The Cell

Cells are Us

Cells are Us

Cilia on a protozoan. Sperm meets egg.

Cells are UsA person contains about 100 trillion cells. That’s 100,000,000,000,000 or 1 x 1014 cells.

There are about 200 different cell types in mammals (one of us).

Cells are tiny, measuring on average about 0.002 cm (20 um) across. That’s about 1250 cells, “shoulder-to-shoulder” per inch.

nerve cell

Red and white blood cells above vessel-forming cells.

The Cell Theory

*All organisms are composed of one or more cells.

*Cells are the basic unit of structure and function.

All organisms living today are descendents of an ancestral cell.

* Cells arise only by division of previously existing cells.

The Cell Theory (proposed independently in 1838 and 1839) is a cornerstone of biology.

A Sense of Scale and Abundance – Bacteria on the Head of a Pin

Two Fundamentally Different Types of Cells

A prokaryotic cell

A eukaryotic cell

Them vs. Us -Prokaryotes and Eukaryotes

An Idealized Animal Cell

The Plasma Membrane – Gateway to the Cell

The Plasma Membrane is Selectively permeable

Small molecules and larger hydrophobic molecules move through.

Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

The physical properties of phospholipids account for membrane assembly and many of its properties.

Plasma Membrane Functions

Maintain a high concentration of materials in the cell.

Keep harmful materials out.

Control the movement of materials into and out of the cell.

Let the cell sense its environment.

Phospholipids

Cholesterol

Proteins (peripheral and integral)

Carbohydrates

Membrane Components

Proteins Are Critical to Membrane Function

Transport Processes - Diffusion

Solutes move down a concentration gradient until they are evenly distributed. This is diffusion.

Another way of saying this is that solutes move from a region of higher concentration to a region of lower concentration until there is no difference in concentration.

Diffusion of water= Osmosis

Three Forms of Transport Across the Membrane

Three Forms of Transport Across the Membrane

Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out.

Three Forms of Transport Across the Membrane

Examples: Glucose or amino acids moving from blood into a cell.

Three Forms of Transport Across the Membrane

Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients.

Moving the “Big Stuff”

Molecules are moved out of the cell by vesicles that fuse with the plasma membrane.

Exocytosis: moving things out.

This is how many hormones are secreted and how nerve cells communicate with one another.

ExocytosisExocytic vesicle immediately after fusion with plasma membrane.

Moving the “Big Stuff”

Large molecules move in via one of two forms of endocytosis.

Pinocytosis

This is the most common form of endocytosis.

Pinocytosis takes up most proteins and other large molecules.

Pinocytosis

pinocytic vesicles forming mature transport vesicle

Transport across a capillary cell (blue).

Endocytosis – Phagocytosis Transports Large Particles

Phagocytosis About to Occur

The Threshold of Phagocytosis - Capture of a Yeast Cell (yellow) by Membrane Extensions of an Immune System Cell (blue)

Effects of Osmosis on Water Balance

• Osmosis is the diffusion of water across a selectively permeable membrane

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

• This will “dilute” the higher concentration.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Lowerconcentrationof solute (sugar)

Fig. 7-12

H2O

Higher concentrationof sugar

Selectivelypermeablemembrane

Same concentrationof sugar

Osmosis

Water Balance of Cells Without Walls

• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water and will shrivel.

• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water and expands.

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

Fig. 7-13

Hypotonic solution

(a) Animal cell

(b) Plant cell

H2O

Lysed

H2O

Turgid (normal)

H2O

H2O

H2O

H2O

Normal

Isotonic solution

Flaccid

H2O

H2O

Shriveled

Plasmolyzed

Hypertonic solution

It’s Crowded In There

An artist’s conception of the cytosol - the region of a cell that’s not in the nucleus or within an organelle.

Animal and Plant Cells Have More Similarities Than Differences

What Do Those Parts Do?

We’ll start by seeing what role these parts play in making and moving proteins.

A Focus of Our Tour of the Cell: The Path of Protein Synthesis and Export

The Nucleus

Think of the nucleus as the cell’s control center.

Two meters of human DNA fits into a nucleus that’s 0.000005 meters across.

Ribosomes and the Endoplasmic Reticulum

The Rough Endoplasmic Reticulum

Protein movement (trafficking)

Protein synthesis (about half the cell’s proteins are made here).

Protein “proofreading”

Functions:

The Golgi Apparatus (or Complex)

Think of the Golgi apparatus as a busy airport receiving passengers (proteins) and sending them to their destinations.

The Golgi Apparatus

Proteins moved through the Golgi apparatus can travel to the lysosome, the plasma membrane, or outside the cell.

Proteins moved through the Golgi apparatus travel to the lysosome, the plasma membrane, or outside the cell.

Note how protein cargo moves in vesicles.

Off the Path of Protein Production: Lysosomes, Mitochondria, Chloroplasts and the Cytoskeleton

The Lysosome

Cell suicide (suicide is bad for cells, but good for us!)

Many diseases (e.g. Tay-Sachs) are caused by lysosome malfunction

Recycling cellular components

Functions:

Digesting food or cellular invaders

(The lysosome is not found in plant cells)

The Lysosome

This bacterium about to be eaten by an immune system cell will spend the last minutes of its existence within a lysosome.

The MitochondrionThink of the mitochondrion as the powerhouse of the cell.

Both plant and animal cells contain many mitochondria.

(Mitochondria is the plural of mitochondrion)

The Mitochondrion

A class of diseases that causes muscle weakness and neurological disorders are caused by malfunctioning mitochondria.

Worn out mitochondria may be an important factor in aging.

Animal vs. Plant Cells – Chloroplasts Are a Big Part of the Difference

Two Other Differences

Cells In a Leaf

A Consequence of Cell Walls – the Great Strength of Woody Plants

The Chloroplast

Think of the chloroplast as the solar panel of the plant cell.

Only plants have chloroplasts, but animals reap the benefits too.

An animal cell cytoskeleton

The name is misleading. The cytoskeleton is the skeleton of the cell, but it’s also like the muscular system, able to change the shape of cells in a flash.

The Cytoskeleton

The Cytoskeleton in Action

Cilia on a protozoan. Beating sperm tail at fertilization.