Chapter 3

Cells: The Living UnitsIntro and Membrane

Cell

• The basic structural and functional unit of all living things

4 Concepts of Cell Theory

– Cell is the basic structural and functional unit of life

– The activity of an organism depends on the individual and collective activities of cells

– The function of the cell depends on its organelles (principle of complementarity)

– Continuity of life has a cellular basis

Characteristics of Cells

– Cells vary greatly in their size, shape, and function– A cells shape reflects its function– All cells are composed of carbon, hydrogen,

nitrogen, oxygen and trace amounts of other elements

– A generalized human cell contains the plasma membrane, cytoplasm, and nucleus

3 Main Parts of Human Cells

– Plasma membrane– Cytoplasm– Nucleus

Plasma Membrane

• Membrane, composed of phospholipids, cholesterol, and proteins, that encloses cell contents; outer limiting cell membrane

Plasma Membrane

• Intracellular fluid• Fluid inside the cell

• Extracellular fluid– Fluid outside of the cell

Plasma Membrane

• Fluid Mosaic Model• Because the proteins that float in the lipid bilayer form a

constantly changing mosaic pattern

Plasma Membrane

• Composition– Phospholipids – most– Cholesterol – small– Glycolipids – small

Plasma Membrane

• Phospholipids– Head – polar

hydrophilic (water loving) – faces the intra/extracellular fluid

– Tail – nonpolar hydrophobic (water fearing) – tails are facing towards the inside

Plasma Membrane

• Glycolipids– Phospholipids with attached sugar groups– Only found on the outer surface of the plasma

membrane

Plasma Membrane

• Lipid Rafts– Groups of saturated phospholipids that are packed

tightly together. Associated with shphingolipids and cholesterol.

– More stable and less fluid that the rest of the membrane

– Assumed that they are needed for cell signaling

Plasma Membrane

• Integral Protein– Protein that is inserted into

the lipid bilayer.– They have both hydrophobic

and hydrophilic regions so that they can interact with the different regions of the lipid bilayer.

Plasma Membrane

• Integral Protein• Can protrude from one

surface, but most are transmembrane which means that they go through the entire membrane.

Plasma Membrane

• Integral Protein– Transmembrane proteins are

mainly involved with transport – when many cluster together they form channels or pores so “stuff” can flow into and out of the membrane (water soluble ions or molecules)

– Also act as carries that bind a substance and then move it through the membrane

Plasma Membrane

• Peripheral Protein– Proteins that aren’t

embedded in the lipid bilayer; attached loosely to integral proteins or membrane lipids. Removed easily without disrupting the plasma membrane

Plasma Membrane

• Peripheral Protein• Some are enzymes and

others are involved in mechanical functions (changing cell shape during mitosis, muscle cell contraction, or linking cells together).

Plasma Membrane

• Glycocalax– “sugar covering”– Fuzzy sticky carbohydrate rich area surrounding

the cell

Plasma Membrane

• Microvilli– “little shaggy hairs”– Small fingerlike

projections of the plasma membrane.

– Increase the surface area.

– Found in areas where lots of absorption take place (intestine and kidneys)

Factors that Join Cells

– Glycoproteins act as an adhesive– Wavy contours of the membranes of adjacent cells

fit together in a tongue and groove fashion– Special membrane junctions (tight junction,

desmosomes, gap junctions)

Special Membrane Junctions

• Tight Junction– Integral proteins of adjacent

cells fuse together to form an impermeable junction that encircles the cell.

– Help to prevent molecules from passing through the extracellular space between cells.

– Found in lining of the digestive tract.

Special Membrane Junctions

• Desmosomes– “binding bodies” or

anchoring junctions– Like rivets to prevent

cells from separating and distributes tension throughout a cellular sheet to reduce the chance of tearing.

– Present in skin and heart muscle

Special Membrane Junctions

• Gap Junction– “bond” or nexus– Communicating junction

between adjacent cells– Cells are connected by hollow

cylinders called connexons.– Small molecules pass through

the water filled channels from one cell to the next.

– Present in electrically excitable tissues like heart and smooth muscle

Functions of the Plasma Membrane

• Cells are bathed in an extracellular fluid called interstitial fluid.– Derived from the blood.– Contains thousands of ingredients• Amino acids, sugars, fatty acids, vitamins, hormones,

neurotransmitters, salts, waste products, etc.

– Cells “take” from the interstitial fluid what they need when they need it.