Biology
Cell Membrane and Transport
NOT drawn to scale!!
Explain how eukaryotic cells are similar to and different from prokaryotic cells. (ON YOUR OWN)
Explain how each eukaryotic organelle/structure (cell membrane, nucleus, mitochondria, chloroplasts, endoplasmic reticulum and plant cell walls, and vacuoles) contribute to the overall function of the cell.
(ON YOUR OWN)
Review all of the cell parts
Cell Membrane and Cell Transport
Learning Outcomes
Understand the fluid mosaic model of membranes
Describe the functions of phospholipids, proteins, and carbohydrates in membranes
Explain what substances are permeable to the cell membrane and how they are transported
describes membranes as a fluid lipid bilayer with floating proteins and carbohydrates.
Fluid – the phospholipid bilayer is viscous and individual phospholipids can move position
Mosaic – the phospholipid bilayer is embedded with proteins, resulting in a mosaic of components
The fluid mosaic model of the plasma membrane structure describes the plasma membrane as a fluid combination of phospholipids, cholesterol, proteins, and carbohydrates
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All membranes in and around a cell are made of a phospholipid bilayer and proteins
Membranes are assemblies of carbohydrates, proteins, and lipids held together by non-covalent forces. They regulate the transport of molecules, control information flow between cells, generate signals to alter cell behavior, contain molecules responsible for cell adhesion in the
formation of tissues, and can separate charged molecules for cell signaling and energy generation.
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Phospholipid Bilayer
The plasma membrane is composed mainly of
phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in the Figure, each phospholipid molecule has a head and two tails. The head “loves”
water (hydrophilic) and the tails “hate”
water (hydrophobic). The
water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell.
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The plasma membrane is a phospholipid bilayer with embedded proteins. There are other components, such as cholesterol and carbohydrates, which can be found in the membrane in addition to phospholipids and protein.
Figure 3.8
Membrane proteins determine functions of cell membranes, including serving as pumps, gates, receptors, cell adhesion molecules, energy transducers, and enzymes.
Carbohydrates covalently linked to proteins (glycoproteins) or lipids (glycolipids) are also a part of cell membranes, and function as adhesion and address loci for cells.
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HIV docks at and binds to the CD4 receptor, a glycoprotein on the surface of T cells, before entering, or infecting, the cell.
Figure 3.19
An example of a glycoprotein that is embedded in the cell membrane of immune cells such as helper T cells.
Found primarily on the surface of CD4 T lymphocytes (CD4 cells). To enter a host cell, HIV binds to a CD4 receptor and a coreceptor (either CCR5 or CXCR4) on the host cell.
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There are many metabolic diseases caused by a recessive allele (version of a gene) that makes a defective protein.
Cystic fibrosis requires constant treatments, and the life expectancy is about 35. It is caused by a defective gene for a chloride channel (a protein).
Transport mechanisms
| Mechanism | Good for… |
| Diffusion across bilayer | Small, nonpolar molecules |
| Facilitated diffusion by membrane proteins | Medium size polar molecules or ions |
| Endocytosis or exocytosis | Large molecules |
| Active transport by proteins | Going against concentration gradient |
This table will be given to you on the exam
Oxygen, Carbon Dioxide, and water are some molecules that enter and exit the cells.
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Figure 3.10 Passive transport.
(a) Diffusion of molecules across the plasma membrane occurs with the concentration gradient and does not require energy. Small hydrophobic molecules, carbon dioxide, and oxygen can diffuse across the membrane.
Transport of medium-sized molecules
Water always moves to “even” out the concentration of solutes on both sides of a membrane.
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Figure 3.10 Passive transport.
(b) Facilitated diffusion is the diffusion of molecules through proteins. Molecules move with their concentration gradient, which does not require energy.
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Figure 3.11 Active transport.
Active transport moves substances against their concentration gradient and requires energy (ATP) to do so.
Transport of LARGE molecules
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Figure 3.12 Movement of large substances.
(a) Exocytosis is the movement of substances out of the cell. (b) Endocytosis is the movement of substances into the cell.
Transport mechanisms
| Mechanism | Good for… |
| Diffusion across bilayer | Small, nonpolar molecules |
| Facilitated diffusion by membrane proteins | Medium size polar molecules or ions |
| Endocytosis or exocytosis | Large molecules |
| Active transport by proteins | Going against concentration gradient |
This table will be given to you on the exam
Oxygen, Carbon Dioxide, and water are some molecules that enter and exit the cells.
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In osmosis, water always moves from an area of higher concentration (of water) to one of lower concentration (of water). In this system, the solute cannot pass through the selectively permeable membrane.
Figure 3.21
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Osmotic pressure changes the shape of red blood cells in hypertonic, isotonic, and hypotonic solutions. (credit: modification of work by Mariana Ruiz Villarreal)
Figure 3.22
1. throughout the cytoplasm
2. from an area with a high concentration of other solutes to a lower one
3. from an area with a low concentration of solutes to an area with a higher one
4. from an area with a low concentration of water to one of higher concentration
Water moves via osmosis ________.
Case study – 2 types of cells
Bioluminescent algae and sperm cells
egg cell many sperm cells
Some living organisms require unique abiotic conditions! Laguna Grande in Fajardo, Puerto Rico
These are two different representations of the reaction that produces bioluminescence in dinoflagellates.
What are the reactants? What are the products?
What is the role of luciferase?
light
Sperm cell case study
Tom and Tami have been married for three years and are ready to start a family, but after a year of trying, Tami was still not pregnant. Their doctor told them that when a couple has trouble conceiving a child, the easiest thing to check first is to determine if the husband is producing semen with a normal number of healthy sperm cells per milliliter (approximately 20-60 million/ml). Semen consists of living sperm cells in a solution of sugars, buffer molecules to regulate the pH, and hormones.
