La nuova biologia.blu Le cellule e i viventi David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum

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 SymporterAntiporter

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