Biology Notes

Key Points
 Gametogenesis, the production of sperm (spermatogenesis) and eggs
(oogenesis), takes place through the process of meiosis.
 In oogenesis, diploid oogonium go through mitosis until one develops into a
primary oocyte, which will begin the first meiotic division, but then arrest; it
will finish this division as it develops in the follicle, giving rise to a haploid
secondary oocyte and a smaller polar body.
 The secondary oocyte begins the second meiotic division and then arrests
again; it will not finish this division unless it is fertilized by a sperm; if this
occurs, a mature ovum and another polar body is produced.
 In spermatogenesis, diploid spermatogonia go through mitosis until they
begin to develop into gametes; eventually, one develops into a primary
spermatocyte that will go through the first meiotic division to form two
haploid secondary spermatocytes.
 The secondary spermatocytes will go through a second meiotic division to
each produce two spermatids; these cells will eventually develop flagella and
become mature sperm.

Key Terms
 spermatocyte: a male gametocyte, from which a spermatozoon develops
 oocyte: a cell that develops into an egg or ovum; a female gametocyte
 polar body: one of the small cells that are by-products of the meiosis that
forms an egg
 mitosis: the division of a cell nucleus in which the genome is copied and
separated into two identical halves. It is normally followed by cell division
 meiosis: cell division of a diploid cell into four haploid cells, which develop to
produce gametes
Gametogenesis (Spermatogenesis and
Oogenesis)
Gametogenesis, the production of sperm and eggs, takes place through the process
of meiosis. During meiosis, two cell divisions separate the paired chromosomes in
the nucleus and then separate the chromatids that were made during an earlier
stage of the cell’s life cycle, resulting in gametes that each contain half the number
of chromosomes as the parent. The production of sperm is called spermatogenesis
and the production of eggs is called oogenesis.

Oogenesis
Oogenesis occurs in the outermost layers of the ovaries. As with sperm production,
oogenesis starts with a germ cell, called an oogonium (plural: oogonia), but this cell
undergoes mitosis to increase in number, eventually resulting in up to one to two
million cells in the embryo.

Oogenesis: The process of oogenesis occurs in the ovary’s outermost layer. A

primary oocyte begins the first meiotic division, but then arrests until later in life
when it will finish this division in a developing follicle. This results in a secondary
oocyte, which will complete meiosis if it is fertilized.
The cell starting meiosis is called a primary oocyte. This cell will begin the first
meiotic division, but be arrested in its progress in the first prophase stage. At the
time of birth, all future eggs are in the prophase stage. At adolescence, anterior
pituitary hormones cause the development of a number of follicles in an ovary. This
results in the primary oocyte finishing the first meiotic division. The cell divides
unequally, with most of the cellular material and organelles going to one cell, called
a secondary oocyte, and only one set of chromosomes and a small amount of
cytoplasm going to the other cell. This second cell is called a polar body and usually
dies. A secondary meiotic arrest occurs, this time at the metaphase II stage. At
ovulation, this secondary oocyte will be released and travel toward the uterus
through the oviduct. If the secondary oocyte is fertilized, the cell continues through
the meiosis II, completing meiosis, producing a second polar body and a fertilized
egg containing all 46 chromosomes of a human being, half of them coming from the
sperm.

Spermatogenesis
Spermatogenesis occurs in the wall of the seminiferous tubules, with stem cells at
the periphery of the tube and the spermatozoa at the lumen of the tube.
Immediately under the capsule of the tubule are diploid, undifferentiated cells.
These stem cells, called spermatogonia (singular: spermatagonium), go through
mitosis with one offspring going on to differentiate into a sperm cell, while the other
gives rise to the next generation of sperm.

Spermatogenesis: During spermatogenesis, four sperm result from each primary

spermatocyte, which divides into two haploid secondary spermatocytes; these cells
will go through a second meiotic division to produce four spermatids.
Meiosis begins with a cell called a primary spermatocyte. At the end of the first
meiotic division, a haploid cell is produced called a secondary spermatocyte. This
haploid cell must go through another meiotic cell division. The cell produced at the
end of meiosis is called a spermatid. When it reaches the lumen of the tubule and
grows a flagellum (or “tail”), it is called a sperm cell. Four sperm result from each
primary spermatocyte that goes through meiosis.
Stem cells are deposited during gestation and are present at birth through the
beginning of adolescence, but in an inactive state. During adolescence,
gonadotropic hormones from the anterior pituitary cause the activation of these cells
and the production of viable sperm. This continues into old age.

Gametogenesis is the process whereby a haploid cell (n) is formed from a diploid cell (2n)
through meiosis and cell differentiation. Gametogenesis in the male is known
as spermatogenesis and produces spermatozoa. Gametogenesis in the female is known
as oogenesis and result in the formation of ova. In this article we shall look at both
spermatogenesis and oogenesis.

Spermatogenesis
Males start producing sperm when they reach puberty, which is usually from 10-16 years old. Sperm are
produced in large quantities (~200 million a day) to maximise the likelihood of sperm reaching the egg.
Sperm are continually produced as males need to be ready to utilise the small window of fertility of the
female.

Sperm production occurs in the testes of the male, specifically in the seminiferous tubules. The
tubules are kept separate from the systemic circulation by the blood-testis barrier.
The blood-testis barrier is formed by Sertoli cells and is important in preventing hormones and
constituents of the systemic circulation from affecting the developing sperm, and also in preventing the
immune system of the male from recognising the sperm as foreign – as the sperm are genetically
different from the male and will express different surface antigens. Sertoli cells also have a role in
supporting the developing spermatozoa.

Spermatogonia are the initial pool of diploid cell that divide by mitosis to give two identical cells. One of
these cells will be used to replenish the pool of spermatogonia – these cells are A1
spermatogonia. This replenishment of spermatogonia means that males are fertile
throughout their adult life. The other cell – type B spermatogonium – will eventually form
mature sperm.
Type B spermatogonia replicate by mitosis several times to form identical diploid cells
linked by cytoplasm bridges, these cells are now known as primary spermatocytes.
Primary spermatocytes then undergo meiosis.

 Meiosis I produces two haploid cells known as secondary spermatocytes

 Meiosis II produces four haploid cells known as Spermatids
The cytoplasmic bridges break down and the spermatids are released into the lumen of
the seminiferous tubule – a process called spermiation. The spermatids
undergo spermiogenesis (remodelling and differentiation into mature spermatozoa) as
they travel along the seminiferous tubules until they reach the epididymis.
From the seminiferous tubule they travel to the rete testis, which acts to “concentrate” the sperm by
removing excess fluid, before moving to the epididymis where the sperm is stored and undergoes the
final stages of maturation.

Spermatogenesis takes approximately 70 days, therefore in order for sperm production to be continuous
and not intermittent, multiple spermatogenic processes are occurring simultaneously within the same
seminiferous tubule, with new groups of spermatogonia arising every 16 days (spermatogenic cycle).
Each of these populations of spermatogenic cells will be at different stages of spermatogenesis.
Note that once sperm leave the male body and enter the female reproductive tract, the conditions there
cause the sperm to undergo capacitation, which is the removal of cholesterol and glycoproteins from
the head of the sperm cell to allow it to bind to the zona pellucida of the egg cell.

Oogenesis
Oogenesis differs from spermatogenesis in that it begins in the foetus prior to birth. Primordial germ cells
(which originate in the yolk sac of the embryo) move to colonise the cortex of the primordial gonad and
replicate by mitosis to peak at approximately 7 million by mid-gestation (~20 weeks). Cell
death occurs after this peak to leave 2 million cells which begin meiosis I before birth and are known
as primary oocytes. Therefore, a human female is born with approximately 2 million primary
oocytes arrested in meiosis and these make up a finite supply of potential ova.
The primary oocytes are arranged in the gonads in clusters surrounded by flattened epithelial cells called
follicular cells and these form primordial follicles. The primary oocytes are arrested in prophase
stage of meiosis I.
During childhood, further atresia (cell death) occurs, leaving ~40,000 eggs at puberty.
Once puberty begins, a number of primary oocytes (15-20) begin to mature each month,
although only one of these reaches full maturation to become an oocyte.

The primary oocytes undergo 3 stages:

 Pre-antral

 Antral

 Preovulatory

Pre-antral Stage
The primary oocyte grows dramatically whilst still being arrested in
meiosis I. The follicular cells grow and proliferate to form a stratified
cuboidal epithelium. These cells are now known as granulosa cells and
secrete glycoproteins to form the zona pellucida around the primary oocyte.
Surrounding connective tissue cells also differentiates to become the theca folliculi, a specialised layer
of surrounding cells that is responsive to LH and can secrete androgens under its influence.
Antral Stage
Fluid filled spaces form between granulosa cells, these eventually combine together to form a central fluid
filled space called the antrum. The follicles are now called secondary follicles. In each monthly cycle
one of these secondary follicles becomes dominant and develops further under the influence of FSH, LH
and oestrogen. (See article on the menstrual cycle).
Pre-Ovulatory Stage
The LH surge induces this stage and meiosis I is now complete. Two haploid cells are formed within the
follicle, but they are of unequal size. One of the daughter cells receives far less cytoplasm than the other
and forms the first polar body, which will not go on to form an ovum. The other haploid cell is known
as the secondary oocyte. Both daughter cells then undergo meiosis II, the first polar body will replicated
to give two polar bodies but the secondary oocyte arrests in metaphase of meiosis II, 3 hours prior to
ovulation.
Ovulation
The follicle has grown in size and is now mature – it is called a Graafian follicle. The LH surge increases
collagenase activity so that the follicular wall is weakened, this combined with muscular contractions of
the ovarian wall result in the ovum being released from the ovary and being taken up into the fallopian
tube via the fimbriae (finger-like projections of the fallopian tube).
Fertilisation
The secondary oocyte will only complete meiosis II on fertilisation, giving off a third polar body once
meiosis II is completed and a fertilised egg. If fertilisation never occurs, the oocyte degenerates 24 hours
after ovulation, remaining arrested in meiosis II.

If the egg is fertilised however, the peristaltic movements of the fallopian tube move the egg to the
uterus where it can implant into the posterior uterine wall.

Structure of Ovum
Despite its large size - it's the only animal cell you can see with the naked eye and is as
big as the period at the end of this sentence - most of the egg cell is padding, layers of
which protect the valuable information in its nucleus.

Most of the inner structures of

the egg cell are the same as
those in any other animal cell,
but they are given special
names. For example, the nucleus
is referred to as the 'germinal
vesicle' and the nucleolus as
the 'germinal spot.'

The cytoplasm of the ovum is

called the 'ooplasm' (meaning
'egg material') or 'vitellus.' As if
two names were not enough, it
is also known as the 'yolk' of
the egg. This can be a bit
confusing when you think of one of the most common, visible and edible ovum around
the chicken egg, in which the yolk looks like the nucleus of the cell but actually contains
most of the egg cell. The yolk supplies nutrients to the growing embryo, a smaller
amount in mammals compared to that of egg-laying animals.
?cSperm
A sperm has three main parts:

1. The head of the sperm contains

the nucleus. The nucleus holds
the DNA of the cell. The head also
contains enzymes that help the
sperm break through the cell
membrane of an egg.

2. The midpiece of the sperm is

packed with mitochondria. Mitochondria are organelles in cells that
produce energy. Sperm use the energy in the midpiece to move.

3. The tail of the sperm moves like a propeller, around and around. This tail is a
long flagella that pushes the sperm forward. A sperm can travel about 30 inches
per hour. This may not sound very fast, but don’t forget how small a sperm is.
For its size, a sperm moves about as fast as you do when you walk briskly.

Sperm Production
To make sperm, cells start in the testes and end in the epididymis. It takes up to two
months to make sperm. The steps are explained below:

1. Special cells in the testes go through mitosis (cell division) to make identical
copies of themselves.

2. The copies of the original cells divide by meiosis, producing cells

called spermatids. The spermatids have half the number of chromosomes as
the original cell. The spermatids are immature and cannot move on their own.

3. The spermatids are transported from the testes to the epididymis. Involuntary
muscular contraction moves the spermatids along.

4. In the epididymis, spermatids slowly grow older and mature. They grow a tail.
They also lose some of the cytoplasm from the head. It is here that the
spermatids mature, becoming sperm cells.

5. When sperm are mature, they can “swim.” The mature sperm are stored in the
epididymis until it is time for them to leave the body.
Sperm leave the epididymis through the vas deferens. As they travel through the vas
deferens, they pass by the prostate and other glands. The sperm mix with liquids from
these glands, forming semen. The semen travels through the urethra and leaves the
body through the penis. A teaspoon of semen may contain as many as 500 million
sperm!

Cell Transport
Introduction

Imagine living in a house that has walls without any windows or doors. Nothing could
enter or leave the house. Now imagine living in a house with holes in the walls instead
of windows and doors. Things could enter or leave the house, but you wouldn’t be able
to control what came in or went out. Only if a house has walls with windows and doors
that can be opened or closed can you control what enters or leaves. For example,
windows and doors allow you to let the dog in and keep the bugs out.

Transport Across Membranes (Diffusion)

If a cell were a house, the plasma membrane would be walls with windows and doors.
Moving things in and out of the cell is an important role of the plasma membrane. It
controls everything that enters and leaves the cell. There are two basic ways that
substances can cross the plasma membrane: passive transport and active transport.

Passive Transport
Passive transport occurs when substances cross the plasma membrane without any
input of energy from the cell. No energy is needed because the substances are moving
from an area where they have a higher concentration to an area where they have a
lower concentration. Concentration refers to the number of particles of a substance per
unit of volume. The more particles of a substance in a given volume, the higher the
concentration. A substance always moves from an area where it is more concentrated
to an area where it is less concentrated. It’s a little like a ball rolling down a hill. It goes
by itself without any input of extra energy.

There are several different types of passive transport, including simple diffusion,
osmosis, and facilitated diffusion. Each type is described below.
Simple Diffusion
Diffusion is the movement of a substance across a membrane, due to a difference in
concentration, without any help from other molecules. The substance simply moves
from the side of the membrane where it is more concentrated to the side where it is
less concentrated. Figure belowshows how diffusion works. Substances that can
squeeze between the lipid molecules in the plasma membrane by simple diffusion are
generally very small, hydrophobic molecules, such as molecules of oxygen and carbon
dioxide.

Diffusion Across a Cell Membrane. Molecules diffuse across a membrane from an area of higher
concentration to an area of lower concentration until the concentration is the same on both sides
of the membrane.

Osmosis
Osmosis is a special type of diffusion — the diffusion of water molecules across a
membrane. Like other molecules, water moves from an area of higher concentration to
an area of lower concentration. Water moves in or out of a cell until its concentration is
the same on both sides of the plasma membrane.
Facilitated Diffusion
Water and many other substances cannot simply diffuse across a membrane.
Hydrophilic molecules, charged ions, and relatively large molecules such as glucose all
need help with diffusion. The help comes from special proteins in the membrane known
as transport proteins. Diffusion with the help of transport proteins is
called facilitated diffusion. There are several types of transport proteins, including
channel proteins and carrier proteins.

 Channel proteins form pores, or tiny holes, in the membrane. This allows water

molecules and small ions to pass through the membrane without coming into

contact with the hydrophobic tails of the lipid molecules in the interior of the

membrane.

 Carrier proteins bind with specific ions or molecules, and in doing so, they

change shape. As carrier proteins change shape, they carry the ions or molecules

across the membrane.

[Figure 2]
Facilitated Diffusion Across a Cell Membrane. Channel proteins and carrier proteins help
substances diffuse across a cell membrane. In this diagram, the channel and carrier proteins are
helping substances move into the cell (from the extracellular space to the intracellular space).

Active Transport
Active transport occurs when energy is needed for a substance to move across a
plasma membrane. Energy is needed because the substance is moving from an area of
lower concentration to an area of higher concentration. This is a little like moving a ball
uphill; it can’t be done without adding energy. The energy for active transport comes
from the energy-carrying molecule called ATP. Like passive transport, active transport
may also involve transport proteins.

Sodium-Potassium Pump
An example of active transport is the sodium-potassium pump. When this pump is in
operation, sodium ions are pumped out of the cell, and potassium ions are pumped into
the cell. Both ions move from areas of lower to higher concentration, so ATP is needed
to provide energy for this “uphill” process. Figure below explains in more detail how
this type of active transport occurs.

[Figure 3]

The sodium-potassium pump. The sodium-potassium pump moves sodium ions (Na+) out of the
cell and potassium ions (K+) into the cell. First, three sodium ions bind with a carrier protein in
the cell membrane. Then, the carrier protein receives a phosphate group from ATP. When ATP
loses a phosphate group, energy is released. The carrier protein changes shape, and as it does, it
pumps the three sodium ions out of the cell. At that point, two potassium ions bind to the carrier
protein. The process is reversed, and the potassium ions are pumped into the cell.

Vesicle Transport
Some molecules, such as proteins, are too large to pass through the plasma membrane,
regardless of their concentration inside and outside the cell. Very large molecules cross
the plasma membrane with a different sort of help, called vesicle transport. Vesicle
transport requires energy, so it is also a form of active transport. There are two types
of vesicle transport: endocytosis and exocytosis.

Bulk Transport

Imagine you are a macrophage: a merciless white blood cell that stalks, amoeba-like,
through the tissues of the body, looking for pathogens, dead and dying cells, and other
undesirables. When you encounter one of these, your task is not just to destroy it, but
to devour it whole. (Chomp!)

This complete annihilation may seem a bit over the top, but it serves two useful
purposes. First, it recovers valuable macromolecules for the body’s use. Second, in the
case of foreign pathogens, it allows the macrophage to present fragments of the
pathogen on its surface. This display alerts other immune cells that the pathogen is
present and triggers an immune response.

Let’s take a step back, though. How does a macrophage “eat” a pathogen or a piece of
cellular debris? In the past few sections, we’ve talked about ways that ions and small
molecules, such as sugars and amino acids, can enter and exit the cell via channels and
transporters. Channels and carrier proteins are great for letting specific small molecules
cross the membrane, but they are too small (and too picky about what they transport)
to let a cell take up something like an entire bacterium.

Instead, cells need bulk transport mechanisms, in which large particles (or large
quantities of smaller particles) are moved across the cell membrane. These mechanisms
involve enclosing the substances to be transported in their own small globes of
membrane, which can then bud from or fuse with the membrane to move the
substance across. For instance, a macrophage engulfs its pathogen dinner by extending
membrane "arms" around it and enclosing it in a sphere of membrane called a food
vacuole (where it is later digested).

Macrophages provide a dramatic example of bulk transport, and the majority of cells in
your body don’t engulf whole microorganisms. However, most cells do have bulk
transport mechanisms of some kind. These mechanisms allow cells to obtain nutrients
from the environment, selectively “grab” certain particles out of the extracellular fluid,
or release signaling molecules to communicate with neighbors. Like the active
transport processes that move ions and small molecules via carrier proteins, bulk
transport is an energy-requiring (and, in fact, energy-intensive) process.

Here, we’ll look at the different modes of bulk transport: phagocytosis, pinocytosis,
receptor-mediated endocytosis, and exocytosis.

 Endocytosis is the type of vesicle transport that moves a substance into the

cell. The plasma membrane completely engulfs the substance, a vesicle pinches

off from the membrane, and the vesicle carries the substance into the cell. When

an entire cell is engulfed, the process is called phagocytosis. When fluid is

engulfed, the process is called pinocytosis.

 Exocytosis is the type of vesicle transport that moves a substance out of the

cell. A vesicle containing the substance moves through the cytoplasm to the cell

membrane. Then, the vesicle membrane fuses with the cell membrane, and the

substance is released outside the cell.

[Figure 4]

Illustration of the two types of vesicle transport, exocytosis and endocytosis.

Phagocytosis

Phagocytosis (literally, “cell eating”) is a form of endocytosis in which large particles,

such as cells or cellular debris, are transported into the cell.
We’ve already seen one example of phagocytosis, because this
is the type of endocytosis used by the macrophage in the
article opener to engulf a pathogen.

Single-celled eukaryotes called amoebas also use phagocytosis

to hunt and consume their prey. Or at least, they try to – the
image series below shows a frustrated amoeba trying to
phagocytose a yeast cell that’s just a tiny bit too big.

Once a cell has successfully engulfed a target particle, the

pocket containing the particle will pinch off from the
membrane, forming a membrane-bound compartment called a
food vacuole. The food vacuole will later fuse with an organelle
called a lysosome, the "recycling center" of the cell. Lysosomes have enzymes that
break the engulfed particle down into its basic components (such as amino acids and
sugars), which can then be used by the cell.

Pinocytosis

Pinocytosis (literally, “cell drinking”) is a form of endocytosis in which a cell takes in

small amounts of extracellular fluid. Pinocytosis occurs in many cell types and takes

place continuously, with the cell sampling and re-sampling the

surrounding fluid to get whatever nutrients and other

molecules happen to be present. Pinocytosed material is held

in small vesicles, much smaller than the large food vacuole

produced by phagocytosis.

Homeostasis and Cell Function

For a cell to function normally, a stable state must be maintained inside the cell. For
example, the concentration of salts, nutrients, and other substances must be kept
within a certain range. The process of maintaining stable conditions inside a cell (or an
entire organism) is homeostasis. Homeostasis requires constant adjustments, because
conditions are always changing both inside and outside the cell. The processes
described in this lesson play important roles in homeostasis. By moving substances into
and out of cells, they keep conditions within normal ranges inside the cells and the
organism as a whole.

Lesson Summary
 A major role of the plasma membrane is transporting substances into and out of
the cell. There are two major types of cell transport: passive transport and active
transport.

 Passive transport requires no energy. It occurs when substances move from

areas of higher to lower concentration. Types of passive transport include simple
diffusion, osmosis, and facilitated diffusion.

 Active transport requires energy from the cell. It occurs when substances move
from areas of lower to higher concentration or when very large molecules are
transported. Types of active transport include ion pumps, such as the sodium-
potassium pump, and vesicle transport, which includes endocytosis and
exocytosis.

 Cell transport helps cells maintain homeostasis by keeping conditions within

normal ranges inside all of an organism’s cells.