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Chapter 5
Section I:
The Cell and In Its Environment
Introduction

All cells are surrounded by a cell
membrane that separates the cells from
the outside environment.

Cells have structures that protect them
from the outside environment.

The cell has to take in needed materials
and get rid of waste.

The cell membrane controls what goes in
and out of the cell.
The Cell Membrane as a Gatekeeper


The selectively permeable (able to pass
through) structure of the cell
membrane enables it to regulate
the materials that enter and leave
the cell.
A cell membrane is usually
permeable to substances such as
oxygen, water, and carbon dioxide.
The Cell Membrane as a Gatekeeper

Substance move in and out of the
cell in one of three methods:
1.
2.
3.
Diffusion
Osmosis
Active Transport
Diffusion – Molecules in Motion


Diffusion is the main method in which
small molecules move into and out of
cells.
The process by which molecules move
from higher concentration to an area of
lower concentration.
(Crowded Room to Less Crowded Room)

The concentration of a substance is the
amount of the substance in a given
volume.
(How Strong)
What causes diffusion?




Molecules are always moving.
Collisions cause molecules to push away
from one another.
The more molecules, the more collisions
that occur.
Naturally, they will spread out evenly.
Diffusion of Cells



In the pond water there are many
more molecules of oxygen in the
water than in the cell.
The structure of the cell membrane
is permeable to oxygen molecules.
The oxygen molecules diffuse from
an area of higher concentration to
an area of lower.
Osmosis – The Diffusion of Water
Molecules



The Diffusion of water molecules through
a selectively permeable membrane is
called osmosis.
Osmosis is important to sustain life
because cells cannot function properly
without adequate water.
In osmosis, water molecules move by
diffusion from an area where they are
highly concentrated through the cell
membrane to an area where they are less
concentrated.
Osmosis – The Diffusion of Water
Molecules


Red blood cells are bathed in a
solution in which concentration of
water is the same as it is on the
inside of the cells.
Salt water pulls water from the
blood cells.
Active Transport



Passive Transport = Is the movement of
materials through a cell membrane
without using energy.
Active Transport = is the movement of
materials through a cell membrane using
energy.
The main difference between passive
transport and active transport is that
active transport requires the cell to use
energy while passive transport does not.
Transport Proteins



Transport proteins are located in the cell
membrane.
Transport proteins pick up molecules
outside the cell and carry them in using
energy.
Transport proteins carry some substances
in and out of cells:



Calcium,
Potassium
Sodium.
Transport By Engulfing



First the cell membrane surrounds,
or engulfs, a particle.
Once the particle is engulfed, the
cell membrane pinches off and
forms a vacuole within the cell.
The cell must use energy in this
process.
Why Are Cells So Small



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One reason is related to how materials move
into and out of cells.
As a cell’s size increases, more of its cytoplasm
is located farther from the cell membrane.
Once a molecule enters a cell, it is carried to its
destination by a stream of moving cytoplasm.
But in a very large cell, the streams of
cytoplasm must travel farther to bring materials
to all parts of the cell = more time for to
complete tasks and remove wastes.
Why Are Cells So Small


If a cell grew too large, it could not
function well enough to survive.
When a cell reaches a certain size,
it divides into two new cells.