B2 Cells, tissues and organs

All cells have:

• a nucleus - controls all the activities of the cell and contains the genes on the chromosomes, which carry the instructions for making new cells or organisms
• the cytoplasm - a liquid gel in which most of the chemical reactions needed for life take place
• a cell membrane - controls the passages of substances into and out of the cell
• the mitochondria - structures in the cytoplasm where oxygen is used and most of the energy is released during aerobic respiration
• ribosomes - where protein synthesis takes places, and all the proteins needed in the cell are made

DIAGRAM:

In addition, most plant and algal cells also have:

• a cell wall made of cellulose that strengthens the cell and gives it support
• chloroplasts - contain the green substance chlorophyll, which absorb light energy to make food by photosynthesis - not all plant cells e.g. root cells, have these
• a permanent vacuole - a space in the cytoplasm filled with cell sap. This is important for keeping the cells rigid to support the plant
• Plants need nitrate ions to produce proteins
• DIAGRAM

Bacteria:

• are much smaller than plant and animal cells
• have a cell membrane and a cell wall which surround the cytoplasm
• do not have a nucleus, so the genetic material is in the cytoplasm
• form a colony when they multiply
• have plasmids, circular bits of DNA that carry extra genetic information
• have long strands of protein called flagella which they use to propel themselves around

Yeast:

• a single-celled organism
• cells have a nucleus, cytoplasm and a membrane surrounded by a cell wall
• are bigger than bacteria, but still small
• reproduces by asexual budding
• can respire aerobically and anaerobically, so can survive for ages without oxygen
• when breaking down sugar without oxygen, produces ethanol and carbon dioxide - this is known as fermentation

Cells may be specialised to carry out a particular function:

• cells that have many mitochondria need a lot of energy e.g. muscle cells
• cells that have many ribosomes make a lot of protein e.g. gland cells
• cells with tails are able to move e.g. sperm cells
• receptor cells have special structures which enable them to detect stimuli e.g. the cone cells in the eye are light sensitive
• neurons are specialized to carry impulses from receptors to the CNS
• plant cells with many chloroplasts will be photosynthesizing
• root hair cells increase the surface are of the root so it cal absorb water and mineral ions efficiently

Diffusion is the net movement of particles of a gas or solute from an area of high concentration as a result of the random movement of the particles.

The greater the difference in concentration, the faster the rate of diffusion.

The net movement into or out of cells depends on the concentration of the particles on each side of the cell membrane.

The difference in concentration between the areas is called the concentration gradient.

A tissue is a group of cells with similar structure and function

During the development of multi-cellular organisms the cells differentiate

Animal tissues include:

• muscle tissue which can contract
• glandular tissue to produce substances
• epithelial tissue which cover the body

Plant tissues include:

• epidermal tissue which covers the plant
• mesophyll which can photosynthesize
• xylem and phloem which transport substances around the plant

Organs are made of tissues. One organ may contain several tissue types.

Cells-tissues-organs-organ systems-whole body.

Organ systems are groups of organs that perform a particular function.

The sequence for multi-cellular organisms:

Cells-tissues-organs-organ systems-organism

Digestion happens in the mouth, the stomach and the small intestine.

Water is mostly absorbed in the large intestine.

Bile is produced by the liver and stored in the gall bladder