Biology II – Chapter 4: Cell Membrane Structure and Function
How Is the Structure of a Membrane Related to Its Function?
• Each cell is surrounded by a thin plasma membrane that acts as a gatekeeper – allowing only specific
substances in or out and passing chemical messages from the external environment to the cell’s
interior.
• 3 General Functions
1. Selectively isolates the cell’s contents from the external environment
2. Regulates the exchange of essential substances between the cell and external environment
3. Communicates with other cells
• The key to membrane function lies in its structure – complex, heterogeneous structures who different
parts perform very distinctive functions and also change dynamically in response to their surroundings.
• Most cells have internal membranes as well as plasma membranes that surround them.
–Internal membranes form compartments where specialized biochemical activities occur
–All the membranes of a cell have a similar basic structure: Proteins floating in a double layer
of lipids
• Lipids are responsible for the isolating function of membranes
• Proteins regulate the exchange of substances and communication
• The fluid mosaic model of cellular membranes was developed in 1972 by cell biologists S.J. Singer
and G.L. Nicolson.
• According to this model, a membrane looks like a lumpy, constantly shifting mosaic of tiles:
–A double layer of phospholipids form a fluid “grout”
–Assorted proteins are the “tiles” that move about within the phospholipid layers
• A phospholipid consists of two hydrophobic tails and one hydrophilic head
–The double bond (keeps the lipid unsaturated) creates a kink in the tail which helps keep the
membrane fluid at lower temperatures
• The components within the plasma membrane remain relatively constant
• The overall distribution of proteins and various types of phospholipids can change over time
• All cells are surrounded by a watery medium
–Animal cells are in a weakly salty extracellular fluid
• Cytoplasm is made mostly of water and consists of all of a cell’s internal contents
–Plasma membranes separate the watery cytoplasm from its water external environment
• Phospholipids spontaneously arrange themselves into a double layer called a phospholipid bilayer –
hydrophilic heads form the outer borders and the hydrophobic tails “hide” inside
–Because individual phospholipid molecules are not bonded to one another and the lipid tails
contain saturated bonds – this double layer is quite fluid allows them to move about easily within each
layer
• Most biological molecules are water soluble, therefore hydrophilic
–Most substances that contact a cell are water soluble and cannot easily pass through the
hydrophobic tails of the phospholipid bilayer
• Largely responsible for the first of 3 membrane functions
• In most animal cells, the phospholipid bilayer of membranes contain cholesterol
–Some just have a few cholesterol molecules
–Others have many cholesterol molecules as they do phospholipids
• Cholesterol affects membrane structure and function in several ways:
–Makes the bilayer stronger
–More flexible but less fluid
•Very important for membrane function: allows the cell to change shape
when necessary to ensure survival
–Less permeable to water-soluble substances
• Thousands of proteins are embedded within or attached to the surface of a membrane’s phospholipid
bilayer
–These proteins regulate the movement of substances through the membrane and communicate
with the environment
–Many in the plasma membrane by carbohydrate groups attached to them – these proteins and
their carbohydrates are called glycoproteins
• Many membrane proteins can move about within the fluid bilayer, others are anchored in place to
protein filaments within the cytoplasm.
–The attachments between these proteins and filaments produce the characteristic shapes of
animal cells
• 3 Major Categories of Membrane Proteins
1. Transport proteins – regulate the movement of hydrophilic molecules through the plasma
membrane
–Channel proteins: Form pores or channels that allow small molecules to pass through
• Large assortment of channel proteins that are selective for specific ions
–Carrier proteins: Have binding sites that can temporarily attach to specific molecules
on one side of the membrane, the protein changes shape (sometimes with the help of energy),
and moves across the membrane
2. Receptor proteins – trigger cellular responses when specific molecules in the extracellular
fluid bind to them
–Dozens of types of receptors on the plasma membrane
• When activated by the appropriate molecule some receptors set off sequences
of cellular changes (increase metabolic rate, cell division, movement, secretion)
–Other receptors act like gates on channel proteins – activating the receptor opens the
gate
3. Recognition proteins – serve as identification tags and cell-surface attachment sites
–Cells of the immune system recognize bacterium as a foreign invader while
recognizing normal cells
How Do Substances Move Across Membranes?
• 3 Characteristics of Fluids
1. Fluid: any substance that can move or change shape in response to external forces without
breaking apart
2. Concentration: the number of molecules in a given unit of volume
3. Gradient: a physical difference in properties such as temperature, pressure, electrical charge,
or concentration of a particular substance between two adjoining regions
• Concentration gradient: a difference in concentration of those substance between one region and
another
–The individual molecules in a fluid move continuously, bouncing off one another in random
directions
–The net movement of molecules from regions of high concentration to regions of low
concentration is a process called diffusion
• The greater the concentration gradient, the faster the rate of diffusion
• Diffusion cannot move molecules rapidly over long distances
• If no other processes interfere, diffusion will continue until dynamic equilibrium is reached– the
concentration gradient no longer exists
–Molecules continue their random movements and collisions but there is no longer any change
is concentration
**Copy Table 4-1 on page 62 into notes!
• 2 Types of Movement
1. Passive transport: substances move into or out of cell down concentration gradients without
the expenditure of energy
• Simple diffusion
• Facilitated diffusion
• Osmosis
2. Energy-requiring transport: the cell uses energy to move substances against a
concentration gradient
• Active transport
• Endocytosis
• Exocytosis
• Because of the properties of the plasma membrane, different molecules cross the plasma membrane at
different locations and at different rates – therefore it is said to be selectively permeable
–Allows some molecules to pass through but prevents others
PASSIVE TRANSPORT
• Lipid-soluble molecules (ethyl alcohol, vitamins A & E, steroids) and small molecules (water,
oxygen, CO2) easily diffuse across the phospholipid bilayer – a process called simple diffusion
–The rate of simple diffusion is a function of the concentration gradient across the membrane,
the size of the molecule, and how easily it dissolves in lipids
• Most water-soluble molecules (ions, amino acids, monosaccharides) cannot move through the bilayer
on their own
–Must have the aid of either channel proteins or carrier proteins – process called facilitated
diffusion
• Most channel proteins have a specific interior diameter and distribution of electrical charge that allow
only particular ions to pass through
• Carrier proteins bind specific molecules from the cytoplasm or extracellular fluid triggering a change
in shape of the carrier that allows the molecules to pass through the protein and across the membrane
without using energy
–Molecules that cross the membrane by facilitated diffusion usually do so more slowly than do
those that cross by simple diffusion
• The diffusion of water across selectively permeable membranes is called osmosis
–Crucial to the functioning of many biological systems
• Water moves across the membrane down its concentration gradient
• Pure water has the highest water concentration
• Any substance added to pure water displaces some of the water molecules
–Results: solution with a lower water content
• Dissolved substances reduce the concentration of free water molecules in a solution - the higher the
concentration of dissolved substances, the lower the concentration of water
• 3 Solutions due to Osmosis
1. Isotonic: equal movement of water into and out of the cell
2. Hypertonic: net water movement out of cells
–Solution outside the cell has a greater concentration than inside the cell – causes the
water to move outside the cell
3. Hypotonic: net water movement into cells
–Solution inside the cell has a greater concentration than outside the cell – causes the
water to move inside the cell
ACTIVE TRANSPORT
• All cells need to move some materials “uphill” across the membrane against concentration gradients
• Every cell requires some nutrients that are less concentrated in the environment than in the cell’s
cytoplasm – diffusion would cause the cell to lose these nutrients
• Other substances must be maintained at much lower concentrations inside the cells than in the
extracellular fluid
• In active transport, membrane proteins use cellular energy to move individual molecules or ions
across the plasma membrane, usually against their concentration gradient
• 2 Active Sites
1. On the face of the plasma membrane in contact with the cytoplasm or one the face in contact
with the extracellular fluid – depending on the transport protein
2. 2nd site is always on the inside of the membrane – binds an energy-carrier molecule (ATP)
that donates energy to the protein that causes it to change shape and move across the membrane
• Active-transport proteins are often called pumps – because they use energy to move molecules
against the concentration gradient
–Vital in mineral uptake by plants
–Mineral absorption in your intestines
–Maintaining concentration gradients essential to nerve cell functioning
• Many cells acquire or expel particles or substances that are too large to diffuse regardless of
concentration gradients
• Cells can acquire fluids or particles from their extracellular environment through the process of
endocytosis (“into the cell”)
–During this process, the plasma membrane engulfs the substance and pinches off a
membranous sac called a vesicle – with the substance inside – and moves it into the cytoplasm
• 3 Types of Endocytosis
1. Pinocytosis – “cell drinking” – moves extracellular fluid into the cytoplasm
2. Receptor-Mediated Endocytosis – selectively moves specific molecules into the cells with
the aid of receptor proteins at coated pit sites
3. Phagocytosis – “cell eating” – large particles are moved into the cytoplasm
–White blood cells use this to engulf and destroy invading bacteria
• Cells often use energy to reverse endocytosis to remove unwanted materials through the process of
exocytosis (“out of the cell”)
–During this process, a membrane-enclosed vesicle carrying material to be expelled moves to
the cell surface, where the vesicle fuses to the membrane then opens to the extracellular fluid and the
contents diffuse out
How Are Cell Surfaces Specialized?
• In multicellular organisms, plasma membranes hold together clusters of cells and provide avenues
that cells communicate with each other
–Depending on the organism and cell type determines the type of connection
• 4 Types of Cell Connections
1. Desmosomes – hold adjacent cells together through proteins and carbohydrates; proteins
filaments attached to the insides of the desmosomes extend into the interior of each cell to further
strengthen the attachment
2. Tight junctions – seals adjacent cells together with strands of protein; membranes nearly
fuse along a series of ridges that form leakproof gaskets between cells – prevents molecules from
escaping between cells
3. Gap junctions – cell-to-cell channels made of protein channels that connect the insides of
adjacent cells
4. Plasmodesmata – opening in the walls of adjacent plant cells that are lined with plasma
membrane and filled with cytoplasm; creates continuous cytoplasmic bridges between the insides of
adjacent cells
• Plasmodesmata is restricted to plant cells
• Some animal cells can posses all three of the other types of junctions – due to the flexible, mobile
organism
• The outer surfaces of the cells of bacteria, plants, fungi, and some protists are covered with nonliving,
stiff coatings called cell walls.
-Plant cell walls are composed of cellulose and other polysaccharides
-Fungal cell walls are made of polysaccharides and chitin.
-Bacterial cell walls have a chitin-like framework to which short chains of amino acids and
other molecules are attached.
• Plant cells secrete cellulose through their plasma membranes and form the primary cell wall
• Later they secrete more cellulose and other polysaccharides beneath the primary wall to form a thick
secondary cell wall – pushes the primary wall away from the plasma membrane
• The primary cell walls of adjacent cells are joined by the middle lamella – a layer made of mostly
pectin
–A polysaccharide that makes jelly solidify
• Cell walls support and protect fragile cells
• Tree trunks are the ultimate proof of cell wall strength
–Composed almost entirely of cellulose and other materials laid down over the years and
capable of supporting impressive loads
• Cell walls are usually porous – permits easy passage of small molecules such as minerals, water,
oxygen, CO2, amino acids, and sugars
• The structure that really governs the interactions that occur between a cell and its external
environment is the plasma membrane.