1.21 · Cell Structure
Cells as the Basic Units of Living Organisms — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
1.2 Cells as the Basic Units of Living Organisms — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe eukaryotic organelles, compare plant, animal and prokaryotic cells, explain the role of ATP and describe viruses.Nucleus — surrounded by a double-membrane nucle…
- Golgi body
- Mitochondria
- Chloroplasts
- Cell surface membrane
- Microtubules
- Centrioles, cilia and microvilli
Section 1 of 12Nucleus and ribosomes
Nucleus
- Nucleus surrounded by a double-membrane nuclear envelope with pores, and contains chromatin.
- (DNA and histone proteins)
- It controls cell activities by controlling protein synthesis.
- The nucleolus makes ribosomes.
Ribosomes
- 80S in eukaryotes and not membrane-bound.
- (rRNA and protein, two subunits)
- Free ribosomes make proteins used inside the cell.
- Ribosomes on the RER make proteins for secretion, membranes and lysosomes.
Section 2 of 12Endomembrane organelles
Rough ER
- Rough ER covered in ribosomes; it folds and transports proteins.
Smooth ER
- Smooth ER no ribosomes; it makes lipids and steroids and stores calcium ions in muscle.
Golgi body
- Golgi body modifies proteins, sorts and packages them into vesicles.
- (for example adding sugars to make glycoproteins)
- It also forms lysosomes, and in plants makes materials for the cell wall.
Lysosomes
- Lysosomes vesicles of hydrolytic enzymes that break down worn-out organelles and engulfed pathogens.
Section 3 of 12Mitochondria and chloroplasts
Mitochondria
- Mitochondria double membrane, inner membrane folded into cristae, matrix containing enzymes, 70S ribosomes and circular DNA.
- Cristae give a large surface area for ATP synthase and electron transport.
- Cells with a high energy demand, such as muscle and sperm, have many mitochondria.
Chloroplasts
- Chloroplasts double envelope, thylakoids stacked into grana, and stroma (Calvin cycle).
- (light-dependent reactions)
- They also contain 70S ribosomes, circular DNA, starch grains and lipid droplets.
Section 4 of 12Cytoskeleton, surface structures and plant features
Cytoskeleton, surface structures and plant features
Cell surface membrane
- Cell surface membrane a partially permeable phospholipid bilayer that controls what enters and leaves.
Microtubules
- Microtubules hollow tubes of tubulin that form the cytoskeleton and spindle.
Centrioles, cilia and microvilli
- Centrioles, cilia and microvilli centrioles organise the spindle, cilia move substances over the surface, and microvilli increase surface area.
Plant cell wall
- Plant cell wall cellulose, freely permeable, gives strength and prevents bursting.
Plasmodesmata and vacuole
- Plasmodesmata and vacuole plasmodesmata link neighbouring plant cells,
- and the large permanent vacuole with its tonoplast holds cell sap and maintains turgor.
Section 5 of 12Prokaryotes and eukaryotes
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Size | 0.5–5 µm | Up to 100 µm |
| DNA | Circular, free in cytoplasm | Linear chromosomes in nucleus |
| Ribosomes | 70S | 80S (70S in mitochondria and chloroplasts) |
| Organelles | None membrane-bound | Many membrane-bound |
| Cell wall | Peptidoglycan | Cellulose (plants) or chitin (fungi) |
| Division | Binary fission | Mitosis or meiosis |
- Extra prokaryote features
- plasmids, and sometimes a slime capsule, pili and flagella.
Section 6 of 12Viruses
Viruses
Structure
- Structure nucleic acid (DNA or RNA) inside a protein capsid made of capsomeres.
- Some have a phospholipid envelope from the host cell with glycoproteins.
Not cells
- Not cells they have no cytoplasm, ribosomes or metabolism and reproduce only inside a host cell.
Attachment proteins
- Attachment proteins bind to complementary receptors on host cells, so each virus infects only certain cell types.
- HIV binds to CD4 on helper T cells.
Section 7 of 12ATP as the energy currency
Role
- ATP transfers energy for active transport, muscle contraction, protein synthesis, DNA replication, cell division, exocytosis and nerve impulses.
Why it suits this role
- Why it suits this role it releases energy in small manageable amounts in a single hydrolysis step.
- It is small and soluble, so it moves easily in the cell, and it is quickly regenerated.
Section 8 of 12Pathway for secreting a protein
1
Ribosomes on the RER make the polypeptide using mRNA copied from DNA in the nucleus.
2
The polypeptide enters the RER cisternae, where it folds and may be modified.
3
Transport vesicles bud off the RER and fuse with the Golgi body.
4
The Golgi body modifies, sorts and packages the protein into secretory vesicles.
5
Vesicles move along microtubules and fuse with the cell surface membrane.
6
Exocytosis releases the protein, using ATP from mitochondria.
Section 9 of 12More from the specification
1Endoplasmic reticulum
- Endoplasmic reticulum rough ER carries ribosomes and makes proteins for export, while smooth endoplasmic reticulum makes lipids and steroids
2Genetic material
- Genetic material the nucleus holds DNA in chromatin, with the nucleolus making ribosomal RNA
3Aerobic respiration
- Aerobic respiration takes place in mitochondria, which have a double membrane and cristae
4Electron micrographs
- Electron micrographs transmission electron microscopy gives a high resolution of ultrastructure, so organelles can be identified by their appearance
5Respiration
- Chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
6Micrograph
- A photograph taken through a microscope.
7Endoplasmic reticulum
- Rough and smooth (lipids, detoxification).
- (ribosomes, proteins)
8Electron micrograph
- A photograph taken through an electron microscope.
9Aerobic respiration
- Respiration that uses oxygen to release a relatively large amount of energy.
10Spindle fibres
- Protein microtubules that move chromosomes in mitosis.
Section 10 of 12More on this topic
| Feature | Prokaryote | Eukaryote |
|---|---|---|
| Size | 0.5–5 µm | Up to 100 µm |
| DNA | Circular, free, no proteins | Linear in nucleus with histones |
| Ribosomes | 70S | 80S |
| Organelles | None membrane-bound | Many |
| Cell wall | Peptidoglycan | Cellulose or chitin |
| Division | Binary fission | Mitosis or meiosis |
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Eukaryote | An organism whose cells have a nucleus and membrane-bound organelles. |
| Prokaryote | A unicellular organism with no nucleus or membrane-bound organelles. |
| Capsid | The protein coat surrounding viral nucleic acid. |
| ATP | Adenosine triphosphate, the universal energy currency of cells. |
| Exocytosis | Release of substances from a cell by vesicles fusing with the surface membrane. |
| Plasmid | A small circular DNA molecule found in many prokaryotes. |
| Glycoprotein | A protein with sugar chains attached. |
Section 12 of 12Exam technique
1
Saying prokaryotes have no DNA — they have DNA, but it is not in a nucleus.
2
Calling viruses cells or living organisms — they are non-cellular particles.
3
Saying all cell walls are cellulose — bacteria have peptidoglycan and fungi have chitin.
4
Saying ATP is a long-term energy store — it is a short-term energy carrier.
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