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11 · Cell Structure

· Cell Structure — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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1 · Cell Structure — Organelles of the Eukaryotic Cell — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Identify the organelles of eukaryotic cells, state their structure and functions, and recognise them in micrographs.Eukaryotic ultrastructure — the cytoplasm is di…

  • Cell surface membrane
  • Rough endoplasmic reticulum (RER)
  • Smooth endoplasmic reticulum (SER)
  • Golgi body
  • Mitochondria
  • Chloroplasts

Section 1 of 10Membrane-bound organelles of the cell

Membrane-bound organelles of the cell
Eukaryotic ultrastructure
  • Eukaryotic ultrastructure the cytoplasm is divided into membrane-bound compartments called organelles.
  • Ultrastructure means the internal structure of cells seen with an electron microscope.
Cell surface membrane
  • Cell surface membrane a partially permeable phospholipid bilayer about 10 nm wide that controls exchange with the surroundings.
Nucleus
  • Nucleus a large organelle surrounded by a double membrane that contains the DNA.
    • (nuclear envelope)
  • Chromosomes are DNA and protein, together called chromatin.
  • Nuclear pores let mRNA and ribosomes leave and let enzymes and signalling molecules enter.
  • The nucleolus is the site of ribosome production.

Section 2 of 10Protein synthesis and processing organelles

1Rough endoplasmic reticulum (RER)
  • Rough endoplasmic reticulum (RER) flattened membrane sacs continuous with the nuclear envelope and covered with ribosomes.
  • It processes proteins made on the ribosomes.
2Smooth endoplasmic reticulum (SER)
  • Smooth endoplasmic reticulum (SER) membrane sacs without ribosomes.
  • It makes lipids and steroid hormones such as oestrogen and testosterone.
3Golgi body
  • Golgi body a stack of flattened membrane sacs not connected to other compartments.
  • It modifies proteins and packages them into vesicles.
4Ribosomes
  • Ribosomes complexes of rRNA and protein, made in the nucleolus, where translation occurs.
  • Eukaryotic ribosomes are 80S (60S + 40S subunits).
5Vesicles
  • Vesicles small membrane-bound sacs that transport and store substances.
  • They bud from the Golgi body, fuse with the cell surface membrane in exocytosis, or form during endocytosis.
6Lysosomes
  • Lysosomes specialised vesicles containing hydrolytic enzymes.
  • They digest worn-out organelles, engulfed pathogens (phagocytosis) and cell debris in apoptosis.

Section 3 of 10Energy-transforming organelles

Mitochondria
  • Mitochondria double-membrane organelles that are the site of aerobic respiration.
  • The inner membrane is folded into cristae, giving a large surface area for embedded respiratory proteins.
  • The matrix contains enzymes, small circular DNA and 70S ribosomes.
Chloroplasts
  • Chloroplasts larger than mitochondria, with a double membrane, found in the green parts of plants.
  • Thylakoids stack to form grana, joined by lamellae.
  • Chlorophyll in thylakoid membranes absorbs light for photosynthesis.
  • They contain small circular DNA and 70S ribosomes used to make proteins for replication and photosynthesis.

Section 4 of 10Cytoskeleton and cell surface structures

Cytoskeleton and cell surface structures
Microtubules
  • Microtubules hollow tubes of tubulin that make up the cytoskeleton.
    • (13 protofilaments in a cylinder)
  • The cytoskeleton gives support and movement.
Centrioles
  • Centrioles hollow fibres made of microtubules;
    • two at right angles form a centrosome that organises the spindle during cell division.
  • They are absent from flowering plants and fungi.
Cilia
  • Cilia hair-like projections made of microtubules that move substances over the cell surface.
  • Ciliated cells in the airways waft mucus away from the lungs.
Microvilli
  • Microvilli projections of the cell surface membrane that increase the surface area for absorption.
  • They occur in the small intestine lining and kidney tubules.

Section 5 of 10Plant cell structures

Plant cell structures
Cell wall
  • Cell wall lies outside the cell surface membrane and gives structural support.
  • It is made of cellulose in plants and chitin in fungi, and is freely permeable.
Large permanent vacuole
  • Large permanent vacuole stores cell sap and gives extra support.
  • It is surrounded by a partially permeable membrane called the tonoplast.
Plasmodesmata
  • Plasmodesmata bridges of cytoplasm between neighbouring plant cells that allow substances such as sucrose to pass.
Animal-only structures
  • Animal-only structures centrioles and microvilli are found in animal cells but not in plant cells.

Section 6 of 10Recognising organelles in micrographs

Nucleus
  • Nucleus always the largest organelle in an animal cell and the largest dark region in a plant cell.
Mitochondria
  • Mitochondria next largest, often cylindrical with folded inner membrane, but they can look round depending on the angle of section.
RER
  • RER sits near the nucleus and may show ribosomes as dots.
Lysosomes and vesicles
  • Lysosomes and vesicles smaller than mitochondria.
Plant cells
  • Plant cells large empty-looking spaces are vacuoles, and chloroplasts often show grana.

Section 7 of 10More on this topic

More on this topic
State whether it is found in prokaryotes, eukaryotes or both.
Identify by features
  • Identify by features shape, position and relative size.
  • The nucleus is the largest organelle in an animal cell.
  • Mitochondria are smaller, rod-like or oval, with internal folds.
  • RER appears as flattened sacs studded with dots (ribosomes).
Reliable clues
  • Reliable clues number of membranes, internal folding, and presence or absence of ribosomes.
Absorptive cells
  • Absorptive cells microvilli, as in the gut epithelium.

Section 8 of 10More from the specification

Prokaryotic vs Eukaryotic Cellular Architecture
  • Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, possessing naked circular DNA in the cytoplasm and 70S ribosomes, unlike compartmentalized eukaryotic cells.
Energy Generation and Cellular Respiration
  • All living organisms require metabolic energy in the form of ATP, produced via aerobic cellular respiration in eukaryotic mitochondria or across the prokaryotic cell surface membrane.
Organization of Genetic Material
  • In prokaryotes, genetic material exists as a circular genophore in the nucleoid alongside autonomous plasmids,
  • whereas eukaryotes organize nuclear DNA into linear chromatin associated with histone proteins.
Viruses as Acellular Biological Entities
  • Viruses are non-cellular infectious particles consisting of genetic material (DNA or RNA) enclosed within a protein coat (capsid);
    • lacking metabolic machinery, they cannot generate energy and rely entirely on host cells.
Prokaryotic vs Eukaryotic Cellular Architecture
  • Prokaryotic cells lack a membrane-bound nucleus and membrane-bound organelles, possessing naked circular DNA in the cytoplasm and 70S ribosomes, unlike compartmentalized eukaryotic cells.
Energy Generation and Cellular Respiration
  • All living organisms require metabolic energy in the form of ATP, produced via aerobic cellular respiration in eukaryotic mitochondria or across the prokaryotic cell surface membrane.
Organization of Genetic Material
  • In prokaryotes, genetic material exists as a circular genophore in the nucleoid alongside autonomous plasmids,
  • whereas eukaryotes organize nuclear DNA into linear chromatin associated with histone proteins.
Viruses as Acellular Biological Entities
  • Viruses are non-cellular infectious particles consisting of genetic material (DNA or RNA) enclosed within a protein coat (capsid);
    • lacking metabolic machinery, they cannot generate energy and rely entirely on host cells.

Section 9 of 10Key terms

TermMeaning
UltrastructureThe internal structure of a cell as seen with an electron microscope.
OrganelleA membrane-bound compartment in the cytoplasm of a eukaryotic cell.
ChromatinDNA combined with proteins in the nucleus.
Nuclear poreA channel in the nuclear envelope controlling the passage of molecules.
Golgi bodyA stack of membrane sacs that modifies proteins and packages them into vesicles.
LysosomeA vesicle containing hydrolytic enzymes.
CristaeThe folds of the inner mitochondrial membrane.
TonoplastThe membrane surrounding the plant vacuole.
PlasmodesmataCytoplasmic bridges linking neighbouring plant cells.

Section 10 of 10Exam technique

1
Saying mitochondria produce energy — they transfer energy to ATP during aerobic respiration.
2
Placing ribosomes only on the SER — ribosomes are on the RER and free in the cytoplasm.
3
Saying centrioles occur in all eukaryotes — they are absent from flowering plants and fungi.
4
Calling the cell wall a membrane — it is a freely permeable layer outside the cell surface membrane.
5
Confusing microvilli with villi — microvilli are projections of single cells, villi are larger multicellular structures.

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