1.2 Cells and cell structure

How can you distinguish between smooth endoplasmic reticulum and Golgi apparatus in electron micrographs?

The Golgi apparatus has vesicles around its edges, while the smooth endoplasmic reticulum does not.

Describe the structure of an animal cell.

Has a cell-surface membrane, membrane-bound organelles, and cytoplasm; no cell wall.

How are plant cells different from animal cells?

They have a cellulose cell wall and chloroplasts; often have a large central vacuole.

What is the function of the nucleus?

Stores genetic material (DNA); site of transcription; contains nucleoli that make ribosomes.

What are the features of eukaryotic DNA?

Linear, associated with histone proteins, contains introns and exons, and forms chromosomes.

State the function of mitochondria.

Site of aerobic respiration; produces ATP.

What is the function of chloroplasts?

Site of photosynthesis in plant cells.

What is the smooth endoplasmic reticulum (SER) and its function?

Membrane system without ribosomes; synthesises, modifies, and packages lipids.

What is the function of the rough endoplasmic reticulum (RER)?

Produces and folds proteins; studded with ribosomes.

What is the Golgi apparatus and what does it do?

Modifies and packages proteins into vesicles; makes glycoproteins and lysosomes.

What are lysosomes?

Golgi vesicles containing lysozymes; hydrolyse pathogens and old organelles.

What is the function of ribosomes?

Site of protein synthesis (translation); found in the cytoplasm or on RER.

How are plant vacuoles structured and what is their function?

Surrounded by the tonoplast; contains cell sap; maintains cell rigidity.

Describe the structure of a prokaryotic cell.

No nucleus or membrane-bound organelles; circular DNA, cell wall (murein), small ribosomes, sometimes plasmids and flagella.

What is murein?

A glycoprotein found in the cell wall of prokaryotes.

How is DNA organised in prokaryotes?

Single circular DNA molecule free in the cytoplasm; not associated with histone proteins; no introns.

What are plasmids?

Small loops of DNA; may contain useful genes such as antibiotic resistance.

What is the role of flagella in prokaryotic cells?

For propulsion (movement).

List three structural differences between eukaryotic and prokaryotic cells.

Eukaryotic cells have membrane-bound organelles, linear DNA with histones, and larger ribosomes.

What is magnification?

How many times larger the image is compared to the actual object.

What is resolution?

The ability to distinguish two separate points.

Describe optical (light) microscopes.

Use light and glass lenses; lower resolution; can view living cells; produces 2D colour images.

Describe scanning electron microscopes (SEM).

Use electrons to scan surfaces; produce 3D black and white images; higher resolution than light microscopes, but lower than TEM; only surface view.

Describe transmission electron microscopes (TEM).

Use electrons to view internal structures in high detail; 2D black and white image; highest resolution; specimen must be dead and thin.

Why do electron microscopes have higher resolution than light microscopes?

Electrons have a shorter wavelength than light.

Outline how to prepare a temporary mount of starch grains.

Place water on slide → add thin section of plant tissue → add iodine in potassium iodide → lower cover slip with a mounted needle.

How do you calibrate an eyepiece graticule?

Compare graticule divisions to a stage micrometer; calculate length per division.

What is the purpose of cell fractionation?

To isolate and study individual organelles.

What are the three key conditions during homogenisation?

Ice-cold (reduce enzyme activity), isotonic (prevent osmotic damage), buffered (maintain pH).

What happens during homogenisation?

Cells are broken up to release organelles.

What is the purpose of filtering after homogenisation?

To remove large cell debris.

What is differential centrifugation?

Spinning at increasing speeds to separate organelles by density.

In what order are organelles separated by centrifugation (from most to least dense)?

Nucleus → chloroplasts (plants) / mitochondria (animals) → lysosomes → endoplasmic reticulum → ribosomes.

Why do denser organelles pellet first in centrifugation?

They experience greater centrifugal force and settle faste.

Why is the electron microscope able to resolve objects better than the light microscope?

The EM uses a beam of electrons that has a much smaller wavelength than light.

Why do specimens have to be kept in a near vacuum in order to be viewed effectively using an electron microscope?

Electrons are absorbed by the molecules in the air and, if present this would prevent the electrons reaching the specimen.

List three carbohydrates that are absorbed by an epithelial cell of the small intestine.

Glucose, fructose, galactose.

Why is an artery described as an organ whereas a blood capillary is not?

An artery is made up of more than one tissue whereas a blood capillary is made up of only one tissue.