biology4alevel600

As the radius of a cell ↑ from 1x to 3x (left), the surface area ↑ from 1x to 9x, and the volume ↑ from 1x to 27x. Source: Nature Education  As a cell increases in size, there is less surface area in proportion to its volume.  Relatively there is less surface area of cell mebrane over which inffusion can coccur.  As the cell grows and increases in size, their surface area - and thus their ability to take in nutrients and transport electrons - does not increase to the same degree as their volume. The volume  ↑  faster than the surface area --> surface area/volume (SA/V) ratio  ↓ . So, with increasing size of a cell, less of the cytoplasm has access to the cell surface for exchange of gases, supply of nutrients, and loss of waste products --> the smaller the cell is, the more quickly and easily can materials be exchanged between its cytoplasm and environment. That's why cells cannot continue growing larger, indefinitely. When a maximum size is reached, ...

Substances can enter or leave a cell in 2 ways: 1) Passive a) Simple Diffusion b) Facilitated Diffusion c) Osmosis (water only) 2) Active a) Molecules b) Particles I. Passive transport across cell membranes 1. Diffusion   Molecules and ions move freely in gases and liquids, each type of these particles tends to spread out evenly within the space available. This is diffusion . Diffusion is:           + the net movement of molecules           +  from a region of its higher concentration to a region of its lower concentration.            + down a concentration gradient           + no energy is used. Source: northlandcollege.edu Some molecules and ions are able to pass through cell membranes --> The membrane is permeable. Some substances cannot pass through cell membranes --> The membrane is partially permeable. Example: O 2 is at a higher concentration outside a cell (in...

The fluid-mosaic model describes the plasma membrane  that surrounds animal cell. The membrane has 2 layers of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid). 1. Fluid Mosaic Model Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), both layers of phospholipids  (1)  have the  hydrophilic  heads (2) facing outwards into the water and the  hydrophobic  tails (3) facing inwards, avoiding contact with water. Cholesterol  molecules are among the phospholipids. Protein  molecules (4) float in the phospholipid bilayer.  Many of the phospholipids and proteins have short chains of  carbohydrates  (5) attached to them, on the outer surface of the membrane. They are known as  glycolipids  (6) and  glycoproteins  (7). There are also other types of glycolipid with no phosphate groups. The...

4.1 Fluid mosaic membranes  4.2 Movement of substances  into and out of cells The fluid mosaic model introduced in 1972 describes the way in which biological molecules are arranged to form cell membranes. The model has stood the test of time as a way to visualise membrane structure and continues to be modified as understanding improves of the ways in which substances cross membranes, how cells interact and how cells respond to signals. The model also provides the basis for our understanding of passive and active movement between cells and their surroundings, cell to cell interactions and long distance cell signalling. Investigating the effects of different factors on diffusion, osmosis and membrane permeability involves  an understanding of the properties of phospholipids and proteins covered in the section on Biological  molecules. Candidates will be expected to use the knowledge gained in this section to solve problems in familiar and ...

• Fluid mosaic model of membrane structure • Movement of substances into and out of cells Learning Outcomes Candidates should be able to: (a) describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of  phospholipids, cholesterol, glycolipids, proteins and glycoproteins; (b) outline the roles of cell surface membranes; (c) describe and explain the processes of diffusion, facilitated diffusion, osmosis, active transport, endocytosis and exocytosis (terminology described in the IOB’s publication Biological Nomenclature should be used; see also section 5; no calculations involving water potential will be set); (d) [PA] investigate the effects on plant cells and the effect on animal cells of immersion in solutions of  different concentrations of solutions (with different water potentials); (e) use the knowledge gained in this section in new situations or to solve related problems.

1  Enzymes are globular proteins which catalyse metabolic reactions. 2  Each enzyme has an active site with a specific shape, into which the substrate molecule or molecules fit precisely. This is the lock and key hypothesis – the substrate is compared with a key which fits precisely into the lock of the enzyme. 3  The lock and key hypothesis has been modified. The modern hypothesis is called the induced fit hypothesis. The active site is no longer regarded as a rigid structure like a lock, but as a flexible structure which can change shape slightly to fit precisely the substrate molecule. 4  When the substrate enters the active site, an enzyme–substrate complex is temporarily formed in which the R groups of the amino acids in the enzyme hold the substrate in place. 5  Enzymes may be involved in reactions which break down molecules or join molecules together. 6  Enzymes work by lowering the activation energy of the reactions they catalyse. 7  The course...