Biology 101
28 January, 1998
Life on Planet Earth

A Brief Outline

1. Cell Walls
2. ThePlasma Membrane

3.Transportacross Membranes

• Passive Transport:

Movement down Concentration Gradients

• Energy-Requiring Transport across Membranes


• Transport across Intracellular Membranes

Plant cells have a cell wall, large central vacuole, and plasmodesmata.

Like other eukaryotes, plants have a nucleus, plasma membrane, mitochondria, and other organelles, but plant cells differ from animal, fungal, and protist cells in a number of important ways. The most obvious difference is the presence of a cell wall which surrounds each cell. It is composed primarily of cellulose, a complex carbohydrate made from glucose. Cellulose provides structural support while remaining flexible; flower petals, for instance, are flexible but will snap if folded too far.

When plant cells divide, they must form a new cell wall between them, and this is accomplished by the formation of a phragmoplast, a system of microtubules oriented along the axis of cell division which helps to guide the deposition of cellulose. This is also an important difference from the "green algae", which produce a phycoplast during mitosis, in which the microtubules lie perpendicular to the axis of division. Only plants and charophytes (green-algae) divide their cells with the aid of a phragmoplast.

Neighboring cells in plants are connected across their cell walls by extensions of the cells through pores called plasmodesmata. A plasmodesma allows cells to transfer nutrients, water, and some other diffusible materials without having to pass them across membranes or other barriers. They are also important for understanding how diseases spread through the tissues of a plant.

At maturity, most plant cells are filled with a single large vacuole, such that the nucleus, plastids, and other organelles are all close to the cell membrane. This can be seen in the picture at right. At the far right is the nearly transparent nucleus, and clusters of plastids can be seen toward the bottom and left. Those plastids which appear to be in the "middle" of the cell are actually at the periphery; remember that there is a cell wall toward you, and a wall on the far side of the cell, with the vacuole filling most of the space in between.

The vacuole is a membrane-bound bag of fluid, containing ions, stored nutrients, and waste materials. The storage of waste materials is a very important function of the vacuole, since plants cannot excrete their wastes the way animals do. The vacuole also assists the cell in maintaining turgor pressure. The high concentration of materials in the vacuole causes water to diffuse into the cell, increasing its volume. Because of the cell wall, however, the amount of space into which the cell can expand is limited. The cell thus exerts outward pressure on its cell wall, and when all cells are maintaining this pressure, the plant gains rigidity. You can notice this by comparing crisp fresh celery with celery that has sat around and grown limp. The old celery has lost fluids, and is no longer rigid.

The fluid mosaic model describes the structure of the plasma membrane.Different kinds of cell membrane models have been proposed, and one of the most useful is the Fluid-mosaic model. In this model the membrane is seen as a bilayer of phospholipids in which protein molecules are embedded.
An illustration of the Fluid mosaic model

• the plasma membrane (flexible bilayer of phospholipids and proteins that the defines the boundary of the cell) and all membranes have a similar basic structure;


• each membrane is referred to as a unit membrane and is composed of two layers of protein and lipid molecules;


• there is an intervening gap between these two layers;




• minute pores are formed in a cell membrane, through which molecules can pass through the membrane;


• it is said to be selectively, or differentially, or semi-permeable;


• on the cell surface of animal cells is an adhesive, cement-like substance that holds neighbouring cells together;


• the surface membrane of certain cells , e.g. the kidney cells, can be extended into minute finger-like processes called microvilli that increase the surface area for the functioning of these cells;

• cilia are thread-like projections of certain cells that beat in a regular fashion to create currents that sweep materials along;




• the two membranes that form the nuclear envelope, are typical cell membranes, with pits or pores that are larger than those of the plasmalemma, to allow for the free movement of RNA and other large molecules into and out of the nucleus;


• the mitochondria and chloroplasts are surrounded by double membranes;




• the lysosomes are surrounded by single unit membranes;


• the endoplasmic reticulum (ER) is formed by a complex system of membranes forming hollow sheets, or tubes, branching through the cytoplasm;




• and the Golgi body is also formed by branching tubes formed by membranes.

• The fluid portion of membranes is produced by a double layer of phospholipids: the phospholipid bilayer.

• A variety of proteins in the cell membrane form a protein mosaic.

3.Transport across Membranes

• Molecules in fluids move in response to gradients.


• Movement across membranes occurs by both passive and energy-requiring transport.

Passive Transport:
Movement down Concentration Gradients

• Molecules diffuse from areas of high concentration to areas of low concentration.


• Molecules diffuse across membranes down their concentration gradients.


• Osmosis is the diffusion of water across membranes.


• Osmosis across the cell membrane plays an important role in the lives of cells.

Energy-Requiring Transport across Membranes

• Active transport uses energy to move molecules across membranes.

Tetrodotoxin is a TOXIN that works by inhibiting a specific
voltaged-regulated Na⁺ channel. This toxin is made by the pufferfish.

Link to the pufferfish website

• Cells engulf particles or fluids by endocytosis.

Transport across Intracellular Membranes

• Vacuoles help regulate cell volume.

• Desmosomes attach cells together.
• Tight junctions leakproof the cell.
• Gap junctions and plasmodesmata allow communications between cells.

Brazil Says Recent Burning of Amazon Is Worst Ever
(read the New York Times article from 27-Jan-98)

Language Development Is Gene-Based, Study Says
(Washington Post, 27 January, 1998).

Last modified on: 4 February, 2000 by Dave Ussery