Movement through the cell membrane

Characteristics of the cell membrane 1.The cell membrane is a double layer called a phospholipid bilayer 2. The electron microscope has revealed that the cell membrane is composed of layers of protein and lipids

3.The cell membrane is selectively permeable

A selectively permeable cell membrane is one that allows certain molecules to pass through it and it has some ability to select what can and cannot pass through

The method used by a cell to pass molecules in or out is determined by the size of the molecule: If molecules are very small AND CAN dissolve, then they can pass through the phospholipid bilayer If the molecules are too big AND CANNOT dissolve, then they CANNOT pass through

The small soluble molecules that CAN PASS through the cell membrane are: 1. glucose 2. oxygen 3. Carbon dioxide 4. water

The larger molecules that CANNOT PASS through the cell membrane are: 1. starch 2. fats 3. proteins They have to be digested first, then they can go through the cell membrane

Transport across the cell membrane There are 2 main ways that materials move across the cell membrane: 1. Passive transport 2. Active transport

What is passive transport? Passive transport means when small molecules move automatically, without energy, from an area of high concentration to an area of low concentration

Why does passive transport occur across the cell membrane? To maintain dynamic equilibrium What is dynamic equilibrium? When there is an equal amount of molecules on both sides of the cell membrane

Types of Passive transport: 1. simple diffusion 2. facilitated diffusion 3. osmosis

Simple diffusion is the movement of oxygen or carbon dioxide across the cell membrane with no energy and no “assistance”

Facilitated diffusion is a form of transport that uses transport proteins to move sugar molecules across the cell membrane

The movement of water molecules across a cell membrane is called OSMOSIS

Diffusion lab mini preview In our diffusion lab you will see passive transport of glucose molecules and Iodine molecules moving across the model of a cell membrane

Diffusion lab mini preview You will also witness osmosis of a red onion cell by using different types of solutions: 1. hypertonic solution 2. hypotonic solution

Saltwater is an example of hypertonic solution Solutions Hypertonic : a strong solution; when water has extra amount of solutes in it Saltwater is an example of hypertonic solution

What does hypertonic solution do to the cell? 1. it dehydrates the cell 2. water molecules move out of the cell 3. the cell shrinks 4. the cell membrane pulls away from the cell wall {plasmolysis}

Hypotonic : when water has low or no solutes in it Distilled water is an example of a hypotonic solution

What does hypotonic solution do to the cell? 1.Osmosis occurs, water molecules move into the cell 2.The cell expands 3.Then the cell membrane push against the cell wall 4.Making the cells firm again

Isotonic: when 2 solutions are the same strength Isotonic solutions have the same concentration as the surrounding cells Does not affect the cell; the cell remains the same

Red Onion Observation Before and after observations of red onion epidermis under the microscope (400X) Red onion under in hypertonic (salt) solution. Note cell membrane has “withdrawn” and the cytoplasm has lost water to the salty environment, making it appear smaller and darker. Red onion under in isotonic (normal) solution. Note cell membrane and cytoplasm almost completely “fill” the boundary of the cell wall.

Active transport This type of movement uses cellular energy {ATP} to force materials to go across the cell membrane

Active transport Active transport moves LARGE molecules from areas of lower concentration to areas of higher concentration It requires energy{ATP} This is the opposite of passive transport

The concentration of potassium is higher in red blood cells than in the surrounding blood plasma. This higher concentration is maintained by the process of ACTIVE TRANSPORT

Exocytosis: substances moving out the cell Endocytosis: substances moving into the cell