biology4alevel600

1 The basic structure of a membrane is a 7 nm thick phospholipid bilayer with protein molecules spanning the bilayer or within one or other layer. Phospholipids and some proteins move within the layers. Hence the structure is described as a fluid mosaic – the scattered protein molecules resemble pieces of a mosaic. 2 Phospholipid bilayers are a barrier to most watersoluble substances because the interior of the membrane is hydrophobic. 3 Cholesterol is needed for membrane fluidity and stability. 4 Some proteins are transport proteins, transporting molecules or ions across the membrane. They may be either channel proteins or carrier proteins. Channel proteins have a fixed shape; carrier proteins change shape. 5 Some proteins act as enzymes – for example, in the cell surface membranes of microvilli in the gut. 6 Glycolipids and glycoproteins form receptors – for example, for hormones or neurotransmitters. They also form antigens, which are cell recognition markers. 7 The cell surface mem...

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...