Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
secretory
vesicle
88
Figure 4.20 Exocytosis in a secretory cell. If the product
being secreted is a protein, the Golgi body is often involved in
chemically modifying the protein before it is secreted, as in
the secretion of digestive enzymes by the pancreas.
Active transport is the movement of molecules or ions
through transport proteins across a cell membrane, against
their concentration gradient, using energy from ATP.
Endocytosis is the bulk movement of liquids (pinocytosis)
or solids (phagocytosis) into a cell, by the infolding of the
cell surface membrane to form vesicles containing the
substance; endocytosis is an active process requiring ATP.
Figure 4.21 Transmission electron micrograph of pancreatic
acinar cell secreting protein. The outside of the cell is coloured
green. Golgi (secretory) vesicles with darkly stained contents
can be seen making their way from the Golgi body to the cell
surface membrane.
Exocytosis is the bulk movement of liquids or solids out of a
cell, by the fusion of vesicles containing the substance with
the cell surface membrane; exocytosis is an active process
requiring ATP.
Summary
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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. Phospholipid bilayers are a barrier to most
water-soluble substances because the interior of the
membrane is hydrophobic. Cholesterol is needed for
membrane fluidity and stability.
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. Some proteins act as enzymes – for example,
in the cell surface membranes of microvilli in the gut.
Glycolipids and glycoproteins form receptors – for
example, for hormones or neurotransmitters.
Glycolipids and glycoproteins also form antigens,
which are cell recognition markers. Membranes play
an important role in cell signalling, the means by
which cells communicate with each other.
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The cell surface membrane controls exchange between
the cell and its environment. Some chemical reactions
take place on membranes inside cell organelles, as in
photosynthesis and respiration.
Diffusion is the net movement of molecules or ions from
a region of their higher concentration to one of lower
concentration. Oxygen, carbon dioxide and water cross
membranes by diffusion through the phospholipid
bilayer. Diffusion of ions and larger polar molecules
through membranes is allowed by transport proteins.
This process is called facilitated diffusion.
Water moves from regions of higher water potential to
regions of lower water potential. When water moves
from regions of higher water potential to regions of
lower water potential through a partially permeable
membrane, such as the cell surface membrane, this
diffusion is called osmosis.
Pure water has a water potential (Ψ) of zero. Adding
solute reduces the water potential by an amount known
as the solute potential (Ψ s
), which has a negative
value. Adding pressure to a solution increases the
water potential by an amount known as the pressure
potential (Ψ p
), which has a positive value. The following
equation is used: Ψ = Ψ s
+ Ψ p