B2: Cells

Yeast is a single-celled fungus Its cells have cell walls but they are not made of cellulose like plant cell walls. Fungi cannot photosynthesise as they have no chloroplasts.

Bacteria do not have a nucleus. Their genes are in the cytoplasm.

• Viruses are not made of cells. They do not have cell membanes, cytoplasm or a nucleus.
• Most viruses are made of a spher of protein, with some DNA inside it.
• They are hundreds of times smaller than a cell.

• The human body contains hundreds of different kinds of specialised cells.

In Animals...

Red Blood Cells are designed to carry oxygen around the body. They have no nucleus and a large surface area to maximise the area to cary oxygen.

A Sperm Cell is specialised as it has a tail which allows it to swim better. It also has a vesicle containing enzymes, which are used to digest the outside of the egg allowing for the nucleus of the sperm to fuse with the nucleus of the egg durig fertilisation.

In Plants...

Root Hair cells are adapted to do its job of taking in water and nutrients by having a large surface area and a thin cell membrane to maximise their absorption of water from the soil.

Leaf cells are adapted to absorb light for photosynthesis. To do this they are packed ith chloroplasts. They are also reglarly shaped and closely packed to form a continuous layer for efficient absorption of sunlight

• Diffusion is the net movement of particles from a region of high concentration to a region of low concentration.
• Most cells need oxygen so that they can respire. Oxygen diffuses into the cells from a higher concentration outside to a lower concentration inside.
• The concentration of oxygen inside a cell is kept low because the cell keeps on using it up.

The speed of diffusion can be increased by by increasing the difference in concentration (the concentration gradient) and increasing the temperature (the faster that particles move around, the faster they will diffuse).