The Cell Membrane and Mechanisms of Cell Membrane Transport

Biology 11

homeostasis

• The internal environment of an organism (cells) remains unchanged, even when there are changes to the external environment. This balance is called homeostasis.

• the cell membrane is responsible

selective permeability

• The property that permits the cell to allow entry of certain substances while denying it to others. 

• The cell membrane controls the passage of solutes, nutrients and wastes.

• This permeability varies between organisms and cells.

plasma membrane

• Describes the structure of the plasma membrane as a mosaic of: 

phospholipids, cholesterol, proteins, and carbohydrates

• These components give the membrane a fluid character (wavelike movements).

• Plasma membranes range from 5-10 nm thickness (that’s about 10-20 large atoms thick!). 

  • This is small...For comparison, human red blood cells are approximately 1,000 times wider than a plasma membrane. 

    The proportions of proteins, lipids, and carbohydrates in the plasma membrane vary with cell type. 

  • For example, the inner membrane of a mitochondria contains 76% protein and 24% lipid while some nerve cell types are 76% lipid and 18% protein. 

phospholipid structure

• Made up of a phospholipid bi-layer with proteins embedded throughout.

• The two layers of phospholipids contain a polar head which attracts water.  

• The tail is nonpolar and repels water.

• Water is part of the fluid that surrounds the cell as well as being inside.

• The proteins help move materials across the cell membrane.

what is 30% of the membrane surface covered by

• About 30% of the membrane surface is covered by proteins such as protein channels, peripheral proteins, or integral proteins. Integral proteins, ex. Carrier proteins, gated channels, help move molecules in and out of the cell. 

what can pass through the plasma membrane and what can’t

can: gases (O2), hydrophobic molecules (Benzene), and small polar molcues (H2O)

cant: large polar molucules (glucose) and charged molecules (Cl-)

glycoprotein and glycolipid

protein with carb attached

lipid with carb attached

passive transport

the movement of materials across a cell membrane without using energy.

active transport

the movement of materials from an area of lower concentration to an area of higher concentration. (This requires energy. You are pushing against the high to low gradient).

three types of passive transport

1. Diffusion 

2. Osmosis

3. Facilitated Diffusion 

diffusion

• The random movement of molecules (called Brownian motion) that results in molecules of high concentration moving to areas of low concentration.

• Diffusion rate depends on factors such as size of molecule, shape, concentration, charge, temperature, and, whether it is lipid soluble. 

• Diffusion takes place until the molecules are evenly spread throughout the space (equilibrium).    ***Think of spraying room spray…scent molecules travel to areas with low concentration of molecules … so high to low. 

• Ex: Blood with a high concentration of O₂ travels to the cells throughout the body. 

• The cells have used their O₂ so they have a low concentration. 

• The oxygen diffuses from the blood to the cells continuously.

osmosis

the diffusion of water through a selectivley permeable membrane

isotonic

1. the solute concentration inside 

   and outside the cell are equal.

2. equillibirum, no net movement

hypotonic

lower concentration of solute outside the cell than inside

water moves into the cell, the cell expands

hypertonic

greater concentration of solute outside the cell than inside

water moves out of the cell, cell shrinks

turgor pressure

turgor pressure is a type of water pressure that applies only to plant cells

turgor pressure pushes the cytoplasm of the plant cell against the cell wall, it is the reason that plants are rigid

hypotonic: water moves into the cell, turgor pressure increases

hypertonic: water moves out of the cell, turgor pressure decreases

facilitated diffusion

• Considered another form of passive transport (no cell energy required)

• Only small uncharged nonpolar molecules, such as oxygen, can diffuse easily across the membrane. 

• Some particles require transport proteins to act entry ways to and from the cell.

3 transport proteins

1. channel proteins

2. carrier proteins

3. gate proteins

channel proteins

1. (sometimes called “pores”): Different channel proteins allow different ions to enter the cell depending on their size and charge.

2. Provide water-filled passages through which small dissolved particles, such as ions, can diffuse with the concentration gradient

carrier proteins

Particles of a specific shape fit into one side of a specific protein. If it fits, the protein changes shape to allow the particle to move to the other side of the membrane.

gate proteins

signal particles (hormones) attach to the protein which opens the gate to allow other particles (ions) to enter.

active transport

• Movement of particles from an area of low concentration to an area of high concentration.

• Requires cellular energy to move the particles against the natural gradient.

endocytosis

the process by which the plasma membrane engulfs a particle and takes it into the cell.

pinocytosis

liquid droplets are taken into the cell from the extracellular fluid.

phagocytosis

the cell membrane reaches out and engulfs a large solid particle to bring it into the cell. The cell forms a vesicle around the particle. This is common in some unicellular organisms and white blood cells.

receptor aided endocytosis

molecules outside the cell link up with receptors on the plasma membrane. The receptors and the molecule move to one area of the cell. An indentation forms in the membrane eventually breaking away into a vesicle. The vesicle still containing the receptors returns to the cell membrane. (hormones can work this way).

exocytosis

the process by which the cell gets rid of particles or substances. The particles contained in a vesicle move to the plasma membrane, fuse with it and then it expels its contents into the environment of the cell.

types of proteins embedded in the plasma membrane

peripheral membrane protein

integral membrane protein