need to take in useful substances and release others. The important structure–function link is that the membrane is selectively permeable, allowing some substances to cross more readily than others. Plant cells have this membrane too, although their cell wall sits outside it.

Inside the membrane is the cytoplasm, a mostly water-based material containing dissolved substances and cell structures. Many chemical reactions occur there. Describing it only as “jelly” identifies its appearance; explaining that it provides a medium in which substances and reactions can be found states why it matters.

The nucleus contains most of the cell’s DNA. DNA carries instructions used to make proteins, and those proteins help determine what the cell does. A nuclear envelope surrounds the nucleus; pores allow materials to pass between the nucleus and cytoplasm. Saying “the nucleus controls the cell” is a useful shorthand, but the stronger explanation is that its DNA supplies instructions that influence cell activities.

Ribosomes are the sites of protein synthesis. They are very small structures, found free in the cytoplasm or attached to the rough endoplasmic reticulum near the nucleus. Proteins have many jobs in cells, including forming structures and acting as enzymes. Ribosomes are therefore essential even though they are much less conspicuous than a nucleus in a diagram. They are not membrane-bound organelles.

Finally, look for the bean-shaped mitochondria in both diagrams. Much of aerobic respiration takes place in mitochondria. During respiration, cells release energy from substances such as glucose and transfer it into a usable form for cellular work. The folded inner membranes of a mitochondrion provide a large surface area for the processes involved. A precise explanation is therefore: its folded inner membranes support respiration, which supplies usable energy for cell activities. Avoid saying that mitochondria “create energy” from nothing.

Notice an important point: plant cells have mitochondria too. Making sugars by photosynthesis does not remove a plant cell’s need to release usable energy by respiration.

What the characteristic plant structures do

A typical plant cell has three prominent features that the animal cell diagram lacks: a cell wall, chloroplasts, and a large permanent vacuole. Each has a different job.

The cell wall lies outside the cell membrane and is made mainly of strong cellulose fibres. It helps maintain the cell’s shape and supports the plant. Unlike the membrane, the wall is not the main structure controlling which dissolved substances enter and leave. If asked to explain support, link the wall’s strength and position around the cell to its ability to resist changes in shape.

Chloroplasts contain chlorophyll, a pigment that absorbs light energy. They are sites of photosynthesis, in which light energy is used to make sugars. Their structure and contents suit that function: they contain the light-absorbing pigment and internal membranes on which key steps of photosynthesis occur. Chloroplasts are not simply “green mitochondria.” Photosynthesis makes energy-rich sugars; respiration releases usable energy from sugars. These are different processes, and a plant cell may carry out both.

Not every plant cell has chloroplasts. A cell in a root, for example, may receive little light and usually lacks them. This is why “plant cells can have chloroplasts” is more reliable than “every plant cell has chloroplasts.”

The large permanent vacuole is a membrane-enclosed space containing cell sap, a solution of water and dissolved substances. When it is full, it presses the cytoplasm and cell membrane against the cell wall. The wall resists this pressure, helping keep the cell firm, or turgid. Here the two structures work together: the fluid-filled vacuole provides pressure, while the strong wall provides resistance. Animal cells can have small vacuoles, but they do not typically have the one large, support-giving central vacuole shown in the plant diagram.

Smaller structures complete the picture

The diagrams include components that are harder to see in a simple microscope image but explain how a cell handles the substances it makes.

Near the nucleus, the rough endoplasmic reticulum (rough ER) is a network of membranes with ribosomes attached. Those ribosomes make proteins; the extensive membrane network helps process and move proteins intended for membranes or for release from the cell. The smooth ER lacks attached ribosomes and is involved in making lipids. The difference between rough and smooth is therefore a visible structural difference associated with different work.

The Golgi apparatus consists of flattened membrane sacs. It modifies and packages substances, including proteins, for delivery to other parts of the cell or outside it. Think of ribosomes, rough ER, and Golgi apparatus as cooperating structures rather than three interchangeable “protein makers”: ribosomes synthesise proteins, while the other structures help process and distribute some of them.

The animal cell diagram also labels lysosomes, membrane-enclosed compartments containing enzymes that break down materials. Keeping those enzymes within a compartment helps the cell digest unwanted material in a controlled place. Both diagrams show a cytoskeleton, a network of protein fibres that helps maintain shape and position internal structures.

You may notice still more labels, such as peroxisomes and centrosomes. For this first lesson, prioritise the shared components and the three characteristic plant structures. You should, however, recognise that a real cell contains many interacting parts, not just the few large shapes visible in a basic textbook drawing.

Turn a label into an explanation

In an assessment, a question asking for a function may need only a concise statement: “Ribosomes make proteins.” A question asking you to relate structure to function calls for a reason that connects a physical feature to that job.

For example, “The vacuole supports the plant cell” names an outcome but leaves out the mechanism. A fuller answer explains that the large vacuole holds cell sap; when full, it presses the cell contents against the strong cell wall, helping the cell stay firm. Similarly, “chloroplasts do photosynthesis” can be strengthened by explaining that their chlorophyll absorbs the light energy required for photosynthesis.

When reading an unfamiliar cell diagram, first identify the boundary, then find the nucleus and other shared structures, and finally examine features such as a wall, a large vacuole, or chloroplasts. Do not rely on shape alone: textbook plant cells are often drawn rectangular, but their components provide stronger evidence. The next lesson will develop that skill using diagrams and micrographs.

Key takeaways

Plant and animal cells share a membrane, cytoplasm, nucleus, ribosomes, and mitochondria. A typical plant cell also has a cellulose wall, may contain chloroplasts, and commonly has a large permanent vacuole. The most valuable explanations connect a specific structural feature—such as a selectively permeable boundary, chlorophyll, folded membranes, or a strong wall—to what that feature enables the cell to do.

Next, you’ll use these features to distinguish plant and animal cells even when an image is less tidy than a textbook diagram.