Cell structure

All living things are made of **cells**; the **cell is the basic unit** of life; new cells come only from **pre-existing cells**.

The (disproved) idea that living organisms can form from **non-living material**.

Broth in a **swan-neck flask** stayed clear; only when the neck was **broken** (letting air-borne microbes in) did it go cloudy — so cells come from cells.

The **cell** — there is no smaller living unit.

**Metabolism, Reproduction, Sensitivity, Growth, Respiration, Excretion, Nutrition** (MRS GREN).

**MRS GREN**.

The removal of the **waste products of metabolism** from a cell or organism.

Taking in (or making) the **food and nutrients** an organism needs.

An organism made of a **single cell** that carries out **all seven functions of life** by itself.

It is a complete organism with **no other cells to help**, so that one cell must do every job needed to stay alive.

Its **metabolism produces waste** (e.g. CO₂) that would **build up and become toxic** if not removed.

**Sensitivity**.

How many **times bigger** the image looks than the real object.

The ability to show two close points as **separate** — in short, how much **fine detail** can be seen.

Magnification = how much **bigger**; resolution = how much **detail**. Magnifying a blurry image just makes a bigger blur.

A beam of **light**.

A beam of **electrons**.

The **light microscope** — electron samples are killed and prepared first.

The **electron microscope** — much higher resolution than the light microscope.

Its **higher resolution** revealed **organelles and ultrastructure** not visible with the light microscope.

The **fine internal detail** of a cell (e.g. membranes, ribosomes) that only the electron microscope can reveal.

**Cryogenic electron microscopy** — it **freezes** a sample to capture a sharp snapshot of proteins and delicate structures.

**Cryo-EM** (a type of electron microscopy).

The sample must be **killed and specially prepared**, so you cannot view living cells.

**DNA, cytoplasm, plasma membrane and ribosomes** (memory hook: D-C-M-R).

A **specialised structure inside a cell** that carries out a **particular function** (for example a ribosome or nucleus).

**Ribosomes** — found in all cells and counting as an organelle.

It is the **genetic material** — the cell's stored instructions.

The **watery jelly** inside the cell where chemical reactions take place.

It is the cell's **outer boundary**, controlling what **enters and leaves**.

They **build proteins** by joining amino acids together.

**No** — prokaryotic cells have **no nucleus**; their DNA floats free in the cytoplasm.

The **nucleus** (in eukaryotes). **Mitochondria** also contain DNA, and **chloroplasts** do in plant cells.

Free in the **cytoplasm**, in a region called the **nucleoid** — there is no nucleus.

Any two of: **nucleus, cell wall, mitochondria, chloroplasts** (these are not universal).

A cell with **no nucleus** and **no membrane-bound organelles** (for example a bacterium).

A cell **with a nucleus** and membrane-bound organelles (for example animal, plant and fungal cells).

**Ribosomes** — universal and also an organelle; never answer 'nucleus'.

A cell with **no nucleus** and **no membrane-bound organelles**; its DNA lies free in the cytoplasm. All **bacteria** are prokaryotic.

A cell with a **true (membrane-bound) nucleus** holding the DNA, plus other membrane-bound organelles. **Animal, plant and fungal** cells are eukaryotic.

Prokaryotes have **no nucleus**; eukaryotes have a **true nucleus** enclosing the DNA.

As **one circular loop**, lying **naked** (no histones) in the cytoplasm, often with small extra rings (**plasmids**).

As **linear chromosomes** wound around **histones**, sealed inside the **nucleus**.

A **protein** that eukaryotic DNA wraps around to package its long chromosomes. Prokaryotic DNA has **no histones**.

A **small extra ring of DNA** in many prokaryotes, separate from the main DNA loop.

**Plasma membrane**, **cytoplasm**, **DNA** and **ribosomes**.

It rotates like a propeller to **move the cell** through liquid.

They are the site of **protein synthesis** (building proteins from amino acids).

Prokaryote ribosomes are **smaller (70S)**; eukaryote ribosomes are **larger (80S)**.

**Eukaryotic** (about 10–100 µm) vs **prokaryotic** (about 1–5 µm).

All are **eukaryotic** — they have a nucleus and membrane-bound organelles.

A **rigid layer outside the cell membrane** that gives a cell a fixed shape and support.

**Cellulose**.

**Chitin** (not cellulose).

**Animal cells** — they have only a flexible cell membrane.

**Chloroplasts** — the site of photosynthesis.

The plant cell has a **rigid cell wall** holding a fixed shape; the animal cell has **no wall**, so its membrane is easily squashed.

It is a fluid-filled sac that **keeps the cell firm**; animal cells have only small temporary vacuoles.

A cell that breaks the usual rule of having **one nucleus** — either anucleate or multinucleate.

A mature **red blood cell** — it **loses its nucleus**, leaving more room to carry oxygen.

**Skeletal muscle fibres** and **fungal hyphae** — one long cell with **many nuclei**.

A **photograph** of a specimen taken through a **microscope**.

A micrograph taken with an **electron microscope** — high enough magnification to see small organelles such as mitochondria.

A structure inside a cell that does a **specific job** (for example the nucleus or a mitochondrion).

Whether there is a **nucleus** — no nucleus means **prokaryotic**.

**No nucleus** and **no membrane-bound organelles**; the DNA lies **free in the cytoplasm**, and the cell is **small**.

It has a **nucleus** and **membrane-bound organelles** (such as mitochondria).

A **plant** cell has a **cell wall** and often **chloroplasts**; an **animal** cell has **neither**.

A **fungal** cell (its wall is made of **chitin**).

The **cell type** AND a **visible feature** as the reason (for example 'no nucleus, so prokaryotic').

The **double membrane (nuclear envelope)**, the **nuclear pores**, the **chromatin** and the **nucleolus**.

Because it is a **double membrane** — drawing a single line is the most common lost mark.

Small — about **1–5 μm** across (eukaryotic cells are usually much larger).

It is **oval** (sausage-shaped) with folded inner membranes called **cristae**.

**Rough ER** has membranes **studded with ribosomes** (dots); **smooth ER** has a **plain surface with no dots**.

It looks like a **stack of flattened, curved sacs**, often with small vesicles nearby.