Biology Cell Structure & Function

What is a cell? The smallest unit that is capable of performing life functions. After observing many cells, scientists came up with the cell theory: All living things are made of 1 or more cells. Cells are the basic units of structure and function in organisms. All cells come from existing cells.

Cell Size To function efficiently, cells must be small. Why? If a cell is the shape of a cube and has 1 mm sides, calculate the surface area of the cube using the formula 6S2. Also, calculate the volume using the formula S3. What is the surface area to volume ratio? Do the same calculations for a cell having 2 mm sides. What happens to the SA/volume ratio as the cell gets bigger? Why is this bad? As a cell gets bigger, the SA/vol ratio gets smaller. This is bad because a cell brings in nutrients through its surface area. If it doesn’t have a high SA/vol ratio then some of the volume won’t get the necessary nutrients.

2 Main Cell Types Prokaryotic Eukaryotic Originated approximately 1.5 billion years ago 2-100µm; typical plant/animal cell is 10-50µm Organized- Have a nucleus and other organelles (structures that carry out specific functions for the cell) DNA is in the form of chromosomes Example- animal and plant cells Originated approximately 3.5 billion years ago 0.5- 2µm (µ is the symbol for “micro” which means 1/1,000,000) Simple- no nucleus nor other special compartments Single, circular DNA molecule (plasmid) Example- bacterium

Prokaryotic Bacterial Cell

Eukaryotic Plant Cell

Eukaryotic Animal Cell

Characteristics of ALL Cells All pro and eukaryotic cells have the following: Cell membrane- An outer boundary which is also called the plasma membrane Cytoplasm- The interior of a cell; just the liquid is called cytosol. Cytoskeleton (microtubules/microfilaments)- Protein filaments (thread-like structures) that are important in cell movement, shape and division Ribosomes- Structures that do not have a membrane and make proteins DNA- hereditary information

Eukaryotic Organelles

Organelles that Surround the Cell Cell membrane- “Selects” what can enter and leave the cell. More about this later! Cell wall Most commonly found in plant cells and bacteria- NOT IN ANIMAL CELLS! Surrounds the cell membrane and provides structure and support Made of cellulose- long carbohydrate chain Cilia- Short, numerous hairs that move the cell and/or substances outside the cell What human cells have cilia?

Flagella- Long tail(s) that moves the cell and/or substances outside the cell Both cilia and flagella are made of protein filaments like the cytoskeleton. What human cell has a flagellum?

Organelles inside the cell Nucleus Directs the cell’s activities Houses DNA Houses the nucleolus where ribosomes are made Surrounded by a double membrane called the nuclear envelope controls what exits the nucleus like ribosomes and RNA has openings called nuclear pores

Endoplasmic reticulum System of membranes that move proteins around inside the cell- “intracellular highway” Can be smooth or rough rough has ribosomes attached to it and smooth does not ribosomes can also be found in the cytosol Golgi apparatus Flattened sacs Processes proteins into vesicles (membrane bound sacs)

Mitochondria Chloroplast Produces ATP- form of energy that can be used by the cell. What cells of the body have lots of mitochondria? Has a double membrane Outer membrane is smooth and the inner membrane (cristae) is folded. Has its own DNA Chloroplast In plants, some protists and some bacteria- NOT IN ANIMAL CELLS! Where photosynthesis occurs Has its own DNA and a double membrane

Mitochondria

Chloroplast

Lysosome Central Vacuole Spherical organelle where digestion occurs Remove the web between fingers of a fetus Usually in animal cells Central Vacuole Large Stores water, nutrients and waste Only in plant cells Animal cells have vacuoles- but not central vacuoles

“Typical” Animal Cell http://web.jjay.cuny.edu/~acarpi/NSC/images/cell.gif

“Typical” Plant Cell

How do animal and plant cells differ? Spherical Might have flagella or cilia Have lysosomes Rectangular Have a cell wall Have chloroplasts Have a central vacuole

Evolution of the Eukaryotic Cell A possible explanation for how eukaryotic cells came into existence is the endosymbiotic theory. Symbiosis is a relationship between organisms where every member benefits. The endosymbiotic theory states that chloroplasts and mitochondria were once prokaryotic bacteria. They entered a larger prokaryotic cell and all parties benefited. The chloroplast and mitochondria got extra protection and the larger cell developed the ability to make its own food and energy. Evidence for the endosymbiotic hypothesis is the following: Mitochondria and chloroplasts are similar to bacteria in both size and structure. Both organelles are bounded by a double membrane – the outer membrane may represent the engulfing vesicle, and the inner membrane from the prokaryote. Mitochondria and chloroplasts each contain a limited amount of genetic material and divide by splitting. The DNA is a circular loop like that of prokaryotes. Most proteins of mitochondria and chloroplasts are now produced by the host cell, but these organelles do have their own ribosomes and do produce some proteins. Their ribosomes resemble those of prokaryotes. The RNA base sequence of the ribosomes in chloroplasts and mitochondria suggests a prokaryotic origin.

Evidence of the endosymbiotic theory Both chloroplasts and mitochondria have their own circular DNA (like prokaryotic cells). Both chloroplasts and mitochondria have a double membrane. Mitochondria are approximately the size of bacteria.

Game to Review Organelles