Cell structure and function explains how cells are organized and how each part helps the cell stay alive. This cheat sheet helps students compare prokaryotic and eukaryotic cells, identify major organelles, and connect structure to function. It also supports quick review of cell size, microscopy scale, and the big idea that cells are the basic unit of life.
The most important concepts include the role of the cell membrane, the difference between plant and animal cells, and how organelles work together. Students should know that the nucleus stores DNA, ribosomes build proteins, mitochondria release usable energy, and chloroplasts perform photosynthesis in plants. Transport rules such as diffusion, osmosis, and active transport explain how materials move across membranes.
Key Facts
- All living things are made of one or more cells, and the cell is the basic unit of structure and function in organisms.
- Prokaryotic cells do not have a nucleus or membrane-bound organelles, while eukaryotic cells do have a nucleus and membrane-bound organelles.
- The cell membrane is selectively permeable, meaning it allows some substances to pass through more easily than others.
- Diffusion moves particles from high concentration to low concentration without using cellular energy.
- Osmosis is the diffusion of water across a selectively permeable membrane from high water concentration to low water concentration.
- Active transport moves substances from low concentration to high concentration and requires energy, usually ATP.
- Surface area to volume ratio can be written as SA:V = surface area / volume, and smaller cells usually exchange materials more efficiently.
- The mitochondria release energy from food during cellular respiration, summarized as glucose + oxygen -> carbon dioxide + water + ATP.
Vocabulary
- Organelle
- A specialized structure inside a cell that performs a specific job.
- Nucleus
- The membrane-bound organelle in eukaryotic cells that contains DNA and controls many cell activities.
- Cell membrane
- A flexible barrier around the cell that controls what enters and leaves.
- Cytoplasm
- The jellylike material inside the cell where organelles are located and many chemical reactions occur.
- Ribosome
- A small cell structure that builds proteins from amino acids.
- Chloroplast
- A plant and algae organelle that uses light energy to make glucose during photosynthesis.
Common Mistakes to Avoid
- Confusing prokaryotic cells with eukaryotic cells is wrong because prokaryotes lack a nucleus and membrane-bound organelles, while eukaryotes have them.
- Saying only animal cells have mitochondria is wrong because both plant and animal cells use mitochondria for cellular respiration.
- Mixing up diffusion and active transport is wrong because diffusion moves down a concentration gradient without energy, while active transport moves against the gradient using energy.
- Calling the cell wall the same as the cell membrane is wrong because the cell wall provides rigid support, while the membrane controls movement into and out of the cell.
- Forgetting that structure matches function is wrong because organelles have shapes and features that help them perform specific jobs, such as folded mitochondrial membranes increasing reaction surface area.
Practice Questions
- 1 A cube-shaped cell has a surface area of 54 square micrometers and a volume of 27 cubic micrometers. What is its surface area to volume ratio?
- 2 A microscope image is 40 mm wide, and the real cell is 0.04 mm wide. What is the magnification?
- 3 A plant cell is placed in a solution with a lower water concentration than the inside of the cell. In which direction will water move by osmosis?
- 4 Explain why a cell with many mitochondria is likely to need a large amount of energy.
Understanding Cell Structure & Function
Cells work as linked systems, not as a set of isolated parts. A protein may begin at a ribosome, enter the rough endoplasmic reticulum, then travel in a small vesicle to the Golgi apparatus. The Golgi modifies, sorts, and packages it for use inside the cell or for release.
Lysosomes contain digestive enzymes that break down worn-out cell parts and large food molecules. This recycling matters because cells must recover useful materials instead of letting waste build up.
In plant cells, a large central vacuole stores water and helps maintain firmness. Loss of water can make plant tissue wilt because the vacuole no longer presses outward on the cell wall.
Membrane transport depends on particle motion and on the properties of the membrane itself. Small nonpolar molecules such as oxygen can pass through the phospholipid layer fairly easily. Charged particles and many larger molecules need transport proteins.
Some proteins form channels, while others bind to a substance and change shape to move it across. Water balance is especially important. If an animal cell is placed in a solution with much more dissolved material than its cytoplasm, water leaves the cell and it shrinks.
In a weaker solution, water enters and the cell may swell. Plant cells are protected from bursting by their rigid walls, though too much water loss still damages their function.
Cell size creates a practical limit on growth. Materials cross the cell surface, but the materials are needed throughout the cell interior. As a cell gets larger, its volume rises faster than its surface area.
This leaves less membrane area available for each unit of cytoplasm. A small cell therefore exchanges oxygen, nutrients, carbon dioxide, and wastes more efficiently. Long thin nerve cells and root hair cells show another solution.
Their shapes give them a large surface area without making the whole cell bulky. When comparing cells, pay attention to shape as well as overall size. Surface area to volume ratio is a reason many organisms have many small cells rather than a few giant ones.
Microscopy requires careful thinking about scale. Most cells are measured in micrometres, which are one millionth of a metre. Organelles are smaller still, and many cannot be clearly seen with a school light microscope.
Magnification tells how much larger an image appears, but resolution tells whether two close details can be distinguished. A blurry large image is not necessarily more useful. Students often confuse image size with actual size, so use a scale bar whenever one is provided.
To estimate actual size, divide the measured image size by the magnification, keeping the units consistent. Drawings from microscope observations should use clear outlines, accurate proportions, and labels that point to structures without crossing lines.