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Chapter 7
Membrane
Structure
and Function
Ā© 2021 Pearson Education, Inc.
Chapter 7
Membrane Structure and Function
Key concepts or Important topics:
- Cell membranes are fluid mosaic model
- Membrane structure and selective permeability
- Membrane lipids
- Membrane proteins
- Membrane carbohydrates
- Membrane cholesterol
- Passive transport
- Diffusion
- Osmosis
- Facilitated diffusion
- Active transport
- Bulk transport
Plasma Membrane:
- Plasma membrane is the outermost covering that separates the cell from its surrounding
- is selectively permeable : allows some substances to cross it more easily than others
Phospholipids
- Phospholipids are the most abundant lipid
in the plasma membrane
- Phospholipids are amphipathic
molecules , containing
- hydrophilic heads (phosphate) on the outside and
- hydrophobic tails (fatty acids) on the inside facing each other - Lipids and proteins are the main components of membranes, but carbohydrates are also important - Membranes are composed mainly of phospholipids - Phospholipids are amphipathic molecules, containing hydrophobic (āwater-fearingā) and hydrophilic (āwater-lovingā) regions
CONCEPT 7.1: Cellular membranes are fluid
mosaics of lipids and proteins
Fluid Mosaic Model
- The fluid mosaic model of membrane
structure
- States that a membrane is a fluid structure (moving and not stationary) with a ā mosaic ā of various proteins embedded in it
- Proteins are not randomly distributed in the membrane, they often form groups that carry out common functions
Figure 7. Current model of an animal cellās plasma membrane
Fluidity of membranes
- Phospholipids in the plasma membrane
- Can move laterally within the bilayer Fig. 7-5a
(a) Movement of phospholipids
Lateral movement ( ļ¾ 107 times per second)
Flip-flop ( ļ¾ once per month)
Do membrane proteins move?
Proteins in the plasma membrane can drift within the membrane
Does the type of fatty acid affect membrane fluidity?
- The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane
- Membranes with more unsaturated fatty acids are more fluid than those rich in saturated fatty acids
Figure 5.5 A
Fluid Viscous
Unsaturated hydrocarbon tails with kinks prevent packing and increase membrane fluidity
Saturated hydrocarbon tails pack together increasing membraneviscosity (a) Membrane fluidity
Presence of cholesterol in animal cells
- resists change in membrane fluidity that can be caused by change in temperature.
Figure 5.5 b
(b) Cholesterol within the animal cell membrane
Cholesterol
Membrane structure is responsible for
permeability of the lipid bilayer
- Hydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and can pass through the membrane easily - Examples, hydrocarbons, CO 2 , O 2 can easily pass through the membrane
- Hydrophilic (polar) molecules such as sugars, amino acids, ions do not cross the membrane easily - Transport proteins allow passage of hydrophilic substances across the membrane
Passive transport is diffusion of the substance across the membrane without using energy
- Diffusion is the tendency for molecules to spread out evenly into the available space
- 3 Types of Passive Transport
- Diffusion
- Osmosis
- Facilitated diffusion
Passive Transport
Diffusion
- Movement of molecules from a more concentrated region to a less concentrated region
- movement down the concentration gradient
- no work is done and energy is not required
Figure 7. Osmosis: Diffusion of water Hyptonic solution to Hypertonic solution
Hypotonic Hypertonic
Hypotonic Hypertonic
Osmosis
- Osmosis is the movement of water across a selectively permeable membrane
- Water diffuses across a membrane from the region of lower solute concentration ( hypotonic ) to the region of higher solute concentration ( hypertonic )
Ā© 2016 Pearson Education, Inc.
Hypotonic solution Isotonic solution Hypertonic solution
Lysed Normal Shriveled
H 2 O H 2 O H 2 O H 2 O
(a) Animal cell
Cells gain water and burst Hemolysis of Red blood cells
Equal exchange of water. No net loss of water from the cell
Cells lose water and shrivel Crenation of Red blood cells
Osmosis in Animal Cells Cell Survival depends on water uptake and loss
Tonicity is the ability of a solution to cause a cell to gain or lose water Three types of solutions:
1. Isotonic solutions: Solute concentration is the same as inside the cell; no net water movement across the plasma membrane 2. Hypertonic solutions: Solute concentration is greater than that inside the cell; **cell loses water
- Hypotonic** solutions: Solute concentration is less than that inside the cell; cell gains water
Water Balance of Cells Without Cell Walls
Plasma membrane
H 2 O
Flaccid Plasmolyzed
H 2 O H 2 O H^2 O
Turgid (normal)
Plasma membrane
Cell wall
(b) Plant cell
Osmosis in Plant cells
Hypotonic solution Isotonic solution^ Hypertonic solution
In hypotonic solutions, cells gain water and become turgid
In Isotonic solution, equal exchange of water. No net loss of water from the cell
In hypertonic solutions, plant cells lose water , shrivel, membrane pulls away from the cell wall and causes the plant to wilt resulting in Plasmolysis
Ā© 2016 Pearson Education, Inc.
- Osmoregulation , the control of solute concentrations and water balance, is a necessary adaptation for life in such environments
- The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that can pump excess water out of the cell
Figure 5.
Contractile vacuole 50 ą« m
Osmosis
If a marine algal cell is suddenly transferred from seawater to freshwater, the algal cell will initially
A. lose water and decrease in volume. B. stay the same: neither absorb nor lose water. C. absorb water and increase in volume.
Facilitated Diffusion: Passive Transport with the help of Proteins
- In facilitated diffusion , transport proteins speed the movement of molecules across the plasma membrane - No energy is used
- Transport proteins: Specific for the substances they move
- Channel proteins:
- Aquaporin , for facilitated diffusion of water
- Ion channels that open and close
- Carrier proteins
- bind to molecules and change shape to shuttle them across the membrane
Figure 7. Two types of transport proteins that carry out facilitated diffusion
Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria. Amino acid cysteine accumulates & causes painful stones.
In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle In pinocytosis , molecules are taken up when extracellular fluid is āgulpedā into tiny vesicles
Exploring endocytosis in animal cells Receptor Mediated Endocytosis Receptor proteins bound to specific solutes from the extracellular fluid are clustered in coated pits that form coated vesicles Human cells use receptor-mediated endocytosis to take in cholesterol, which is carried in particles called low-density lipoproteins (LDLs) Individuals with familial hypercholesterolemia have missing or defective LDL receptor proteins Cholesterol accumulates in the blood, building up lipids and narrowing the space in the blood vessels , resulting in potential heart damage or stroke
Concept Check
Cells (e.g., bacteria) are taken up by other cells (e.g., an immune cell) by which of the following? A. pinocytosis B. exocytosis C. receptor-mediated endocytosis D. phagocytosis E. facilitated diffusion
You should be able to:
- Define the following terms: amphipathic molecules, aquaporins, diffusion
- Explain how membrane fluidity is influenced by temperature and membrane composition
- Distinguish between the following pairs or sets of terms: peripheral and integral membrane proteins; channel and carrier proteins; osmosis, facilitated diffusion, and active transport; hypertonic, hypotonic, and isotonic solutions
- Explain how transport proteins facilitate diffusion
- Explain how large molecules are transported across a cell membrane
