Cell Structure

All cells have structures inside them - these are called sub-cellular structures.

Animal cells have:

A nucleus, which controls the activities of the cell and contains the genetic material.

A cytoplasm, in which most of the chemical reactions take place.

A cell membrane, which controls the passage of substances into and out of the cell.

Mitochondria, where aerobic respiration takes place.

Ribosomes, where proteins are synthesised (made).

Plant cells have everything the animal cells have, and they also have these sub cellular structures:

A cell wall made of cellulose, which strengthens the cell.

A permanent vacuole filled with cell sap, which supports the plant.

Some have chloroplasts, which absorb light to make glucose by photosynthesis.

Eukaryotic cells: Plant, animal and fungal cells.

Prokaryotic cells: Bacterial cells.

Differences:

Prokaryotic cells are much smaller in size.

The genetic material is not enclosed in a nucleus (they do not have a nucleus).

The genetic material is a single DNA loop and there may be one or more small rings of DNA, called plasmids.

They do not contain Mitochondria or chloroplasts. 

They have many different shapes; some are round, some are rod-shaped and some are spiral.

The roles of mitochondria and chloroplasts are taken over by the cytoplasm.

There may be one or more flagella, which are tail-like structures that move the bacterium.

They contain plasmids, which are loops of DNA that can be transferred from one cell to another.

A typical plant cell: 0.1mm in diameter. Typcial animal cell: 0.02mm in diameter. Prokaryotic cells: often about 0.002mm long.

Yoi use different units to describe the size of cells and Sub-cellular structures.

Centimetre (cm): 100 units in one metre.

Millimeter (mm): 1000 units in one metre.

Micrometre (μm): 1 000 000 units in one metre.

Nanometre (nm): 1 000 000 000 units in one metre.

Magnification is how many times larger the image is than the real object.

You calculate magnification by doing: size of image / size of real object.

Bacteria divide via binary fission, an example of asexual reproduction.

They can multiply as frequently as once every 20 minutes in the right conditions.

They can be grown of a type of jelly called Agar. This is called a culture.

The agar is in the bottom of a flat dish called a Petri Dish.

1. Petri dish + agar must be sterilised before use to kill unwanted microorganisms.

2. Incoulating loop must be sterilised by passing it through a flame.

3. Loop used to transfer bacteria to agar, and lid of petri dish must be quickly removed and replaced and secured with tape to stop it coming off.

4. Dish is stored upside down to stop condenation dripping onto the agar surface, and is incubated at a maximum temperature of 25 degrees celsius. 

Chromosomes are made of DNA and are found in the nucleus of a cell.

They carry hundreds to thousands of genes, and each gene contrains the code to make different proteins.

They are found in pairs, and different species have different numbers of pairs of chromosomes, eg. humans have 23 pairs and dogs have 39 pairs.

Mitosis is when the cell divides into two identical cells.

1. DNA replicates to form two copies of each chromosome.

2. One set of chromosomes is pulled to each end of the cell.

3. The nucleus divides.

4. The cytoplasm and cell membranes divide to form two identical cells.

Stem cells are undifferentiated - they have not yet become specialised.

Found in human embryos, in the umbilical cord of a new born baby, in some organs and tissues and in the meristems of a plant.

Embryonic stem cells: from human embryos. Can make all types of cells.

Adult stem cells: found in some organs and tissues, eg. bone marrow. Can only make certain types of cells and their capactity to divide is limited.

Uses: Can be used to replace damaged cells, eg. in diabetes and paralysis. 

Therapeutic cloning is where a cloned embryo of the patient may be made and used as a source of stem cells.

There may be risks with this, such as the transfer of viral infection and some people may have ethical or religious objections.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration until they are evenly spread out.

Examples: oxygen and carbon dioxide diffuse during gas exchange in lungs, gills and plant leaves, and digested food molecules from the small intestine diffuse into the blood.

Factors that affect the rate of diffusion:

- The concentration gradient.

- The temperature.

- The surface area of the membrane; A larger surface area to volume ratio allows more molecules to diffuse into and out of the cell.

Osmosis is the diffusion of water from a dilute solution (lots of water) to a concentrated solution (less water) through a partially permeable membrane.

Active transport moves substances against a concentration gradient; from an area of low concentration to high concentration.

This requires energy from respiration.

This allows mineral ions to be absorbed into plant root hairs from very dilute solutions in the soil.

It allows sugar molecules to be absorbed from lower concentrations in the gut into the blood, which has a higher concentration.