Cell Membrane & Homeostasis

Cell Membrane All cells are surrounded by a thin, flexible barrier known as the cell membrane. Plant cells also have a strong supporting layer around the membrane called the cell wall. The cell membrane regulates what enters and leaves the cell and also provides protection and support.

Cell Membrane The cell membrane is made of phospholipids arranged in a double- layered sheet called a lipid bilayer. This means there are two layers of lipids. A lipid bilayer gives cell membranes a flexible structure that forms a strong barrier between the cell and its surroundings.

Cell Membrane Each phospholipid molecule has a polar phosphate head that is hydrophillic (attracts water) and a non-polar fatty acid tail that is hydrophobic (repels water).

Cell Membrane In addition to lipids, most cell membranes also contain protein molecules that act as channels or pumps. These channel proteins help move materials across the cell membrane. They can also contain carbohydrate chains that act like chemical identification cards that allow cells to identify each other. Cholesterol is another molecule found in the cell membrane. It adds flexibility to the cell membrane.

Cell Membrane

HOmeostasis Homeostasis is the maintenance of a stable internal environment, even though the outside environment is constantly changing. The selective permeability of the cell membrane is closely associated with homeostasis, because it regulates particle flow into or out of the cell. Your body has certain mechanisms it uses to keep cells in a stable environment.

HOmeostasis Controlling body temperature All mammals maintain a constant body temperature, this is called being endothermic. Human body temperature is around 37oC. Your body must maintain this temperature if you are in a hot environment or a cold environment. What mechanisms are there to cool the body down?

HOmeostasis Sweating Vasodilation When your body is hot, sweat glands release sweat, which evaporates and cools down your body. Vasodilation Your blood carries most of the heat energy around your body. There are capillaries under your skin that fill with blood if you get too hot. This brings the blood closer to the surface of the skin so more heat can be lost and cools you down.

HOmeostasis What mechanisms are there to warm the body up? 1. Vasoconstriction This is the opposite of vasodilation. The capillaries under your skin constrict (shut off), this takes the blood away from the surface of the skin so less heat can be lost. 2. Shivering and goose bumps This is when the hairs on your skin stand up. The hairs trap a layer of air next to the skin which is then warmed by the body heat. You shiver to create movement in your body. This muscle movement results in heat generation.

HOmeostasis Controlling glucose levels Your cells need an exact level of glucose (sugar) in the blood. Excess glucose gets turned into glycogen and is stored in your liver for later use. This is regulated by two hormones produced by the pancreas called insulin and glucagon. If there is too much glucose in the blood, insulin converts some of it into glycogen for storage. If there is not enough glucose in the blood, glucagon converts some glycogen into glucose.

HOmeostasis Diabetes is a condition in which a person’s body does not produce enough, or any, insulin. When they eat food, the glucose levels in their blood cannot be reduced, so diabetics sometimes have to inject insulin into their blood.

HOmeostasis Controlling water and waste levels The kidneys control the water levels in the body and are closely linked to the excretion of urine. Urea is a waste product that is made when the liver breaks down proteins that are not needed by the body. Urea is mixed with water filtered from the blood to create urine.

HOmeostasis Controlling pH level When our bodies are active, our muscles require more oxygen. By using more oxygen, we create more carbon dioxide as a waste product which can cause our pH to become more basic than neutral. This is why we breathe heavily during exercise: to get more oxygen in for our cells to use and to get rid of excess carbon dioxide and bring our pH back to 7.

Transport through the cell membrane & atp 3/6/2012 Transport through the cell membrane & atp

Transport through the cell membrane Every living cell exists in a liquid environment. One of the most important functions of the cell membrane is to move dissolved molecules from the liquid on one side of the membrane to the liquid on the other side. Remember: a solution is a mixture of two or more substances. The substances dissolved in a solution are called solutes. The concentration of a solution is the mass of the solute in a given volume of solution, or mass/volume.

Diffusion Diffusion = high  low In solutions, particles tend to move from an area where they are more concentrated to an area where they are less concentrated in a process known as diffusion. Diffusion = high  low When the concentration of the solute is the same throughout the system, the system has reached equilibrium. The concentrations on both sides of the membrane are equal.

diffusion The process of diffusion does not require the cell to use any energy because it depends upon random particle movements. Any time particles move with the concentration gradient, they move without using energy from high concentration to low concentration. If a substance can diffuse across a membrane, the membrane is said to be permeable to it. A membrane is impermeable to substances that cannot pass across it. Most biological membranes are selectively permeable, meaning some substances can pass across them and some cannot.

osmosis Osmosis is the diffusion of water through a selectively permeable membrane. In this example, the membrane is permeable to water but not to sugar. This means that at first, the concentration of water is lower on the right side and higher on the left. Osmosis occurs, which means that water crosses the membrane to make the water concentrations on each side equal = equilibrium.

Facilitated diffusion Some molecules pass through cell membranes very quickly by using protein channels. These cell membrane proteins are said to facilitate, or help, the diffusion of the molecule across the membrane. This process is called facilitated diffusion. This process does not use cell energy, since it is still a type of diffusion in which particles move from an area of high concentration to an area of low concentration. Facilitated Diffusion = high  low Since diffusion, facilitated diffusion, and osmosis do not require energy, they are considered types of passive transport.

Active Transport Active transport, however, requires energy. Molecules are moved across a cell membrane using transport proteins or pumps found in the membrane. A lot of the energy used by our cells is used to power active transport because it moves molecules from areas of low concentration to areas of higher concentration. Active Transport = low  high (using energy)

Endocytosis & exocytosis Sometimes very large molecules need to be moved into or out of cells that are too big to cross the membrane. These molecules are moved in different ways than the smaller particles we have been discussing.

Endocytosis & exocytosis Large molecules are moved into a cell by a process called endocytosis. The cell membrane makes a fold or a pocket that forms a vacuole that takes up a molecule. Large molecules are moved out of a cell by exocytosis, in which a vacuole pushes molecules out of the cell. This occurs in the reverse direction of endocytosis.

Energy for life processes All living things use chemical fuels. One of the main compounds that cells use to store and release energy is called adenosine triphosphate, or ATP. ADP is a compound that looks just like ATP, only with two phosphate groups instead of three.

Energy for life processes When a cell has energy, it can store it by adding a phosphate group to ADP, producing ATP. The energy that is stored in ATP is released by breaking the bond between the 2nd and 3rd phosphate groups, turning ATP back into ADP.

Energy for life processes

Energy for life processes What do we use the energy stored in ATP for? Cells use this energy to carry out active transport to move molecules across the cell membrane. ATP also powers the synthesis of proteins and nucleic acids and in some organisms can produce light.

Energy for life processes ATP can only store enough energy to last for a few seconds of activity. ATP is great for storing energy to transfer it somewhere else, but organisms use carbohydrates and lipids to store large amounts of energy.