Biology I The Cell and its Environment Homeostasis

What is homeostasis? Organisms must constantly make adjustments to stay in balance with their environment. The balance between an organism and its surroundings is called homeostasis. Muscles help regulate body temperature. Contracting muscles produce heat. That is why when you’re very cold, you shiver. When humans sweat, the moisture released on the skin's surface cools as it evaporates and in this way, humans can lose excess heat from their body. Unlike a human sweat system, a dog's body can only cool itself mainly through an evaporation system that involves the dog's tongue. Although a dog's body does have sweat glands located on the pads on a dog's feet and ears, the dog's body is cooled off mainly through its tongue.

Homeostasis also occurs on a cellular level. Cells are affected by their surroundings as materials move in and out through the cell membrane. Fresh water plants cannot survive in salt water and salt water plants cannot survive in fresh water.

Animal cells are also affected by their surroundings. Red blood cells require a certain environment in which to survive. When salt water is added to red blood cells they will shrink. Cells in their normal Environment. When cells are placed in distilled water they will swell and burst.

Homeostasis greatly depends on the movement of materials in and out of the cell. Cell membranes help organisms maintain homeostasis by controlling what substances may enter or leave cells. The cell membrane is located at the outer edge of the cytoplasm and separates one cell from another and from surrounding fluids. Nutrients, oxygen and water enter into the cell and wastes are removed. The cell membrane is semi or selectively permeable. This means that some substances can move in and out of cell, and others cannot.

Types of Cell Transport Passive Transport • Diffusion • Osmosis • Facilitated Diffusion Active Transport • Endocytosis • Phagocytosis • Pinocytosis • Exocytosis

Passive Transport Diffusion Osmosis Facilitated Diffusion

• Passive Transport: process by which substances can cross the cell membrane without any input of energy by the cell

Diffusion • Simplest type of passive transport • The movement of molecules from an area of higher concentration to an area of lower concentration. – Concentration gradient: the difference in the concentration of molecules across a distance. • Driven by the molecules’ kinetic energy (constant motion of the molecules bumping into each other)

Molecules naturally tend to move “down” their concentration gradient as they spread out and reach equilibrium.

• Equilibrium: the concentration of molecules is the same throughout the space they occupy. – Even at equilibrium, random molecule movement continues.

Diffusion Across a Membrane / Simple Diffusion • If a molecule can pass through a cell membrane, it will diffuse from an area of higher concentration on one side to an area of lower concentration on the other side. • Depends on: – Size: very small molecules may pass through pores made by integral proteins. – Type of Molecule: substances such as CO 2 and O 2 are nonpolar and can dissolve in lipids. – Chemical Nature of the Membrane

• Depends on: – Size: very small molecules may pass through pores made by integral proteins. – Type of Molecule: substances such as CO 2 and O 2 are nonpolar and can dissolve in lipids. – Chemical Nature of the Membrane

Osmosis • A solution is composed of a solute dissolved in a solvent. – Ex. Sugar (solute) in water (solvent) – In cells, organic and inorganic compounds are the solutes and water is the solvent. – Both solutes and solvents can diffuse.

• Osmosis: the process by which water molecules diffuse across a cell membrane from an area of higher concentration to an area of lower concentration

Direction of Osmosis • The net direction of osmosis depends on the relative concentration of solutes on the two sides of the membrane.

• Hypotonic Solution: when the concentration of solute molecules outside the cell is lower than the concentration inside the cell. Water diffuses into the cell until equilibrium is established.

• Hypertonic Solution: when the concentration of solute molecules outside the cell is higher than the concentration in the cytosol. Water diffuses out of the cell until equilibrium is established.

• Isotonic Solution: when the concentration of solutes outside and inside the cell are equal. Water diffuses into and out of the cell at equal rates. No net movement of water.

• Ex. Many unicellular freshwater organisms. Paramecium have contractile vacuoles (organelles that remove excess water using active transport) The paramecia shown live in fresh water, which is hypotonic to their cytoplasm. Contractile vacuoles collect excess water that moves by osmosis into the cytoplasm. The vacuoles then contract, returning the water to the outside of the cell.

These two photographs show cells in the skin of a red onion. In a hypotonic environment, the cells are pressed against the cell walls. In the hypertonic environment, the cells contract and pull away from the cell walls.

In an environment that is isotonic to the cytoplasm, a human red blood cell keeps its normal shape – round and dimpled. In a hypotonic environment, the cell gains water and swells. In a hypertonic environment, the cell loses water and becomes shriveled Cell Membrane Video. Egg Video

Facilitated Diffusion • Process used for molecules that cannot readily diffuse through cell membranes even when a concentration gradient exists. – May not be soluble in lipids, or too large to pass through pores. • Specific carrier proteins assist movement of molecules. Different carrier proteins are used for each different type of substance.

Active Transport • Movement of materials “up” their concentration gradient from an area of low concentration to an area of higher concentration. • Requires a cell to expend energy • Picture • Video

Sodium/Potassium pump • pumps out 3 sodiums for every 2 potassium's taken in against gradient • A huge amount of energy in our bodies is used to power this pump and prevent sodium from building up within our cells. • What would happen if you had too much sodium in your cells?

Animation • http: //highered. mheducation. com/sites/007 2495855/student_view 0/chapter 2/animatio n__how_the_sodium_potassium_pump_w orks. html

Endocytosis • The process by which cells ingest external fluid, macromolecules, and large particles, including other cells. • External materials are enclosed by a portion of the cell’s membrane, which folds into itself and forms a pouch that is pinched off into a membrane-bound organelle called a vesicle.

Two Main Type of Endocytosis • Based on the kind of material that is taken in. • Pinocytosis: involves the transport of solutes or fluids. • Phagocytosis: the movement of large particles or whole cells. (ex. feeding by unicellular organisms, immune system cells called phagocytes ingest virus and bacteria) • http: //vimeo. com/70326148 • https: //www. youtube. com/watch? v=Jnl. UL Oj. Uh. SQ

Two Main Type of Endocytosis • Based on the kind of material that is taken in. • Pinocytosis: involves the transport of solutes or fluids. • Phagocytosis: the movement of large particles or whole cells. (ex. feeding by unicellular organisms, immune system cells called phagocytes ingest virus and bacteria) • Video

Exocytosis • The process by which a substance made by the cell is released from the cell through a vesicle. • Reverse of endocytosis.

• Used to release large molecules such as proteins waste products, or toxins that would damage the cell if they were released within the cytoplasm. • Used by nervous and endocrine system cells to release small molecules that control the activities of other cells (hormones, neurotransmitters, etc. )