cells - Cells Cells are the smallest unit capable of...

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Cells Cells are the smallest unit capable of carrying out physiological functions. .. all living things are made from cells. All cells come from the division of other cells. Cells can either be prokaryotic or eukaryotic: Prokaryotic cells have no nucleus (bacteria) Eukaryotic cells do have a nucleus (everything else, including us) The cell can broadly be divided into three parts: - the cell (or plasma) membrane, an outer membrane which divides inside the cell from outside (much as your skin divides outside from inside of you ) - the nucleus, which is a membrane-enclosed structure protecting the DNA - the cytoplasm, a thick fluid which contains many dissolved substances as well as various organelles (little organs, each of which carry out a particular function for the cell): o cytoskeleton, internal structure for the cell o ribosome, makes protein o rough endoplasmic reticulum (ER), processes protein o Golgi, packages protein for export o smooth ER, synthesizes lipids and detoxifies o lysosomes/peroxisomes, waste disposal organelles o mitochondria, synthesizes a form of useable energy for the cell called ATP Cell membrane The membrane is how a cell maintains physical boundaries. It allows for: - physical separation from interstitial fluids - regulation of exchange with environment - sensing of environment and response to hormones, etc., via receptors - structural support Composition of membrane: 1. Phospholipid bilayer: the major component of a cell membrane. Remember that phospholipids are polar on one end and nonpolar on the other; the heads (polar) face out, nonpolar heads face in away from the water. Because you form a lipid skin, this creates a water-resistant membrane, across which polar molecules cannot easily move (we will explain later how they DO move across); however, nonpolar molecules can cross easily (oil will mix with oil). 2. Embedded in the membrane are PROTEINS, of four major classes: - Channels/pores: allow water and small ions to pass through, do not use energy. - Transporters: move substances across the membrane, may or may not use energy - Receptors: allow signals to be transmitted across the membrane (somewhat like nerve endings allow signals to be transmitted across your skin) - Identification: just as they sound – identify certain kinds of cells by the protein that sticks out of it. 3. Some cells have sugar (carbohydrate) molecules that stick out; also, most cells have a certain amount of cholesterol as part of the membrane.
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Composition of the membrane can change and be adjusted as the need of the cell changes… for example, new receptors can be inserted, or removed. Attached to the inside of the membrane is the CYTOSKELETON – a network of protein fibers that: - give the cell structural integrity (internal framework, not unlike your own skeleton) - allow the cell to divide - provide a “highway” for molecules to be moved around inside the cell (protein motors walk on the cytoskeleton) Cell membrane is a barrier that effectively keeps things on 2 sides – consequently, inside
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cells - Cells Cells are the smallest unit capable of...

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