Cell Wall, Cell Membrane, and Vacuole

Cell Membrane


In contrast to a plant's cell wall, animal cells have flexible lipid bilayer called the cell membrane a.k.a plasma membrane. Within this double layer contains the material of the inside of the cell. The cell membrane protects the cell from the harsh conditions of the outside world.

Cell Membrane

Phospholipids are lipids with hydrophilic heads and two fatty acid hydrophobic tails that make up the cell membrane. In an aqueous solution, the phopholipids arrange themselves so that their tails are facing away from water and the heads are left exposed. When not in the layer formation, phosphlipids form a micelle, which is an aggregation of phopholipids.


Other lipids found in the cell membrane are cholesterol and glycolipids. Depending on the cell membrane, the ratio of these lipids can be found in different proportions.

Carbohydrates are attached to the outside of the cell membrane in chains extending to be 15 or fewer sugar molecules long and give the cell identity.
Passive and Active transport

Different membranes contain different membranes that make the membrane unique. Different types of membranes found in the cell membrane include enzymes, cell surface receptors, transportors and surface antigens. Proteins are anchored to the inside of membrane because of their hydrophobic and hydrophilic behaviors.

Cell Surface Receptors- bind molecules to the specific cell and alters the behavior of the target cell by sending a signal to the molecule

Surface Antigens- detect foreign material in the cell and trigger killer-T cells in the immune system to destroy the unwanted material.

Transport proteins are each specialized for a certain molecule and can transport polar molecules across the cell membrane.

Passive transport proteins work nauturally sending the solute into or out of the cell without requiring energy. The solute moves from high to low concentration. (for example channel proteins- allow solutes to cross if they are the corresponding size and shape, and carrier proteins- that bind to the solute and carry them through the membrane.)

Active transport is the transport of a solute across a cell membrane but requires energy.
An example of active transport is the sodium-potassium pump. This pump regulates the the concentration of potassium into the cell and sodium out of the cell.

The potassium-leak channel allows potassium to leak out of the cell in order to keep the right amount of sodium inside the cell.

There are two main types of active transport:

macromolecules are inclosed in a vesicle that separates them from the rest of the cell, until reaching the cell membrane and fusing with it until it is opened up and its contents are released from the cell.

There are two types of Exocytosis:
- Pinocytosis involves ingesting small molecules or fluids

- Phagocytosis involves the ingestion of large molecules, such as microorganisms or cell debris using large vesicles, or vacuoles.

There are several types of transport proteins:

Uniport Proteins- move solutes from one side of the membrane to the other.

Cotransport- work by sending two solutes across the lipid bilayer at the same time. Cotransport systems can be further categorized by two types:

Symport - solutes are sent in the same direction of the membrane.

Antiport- solutes are sent in the opposite direction of the membrane.

The cell membrane is collaged with the many different proteins that are embedded in the "fluid matrix" of the lipid bilayer. The structure is a semi-permeable membrane. Proteins determine most of the membrane's specific functions. The plasma membrane and the membranes of the various organelles each have unique collections of proteins.

Comparison of Transport Proteins

Quiz yourself on the cell membrane:
Cell Membrane Quiz

Compare plant and animal cell:
Interactive Plant and Animal cell

cell membrane active transportimage036.gif


Cell WallSecwall.gif
A feature that separates plant cells from animal cells is that plant cells have a relatively thick cell wall surrounding the cell's plasma membrane, while animal cells do not.

Plant Cell Walls
Plant cell walls are thick walls that encase the cell, which can be numerous micrometers thick. Cell walls are made of microfibrils of cellulose set in a base of proteins and other polysaccharides. The wall itself consists of a primary cell wall, a secondary cell wall, and a middle lamella. The plant cell also has many holes on its perimeter as well.

CELLULOSE- the molecules that make up the microfibrils found in cell walls; the microfibrils are groups of cellulose polysaccharides (which do not branch), but are held together by hydrogen bonds to other cellulose polymers that are parallel to each other. Cellulose is in beta configuration making it possible for the cellulose strands to be very extensive.
fiber.gif Cellulose_strand.jpg

PRIMARY WALL- when a plant cell first forms, it creates a primary cell wall that is polysaccharide-rich, flexible, and thin. The new wall surrounds the whole cell. It contains cellulose, but also forms of hemicellulose, which is homologous to cellulose but are branched instead of linear. The forms of hemicellulose that are involved are glucomannan, arabinoxylan, glucuronoxylan, and galactomannan. In addition, the wall also contains a complex polysaccharide called pectin.

SECONDARY WALL- the secondary wall is produced when the cell has completed its growth. It forms between the primary cell wall and the plasma membrane.. The secondary wall is where the cell wall gets most of its rigidity, weight, and thickness; sometimes however, secondary walls are not produced at all, but the primary wall is simply exposed to substances that harden it and a secondary wall is not needed. Like the primary wall, the secondary wall contains cellulose, hemicellulose, and pectin; but it also can contain tracheids, sclereids, and high amounts of xylan fibers and carbohydrates.

MIDDLE LAMELLA- A thin layer that lies between two adjacent plant cells made of pectin.

First of all the cell wall offers structural support, because since plants do not have skeletons to allow them to stand upright, the rigidity that the primary and secondary walls allows the plant to have a stiff base to support it. Its rigidity also allows the cell to retain its shape. The cell wall also decides how fast the plant will grow, and the way in which it grows, through the secondary wall being formed at the end of cell growth. The holes in the cell wall: allow certain things to come in and out, protect against harsh environmental factors and diseases, allows cells to interact, and help regulate turgor pressure. The middle lamella contains sticky pectin which makes it possible for neighboring cells to stick together.


Wood is mostly all made from the hard cellulose in the secondary cell wall, showing how truly powerful the cell wall is because it can hold up extremely trees no problem.
Cellulose is so stiff that you can actually hear it breaking when you snap or bite into celery or other vegetables.bacteria2.jpg

Other Cell Walls
Plant cells are not the only cells that have cell walls however, fungi, some prokaryotes and protests also have cell walls.

STRUCTURE- have a semirigid cell wall made out of peptidoglycan.

FUNCTION- to prevent osmotic lysis and make sure the cell does not burst from too much water flow.

STRUCTURE- made of chitin or glucan

FUNCTION- to provide structure and protection for the cell

Vaucuoles are membrane-bound sacs which are found in the cytoplasm of plant cells and some animal cells.

ch13f20.jpg plantcell.jpg

A plant cell contains a large, single central vacuole which occupies almost 90% the cells volume.
The vacuole consists of a material called sap. Because the sap is made of of a different chemical constitute than the surrounding cytoplasm, the central vacuole can maintain selectivity in transportion of materials through the vacuole's membrane.

In plant cells central vacuole is enclosed by a membrane called the tonoplast. As the cell grows, the smaller vacuoles join together forming a large vacuole. These smaller vacuoles come from the Golgi apparatus and endoplasmic reticulum.

CE725300FG0010.gif anatomy.GIF.jpg

The central vacuole has the ability to maintain turgor pressure.
The vacuole swells with an adequate amount of water supply; as it accumulats the liquid creates a high level of turgor pressure.


PLANT CELL: Central Vacuole

  • Stores minerals, salts, proteins, nutrients, as well as pigments to attract polinators
  • Aids in growth of a plant
  • Exert turgor pressure on the cell wall to provide structural support
  • Defense mechanisms- hold bitter wastes to stop preditors from eating thime
  • Less cytoplasm needed because its so big

ANIMAL CELL: Small Vacuoles
Animal cells have much smaller vacuoles which are used as temporary store house of materials.
They also expel items from the cell such as lipids, proteins, and waste products

Some protists have specialized contractile vauoles that eliminate excess water in the cell. If they did not have this type of vacuole, the water from the outside would automatically flow in with no end and the protist would explode.

Contractile vacuoles

AMOEBA and PROTOZOA- use food vacuoles to obtain molecules from outside the cell through endocytosis then excrete the waste back out through exocytosis. http://www.dnatube.com/video/358/Endocytosis


Inside of the cell

Interactive Cell Diagrams <<<<<<<<<<<<<<CHECK THIS OUT IT'S REALLY COOL


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