Transport across the plasma membrane
2.4
KEY IDEAS ✚ ✚
Study tip
SA LE
In this topic, you will learn that:
the structure of the plasma membrane is described by the fluid mosaic model substances are transported across the plasma membrane by osmosis, facilitated diffusion and active transport.
Every cell is enclosed by a plasma membrane that acts as a boundary, protecting and separating the cytoplasm from the extracellular fluid that surrounds the cell. The plasma membrane is semipermeable, which means it allows some substances to move in and out of the cell but not others. Substances are transported across the plasma membrane by different modes of passive or active transport.
FO
R
In the term ‘plasma membrane’, ‘plasma’ describes the fluid of the cytoplasm. ‘Plasma membrane’ is preferred to ‘cellular membrane’ because many organelles also have membranes.
T
Fluid mosaic model of the plasma membrane
a molecule consisting of a phosphate head and a lipid tail; a key molecular component of the plasma membrane
N LY
phospholipid
-N
O
The fluid mosaic model was proposed in the early 1970s and describes the composition and structure of the plasma membrane (Figure 1). The model describes the plasma membrane as being made of thousands of lipids, proteins and carbohydrate molecules. The composition of every plasma membrane is a mosaic of different molecules that are unique to a particular cell type. The plasma membrane has a fluid nature, where molecules are free to move laterally within their side of the bilayer. This is important for diffusion and when particular proteins need to congregate together in a specific area. The main molecule is a type of lipid called a phospholipid.
OUTSIDE OF THE CELL
O
Glycolipid
Protein (of glycoprotein)
D
R AF
T
Carbohydrate (of glycoprotein)
Filaments of the cytoplasm
Protein
Phospholipid Cholesterol
INSIDE OF THE CELL (cytoplasm) FIGURE 1 The fluid mosaic model describes the structure of the plasma membrane.
58
BIOLOGY FOR VCE UNITS 1 & 2
OXFORD UNIVERSITY PRESS
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