Cells and Their Environment

Structure of the Plasma Membrane

The plasma membrane (cell membrane) is made of two groups of organic molecules--phospholipids and proteins.

A phospholipid is a molecule shaped like a head with two tails. The head is polar and the two tails are nonpolar.

Water molecules form hydrogen bonds with the polar heads, but push away the nonpolar tails, thus forming a two-layer structure with the heads facing outward and the tails facing inward. This is the plasma membrane.

The plasma membrane is fluid, like a soap bubble and has various proteins stuck in it.

Polar head

Nonpolar tails

Outside of Cell

Inside of Cell Channel protein Receptor protein Marker protein

Cell Surface Proteins The proteins within the plasma

membrane of cells are called cell surface proteins. There are several kinds.

Channel proteins have a structure that makes them doughnut shaped, with a hole in the middle. Polar sugars, amino acids, ions, and other particles can pass through these channels.

Receptor proteins transmit information from the world outside the cell to the cell’s interior. The outside part of a receptor protein fits only a particular type of molecule. If that molecule attaches to a receptor, the shape of the other end of the receptor is changed, thus passing “information” inside the cell.

Marker proteins have long carbohydrate arms attached to them that identify the cell to other cells.

Channel protein Receptor Protein

Marker protein with carbohydrate arm

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Moving Materials Into and Out of Cells

Materials can move through the cell membrane without using any of the cell’s energy. This is called passive transport.

One kind of passive transport is diffusion. Particles in a solution tend to move from an area of greater concentration to an area where there are less of them (a lesser concentration). This movement is called diffusion.

Water also moves from a greater to a lesser concentration. When water travels across a membrane from a greater to a lesser concentration of water, this is called osmosis.

Cell

Cell’s Environment

Water molecule - will move into the cell

Solute molecule - will move out of the cell.

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Water and Solution Concentrations

Hypotonic Solution- the environment’s solute concentration is less than inside the cell. Water will move into the cell.

Hypertonic Solution- the environment’s solute concentration is greater than inside the cell. Water will move out of the cell.

Isotonic Solution- the solute concentration is the same inside and outside the cell. Water movement into the cell equals movement out of the cell. No net water movement.

Water Molecule Solute Molecule

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Facilitated Diffusion Facilitated diffusion is the

movement of specific particles through a membrane by a channel protein.

The molecule that is to be transferred locks onto the channel protein on the outer surface of the cell membrane and then passes through the channel to the inside of the cell.

The plasma membrane is selectively permeable. It allows some particles to pass through and prevents the passage of other particles.

Many of the selective protein channels through the plasma membrane are two-way channel proteins called pores. Different particles fit into different sized pores.

Interior of Cell

Plasma Membrane

Channel Protein

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Active Transport Cells must be able to take in

molecules that are in low concentration outside of the cell. To do this, cells need to spend energy. These processes are called active transport.

Proton pumps carry protons (H+ ions) across membranes to make ATP, the energy currency of the cell. The active transport of protons to make ATP is called chemiosmosis.

Sodium-potassium pumps use some of the ATP produced by the proton pumps to build up a large concentration of Na+ ions outside of the cell.

Coupled channels carry the sodium ions back into the cell, along with some food molecules, that cannot pass through the plasma membrane by themselves.

Sodium ions along with food molecules enter the cell through coupled channels.

Energy from food molecules is used to drive

Proton pumps that cause the production of ATP. ATP molecules drive

Sodium-potassium pumps, which transport sodium ions back outside the cell.

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Endocytosis and Exocytosis

Food particles that are too large to go through protein channels in the cell membrane enter the cell by endocytosis.

In endocytosis, the cell surrounds and then engulfs the food particle. This requires energy. There are two kinds of endocytosis. If the material being engulfed is liquid, the endocytosis is called pinocytosis. If the material is solid, the process is called phagocytosis.

The opposite of endocytosis is exocytosis. Cells use this process to get rid of wastes. Gland cells dump hormones into the blood by exocytosis.

Endocytosis

Exocytosis

1. 2. 3.

1. 2. 3.

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How Cells Communicate Your cells communicate with

each other directly and indirectly to coordinate growth, development, and other activities.

Direct communication happens between some cells. These cells have tiny openings in their cell membranes called gap junctions, that let the cytoplasm of the two cells join together.

Indirect communication between cells is done two ways--through the endocrine system and the nervous system.

The endocrine system communicates using chemicals called hormones.

Nerve cells communicate using chemicals called neurotransmitters.

Special proteins on the cell’s surface, called receptors, signal the cytoplasm when a particular particle has bumped into the cell’s surface.

Cell #1 cytoplasm

Cell Membrane

Cell Membrane

Cell #2 cytoplasm

Gap Junction

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How Cells Receive Communications

The binding of a signal particle to a cell’s surface receptor can influence the cell’s cytoplasm in one of three ways:

The receptor can become an enzyme that chemically changes molecules in the cell’s cytoplasm.

The receptor can create a second messenger which will have an effect somewhere else in the cytoplasm.

The receptor can open a channel through the plasma membrane. Neurotransmitters do this.

Some channel proteins can be opened or closed. They are called gated channels. Gated channels may be opened either chemically or electrically.

Cell cytoplasm

Cell Membrane

Gated channel is closed. Ions cannot enter.

Na+

Na+

Signal molecule

Signal molecule

Cell Membrane

Signal molecule changes shape of gate, which opens. Ions pass through gate to cytoplasm.·