Cells

:CELLS: All living organisms on Earth are divided in pieces called cells. Cells are small compartments that hold all of the biological equipment necessary to keep an organism alive and successful on Earth.

:CELLS: A main purpose of a cell is to organize. Cells hold a variety of pieces and each cell has a different set of functions. It is easier for an organism to grow and survive when cells are present. If you were only made of one cell, you would only be able to grow to a certain size. You don't find single cells that are as large as a cow. Also, if you were only one cell you couldn't have a nervous system, no muscles for movement, and using the internet would be out of the question. The trillions of cells in your body make your life possible.

Plant vs. Animal Cells • There are many types of cells. In biology class, you will usually work with plant-like cells and animal-like cells. We say animal-like because an animal type of cell could be anything from a tiny microorganism to a nerve cell in your brain. Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall; animals do not. Plants also have organelles like the chloroplast (the things that make them green) or large water-filled vacuoles.

Organelles Cells have organelles that carry out various life processes. Organelles are structures that perform specific functions within the cell. Different types of cells have different organelles.

Cell Wall - What's it for? • A cell wall is a rigid structure that gives support to a cell. The cell wall is the outermost structure of a cell. Plants and algae have cell walls made of cellulose (SEL yoo LOHS) and other materials. Cellulose is a complex sugar that most animals can’t digest. The cell walls of plant cells allow plants to stand upright. • In some plants, the cells must take in water for the cell walls to keep their shape. When such plants lack water, the cell walls collapse and the plant droops.

Cell Membranes We have been talking about cells being a unit of organization in biology. Let's look at the cell membrane and see how that membrane keeps all of the pieces inside. When you think about a membrane, imagine it is like a big plastic bag with some tiny holes. That bag holds all of the cell pieces and fluids inside the cell and keeps any nasty things outside the cell. The holes are there to let some things move in and out of the cell.

Cytoplasm - Filling Fluid Cytoplasm is the fluid that fills a cell. Early on, they didn't know about the many different types of fluids in the cell.

Cell Nucleus - Commanding the Cell The cell nucleus acts like the brain of the cell. It helps control eating, movement, and reproduction. If it happens in a cell, chances are the nucleus knows about it. The nucleus is not always in the center of the cell. It will be a big dark spot somewhere in the middle of all of the cytoplasm. You probably won't find it near the edge of a cell because that might be a dangerous place for the nucleus to be.

Ribosomes: Protein Construction Teams • Organelles that make proteins are called ribosomes. Ribosomes are the smallest of all organelles. And there are more ribosomes in a cell than there are any other organelles. Some ribosomes float freely in the cytoplasm. Others are attached to membranes. Unlike most organelles, ribosomes are not covered by a membrane. All cells have ribosomes.

Mitochondria - Turning on the Powerhouse • Mitochondria are known as the powerhouses of the cell. A mitochondrion is the organelle in which sugar is broken down to produce adenosine triphosphate (ATP). They are organelles that act like a digestive system that takes in nutrients, breaks them down, and creates energy for the cell. A mitochondrion is shaped perfectly to maximize its efforts. Mitochondria are very small organelles. You might find cells with several thousand mitochondria. The number depends on what the cell needs to do. If the purpose of the cell is to transmit nerve impulses, there will be fewer mitochondria than in a muscle cell that needs loads of energy. If the cell feels it is not getting enough energy to survive, more mitochondria can be created. Sometimes they can even grow, move, and combine with other mitochondria, depending on the cell's needs.

Chloroplasts - Show me the Green • Animal cells cannot make their own food. Plants cells are different. Some of their cells have chloroplasts. Chloroplasts are organelles in plant and algae cells in which photosynthesis takes place. Chloroplasts are the food producers of the cell.

• Like mitochondria, chloroplasts have two membranes and their own DNA. Chloroplasts are green because they contain chlorophyll, a green pigment found inside their inner membrane. • Chlorophyll traps the energy of sunlight, which is used to make sugar. The sugar produced by photosynthesis is then used by mitochondria to make ATP.

Endoplasmic Reticulum Wrapping it Up Many chemical reactions take place on or in the endoplasmic reticulum. The endoplasmic reticulum, or ER, is a system of folded membranes in which proteins and other materials are made. The ER is part of the internal delivery system of the cell. Substances move through the ER to different places in the cell. Endoplasmic reticulum is either rough ER or smooth ER. The part of the ER covered in ribosomes is rough ER and is usually found near the nucleus. Ribosomes on rough ER make many of the cell’s proteins. The ER delivers these proteins throughout the cell. ER that lacks ribosomes is smooth ER. The functions of smooth ER includes making lipids and breaking down toxic materials that could damage the cell

Vacuoles - Storage Bins to the Cells • Vacuoles are storage bubbles found in cells. A vacuole is a large vesicle. In plant and fungal cells, some vacuoles act like large lysosomes. They store digestive enzymes and aid in digestion within the cell. Vacuoles in plant cells also store water and other liquids. Vacuoles that are full of water help support the cell. Some plants wilt when their vacuoles lose water.

Lysosomes - Little Enzyme Packages • Lysosomes are vesicles that contain digestive enzymes and are responsible for digestion or to break down the cell when it dies. They destroy worn-out or damaged organelles and get rid of waste materials. In addition, they protect the cell from foreign invaders.

Golgi Complex • The organelle that packages and distributes proteins is called the Golgi complex. Lipids and proteins from the ER are delivered to the Golgi complex. There, the lipids and proteins may be modified to do different jobs. The final products are enclosed in a piece of the Golgi complex’s membrane. This membrane pinches off to form a small bubble. The bubble transports its contents to other parts of the cell or out of the cell.