Biology (Holt)

Carbohydrate

portion

1. Cell-surface marker:
   Identifies cell type

Phospholipid heads

Protein

portion

Phospholipid tails

2. Receptor protein:
   Recognizes and
   binds to substances
   outside the cell
   3. Enzyme:
   Assists chemical
   reactions inside
   the cell
   4. Transport protein:
   Helps substances
   move across
   cell membrane

Lipid

bilayer

Outside of cell

Inside of cell

Membrane Proteins
Various proteins are located in the lipid bilayer of a cell membrane.
What keeps these proteins within the lipid bilayer? You may recall
that proteins are made of amino acids and that some amino acids
are polar, while others are nonpolar. The nonpolar part of a mem-
brane protein is attracted to the interior of the lipid bilayer but is
repelled by the water on either side of the lipid bilayer. In contrast,
the polar parts of the protein are attracted to the water on either
side of the lipid bilayer. This dual attraction holds the protein in the
lipid bilayer. However, the motion and fluidity of phospholipids
enable membrane proteins to move around within the lipid bilayer.
As shown in Figure 12,cell membranes contain different types of
proteins. Marker proteins attached to a carbohydrate on the cell’s sur-
face advertise cell type—such as a liver cell or a heart cell. Receptor
proteins bind specific substances, such as signal molecules, outside
the cell. Enzymes embedded in the cell membrane are involved in
important biochemical reactions in the cell. Transport proteins aid the
movement of substances into and out of the cell.

SECTION 2 Cell Features 61

The cell membrane contains various proteins with specialized functions.

Figure 12 Membrane proteins

Section 2 Review

Describethe importance of the surface-area-to-

volume ratio of a cell. 4B

Comparethe structure of a eukaryotic cell with

that of a prokaryotic cell. 4A

Critical ThinkingComparing Functions

Describe the functions of two types of cell-

membrane proteins. 4A 9A

Analyzethe three parts of the cell theory and

describe two observations of early scientists that

support it. 3A 3F

A bacterium that lost its

flagella would be unable to 4B

Adivide Cmaintain its shape

Bmove Dmake proteins

TAKS Test PrepTAKS Test Prep

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Topic: Cell Biology

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Keyword: HXX4006