Structure of eukaryotic cells
AQA
A-Level
Eukaryotic cells contain membrane-bound organelles. The cell-surface membrane is a phospholipid bilayer controlling exchange. The nucleus has a double-membrane envelope with pores, containing linear DNA bound to histones, and nucleoli. Mitochondria have a folded inner membrane (cristae) and matrix, releasing ATP. Chloroplasts, found in plants and algae, contain thylakoids stacked into grana and stroma for photosynthesis. The Golgi apparatus modifies, packages and transports proteins and lipids, forming vesicles and lysosomes (containing hydrolytic enzymes). Ribosomes synthesise proteins. Rough ER has ribosomes and transports proteins; smooth ER synthesises lipids. Plant, algal and fungal cell walls provide support. Plant cell vacuoles maintain turgor.
13
Objectives
11
Exam Tips
19
Pitfalls
24
Key Terms
20
Mark Points
Subtopics in this area
The structure of eukaryotic cells, restricted to the structure and function of: cell-surface membrane nucleus (containing chromosomes, consisting of protein- bound, linear DNA, and one or more nucleoli) mitochondria chloroplasts (in plants and algae) Golgi apparatus and Golgi vesicles lysosomes (a membrane-bound organelle that releases hydrolytic enzymes) ribosomes rough endoplasmic reticulum and smooth endoplasmic reticulum cell wall (in plants, algae and fungi) cell vacuole (in plants).
In complex multicellular organisms, eukaryotic cells become specialised for specific functions.
Specialised cells are organised into tissues, tissues into organs and organs into systems.
Students should be able to apply their knowledge of these features in explaining adaptations of eukaryotic cells.
Structure of eukaryotic cells Revision Guide
Learning Objectives
What you need to know and understand
- Name one structure in a micrograph or diagram of an unfamiliar cell that shows it is a eukaryote, choosing from mitochondrion, nuclear envelope, rough endoplasmic reticulum, vesicle or lysosome, and give one function of that structure.
- Describe two functions of the Golgi apparatus and state the structures it forms.
- Explain how one structural feature of each of the chloroplast, the cell wall, the permanent vacuole and the smooth endoplasmic reticulum lets it do its job, and state which three of the four are absent from an animal cell.
- State that chloroplasts are found in plants and algae.
- Explain how differential gene expression allows cells to develop into specialised types.
- Relate specific structural features and organelle proportions of a named specialised cell to its function.
- Deduce the specialised function of an unfamiliar cell from data regarding its organelle distribution.
- Define tissue, organ and organ system, and place a named example at each level.
- Identify, from a description of a structure, whether it is a tissue or an organ, and justify the decision.
- Explain why multicellular organisms of large size require organ systems rather than relying on diffusion.
- Explain how three features of an epithelial cell in the proximal convoluted tubule allow rapid reabsorption of glucose.
- Interpret an electron micrograph of an unfamiliar cell to suggest its function, citing two organelles as evidence.
- Justify why the number of an organelle, rather than its presence, is what makes a cell adapted to its role.
Marking Points
Key points examiners look for in your answers
- identifying a membrane-bound organelle as evidence that a cell is eukaryotic, such as a mitochondrion, nucleus, endoplasmic reticulum, Golgi apparatus, lysosome or vesicle
- describing the nucleus as having a double membrane or nuclear envelope with pores, and containing chromosomes of DNA bound to histones plus a nucleolus
- stating the Golgi apparatus modifies, packages or transports proteins or lipids, or forms vesicles and lysosomes
- identifying that lysosomes contain hydrolytic enzymes that are released to digest material
- linking mitochondria to the release of ATP and ribosomes to the production of protein
- stating that chloroplasts are found in plants and algae
- Describe how differential gene expression leads to specialisation by altering cell shape and organelle proportions without changing the genome of somatic cells.
- Identify the specific organelle that is abundant or modified in a named specialised cell, such as extensive rough endoplasmic reticulum in pancreatic cells.
- Link the abundance of a specific organelle directly to the cell's function, for example, mitochondria releasing ATP for active transport or movement.
- Explain structural losses as functional adaptations, such as the absence of a nucleus in a mature red blood cell to accommodate more haemoglobin.
- Award credit for defining a tissue as a group of similar or specialised cells with a shared function.
- Award credit for defining an organ as different tissues working together to perform a particular function.
- Award credit for defining an organ system as a group of organs working together.
- Award credit for a correct worked example running through all the levels, such as epithelial cell, epithelium, ileum, digestive system.
- Award credit for linking the level of organisation to body size and a small surface area to volume ratio, so that diffusion alone is insufficient.
- one mark for naming the feature seen in the cell rather than restating the data given
- one mark for saying there are many of the organelle, or that the membrane is folded, where number or surface area is the adaptation
- one mark for the process the feature carries out, such as mitochondria releasing ATP in aerobic respiration
- one mark for linking that process to the overall function of the cell, such as the co-transport of glucose into the cell from the lumen
- one mark for a second, genuinely different adaptation where the question asks for more than one
Examiner Tips
Expert advice for maximising your marks
- 💡Answer structure and function in matched pairs; a list of organelle names without functions rarely provides a complete explanation.
- 💡Read exclusions carefully, as in 'other than the Golgi apparatus, name one structure', because an excluded answer cannot be credited.
- 💡When identifying chloroplasts, state their occurrence in plants and algae to match the specification clause.
- 💡When describing cellular adaptations, explicitly link the structural feature to the process it performs and its ultimate function for the cell.
- 💡If presented with an unfamiliar cell in a table or micrograph, identify which organelles are unusually abundant to deduce the cell's primary function.
- 💡Give one worked example at every level in the same answer; examiners credit the chain, not the definitions alone.
- 💡Use plant examples as readily as animal ones, since leaf and root questions are common alongside gut questions.
- 💡If asked why large organisms need organ systems, bring in surface area to volume ratio explicitly.
- 💡Count the marks and give that many separate feature-and-function pairs; three marks means three distinct adaptations.
- 💡Where transport proteins are involved, say which kind of transport they carry out, since channel, carrier and co-transport proteins function differently.
- 💡Answer 'describe and explain' in two halves for every point: what the feature is, then why it helps.
Common Mistakes
Pitfalls to avoid in your exam answers
- naming the nucleolus or the cell-surface membrane as evidence that a cell is eukaryotic; prokaryotes also have a cell-surface membrane, and the nucleolus is a region inside the nucleus rather than a membrane-bound organelle
- writing 'mitochondria for ATP' without specifying that they release ATP, or stating that they 'produce energy'
- saying the Golgi apparatus makes proteins, when it modifies, packages and transports them
- describing ribosomes as membrane-bound structures, whereas they lack a membrane
- stating that chloroplasts are found in all eukaryotic cells; correct by specifying that they occur in plants and algae
- Stating that all cells in a multicellular organism contain identical DNA; correct by noting that mature mammalian red blood cells lack a nucleus and DNA, while gametes are haploid.
- Writing that sperm cells or muscle cells 'produce energy'; correct by specifying that their abundant mitochondria release ATP during aerobic respiration.
- Claiming a red blood cell 'has no organelles' when asked about a specific missing structure; correct by explicitly naming the absent structure, such as the nucleus.
- Confusing differentiation with mutation; correct by explaining that specialisation usually involves changes in gene expression via transcription factors, not changes to the DNA base sequence.
- Calling blood, or xylem, an organ when each is a tissue.
- Defining a tissue as 'lots of cells' without saying that they are similar and share a function.
- Assigning organs to only one system, for example placing the pancreas in the endocrine system alone when it is also a digestive organ.
- Stating that an organ is made of one type of tissue.
- Mixing plant and animal examples within the same chain, so the hierarchy no longer holds together.
- listing the organelles visible in a figure without linking any of them to the function of the cell
- omitting 'many', since it is the number rather than the presence of an organelle that is being credited
- writing that mitochondria 'give energy' rather than release ATP in aerobic respiration
- giving two versions of the same adaptation, such as microvilli and folded membrane, when two different features are required
- conflating channel proteins, which only perform facilitated diffusion, with carrier proteins that can also perform active transport and co-transport