Cells, Tissues and Organs

Bacterial Cells:

• Single-celled organism
• Different structure to animal abd plant cells
• It has cytoplasm, a cell membrane, all surrounded by a cell wall but the genetic material is not contained in a distinct nucleus.

Yeast:

• Single-celled organism
• Contains: cytoplasm, a cell membrane, a cell wall and a nucleus.

Leaf cell

Absorbs light energy for photosynthesis Packed with chloroplasts. Regular shaped, closely packed cells form a continuous layer for efficient absorption of sunlight.

Root hair cell

Absorbs water and mineral ions from the soil Long 'finger-like' process with very thin wall, which gives a large surface area.

Sperm cell

Fertilises an egg cell - female gamete The head contains genetic information and an enzyme to help penetrate the egg cell membrane. The middle section is packed with mitochondria for energy. The tail moves the sperm to the egg.

Red blood cells

Contains haemoglobin to carry oxygen to the cells. Thin outer membrane to let oxygen diffuse through easily. Shape increases the surface area to allow more oxygen to be absorbed efficiently. No nucleus, so the whole cell is full of haemoglobin.

• Diffusion is the spreading of the particles of any substance in solution, or particles of a gas, resulting in a net movement from a region where they are of a higher concentration to a region with a lower concentration. The greater the difference in concentration, the faster the rate of diffusion.
• Dissolved substances can move into and out of cells by diffusion.
• Oxygen required for respiration passes through cell membranes by diffusion.

• Osmosis is the diffusion of water from a dilute to a more concentrated solution through a partially permeable membrane that allows the passage of water molecules.
• Differences in the concentrations of the solutions inside and outside a cell cause water to move into or out of the cell by osmosis.

• Substances are sometimes absorbed against a concentration gradient. This requires the use of energy from respiration. The process is called active transport.
• Active transport enables plants to absorb ions from very dilute solutions, eg by root hair cells. Similarly, sugar may be absorbed from low concentrations in the intestine and from low concentrations in the kidney tubules.
• A single-celled organism has a relatively large surface area to volume ratio. All the necessary exchanges occur via its surface membrane.
• The size and complexity of an organism increase the difficulty of exchanging materials.

In multicellular organisms many organ systems are specialised for exchanging materials. The effectiveness of an exchange surface is increased by:

• having a large surface area that is thin, to provide a short diffusion path
• (in animals) having an efficient blood supply
• (in animals, for gaseous exchange) being ventilated

Candidates should be able to explain how the small intestine and lungs in mammals, and the roots and leaves in plants, are adapted for exchanging materials.

Gas and solute exchange surfaces in humans and other organisms are adapted to maximise effectiveness.

• Cell division is necessary for the growth of an organism, or for the repair of damaged tissues
• Mitosis results in two identical cells being produced from the original cell
• The chromosone contain the genes (alleles) which must be passed on to each new cell.
• A copy of each chromosone is made before the cell divides and one of each chromosone goes to each new cell
• In early development of animal and plant embryos the cells are unspecialised and are called stem cells.
• Most animal cells differentiate early in development and cell division is mainly for repair and replacement.
• Plant cells can differentiate throughout the life of the plant as it continues to grow
• Cells of offspring produced by asexual reproduction are produced by mitosis from the parent cell. They contain the same alleles as the parents.

• Cells in reproductive organs divide by meiosis to form sex cells (gametes). In humans the gametes are the sperm and ova.
• Each gamete has only one chromosone from each original pair. All of the cells are different from each other and the parent cell.
• Sexual reproduction results in variation as the gametes from each parent fuse. So half the genetic information comes from the father and half from the mother.
• When gametes join at fertilisation, a single body cell with new pairs of chromosones is formed.
• A new individual then develops by this cell repeatedly dividing by mitosis.

• Large multicellular organisms develop systems for exchanging materials. During the development of a multicellular organism, cells differentiate so that they can perform different functions.
• A tissue is a group of cells with similar structure and function.
• Organs are made of tissues. One organ may contain several tissues.
• Organ systems are groups of organs that perform a particular function.

Examples of animal tissues include:

• muscular tissue, which can contract to bring about movement
• glandular tissue, which can produce substances such as enzymes and hormones
• epithelial tissue, which covers some parts of the body.

An example of an animal organ is the stomach, which contains:

• muscular tissue, to allow contents to move through the digestive system
• glandular tissue, to produce digestive juices
• epithelial tissue, to cover the outside and the inside of the stomach.

An example of an animal organ system is the digestive system, a system in which humans and other mammals exchange substances with the environment. The digestive system includes:

• glands, such as the pancreas and salivary glands, which produce digestive juices
• the stomach and small intestine, where digestion occurs
• the liver, which produces bile
• the small intestine, where the absorption of soluble food occurs
• the large intestine, where water is absorbed from the undigested food, producing faeces.

Examples of plant tissues include:

• epidermal tissues, which cover the plant
• palisade mesophyll, which carries out photosynthesis
• spongy mesophyll, which has air spaces to facilitate diffusion of gases
• xylem and phloem, which transport substances around the plant.

Plant organs include stems, roots and leaves.