Cells

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Lesson notes

Cell Theory and Discovery

Cells are the **basic structural and functional unit** of all living organisms.
Robert Hooke discovered cells in **1665** after observing cork under a microscope.
Cell theory (Schleiden & Schwann, 1839): all organisms are made of **one or more cells**, cells are the **fundamental unit** of life, and all cells arise from **pre-existing cells**.

Prokaryotic vs Eukaryotic Cells

**Prokaryotes** (bacteria & archaea) lack a membrane-bound nucleus; their DNA is in a **nucleoid** region.
**Eukaryotes** (animals, plants, fungi, protists) have a **membrane-bound nucleus** and other **organelles**.
Prokaryotes are generally **smaller** (0.1–5.0 μm) and simpler; eukaryotes are larger and more complex.
All prokaryotes are **single-celled**; eukaryotes can be **single-celled** (e.g., yeast) or **multicellular**.

Plant vs Animal Cells

Both have a **cell membrane**, **cytoplasm**, **nucleus**, **mitochondria**, **ribosomes**, and **endoplasmic reticulum**.
Plant cells have a **cell wall** (cellulose), **chloroplasts** (photosynthesis), and a large central **vacuole**.
Animal cells have **no cell wall**, **no chloroplasts**, and **smaller vacuoles** (if any).
Plant cells are typically **rectangular**; animal cells are **irregular** or **rounded**.

Key Organelles and Their Functions

**Nucleus**: contains genetic material (DNA); controls cell activities.
**Cell membrane**: semipermeable; controls entry/exit of substances.
**Cytoplasm**: jelly-like fluid where chemical reactions occur.
**Mitochondria**: site of **aerobic respiration**; produce ATP (energy).
**Chloroplasts** (plants): contain chlorophyll; site of **photosynthesis**.
**Vacuole** (plants): stores sap; maintains turgor pressure.
**Ribosomes**: synthesise **proteins**.
**Cell wall** (plants, bacteria): provides structural support and protection.

Prokaryotic Cell Structure

Prokaryotes have a **cell envelope**: plasma membrane + cell wall (peptidoglycan); some have a **capsule**.
DNA is a **single circular chromosome** in the **nucleoid**; **plasmids** (extra DNA) may be present.
**Ribosomes** are smaller than eukaryotic ribosomes (70S).
Some have **flagella** (movement) and **pili** (attachment/conjugation).
No membrane-bound organelles; some have **microcompartments** (e.g., gas vesicles, magnetosomes).

Microscopy

Most cells are **microscopic** and require a microscope to be seen.
**Light microscopes** use visible light; can magnify up to ~1000×.
**Electron microscopes** use electron beams; much higher magnification and resolution.
Stains (e.g., methylene blue) help highlight cell structures.

Animal cell structure showing key organelles.

Plant cell structure with cell wall, chloroplasts and large vacuole.

Prokaryotic cell (bacterium) with key features.

Slides

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Practice questions

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1.What is the basic structural and functional unit of all living organisms?
Easy
- ACell
- BNucleus
- COrganelle
- DTissue
2.Which structure is found in plant cells but not in animal cells?
Easy
- ACell wall
- BCell membrane
- CNucleus
- DMitochondrion
3.The diagram shows a plant cell. Which labelled structure is the chloroplast?
Medium
- AA
- BB
- CC
- DD
4.Which of the following is a function of the cell membrane?
Medium
- AControls what enters and leaves the cell
- BProvides energy for the cell
- CContains the genetic material
- DSite of protein synthesis
5.Who first discovered cells and named them after their resemblance to monastery cells?
Easy
- ARobert Hooke
- BMatthias Schleiden
- CTheodor Schwann
- DAntonie van Leeuwenhoek
6.A student observes a cell under a microscope. It has a nucleus, mitochondria, and a cell wall. What type of cell is it most likely to be?
Hard
- APlant cell
- BAnimal cell
- CBacterial cell
- DFungal cell
7.What is the function of the mitochondria in a cell?
Medium
- ATo produce energy (ATP) through respiration
- BTo control cell activities
- CTo carry out photosynthesis
- DTo store water and nutrients
8.The diagram shows a bacterium. Which labelled part contains the genetic material?
Hard
- AA (cell wall)
- BB (plasma membrane)
- CC (nucleoid region)
- DD (ribosome)

Revision notes

## Cells – Revision Notes ### Cell Theory - All organisms are composed of one or more cells. - The cell is the fundamental unit of structure and function. - All cells arise from pre-existing cells. ### Two Main Cell Types - **Prokaryotes**: No membrane-bound nucleus; nucleoid region. Single-celled (bacteria, archaea). Smaller (0.1–5.0 μm). - **Eukaryotes**: Have a membrane-bound nucleus and other organelles. Can be single-celled or multicellular (animals, plants, fungi, protists). ### Organelles (Eukaryotic Cells) - **Nucleus**: Contains DNA; enclosed by nuclear membrane. - **Cell membrane**: Semipermeable; controls what enters/leaves. - **Cytoplasm**: Jelly-like substance where organelles are suspended. - **Mitochondria**: Powerhouse; produces energy (ATP). - **Chloroplasts** (plant cells): Site of photosynthesis; contain chlorophyll. - **Cell wall** (plant cells): Rigid outer layer; provides support. - **Vacuole** (plant cells): Large central vacuole for storage and turgor. - **Ribosomes**: Protein synthesis; present in both prokaryotes and eukaryotes. ### Plant vs Animal Cells | Feature | Plant Cell | Animal Cell | |---------|------------|-------------| | Cell wall | Yes (cellulose) | No | | Chloroplasts | Yes | No | | Vacuole | Large central | Small or absent | | Shape | Fixed, rectangular | Irregular, round | ### Prokaryotic Cell Features - No nucleus; DNA in **nucleoid**. - **Plasmids**: Small circular DNA (e.g., antibiotic resistance). - **Cell wall**: Peptidoglycan; prevents bursting. - **Capsule** (some bacteria): Polysaccharide or polypeptide layer. - **Flagella**: Movement. - **Pili/Fimbriae**: Attachment to surfaces or other cells. - **Ribosomes**: Smaller than eukaryotic ribosomes. ### Microscopy - Cells are typically visible only under a microscope. - Robert Hooke discovered cells in 1665. ### Key Points - All living things are made of cells. - Prokaryotes are simpler and lack membrane-bound organelles. - Eukaryotes have compartmentalized organelles. - Plant cells have cell wall, chloroplasts, and large vacuole; animal cells do not.

Cell: The basic structural and functional unit of all living organisms.
Prokaryote: A single-celled organism without a membrane-bound nucleus or other membrane-bound organelles; includes bacteria and archaea.
Eukaryote: An organism whose cells contain a membrane-bound nucleus and other membrane-bound organelles; can be single-celled or multicellular.
Nucleus: A membrane-bound organelle in eukaryotic cells that contains the cell's genetic material (DNA).
Cell membrane: A semipermeable barrier surrounding the cell that controls the movement of substances in and out.
Cell wall: A rigid outer layer found in plant cells (and some prokaryotes) that provides structural support and protection.
Cytoplasm: The jelly-like substance within the cell membrane that contains organelles and is the site of many cellular processes.
Chloroplast: An organelle in plant cells where photosynthesis occurs; contains chlorophyll.
Mitochondrion: An organelle that produces energy (ATP) for the cell through cellular respiration.
Vacuole: A membrane-bound sac used for storage, waste management, and maintaining turgor pressure; large in plant cells.
Ribosome: A non-membrane-bound organelle that synthesizes proteins; present in both prokaryotes and eukaryotes.
Nucleoid: The region in a prokaryotic cell where the circular DNA is located, not enclosed by a membrane.
Plasmid: A small, circular piece of extrachromosomal DNA in bacteria that often carries genes for antibiotic resistance.
Flagellum: A long, whip-like appendage used for movement in some cells (e.g., bacteria, sperm).
Pilus: A hair-like appendage on the surface of bacteria used for attachment or conjugation.

Flashcards

What is the basic structural and functional unit of all living organisms?

The cell.

What does the term 'cell' mean in Latin?

Cellula, meaning 'small room'.

What are the two main types of cells based on the presence of a nucleus?

Prokaryotic cells (no membrane-bound nucleus) and eukaryotic cells (have a membrane-bound nucleus).

Name three structures found in both plant and animal cells.

Cell membrane, cytoplasm, and nucleus.

Which organelle is responsible for photosynthesis in plant cells?

Chloroplast.

What is the function of the cell wall in plant cells?

Provides structural support and protection; prevents the cell from bursting in hypotonic conditions.

How do plant cells store water and maintain turgor pressure?

Through a large central vacuole.

What is the role of the nucleus in a cell?

Contains genetic material (DNA) and controls cell activities.

What is the function of the cell membrane?

Controls what enters and leaves the cell; semipermeable barrier.

Where does protein synthesis occur in a cell?

At ribosomes (found in cytoplasm or on rough endoplasmic reticulum).

What is the main difference between prokaryotic and eukaryotic cells?

Prokaryotes lack a membrane-bound nucleus and membrane-bound organelles; eukaryotes have both.

Give an example of a prokaryotic organism.

Bacteria (e.g., Escherichia coli) or archaea.

What is the nucleoid in prokaryotic cells?

The region where the circular chromosome is located, not enclosed by a membrane.

What are the three parts of cell theory?

All organisms are composed of one or more cells; the cell is the fundamental unit of structure and function; all cells come from pre-existing cells.

Who discovered cells and in what year?

Robert Hooke in 1665.

What is the function of mitochondria?

Provide energy for cell functions through cellular respiration.

Why are most cells not visible to the naked eye?

They are microscopic; most are less than 0.1 mm in diameter.

What is the difference between somatic cells and gametes?

Somatic cells are diploid body cells; gametes are haploid sex cells (sperm and egg).

What is the main structural difference between plant and animal cells?

Plant cells have a cell wall, chloroplasts, and a large central vacuole; animal cells do not.

What is the function of ribosomes?

Synthesize proteins.

What is the cytoplasm?

The jelly-like substance inside the cell that contains organelles and is the site of many chemical reactions.

What is the approximate size range of most prokaryotic cells?

0.1 to 5.0 μm in diameter.

What is the largest known bacterium?

Thiomargarita magnifica, up to 2 cm long.

What is the function of the bacterial capsule?

Protection; helps bacteria attach to surfaces and evade immune responses.

What are plasmids?

Small, circular extrachromosomal DNA molecules in bacteria that often carry antibiotic resistance genes.

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