Cell Structure Revision

Cell Membrane

Introduction The cell membrane: The cell membrane: –surrounds the outside of the cell. –is semi-permeable (allows certain molecules in and out) –Is composed of a double layer (bilayer) of molecules called phospholipids  These lipids have a hydrophilic head part and a hydrophobic tail The cell membrane can be explained using the Fluid Mosaic model. The cell membrane can be explained using the Fluid Mosaic model. –This model says that membrane is a lipid bilayer that has various proteins bobbing in it.

Fluid Mosaic Model

Diffusion and Osmosis

Diffusion Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration. Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration.

The difference in concentration between 2 different areas is called the concentration gradient The difference in concentration between 2 different areas is called the concentration gradient The larger the concentration difference between the 2 areas the steeper the gradient. The larger the concentration difference between the 2 areas the steeper the gradient. –Therefore a smaller concentration difference will have a less steep gradient.

Diffusion is also called passive transport Diffusion is also called passive transport –The cell membrane doesn’t use up any energy to make diffusion happen.

Rate of Diffusion The rate of diffusion depends on: The rate of diffusion depends on: –The size of the molecules –Smaller molecules move faster –The temperature of the liquid or gas –Warmer matter moves faster –The state of the matter that is diffusing –Gases move faster than liquids –The concentration of the chemicals –More concentrated chemicals move faster

Concentration Gradient Membrane A B 65% 90% 95%

Facilitated Diffusion Facilitated diffusion is a form of passive transport that uses transport proteins to help certain molecules across the cell membrane faster. Facilitated diffusion is a form of passive transport that uses transport proteins to help certain molecules across the cell membrane faster.

Summary

Osmosis Osmosis is the net movement of WATER molecules from an area of high water concentration to an area of low WATER concentration through a semi-permeable membrane. Osmosis is the net movement of WATER molecules from an area of high water concentration to an area of low WATER concentration through a semi-permeable membrane. Osmosis is also passive transport Osmosis is also passive transport

Osmosis Definitions Osmosis always refers to the water concentration. Osmosis always refers to the water concentration. Therefore: Therefore: –A solution with a high water concentration is called hypotonic –A solution with a low water concentration is called hypertonic –If both solutions involved in osmosis have the same water concentration, they are called isotonic

Osmosis in Action The process of osmosis works in cells because: The process of osmosis works in cells because: –Cell membranes are semi-permeable –The cytoplasm consists of about 90% water, but the remaining 10% is large molecules that cannot pass through by diffusion

Water will diffuse into a cell when the water outside is at a higher concentration. Water will diffuse into a cell when the water outside is at a higher concentration. –This will make the cell turgid Water will diffuse out of the cell when the outside concentration is less than inside the cell. Water will diffuse out of the cell when the outside concentration is less than inside the cell. –This loss of water in animal cells makes the cell shrink and become flaccid –If this happens to plant cells then the cytoplasm will shrink away from the cell wall  This is called plasmolysis

Osmoregulation The movement of water into a cell can build up pressure. If cells continue to take on water by osmosis they keep swelling till the burst. The movement of water into a cell can build up pressure. If cells continue to take on water by osmosis they keep swelling till the burst. In plants the cell wall prevents this from happening. In plants the cell wall prevents this from happening. This is a problem for unicellular organisms such as amoeba and paramecium who live in fresh water. This is a problem for unicellular organisms such as amoeba and paramecium who live in fresh water. –These organisms have a special contractile vacuole the actively pumps out excess water. –This process of osmoregulation keeps the water content of the organism relatively constant.

Textbook Copy the diagram at the bottom of page 89. Copy the diagram at the bottom of page 89. Using the paragraph beside it – name 3 body systems that need to have the correct osmotic balance Using the paragraph beside it – name 3 body systems that need to have the correct osmotic balance

Revision Question 1. Give the name and function of both organelle A and organelle B. 2. Cell membranes are semi-permeable. Explain what this means. 3. Describe how the structure of a cell membrane is related to its selective permeability AB

Active Transport In certain circumstances molecules can pass in and out of cells by moving from a low to high concentration. In certain circumstances molecules can pass in and out of cells by moving from a low to high concentration. –That is it moves against the concentration gradient. This process is called active transport and uses energy to actively move the molecules. This process is called active transport and uses energy to actively move the molecules.

There are several ways to tell if active transport is occuring: There are several ways to tell if active transport is occuring: –Heat is given off –Oxygen is used up –Carbon dioxide is produced –Glucose is used up Another way to tell whether a lot of active transport is taking place is the number of mitochondria present in the cells. Another way to tell whether a lot of active transport is taking place is the number of mitochondria present in the cells. –Why?

High Low High Low Passive Transport Active Transport Conc

Revision Question 1. What method of transport is responsible for moving sodium ions out of, and potassium ions into, the red blood cell? Explain your answer. 2. When red blood cells are separated from blood plasma and placed into distilled water they burst open. Explain why the blood cells burst open. ionplasmaRed blood cell K+K+ Na Concentrations of sodium and potassium ions in human red blood cells and in the surrounding plasma.

Surface Area to Volume