Absorption & secretion of materials

Plant & animal cells in solutions of different strength.

Absorption and secretion of materials Cell Walls The cell wall is a non-living layer composed mainly of cellulose – a fibrous criss-crossing carbohydrate. Cell walls allow free movement of most substances into or out of the cell and provide a continuous water-conducting path around the plant. The cell membrane is selectively permeable !

 The molecules of liquids and gases move about freely all the time.  Molecules move from a region of high concentration of that substance to a region of low concentration of that substance.  Diffusion continues until the concentration becomes equal.

Diffusion Factors affecting the rate of diffusion across membranes include: Temperature: Molecules have more kinetic energy at high temperatures and diffuse faster Surface area: the greater the surface area, the more molecules or ions that can cross it. The type of molecule or ion: Large molecules diffuse more slowly than small ones. Non-polar molecules diffuse faster The steepness of the concentration gradient. The greater the difference in concentration, the faster the rate. Diffusion is the net movement of molecules down a concentrated gradient.

Osmosis in animal cells – red blood cells Movement of water into or out of red blood cells by osmosis in solutions of different concentration Red cell shrinksRed cell remains normal Red cell bursts In (hypotonic) pure water or dilute solution In a (isotonic) solution with the same concentration as the red cell In a (hypertonic) more concentrated solution Low concentration of solute molecules, high concentration of water molecules High concentration of solute molecules, low concentration of water molecules

Hypotonic=contains more water Isotonic=contains equal water Hypertonic=contains less water

Lipids Simple lipids (Triglycerides) The most common forms of lipids are triglycerides (fats and oils) These are made by the combination of 3 fatty acid molecules with 1 glycerol molecule. GLYCEROLGLYCEROL FATTY ACID

Lipids Phospholipids The third fatty acid has been replaced by a phosphate group. Phosphate groups are hydrophilic. Polar head 2 representations of a phospholipid molecule Non-polar tail

Lipids Phospholipids and membranes Cell membrane cross section A cell membrane is made up of a double phospholipid layer 1 phospholipid molecule

Membrane structure When mixed with water, phospholipid molecules spontaneously assemble to form membrane-like structures. Their polar heads point outwards towards the surrounding charged water molecules, and their non-polar tails point inwards. Under certain conditions they form bilayers, the basis of cell membranes PHOSPHOLIPID MOLECULE HYDROPHOBIC TAIL HYDROPHILIC HEAD DETAILED VIEW OF BILAYER

Membrane structure Features of the fluid mosaic model The double line seen at very high power is thought to be the 2 phospholipid layers. The bilayer is about 7 nm wide. Membranes also contain proteins and the model of membranes accepted at present is called the ‘fluid mosaic’ model. The protein molecules form a mosaic pattern set in the phospholipid bilayer The phospholipid bilayer is the fluid part because phospholipid molecules can move around

Membrane structure Features of the fluid mosaic model inside outside Most protein molecules are mobile, moving around freely. Others are fixed like islands to structures in the membrane and do not move EXTRINSIC PROTEIN 3 INTRINSIC PROTEINS PHOSPHOLIPID LAYER

Membrane structure: the fluid mosaic model Phospholipids consist of a phosphate group ‘head’ and two fatty acid tails. The phosphate head is hydrophilic and the tail is hydrophobic; in water the molecules spontaneously arrange themselves into membrane like structures. Under certain conditions they form bilayers. Membranes also contain various proteins, set in the phospholipid layer in a mosaic pattern.

Torrance p.8 Questions 1 – 3

Transport across the plasma membrane Active transport Active transport is the pumping of ions across membranes against a diffusion gradient. ATP is required to change the shape of the protein and move the ion or molecule across.

Transport across the plasma membrane Bulk transport Diffusion, osmosis and active transport refer to the movement of individual particles across membranes Mechanisms also exist for the bulk transport of materials in and out of cells (endo- and exocytosis). Ingestion Destruction of microbe Release of microbial debris Phagocytosis or ‘cell eating’. The bulk uptake of solid materials. Cells which do this are phagocytes, e.g. some white blood cells Stages in phagocytosis of a bacterium by a white blood cell - Endocytosis

Transport across the plasma membrane Bulk transport - Exocytosis Exocytosis is the reverse of endocytosis It happens, for example, in the secretion of digestive enzymes from the pancreas Secretory vesicles from the Golgi body carry the enzymes to the cell surface and release them to the outside of the cell Diagram of Golgi apparatus secretion

Exocytosis Golgi apparatus Secretory vesicle containing secretory product, e.g. enzyme EM of pancreatic acinar cell secreting protein

Torrance pp. 15/16 Questions 1 – 3 Do either 1. a) or 1. b) and either 3. a) or 3. b)

ATP and energy release

Aerobic respiration

ATP and energy release In respiration, glucose is broken down to release a large amount of energy. This energy must be transferred to the point in the cell where it is needed. Adenosine triphosphate (ATP) is the molecule which acts as an energy carrier in all living cells.

Structure of ATP

ATPADP + Pi (high energy state)(Low energy state) energy released energy required

Turnover of ATP The total ATP in the body would be used up, at rest, in about 90 seconds. Therefore, ATP is constantly being reformed from ADP using energy released during the breakdown of glucose. The net result is a constant quantity of ATP in the body; about 50g.

Torrance pp. 22/23 All questions