Unit 2A Human Form & Function Cells, metabolism & regulation Transport

Further information Further information about this topic can be found in Our Human Species (3rd edtn) Chapter 3, section 5

The cell membrane Copyright - National Institute of Standards and Technology Drawing by Dana Burns

The cell membrane The cell membrane (or plasma membrane) is the outer boundary of a cell. The membrane is a phospholipid bi-layer. Proteins and other molecules are embedded in the membrane. The cell membrane is selectively permeable – i.e. it allows some substances to cross more easily than others.

The bi-layer TISSUE FLUID CYTOPLASM Hydrophilic phosphate Membrane protein TISSUE FLUID Hydrophobic lipid tail PHOSPHOLIPID BI-LAYER CYTOPLASM

Diffusion through the membrane Because the cell membrane is fatty, most water soluble substances cannot diffuse through it. Exceptions include oxygen & carbon dioxide.

Membrane proteins A variety of proteins are embedded in the bi-layer. These serve various functions including the movement of substances in and out of the cell.

Membrane transport proteins Membrane proteins that aid the movement of substances in and out of the cell include: Channel proteins (ion channels) – open channels that allow simple diffusion. Carrier proteins that allow facilitated diffusion (e.g. glucose) and active transport (specific membrane pumps).

Membrane transport Transport processes are either passive or active. Passive processes require no cellular energy and include diffusion, osmosis & facilitated diffusion. Active processes require ATP and include specific membrane pumps and phagocytosis/pinocytosis.

Passive processes Require no cellular energy (ATP). Substances move from high concentration to low concentration. E.g. – diffusion, facilitated diffusion & osmosis.

Diffusion Diffusion is the tendency for particles to move from an area of higher concentration to an area of lower concentration. Diffusion results from the random motion of atoms and molecules due to their kinetic energy. Diffusion involves the movement of atoms & molecules in gases and liquids (specifically, solutes).

Diagram created by LadyofHats Diffusion Simple diffusion is the movement of particles from an area of high concentration to an area where their concentration is lower. Diagram created by LadyofHats

Concentration gradient When the concentration of a substance is different at two places, the substance will diffuse along the concentration gradient until the concentration of the two areas becomes equal. High concentration Diffusion gradient Low concentration

Rate of diffusion slows Uniform concentration Concentration at A Concentration at B 1 Diffusion gradient High concentration Low concentration Rate of diffusion slows 2 Equilibrium 3 Uniform concentration

Osmosis This is the diffusion of water across a selectively permeable membrane. Water molecules move from high concentration to low concentration. This is a passive process (does not need ATP).

Osmosis Net water movement from high conc. To low conc. Low water concentration (concentrated soln.) High water concentration (dilute soln.) Net water movement from high conc. To low conc. Semipermeable membrane

Facilitated diffusion Facilitated diffusion is a process of diffusion where molecules diffuse across cell membranes with the assistance of transport proteins. Diffusion takes place from high concentration to low concentration and does not require ATP. Examples: glucose & amino acids.

Simple diffusion (left) & facilitated diffusion Diagram created by LadyofHats

Active transport Require energy (ATP). Substances move from low concentration to high concentration (i.e. against the concentration gradient). Example: membrane pumps.

Active transport (sodium pump) Diagram created by LadyofHats

Endocytosis Pinocytosis & phagocytosis are specific types of endocytosis. Both processes involve cells absorbing large particles such as proteins (or even whole organisms, such as bacteria & viruses) from the outside by engulfing them with their cell membrane to form a vesicle (like a bubble) within the cytosol.

Endocytosis & exocytosis Endocytosis = things entering the cell Phagocytosis = cell eating Pinocytosis = cell drinking Exocytosis = things leaving the cell

Diagram created by LadyofHats Endocytosis Diagram created by LadyofHats

Diagram created by LadyofHats Exocytosis Diagram created by LadyofHats

Study Guide Read: Our Human Species Chapter 3, section 5 Complete: Workbook Topic 4 - Transport