David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova
biologia.blu
Le cellule e i viventi
Cell Membranes
What Is the Structure of a Biological Membrane?
The general structure of biological
membranes is known as the fluid mosaic
model.
Phospholipids form a bilayer, which is like a
“lake” in which a variety of proteins “float”.
A Phospholipid Bilayer
Phospholipids have a
polar, hydrophilic head
and hydrophobic fatty
acid tails.
In an aqueous
environment,
phospholipids form
a bilayer.
What Is the Structure of a Biological Membrane?
Animal cell membranes may be up to
25% cholesterol, which is important for
membrane integrity.
What Is the Structure of a Biological Membrane?
Membranes also contain proteins; the number varies
depending on membrane function.
Integral membrane
proteins have
hydrophobic and
hydrophilic regions or
domains.
Some extend across
the lipid bilayer;
others are partially
embedded.
What Is the Structure of a Biological Membrane?
Membranes also have carbohydrates
on the outer surface that serve as
recognition sites for other cells and
molecules.
Glycolipids—carbohydrate + lipid
Glycoproteins—carbohydrate + protein
The Endomembrane System
Membranes are dynamic.
How Is the Plasma Membrane Involved In Cell Adhesion and Recognition?
Cells arrange themselves in groups by
cell recognition and cell adhesion.
Cell junctions are specialized
structures that hold cells together:
• Tight junctions
• Desmosomes
• Gap junctions
Junctions Link Animal Cells Together (Part 1)
Tight junctions help ensure directional
movement of materials.
Junctions Link Animal Cells Together (Part 2)
Desmosomes are like “spot welds.”
Junctions Link Animal Cells Together (Part 3)
Gap junctions allow communication.
What Are the Passive Processes of Membrane Transport?
Membranes have selective
permeability—some substances can
pass through, but not others.
Passive transport—no outside energy
required (diffusion).
Active transport—energy required.
What Are the Passive Processes of Membrane Transport?
Diffusion: the process of random movement
toward equilibrium.
Diffusion rate depends on:
•Diameter of the molecules or ions
•Temperature of the solution
•Concentration gradient
Simple diffusion: Small molecules pass
through the lipid bilayer.
Osmosis Can Modify the Shapes of Cells
Osmosis: the diffusion of water.
What Are the Passive Processes of Membrane Transport?
Facilitated diffusion of polar molecules (passive):
• Channel proteins—
integral membrane
proteins that form
a channel.
A Carrier Protein Facilitates Diffusion
• Carrier proteins—membrane proteins that bind
some substances and speed their diffusion
through the bilayer.
What Are the Active Processes of Membrane Transport?
Active transport: moves substances
against a concentration and/or electrical
gradient. Requires energy.
The energy source is often adenosine
triphosphate (ATP).
• Uniporter—moves one substance in
one direction
• Symporter—moves two substances
in one direction
• Antiporter—moves two substances
in opposite directions
Three Types of Proteins for Active Transport
Uniporter
Symporter
Antiporter
How Do Large Molecules Enter and Leave a Cell?
Endocytosis: processes that brings
molecules and cells into a eukaryotic cell.
The plasma membrane folds in or
invaginates around the material, forming a
vesicle.
Exocytosis: material in vesicles is expelled
from a cell.
Indigestible materials are expelled.
Other materials leave cells such as
digestive enzymes and neurotransmitters.
Endocytosis and Exocytosis
How Do Large Molecules Enter and Leave a Cell?
• Phagocytosis
• Pinocytosis
• Receptor mediated endocytosis
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved