Bio week3

Why is the cell called the fundamental unit of life?

Every living organism is composed of one or more cells, and nothing smaller than a cell is capable of performing all of the activities required for life.

Complete the sentence: "Nothing smaller than a cell is capable of ______."

Performing all of the activities required for life.

Name the two fundamental kinds of cells and which organisms have each.

Prokaryotic - bacteria and archaea. Eukaryotic - plants, animals, fungi, and protists.

What is the single most important difference between prokaryotic and eukaryotic cells?

Eukaryotic cells have membrane-enclosed organelles, including a nucleus that encloses the DNA. Prokaryotic cells have neither.

How much larger is a typical eukaryotic cell than a prokaryotic one?

About 10-fold bigger, commonly quoted as 10 to 100 times larger.

When did prokaryotes first appear, and when did eukaryotes evolve?

Prokaryotes about 3.5 billion years ago; eukaryotes about 1.8 billion years ago.

Prokaryote vs. eukaryote - complexity and cell number

Prokaryotic cells are simpler and always unicellular. Eukaryotic cells are more structurally complex and can be unicellular or multicellular.

Prokaryote vs. eukaryote - where is the DNA?

In a prokaryote the DNA is not contained within any cellular structure. In a eukaryote it is housed in a membrane-enclosed nucleus.

Nucleoid

A "nucleus-like" region in a prokaryotic cell where the DNA is coiled. It is not surrounded by a membrane.

Where is the DNA in a prokaryotic cell, and what does that region lack?

Coiled in the nucleoid, a nucleus-like region that is not surrounded by a membrane.

Name four structures found only in prokaryotic cells.

Nucleoid, plasmids, capsule, and pili.

Plasmid

A small ring of DNA that duplicates independently and can be exchanged among cells.

Capsule (prokaryote)

A sticky outer coat surrounding the cell wall. It provides protection and helps the cell stick to surfaces.

Pilus (plural pili)

Short projections on a prokaryote that attach to surfaces or to other prokaryotes.

Plasma membrane - function in one line

A thin layer that regulates the passage of substances between the cell and its surroundings.

Cell wall - function in one line

A rigid outer layer that protects the cell and provides shape.

Ribosomes - function in one line

Small structures that build proteins using instructions encoded in DNA.

Flagellum - function in one line

A long cellular appendage that aids in movement.

Do all prokaryotes have all of these structures?

No. Not all prokaryotes carry every one of them.

Which two domains are prokaryotes?

Bacteria and archaea. Both are single-celled and both lack a nucleus.

Give two named examples of prokaryotes.

Pseudomonas aeruginosa, a disease-causing bacterium, and Sulfolobus, an archaean that grows only in very hot, acidic environments.

Are archaea prokaryotic or eukaryotic?

Prokaryotic. They are a separate domain from bacteria, but they still lack a nucleus.

True or false: prokaryotes have no ribosomes. Explain.

False. They have ribosomes. Ribosomes have no membrane, and the rule is that prokaryotes lack membrane-enclosed organelles.

Precision point: nucleoid vs. nucleus

A nucleoid is a region, not an organelle, and it has no membrane. Saying "prokaryotes have a nucleus" is wrong; saying "their DNA is in a nucleus-like region called the nucleoid" is right.

Precision point: what is the safe wording for what prokaryotes lack?

"No membrane-enclosed organelles." Saying "prokaryotes have no organelles" is the sloppy version, since ribosomes are present in both cell types.

Precision point: do prokaryotes have DNA?

Yes. Lacking a nucleus does not mean lacking DNA - it means the DNA is not enclosed.

Precision point: is a cell wall a prokaryote-only structure?

No. Prokaryotes, plants, fungi, and some protists all have walls. Only animal cells lack one. It is not a prokaryote/eukaryote distinguisher.

Precision point: is a flagellum prokaryote-only?

No. Both bacteria and animal cells can have one.

How do bacterial and eukaryotic flagella differ?

Bacterial flagella rotate like a propeller; eukaryotic ones whip or bend. They are structurally unrelated despite sharing a name.

Precision point: which organisms are multicellular?

Only eukaryotes. Prokaryotes are always unicellular. Eukaryotes are unicellular or multicellular - a yeast or an amoeba is a single eukaryotic cell.

How old are eukaryotes?

Roughly 1.8 billion years - give that figure. Estimates run up to 2.1 billion depending on what counts as the first true eukaryote. Prokaryotes at 3.5 billion is not disputed.

Trivia: bacteria in your mouth

There are more bacteria in your mouth right now than the total number of humans who have ever lived.

Which of the four cell types contain a nucleus?

Plant and animal cells contain nuclei. Bacteria and archaea are prokaryotic and so lack nuclei.

How alike are eukaryotic cells?

All eukaryotic cells are fundamentally alike. Compared to prokaryotic cells they are relatively large, about 10-fold bigger, and more complex.

Cytoplasm

The fluid-filled region of the cell between the nucleus and the plasma membrane.

Organelle

"Little organs" - membrane-enclosed structures found only in eukaryotic cells that perform specific functions.

Nucleus

The most prominent organelle; a membrane-bound region where most of a cell's DNA is housed.

Name eight structures found in both plant and animal cells.

Plasma membrane, nucleus with nuclear envelope, ribosomes, endoplasmic reticulum, Golgi apparatus, vesicles, mitochondria, and cytoskeleton - plus cytoplasm.

Which organelle is found only in animal cells?

The lysosome.

Which three structures are found only in plant cells?

Chloroplast, cell wall, and central vacuole.

State the plant/animal split in one line.

Only animal cells have lysosomes; only plant cells have chloroplasts, cell walls, and central vacuoles.

Do plant cells have mitochondria? Why does the answer matter?

Yes. Plants photosynthesize and respire, so they need both organelles. It matters because "plants have chloroplasts instead of mitochondria" is the single most common wrong answer.

Which organelle(s) produce energy in an animal cell? In a plant cell?

Mitochondria in animal cells; mitochondria and chloroplasts in plant cells.

Precision point: what is the animal-only / plant-only count?

One animal-only (lysosome) versus three plant-only (chloroplast, cell wall, central vacuole). Getting the count right is a cheap mark.

Precision point: vacuole vs. central vacuole

Vacuoles occur in plants, protists, and fungi. It is the large central vacuole that is specifically the plant feature.

Are lysosomes really animal-only?

Give animal only - that is the expected answer. Strictly, plant and fungal cells have lytic compartments doing the same digestive job under different names.

Precision point: is the cell wall a replacement for the plasma membrane?

No. A plant cell has both a wall and a plasma membrane, in that order from outside in.

Is a ribosome an organelle?

An organelle is normally defined as a membrane-enclosed structure, and a ribosome has no membrane - so "cellular structure" or "non-membranous organelle" is the safer wording. Never claim ribosomes are membrane-bound.

Trivia: how many cells are in an adult human?

A recent study put the total at 37.2 trillion.

Why must a cell have a plasma membrane at all?

To maintain a relatively constant internal environment even when the external environment changes, by regulating the passage of materials into and out of the cell.

Plasma membrane - physical description

A thin, flexible, oily sheet forming the boundary between the cell and its surroundings. Simple structure, many functions.

Describe a phospholipid molecule. Which part is hydrophilic and which hydrophobic?

A phosphate head and two fatty acid tails. The head is hydrophilic, "water loving"; the tails are hydrophobic, "water fearing."

Which way do the phospholipid heads and tails point?

The heads always face the watery environment on either side of the membrane. The tails always face the interior of the bilayer, forming an oily barrier.

Why do phospholipids spontaneously form two layers?

Because the heads are attracted to water and the tails are repelled by it, they organize themselves so the tails point inward at each other and the heads face the water on both sides. Nobody has to assemble it - the geometry does it.

Why is a membrane called a "fluid mosaic"? Explain both halves.

Fluid because the bilayer and most of its proteins drift about in the plane of the membrane, so it flexes and undulates and molecules move freely past one another. Mosaic because of the diversity of proteins that float like icebergs in the phospholipid sea.

What does cholesterol do in an animal cell membrane?

It sits within the bilayer and provides additional flexibility.

Name four jobs performed by membrane proteins.

Regulate the passage of materials into and out of the cell; aid communication between neighbouring cells; facilitate chemical reactions; and anchor the cell or its components.

Are membrane proteins fixed or mobile?

Some are fixed in place; others float about within the bilayer.

What do membrane carbohydrate chains do?

Some proteins extending from the plasma membrane carry carbohydrate chains that serve as markers other cells can recognize.

Extracellular fluid

The liquid environment outside the cell. The plasma membrane regulates passage between it and the cell interior.

Cytosol

The watery liquid that makes up the interior of the cell, before you count organelles and dissolved molecules.

What is the difference between cytoplasm and cytosol?

Cytosol is the fluid only. Cytoplasm is the whole interior - cytosol plus organelles plus dissolved molecules.

Is the nucleus part of the cytoplasm?

No. Cytoplasm is defined as the region between the nucleus and the plasma membrane.

Why is the structure of a membrane called a phospholipid bilayer?

Because it consists of two stacked layers of phospholipid molecules. The answer really is that literal.

Precision point: what is a membrane mostly made of?

Mostly phospholipids and proteins. Cholesterol and carbohydrates are additions, not the base.

Precision point: are membrane carbohydrates transporters?

No. Their job is cell recognition, not moving substances.

What are glycoprotein and glycolipid?

A protein or a lipid with a carbohydrate chain attached - the cell-recognition markers on the membrane surface.

Does the bilayer structure apply to organelle membranes too?

Yes. The plasma membrane and the other membranes inside the cell are all built this way - the nuclear envelope, the ER, the Golgi, all of it.

Where does the term "fluid mosaic" come from?

The fluid mosaic model, proposed by Singer and Nicolson in 1972.

Which membranes does a eukaryotic cell have besides the plasma membrane?

Many organelles such as the nucleus are surrounded by their own membranes, and some organelles such as mitochondria and chloroplasts have internal membranes.

What is the most important function of all cellular membranes?

To regulate the flow of materials.

Selectively permeable

The membrane allows some substances to flow freely, others only under certain circumstances, and some not at all.

Define passive transport and active transport, and give the one difference that separates them.

Passive transport is movement down the concentration gradient with no energy required. Active transport is movement against the gradient and always requires energy. The separating fact is the direction relative to the gradient.

What drives passive transport versus active transport?

Passive transport is driven by the molecules' own motion. Active transport is usually driven by a protein sitting within the membrane.

Name the three forms of passive transport.

Diffusion, osmosis, and facilitated diffusion.

Define diffusion and give two examples.

The movement of molecules from an area of higher concentration to an area of lower concentration. Examples: perfume molecules leaving a bottle and filling a room, and glucose diffusing from the bloodstream into body cells.

Define osmosis. In which direction does water flow?

Osmosis is the diffusion of water. Water always flows from an area of higher water concentration to an area of lower water concentration.

Give an everyday example of osmosis.

Placing meat in salt "cures" it by drawing out the water.

Facilitated diffusion

The passage of substances that cannot cross on their own, for example because they are too large, via specific transport proteins acting as selective channels. It is still passive and still moves down the gradient.

Which form of passive transport always requires a membrane protein, and which only sometimes does?

Facilitated diffusion always requires a transport protein. Osmosis sometimes but not always does - water can also cross the membrane itself.

Trap alert: which passive transport ALWAYS needs a protein?

The word "always" is the trick. The answer is facilitated diffusion. If you answer "osmosis" you lose the mark, because osmosis sometimes but not always requires protein channels.

Give an example of passive transport in your lungs.

O2 diffuses from lung air into blood and CO2 from blood into lungs - no work required by your lungs.

What does the sodium-potassium pump do, how many ions, and why does it matter?

It moves three sodium ions out of the cell against their gradient, using energy. This active transport of ions is essential in the formation of nerve signals.

Endocytosis

Bulk transport into the cell. Large substances such as food or foreign material are packaged into vesicles that bud inward from the plasma membrane, then travel through the cytoplasm.

Exocytosis

Bulk transport out of the cell. Vesicles containing material fuse with the plasma membrane, dumping their contents into the environment.

Give an example of exocytosis.

Cells in your tear glands exporting salty tears.

What do the prefixes endo- and exo- tell you?

Endo- means into, exo- means exit. Both processes use vesicles - bubbles surrounded by membrane.

Precision point: what does "passive" actually mean?

No energy - not "no protein." Facilitated diffusion uses a protein and is still passive, because the substance still moves down its gradient.

Precision point: what makes transport active?

Going against the gradient. That is the definition; the energy cost follows from it.

Precision point: is osmosis a separate category from diffusion?

No. It is diffusion where the thing diffusing is water.

How else can you state the direction of osmosis?

Water moves from lower to higher solute concentration - the same statement as higher to lower water concentration, just from the other side.

Why do salt and honey preserve food?

Salt or sugar outside the cell means less free water outside, so water leaves the cell. Honey has been used as a preservative for millennia for exactly this reason.

Precision point: where do endocytosis and exocytosis belong?

Neither is passive transport nor "active transport" as such - they are bulk transport by vesicle, a third mechanism. Do not file them under the pump heading.

Core question 3.4: What form of passive transport always requires membrane-bound proteins?

Facilitated diffusion. Osmosis sometimes, but not always, requires protein channels.

What is the most prominent membrane-enclosed organelle?

The nucleus.

The factory analogy

If a cell is a factory, the nucleus is the executive board room - orders are issued here that result in the cell's processes being performed. DNA is the executive director: it issues orders via RNA rather than doing the hands-on work.

Nuclear envelope

A double phospholipid membrane around the nucleus, also called the nuclear membrane. Each of its two layers is structured like the plasma membrane. It controls the passage of materials into and out of the nucleus.

Nuclear pore

A protein-lined opening in the nuclear envelope through which certain molecules, such as RNA, may pass in and out.

What is a chromosome made of, and how many are in a human body cell?

One DNA molecule and its associated protein molecules. A human body cell has 46.

What do chromosomes do?

They store most of the cell's genetic information and control the cell's activities by directing protein production.